Engineering Metallurgy Engineering

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PITMAN METALLURGY SERIES

FRANK T. SISCO, Advisory Editor

pitman PUBLISHING CORPORATION >*

Metallurgy

By THE COMMITTEE ON METALLURGY

-4 collaborative writing group of metallurgy professors.

1.2

Associated Companies

Sir Isaac Pitman & Sons, Ltd. London Mcllraurne Johannesburg Sir Isaac Pitman & Sons (Canada), Ltd. Toronto / t &9 /

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PRINTED in the United States of America Coauthors

Theodore Allen, Jr., M.S.M.E., Associate Professor of Mechanical En- gineering, University of Houston, Houston, Texas; Engineer Associated with Anderson, Greenwood and Co., Bellaire, Texas

Lee L. Amidon, M.S.M.E., Professor and Head, Department of Mechani- cal Engineering, South Dakota State College, Brookings, South Dakota

John K. Anthony. M.S.. Associate Professor of Physical Metallurgy, Uni- versity of Arizona, Tucson, Arizona

Robert E. Bannon, S.M., Professor of Metallurgy, Newark College of Engineering, Newark, New Jersey

Francis William Brown, Ph.D., Associate Professor, Clarkson College of Technology, Potsdam, New York

Frederick Leo Coonan, D.Sc, Professor and Chairman, Department of Metallurgy and Chemistry, U.S. Naval Postgraduate School, Monterey, California

Howard P. Davis, M.S., Associate Professor, Department of Mechanical Engineering, University of Wyoming, Laramie, Wyoming

Harold Vincent Fairbanks, M.S., Professor of Metallurgical Engineering, West Virginia University, Morgantown, West Virginia

Mars G. Fontana, Ph.D., Professor and Chairman, Department of Metal- lurgical Engineering, The Ohio State University, Columbus, Ohio

Arthur R. Foster, M.Eng., Associate Professor of Mechanical Engineer- ing, Northeastern University, Boston, Massachusetts

Arthur C. Forsyth, Ph.D., Associate Professor of Metallurgical Engi- neering, University of Illinois, Urbana, Illinois

Richard Edward Grace, Ph.D., Associate Professor of Metallurgical En- gineering, Purdue University, Lafayette, Indiana

Leonard B. Gulbransen, Ph.D., Associate Professor, Washington Uni- versity, St. Louis, Missouri

Joseph Gurland, D.Sc, Assistant Professor, Division of Engineering, Brown University, Providence, Rhode Island

Walter R. Hibbard, M.S., Adjunct Associate Professor of Metallurgy, College of Engineering, University of Bridgeport, Bridgeport, Con- necticut vi Coauthors

Walter M. Hirthe, M.S.M.E., Assistant Professor of Mechanical En- gineering, College of Engineering, Marquette University, Milwaukee, Wisconsin

Abraham Eldred Hostetter, Ph.D., Professor of Metallurgy, Kansas State College, Manhattan, Kansas

John J. Kaufman, Metallurgy Department, Academy of Aeronautics, Flushing, New York

J. Edward Krauss, M.S., Head, Department of Mechanical Technology, New York City Community College, Brooklyn, New York

Hollis Philip Leighly, Jr., Ph.D., Chairman, Department of Metallurgy, University of Denver, Denver, Colorado

Irving J. Levinson, M.S., Professor of Mechanical Engineering, Lawrence Institute of Technology, Detroit, Michigan

Jules Washington Lindau, III, M.E., Associate Professor of Mechanical Engineering, The University of South Carolina, Columbia, South Carolina

James R. MacDonald, Ph.D., Chairman, Department of Mechanical En- gineering, School of Engineering, The University of Mississippi, Uni- versity, Mississippi

Omar C. Moore, M.S., Associate Professor of Chemical Engineering, Alabama Polytechnic Institute, Auburn, Alabama

Don R. Mosher, B.S., Assistant Professor of Mechanical Engineering, University of Colorado, Boulder, Colorado

Charles Arthur Nagler, Ph.D., Associate Professor, Department of Chemical and Metallurgical Engineering, Wayne State University, Detroit, Michigan

Richard O. Powell, College of Engineering, Tulane University, New Orleans, Louisiana

Oran Allan Pringle, M.S.M.E., Associate Professor of Mechanical En- gineering, University of Missouri, Columbia, Missouri

Kenneth E. Rose, M.S., Professor of Metallurgical Engineering, Uni- versity of Kansas, Lawrence, Kansas

Philip C. Rosenthal, M.S., Professor of Metallurgical Engineering, Uni- versity of Wisconsin, Madison, Wisconsin

Robert E. Shaffer, M.S., Associate Professor of Engineering, University of Buffalo, Buffalo, New York Coauthors vii

Walter E. Short, M.S.M.E., Associate Professor of Mechanical Engineer- ing, Bradley University, Peoria, Illinois

Floyd Sheldon Smith, M.S., Associate Professor of Mechanical Engineer- ing, Alabama Polytechnic Institute, Auburn, Alabama

GEORGE V. Smith, Ph.D., Assistant Director for Metallurgical Engineer- ing, School of Chemical and Metallurgical Engineering, College of Engineering, Cornell University, Ithaca, New York

Sicmund Levern Smith, M.Met.E., Professor of Metallurgy, College of Mines, University of Arizona, Tucson, Arizona

Joseph William Spretnak, Ph.D., Professor of Metallurgical Engineer- ing, The Ohio State University, Columbus, Ohio

Rocer Greenleaf Stevens, Ph.D., Head, Department of Chemical En- gineering, Southwestern Louisiana Institute, Lafayette, Louisiana

William H. Tholke, B.S., Instructor of Metallurgy, University of Cin- cinnati, Cincinnati, Ohio

John Stanton Winston, M.A., M.S., Chairman, Department of Metal- lurgy, Mackay School of Mines, University of Nevada, Reno, Nevada

Preface

Engineering Metallurgy was developed to present in a concise, understandable manner the principles of ferrous and nonferrous metallurgy for all engineers—student and practicing. Both graduate and undergradu- ate student engineers need a fundamental knowledge of the metals they will employ in their work. The emphasis, throughout therefore, is on metallurgical principles rather than on handbook information; however, specific data are given so as to provide a realistic structure to reinforce

the theoretical presentations. The practicing engineer who has had little contact with the field of metallurgy, or who has had no formal work in

metallurgy, will find in this book a sufficiently complete summary of all of the essentials he needs to know to obtain a broad understanding of the field.

Keeping up with metallurgical developments in all branches of the

art, as reported in the technical literature of the world, is difficult for those actively engaged in the manufacture, processing, or the industrial use of engineering metals and alloys. For the thousands of such indi- viduals, who work with or who use metallic materials but who cannot possibly find time to read everything, summaries such as this book have a well-defined place in the scheme of things.

Because metallurgy is such a dynamic and diversified art and science, the preparation of a definitive, up-to-date, authoritative work in this field required a bold approach. Forty professors actually engaged in teaching engineering metallurgy in universities across the country were asked to pool their knowledge and research to produce this text. Through intensive questionnaire techniques, the scope and content of the book were first defined and outlined by the group. Once the basic content had been selected, ideas for all chapters were channeled to experts selected to serve on individual chapter committees. From these ideas and their own combined experience and research, each committee built chapter out- lines. Overlaps and omissions were detected by the editorial staff and referred to the committees for alteration and preparation of rough draft. The coauthors read and checked the smoothness of presentation of the chapter, adding to and refining the draft. Thus were built the twenty- three chapters of Engineering Metallurgy by the forty coauthors. Chapters 1 through 6 deal with the general principles of metallurgy as they are related to engineering. Chapter 3 (Factors Affecting En- gineering Properties) will be especially useful to the student in gaining

ix x Preface

an appreciation of the over-all study of engineering metallurgy. In Chap-

ter 6 (Phase Diagrams and the Simply Alloy Systems) , the student is introduced to basic problems of equilibrium and alloying. Chapters 7 through 10 treat of the nonferrous metals and alloys. To these important materials, a generous amount of space has been devoted so that com-

plete coverage could be obtained. Chapters 1 1 through 20 arc concerned with iron and steel—with special attention given to the subjects of heat treatment and ferrous alloys. Machinability, corrosion, and the effects of temperature are fully covered in Chapters 21 to 23.

All technical terms are defined as they are introduced, and stress is

laid upon fundamental concepts. At the end of each chapter there is

a set of questions and exercises constructed to help the student focus attention on the important definitions and principles presented in the chapter.

Principles, definitions, and illustrative examples are set down with precision and clarity. Drawings and photographs are used without re- serve to amplify the discussion. In certain chapters, detailed tables are included for the convenience of the reader.

The Committee on Metallurgy is aware that the usage of this text and developments in the field will indicate areas needing revision. Professors and students are therefore urged to send comments on chapters to the publisher or chapter committees so that appropriate changes may be made in the next edition. The Editor Contents

Preface v

Chapter 1. Metallurgy and Engineering 1

1.1. Metallurgy as an Art 1

1.2. Metallurgy as a Science 2

1.3. Metallurgy and Engineering 3

Chapter 2. Fundamental Structure of Metals and Alloys 5

2.1. Building Blocks of Matter 5

2.2. The Building-Up Principle 9

2.3. Types of Bonding in Solids 9

2.4. Assemblages of Atoms 11

2.5. Defects in Crystals 14

2.6. Polycrystalline Aggregates 16

2.7. Interactions in Metallic Solutions 18

2.8. Liquid Solutions 19 2.9. Solid Solutions 20 2.10. Intermediate Phases 20

Chapter 3. Factors Affecting Engineering Properties ... 23 3.1. Grain Size Control 24

3.2. Effect of Grain Size on Properties 29 3.3. Deformation of Metals 32 3.4. Slip in Single Crystals 33 3.5. Twinning 36 3.6. Deformation of Polycrystalline Metals 37 3.7. Hot Working 38 3.8. Cold Working 40 3.9. Annealing Cold Worked Metal 42 3.10. Factors Affecting Rccrysiallization Temperature and Grain Size 45

xi xii Contents

3.11. Summary of Hot and Cold Working : 46 3.12. Solid Solution Effects 47 3.13. Polyphase Structures 49

3.14. Allotropic Transformation 51 3.15. Precipitation Hardening 52

Chapter 4. Static Properties of Metallic Materials ... 57 4.1. Properties of Metallic Materials 57 4.2. The Relative Standardization of Static Tests 60

4.3. Tensile Strength 61

4.4. Elastic Limit, Proportional Limit, and Modulus of Elas- ticity 63 4.5. Yield Point and Yield Strength 66 4.6. Elongation and Reduction of Area 67 4.7. Hardness 69 4.8. Comparison of the Various Hardness Tests 72 4.9. Relation among Hardness Tests and between Hardness and Tensile Strength for Steel 73 4.10. Relation among Hardness Tests and between Hardness and Tensile Strength for Nonferrous Alloys .... 74 4.11. Shear, Compression, and Bend Tests 77 4.12. Sonic Testing 78

Chapter 5. Dynamic Properties of Metallic Materials . . 81

5.1. Notch Brittleness and Transition from Ductile to Brittle Fracture 81 5.2. Value of the Notched-Bar Impact Test 85 5.3. The Mechanism of Fatigue 86 5.4. The Endurance Limit 87

5.5. Relation of the Endurance Limit to Other Properties . . 88 5.6. The Effect of Notches on the Endurance Limit .... 89 5.7. Corrosion Fatigue 92 5.8. Increasing the Endurance Limit by Shot Pcening ... 92 5.9. Significance of Endurance Data 93

5.10. Damping Capacity 94

Chapter 6. Phase Diagrams and the Simple Alloy Systems 97 6.1. Solid State Thermodynamics and Thermostatics ... 97 Contents xiii 6.2. Concept of Dynamic Equilibrium 97 6.3. Cooling Curves 98 6.4. Solid State Mass Transfer 99 6.5. The Phase Rule 100 6.6. Solid State Solubility 101

6.7. Binary Systems 101 6.8. Intermetallic Compounds 106

6.9. The Peritectic Reaction . • 108 6.10. Closure 109

Chapter 7. Heat Treatment of Alloys by Precipitation Hardening Ill 7.1. Alloy Requirements . Ill 7.2. Step I—Solution Heat Treatment 112 7.3. Step II— Precipitation Heat Treatment (Aging) .... 1 14 7.4. Theory of Precipitation Hardening 114 7.5. The Effect of Time and Temperature During Precipitation Heat Treatment 117 7.6. Precipitation Hardening of Aluminum Alloys .... 119 7.7. Precipitation Hardening of Magnesium Alloys .... 121 7.8- Precipitation Hardening of Copper Base Alloys .... 124 7.9. Precipitation Hardening in Low Carbon Steel .... 125 7.10. Strain Aging and Its Consequences in Low Carbon Steel 126

7.11. Precipitation Hardening in Alloy Steels 126

Chapter 8. Light Alloys as Engineering Materials .... 129 8.1. History of Aluminum 129 8.2. Economics of the Aluminum Industry 130

8.3. Aluminum Ores—Occurrence and Concentration . . . . 131 8.4. Manufacture of Aluminum 132 8.5. Physical Properties of Aluminum 133 8.6. Chemical Properties of Aluminum 134 8.7. Aluminum Alloys 135 8.8. The Wrought Alloys 136 8.9. Aluminum Casting Alloys 140 8.10. Functions of Alloying Elements 143 8.11. Cold and Hot Working of Aluminum Alloys 147 xiv Contents 8.12. Heat Treatment of Aluminum Alloys 148 8.13. Corrosion Resistance of Aluminum Alloys 150 8.14. Joining of Aluminum Alloys 152

8.15. Magnesium and Its Alloys 155 8.16. Beryllium 158

8.17. Titanium 159

Chapter 9. Copper and Copper-Base as Engineering Materials 161

9.1. Properties and Uses of High-Purity Copper 164

9.2. Constitution of the Common Copper-Rich Alloys . . . 165 9.3. Nomenclature of the Copper-Rich Alloys 167 9.4. Characteristics and Uses of the High Brasses 168

9.5. Characteristics of the Low Brasses and the Nickel Silvers 170

9.6. Properties of the Wrought Brasses 171

9.7. Cast Brass and Cast Nickel Silver 172

9.8. The Copper-Base Bearing Metals 172

9.9- The Tin Bronzes 174 9.10. Aluminum Bronze and Copper-Silicon Alloys 176

9.11. Copper-Beryllium Alloys 177

9.12. The Copper-Rich Copper-Nickel Alloys 184

9.13. Copper and Copper-Base Alloys in Powder Metallurgy . . 186

Chapter 10. Miscellaneous Heavy Nonferrous Metals and Alloys 190

10.1. The White Metals 191 10.2. Lead and Tin as Engineering Materials 196

10.3. Zinc and Zinc Base Alloys as Engineering Materials . . . 197 10.4. Nickel and Nickel Base Alloys 199 10.5. Cobalt Base Alloys 201 10.6. Other Miscellaneous High Temperature Metals and Alloys 201

10.7. The Precious and Semi-Precious Metals and Alloys . . . 202 10.8. Metals and Alloys in Atomic Power Applications .... 203

Chapter 11. The Manufacture and Composition of Carbon and Alloy Steels 208

11.1. Definitions of Ferrous Engineering Materials 210

11.2. Iron Ore and the Manufacture of Pig Iron 212 11.3. Acid and Basic Processes 216 Contents xv

11.4. Bessemer Processes 217 11.5. Open-Hearth Processes 220

11.6. Manufacture of High-Quality Steels by the Electric Processes 226 11.7. Wrought Iron 227 11.8. Special Steel-making Processes 229 11.9. Mechanical Treatment of Steel 231 11.10. Harmful Elements in Carbon and Alloys Steels .... 233 11.11. Manganese in Carbon and Alloy Steels 236 11.12. Carbon Monoxide, and Rimming and Killed Steels . . . 237 11.13. Silicon and Other Degasifiers 239 11.14. Other Elements 239 11.15. Low-Alloy Steels . . 240 11.16. High-Alloy Steels 241

Chapter 12. The Constitution of Steel 244 12.1. The Allotropy of Iron 244 12.2. Iron-Carbon Phase Diagram 245 12.3. Phase Changes and Microstructures of Slowly Cooled Plain Carbon Steels 247 12.4. Isothermal Transformation in Steel 251 12.5. The Effect of Cooling Rate Upon the Resulting Structure 254 12.6. Effect of Alloying Elements Upon the Iron-Carbon Dia- g™™ 256 12.7. Effect of Alloying Elements on the Isothermal Transfor- mation of Steel 257 12.8. Effect of Hot Working on Structure 258 12.9. Effect of Cold Working on Structure 259

Chapter 13. Fundamentals of Heat Treatment of Steel . . 263 13.1. Grain Size and Grain Growth 264 13.2. Controlling and Classifying Grain Size 265 13.3. Effects of Hot Working on Grain Size 267 13.4. Hardenability 269 13.5. Grain Size and Hardenability 270 13.6. Quenching and Properties of Martensite 270 13.7. The Instability of Quenched Carbon Steels 273 XVI Contents

13.8. Retained Austenite and Cold Treatment 274 13.9. Structural and Other Changes in Tempering 274

Chapter 14. The Operations of Heat Treatment 278 14.1. Heating Cycle 279 14.2. Annealing 280 14.3. Normalizing 280 14.4. Spheroidizing 281 14.5. Quenching for Hardening 282 14.6. Tempering 284 14.7. Isothermal Treatments 285 14.8. Case Hardening Processes ' 289 14.9. Flame Hardening and Induction Hardening 293

Chapter 15. Carbon Steel as an Engineering Material ... 297 15.1. Carbon-Steel Castings as Engineering Materials .... 298 15.2. Factors Affecting the Properties of Carbon-Steel Castings 300 15.3. Hot-Worked Carbon Steels as Engineering Materials . . 302 15.4. Effect of Composition on Static Properties of Hot-Worked Carbon Steels 303 15.5. Effect of Composition on Other Properties 305

15.6. Cold-Worked Carbon Steels as Engineering Materials . . 306 15.7. The Important Properties of Cold-Worked Steel ... 307 15.8. General Effects of Cold Working on Strength and Ductility 308 15.9. Variables Affecting the Properties of Cold-Worked Wire . 311 15.10. Effect of Cold-Working on Dynamic Properties . . . . 312 15.11. Heat-Treated Carbon Steels as Engineering Materials . . 312 15.12. Effect of Section Size on the Properties of Heat-Treated Medium-Carbon Steels 315

Chapter 16. Low-Alloy Steels as Engineering Materials . . 318

16.1. Balanced Compositions in Low-Alloy Steels 319 16.2. General Effects of the Alloying Elements on Carbon Steel 320 16.3. Effects of Phosphorus, Manganese, and Silicon .... 322 16.4. Effects of Nickel and Chromium 324 16.5. Effects of the Other Common Alloying Elements .... 325

16.6. Low-Alloy Structural Steels as Engineering Materials . . 326 Contents xvii

16.7. Composition and Properties of the Low-Alloy Structural Steels 327 16.8. The S.A.E. Low-Alloy Steels 328 16.9. The S.A.E. Low-Alloy Steels as Engineering Materials . . 331 16.10. The New Metallurgy of Low-Alloy Steels 333 16.11. Similarity of Properties of Heat-Treated S.A.E. Low-Alloy Steels 334

16.12. The Engineering Properties of the S.A.E. Low-Alloy Steels 337

Chapter 17. Hardf.nability 342 17.1. Hardness and Hardenability in Carbon Steels .... 343 17.2. Hardness and Hardenability in Low-Alloy Steels ... 344 17.3. Cooling Rate and Hardenability 344 17.4. Time Delay and Hardenability 346 17.5. Variables Affecting Hardenability 346 17.6. Methods of Determining Hardenability 348 17.7. The Jominy End-Quench Hardenability Test 349 17.8. Relation of the End-Quench Test to Actual Cooling Rates and the Selection of Steel by Hardenability 350 17.9 Virtues and Shortcomings of the Jominy End-Quench Test 352 17.10. Hardenability Bands 353 17.11. Relation of Hardenability to Engineering Properties . . 354 17.12. Relation of Tempering to Hardenability 356 17.13. Fundamentals of Calculated Hardenability 356 17.14. The Accuracy of Calculated Hardenability 360 17.15. The Effect of Boron on Hardenability 361

Chapter 18. Special Purpose Steels 364 18.1. Classes of Stainless Steels 364 18.2. Constitution of High-Chromium Steels 366

18.3. Relation of the Constitution of High-Chromium Steels to Their Heat Treatment 367 18.4. Mechanical Properties of High Chromium Steels ... 369 18.5. Corrosion and Oxidation Resistance of High-Chromium Steels 370 18.6. The Constitution of 18-8 372 18.7. The Role of Carbon in 18-8 374 xviii Contents

18.8. Properties of 18-8 376 18.9. Recent Developments in Stainless Steels 378

18.10. Superstainless Steels 381 18.11. High-Nickel Steels and Special Iron-Nickel Alloys ... 384 18.12. Austenitic Manganese Steel 385

Chapter 19. Tool Steels, Die Steels, and Cemented Hard Carbides 389 19.1. High-Carbon Tool Steels 390 19.2. Low Alloy Tool Steels 395

19.3. Medium Alloy Tool and Die Steels 396 19.4. High-Alloy Tool and Die Steels 397

19.5. High Speed Steels. 398

19.6. Function of Alloy Additions in Tool and Die Steels . . 402 19.7. Cast Alloys 404

19.8. Cemented Carbide Tools 405

Chapter 20. Cast Iron 408

20.1. White Cast Iron as an Engineering Material 410 20.2. Malleable Cast Iron as an Engineering Material .... 410 20.3. Engineering Properties of Malleable Cast Iron .... 410 20.4. Gray Cast Iron as an Engineering Material 412 20.5. Structure of Gray Cast Iron 414 20.6. Relation between Properties and Structure of Gray Cast Iron 415

20.7. Effect of Cooling Rate 416

20.8. Effect of Graphite Size on Structure and Properties . . 417 20.9. Evaluation of Gray Cast Iron for Engineering Applications 419

20.10. Nodular or Ductile Cast Iron as an Engineering Material 420 20.11. Melting and Casting of Irons 422 20.12. Ternary System of Iron, Carbon, and Silicon 425

20.13. Heat Treatment of Cast Iron 430 20.14. Normal and Alloy Elements in Cast Iron 434

Chapter 21. Machinability, Wear Resistance, and Deep-Draw- ing Properties 438

21.1. Variables Affecting Machinability 438

21.2. Evaluation of Metallic Materials for Machinability . . . 438 Contents xix

21.S. Free Machining Steels 440 21.4. Relative Machinability of Steel and Nonferrous Alloys . 441 21.5. Types of Wear 442 21.6. Variables Affecting Wear Resistance 443 21.7. Evaluation of Steel for Wear Resistance 444 21.8. Importance of Deep-Drawing Properties 445 21.9. Evaluation of Steels for Deep Drawing 446 21.10. Yield-Point Elongation, Stretched Strains, and Deep Draw- ing Properties 448 21.11. Deep-Drawing Properties of Nonferrous Alloys .... 449

Chapter 22. Corrosion and Oxidation 452 22.1. Electrochemical Corrosion 452 22.2. EMF Series and Passivity 454 22.3. Uniform Corrosion 456 22.4. Galvanic or Two-Metal Corrosion 456 22.5. Concentration Cell Corrosion 458 22.6. Pitting 459 22.7. Intergranular Corrosion 460 22.8. Stress Corrosion 462 22.9. Dezincification . 466 22.10. Erosion-Corrosion 468 22.11. Methods for Combating Corrosion 469 22.12. Corrosion Testing 471 22.13. Liquid-Metal Corrosion 472 22.14. High-Temperature Oxidation 472 22.15. Formation of Oxides 473 22.16. Dependence of Oxides Growth upon Gas Pressure . . . 474 22.17. Dependence of Oxide Growth Upon Time 477 22.18. Dependence of Oxide Growth Upon Temperature . . . 479 22.19. Oxidation Prevention 480

Chapter 23. Effect of Temperature on Mechanical Properties of Metals 485 23.1. The Importance of Creep 486 23.2. The Engineering Significance of Creep 486 23.3. The Creep to Rupture Curve 488 xx Contents

23.4. Determination of Creep 489 23.5. Effect of Variables on Creep 491 23.6. Fatigue and Combined Fatigue and Creep 493 23.7. Structural Changes During Creep 495

23.8. Variation of Other Properties with Temperature . . . 498 23.9. Design for Elevated Temperature Service 499

23.10. Variation Mechanical Properties at Reduced Tempera- tures 500 23.11. Effects of Metallurgical Variables 502 23.12. Design for Low-Temperature Service ...... 503

Index 507 Engineering Metallurgy

Metallurgy and

Engi nee r i CHAPTER ng

1 Don R. Mosher, B.S., Assistant Professor of Mechanical Engineering, University of Colorado, Boulder, Colo- rado

1.1. Metallurgy as an Art

METALLURGICAL knowledge had its begin-

nings in the stone age when some ancient craftsman first recognized the difference in behavior amongst certain of the "rocks" with which he was working. The stones familiar to this primitive artisan were hard and brittle, and were capable of being fashioned into tools and weapons only by tedious shaping, chip by chip. His first encounter with metal then, undoubtedly native copper or gold, must have been an exciting experience. Here was a substance which, instead of fracturing, yielded to the blows of his stone hammer. Here was a material which could be formed much more precisely, which was pleasing to the eye, which could be worked to a keener edge, and which, moreover, somehow acquired greater strength the more it was worked. From the use of native metal to the deliberate reduction of an ore by carbon is a long step, and one which is the subject of much interesting speculation. It seems likely that the first such reduction was accidental- possibly the result of a fortunate combination of circumstances in which the heat of a campfire, together with carbon from the charred logs succeeded in reducing copper ore contained in the surrounding stones, On other occasions the stones may have contained additional metals such as tin, and the result was a remarkably stronger metal. In time, the relationship between the fire, the charred logs, and the particular types of stones was recognized, and the discoverer, the first metallurgist, began to produce metals at will. He and his progeny of the next several thousand years have accumulated a mass of information concerning the effects of variables in processing upon the properties of the final product.

1 2 Engineering Metallurgy

1.2. Metallurgy as a Science

Questions concerning the reasons why these variables resulted in the observed effects must certainly have been present in the minds of these pioneers long before the means were available to answer them. With the advent of the microscope and the X-ray, these inquiring minds began to supply the answers.

The science of metallurgy really began when Sorby, a British scientist, reported in 1864 the results of his investigations on the use of the microscope to study the structure of meteoric iron. This was followed by studies in the same general field by Martens in Germany, reported in 1878. The work of these two scientists, however, aroused little interest at the time, and nothing further was accomplished until Sorby showed to the British Iron and Steel Institute in 1886 some photomicrographs of iron and steel. This aroused much interest in the internal structure of metals, and from 1890 to 1920 many distinguished metallurgists devoted themselves to developing a science of physical metallurgy. The paramount early problem of metallurgy, which fairly cried for solution, was that of the hardening of steel—why steel containing considerable carbon was soft when cooled slowly from a red heat but hard when cooled rapidly from the same temperature. This problem occupied most of the workers in the science of metals for more than two decades. Despite the efforts of many brilliant minds, which resulted in a volume of published literature that amazes present-day metallurgists, little that was wholly decisive was accomplished until confirmatory X-ray crystallography methods came into use about forty years ago. Although some cynics say that the X-ray has created more problems than it has solved, X-ray crystallography has been a useful tool in the study of the structure of metals and the constitution of alloys. In the past three decades the science of physical metallurgy has changed remarkably. Always closely related to chemistry and physics, it has been greatly affected by the revolution that has occurred in these two sciences since 1920. The gap between chemistry and physics has been largely eliminated, and, as these sciences came together, the science of metallurgy changed from simple speculations on the structure of metals and alloys, as affected by composition or mechanical or thermal treatment and as observed by the microscope, to speculations which involve such complex abstractions as spinning electrons, statistical mechanics, electromagnetic theory, quantum theory, wave mechanics, and thermodynamics. Present-day physical metallurgists are inclined to smile condescendingly at the battles over beta iron, cement carbon, and amorphous metal which Metallurgy and Engineering 3

filled the transactions of the metallurgical societies forty or fifty years ago. It is not at all certain that even broader smiles will not be in order thirty years from now over the discussions of free energy, entropy, and mosaic structure which are filling our journals at the present time. Especially apropos in this connection are the words of a venerable man of science, Sir Ambrose Fleming, who presented a paper to the first meeting of the Physical Society of London in 1873 and who, in a formal address to the same body of scientists on "Physics and Physicists of the Eighteen Seven- ties,"* summarized his seventy years of experience by saying:

When we come to look back then on the world of physicists during the eighteen seventies, what we is find that their inventions, discoveries of fact, and ascertained principles remain with us today of permanent value, forming part of our useful knowledge. But their theories and speculations as to underlying causes and nature have nearly all passed away. Perhaps it will also be the same with our present-day work. If some sixty years hence a fellow of the Physical Society gives a talk on the physics of the nineteen thirties, he will have to record the great additions then made to knowledge of physical facts. But he may also have to say that our explanations and theories concerning them have all vanished, or at least been replaced by others also destined in turn to pass away.

1.3. Metallurgy and Engineering Until about forty years ago there was little need for the engineer to know anything about metallurgy since untreated carbon steel, hot- rolled or cold-drawn, was used for at least 95 per cent of steel structures and machines. The engineer was interested primarily in four properties- tensile strength, yield point, elongation, and reduction of area-and in having available an ample supply of cheap steel which, in addition to meeting specifications for tensile properties, would machine easily and fabricate readily. It was considered sound engineering practice to build machines and structures that would carry a much higher load than was anticipated; weight was synonymous with quality, and the heavier the structure the better the design. High factors of safety were used; consequently slight variations in quality, such as lack of structural homogeneity, surface irregularities, and numerous others, made little or no difference in designing.

This is no longer true. Weight and the strength-weight ratio (tensile strength divided by specific gravity) have become very important. Under the leadership of the automotive and aircraft industries engineers have come to realize that excess weight not only indicates poor design but is an inexcusable economic loss. The experience of the automotive and

•Nature, v. 143, 1939, pp. 99-102. 4 Engineering Metallurgy the aeronautical engineer in designing lightweight structures and machines stimulated similar efforts in other fields of engineering. This is shown by the recent developments in machine tools, in lightweight railway rolling stock, and even in bridge and building structures.

It is, of course, self-evident that the present-day emphasis on light weight in engineering design as exemplified by the automobile, airplane, and the streamlined train is directly related to the development of new types of steels and light nonferrous alloys and to new treatments for these materials. It is a moot question whether the metallurgist or the engineer was responsible for most of this development. Enthusiastic metallurgists insist that engineering progress has been the direct result of metallurgical progress; that engineers only improved their tools, machines, and structures because metallurgical art and science had produced new metallic materials for the engineer to use. There is no doubt that many engineers are too conservative and that engineering progress has at times lagged behind progress in metallurgy. On the other hand, examples could be cited where the metallurgist did not improve his product until insistent engineering demand forced it upon him. A discussion of whether this advance was pioneered by the metallurgist or the engineer is as futile as arguing whether the egg or the chicken came first; the essential fact is that important changes have taken place and that the engineer should know something of the metallurgical progress that has accompanied his changes in design.

It is, therefore, the purpose of this book to outline the recent developments in metallurgical art and in metallurgical science. This does not mean that there are long descriptions of melting and refining, or of mechanical and heat treatment, or of thermodynamics and wave mechanics; it does mean, however, that sufficient details of the present state of metallurgical art and science are given so that engineers may recognize the importance of the variables, inherent in the manufacture and treatment of metals and alloys, that affect the engineering properties and the suitability of these materials for engineering applications.

QUESTIONS

1. Distinguish between the art and the science of metallurgy. 2. What research tools have most profoundly influenced the development of metallurgical science?

3. To what other sciences is metallurgy closely related?

4. How is progress in engineering and in metallurgy inter-related?

5. Why is it becoming increasingly important for engineers to understand the fundamentals of metallurgy? Fundamental Structure

CHAPTER of Metals and Alloys

Leonard B. Gulbransen, Ph.D., Associate Professor, Washington University, St. Louis, Missouri Joseph William Spretnak, Ph.D., Professor of Metal- lurgical Engineering, The Ohio State University, Columbus, Ohio

In RECENT years the study of the solid state and the application of wave mechanics to solid state physics and metallurgy has resulted in a much clearer picture of the structure of metals and alloys. Physical and mechanical properties of the metals, such as tensile strength, ductility, electrical conductivity, diffusion, etc are dependent on structure, sometimes to a marked degree. For this reason it is desirable to discuss the structure of solids, and in particular the structure of metals and alloys.

2.1. Building Blocks of Matter

All metals are aggregates of atoms. Atoms consist of a nucleus and one or more planetary electrons. In general, except for applications of the nuclear reactions, it can be assumed that the atomic nucleus consists of positively charged protons, and neutrons with no electrical charge. Most of the mass of the atom is concentrated in the nucleus and is equal to the sum of the masses of the protons and neutrons in the nucleus. Negatively charged electrons sufficient to balance the positively charged protons in the nucleus, resulting in an electrically neutral atom, are found outside the nucleus. Electrons may be thought of as point particles, with a definite mass and electrical charge; however, their motion must be described in terms of an associated wave. The part of science that deals with this problem is known as wave mechanics. The fundamental equation de- 6 Engineering Metallurgy

scribing the motion of an electron and accompanying wave is De Broglie's equation: A = h/mV

27 where h is Planck's constant (6.62 x 10— erg-sec) , m is the mass of the electron, V is the velocity of the electron, and A is the associated wave- length. Practical use of this equation is made in electron diffraction equipment. Application of De Broglie's equation, and Heisen berg's uncertainty principle, which states that the position and momentum (mV) of a particle cannot be exactly determined simultaneously, result in a somewhat different idea of atomic structure than the classical picture of a nucleus and planetary electrons in definite fixed orbits. Application of the equations of wave mechanics results in a picture of the atom as a positively charged nucleus, with electrons in discrete, but "smeared out" orbits. The limits of the orbit can be described in terms of a probability function, in which, the probability of finding the electron at the center of the orbit is maximum, but finite and small probabilities exist for finding the electron at the limits of the "smeared out" orbit. This description of

atomic structure is sometimes described as an electron cloud picture.

1 s State 2 p State 3 d State

Fig. 2.1. Electron cloud diagrams of hydrogen atom.

In the solution of the wave equation for a given atom, the electrons are

characterized by four quantum numbers; n, I, m and m . The quantum t s number n is called the principal quantum number, and is related to the total energy of the electron. Number n may have any integral value from

+ 1 to infinity. The quantum number / is related to the angular mo-

mentum of the electron and may have any integral value from to (n-1) .

The quantum number m, is related to the magnetic moment of the elec- Fundamental Structure of Metals and Alloys 7

tron and may have any value from to ± I. The quantum number m, is related to the rotation of the electron about its own axis, and may have

values of i/ . ± 2 The four quantum numbers determine the energy of the electron in various states, with the result that an atomic system may be fully described by specifying the values for the quantum numbers for each electron.

A further result of wave mechanics is the Pauli Exclusion Principle which states that no two electrons in the same atom can have the same four quantum numbers. This principle in effect restricts a given electron to one and only one energy slate in a single atom. If then, two atoms are brought together to form a molecule, the electrons of each atom must occupy different energy states or energy levels. The idea of energy levels is important with regard to molecule formation and also to the formation of the solid state.

MAXIMUM NUMBER OF ELECTRONS

5P 10 to 6 5s 2

Up 3D "i Us

Fig. 2.2. Energy level diagram of an atom.

As an example of the above principles, consider the hydrogen atom.

The hydrogen atom consists of a proton, + 1 charge, and a mass of approximately one atomic unit. The hydrogen atom must then possess one electron. In the lowest energy state, this electron would have an energy described by the quantum numbers / = = n = 1, 0, 0, and m, = . m ; + \/2 The next atom in the periodic system, helium, with a + 2 charge on the 8 Engineering Metallurgy nucleus must possess two electrons. The quantum numbers describing these electrons, in order to fulfdl Pauli's exclusion principle, would then be: n, =1, I, = mj, = n, 1, l„ = wijj = 0, and 0, 0, m, l = + \/z , and = 0, m sl = — \/z . These two electrons have very nearly the same energy, the only difference being related to the spin quantum numbers m„, and m tl .

Both electrons are confined to the same major energy level with n = 1, and only these two electrons can occupy this level. The next atom in the periodic system, lithium, must possess three electrons. Two of the electrons have the same quantum numbers as those for helium, but the third, n = 2, I = 0, m = 0, and m, = + i/2 . A more common method of representing the electronic configuration of an atom is to use the terminology of the spectroscopist, in which the principal quantum number n is listed, followed by a letter designating the quantum number /, and indicating by a superscript the number of electrons in this particular energy state. For most metallurgical applica-

tions, the difference between energy states with different values for m ( and m, is so small that it may be neglected. According to this method of representation the hydrogen atom in its lowest energy state may be described as being in a (Is) 1 state; the helium atom in a (Is) 2 state; the

lithium atom in a (Isf (2s) • state etc. The quantum number I is designated by s for I = 0, p for / = 1, d for / = 2, and / for / = 3. By applying Pauli's exclusion principle, the s state may contain a maximum of two electrons, the p state a maximum of six electrons, and the d state a maximum of ten electrons.

ATOMIC NUMBER ELEMENT ELECTIION CONFICUR ATION

1 1 Hydrogen (Is) 2 Helium (Is)' 3 Lithium (Is)' (2s) 1 1 i Beryllium (Is)' (2s)

5 Boron (Is)' (2s)'(2p)'

6 Carbon (Is)' (2s)'(2p)'

7 Nitrogen (Is)' (2s) '(2p) 3

fi Oxygen (Is)' (2s)'(2p)< 9 Fluorine (Is)' (2s)'(2p) s 10 Neon (Is)' (2s)'(2 P)«

1 ii Sodium (Is)' <2s)'(2p)« (3s)

12 Magnesium (Is)' (2s)'(2p)« (3s)»

Fic. 2.S. Table of atoms through atomic number 12 with energy states. Fundamental Structure of Metals and Alloys 9

2.2. The Building-Up Principle

Using the principles outlined in the previous section, the electron configuration of the elements can be determined through argon, atomic number 18. However with the element potassium, atomic number 19, instead of progressing from a completed 3p energy state to the 3d state, the next electron falls into a 4s state. Calcium follows with two electrons in the 4s state. This discontinuity in the building up principle results from the fact that as electrons are added to energy levels further removed from the nucleus a resultant shielding of the positively charged nucleus takes place due to the intervening electrons, causing the energy of the 4s state to actually have a lower value than that of the 3d state. Following calcium in the periodic table, the building up of the 3d state occurs in a normal manner, inside of a completed Is state. The electron configura-

tion of nickel, atomic 2 2 2 6 number 28, being (Is) (2s) (2p) " (3s) (3p) (3d) 8 2 (4s) . The electron configuration of element atomic number 29, copper, then becomes normal with a completed 3d state and a (4s) 1 valence state. Elements with atomic number 21 through 28 are known as transition elements and are characterized by incomplete 3d energy states. In the second long period of the periodic table a similar series of elements occur with incomplete 4d energy states. The group of transition elements is important to the metallurgist because it includes many of our most important metals, such as iron, titanium, vanadium, chromium, manganese, cobalt, and nickel.

ATOMIC NUMBER TRANSITION ELEMENT ELECTRON CONFIGURATION 21 Scandium (Is)' (2s)'(2p)* (3s)'(3p)«(3d)> (4s)' 22 1 2 Titanium (Is) (2s) (2p)' (3s)'(3p)«(3d) 2 (4s)* 23 Vanadium (ls)« (2s)*(2p)« (3s)»(3p)«(3d)' (4s) 8

24 1 2 s Chromium (Is) (2s)*(2p)° (3s) (3p)«(3d) (4s)'

25 ' 2 Manganese (Is)' (2s)'(2p)° (3s) (3p)«(3d)» (4s)' 26 2 6 Iron (Is) (2s)'(2p) (3s)'(3p)"(3d)« (4s) 2

27 Cobalt (Is) 2 2 6 2 2 (2s) (2 P) (3s) (3p)«(3d)' (4s)

28 2 8 2 Nickel (Is) (2s)*(2p)« (3s)'(3p)°(3d) (4s)

Fig. 2.4. Table of atoms of the transition elements.

2.3. Types of Bonding in Solids

All solids can be classified in groups according to the bond formed between atoms in the solid state. The four types of bonds that may be formed in solids are: 10 Engineering Metallurgy

(1) The Van der Waal's bond

(2) The ionic bond

(3) The covalent bond

(4) The metallic bond. These various types of bonds may all be analyzed in terms of electron- electron, electron-proton, proton-proton interaction between atoms. Van der Waal's bonding occurs between atoms or molecules in solids due to an induced polarity on one molecule with respect to another. This type of bond is electrostatic in character and the binding force is relatively weak. Solids formed by Van der Waal's bonding have low melting and sublimation temperatures, and low strength. Solids formed by the atoms or molecules of neon, argon, and iodine are examples of Van der Waal's solids. Ionic bonds can be considered to be the result of the transfer of an electron, or electrons from one atom to another with the net result that one atom becomes positively charged, and the other negatively charged. On the basis of the building-up principle, such bonds should tend to form between atoms with one or two electrons in the valence state and atoms with one or two vacancies in the valence state. More stable electronic structures occur for completed energy levels; thus, when two atoms such as sodium and chlorine are brought together to form a solid, the valence electron of sodium is accepted by the chlorine atom to complete the valence state of chlorine, leaving a net charge of + 1 on the sodium atom, and resulting in a net charge of — 1 on the chlorine atom. The ionic bond is electrostatic in nature and results in a strong bond. Examples of ionic solids are sodium chloride, magnesium fluoride, magnesium oxide, and calcium sulfide.

The covalent bond is formed by forces arising from the sharing of electrons between atoms. The simplest example of this type of bond is the formation of the hydrogen molecule from two hydrogen atoms. The lowest energy state for the hydrogen atom is the h state in which m„ = this state can be occupied by two electrons, it follows that ± \/2 , and since if a molecule is to be formed between two hydrogen atoms, their electrons must share this lowest energy level. Many elements with more than one valence electron form crystal structures in the solid state based on this type of electron bond. To complete an octet of electrons, required for a stable structure, electrons are shared with 8-N neighboring atoms, where N is the number of valence electrons in a given element. The covalent bond is a strong bond, because of the completion of an energy level; so that covalent solids are generally speaking hard and strong. Examples of covalent solids are diamond, silicon, and graphite. Fundamental Structure of Metals and Alloys 11

The metallic bond has some of the features of both the ionic and covalent bond. When metal atoms are brought together to form a solid, a broadening and possible overlapping of energy levels occurs; so that, although a discrete number of electrons must occupy a given energy level, according to the Pauli principle, the electrons in a given energy level may have more freedom to move throughout the solid lattice under an applied electric field.

Solid

Decreasing interatomic distance

Fig. 2.5. Broadening of energy levels in a solid.

The metallic state is sometimes visualized as a structure consisting of positive ions arranged periodically in space, with a negatively charged cloud of electrons surrounding the positively charged core. An explanation of the high electrical conductivity of the metals is based on the idea of unfilled or overlapping broad energy levels. An applied electric field in either case may increase the energy of electrons in such bands and promote them to a conduction band, resulting in a net flow of electrons in the direction of the applied field. Insulators on the other hand are characterized by filled and empty levels. Thus, a normal electric field does not possess enough energy to promote electrons onto a conduction band.

2.4. Assemblages of Atoms

Of the seven crystal systems used by the crystallographer, the two most important to the metallurgist are the cubic and the hexagonal 12 Engineering Metallurgy systems. Nearly all of the common metals and alloys of commercial importance belong to these two systems. Before discussing metal structures,

it is important to define certain terms used in describing solid structures.

The Space [.attire. A space lattice is defined as a lattice of lines dividing space into equal sized prisms which stand side by side with all faces

in contact, so as to fill space with no voids. All of the points of a space lattice have identical surroundings. It can be shown that there are only fourteen possible space lattices. The Space Group. Starting with a given space lattice, there are many ways of building up actual crystals by placing atoms or molecules at various points on the space lattice. Actually there are 230 such ways, called space groups, of forming actual crystals based on the fourteen space lattices.

The Unit Cell. The unit cell is defined as a network of lines through

the points of a space lattice, dividing it into prisms. Each unit cell in a space lattice is identical in size, shape, and orientation with every other unit cell. The unit cell is the building block from which the actual crystal is constructed by repetition in three dimensions. The three most important unit cells for the metallurgist are the body-centered cubic, the face-centered cubic, and the hexagonal close-packed.

/' / 7 y «/ y \ i

/• / . f / / > / / ¥ 7 /* / r y / y

Fie. 2.6. The three unit cells.

In the cubic system all of the cube edges of the unit cell are of equal length and stand at 90° to one another. In the hexagonal system, three axes on the basal plane of equal length stand at 120° to one another with the third axis of different length at 90° to the basal plane. Special planes and special directions in crystals are of great importance in plastic deformation of metals, hardening reactions, and other aspects

of the behavior of metal crystals; therefore, it is necessary to have some method of describing such planes and directions. Fundamental Structure of Metals and Alloys 13

The most common method is to use Miller indices and indices of direction to describe the position of a plane and its direction from a fixed reference point. The Miller index system is based on the space lattice

and unit cell of the structure in question, and is derived from the intercepts of the plane along the axes of the space lattice. The three steps involved in the determination of the Miller indices of a particular plane are as follows:

(1) Determine the intercepts of the plane on the three-crystal axes as a multiple or fraction of the unit cell edge.

(2) Determine the reciprocals of these three numbers.

(3) Reduce the reciprocals to the smallest integers that are in the same ratio.

The three indices are then enclosed in parentheses. Planes that have the

Plane I Plane II

Intercepts x axis = a,, x axis = a„

y axis = a„ y axis = oo

z axis = ao z axis = oo

= Reciprocals x axis = — =1 x axis — = 1 a.

y axis = = 1 y axis = - = a„ oo

z axis — 1 /. axis =-r— = oo

Miller Index (111) (100)

Fig. 2.7. Determination of Miller indices.

same integers, either positive or negative, but in a different sequence

to planes. The (OlO) etc. belong the same family of planes (010) , , (100) belong to the same family of planes, the [100] family. Indices of direction are derived by considering the motion that must

be imparted to a point at the origin to move it in a required direction. 14 Engineering Metallurgy

The required indices are found by moving the point first along the X axis, then along the Y axis, then along the Z axis, in terms of the unit distance along these axes. The three smallest integers in the same ratio that will impart die required motion to the point are the indices of direction. Indices of direction are enclosed in square brackets. Direc- tions having the same indices, positive or negative, but in a different sequence belong to the same family. The indices of direction [100],

[O'TO], [001] belong to the same family of equivalent directions <100>. Note that reciprocals are not used in finding indices of direction.

2.5. Defects in Crystals

Tn idealized representations of metal crystals, a model of perfect crystals is generally used. In this model, a unit cell is used that possesses no imperfections and is repeated indefinitely through space with no disregistry among the unit cells. Such a model clearly is not representative of a real metal crystal. If it were, we should commonly experience tensile strengths of the order of magnitude of millions of pounds per square inch.

Real metal crystals and |>olycrystalline aggregates are characterized by

the presence of several types of defects. It is convenient to classify these defects in three categories:

1. Surface imperfections

2. Line imperfections

3. Point or atomic imperfections.

Surface imperfections include the free surface of a crystal, the grain boundary or interface between two adjoining grains in a polycrystalline aggregate, interfaces between two solid phases, the interface of a twin and the parent crystal, and sub-boundaries of the mosaic block structure. Metal crystals are made up of cubic crystallites about 10 * cm. edge

length, called mosaic blocks. There is a slight disregistry among the blocks, giving rise to interfaces formed by adjacent blocks. Line imperfections are termed dislocations. There are two basic types of dislocations, the "edge" type and the "screw" type. These dislocations are illustrated in Figure 2.8. The dislocation concept is the basis of the modern theories of plastic flow in metals.

Atomic imperfections include several types. The lattice vacancy is a normal lattice point from which the atom is missing. Interstitial defects consist of either a foreign atom in the interstitial space between normal

lattice sites, or an atom from a normal lattice site removed to an intersti- Fundamental Structure of Metals and Alloys 15

POSITIVE DISLOCATION

SCREW DISLOCATION

Fie. 2.8. Dislocations in Metals.

tial site at a considerable distance from the vacancy so formed. Finally a foreign atom may substitute for a host atom on a normal lattice site, giving rise to a substitutional defect. The defects listed above account for the flow and fracture characteristics, electrical properties including semi-conducting behavior, diffusion mechanisms, and creep characteristics of real metals and alloys. These defects are the basis of the "prior history" effect so well known in metals and alloys. The study of these defects by metal scientists has a two-fold purpose; namely (1) a better understanding of them and how they affect properties, and (2) exploration of possibilities of minimizing or eliminating these defects. For example, metal whiskers of diameter of about 10— ' cm. exhibit essentially theoretical strengths and arc thought to be free of dislocations. 16 Engineering Metallurgy

Fie. 2.9. Schematic representation of crystallization. (Rosenhain)

2.6. Polycrystalline Aggregates

In technical freezing processes such as the solidification of ingots in metal molds or castings in sand molds, many nuclei or starting sites become active. A nucleus consists of a crystallite which is large enough to be thermodynamically stable in the presence of the liquid. Each nucleus grows until it impinges on its growing neighbors. This process is illustrated schematically in Figure 2.9. The impingement process yields metal grains at the completion of freezing, since on the average, two neighboring nuclei have different orientations in space. The structure developed

is generally termed a polycrystalline aggregate. Fundamental Structure of Metals and Alloys 17

The junction of two neighboring grains gives rise to a zone of dis-

registry termed the grain boundary. The grain boundary is a few atoms in thickness and is essentially disordered in structure, showing viscous properties since the atoms in this zone belong to the lattice of neither of the two neighboring grains. Some important manifestations of grain boundaries are the following:

(1) They are barriers to the propagation of slip, the basic mechanism of plastic deformation. The smaller the grain size, the harder is the metal.

(2) Grain boundaries are high energy sites; thus, they are favored sites for nucleation of solid state reactions. They may also attract solute atoms in some cases, resulting in a segregation of the foreign atom at the boundary.

(3) Diffusion is faster along grain boundaries than in the bulk of the grain.

(4) At elevated temperatures, grain boundaries may contribute to deformation in creep by grain rotation.

The grain size range usually encountered is from 10 to 250 microns (micron = 10—* cm.) in diameter. The grain structure is usually studied with the aid of a microscope. In certain cases such as cast structures and electrical sheet, grains are easily discernible by the naked eye. An example of a polycrystalline metal is given in Figure 2.10.

Fie. 2.10. Polygonal grains of high purity iron, etched. 500x- »

18 Engineering Metallurgy

2.7. Interactions in Metallic Solutions

The initial addition of solute atoms of species B to a pure liquid or solid metal of species A results in a liquid solution or solid solution respectively. The solubility of B in A may be extensive or restricted.

If B is completely soluble in A, then it is said that B is completely miscible

in A. If B is not completely miscible in liquid A, then a second liquid phase will appear. In solid metal A, limited solubility will give rise to the appearance of a second phase, which may occur in the form of another solid solution, or as an intermetallic compound with a characteristic crystal structure. Two characteristics mainly determine the miscibility of B in A, namely the size factor and the chemical interaction among the species. The ratio of the diameter of the solute atom B to the diameter of the solvent

atom A must be such that B can either (1) substitute for A at a lattice site forming a substitutional solid solution, or (2) such that B ran fit in the interstitial space of the A lattice, giving rise to an interstitial solid

solution. The size factor is illustrated in Figure 2.11.

. Favorable I Favorable for substitutional for i solid solutions interstitial i

solid solutions i j

0.59 0.85 1.0 1.15

Ratio of size of solute atom to solvent atom—

Fie. 2.11. The size factor in the formation of solutions in metals.

Chemical interaction has to do with the strength of the bond between atoms. In an assemblage of A and B atoms, the possible pairs of nearest neighbors are A-A, B-B, and A-B. We can also refer to bond strengths by these symbols. In solutions, three general types of behavior may be experienced: ideal solution behavior, positive deviation from ideality, and negative deviation from ideality. In the case of ideal solution behavior,

the situation in bond strengths is A-A — B-B — A-B. This is to say that a given A atom has no preference as to an A or a B atom as its nearest neighbor. (The number of nearest neighbors depends on the symmetry

of the space lattice, and is expressed as the coordination number. Values for the common lattices are 12 for the face centered cubic and hexagonal mis- metals, and 8 for the body centered cubic) . In this case complete »

Fundamental Structure of Metals and Alloys 19

cibility in the liquid and solid state is expected, yielding the simple

A A 1 HH isomorphous diagram. For the situation AB > , positive deviation is experienced, meaning A atoms prefer to cluster with other A atoms, and B atoms prefer to cluster with B atoms. This gives rise to the tendency for phase separation. With increasing positive deviation, the sequence of binary diagrams is the following:

Isomorphous —» phase separation in the solid state —

peritectic with a minimum —» peritectic -h> eutectic -» eutectic with a miscibility gap in the liquid region.

For the situation AB < , negative deviation from ideality is experienced, meaning A's and B's prefer to be associated. This leads to the tendency for compound formation. The sequence of phase diagrams with increasing negative deviation is the following:

Isomorphous —» ordering —» compound formation.

An ordered solid solution is one in which the solute occupies specific lattice sites and is not distributed randomly.

2.8. Liquid Solutions

The concept of liquid solutions of metals is easy to grasp since we have common counterparts in our everyday experiences such as sugar dissolved in coffee, salt dissolved in water. A liquid solution has some important differences from the solid solutions. Probably the most important difference is that only "short-range order" exists. In a true solid, the crystalline arrangement is repeated indefinitely in space giving rise to "long-range order." When a metal melts, an increase in volume occurs (only bismuth and gallium among the pure metals contract on melting) . This increase in volume is accounted for by the formation of additional lattice vacancies and an increase in the interatomic distances, with a loss of long-range ordering. It is believed that the liquid configuration is 12-15 atoms clustered about a lattice vacancy. As the temperature of the liquid is increased, the short-range order begins to disappear and the structure approaches complete randomness.

Liquid metals and solutions in general have no ability to sustain shearing stresses. The thermal and electrical conductivities are about 50% of the values for the solid structure. The solubility of gases such as hydrogen increases with increasing temperature. When immiscibility occurs in the liquid region of an alloy, generally two liquid layers are formed because of difference in density of the two liquid phases. 20 Engineering Metallurgy

2.9. Solid Solutions

The concept of solid solutions is somewhat harder to grasp. These solutions are in reality solute atoms dissolved in the crystal lattice of the host, or solvent metal. As previously mentioned, the solute atom may substitute for a solvent atom on a lattice site, giving a substitutional solid solution, or it may lodge in the interstitial space between the solvent atoms, giving an interstitial solid solution. These two types of solid solution are illustrated schematically in a two dimensional array in

/' 7 s / yt s i —

<- / , ' /* / } / yC

/• / } ,/ / y B

Fig. 2.12. Schematic representation of solid solution.

Figure 2.12. Solid solutions are produced either by the freezing of the liquid solution, or by the diffusion of the solute atom into the lattice of the solvent atom in the solid state. The introduction of solute atoms into the lattice of the solvent metal brings about some important changes in properties:

(1) The electrical and thermal conductivities are reduced.

(2) The thermoelectric power is reduced.

(3) The strength and hardness are increased.

(4) The ductility is decreased. The formation of solid solutions is one of the fundamental methods of hardening a metal.

2.10. Intermediate Phases

When the solid solubility of a solute species is exceeded, a second phase will appear in the microstructure of the alloy. These intermediate phases can be classified into six categories: (I) Intermediate solid solutions.

These solid solutions exhibit a range of composition and lie intermediate to the components A and B. Terminal solid solutions, on the other hand, start with pure A or pure B. Fundamental Structure of Metals and Alloys 21

(2) Normal Intermetallic compounds. These are often called valency compounds since they show a definite stoichiometric ratio of A to B and a definite chemical composition. Ex- amples are Mg Pb, 2 Mg3 Bi2, PtSn 2 , AlSb, MgTe, MnSn, and AuGa. (3) Electron Compounds. These compounds are characterized by a definite valence electron to atom ratio, a definite structure, and usually a range of composition. The electron/atom ratios encountered are 3/2, 21/13, and 7/4. A common example of electron compounds is the Cu-Zn, or brass series of phases. These electron compounds are found in alloys of copper, silver, and gold. (4) The Sigma Phase.

The sigma is phase a hard, brittle phase found in certain alloy systems such as for example Fe-Cr. It has a tetragonal structure with 30 atoms in the unit cell.

(5) Interstitial Compounds. These compounds involve small solute atoms, usually hydrogen, nitrogen, and carbon, which lie in orderly arrays between layers of the larger solvent atoms. They are characterized by their high melting points, high hardness, and metallic characteristics. (6) The Laves Phases. The Laves phases are compounds which form when the difference in atomic size between solute and solvent is intermediate. They show the composition AB2 with a size ratio of 1.2/1. The formation of intermediate phases is another important method of hardening metals. The intermediate phases can further be classified as to being coherent or non-coherent. Coherent phases are those whose structure is continuous with the matrix structure and lead to precipitation hardening (age hardening) . Non-coherent phases are those whose lattice structures are discontinuous with that of the matrix and are simply mechanical mixtures. This class of intermediate phase leads to dispersion hardening. The hardness of such a mixture is mainly dependent on the average distance between the precipitate particles, or expressing it differently, the mean free path in the matrix.

QUESTIONS

1. Calculate the wave-length of an electron (m = 9.1 x 10-28 grams) with a velocity of 1.8 X 109 cm/sec. 2. Calculate the wave-length of a particle of mass 2000 pounds and a velocity of 50 miles per hour. Would it be necessary to use wave mechanics to interpret the motion of this mass? Explain. 3. Write the electron configuration of silver, molybdenum, and tungsten. 4. Discuss the difference between the ionic and Van der Waal's bond. 22 Engineering Metallurgy

5. How do the metallic bond and the covalent bond differ? 6. Show how two different space groups may be constructed on a body- centered cubic lattice. 7. Determine the number of atoms in a body-centered cubic unit cell; a face- centered cubic cell. (Note that corner and face-centered atoms are shared by other unit cells.) 8. Determine the Miller indices of a plane in the cubic system with intercepts

a„, . (a is the length of the unit cell edge) 'A a , V-i and \i a 9. Determine the intercepts of a plane in the cubic system with Miller indices of (235). 10. Sketch the direction [101] in a face-centered cubic cell. 11. Enumerate the three classifications of defects encountered in real metal crystals and give examples of each type. 12. What is the influence of these defects in metals and alloys? 13. Define a grain boundary and list some important manifestations of grain boundries. 14. Describe the two types of solid solutions formed in metal systems.

15. What is meant by the size factor in solid solutions? Give the favorable size factor range for the two types of solid solutions. 16. Discuss the arrangement of atoms in solid solutions exhibiting (a) ideal behavior, (b) positive deviation from ideality, and (c) negative deviation from ideality. 17. Give the sequence of phase diagrams expected for increasing (a) positive deviation from ideality, and (b) negative deviation from ideality. 18. Discuss the properties of liquid solutions of metals. 19. What changes occur in physical and mechanical properties when B atoms arc dissolved in the crystalline lattice of A, forming a solid solution? 20. Enumerate the six categories of intermediate phases and describe the characteristics of each. Factors Affecting Engineering Properties CHAPTER 3 Philip C. Rosenthal, M.S., Professor of Metallurgical Engineering, University of Wisconsin, Madison, Wis- consin

IF A "perfect" single crystal of metal is tested, it is found that its strength is in the neighborhood of a million pounds per square inch, exceeding by a hundred-fold or more the strength of the same metal in its ordinary polycrystalline form. According to dislocation theory, this great difference in strength between the "perfect" crystal and the ordinary polycrystalline metal can be attributed to the presence in the latter of dislocations within the grains, and to the grain boundaries themselves, which in effect, represent a series of dislocations. Therefore, the properties of polycrystalline metals are limited at the start by this characteristic atomic disregistry; and whatever means we have available for controlling their properties cannot eliminate this situation. In addition to the inherent atomic imperfections that are present, other microscopic or even macroscopic imperfections may be introduced during the manufacture of the metal part. These are such imperfections as segregation of alloying elements, presence of non-metallic inclusions, seams, porosity, voids, dissolved gas, etc., which will alter the properties of the metal and determine its "quality." These "built-in" factors cannot always be prevented and must be recognized as affecting the engineering properties to a greater or lesser degree.

Starting with a material that is already limited in its properties because of these imperfections, we can alter its properties within limits, through the utilization of one or more of the following processes or "unit operations" of engineering property control that are available to the engineer for his manipulation. These are:

23 .

24 Engineering Metallurgy

1 Grain size control 2. Mechanical working 3. Alloying 4. Heat treatment 5. Nuclear radiation. The final engineering properties of a metal may therefore be thought of as resulting from a combination of positive and negative factors. The negative factors are the dislocations and the additional imperfections of segregation, inclusions, porosity, and any other undesirable factors that

are picked up by the metal during the course of its preparation and that detract from the properties of the metal. The positive factors are those combinations of the first four of the process variables listed above that are used to obtain the properties desired. To be most effective, any one or combination of these operations should be performed on metal having a "quality" consistent with the application and costs of the part. To illustrate the extremes possible, the metal going into an ordinary boat anchor and that going into a highly stressed landing gear of an airplane can be compared. In the first instance, the most important requirement is shape and weight, and strength is decidedly secondary. Metal going into this part could contain a number of the imperfections pre-

viously listed without seriously detracting from its serviceability. Further- more, no strengthening would be required by any further processing. On the other hand, the highly stressed part of an airplane landing gear would need to be very carefully produced to avoid any imperfections and excessive inclusions. In addition, the metal would be alloyed and properly heat treated and possibly mechanically worked to get the best possible serviceability for the minimum weight of the part. Here the negative factors would be kept at a minimum and the positive factors derived from the processing developed to the highest degree. While knowledge concerning the control of all the variables affecting engineering properties is important to the engineer, it is with respect to

the processes used to manipulate the engineering properties* that he is generally concerned and these will be discussed primarily herein. Refer- ences to the factors affecting the "quality" of the metal will be made when necessary.

GRAIN SIZE AND ENGINEERING PROPERTIES

3.1. Grain Size Control Although other factors such as alloying or heat treatment may have a more profound effect on metal properties, nevertheless control of the

• The metallurgical engineer, of course, is expected to be familiar with all. Factors Affecting Engineering Properties 25

Fie. 3.1. Partially solidified iron crystals showing their dendritic appearance (Howe).

grain size of the metal can be important because such properties as toughness, high temperature strength, machinability, and formability can vary appreciably with the grain size. How grain size can be controlled will be discussed first, following which some of the more important engineering aspects of grain size control will be discussed. Casting Process. When metal is melted and poured into castings, the metal solidifies as a mass of interlocked grains. The way the metal freezes and the final form it takes is related to its composition. It is not possible to consider here all the ramifications of solidification, and further consideration of this point is given in Chapter 6, but those factors affecting the final grain size can be considered. As the temperature of the molten met;il is lowered by loss of heat to the mold walls, freezing commences by the nucleation of solid metal grains along the sides of the mold. The grains so nucleated are probably randomly oriented but a number will be so oriented that growth inward .

26 Engineering Metallurgy

Fig. 3.2. Completely frozen gold alloy showing dendritic character of each grain and grain boundaries. 100X.

will be favored, i.e. in the direction of the temperature gradient. At the same time that they grow inward, these grains will also grow at lower rates in their other dimensions as well. The growth, however, will not be like that of an expanding block but more on the order of a growing tree, with the main stem being represented by the growth inward and the branches representing the lateral expansion. This tree-like structure is related to the crystallographic structure of the metal since the lateral growth is not random (as in a tree) but confined to definite preferred directions. In the growing crystals illustrated in Figure 3.1, for instance, the lateral growth in any one grain is confined to two major directions at right angles to each other. In addition to the secondary branches, there are also tertiary branches extending therefrom. Crystals which grow in this manner are referred to as dendrites. The tree-like or dendritic growth rather than a wall-like growth in these crystallographic directions is attributed to the fact that heat can diffuse away from a protuberance much more readily than from a flat surface; consequently, there will be a natural tendency toward local extension of the growing interface.

The crystals in Figure 3.1 are only partially solidified. If allowed to freeze completely, and assuming enough liquid metal available to satisfy their needs, the grains would eventually fill out until all available space would be occupied by solid metal and each grain would be separated from the other by an irregular grain boundary (Fig. 3.2) Factors Affecting Engineering Properties 27

Fie. Polygonal 3.3. or equi-axed grains of high-purity iron etched. 500 x.

If the metal is pure, the dendritic structure displayed in Figure 3.2 will not be visible, but the grain boundaries will still be present. The dendritic structure shown in Figure 3.2 occurs because chemical heterogeneity is created during freezing and can be revealed by etching the solidified alloy. Such chemical heterogeneity would not be possible in a pure substance; therefore, evidence of the dendritic structure can only be obtained by draining liquid away from a partially solidified metal as was necessary to reveal the dendrites in figure 3.1. Whenever grains grow more rapidly in one direction than the others, they are called columnar grains. If they grow equally fast in all directions they are called equi-axed grains. Equi-axed grains of iron are shown in Figure 3.3. The final size of these grains is dependent on two factors: (1) the rate of nucleation, N, or in other words, the number of growth centers that form in a given amount of time, and (2) the rate of growth, G, of these nuclei. It should be quite obvious that if conditions are adjusted to give many nuclei and their growth is at a moderate rate, grain size will be small. On the other hand, only a few nuclei, growing at an equal rate, will produce a coarser grain structure. Both rates of nucleation and the rates of growth can be represented by complex thermodynamic functions involving a number of factors. The rate of nucleation in particular is usually affected by impurity particles or the container walls. Practically, 28 Engineering Metallurgy however, both of these rates are dependent on the cooling rate of the metal. As the cooling rate increases, the greater is the degree of super- cooling of the liquid and the greater the rates of nucleation and growth.

The relative changes of these two factors, however, is such that for most metals, rapid cooling produces a finer grain structure than slow cooling.

It is apparently impossible to repress completely nucleation in metal as is the case for glass which represents a material which does not develop an appreciable rate of nucleation until it is cooled to a temperature where

the rate of growth is too low to be effective and the glass remains a super- cooled liquid. Because nucleation of metals can be affected by impurities, some metals can be produced in a fine grain structure by the addition of certain nucleating or "inoculating" agents. The addition of sodium to an aluminum-silicon alloy, for example, produces a fine grained alloy. Mechanical vibration can also induce nucleation. During the cooling of liquid metal in a mold other changes occur that may have a bearing on the properties and behavior of the solid. Most metals contract on freezing. Since this normal contractual process occurs simultaneously with dendritic growth, interdendritic void areas may develop that cannot be fed properly. Also as metal cools, the solubility for certain dissolved gases or solids may change, resulting in their precipitation and entrapment within the grains or on grain boundaries depending on the degree and time of these changes. These conditions affect the "quality" of the metal as discussed earlier.

Mechanical Working. The previous section has described how grain size of cast metals is controlled. Many metal parts, however, are produced from metal that has been cast into an ingot and then subsequently rolled or otherwise mechanically worked into a desired shape such as a forging, bar stock, plate, angle or some other form. This mechanical working can also change the grain sue of the metal.

If the working is done at an elevated temperature (hot working) the combined effects of the heat and working action will cause recrystallization, the size of these new grains being dependent on the amount of this mechanical action and the temperature at which it is performed. As long as the temperature remains high enough to assure spontaneous recrystallization, the lower that temperature, the finer will be the grain size.

is If the metal worked at room temperature (or below for some metals) , the original grains are severely deformed and distorted. This is called cold working. However, insufficient energy is available to cause this metal to seek a new orientation (nucleate) and grow into a new set of Factors Affecting Engineering Properties 29

grains. By raising the temperature of the metal (annealing) , a temperature is reached where recrystallization docs occur and a new set of grains develops as a result of nuclcation and growth in the solid state. The size of these grains can be controlled, as described later in this chapter.

Powder Metallurgy. Metal powders can be produced by reducing oxides or other compounds, by electro-deposition, grinding, or by other means. If these are subsequently compressed and heated (sintered) a metallic compact is produced by interdiffusion of atoms between the particles, producing ultimately a metallic mass composed of grains having a grain size that is determined by the processing variables involved. Allotropic Transformation. In metals which undergo an allotropic transformation in the solid state a new set of grains is produced as a result of the transformation. Thus, in iron, which is the best example of this situation, cooling it from the high temperature face-centered cubic form to the room temperature body-centered structure results in nucleation and growth of the body-centered structure from the original face- centered structure. The size of the grains is again determined by the relative rates at which nucleation and growth occur. Slow rates of cooling promote large grain size. Other Methods for Controlling Grain Size. In addition to the foregoing methods, it is also possible to produce a solid metal of a certain grain size by deposition from a vapor or by electrodeposition from a solution. In the latter case the characteristics of the plating bath determine the grain size of the deposited metal.

3.2. Effect of Grain Size on Properties

Table 3.1 gives selected values showing the influence of grain size on the strength and ductility of pure iron.

Table 3. 1 Effect * . of Grain Size on Tensile Properties of Iron.

Grain Size, Grains Tensile Strength Yield Strength Elongation in 2 in. per sq. mm. psi ps ; m

2.5 to 9.7 23,900 to 33,700 5,900 to 6,500 28.8 to 35.3

35.6 to 77.5 38,200 to 41,600 8 , 300 to 20 , 200 44.8 to 50.7 109 to 135 42,200 to 43,300 15,100 to 22,200 42.8 to 45.3

• Metals Handbook, ASM, 1948 ed., p. 433. 30 Engineering Metallurgy

These data show that there is a substantial increase in strength as a result of decreasing grain size, with an accompanying rise in ductility. However, once the grain size reaches approximately 100 grains per sq. mm. little additional change in properties is apparent. Since metal failure usually occurs by a process of slip (described later) along certain crystallographic planes of the grains, followed by eventual fracture, the effect of decreasing grain size is to cause interference to slip among the many grains composing a fine-grained material thereby raising its strength and altering the ductility, as compared to coarse grained metal where each individual grain is freer to slip and distort with less interference from neighboring grains. Figures 3.4 and 3.5 show further examples of the effect of grain size on the strength and ductility of metals. In the case of pure copper, very little change in properties is evident, but for the 65-35 Cu-Zn brass the strength and ductility are noticeably affected by grain size. In this case, the strength decreases and ductility increases with increase in grain size.

In addition to affecting strength and ductility, grain size is also important in forming operations. Table 3.2, for example, shows the applications for brass of various grain sizes. The coarser grained material is more readily deformed but gives a rougher surface than the fine-grained material. Similar effects are noticed in deep-drawing of low-carbon steel where grain size in excess of about 0.0012 inches average diameter may lead to a surface roughness after drawing referred to as an orange peel effect (Figure 3.4) . On the other hand, extremely fine-grained or annealed low-carbon steel may show another type of defect after deep-drawing

called stretcher strains. This defect is characterized by fine wavy lines that appear on the surface of the part after the deformation (Figure

3.5) . Also, a grain size that is too small is likely to make deep-drawing difficult because of high strength and low ductility.

* Table 3.2. Grain Size of Cold Worked and Annealed Copper Alloys.

Nominal Grain Size, mm. Typical Use

0.015 Slight forming operations.

0.025 Shallow drawing.

0.035 For best average surface combined with drawing.

0.050 Deep drawing operations.

0. 100 Heavy drawing on thick gages.

• Loc, cit. Factors Affecting Engineering Properties 31

Fie. 3.4. Microstructure and surface structure of a fine-grained steel (A) and a coarse-grained steel (B). Orange-peel eficct is revealed in (B) after Erichsen cupping tests. (F Korber, . Staht & Eisen, v, 47, 1927, p. 1158).

Machining operations are also influenced by grain size. Coarse-grained steels, for example, allow for higher cutting speeds, but if finish is important a fine-grained steel is recommended. Metals under stress at elevated temperatures tend to elongate slowly with time (creep) This . behavior is associated with grain boundary movement, hence resistance where to stress at high temperatures is important, a coarse-grained metal will be superior to a fine-grained metal. The tungsten filament in a lamp bulb, for example, is produced as a coarse-grained wire because of its better life. In other high temperature applications, such as turbines, jet engines, etc, the grain size of the metal composing the high temperature components will be a factor in determining its resistance to creep. This is discussed in Chapter 23. 32 Eiigineering Metallurgy

Fie. 3.5. Stretcher strains in low-carbon steel. % actual size (1955 Supplement to Metals Handl>ook).

EFFECT OF MECHANICAL WORKING ON ENGINEERING PROPERTIES

In a previous section the effects of mechanical working on grain size were described. In addition to these effects there are other important effects of mechanical working on the engineering properties that should be considered. Before these can be discussed, however, it is necessary to review briefly the principles dealing with the deformation of metals because some knowledge of this subject is helpful for an understanding of the engineering applications of metal working.

3.3. Deformation of Metals

When a stress of a magnitude to cause only temporary displacement

is applied to a metal, the resultant strain is termed elastic strain. If the stress exceeds the limiting stress for elastic behavior, the permanent

strain resulting is termed plastic strain. With increased application of stress beyond the elastic limit, metals continue to deform until they fail by fracturing. Most nonmetals and occasionally some metals are totally lacking in this plasticity and fail by fracturing with no permanent de-

formation. This is referred to as brittle fracture. Metals, in fact, are unique among structural materials in their ability to be deformed and

it is only under special circumstances that they display a brittle fracture. Demonstrations of elastic and plastic behavior in tensile testing will be given in Chapter 4. From an atomistic viewpoint, a stress that causes elastic strain results in a temporary and small displacement of atoms from their normal posi- Factors Affecting Engineering Properties 33

Fig. 3.6. An octahedral or (111) plane. Eight such planes can be constructed in a given cube.

tions in the lattice so that the lattice is slightly distorted. Release of the stress causes the atoms to return to their normal positions and the lattice regains its original shape. In the case of plastic strain, two basic processes of atomic movement are possible—$/»'/> and twinning. These deformation mechanisms are discussed in the following sections, first with respect to single crystals and then with respect to polycrystalline metal. It will be impossible to cover all phases of this subject here, but the interested reader can find good discussions of this topic elsewhere.

3.4. Slip in Single Crystals

In plastic strain there is a permanent displacement of the atoms, the atoms moving from one atomic position in the lattice to another. The particular crystallographic structure of a single crystal restricts the movement occurring during this straining to certain preferred planes. In the face-centered cubic metals such as copper, aluminum, or nickel the preferred. plane for straining by slip is normally the (111) • plane. This is the octahedral plane that cuts diagonally through the unit cell as illustrated in Fig. 3.6. When this plane is viewed from a position at right angles to it, the atoms are close-packed much in the manner shown in Fig. 3.7b. On the other hand, it should be apparent from the left-hand illustration in Fig. 3.7 that other planes such as those parallel to the faces of the cube have a lower atomic density. Because of its high atomic density

•A system of crystallographic indexing which indicates displacement of unity in the three crystallographic directions as measured from a reference point. .

34 Engineering Metallurgy V \

(111) plane

\ (III) plane -^\ (a)

Fig. 3.7. Arrangement of atoms in the octahedral plane of the face-centered cubic unit cell as viewed from the side of the cube (left) and perpendicular to the face of the plane (right).

the octahedral plane has the greatest interatomic attraction and therefore is said to be a strong plane. As a consequence, slip occurs parallel to this plane rather than along some intersecting plane of lower atomic density. For slip to occur along a plane of lower atomic density would require that the stronger interatomic attraction existing on the octahedral planes would have to be overcome—an unlikely situation.

The direction of slip is likewise limited. These directions are shown by the arrows in Fig. 3.7 and correspond to the directions parallel to the lines of greatest concentration of atoms, and the lines of least resistance to movement of one row of atoms past another. Any other directions within the octahedral planes would require a greater disturbance of atoms since the direction of movement would be tangent to some atoms but pass directly through others. The combination of slip planes and slip directions constitutes the slip system. In face-centered cubic metals, four slip planes are possible as mentioned in Figure 3.6, each having 3 slip directions. There are therefore 12 slip systems. Since slip depends on exceeding a critical resolved shear stress that is specific for a given metal, metals crystals will not flow or deform by slip unless the applied force can be resolved into shear stress parallel to a slip plane and in one of the slip directions. This is always possible in face-centered cubic crystals; therefore flow always precedes fracturing in these crystals (unless a unique condition of equal triaxial stresses exists) A hexagonal close-packed metal such as zinc or magnesium can slip only on the basal or (0001) plane. This plane represents the plane of greatest atomic density in this type of crystal. As in the face-centered metals, there are also three directions of slip. In fact, there is nothing to Factors Affecting Engineering Properties 35

distinguish the basal plane of a hexagonal close-packed metal in atomic arrangement from the octahedral plane of the face-centered cubic metals. The differences come in the way atoms in adjacent planes are stacked above or below the reference plane. Because it has only the one slip plane and three slip directions, a hexagonal close-packed crystal has only three slip systems, and it is possible to orient such a crystal with respect to the applied load so that the slip planes are perpendicular or parallel to this load. In such cases, no flow occurs and a brittle failure ensues. Only when the basal plane is so oriented with respect to the applied load that a shear stress can be resolved in one of the slip directions can flow occur. Metals having this crystallographic structure, therefore, are not so ductile as the face-centered cubic type.

The slip planes in body-centered cubic metals are not so well defined as in the preceding two cases, and one of several possible planes can represent the plane of major slip. The slip direction, however, is always the close-packed direction represented by the cube diagonals— four in number. Around each of these four directions there are arranged 12 possible slip planes; consequently, 48 slip systems exist in these metals. Although this represents a comparatively large number of slip systems, each system is less regular and less densely packed than those of the face-centered cubic metals. In general, then, the body-centered cubic metals require somewhat higher resolved shear stresses to initiate slip, and less permanent deformation is possible in advance of fracture.

Manifestation of Slip. The visual evidence of slip is the appearance of numerous slip lines or slip bands upon microscopical examination of a polished metal surface after the specimen has been deformed plastically. These slip bands represent a permanent displacement of one part of a metal crystal with respect to the rest of the crystal. The resulting microscopical difference in elevation accounts for the appearance of lines traversing the crystal faces when viewed under a microscope. Repolishing and etching will remove this evidence of slip because the surface displacement which produced the slip lines is now removed thereby. In cases of severe distortion, however, the disturbance is so great that slip will be evident even though the distortion preceded the polishing and etching.

This is demonstrated by the cold worked brass in Fig. 3.1 1.

Strain Hardening. The amount of stress required for slip increases with an increase in the amount of prior deformation. In other words, a strain-hardening action occurs which lends to retard or restrict further slip on a given slip plane. As the stress increases, slip is transferred to other slip planes. This is the basis for hardening metals by cold working as described later. This mechanism is unique to metals and 36 Engineering Metallurgy represents one of its outstanding characteristics. Putty or clay, for example, is readily deformed and worked, but no hardening action occurs. Although much has been written about this hardening action and many theories proposed to explain it, the theory of dislocations is generally accepted today as offering the best explanation for the relatively low stress required to initiate slip and increased resistance to the slip as deformation proceeds. The low stress required to start slip is explained on the basis of a wave-like movement of a small dislocation "zone" across the crystal rather than a block-like displacement of the entire grain. The increased resistance to slip as deformation progresses is attributed to an interaction between dislocations created during the slip process. Since the dislocation theory is still in the formative stage, any explanation of strain hardening based on this theory is still somewhat speculative. The theory does, however, offer the best overall explanation for metal behavior yet available.

3.5. Twinning

Another mechanism accounting for plastic deformation of metal crystals is twinning. This process has certain features similar to those of slip and other features which are dissimilar. Like slip, twinning occurs on certain crystallographic planes and in certain directions, but the planes and directions are not necessarily the same as those for slip. Also like slip, a certain level of shear stress is required.

Twinning differs from slip in that the atomic movement is not from one atom position to the next but rather a fractional atomic distance.

This results in a new orientation in part of the grain. This orientation is such that the atoms in the area undergoing twinning present a mirror image of those in the original part of the grain when viewed along the plane dividing the twinned area from the untwinned region as shown in Fig. 3.8. Although the figure depicts an atomic movement which increases the greater the distance from the twinning plane, in reality the movement, of any one atom need only be less than one atom spacing with respect to a neighboring atom in order to accomplish the repositioning necessary for twinning.

Twins produced by mechanical action as just described are referred to as mechanical twins. Cold worked metals that are annealed also will exhibit annealing tivins which probably originated from mechanical twins. Mechanical twins are usually narrow and may be confused with slip lines. Slip lines should appear on an unpolished surface that has been strained; whereas, twin lines are usually evident only after etching. Furthermore, Factors Affecting Engineering Properties 37

Twinning plane*

Original lattice of face-centered cubic metal as Mirror image lattice as viewed parallel to twinning plane (111). created by atomic movement shown by arrows.

Fig. 3.8. Diagram showing atomic movement required for twinning.

repolishing and etching would destroy a slip line but a twin would re- appear. Slip usually shows up as lines; whereas, twins normally appear as bands.

Twinning is not capable of contributing as much permanent deforma-

tion as slip. In face-centered cubic metals, therefore, it represents only a small part of the total deformation. On the other hand, in hexagonal close-packed metals, twinning is important not because of the deformation it permits but because twinning facilitates slip by aligning more slip planes in a favorable direction.

3.6. Deformation of Pol j crystalline Metals

The metals used in engineering are, of course, polycrystalline rather than single crystals. The mechanisms previously described apply to each of the single crystals composing the aggregate. However, the extent of the movement is governed by the presence of the surrounding grains.

Consequently, the situation is a good deal more complex than in the ideal case of a single crystal. Cold working or stressing a polycrystalline aggregate composed of randomly oriented grains causes slip to occur in the most favorably oriented grains and on the most favorably oriented slip planes. As the stress increases, additional grains less favorably oriented are similarly deformed. Severe distortion in one direction will cause the following effects in addition to the slip and twinning. 38 Engineering Metallurgy

1. The lattice of each grain that is deformed tends to rotate into the direction of the applied load.

2. In addition, the constraint by surrounding grains may cause the lattice to be bent.

3. Deformation bands representing different degrees of orientation within a grain appear in the microstructure.

4. A preferred orientation of the grains develops. This is sometimes referred to as crystallographic fibering. The type of fibering that develops depends on the metal being worked and the nature of the

stress system that is operating. This effect is of importance in the cold working processes applied to metals, because the properties

are similarly rendered anisotropic and when the metal is recrystallized by annealing (discussed later) a "recrystallization texture,"

which is also of a preferred orientation, may develop.

3.7. Hot Working

If metals are mechanically worked at a high enough temperature, they do not strain harden, but simply recrystallize into new sets of

grains. When this occurs the working is referred to as hot working. Hot working temperatures depend on the characteristics of the metal

being worked. Thus, lead is essentially hot workable at room tempera-

ture because it will recrystallize during the working operation. Steel, on the other hand, must be heated to above a red heat before the operation can be classified as hot working. In either case recrystallization occurs simultaneously with the mechanical working, and the properties, with the exceptions noted in succeeding paragraphs, are not changed much.

During the working process, the shape of the metal part is usually changed drastically. As an example, in forming bar stock, the metal starts out as a bulky ingot and ends up as an elongated bar. The same relative changes occur to a greater or lesser degree in forming the many other wrought shapes by rolling, forging, extruding, or by other metal working

processes. The coarse ingot structure is refined by this operation and voids and porosity that might have been present in the original ingot are largely eliminated, but the heterogeneity originally existing in the ingot

as a result of the changes occurring during freezing is not completely irradicated. In fact, the metal working tends to elongate areas showing chemical segregation, and the inclusions are also strung out in the rolling direction. This has a tendency of producing a fiber-like structure in the metal so that the properties when tested in the direction of rolling will not be the same as those obtained from tests made at right angles to the rolling direction. This type of directionality developed by the rolling Factors Affecting Engineering Properties 39

>— n* -' iMMH

Fig. 3.9 Elongated slag inclusions in rolled steel, uneiched. IOOX.

Fie. S.10. Elongated sulfide inclusions in steel and evidence of chemical segregation in the region of these inclusions, etched. 100X.

process is sometimes referred to as mechanical fibering to distinguish it from the preferred orientation effect which may occur as the result of cold working and referred to as crystallographic fibering. Examples of the elongation of inclusions and evidence of chemical segregation are given in Fig. 3.9 and Fig. 3.10.

The principal effect of this directionality is to lower the ductility and toughness in the transverse direction as compared to the direction of .

40 Engineering Metallurgy rolling. The data in Table 3.3 illustrates how the V-notch Charpy impact values (which serve as a measure of toughness) vary with the selection of the test piece. .Similar differences but not so pronounced are also encountered in the reduction of area values obtained in tensile testing.

TABLE 3.3. Influence of Directional Effects on V-Notch Charpy Impact Values at 70-F. (9).

Specimen Transverse Specimen Parallel Nature of Steel Tensile to Principal Principal Strength Rolling Direction Rolling Direction

137,500 psi. 34-34 ft-lb. 67-64 ft-lb. Straightaway 142,000 36-37 70-71 rolled steels* 134,000 43-44 75-76 Cross rolled 132,000 54-49 62-61 steels* 155,000 44-47 46-45

Cast steel, as-cast 110,000 53-56 ft-lb. * Same steel forged * 110,000 102-122 ft-lb.

* Notch parallel to plate surface. ** From 2 x 3}^ in. section to AX in. square bar.

Outside of the effects previously noted, hot working does not alter the tensile properties of metals appreciably, particularly when compared to the effects of cold working, which can effect a substantial change in properties.

3.8. Cold Working

Cold working results in a distortion of the grain structure as depicted in the series of photomicrographs of Fig. 3.11. Beginning with the hot worked or soft tempered metals containing equi-axed grains, the succes- sive stages of cold reduction produce increasingly greater distortion and elongation of the grains and generation of slip lines. Because of the grain fragmentation and disturbance of the lattice, there is an accompanying hardening action as shown by the accompanying Rockwell B hardness values. Tensile strength increases in about the same manner as hardness, and there is a simultaneous decrease in ductility and toughness. Changes in the tensile properties of brass and copper with percentage reduction in drawing are shown in Fig. 3.12. This hardening action is attributed to internal barriers to flow such as precipitated particles, impurity atoms, etc., to interactions between dislocations moving on different slip systems, and to formation of subboundaries within the grains (polygoniza- tion) Factors Affecting Engineering Properties 41

s& m ml&E&fmiife

'-' i - - 1*3; M ,

'- S .a. .-=H : :i ----. ::

-J - • -^- ;*- «<

Fie. 3.11. Microstructure of 70% Cu-30% Zn sheet brass of various tempers.

Soft temper: i/ i/ 47 Rockwell B; 4 hard: 73 Rockwell B; 2 hard: 82 Rockwell B; hard: 89 Rockwell B; spring temper: 94 Rockwell B. (Courtesy of R. A. Ragatz, University of Wisconsin)

Cold working therefore provides a means for hardening pure metals or alloys. It is a particularly useful process where the composition is such that hardening by heat treating cannot be used. The terminology used in Fig. 3.11 (such as quarter hard, half hard, "spring temper") are commercial expressions for the degree of cold reduction obtained. De- pending upon the final finishing processing operations required, a metal hardened by cold working can be ordered from the mill in any one of the "tempers" indicated. Where heavy subsequent forming operations are intended, the annealed or partially hardened metals are specified. But if little further forming is to be done and a metal of high hardness and strength desired, the full hard or spring tempers can be specified. The actual hardness and strength for each of the tempers would vary depending on the base metal used. 42 Engineering Metallurgy

Q. O 140 O Q 120

t£ 100 c

Soft 10 20 30 40 50 60 70 80 90 Percentage Reduction by Drawing

Be. 5.12. Eircct of cold reduction on the tensile properties of copper and brass. (ASM Handbook.)

Steel can also be cold worked, and cold finished steel has certain advantages not obtainable by other means. For example, not only does cold finishing provide a smooth, oxide-free finish, but the metal itself is readily machinable due to the increased brittleness induced by the rolling action. This effect makes chip formation easier as compared to a steel of the same hardness obtained in other ways.

3.9. Annealing Cold Worked Metal

Because of its strained condition, cold worked metal can be recrystallized by reheating or annealing. In practice, this probably should be considered as essentially a fabrication procedure inasmuch as the accompanying recrystallization and softening may be necessary to allow further forming operations on the metal, but it produces properties contrary to the usual requirements of engineering materials. However, it is important to be acquainted with the principles of annealing cold worked metals because this knowledge is necessary wherever cold working is used for fabrication.

As cold worked metals are heated, eventually enough energy is supplied to form nuclei for recrystallization. These nuclei grow at the expense of the deformed metal until the entire structure is composed of a new set of grains. It is convenient to separate the process into three distinct stages:

(1) Recovery, (2) Recrystallization, and (3) Grain growth. Recovery. Recovery represents a relief of internal stress and a decrease in thermelectromotive force and electrical resistivity. Internal stresses Factors Affecting Engineering Properties 43

QJ .** co Wmm^/AalS^^^r^ Groin .c 'Recrysloll' D Rtcot/cry G'owin I notion >~ I CD

Annealing Temperature

Fin. S.13. Schematic changes in recovery, strength and recrystallization on annealing cold worked metal. (ASM Handbook)

were originally developed during the cold working process. They represent a balance of compressive and tensile stresses within the metal. Their magnitude and distribution are determined by the degree and type of cold working treatment used. These stresses are detectable only indirectly through the distortion that accompanies annealing or in the loss in

"spring-back" tendencies. "Spring-back" is the increase in diameter of a roll of strip after it has been removed from a mandrel around which it was held. The presence or absence of internal stress is one of the factors that an engineer must constantly bear in mind in design, and these stresses may be beneficial or harmful depending on how they occur within a part.

For example, shot peening a helical spring can markedly improve its service life, i.e., its resistance to fatigue failure. Distortion occurring after machining a cold-worked metal is an example of the harmful effects of residual stresses.

Fig. 3.13 shows schematically what happens to residual stress, strength, and grain size when a metal is heated in the recovery range.

Recrystallization. When the metal is heated sufficiently high, recrystal- lation occurs. As shown by Fig. 3.14 this results in a rapid decrease in strength and increase in ductility. This process is both temperature and time dependent, but the former variable has the more potent effect. 44 Engineering Metallurgy

._... > Lokt T.P. , a Pnoipnorued ° Elee T.P wornc Anneolinq Temperature, deq Fohr 200 HOC 600 800 1000 1200 1400 BO 0.10

70 014

I. *-jt ^ Contract-.™ at A'CQ 6 ^.l— vr rr.Trrytr**.,' ,- 0i2 ?;" SO OlO 1. 65 ?

40 06 S5 ? «3 30 006 ««J E o 5 S 10 004 O IS J

«f (Of -j 0.02 25

(00 ?00 300 4O0 tOO 600 700 800 Annealing Temperature, deq Cent

Fic. 3.14. Elfect of annealing on tensile properties and grain size of four commercial grades of copper. (ASM Handbook)

Grain Growth. Once a new set of grains is established, the growth of these grains will continue if the temperature is raised or the time of heating prolonged. The effect on the mechanical properties as well as on grain size is shown for copper and brass in Figs. 3.14 and 3.15.

Annealing Temperature, deg Fahr ZOO 600 1000 1400 140 70 .g CM 60 g

50 .§ Grain Site- 4Q §,

so Ci

— 63XHard- in —43%Hard 24%Hord Hard ZOO 400 600 800 Annealing Temperature, deg Cent

Fic. 3.15. Effect of annealing on tensile properties and grain size of yellow brass wire. (ASM Handbook) Factors Affecting Engineering Properties 45

3.10. Factors Affecting Recrystallization Temperature and Grain Size

There are a fairly large number of variables affecting the recrystallization temperature and the grain size after recrystallization, and since these effects may be encountered in the engineering utilization of metals it is important that they be recognized.

800 1200

i-00 400 600 800 Annealing Temperature, deg Cent

Fig. 3.16. Influence of the amount of prior cold work on the recrystallization temperature of high-purity copper. Annealed one hour at temperature. (Koster, ASM Handbook)

Percentage of Cold Work. Increasing the percentage of cold work lowers the recrystallization temperature. This is more apparent in Fig. 3.16 than in Fig. 3.15. Increasing the percentage of cold work also decreases the size of the new grains (Fig. 3.17) . There is, however, an intermediate amount of deformation as illustrated by Fig. 3.17 that will lead to excessive grain growth. Apparently, this amount of deformation restricts nucleation to but a few sites, and these nuclei therefore grow to large grains. In an actual part where a variation in the amount of cold working might be experienced, annealing could result in a region of ex-

g f-iq. 7 E

in .c 77 % JWOTTO 20 40 60 80 I Degree of Deformation, %

Fic. 3.17. Influence of degree of deformation on the grain size developed at 950 C (1742 F) in low-carbon steel. (Hanemann, ASM Handbook) 46 Engineering Metallurgy cessively large grains which would effect the properties adversely. A cold punching operation would be a good example of a situation where this could occur. If annealing is required, its temperature should be controlled to keep it below the recrystallization of the less severely worked metal but above that of the more severely deformed areas to avoid large grains in the regions of critical deformation. Composition. The actual temperature at which recrystallization occurs depends on the composition. Thus some metals like zinc may recrystalli/.e by working at room temperature. Other alloys such as those used in high temperature applications may not recrystalli/.e even when heated to 650°C (1200°F) or above.

Annealing Temperature, deg Fohr J00 400 500

too iso koo eso soo

Fie. 3.18. Effect of annealing lime on the softening of copper wire reduced 93% by cold drawing. (Alkins & Cartwright, ASM Handbook)

Effect of Time. Time-tern (jeratu re relations for the annealing of copper previously cold reduced 93% are shown in Fig. 3.18. This shows that increasing time lowers the recrystallization temperature. Effect of Initial Grain Size. The larger the original grain size, the greater the amount of cold deformation to give equivalent recrystallization temperatures and time.

3.11. Summary of Hot and Cold Working

Hot working is used primarily for fabrication purposes and does not cause a large change in tensile properties. A directionality or fibering effect is introduced, however, which has a tendency to reduce ductility and toughness when tested transversely to the rolling or forming direction. Cold working raises strength and hardness and lowers the ductility. It offers an important method for hardening alloys that cannot be hardened otherwise or as an auxilary treatment along with other hardening processes. The effects of the cold rolling can be removed by annealing or Factors Affecting Engineering Properties 47

tempering and can only be reintroduced by additional cold rolling. The

combination of cold rolling and annealing has an effect on the re- crystallizcd grain size of the metal, and the various factors affecting the grain size and recrystallization temperature must be recognized for a proper utilization of this process.

EFFECT OF ALLOYING ELEMENTS ON ENGINEERING PROPERTIES When one or more metals are added to another metal in the liquid state, it is found that upon solidification the atoms of the added elements will either be a part of the lattice structure of the base metal as a solid solution or another phase will appear in the microstructure. Either of these effects will alter the engineering properties of the base metal; therefore, alloying represents one of the important methods for controlling them. Frequently, the effects of alloying are combined with those of heat treatment to further influence the properties.

3.12 Solid Solution Effects

A binary alloy that exists as a solid solution has grains containing atoms of two sizes in the lattice structure. This strains the lattice and increases resistance to deformation, thereby serving to strengthen and harden the alloy. For two metals that are completely soluble in each other in the solid state, the maximum hardening occurs at about 50 atomic percent of each. Copper—nickel alloys are good examples of this case and Fig. 3.19 shows how several properties of these alloys change with composition. Although the maximum strength occurs at about 70% nickel, the maximum hardness (not shown) occurs close to 50 atomic percent (48 wt.%) nickel. In addition to the mechanical properties, other physical properties are altered by solid solution alloying. Fig. 3.20, for example, illustrates how electrical resistivity and the the temperature coefficient of electrical resistance vary with nickel content in copper-nickel alloys. This illustrates the important principle that the maximum heat and electrical conductivity are obtained from pure metals and that the presence of alloys (impurities in this case) is definitely detrimental in those instances where high conductivity is desired. Other alloy systems may have only limited rather than complete solid solubility of the alloying element in the base metal. Such is true for many of the copper-base alloys, for example. In these alloys the solid solution hardening action increases with the amount of the alloy up to the limit of solid solubility, following which the properties are affected by the presence of another phase in the microstructure as described in the next 48 Engineering Metallurgy

40 60 80 Nickel, percent

Etc. 3.19. Effect of nickel on the mechanical properties of copper-nickel alloys. (Wise)

. 40 60 Nickel, per cent

Fie. 3.20. Effect of nickel on the electrical properties of copper-nickel alloys. (Wise) .

Factors Affecting Engineering Properties 49

section. The degree of hardening is usually inversely proportional to the range of solid solubility, so that those alloys of very limited solubility exhibit the greatest percentage-wise influence on the properties. As an illustration of this point, it takes 0.6% Sb, 4.6% Si, 4.7% Al, 12.1 % Zn or 13.2% Ni to increase the hardness of pure copper by an increment of 14 Vickers DPH. (diamond pyramid hardness number) . The order in which these elements are listed is also the order of increasing solubility of these elements in copper.

Besides the aforementioned effects, it might also be mentioned that during the free/ing of alloys that exist as solid solutions at room temperature, there is a tendency for chemical segregation to occur within the growing dendrites as freezing progresses. Also, voids or a general porosity may develop or inclusions may segregate between or within grains. If the metal is further processed by working, the segregation and porosity are fairly well eliminated and the inclusions more or less uniformly distributed and broken. If the metal is retained in the cast condition, however, these imperfections may have some bearing on the properties of the casting. These are examples of some of the factors affecting the quality of the metal.

3.13 Polyphase Structures

If an alloy is prepared which exceeds the limits of solid solubility at room temperature, the second phase may appear as: 1. The second component in essentially pure form. Alloys of aluminum silicon and are of this type, with the silicon phase interspersed in the aluminum phase. The effect of silicon on the properties of Al-Si alloys, is shown in Fig. 3.21.

2. An intermetallic phase, in itself a solid solution, interspersed in the original solid solution. Copper-zinc alloys, with zinc exceeding about 38%, illustrate this behavior. The alloy at room temperature consists of separate grains of alpha brass, the original solid solution, and beta brass, an intermetallic phase containing about 45% Zn. (In the absence of the alpha phase, the beta phase can range in composition from about 45 to 50% Zn., which indicates its solid solution characteristics)

3. An intermetallic phase which is essentially an intermetallic compound. Carbon in steel precipitates as the compound Fe.-,C so that at room temperature steel consists of a mixture or iron and iron carbide.

No matter what these extra phases are, they will influence the engineering properties. There is a tendency to generalize by staling that the effect 50 Engineering Metallurgy

£ 4 6 8 K) % Silicon

Fie. 3.21. Effect of silicon content in annealed and hard rolled aluminum-silicon alloys. (ASM Handbook)

of the second phase is proportional to the amount present, the properties consequently being determined by the properties and amounts of the

individual phases present. This is only true if the second phase is rather uniformly distributed in the parent phase; however, there are so many exceptions to this rule that it is perhaps better to generalize that the effects of the second phase are determined by (1) its properties, (2) the amount present, and (3) its distribution. As will be learned when considering steel, for example, the way the iron carbide is distributed will have a profound effect on the properties of the steel. Present as a net-

work, around the grains it can lead to brittleness and poor machinability.

Finely distributed throughout the steel, it contributes to high strength and toughness. Occasionally, a very small quantity of a given phase can be very harmful to properties due to the fact that it may be distributed in a manner to produce extreme brittleness. Examples of these effects are found with bismuth in copper, iron sulfide in steel, and graphite in cast iron. In the latter case, whether the graphite is distributed as essentially spherical particles or as elongated shapes referred to as flakes will determine whether the metal is strong and ductile or weaker ami brittle.

There are so many different possibilities that it is impossible here to cover each of the situations that may be encountered. However, it can be stated that the presence of a second phase can produce a decided strengthening effect or can be decidedly harmful depending on the three factors listed previously. In any event, by alloying to produce a second Factors Affecting E?igineering Properties 51

phase we have available to us another method for controlling engineering properties, and in succeeding chapters, examples will be given to illustrate its importance.

To have a full understanding of the consequences of alloying it is necessary that one become familiar with the principles of alloying as enunciated in Chapter 6.

EFFECT OF HEAT TREATMENT ON ENGINEERING PROPERTIES

The use of heat treatment to alter mechanical properties depends upon:

1. An allotropic transformation as in the case of steel, or 2. A decrease in solid solubility of a second phase with decrease in

temperature as exemplified by a fairly large number of binary systems having copper, aluminum, magnesium, iron, or a number of

other metals as the base metal. Heat treating based on this effect is referred to as age hardening or precipitation hardening.

3.14. Allotropic Transformation

Iron undergoes an allotropic transformation from face centered cubic to body centered cubic structure at 910°C (1670°F). When iron is alloyed with carbon up to about 0.8%, this transformation is depressed to lower temperatures and occurs over a temperature range. The normal product on slow cooling steels in this range of carbon content is a mixture of iron (ferrite) and a product referred to as pearlite which is actually a mixture of ferrite and iron carbide. Rapid cooling, however, tends to retard or repress this transformation with the result that other microstructural products are produced, and in the instance of very rapid cooling a new metastable phase called martensite is produced instead of iron and pearlite. This phase is the hardest product obtainable for a given carbon content. It is fairly brittle as produced and is usually softened and toughened by a subsequent tempering treatment: the higher the temperature of tempering, the softer and tougher the resultant structure. Utilization of the various heat treating possibilities based on the allotropic transformation in iron produces a variety of microstructural products and properties. Hence, heat treatment represents a basic method for controlling the properties of steel. Inasmuch as this subject is discussed fully in subsequent Chapters 12, 13, and 14, no details need be presented here.

However, it should be pointed out that in addition to the operation of heat treatment, the engineer also has at his disposal the benefits avail- '

52 Engineering Metallurgy able from alloying so that in steel we are usually utilizing a combination of alloying and heat treatment. For example, by varying the carbon content (alloying) the base hardness of the steel can be changed; whereas, varying some of the other alloying elements will alter its susceptibility to heat treatment. Combinations of these effects, therefore, provide a degree of control and versatility not obtainable by either method alone. Superimposed on these effects of alloying and heat treatment are those resulting from the quality of the metal as originally produced in the ingot and from the mechanical processing the steel may have had. These mat- ters must also be kept in mind when designing a part.

. | 105 F 1 /OT___ — CHANCE —/ IN SCALE I t 1 ; 140 F . • ^

1 VVL / 1 -J75 F 32 F r~

a' £' F-' 68 210 P '0 10 20 30 40 SO 60 75 100 ZOO Time al Aging Temperature. hours

Fig. 3.22. Quench-aging of a 0.06% carbon steel. Hardness after quenching from 717C (1325 K) and aging at indicated temperatures. (ASM Handbook)

3.15. Precipitation Hardening

The principles of precipitation hardening are discussed in Chapter 7 and will therefore not be considered in detail here. The process involves heating an alloy of selected composition to a temperature where it is a single phase and then cooling it rapidly to a temperature where it normally would be a two-phase structure as a result of solubility changes with temperature. The rapid cooling, however, retains it as a single phase metal. It is then subsequently "aged" at room temperature or above to initiate a start toward the equilibrium two-phase structure. As a result of the atomic adjustments that occur during this treatment, certain alloys can be hardened appreciably. This hardening action is a function of time and tem|>eraturc as illustrated by Fig. 3.22. The maximum hardness reached is decreased as the aging temperature is raised. Also, the increase in aging temperature results in a rapid drop off in hardness which is referred to as overaging. Fig. 3.22 illustrates the fact that even in iron- carbon alloys, which were previously used to describe the principle of hardening by allotropic transformation, age-hardening can also be encountered. This aging effect depends on a slight solubility of carbon in Factors Affecting Engineering Properties 53

body-centered cubic iron. This solubility increases with increasing temperature. Consequently the necessary {but not exclusive) condition for

age hardening is present.

In addition to the hardness change illustrated by Fig. 3.22, there are also corresponding changes in strength, inverse changes in ductility and toughness, and changes in electrical conductivity and other properties. Aging may also be combined with metal working treatment to provide additional control over engineering properties.

EFFECT OF NUCLEAR RADIATION

For materials going into the construction of nuclear reactors, it is necessary to know about their relative absorption coefficient for neutrons. Materials with low neutron absorption coefficients are necessary where neutron passage is desired, or in other words, where the maximum utilization and minimum waste of neutrons is desired. On the other hand, a high coefficient is necessary for shielding or control purposes. The absorption coefficient, then, is one of the engineering properties that must be established. As examples, elements like boron, cobalt, and cadmium have high absorption coefficients, but elements such as beryllium, sodium, magnesium, aluminum and zirconium have low coefficients. The property of neutron absorption plus related properties of strength, formability.

UNIT CELL OF URANIUM WITH PERTINENT PLANES INDICATED

PREDOMINANT ""* PLANES PREDOMINANT PLANES ' EFFECTS OF PREFERRED ORIENTATION ON DIMENSIONAL STABILITY OF URANIUM DURING IRRADIATION

Fic. 3.23. Effect of nuclear radiation on •'growth" of a nuclear fuel material. (Courtesy of General Electric Co. Hanford Works) 54 Engineering Metallurgy corrosion resistance, heat resistance, etc. must be evaluated for the design of nuclear reactors, and the metallurgy of the materials used to construct the reactor is therefore a very vital part of design.

Besides the aforementioned requirements, it is also necessary that the susceptibility of the metal to radiation damage be known. Fig. 3.23 shows what happened to a nuclear fuel material exposed to an intense neutron atmosphere. The original smooth cylindrical specimen increased in size or elongated and warped depending on crystal orientation. Proper metallurgical treatment can considerably improve resistance to this type of deterioration. Structural metals subjected to neutron radiation are hardened or strengthened and sometimes embrittled by such treatment. It has been found that the face-centered metals are least damaged by such exposure, whereas the hexagonal and body-centered cubic metals tend to become

brittle. Raising the temperature causes less damage. Heat treating by annealing can be used to eliminate the embrittlement.

SERVICE FACTORS

In the preceding sections the factors affecting the engineering properties have largely been those concerning the processing and preparation of the metal for service. There are, however, additional effects that may have an influence after the part is placed in service.

Lack, of attention to the problem of residual stresses as briefly discussed under cold working could lead to a premature failure of a part of fatigue. Besides originating from cold working, these stresses can also be thermally induced as by welding or heat treating. Without going into the

details of this point, since it requires a knowledge of the static and dynamic properties as discussed in Chapters 4 and 5, it should be emphasized nevertheless that the stress distribution created by design or

metallurgy is unfortunately often overlooked and that many of the failures which have been recorded can be attributed to this omission.

Another service factor of extreme importance to be covered in Chapter

22 is corrosion. It should be quite obvious that when a part is designed for corrosive conditions the properties of that part will deteriorate in time and cause failure unless a sufficiently resistant material is used or it

is replaced frequently enough to avoid damage. By the same token, unexpected corrosion can seriously detract from the load-carrying ability of a part in service and possibly cause premature failure. These are additional factors, therefore, that the engineer must observe. Factors Affecting Engineering Properties 55 SUMMARY

There is a very close relationship between the structure and properties

of metals. Part of the structure is established during the manufacture of the metal, and this can carry over in part at least to affect the final properties. Most of the structural modifications, however, depend on subsequent processing, and such operations as grain size control, metal working, alloying, heat treatment, and/or nuclear radiation are available to the engineer for controlling the properties. In certain applications,

these operations may continue while the part is in service as for example,

at high temperatures or under corrosive conditions. Then it is necessary for the engineer to recognize the effect this will have during the service life of the part. It can be said, therefore, that unlike gases and liquids, the properties of solid metals depend on past history, rather than on the variables existing at the moment.

In order to gain an appreciation of the relationships and interrela-

tionships involved in producing a given set of properties, it is necessary for the engineer to learn about the fundamental structure of metals as discussed in Chapter 2, about the measurements of behavior as discussed in Chapters 4 and 5, and about the principles of alloying and heat treatment as discussed in succeeding chapters.

With the exception of corrosion behavior which is dependent on surface conditions and chemical reactions, the behavior of a metal is governed by its internal structural condition.

QUESTIONS

1. What are the processes available to the engineer for controlling metal properties?

2. How does the factor of metal "quality" enter into the problem of property control? 3. Describe the freezing of a dendrite. 4. Why is the dendritic structure absent on etched surfaces of pure metals? 5. What two factors affect the final grain size of a freezing metal? 6. How arc these two factors affected by cooling rate and what additional variables beside cooling rate may be used to control grain size? 7. Beside the growth of dendrites, what additional changes take place during freezing that may affect the quality of a metal? 8. Describe several ways besides the casting process which can be used to control the grain of metals. 9. what Of practical importance is grain size control? Cite a number of examples.

10. Compare the slip behavior in the face-centered cubic, hexagonal close- packed and the body-centered cubic crystals. 56 Engineering Metallurgy

11. How does the twinning mechanism differ from the slip mechanism and in what respects is it similar? 12. Why is the twinning mechanism important in some metals? IS. Describe a number of the structural changes accompanying cold working of polycrystalline metal.

14. Compare the effects of cold working and hot working on such factors as (a) tensile strength, (b) directionality of properties, (c) toughness, and (d) ductility. 15. Suggest possible applications for hard or spring temper brasses.

16. Why is the relief of internal stresses during the recovery period when annealing cold worked metal of industrial importance? 17. Why is annealing a cold-worked metal sometimes considered a fabrication procedure? 18. What damage may result from annealing at too high a temperature for a metal that has experienced a wide variation in the amount of cold work? 19. What are the possible effects on properties when one metal is added to another to (a) form a solid solution? (b) cause another phase to appear? 20. What basic ways are available for altering mechanical properties by heat treatment? 21. Explain how design for nuclear reactors involves factors not encountered in other types of applications. Static Properties of Metallic Materials CHAPTER

Lee L. Amidon, M.S.M.E., Professor and Head, Depart- ment of Mechanical Engineering, South Dakota State College, Brookings, South Dakota Walter E. Short, M.S.M.E., Associate Professor of Mechanical Engineering, Bradley University, Peoria, Illinois

4.1. Properties of Metallic Materials

I HE properties of metallic materials are commonly divided into three general classes: (1) physical constants, (2) electrical and magnetic properties, and (3) engineering properties. The first class includes elastic constants, density and related properties, thermal expansion, vapor pressure, emissivity, and thermal conductivity. The thermal properties are affected by temperature and phase changes. Dimensions change and heat is absorbed or discharged at such times. Problems in heat-treating operations may be anticipated as a result. These properties are of interest to the physicist as well as to the metallurgist. Usually, however, they are of interest to the physicist as a step in his study of theories of matter rather than as properties of metallic materials. Some of the physical constants are of interest to the engineer: the elastic constants surely are, because they are important in the design of structures; density is of interest if the strength-weight ratio must be considered, as in aircraft construction, high speed railway trains, and motor-haulage vehicles; thermal expansion and thermal conductivity may be of importance for some engineering applications and of none for others. The essential fact for the engineer to know about the physical constants as a group is that in general they are determined by standardized methods that yield fairly precise, readily reproducible values that can be used in engineering design without trepidation regarding their meaning and accuracy. Values for the constants are well established and can be obtained in handbooks of physical tables.

57 .

58 Engineering Metallurgy

Electrical and magnetic properties include thermoelectric properties, resistivity, magnetization, induction, permeability, and coercive forces. These properties vary widely between different metals and their alloys and must be investigated thoroughly before making final selection for any given service. Aluminum on a weight basis may be a better conductor than copper and cheaper as well, but its tensile strength may not be sufficient to make it safe for the long spans desired on high-tension transmission lines so we compromise by using a steel core to provide strength and the result is an efficient carrier at low cost. Table 4.1 gives typical values of resistivity and the manner in which these values will change with temperature. The resistor elements for a dynamometer will call for a material with great resistance to the flow of electricity while in a

Table 4.1. Resistivity of Typical Metals and Alloys

Resistivity at 20° C. Temperature Coef. Material Microhm-cm. Increase per ° C.

Copper 1.7241 .00393 (0-100° C)

Aluminum 2.654 .00429 (20° C) Iron 9.8 .0056 (0-100° C)

Constantan (55% Cu, 45% Ni) -19 .0002 (20-100° C)

Nichrome (80% Ni, 20% Cu) . . 107.9 .0019 (20° C)

transmission line the 1 2 R loss should be as near zero as possible. Magnetic properties are essential in the design of motors, generators, transformers, metering equipment, and huge lifting magnets. The magnetic properties are the result of the plus and minus magnetic fields set up by the electrons whirling in their orbits. Ferromagnetic materials such as iron, nickel and cobalt exhibit this property to the greatest extent. Single atoms will not form a permanent magnet but several atoms grouped together will form a magnetization unit. This small group is known as a domain. There will be several domains in one metal grain. Ordinarily the domains of a metal bar will have random orientation in which case the bar will not be magnetic. An external magnetic field will align the orientations found in the many domains and the bar is mag- netized. If upon removal of the external field, the domains remain oriented we have a hard permanent magnetic substance. On the other hand if they tend to lose orientation then we have a soft or temporary magnetic substance. Static Properties of Metallic Materials 59

Temperature has a very decided influence on ferromagnetic materials. Increase in temperature causes the magnetization to decrease and finally to disappear altogether at what is known as the Curie temperature. This temperature is 1414°F for iron, 2039°F for cobalt, and 665°F for nickel.

The domains disappear at these temperatures, which for iron is below the point at which it becomes austenitic. These temperatures may be altered drastically by small additions of alloying elements or impurities.

Electric properties are extremely important to the electrical engineer, and the methods for their determination are precise; but because of their limited application to general engineering, further discussion is beyond the scope of this book.

From the standpoint of engineering in general and of those fields of engineering which deal specifically with the design, erection, and maintenance of structures and machines, the last class, engineering properties, is of vital importance. For convenience, they may be divided into three classes: static mechanical properties, dynamic mechanical properties, and miscellaneous properties. There is a very close relationship between the engineering properties and the metal lographic structure, which will be made clear in later chapters of this book. Because of this relationship between micro-structure and physical properties, metallic materials are structure sensitive. In other words, they are affected by changes in the structure even though the chemical composition is unchanged. As we learn how to produce space lattices which are undistorted during the mechanical processing of our materials, much improvement in strength and physical properties may be anticipated. Precise instructions for the physical testing of engineering materials have been established by the American Society for Testing Materials and summarized in the Handbook of the American Society for Metals. It must be understood that the properties established by these tests are useful primarily in maintaining uniformity of material once a satisfactory design has been established. The most useful form of testing is that made upon a completed structure under actual service conditions. Time and cost usually limit this procedure; therefore, the following pages will evaluate the advantages and shortcomings of the various tests of a given material for use in engineering machines and structures.

It cannot be emphasized too strongly that the data from a small test specimen should be accepted with caution unless it is certain that the test piece is representative of the entire lot of material whose properties are being determined. A large number of specimens reduce the chances for error of even the most accurate and reproducible tests. The shape of the designed part may determine to a large extent the distribution of stress 60 Engineering Metallurgy in a given cross-section. The presence of stress raisers (holes, sharp changes in cross-section, scratches, inclusions) may increase the stress 3 to 5 rimes the average value. Ductile materials under static loading will yield and little harm will be produced. However brittle materials and parts under repeated or reversed loading will start to tear and fatigue failure of the part thus ultimately occurs.

Many of these stress concentrations can be readily corrected by proper design changes. Frequently the magnitude of stresses at concentration points cannot be readily determined by analytical methods. The use of brittle lacquers applied to the structure under simulated service conditions will reveal trouble spots which can then be corrected or eliminated.

This design technique known as stress coating will supplement the standard test data. The use of strain gages in a similar manner will determine the location, direction, and magnitude of the unit strain at locations where stress concentrations are suspected. The unit strain can then be used to calculate the actual stresses rather than the average stress that might be determined by the ordinary methods of design. Another method of determining stress concentrations is based on the optical properties of transparent materials. Some of these materials are optically isotropic when in an annealed condition, but become optically anisotropic when subjected to external stress. This method requires a plastic model, which, of course, limits its use to a certain degree. Hidden stress raisers such as blow holes, slag inclusions, malformed space lattices or grain structures may be revealed by sonic testing described later in this chapter. Other methods frequently used to determine surface and hidden defects are radiography and magnetic particle detection. All of the above mentioned methods have the advantages of being non-destructive tests. Physical properties, then, must always be considered in relation to the structure itself and sound engineering judgment used in its selection, considering always the limitations of the material for a given service or condition.

4.2. The Relative Standardization of Static Tests

Static mechanical properties, which are a measure of the resistance of a material to a steadily applied load, include tensile strength, compressive strength, torsional strength, bending strength, hardness, and their related properties, such as yield strength, elongation, and others determined incidentally to the first named. The determination of some of these properties has been given considerable attention by testing societies in the principal metal-using countries with the result that methods and specimens are fairly well standardized. Static Properties of Metallic Materials 61

The most important and the two most common tests for the evaluation of the static properties of metallic materials are tensile and hardness tests. The former is well standardized throughout the world. If the tensile strength of a certain steel is reported as 60 kg. per sq. mm. in Germany, it is certain that a standard specimen of the same steel will have a strength of about 85,000 lb. per sq. in. in the United States and that either value represents accurately the stress necessary to break a i/4-in. specimen of that particular steel. The tensile strength of a steel of

certain composition, made by a particular process, and of otherwise satis-

factory quality can be used in the design of a bridge in Japan, even if the steel was made and the strength determined in France. Because hardness tests are made rapidly and cheaply and because they do not destroy

the material, they are used widely. That hardness is not a definite property but a combination of several properties makes the interpretation of some of the tests difficult. Hardness tests are not standardized, although the measure of a specific kind of hardness, namely, resistance to indentation, is in a fairly advanced state of development. Hardness tests took on added importance during the war years as new and untried low-alloy steels were substituted freely for some of the older alloy steels on the basis of the hardness of an end-quench hardenability test. Static tests used less frequently, and for which standardization has not proceeded so far, include compressive, torsion, and bend tests. None of these is used so widely as are tensile or hardness tests, although the transverse test, a form of bending, is the most widely used and is probably the most valuable single test for the evaluation of gray cast iron as an engineering material. With this one exception, none of these tests is standardized, and it is not always easy to interpret the values obtained or to apply them to the design of engineering structures.

4.3. Tensile Strength

The commonly determined tensile properties, including tensile strength, yield strength, proportional limit, elongation, and reduction of area, are obtained test on a single specimen (see Fig. 4.5) . The accuracy and reproducibility of the results depend upon the dimensions of the specimen, the sensitivity of the machine, the alignment of the specimen in the machine, and above all on how accurately the specimen represents the whole section. Considerable attention has been paid to these factors by the testing societies and the builders of machines in all countries, with the result that reported values are about as accurate as for any engineering property. 62 Engineering Metallurgy

17.000

16.000 / ^ 15.000 y /' 14,000

13,000 / / 12,000 c 11,000 hn /

« 10.000 TS /, §9000 / o / 1 °:8O0o 1 1 / 1 7000 1 | Is 1 i 1/" 1 ta 6000 ri 1/ 5000 p 1

4000 I 1 3000 1 ^ 2000 _i A / 1000 1 1 1 1 c 0.002 0.004 0.006 0.002 0.004 0006 St rain, inch per inch

Fie. 4.1. Stress-strain curves in tension for (A) annealed low-carbon steel and (B) cold-worked low-carbon steel.

As is well known, the tensile strength of a specimen is the maximum load before rupture (point C, Fig. 4.1 A) divided by the original cross- sectional area. It is reported in pounds per square inch, tons per square inch (England) or kilograms per square millimeter (continental

Europe) . Specimens and other variables have been standardized by the

American Society for Testing Materials *so that further discussion is unnecessary. Owing to the inherent variations in structural constituents and in composition which may cause variations in tensile strength of as much as

± 1,500 lb. per sq. in., there is a growing tendency to test full-sized structural members with large machines. This type of test is especially valuable for structures that are riveted or welded.

If a tensile test is made on a larger or a smaller specimen than is commonly used (0.5 in.) , the tensile strength may be considerably higher or lower than that obtained on the standard bar. For ductile materials, which include most rolled, forged, or heat-treated carbon and alloy

• A. S. T. M. Standards, 1955. Static Properties of Metallic Materials 63

\ STRAIN HARDENING BEGINS

Fic. 42. Greatly enlarged view of stress-strain diagram in region near yield point.

steels, the error is not great; for a brittle material, such as cast iron, the

variation in strength may be considerable if differences in cooling rates of cast specimens of various sizes result in differences in structure. For a number of years, structural design has been based on either the tensile strength or the yield strength with the use of appropriate factors of safety. For example, a working stress for carbon steel may be of the order of 18,000 psi. although the yield point of this material may be approximately twice this value. It might be noted that the portion of the stress strain curve shown in Fig. 4.1 A at point B would have a considerably different appearance if the curve were plotted with a different scale.

It will be noted from Fig. 4.2 that a large amount of yielding at a relatively constant stress will occur before strain hardening begins. This phenomenon is the basis of recent work in the field of plastic design.

4.4. Elastic Limit, Proportional Limit, and Modulus of Elasticity

There is occasionally confusion among engineers and steel men concerning the meaning of the terms Elastic Limit, Proportional Limit, Yield Point, and Yield Strength. The elastic limit is the maximum stress that a material withstands without permanent set. Although it is practically impossible to determine the true elastic limit experimentally, it is possible with present-day extensometers to detect a relatively slight variation from Hooke's law,* and by careful testing the point of this variation can be fixed with considerable exactness on the stress-strain diagram (point P, Fig. 4.1A) . The load which this point represents, divided by

• Hooke's law states that in clastic materials stress is proportional to strain. It is generally held that in metallic materials the application of stress, within the range where deformation is elastic, actually produces some plastic deformation. The amount, however, is so small that for all practical purposes, and especially for engineering design, Hooke's law is valid. .

64 Engineering Metallurgy

0.004 0.012 0.016 0.020 0.024 0.032

Strain, in./in.

Fic. 4.3. Stress-strain diagrams of common metals. (H. W. Gillett, Batelle Memorial Institute, "The Behavior of Engineering Metals," p. 15.)

the cross-sectional area, is the proportional limit. It is frequently determined in testing laboratories but is seldom used in the design of structures. In most annealed and heat-treated carbon and low-alloy steels, the proportional limit is practically identical with the elastic limit and, as

Fig. 4.1 A shows, is slightly below the yield point. If the proportional limit of such steels is found to be much below the yield point, it is likely that internal stresses are present. The determination of proportional limit is frequently made to detect the presence of such stresses.

Modulus of elasticity in tension is the ratio of stress to strain within the elastic range. It is given by the slope of the stress-strain curve (see Fig. 4.1 A) up to point P. For carbon and low-alloy steels of any composition and treatment, the value is approximately 29,000,000 lb. per sq. in.; for aluminum alloys, the modulus in tension is about 10,000,000 lb. per sq. in.; for copper and copper alloys, about 15,000,000 lb. per sq. in. Modulus of elasticity may be determined in compression as well as in tension; values are about the same for either method. Another important quantity is the shear modulus, or modulus of rigidity, which is invariably smaller than the modulus of elasticity (Young's modulus) Static Properties of Metallic Materials 65

A! -*«

KM^se-r'*

0.002 0.004 0.006 0.008 Strain, in. per in.

Fig. 4.4. Stress-strain curves for two gray cast irons tested in tension. (/. W. Bolton, Ptoc. Am. Soc. Testing Materials, v. 32, 1932, II, p. 477)

The modulus of elasticity is a measure of the stiffness of the metal. It is not appreciably affected by heat treatment. A part too springy in the annealed state will not have the condition improved by hardening. The modulus decreases with increases in temperature moderately at first, but may change very rapidly when some specific high temperature is reached. The rate of change varies with the alloy chosen. Some typical values of stress-strain diagrams and moduli of elasticity are shown in Fig.

There are some metallic materials-gray cast iron and austenitic alloy steels are outstanding examples-which when tested in tension have a stress-strain curve that is curved from origin the (Fig. 4.4) . Such materials have no readily determinable proportional limit and yield point. Moreover, they have no true modulus of elasticity; reported moduli are arbitrary values that measure the relative stiffness under specified conditions of loading, usually with a stress equivalent to 25 per cent of the tensile strength. 66 Engineering Metallurgy

4.5. Yield Point and Yield Strength

Engineering structures and machines subject to static loads are designed on the assumption that the stresses that must be withstood will not cause appreciable plastic deformation (permanent set) . As plastic deformation probably starts in a large number of independent locations, it is difficult to determine where it actually begins. Plastic deformation does not become measurable until a large part of the section has been affected, and even then very sensitive equipment is necessary to locate this point—the proportional limit. Fortunately, practical experience has shown that, at ordinary temperature, the limit of usefulness of a metallic material is that stress which produces a plastic deformation that is readily detected. Higher stresses than this may cause considerable damage; with lower stresses the damage is negligible. This stress is the yield point. In ductile materials, it is indicated by a marked elongation of the specimen with no increase in load. It is readily detected with a pair of dividers or by the drop of the beam of the testing machine. The yield point is of considerable value to engineers in the design of structures, but it has the disadvantage that it is not well marked in hard materials, some of which have no true yield point. To overcome this disadvantage the American Society for Testing Materials recommends the use of yield strength instead of yield point.* This is the maximum stress that produces a specified set, usually 0.1 or 0.2 per cent of the gage length. In hard materials, the point can be readily determined from the stress-strain curve (point B, Fig. 4. IB); in soft materials, it corresponds closely to the yield point determined by drop-of-beam. The yield strength is an important and readily determinable value and should replace the yield point in all reports of tensile properties.

The value for yield point is affected by the rate of strain, that is, by the speed of the cross head of the machine. Generally, however, the error is not great for the speeds used in testing to determine whether a material is within specifications. For example, increasing the rate of strain tenfold, as from 0.2 to 2 per cent per min., increases the yield point only about 2,000 lb per sq. in. The American Society for Testing Materials i/ in. specifies that the cross-head speed should not exceed 8 per min. for a 2-in. gage length.

Very soft materials, especially annealed low-carbon sheet, frequently have two yield points: an upper point, at which there is a decrease in stress (A in Fig. 4.1 A) marked by a sudden drop of the beam of the testing machine, and a lower yield point (B in Fig. 4.1 A) , where strain

1 A. S. T. M. Standards, 1955. —

Static Properties of Metallic Materials 67

Radius not less U gj „.' than'/ " 8 \| Parallel section - Nole : The gage length, parallel sect/on, and o— fiO.Ol" - fillets shall be as shown, but the ends may ofany be shape 2"i0.00S"—A to fit the holders of ,k length the testing machine Gage in such a way that the for elongation load shall be axial after fracture

h- ?gm/h mgnps-*^*- 3f mih between grips >j<- 2g/n/n ingrips- "1

< w =o.soo"io.o/o" I ^r , x . w+0.003°to0.0OS" „,» . to +0.003 to 0.005 -t 1 Zj mm. >-} \ , „ (See Note! ''Radius Ifeduced section 0.5"to3" — 6'min.

t = Thickness ofma ferial Note: Gradual taper from ends ofreducedsection to middle. Allmachining dimensions are shown below andtestingdimensions above specimen Fig. 4.5. Standard round and flat tensile specimens. (AS.TM. Standards E8-46)

hardening begins and where additional stress is necessary to produce further strain. The difference between the upper and the lower yield point is considered to be an indication of the deep-drawing capacity of annealed low-carbon sheet.

4.6. Elongation and Reduction of Area In the United States, the standard round tensile-test specimen has a diameter of 0.505 in. (cross-sectional area 0.2 sq. in.) in a gage length of 2 in. (Fig. 4.5 top) . Elongation values determined on round specimens larger or smaller than the standard are directly comparable with the standard if it has a gage length equivalent to G = 4.5vT where G is the gage length and A the cross-sectional area in square inches. Unfortunately, since the standard tensile specimens used in Great Britain, Germany, and France have a different relation between gage length and cross-sectional area from that expressed by the above formula, values obtained in those countries, while always reproducible where determined, cannot be compared directly with elongation values determined in the United States. 68 Engineering Metallurgy

Fie. 4.G. BrineU hardness machines: left, hand-operated laboratory model; right, high-speed production testing model. (Courtesy of Tinius Olsen Testing Machine Company)

For some products, especially for plate and thin sheet, specimens are used that may differ in size and shape from the standard (Fig. 4.5 bottom) . Gage lengths for tensile specimens of such products vary from 2 to 10 in., depending upon the material and its size. As elongation is reported as a percentage of the original length, the percentage elongation of a given material is higher the shorter the gage, owing to the fact that most of the elongation takes place in the "necking-down" that occurs at the point of fracture. When tensile properties are determined on specimens that differ in dimensions from the standard, the gage length is always given. Even if a standard specimen is used, most metallurgists report the gage length (2 in.) so that there will be no misunderstanding. Reduction-of-area values are always reported as a percentage of the original area so that values for this property should be comparable. Static Properties of Metallic Materials 69

Elongation and reduction of area give a rough, but usually a fairly clear, idea of the ductility of the metal tested. Although engineers generally demand a relatively high elongation, metallurgists place more emphasis on reduction of area as a criterion of ductility and believe that the importance of elongation is usually overemphasized. It is doubtful if there are any applications for which an elongation of more than 8 or 10 per cent in 2 in. is needed. As an example of the exaggerated importance sometimes given to ductility, the crankshaft of the internal-combustion engine may be cited. For years crankshafts were made from highly alloyed steels which were carefully heat treated so that they would be as ductile as possible coincident with the high hardness required. The cast-iron crankshaft now used successfully in some automotive engines shows very little elongation in the tensile test and has practically no resistance to single-blow impact.

It is generally true that in carbon steels and many nonferrous alloys high strength values are accompanied by low values of elongation and reduction of area and vice versa. This general relation does not always hold for alloy steels. One of the principal reasons why alloy steels are used in place of carbon steels is that a better combination of yield strength and ductility-as shown by elongation and reduction of area (and impact resistance) -justifies their higher cost for many applications. 4.7. Hardness

Physicists do not recognize hardness as ordinarily determined as a fundamental property of matter; to engineers it is a property of great importance. Everyone thinks he knows what hardness is, but no one has been able to define it satisfactorily. Probably as good a definition as any is that hardness is resistance to permanent deformation. Hardness is complex. First and most important, it is the resistance of a material to indentation. It is also the resistance to cutting with a tool, which involves resistance to indentation plus tensile strength and toughness. It is also resistance to abrasion, which likewise involves indentation resistance, strength, and toughness.

There are least at eight methods for the determination of hardness, of which two-Brinell and Rockwell-are the more frequently used. The literature on hardness testing is extensive. Since standards for making Brinell and Rockwell tests have been set up by the American Society for Testing Materials,* the methods for making these tests can be dismissed with a few words.

* A. S. T. M. Standards, 1955. 70 Engineering Metallurgy

Fie. 4.7- Rockwell hardness lesliug machine. {Courtesy of Wilson Mechanical In- strument Company)

In the Brinell test, a definite load is applied to the piece to be tested by means of a hardened steel (or tungsten carbide) ball. The diameter or the depth of the resulting impression is measured, and the Brinell

hardness number is calculated by dividing the load applied (in kilo-

is grams) by the area of the impression (in square millimeters) , which assumed to be spherical. Owing to a slight deformation of the steel ball

and to the fact that the impression is not truly spherical, the hardness number varies somewhat with the load applied and with the diameter of the ball. These have been standardized in this country: a 10-mm. steel ball and a load of 3,000 kg. are used for steel and a few nonferrous alloys; and the same ball and a load of 500 kg. are used for soft materials. The

load is applied for 20 to 30 sec, the diameter of the impression varies from about 6 mm. for soft materials to about 2.5 for hard materials and

is measured with a micrometer microscope. The Brinell machine is shown in Fig. 4.6. Static Properties of Metallic Materials 71

The Rockwell hardness machine (Fig. 4.7) is designed to test materials of widely varying hardness by interchangeable indenters. For hard materials, a diamond cone is used; for softer metals and alloys, y16 -in. and l/g-in. hardened steel balls are employed. In the Rockwell test, a minor load of 10 kg. is applied to force the penetrator below the surface of the material. The major load is then applied. The hardness, which is inversely proportional to the depth of penetration, is read directly on a scale, several of which are used.* A Rockwell hardness number is meaningless unless the scale symbol accompanies the value. Loads and scales used with the Rockwell machine are as follows:

Major Scale Pencirator load, Materials tested kg-

A Diamond cone 60 Tungsten carbide and other very hard materials. Thin, hard steel, such as razor blades, nitrided steel, and the like B ball Hi-in. 100 Soft steels, hard nonferrous alloys C Diamond cone 150 Steels of 90,000 to 350,000 lb. per sq. in. tensile strength D Diamond cone 100 Materials with thin, hard surface layer; case- hardened and nitrided steel E ball H-in. 100 Soft materials, aluminum alloys, bearing metals, and the like

There is third a hardness test, which is coming into relatively wide use, especially in the laboratory; this is the Vickers test in which a diamond pyramid is pressed into the surface of the material. Other hardness tests include the scleroscope, now rarely used, which determines hardness by measuring the rebound of a diamond-pointed weight falling from a fixed height; and a number of scratch tests in which needles of definite hardness or diamond or sapphire points are dragged over the polished surface. Scratch tests are sometimes useful in the research laboratory to detect differences in the hardness of the various constituents in an alloy but are rarely used industrially. A new laboratory hardness tester has been recently developed by Frederick Knoop at the National Bureau of Standards. A carefully prepared diamond pyramid is forced into a • There are a number of other scales in addition to the ones listed in which the table are used with i/ -, i/ -, and i/ e 4 2-in. ball indenters; these are used occasionally for special hardness determinations. There is also available a special Rockwell machine for the determination of superficial hardness. All of these are discussed in A. S. T. M. Standards, F.18-42. 72 Engineering Metallurgy polished metal surface and the dimensions of the resulting impression are read with a microscope. The Knoop tester is flexible and apparently gives precise results, but at present it is adapted only to the laboratory. A practical qualitative method widely used for the determination of the hardness of steel is the file test. In the hands of an experienced man, relatively slight differences in hardness can be detected by rubbing the material with a file of known hardness. Thus, a steel with a Rockwell C hardness of 54 can be differentiated from one which has a Rockwell C hardness of 60. Obviously, too much depends upon the personal equation for the file test to be useful except in the hands of an experienced man.

4.8. Comparison of the Various Flardness Tests

In making hardness tests by any method a number of precautions are necessary. In the first place, the specimen must be rigidly supported, and the load must be applied perpendicularly. In the second place, the surface tested must be flat and relatively smooth, and any surface scale or decarburized areas must be removed. In the third place, specimens must be thick enough so that the material of the anvil on which the specimen rests does not affect the indentation in the specimen. In general, hardness impressions on thin sections are not accurate unless the material has a thickness equivalent to 10 times the depth of the impression.

All tests for hardness by indentation cause strain hardening to some degree. The Brinell test is as close to an ideal test as possible as it distrib- utes strain hardening over a relatively wide area, but it lias the disadvantage that the amount of strain hardening may not be uniform and, further, that the method is too restricted in application. With a 500-kg. load, the Brinell test is quite satisfactory for aluminum alloys and for brass and bronze; with a 3,000-kg. load its use is restricted to steels with a tensile strength between 45,000 and 250,000 lb. per sq. in. When the test is used on steels that are harder than 500 Brinell (tensile strength greater than 250,000 lb. per sq. in.) , there is likely to be some permanent deformation of the steel ball that causes all subsequent hardness determinations made with this ball to be inaccurate. For steels with high hardness a special tungsten carbide ball is used.

The Rockwell machine is designed to overcome some of the limitations of the Brinell test. It is flexible and can be used for hard materials, it makes a small impression that is not likely to spoil the appearance of the piece, and it can be used to measure the hardness of relatively thin sec- ^

Static Properties of Metallic Materials 7:5

tions. For example, on steel of Scleroscope hardness 150,000 lb. per sq. in. tensile 20 24 30 40 50 60 70 80 90 100 100 strength the minimum thickness 150 that can be tested accurately by 60 200 the Brinell method is 0.15 in.; \ '.onj\j with the Rockwell diamond cone 250 k N 120 (C scale), it is 0.06 in. The ^ 300 WO Rockwell machine also has the \ 350 — 160 advantage that hardness num- L_ Iftfl bers are read directly from the 400 \ £ \ c 1 an scale; it is unnecessary to meas- S450 m ure the width of an impression. 240 — V\0 MU The Rockwell machine is, there- c

ardized, is the most flexible 750 V of \ all hardness tests and can be 800 -10 10 20 30 40 50 60 70 used without change of pene- Rockwell C hardness trator on materials of widely varying hardness by varying the Fie. 4.8. Relation of hardness values to load applied. each other and to tensile strength for carbon The impression is and low-alloy steels. (French and Sands) small and, owing to its diamond shape, can be measured very accurately. The Vickers test, ideal as a laboratory tool, at present is not so well adapted to routine testing as the Rockwell.

4.9. Relation among Hardness Tests and between Hardness and Tensile Strength for Steel All indentation hardness methods fail to measure the hardness of the undeformed metal. An exact mathematical correlation of the various hardness numbers is therefore impossible as the amount of strain hardening varies from test to test and, furthermore, varies from metal to metal. large A amount of time and effort has been expended in studying the relation among the hardness numbers as obtained by the Brinell, Rock- well, Vickers, and scleroscope machines. One of these correlations for carbon and low-alloy steels is shown in Tig. 4.8* A word of caution is

• H. French J. and J. W. Sands (editors), Nickel Alloy Steels, The International Nickel Co., Inc., New York, 1934. 74 Engineering Metallurgy

necessary concerning such a correlation. Brinell values may be converted to Vickers or Rockwell C values, or vice versa, with an expected error of not more than 10 per cent, except for hardness numbers over 500 Brinell where the uncertainty becomes greater. The error in converting Brinell or Rockwell to scleroscope may be considerably greater than 1 per cent. The most recent conversion table and probably the most accurate for carbon and low-alloy steels was compiled by a committee of the Society of Automotive Engineers and the American Society for Testing Mate- rials.* These values are reproduced in part in Table 4.2. The resistance to indentation is directly related to compressive strength. For most carbon and low-alloy steels there is a fairly close relation between compressive strength and tensile strength; hence, indentation hardness values for such steels can be converted into tensile strength with reasonable accuracy. Results of a large number of investigations on this relation indicate that the ratio of tensile strength (in units of 1,000

lb. per sq. in.) to Brinell hardness number varies from 0.45 to 0.58, and heat treatment apparently has no effect on the ratio. The ratio obtained

most frequently is 0.47 to 0.50. Thus, if the Brinell hardness number is given, tensile strength can be calculated by multiplying the hardness number by 480; the result should be accurate within =p 5,000 lb. per sq. in. The relation between the various hardness numbers and the tensile

strength for carbon and low-alloy steels is shown in Fig. 4.8 and in Table

4.2. Although this relation is fairly accurate, there may be considerable variation in individual specimens. The actual ratio depends upon structure, with composition a possible complicating factor, and although in most cases the conversion may be made with an error of ± 5,000 lb. per sq. in., errors as great as ± 12,000 lb. per sq. in. may be encountered.

4.10. Relation among Hardness Tests and between Hardness and Tensile Strength for Nonfcrrous Alloys

In contrast to carbon and low-alloy steels, which all harden approximately the same amount when cold worked (strain hardened) and which show, therefore, a fairly close relation among hardness tests and between hardness and tensile strength, the nonferrous alloys differ considerably in their capacity for strain hardening and no such relation can be worked out for these materials as a class. For example, a high-strength duralumin that has been heat treated will have a tensile strength of around 70,000

lb. per sq. in. and a Brinell hardness (500 kg.) of 115; whereas a cast manganese bronze containing 55 to 60 per cent copper and 38 to 42 per cent zinc will have a tensile strength of 90,000 lb. per sq. in. and a Brinell

1 A. S. T. M. Standards, 1955. Static Properties of Metallic Materials 75

Table 4.2. Approximate Hardness Conversion Numbers for Steel

Brinell. 10 mm. ball, Rockwell hardness nun ber 1 3,000-kg. load Vickers d lamond- Approxi- 1 pyramid Sclero- mate B scale. hardness scope tensile Diameter A-scale. 100-kg. C-scale. D-scale. number hardness strength. of Hardness 60-kg. load. 150kg. 100 kg. (DPH) number lb. per impression number *, load. Hi-in. load, load. sq. in. mm. diamond ball diamond diamond

* 2.40 653 81.2 60.0 70.7 697 81 • 2.45 627 80.5 58.7 69.7 667 79 323,000 wn 2.50 601 • 79.8 57.3 68.7 640 77 309,000 * 2.55 578 79.1 56.0 67.7 615 75 297,000 * 'JfWUf 2.60 555 78.4 54.7 66.7 591 73 285,000

* '>-- 2.65 534 77.8 53.5 65.8 569 71 274,000 • 2.70 514 76.9 52.1 64.7 547 70 263,000 ' 2.75 495 76.3 51.0 63.8 528 68 253,000 - 2.80 477 75.9 50.3 63.2 516 66 247,000 2.85 461 75.1 48.8 61.9 495 65 237,000 ***/tf 2.90 444 74.3 47.2 61.0 474 63 226,000 3.00 415 72.8 44.5 58.8 440 59 210,000 '- 3.10 388 71.4 41.8 56.8 410 56 195,000 3.20 363 70.0 39.1 54.6 383 52 182,000 3.30 341 68.7 36.6 52.8 360 50 170,000

3.40 321 67.5 34.3 51.0 339 47 160,000 3.50 302 66.3 32.1 49.3 319 45 150,000 3.60 285 65.3 29.9 47.6 301 43 141,000 3.70 269 64.1 27.6 45.9 284 40 133,000 3.80 255 63.0 25.4 44.2 269 38 126,000

3.90 241 61.8 100.0 22.8 42.0 253 36 118,000 4.00 229 60.8 98.2 20.5 40.5 241 34 111,000 4.10 217 96.4 17.5 228 33 105,000 x 4.20 207 94.6 '0*i

4.40 187 90.7 196 28 90,000 4.50 179 89.0 188 27 87,000 4.60 170 86.8 178 26 83,000 4.80 156 82.9 163 24 76,000 5.00 143 78.7 150 22 71,000

* By tungsten carbide ball.

hardness of less than 100. Copper hardened with arsenic, in the cold- rolled (hard) condition, will have a Rockwell B hardness of 95 to 100 and a tensile strength of about 60,000 lb. per sq. in.; whereas admiralty metal containing 71 per cent copper, 28 per cent zinc, and 1 per cent tin and of the same Rockwell B hardness will have a tensile strength of 95,000 lb, per sq. in. or higher. 76 Engineering Metallurgy

The only nonferrous alloy for which hardness conversions have been worked out is cartridge brass. Some of these values are collected in Table 4.3.* All the thousands of other alloys, with the possible exception of the high-nickel alloys, which apparently harden much like steel, are quite evidently a law unto themselves, and each must be considered as a special case.

Table 4.3. Approximate Hardness Conversion Numbers for Brass (70 Per Cent Copper, 30 Per Cent Zinc)

Brinell, 10-mm. ball Rockwell hardness number Vickcrs diamond Diameter Hardness number pyramid B scale. F scale. Tscale. t of hardness 100-kg. 60-kg. 30-kg. impres- number 3,000-kg. 500-kg. load, load, load, sion, (DPH) mm. load* load Ke-in. ball Ho-in. ball Hs-in. ball

2.00 158 91.0 108.0 75.5 183 2.10 143 85.5 105.5 72.5 165 2.20 130 79.5 102.5 69.5 149 2.30 119 74.0 99.0 66.0 135 2.40 653 109 68.5 96.0 62.0 123

2.50 601 100 63.5 93.5 59.0 113 2.60 555 93 59.0 91.0 55.5 105 2.70 514 86 54.0 88.0 52.5 98 2.80 477 80 47.5 84.4 48.0 90 2.90 444 74 40.0 80.0 43.0 82

3.00 415 69 33.5 77.0 38.5 77 3.10 388 65 28.7 74.0 34.0 73 3.20 363 61 18.5 68.5 29.0 66 3.30 341 57 12.5 65.0 23.0 62 3.40 321 53 61.0 18.0 58

3.50 302 50 56.5 12.0 54 3.60 285 48 53.5 52 3.70 269 45 47.0 48 3.80 255 42 40.0 45 3.90 241 40 43

* For steel, included for comparison. t Rockwell superficial hardness: initial load 3 kg.; major load 30 kg.

If the proper precautions are observed, indentation hardness tests are accurate and readily reproducible; and if judgment is used, the tests give a reasonable approximation of the tensile strength and a rough idea of machinability and wear resistance for most steels but not for most

• A. S. T. M. Standards, 1955. Static Properties of Metallic Materials 77

nonferrous alloys. Their greatest value lies in their speed and cheapness, which make them ideal for checking uniformity of heat treatment and as an acceptance test for hardened steels, especially tools and dies, and for a number of other metallic materials.

4.11. Shear, Compression, and Bend Tests The determination of the resistance of a material to failure by shear or torsion is important for many engineering applications, especially for shafts subjected to twisting, for rivets holding two plates together, for keys in shafts, the and like. Neither specimens for such tests nor the methods of making the tests are standardized. Stress-strain diagrams may be plotted from a torsion test, from which the maximum torque (torsional or shearing strength) , yield point or proportional limit, and the modulus of elasticity in shear (modulus of rigidity) may be determined. There is no accurate relation between torsional strength and tensile strength, but for carbon steels, the torsional strength is generally about 75 per cent of the tensile strength. Compressive strength can be determined accurately on brittle materials that fracture; on ductile materials, which flatten without fracturing, it can be determined only when a limiting amount of deformation is specified. Compressive strength is a common test for cast iron, as this metal is frequently used in compression, but is rarely made on steel. The compressive strength of cast iron is 3 to 4 times its tensile strength. Bend tests are becoming increasingly valuable for some of the new low-alloy steels. These steels are used in structures primarily to save weight, and the bend is test useful to determine the relative stability of these materials under load. When ductile materials are tested, the specimen does not fracture when bent; hence, the test can also be used to detect deep-seated flaws and other internal defects or to discover brittle areas in and around a weld. A variety of the bend test-the transverse test-is a useful measure of the quality of cast iron and has been used so widely for this purpose that virtual standardization has been effected. Transverse tests of cast iron are made on standardized bars. As discussed in Chap. 20, the results are reported as the load necessary to fracture a specimen of definite diameter held on supports a specified distance apart. In the United States this is usually reported as transverse strength in pounds; abroad, and to some extent in the United States, it is reported as modulus of rupture in pounds per square inch. The test is usually made on a cast bar without machining; removing the surface layers of the bar may cause a slight change in values. The deflection in inches (that is, how the much bar bends before breaking) is also re- 78 Engineering Metallurgy

Fie. 4.9. Oscillograph screen showing diagram obtained by sonic tester.

ported. The transverse test gives an indication of toughness, flexibility, surface condition, and even machinability and is the most valuable single

test for cast iron.

4.12. Sonic Testing

Large forgings, castings, and weldments are inspected for hidden flaws by x-ray radiography. Recently the sonic tester has been developed for

non-destructive testing of all types of machine elements. This apparatus is portable and can be used on a part even when it is completely assembled in the machine. This makes possible inspection during manufacture, but

it also can reveal fatigue cracks and defects in highly stressed machine parts after the machine has been placed in service. High-frequency compression waves are produced by a quartz crystal placed directly on the work surface or the crystal may be carried on an oil film. The beam of vibrations flows through the metals and are reflected back from the bordering surfaces. The reflections are then recorded on an oscilloscope, A on Fig. 4.9 being true top surface reflection and C the bottom surface reflection. An internal crack would also send back a reflection as shown by the pip at B. Location of the defect can also be determined from die oscillograph. The length from A to C represents the work depth. AB is then the distance from top surface to defect on the same scale. This ability to measure depth makes it possible to gage the Static Properties of Metallic Materials 79

SEARCH UNIT SEARCH UNIT AT POSITION 1 AT POSITION 2

Fro. 4.10. Surface Wave Testing of a Shaft. (Courtesy Sperry Products, Inc.) progress of corrosion in piping and pressure vessels even when filled with fluid. The majority of ultrasonic testing is done by x-cut quart/, crystals which send waves directly through the specimen. However, Y-cut crystals or plastic wedges may be used to propogate surface waves which can follow curved surfaces and thus reach otherwise inaccessible locations. Fig. 4.10 shows this technique being used to check the fillet areas of a shaft for fatigue cracks, such a crack being revealed by the surface wave unit in position 2.

QUESTIONS

1. Define static mechanical properties. What are the five most useful static properties? What engineering property is standard throughout the world? 2. What materials, because of their magnetic properties, are most useful in the design of motors, generators, and transformers? Define domain. Define Curie temperature. What happens to the domain when this temperature is reached? 80 Engineering Metallurgy

S. How accurate is the tensile strength of a metallic material as usually determined? What variables affect this accuracy? What factor is most important in using tensile-test data in engineering design? 4. What are the advantages and disadvantages of testing a completed structure under actual service conditions?

5. Discuss the use of aluminum as an electrical conductor.

6. Distinguish between elastic limit and proportional limit. What is modulus

of elasticity, and what does it represent? Can proportional limit and modulus of elasticity be readily determined on all metallic materials? Give the reason for your answer. 7. Give approximate values of modulus of elasticity for (a) steel, (b) aluminum alloys, (c) copper. 8. Describe three methods of testing which will allow the designer to determine the location and magnitude of stress concentration in a given specimen or structure.

9. What modulus is a measure of the stillness of a metal. Can a metal part

which is not stiff enough in the annealed state be improved by hardening? 10. What is the difference between yield point and yield strength? What is the importance of these values to engineers? How does the speed of testing affect the yield point? 11. What phenomenon occurs on the stress-strain curve which has developed into a new science known as plastic design? 12. Define elongation and reduction of area. How are these values determined? What is the effect of gage length on elongation? 13. What properties are components of hardness? What precautions are necessary in the accurate determination of hardness? 14. Describe briefly the method of determining hardness by the Brinell, Rock- well, and Vickers machines, and give the principal advantages and disadvantages of each.

15. When a part is so designed that stress concentration is present, how will a ductile material react as compared with a brittle material? What will be the

reaction if the stress is released and then reapplied? 16. Convert a hardness of 40 Rockwell C to Brinell and Vickers values and to tensile strength. How accurate are these conversions? If the tensile strength of an annealed carbon steel is 85,000 lb. per sq. in., what are its approximate Brinell, Vickers, and Rockwell C hardness values? 17. What do we mean when we say a material is structure sensitive? 18. What hardness tests are commonly used for nonferrous alloys? Why is it impossible to convert hardness values to tensile strength for most nonferrous alloys? 19. What is the value of the shear test? What is the value of the bend test (a) for ductile materials? (b) for brittle materials? What is the relation between the strength of a metallic material in torsion and in tension? 20. How docs the sonic tester function? What is the advantage of this method of testing as compared with x-ray analysis? How is a defect in the specimen located? Dynamic Properties of Metallic Materials CHAPTER

Oran Allan Pringle, M.S.M.E., Associate Professor of Mechanical Engineering, University of Missouri, Columbia, Missouri

iHE design of engineering structures and machines that are subject to static loads is based upon tensile strength and yield strength with an ample factor of safety. Few failures of such

structures and machines occur. This, however, is not the case with moving

parts of machines which fail so frequently that it has been estimated that 95 per cent of all metal failures are caused by dynamic loads. For this reason, metallurgists and engineers are now paying more attention than ever before to the resistance of ferrous and nonferrous materials to dynamic stresses and to methods of determining the dynamic properties. The principal dynamic properties — impact resistance, endurance (fatigue) strength, and damping capacity—measure the ability of the material to withstand suddenly applied or pulsating loads. Endurance strength has been the subject of much study and in some respects might even be called a standardized property. Impact resistance and damping capacity are not standardized; hence, the values obtained by impact and damping tests may mean very little unless the method and the specimen used are known. Such tests are more valuable as a method of comparing various materials or various grades of the same material than as a means to obtain values which can be used in design.

5.1. Notch Brittleness and Transition from Ductile to Brittle Fracture

In applications involving impact loads it is necessary to know the influence of speed of loading or strain rate on the mechanical properties of

81 82 Engineering Metallurgy

. , Stress Stress atMN

M-,

Fie. 5.1. Typical notches or stress raisers occurring in machines and structures. (Moore)

the materials. According to present knowledge, there is always a considerable increase in yield strength of metals at high strain rates; for example, over a range of strain rates from 10—3 to 102 in. /in. per sec. the yield point for mild steel increases by as much as 100 per cent. The ultimate strength also increases at high strain rates but to a lesser degree, and at very high strain rates the yield point of mild steel approaches the value of the ultimate strength. These considerations might lead one to conclude that there is less likelihood of failure under impact loads than static loads. Unfortunately however, it is now well known that some commonly ductile materials can fail in a brittle fashion when subjected to certain insidious combinations of high strain rates, low temperatures, and notch effects.

A ductile fracture is preceded by a considerable amount of plastic deformation; since the force required for fracture thus acts through a comparatively large distance, a relatively large amount of energy is

absorbed. A brittle fracture is preceeded by little or no plastic deforma-

tion; therefore very little energy is required to produce this type of fracture. Some materials, like glass, always fracture in a completely brittle manner. On the other hand, a few metals, among them the plain- carbon and low-alloy steels, behave in a ductile manner under some con- Dynamic Properties of Metallic Materials 83

Charpy Impact specimen -„?-- /-- • ajs.*' —Y-45" 0. J/5" ^0.01-Bad.

Alternate Charpy impact specimen 0.395?' *SM 1 0.001\ -l.l024~iO.OI9P -o.OOO- )

0.3I5~£0.00I- '-O.OI'Rad. . Izod impact specimen

Fie. 5.2. Standard impact specimens. (American Society of Testing Materials)

ditions, such as in the ordinary tensile test; however, they can break in an almost completely brittle manner under certain other conditions. The three most important conditions which tend to bring about this transition from ductile to brittle behavior are high strain rates due to shock loads, low temperatures, and the presence of notches (Fig. 5.1). Thus a certain steel part may bend without breaking under a static load at room temperature if smooth, but may snap with a brittle fracture if a sharp notch is located in the region of stress, and/or the load is suddenly applied, and/or the temperature is sufficiently lowered. In testing, it is often convenient to hold constant the notch condition and strain rate and to use the temperature zone or transition temperature at which ductile behavior ceases as an index of the susceptibility of the material to brittle fracture. Many different types of tests and specimens have been used. The Charpy and Izod tests are common in the United States. The specimens used are shown in Fig. 5.2. The pendulum of the testing machine falls from a definite height and breaks the specimen at the notch. The energy absorbed during fracture is measured from the backswing of the pendulum. Typical results are shown in Fig. 5.3. It can be seen that a drastic decrease in ability to absorb impact energy occurs when the temperature is below the transition range. In order to understand how brittle fracture can occur in ordinarily ductile materials, it must be realized that several entirely different mechanisms can cause fracture. The brittle type of fracture occurs when the highest tensile stress reaches a critical value, and is almost indepen- 84 Engineering Metallurgy

Temperature, deg.C. -I7S -150 -I2S -100 -75 -50 -25 +25 1 tJ ,J I ' 70 r 1 A, !_,

-300 -250 -200 -150 -100 -50 +50 +100 Temperature, deg. F.

Tig. 5.3. Transition temperature curves for various steels: (A) 18-8 austenitic stainless steel; (B) annealed low-carbon 3.5 per cent nickel steel; (C) annealed low- carbon 13 per cent chromium steel; (D) normalized 0.35 per cent carbon, 15 per cent nickel, 0.6 per cent chromium steel; (E) annealed 0.35 per cent carbon steel; and (F) normalized 0.35 per cent carbon steel. Specimens for steels B and E had keyhole notch; others had V notch. (The International Nickel Company)

dent of other types of stress. On the other hand, the ductile type of fracture occurs only after plastic yielding has taken place, which begins when the maximum shear stress reaches a critical value. As the load on a ductile material is increased, the critical value of shear stress is ordinarily reached before the critical value of tensile stress, consequently plastic deformation and ductile fracture result. If by some means the critical shear distress could be raised so high that the critical tensile stress were reached first, brittle fracture would occur. This is exactly what happens at high strain rates, since increasing the strain rate raises the yield strength. Low temperatures have a similar effect, since the yield strength of most metals increases by a factor of around 2 as the temperature is lowered from room temperature to the boiling point of hydrogen.

The brittle tensile strength is also affected by strain rate and temperature, but to a lesser degree. Since brittle fracture results from exceeding a critical tensile stress and ductile fracture from exceeding a critical shear stress, any means by Dynamic Properties of Metallic Materials 85

which shear stress in a part is reduced or suppressed will tend to promote brittle fracture. This is what is accomplished by notches. Consider a cylindrical tensile specimen with a circumferential notch or groove. As

the load is increased, the first effect of the notch is to produce elastic stress concentration at the root of the notch. With further increase in load, the elastic limit is reached, and plastic yielding relieves the stress concentration. As the core of the circumferential notch undergoes plastic elongation, it must also contract in a transverse direction, pulling in on the metal surrounding the notch. The surrounding metal then pulls back out on the notch core, subjecting it to biaxial tension in the transverse direction. Actually, however, the notch core is in a state of triaxial tension, as the external axial load on the specimen is still acting. In a system of equal triaxial tensions, the shear stress is zero. Thus the effect of the notch is to suppress all or part of the shear stress which would ordinarily be present in the tensile specimen, leaving the way open for the tensile stress to become sufficiently high to exceed the critical value for brittle fracture. This effect of the notch is called plastic constraint. In addition to high strain rates, low temperatures, and notches, numerous other factors tend to a lesser degree to promote the transition from ductile to brittle fracture. Some of these factors are: increased size of part, large grain size, pearlitic rather than martensitic microstructure, certain deoxidation practices, and prior cycles of fluctuating load. Although the ferritic steels are the most important alloys which exhibit the phenomonon of a transition from ductile to brittle fracture, other metals, such as zinc alloys, tin alloys, and magnesium have been shown to behave similarly. On the other hand, aluminum, copper, nickel, and austcnitic steels (Fig. 5.3) have thus far failed to show a transition temperature.

5.2. Value of the Notched-Bar Impact Test

It is evident from the foregoing discussion that the chief value of the notched-bar impact test is in classifying materials according to their tendency toward brittle fracture. It is rapid and cheap and gives information that no other test gives at present, provided that common sense and experienced judgement are used in its evaluation. In the present state of development it has a number of disadvantages. Transition temperatures obtained with one type of specimen and machine cannot be directly correlated with values obtained with different specimens and machines; similarly, test results cannot be directly correlated with results on full size parts under service conditions. Furthermore, it is not a reliable test for inherently brittle materials like cast iron and hardened tool steels. De- .

86 Engineering Metallurgy

Fig. 5.4. Typical fractures of fatigue specimens with unclistortcd area (A) in center (Moore) and (B) at side. (Johnson) spite these disadvantages, the engineer and metallurgist have found the notched-bar impact test to be a valuable tool. It is used primarily as an exclusion test to detect steels with high tendency toward brittle fracture.

In industry it is used by manufacturers of ordnance, aircraft, and automobiles, who have found as a result of experience, that steel which exceeds a certain minimum impact resistance in the laboratory gives satisfactory service in applications where low temperatures and shock loads are encountered.

5.3. The Mechanism of Fatigue

Ever since man has used metal for moving parts, he has noticed that after a period of satisfactory service such a part would sometimes fail even though the stresses were relatively low. Usually the failure would be sudden: an axle that had been forged from steel of excellent ductility would snap off short as if it were brittle. Examination of the fracture would show a dull area usually surrounding an oval-shaped area of bright crystals (Fig. 5.4) Although all details of the mechanism of fatigue failure are not thoroughly understood, the main events which occur during this progressive type of failure may be summarized. Under repeated stress the metal yields at one or more local areas of weakness, usually at some surface defect where elastic stress concentration occurs. Eventually the atomic bonds are broken, resulting in submicroscopic cracks at the ends of which Dynamic Properties of Metallic Materials 87

I'io. 5.5. R. R. Moore high-speed machine for rotating-hcam fatigue testing.

there may be high concentrations of stress. As the stress on the piece is repeated, the crack or cracks spread. After a time there is so little sound metal left that the normal stress on the piece is higher than the strength of the remaining material, and complete fracture occurs. The appearance of a typical fatigue fracture is explained by the fact that as failure proceeds the severed surfaces rub and crush each other and produce a dull appearance; while the remaining unfractured portion preserves the normal grain structure up to the moment of fracture.

5.4. The Endurance Limit

Fatigue is perhaps not the best word to use in referring to the type of failure under discussion, but it is too firmly entrenched in metallurgical and engineering terminology to be dislodged. The term endurance is also commonly used. The endurance limit of a metal or alloy is the stress below which the material will not fail if subjected to an infinitely large number of cycles of stress. It is determined in a number of ways, the most common of which is by alternate flexure, usually known as the rotating beam method. A polished specimen of suitable size, which is rotated in a specially designed machine (Fig. is 5.5) , subjected by dead weights to alternate tensile and compressive stresses. A number of identical specimens are prepared from a single material. I he first is run at a stress which is considerably higher than the expected endurance limit, and the number of cycles causing failure is noted. The stress is reduced for the next specimen; this process is continued until, for steel, there is no fracture after 107 cycles of stress. The stress (S) for 88 Engineering Metallurgy

'104 1 105 106 I0 108 Cycles

Flo, 5.6, Typical S-N curves for carbon steels. (Moore and Kommcrs)

each number of cycles (N) is plotted; the result is the S-N diagram

(Fig. 5.6) . The value for S which the horizontal portion of the curve represents is the endurance limit.

Although steels have a definite endurance limit, it is doubtful if the same is true for most nonferrous alloys. At any rate, the S-N diagram for nonferrous alloys usually does not show an abrupt knee at the endurance limit, but rather consists of a gradual curve with the portion at lower stresses becoming approximately horizontal only at a very large number of cycles of stress. In reporting results of fatigue tests on nonferrous alloys the endurance strength (or fatigue strength) corresponding to a certain number of cycles of fatigue life, such as 500,000,000, is usually given, since a well denned endurance limit is not found. The importance of determining the endurance limit or endurance strength, even though the test takes a long period of time, is evident when it is recalled that airplane propellers, turbines, etc. may easily run a billion cycles during their useful life.

5.5. Relation of the Endurance Limit to Other Properties

If the endurance limit is determined by the rotating-beam method using carefully polished specimens of proper design, the value obtained for most carbon and alloy steels is about one-half the tensile strength

(Fig. 5.7) . As there is a fairly close relation between tensile strength and hardness, it follows that there is also a close relation between endurance limit and hardness; the former is about 250 times the Brinell hardness number (Fig. 5.7) . There seems, however, to be no correlation between endurance limit and any other property of ferrous alloys. Dynamic Properties of Metallic Materials 89

Brinell hardness number

,,0 100 200 300 400 500 600

20 40 60 80 100 120 140 160 180 200 220 240 260 280 Tensile strength, thousand lb per sq. in.

Fig. 5.7. Relation of endurance limit of steel to tensile strength and Brinell hardness. (Moore)

The above relations among endurance limit, tensile strength, and hardness hold for carbon and alloy steels of any composition and heat treatment except those treatments which produce very high hardness, and may be used as approximations in design. For refined design, however, the actual endurance limit of the material must be determined, as this may in individual cases vary between 35 and 65 per cent of the tensile strength. Cast steel (not heat treated) has an endurance limit of about 0.4 times the tensile strength, somewhat below the average for wrought steel. Cast iron has a ratio of 0.3 to 0.35. Attempts to correlate endurance strength of nonferrous alloys with tensile strength are not very successful; the endurance strength may be less than 25 per cent or more than 50 per cent of the tensile strength.

5.6. The Effect of Notches on the Endurance Limit

The endurance limit of a carefully polished specimen may be reduced as much as 50 per cent by a notch. In actual machines and structures notches of some sort are almost invariably present in some form such as rough surfaces, tool marks, holes, screw threads, keyways, fillets, inclusions, gas cavities, etc. (Figs. 5.1, 5.8) . The effect varies with the kind of 90 Engineering Metallurgy

Fie. 5.8. Failure of a crankshaft starting in a sharp fillet. The crack started in the

fillet about \/2 in. above and to the left of the arrow.

notch, its location, and the notch sensitivity of the material. For hard steels the effect of the notch can be approximately predicted by the elastic stress concentration as calculated by mathematical theory or as determined

by photoelastic studies. For ductile steels the effect of the notch is usually

less severe. The effect of a number of different surface conditions is shown in Fig. 5.9. As a general rule the damaging effect of a notch in carbon and alloy

steels and in some nonferrous alloys increases with its sharpness, that is,

with increase in the ratio of its depth to the radius of its root. Usually a notch becomes harmful of this ratio exceeds 28 or 30. When shallow notches are present, hard steels with their relatively high endurance limits are superior to low strength steels. On the other hand, with sharp notches present, a soft steel with low notch sensitivity may be superior to a high-strength high-endurance steel with high notch sensitivity. Gray cast Dynamic Properties of Metallic Materials 91

Tens le strength .thousand lb. per sq in. GO 80 100 120 140 160 180 200

' a- 1 1 1 i_

b IC „c 20 \ s s d t in \ s \ - *40 S. J \e P<0 5 i \f "fin

1 o 70 . 9 oB V- notch flO " -»£ y\l O.lmm.

so - 7.5mm. tj>

inn 1 1 1

3 i i i 4 J 6 i i i) a3 9> j 15 3 K D E 3 U > m Tensi e str< ngth kg.per sq. mrn.

Fig. 5.9. Decrease in endurance limit for carbon and low-alloy steel specimens having the following surfaces: (a) polished, (b) ground, (c) roughened, (d) circumferential V notch, (e) decarburized, (f) corroded in tap water, and (g) corroded in salt water. (Z. Ver. deut. Ing., v. 77, 1933)

iron, although inherently brittle, is relatively insensitive to notches. Another important fact to remember is that a decarburized surface, while not acting as a notch, lowers the endurance limit of the whole section to that of a steel whose carbon content is the same as the surface layer, since fatigue failure usually originates at the surface. As Fig. 5.9 shows, effect the of a decarburized surface may be greater than the effect of a sharp notch.

Chafing or fretting the polished surface of steel or nonferrous alloys, a of form wear which occurs in parts which are intended to fit tightly but which slip slightly under fluctuating loads, reduces the endurance limit. 92 Engineering Metallurgy

5.7. Corrosion Fatigue

If a polished metal specimen becomes corroded before testing, the endurance limit is lowered somewhat, an effect which would be expected in view of the roughening of the surface. On the other hand, if a polished specimen is corroded simultaneously with the application of repeated stress, the endurance limit is lowered to a much greater degree. As a matter of fact, there is probably no true endurance limit in corrosion fatigue, since with sufficient time the corrosion alone would produce failure even at very low stresses. Some data indicate that carbon and low-alloy steels subjected to simultaneous reversed stress and corrosion will fail at stresses as low as 5000 lb. per sq. in. if they are run long enough (100,000,000 cycles or more) . The data given in Fig. 5.9 indicate that a combination of corrosion and fatigue causes a loss of about 50 per cent of the endurance limit if the tensile strength is low, and a loss of about 90 per cent if the strength is high. The corrosion fatigue strength of most nonferrous alloys is likewise only a small fraction of their endurance strength in air. For high resistance to corrosion fatigue a material must have an oxide skin which is tightly adherent and at the same time flexible enough to withstand repeated stressing without breaking. Among ferrous alloys, only the high-chromium stainless steels normally have such a skin. Corrosion of carbon and low-alloy steels results in corrosion products which, being brittle, fracture readily and expose the underlying surface to further attack. Since all ferrous materials, except some highly alloyed steels, are reduced by simultaneous corrosion and repeated stressing to a common low level of resistance, there are only two things to be done in such applications: use a stainless steel, or protect the surface by plating with some metal that will form a continuous corrosion-resistant coating. According to present knowledge, electroplated zinc or cadmium coatings are best.

5.8. Increasing the Endurance Limit by Shot Peening

Shot peening, if properly controlled, can increase the endurance limit or fatigue life 25 to 100 per cent or more. Shot peening consists of subjecting the surface of a finished metal part to a hail of metallic shot that has been released by the blades of a revolving wheel or by an air blast. The shot, in striking the surface with force, cold work the outer layer, increasing the strength and hardness and decreasing the ductility of this region. At the same time, the cold working introduces longitudinal compressive stresses near the surface. Surface-rolling is an alternate method that produces the same effects.

The resulting increase in endurance limit is due to two factors: (1) the increase in strength of the surface layer due to the cold work; and (2) Dynamic Properties of Metallic Materials 93

the residual compressive stresses set up in the surface layer. Opposed to these favorable circumstances, however, are the roughening of the surface and the formation of a large number of shallow pits that may cause localized stress concentration. If the shot peening is carefully done and if the size of the shot and the velocity of impact are controlled, the net result of the operation is a marked increase in endurance limit or fatigue life. Shot peening is now used regularly by some automotive and aircraft manufacturers for improving the life of crankshafts, camshafts, connecting rods, steering knuckles, transmission shafts, gears, and other parts.

5.9. Significance of Endurance Data

Endurance limits that have been obtained under controlled laboratory conditions on carefully designed polished specimens are valuable in the design of machines and structures subject to repeated stresses, but they should be used only with full realization of their somewhat limited significance. Such endurance data give information of one kind only; conditions in service are rarely so simple that these data can be used without considering other factors. Most structure and machine parts are subjected to more or less random fluctuations of load; however, most fatigue tests are run under conditions of constant maximum and minimum stress. At the present time there is no completely satisfactory method of correlating the behavior of a specimen under cycles of varying stress amplitude and the behavior of another specimen under cycles of constant stress amplitude. It is certain, however, that the endurance limit of a specimen may be affected by its stress history. If a specimen is understressed, or subjected to a few million cycles of a stress slightly lower than its ordinary endurance limit, its final endurance limit as determined later will be considerably increased. If a repeated stress slightly above the endurance limit is applied for a small number of cycles, an improvement similar to that produced by understressing is sometimes observed. On the other hand, higher values of repeated stress and larger number of cycles may damage the material, and its endurance limit as subsequently determined will be reduced. Another important factor is surface condition. Parts in actual service are rarely as highly polished as most laboratory fatigue specimens, and full consideration should be given to the character of the surface, including the possibility of corrosion and accidental defects, and to how much a roughened surface may reduce the endurance limit. All in all, a great many complicating factors including surface condition, variable stresses, external and internal stress raisers both intentional and unintentional, notch sensitivity, possible decarburization during heat treatment, residual 94 Engineering Metallurgy

Cast iron

mmmtmHmmmmi Carbon s+eel

Aluminum alloy

Fie. 5.10. Damping curves for cast iron, carbon steel, and an aluminum alloy. (Pohl)

stresses, and combined stresses must be considered in problems involving fatigue.

5.10. Damping Capacity

If a metal or alloy bar is supported at one end and if the free end is struck with a hammer, the bar will vibrate for varying lengths of time depending upon the composition, the stress imposed, and other factors, but at a uniformly decreasing rate owing to the internal friction of the material which dissipates the energy as heat and sound. The dissipa- tion of energy per unit volume of the material for one cycle of stress is the damping capacity. It may be reported as inch-pounds per cubic inch, or as centimeter-kilograms per cubic centimeter, per cycle. Damping capacity is usually measured by the area of the stress-strain hysteresis loop. The ratio of this value to the work of deformation is the damping ratio. This increases with the capacity of the material to damp out vibration.

There is apparently no direct relation between damping and any other mechanical property. In most cases the ability of a steel to damp vibration decreases as the tensile strength increases. Of ferrous materials, the strong alloy steels usually have the lowest, and cast iron and low-carbon steels the highest damping capacity. There are, however, many exceptions to this generality as steels of the same composition and tensile strength may have very different damping capacities. Typical curves Dynamic Properties of Metallic Materials 95

showing free vibrations clamped only by internal friction are shown in Fig. 5.10 for cast iron, carbon steel, and duralumin. It has been suggested that the endurance limit might be predicted from damping capacity, although attempts to correlate the two have not been successful. There is, however, some evidence that damping capacity is related to notch sensitivity. For, example, cast iron has a high damping capacity and is relatively insensitive to notches; duralumin, with a high notch sensitivity, has a very low capacity to damp vibrations. There is no question, however, that the damping capacity of a material is an important criterion for its usefulness in many applications. For example, in crankshafts in internal combustion engines resonant vibrations frequently occur. If a material with high damping capacity is used, the energy of vibration is dissipated by internal friction and the peak amplitude of the vibratory stresses is held to a low level, thus minimizing the possibility of fatigue failure.

QUESTIONS

1. Define the terms notch brittleness and notch sensitivity. Give an example to illustrate the importance of each of these properties in engineering. 2. What is the chief value of the information gained from notched-bar impact tests?

3. What are some disadvantages of the common methods of notched-bar impact testing?

4. Why is a fatigue fracture usuallv characterized by a bright area surrounded by a dull area?

5. Define the terms endurance limit and endurance strength. 6. Sketch typical S-N curves for a steel and an aluminum alloy. 7. How many cycles of testing arc usually necessary to determine (a) the endurance limit of a steel, and (b) the endurance strength of an aluminum alloy?

8. recent A expedition exploring Greenland experienced considerable difficulty with breakage of almost new vehicle tracks. What was the probable cause of these failures?

9. What is the approximate endurance limit of (a) quenched and tempered alloy steel with a tensile strength of 150,000 psi and a ground surface, (b) ultra-high strength steel with a tensile strcngdi of 280,000 psi and a ground surface, and (c) annealed steel with a Brinell hardness of 200 and a V- notch?

10. If a suspension spring on your automobile breaks after 60.000 miles of service, what is the most probable cause of failure? 11. What is the approximate endurance limit of a steel having a tensile strength of 140,000 psi if the surface is (a) ground, (b) heat treated with a smooth but decarburized surface, and (c) heat treaded with a rough, pitted, oxidized surface? 96 Engineering Metallurgy

12. The gear teeth of an automobile transmission may fail in fatigue. Assuming that the endurance limit of the steel lias been determined on a rotating* beam testing machine, what factors would you consider in deciding upon a suitable operating stress in the gear teeth?

13. During preliminary testing of a crankshaft, fatigue failure occurs at an abrupt fillet. Due to dimensional restrictions it is undesirable to increase the fillet radius. Can you suggest another method by which the fatigue- strength of the fillet may be substantially increased?

14. What advantage would gray cast iron have over other materials for use as a frame for a precision machine tool? 15. Railroad car axles are frequently surface-rolled over the region where the

wheel is pressed on. What benefit may be gained by this operation?

16. The elastic stress concentration factor of a notch or similar discontinuity can frequently be determined by photoclastic studies of plastic models. To what extent can information of this type be used in predicting fatigue behavior of metal parts with similar discontinuities?

17. Although fatigue data for standard specimens of many different materials are available, fatigue testing of actual parts under simulated service conditions is widely practiced. Can you suggest some reasons for this? 18. Under what conditions might there be no advantage in using a high-

strength steel for a part, even though it is heavily loaded?

19. For certain applications, such as bridge cable and helical springs, cold-

drawn steel wire strengthened by work-hardening is preferred over steel wire strengthened by quenching and tempering. Why?

20. Why can the shank area of a bolt be turned down to considerably less dian the root area of the threads without decreasing die strength of the bolt under fluctuating loads? Phase Diagrams and the Simple Alloy Systems CHAPTER

Theodore Allen, Jr., M.S.M.E., Associate Professor of Mechanical Engineering, University of Houston, Hous- ton, Texas; Engineer Associated with Anderson, Greenwood and Co., Bellaire, Texas John K. Anthony, M.S., Associate Professor of Physical Metallurgy, University of Arizona, Tucson, Arizona

6.1. Solid State Thermodynamics and Thermostatics

1 HE study of physical metallurgy would hardly be complete without the mention of solid state physics; indeed, it is just a small portion of that vast field. For reason of simplicity the metallurgist often refers to a "system." A system may be defined as an arbitrary portion of matter which has been isolated from the rest of the universe by prescribed boundaries. The system is merely separated by these boundaries from its environment for the purpose of analysis. A phase may be defined as a portion of homogeneous matter in a system. When a system undergoes a phase change, this change must take place at a rate which is infinitesimally slow, or equilibrium can not be maintained during the change. This is not novel, for thermodynamic processes which are reversible necessarily require the state point to pass through a series of quasi-static or quasi-equilibrium states. In this sense, then, reversible or equilibrium changes should be classified as thermostatics while actual changes which proceed at a finite rate are truly thermodynamic changes of state. The study of metallurgy includes such non-equilibrium phase changes as those that produce coring, segregation, and other irreversible processes as well as those processes which are carried out slowly enough to produce a "near equilibrium" change of state.

6.2. Concept of Dynamic Equilibrium

From what has been said in the preceding paragraph, one might infer that this concept of equilibrium is a static sort of thing; such is not the

97 98 Engineering Metallurgy

case. From a macroscopic point of view this assumption would be tolerable, but from the microscopic point of view the picture is a dynamic one. To be specific, two phases of a pure substance may be in equilibrium with one another, and the molecules of one phase may be escaping into the other phase, while those of the second phase are escaping into the first, ff this exchange were taking place so that the net transport of mass, momentum, and energy is zero, then the two phases would be in equilibrium. It is a well established fact (the Second Law of Thermodynamics) that nature tends to equalize its potentials; a system, then, would be in equilibrium if no unbalanced tendency existed which would bring about a change of state provided this tendency was active. With respect to the escaping tendency of a component from one phase to a second phase in chemical equilibrium with the first, it may be said that the fugacity of a component in one phase must be equal to the fugacity of that same component in the second phase for equilibrium to persist.

6.3. Cooling Curves

Useful in equilibrium studies are cooling curves. These curves present the temperature history of a system while it is being heated or cooled. Consider the cooling of liquid HaO. The (temperature-time) curve for it, starting from some arbitrary temperature, say 80°F, is a line with a negative slope. The temperature decreases with time until a temperature of approximately 32° F (corresponding to standard pressure of 14.696 psi, abs.) is reached. At this point the formation of another phase (ice) occurs. The cooling curve is then a horizontal line until all of the liquid is frozen (the delay is explained by the phase rule in section 6.5 of this chapter) . Upon completion of solidification, further cooling results in a drop in temperature with time. Such a cooling curve is the simplest possible because only one component, H,0 is present in the system.

The concentration and nature of a component is required to specify the composition of a phase. In the above example, there was one phase (liquid, composed of 100% HaO) at the start of cooling. Next, two phases coexisted (liquid and solid, each composed of 100% H,0) . Finally, a single phase (solid, composed of 100% FLO) existed. Frequently a system may contain two or three components. Such systems are referred to as binary and ternary systems, respectively. The number of components is not limited to three or any number in particular. Suppose the water had been cooled very rapidly from SOT. Chances are very good that ice would not have formed at 32°F, but at a somewhat Phase Diagrams and the Simple Alloy Systems 99

lower temperature. It is possible to cool the water below 32°F without the formation of ice. However, the addition of a small crystal of ice to serve as a nucleus causes solidification. Freezing can also be brought about by disturbing a super-cooled liquid. Super-cooled liquid water in this state is not characterized by stable equilibrium. This illustration serves to point out the influence of cooling rate on equilibrium. The only way to freeze liquid water at exactly 32°F (if this be the "absolutely correct" freezing point) is to cool it infinitesimally slowly; otherwise the solidification will begin at a temperature lower than 32°F, the amount of de- parture being a function of the cooling rate. The depression of the freezing point is brought about because of time necessary for solidification to begin after the freezing point is reached. The student of thermodynamics will again recognize the influence of time on the irreversibility of processes.

When ice is melted, the melting begins at a temperature slightly above 32° F if the heating rate is greater than zero. A way frequently employed to determine the "correct" freezing or melting point is to construct a heating and a cooling curve based on a minimum rate of temperature change and to interpolate between the "depressed" freezing point and the "elevated" melting point.

Cooling curves for alloy systems can be determined experimentally for various concentrations of the components, and these curves may be used to construct equilibrium or constituent diagrams such as presented in Section 6.7 of this chapter.

6.4. Solid State Mass Transfer

The rate of mass transfer or diffusion, as it is commonly referred to, is given by Fick's Law in the following fundamental form m=-DA^ ax in which m is the mass transfer rate, ?£ is the change of concentration dx in the direction of the diffusion path, per unit of path length, A is the cross sectional area of the path (this area is normal to the path) , and D is a coefficient of mass diffusivity. The minus sign indicates that the mass transfer takes place in the direction of decreasing concentration. It is through the process of diffusion that a solid solution of several metals may be homogenized. If a large concentration of one component exists in a localized form in a solid solution, it will diffuse. The concentration provides a gradient which gives rise to the mass transfer. .

100 Engineering Metallurgy

The process of diffusion is very slow for solids at ordinary room temperatures. However, the coefficient of mass diffusivity varies ex- ponentially with the absolute temperature of die material being diffused. Because of this increase in diffusivity at elevated temperatures, a concentration is diffused in a matter of minutes at 1500°F that would probably take several centuries at room temperature. For this reason, alloys which are heterogeneous because of rapid and irreversible cooling are homogenized at elevated temperatures in annealing ovens. By the process of dilfusion (a "self-homogeni/.ing action") solid aggre- gations become homogeneous (and thence become phases)

6.5. The Phase Rule In the latter portion of the nineteenth century, the phase rule was advanced by J. Willard Gibbs. It is largely because of the work of Gibbs that our knowledge of phase equilibria is as advanced as it is today. The following is a short explanation of how the phase rule may be derived. Equilibrium is assumed; therefore, the fugacity of a component in one phase is equal to the fugacity of the same component in any other phase. If there are N p phases in the system, then equilibrium provides

(N p-1) equations per component. In a system of N components, there are Nc (N p-1) fugacity equations. The concentration of all the components in a particular phase must sum to unity. This fact provides Np independent equations. The number of variables that are independent, (also called degrees of freedom) N f are then equal to the difference between the total number of variables and the number of independent equations relating these variables. The total number of variables are N c N„ due to composition plus any two thermodynamic coordinates. It follows then that

N r = [N N 2] — (N -1) ] c p + [N. p — N„ or

N f =N„-N„ + 2

This is the phase rule of Gibbs. By way of illustration, attention is again directed to the freezing of the water. When two phases of the one component were coexistent, the phase rule indicates that there was only one independent variable. This is evidenced by the fact that once the pressure (a thermodynamic coordinate which was arbitrarily taken as 14.696 psi, abs.) is independently chosen, the temperature is fixed. (in this case at 32°F) for two phases and one component. Upon completion of freezing, one phase existed. Now there are two variables that are independent according to the phase rule. After solidification is complete, the temperature varies although the pressure has been arbitrarily selected. Phase Diagrams and the Simple Alloy Systems 101

To examine further application of the phase rule, the following question is advanced. Can three phases of a single component exist simultaneously? The phase rule indicates that this is possible, but that there would be no independent variables. This means that the state where the three phases coexist is fixed ( that is, there is a definite temperature and pressure for a given component where triple phase coexistence can manifest itself) . The temperature and pressure are not arbitrary once the component (the system in this case) is selected. The reader will recognize this concept as the "triple point" in physics.

The phase rule is a valuable tool in the field of experimental metallurgy. It is useful in predicting the phase equilibria once the system is fixed. The phase rule does not apply to the prediction of state configura- tions that result from non-equilibrium effects, nor is the rule applicable to a vacuum. The phase rule is based on the assumption of equilibrium and the existence of a system.

6.6. Solid State Solubility Solid phases form solutions as do liquid and gaseous phases. The mechanism of explanation here is much the same as that for the familiar liquid solutions. Solid solutions can be formed by atoms of the solute replacing atoms in the crystalline structure of the solvent. This type of solid solution is referred to as a substitutional solid solution. Another common type of solution is the interstitial solid solution, in which the atoms of the solute locate in the interstices of the crystallographic planes of the solvent. Iron, iron-carbide alloys are examples of this type of solution. The smaller atoms of carbon are the solute. Carbon is referred to as a metalloid, since it has metal as well as nonmetal characteristics. One of its predominant nonmetallic properties is its negative temperature- resistance slope.

Section 6.7 shows equilibrium diagrams of some binary systems with varying degrees of solubility between components. It is possible to have supersaturated solutions in the solid state; martensite is a common example of a solid solution containing an excess of carbon.

6.7. Binary Systems

The constitution of metallic alloys is studied by preparing a series of high-purity alloys ranging in composition from 100% of one constituent or component to 100% of the other component. Suitable specimens of these alloys are subjected to treatments that produce a state of equilibrium. This step is followed by studying the changes in length, electric resistivity, microscopic structure, atomic structure, absorption or libera- . . . .

102 Engineering Metallurgy

M.P. of Ni

CC = alpha solid solution of Cu+Ni

Cu , Ni f 0%N; 25%Ni 50% Ni 75% Ni lOOZNi 100% Cu 75% Cu 50%Cu 25% Cu 0%Cu Composition (by wt.)

Fig. 6.1. Equilibrium diagram for the Cu-Ni system. tion of heat (thermal analysis) , and other changes that occur when the alloy is heated or cooled to room temperature or sometimes to the melting point and above. The temperatures at which the changes occur are plotted against the composition; the result is an equilibrium diagram (also called a constitution or phase diagram) of the series. A diagram showing changes between two metals is called binaiy equilibrium diagram. Solid Solution Alloys. A few binary alloys exhibit complete solubility or miscibility in all proportions and temperatures in the solid state.

These are called solid-solution alloys. Typical of this type is the Cu-Ni system. Development of an equilibrium diagram for Cu-Ni by thermal analysis is shown in Fig. 6.1. (The cooling curves shown have been previously discussed in Section 8 of this chapter)

The line, 1-2-3-4-5, is the liquidus line, and everything above this line is liquid or melt (also indicated as m)

The line, l-2'-3'-4'5, is the solidus line, and everything below this line is solid solution or alpha (also, indicated as

starting from the extreme left of the a. , . diagram ( , /?, « etc.) To trace the cooling of an alloy of Cu-Ni, refer to Fig. 6.2. Here alloy "A" was chosen, containing 63% Ni and 87% Cu, which represents approximately the composition of Monel metal (shown by the vertical dotted line) Phase Diagrams and the Simple Alloy Systems 103

\rm

Tc- _ftrja Td

Cu Composition Ni

Fig. 6.2. Equilibrium diagram for 63% Ni and 37% Cu.

The following stages on cooling may be noted (cooling must be slow

enough so that equilibrium conditions prevail) :

1. At temperature Tm, the alloy is completely molten. 2. The alloy stays molten until the temperature reaches Ta. 3. At Ta, the first crystals of solid solution appear and will have the composition as shown at a' (by drawing a horizontal line from a until it intersects the solidus line) . The composition may be read by dropping a vertical line from a' to the composition axis. 4. On further cooling to Tb, more solid solution crystals appear, with the composition changing along the solidus line from a' to b', and

the composition of the remaining melt is shown by dropping a vertical line from b on the liquidus line to the composition line. 5. Thus, on cooling between Ta and Td, the composition of the melt will become richer in Cu from a to d and the composition of the

solid solution crystals will vary from a' to d'. At any temperature, Tc, between Ta and Td, the relative amounts of solid and melt may be calculated as follows: Let Wt = total weight of alloy, Wm = weight of melt, Ws = weight of solid.

Then, Wt = Wm + Ws, (or) Ws = Wt — Wm. [6.1] And, Wt X % Cu in the original alloy will represent the total amount of Cu present. Wm x % Cu m will represent the amount of Cu present in the melt. Ws X % Cu, will represent the amount of Cu present in the solid. (Cu represent n , & Cu„ the composition of the melt and solid re- ; .

104 Engineering Metallurgy

spectively as read by dropping vertical lines from c & c', where the horizontal line at temperature Tc intersects the liquidus and solidus lines. Then: Wt Cu Wm Cu Ws Cu X % = X % m X % „ 100 100 + " 100

By substituting the value of Ws from Equation 6.1 in 6.2, Ws is eliminated and Equation 6.2 becomes:

Wt Cu Wm Cu , Wt Cu, Wm Cu X % = X % n X % _ X % , 100 100 + " 100 100

(or)

Wt x % Cu- Wt X % Cu 8 = Wm X % Cum -Wm X % Cus Wt Cu _ Cu ) = Cu - Cu (% % B Wm (% ra % 8) At temperature Tc, the composition of the original alloy is still 37% Cu, and the composition of the melt is 54% Cu,

and the composition of the solid is 28% Cu. Let 100# represent the total weight of the original alloy; then, 100 (37-28) = Wm (54-28) Wm = 1Q0X9 =34.5# or 34.5%; 26 .". Ws = 65.5# or 65.5%.

The Lever Principle. Since the length of the horizontal line is proportional to the composition, the above problem may be solved by the so-called lever principle.

Illustration

The length of the line c-Tc-c' is 26 units

The length of the line c-Tc is 17 units

The length of the line Tc-c' is 9 units

Therefore, % of solid = -1^- 100 = 65.5% .— 100, X 7 X 26 ' c-Tc-c q Tc-c' and % of melt = -^ X 100 = 34.5% = X 100. *6 c-Tc-c'

The ratio for the melt is the distance from the nominal analysis (fulcrum of the lever) to the solidus line over the total distance between

the lines. The ratio for the solid is the distance from the nominal analysis to the liquidus line over the total distance. The lever principle may be used to determine the relative amounts of each phase when two phases are present (both phases may be solid) Phase Diagrams and the Simple Alloy Systems 105

The total horizontal distance is always measured between the two

boundary lines of the two phases present, and the fulcrum is taken at the nominal analysis of the binary alloy.

Application of the phase rule shows that a one-phase region, such as

the cc or m areas, is trivariant: i.e.. there are three degrees of freedom.

N„ + N f = N + 2

1 +N f = 2+2

Nf = 3 One degree of freedom is used in selecting a pressure of one atmosphere, the second degree is used in selecting a temperature which will

locate the vertical position, and the third is used in selecting the composition, which locate the lateral position. Thus, the three degrees are utilized in fixing the position of any point in the single phase area.

When two phases are coexistent, as in the m +

N p + N f = N c + 2

2 + N f = 2 + 2

N f = 2 One degree of freedom is used in fixing the pressure at one atmosphere. Therefore, if the remaining degree of freedom is used in fixing the temperature (vertical position) it follows that the compositions of the

two phases are then fixed; or, if the compositions are selected first, then the temperature becomes fixed.

Binary Eutectic Alloys. When two metals arc soluble in all proportions in the liquid state but only partially soluble in each other in the solid state, a eutectic will be formed between the limits of solid solubility. A binary eutectic is a finely divided mechanical mixture of two solid phases and will exhibit a definite composition and definite melting point. Cu-Ag form alloys of this type as shown in Fig. 6-3. Consider an alloy containing 6% Cu and 94% Ag. On cooling from some temperature above the liquidus line, the first phase change will occur when the temperature reaches the liquidus line. At this point, a, crystals of alpha solid solution will start to separate, with an initial

composition as shown by drawing a horizontal line until it intersects the solidus line at point b. On further cooling, the composition of the alpha will change along the solidus line and the composition of the remaining melt will change along the liquidus line; with more and more alpha crystallizing out and, consequently, less and less of the melt remaining. At point c the last of the melt will crystallize and the alloy will consist of solid alpha. On further cooling to the solubility line at point d, beta solid solution will start to precipitate from the alpha as the solubility of 106 Engineering Metallurgy

X 1200

108.1

1000

800

600 Solvus (or) solubility Solvus (or) line solubility line 400 10 20 30 40 50 60 70 80 90 Ao Cu Wt. % Cu

Fig. 6.3. Equilibrium diagram for the Ag-Cu system.

beta in alpha decreases with a decrease in temperature as shown by the shape of the solubility line. Consider an alloy containing 20% Cu and 80% Ag. Again alpha starts to crystallize at the liquidus line. On further cooling, the composition of the melt approaches the eutectic composition. When the temperature has dropped to the eutectic, the remaining melt will have reached the eutectic composition. At this point, and on further extraction of heat, the melt will crystallize isothermally to Eutectic. Therefore, the various areas may be described as follows:

Area 1. Two phases consisting of alpha plus precipitated beta. Area 2. Two phases consisting of alpha plus eutectic. Area 3. Two phases consisting of beta plus eutectic. Area 4. Two phases consisting of beta plus precipitated alpha. Area 5. A single phase consisting alpha solid solution only. Area 6. A single phase consisting of beta solid solution only.

Since the eutectic is a finely divided mechanical mixture of both alpha and beta, the combined area (1, 2, 3, 4) can be designated as consisting of alpha plus beta, but they will be in different physical states as described above.

The precipitation of one solid phase from a solid solution is a very important factor in the heat treatment of alloys such as those of Al, Mg, Phase Diagrams and the Simple Alloy Systems 107

Atomic % Ca 10 20 30 40 50 60 70 60 90

Fie. 6.4. Equilibrium digram for the Mg-Ca system.

and Gu. This type of treatment is known as "aging" or precipitation hardening and will be discussed in more detail in Section 6.8. Application of the phase rule will show why the eutectic reaction must occur at a constant temperature.

Np + N f = N« + 2 3 + N, = 2 + 2

N, = 1

Again, one degree of freedom is used in fixing the pressure at one atmosphere; therefore, neither the temprature nor the composition of any of the phases can change during the course of the eutectic reaction under equilibrium conditions.

6.8. Intermetallic Compounds

Many alloy systems will exhibit one or more intermetallic compounds.

This type of compound is generally very hard and brittle, and, under proper conditions, may impart additional strength ami hardness to an alloy. Therefore, intermetallic compounds are very important industrially. of Compounds this type have a definite chemical formula, eg. Cu s Al, and have a definite melting point that shows as a maximum on the liquidus line (Fig. G.4) . This melting point may sometimes be higher than the melting point of either component. Phase diagrams of this type may be conveniently studied by considering them as two-part diagrams, i.e. the system Mg-MgXa and the system 108 Engineering Metallurgy

a ^=^-

\ •7s — «!• I i — e is the solidus line. be is the CuAla solubility line.

*/ i < ' i 2 3 4-56 %Cu

Fig. 6.5. Al.-rUh end of Cu-AI phase diagram.

Mg2Ca-Ca. ]Jy this method the diagram simplifies itself into two eutectic types. Fig. 6.5 represents the Al rich end of the Cu-AI phase diagram. If an alloy containing 2% Cu is heated to temperature T, and held for a sufficient length of time, the alloy will become a homogeneous solid solution. Rapid quenching will then retain the alloy in this condition at room temperature. This is known as solution treatment. On reheating this alloy up to temperature T,, (below the solubility line be) , the intermetallic compound, CuAl2 , will precipitate from the solid solution. The amount of this precipitation will depend on time, temperature, and solubility. This step in the heat-treatment is known as aging or precipitation hardening. The physical properties of certain alloys may be markedly changed by the heat-treatments described above.

6.9. The Peritectic Reaction

The peritectic reaction may be called an inverse or fake eutectic reaction, wherein a liquid and a solid react isothermally to form a new solid. By examining Fig. 6.6, it will be noted that alloys of Li and Mg lying between the points a & P will first contain alpha solid solution and melt after crossing the liquidus line on cooling. At the peritectic temperature all of the melt will react with some of the alpha to form the new solid, beta solid solution. Between points P and b and at the Peritectic temperature all of the alpha will react with some of the melt to form beta. If an alloy were chosen such that its composition was ex- Phase Diagrams and the Simple Alloy Systems 109

40 50 60 % Wi of Li

Fig. 6.6 Equilibrium diagram for the Mg-Li system.

acily as shown at the peritectic point P, then, on cooling to the peritectic temperature, all of the solid, alpha, would react with all of the melt to form beta.

6.10. Closure

In recent years considerable progress has been made in the field of solids, and these achievements are reflected in no small measure in the science of metallurgy as well as in other subdivisions of solid state physics. New developments include transitors, transducers, solid rocket fuels, high temperature sealants, metal production technology, "light weight" radiation shields, and a host of other innovations.

The salient feature of the real progress in all of these fields is that success was accomplished by thorough consideration of the fundamental nature of the problem. The metallurgist of the future should be well versed in such items as quantum theory, radiation, and fundamental rheology. The physicist has helped the engineer to understand aspects

of the solid state phenomena, but there is a need for further investigation by the research engineer. 110 Engineering Metallurgy

QUESTIONS

1. With reference to the "lever principle", would such a rule apply to a two phase region in any plane where extensive properties were used as coordinates? Assume equilibrium exists and recall that an extensive property of a multiphase system is the sum of that extensive property for all phases of the system.

2. It is known from thermodynamics that constant pressure boiling of a pure substance (one component) takes place at constant temperature as long as the vapor is in contact and in equilibrium with the liquid. Explain this.

S. Can the "triple point" referred to in paragraph 6.5 ever appear as an area on a plane of thermodynamic properties? Explain.

4. During a certain experiment, pertinent to cooling a binary system, a specimen was "isolated" and observed to contain five phases, apparently co- existing. Give several conclusions and substantiate.

5. With reference to the lever principle in paragraph 6.7, how was equilibrium assumed or implied in the derivation?

6. Using the law of mass transfer as given in paragraph 6.4, derive the partial differential equation for three dimensional, unsteady state diffusion. 7. Using the equilibrium diagram for copper and nickel calculate the proportions of solid and liquid for an alloy containing 30% Ni. at a temperature of 2200 deg.F. What are the concentrations of each component in the liquid and the solid? 8. Compute the volume of a unit mass of mixture containing 80% (by mass) water vapor and 20% liquid water in equilibrium with the vapor. The pressure of the mixture is 50 pounds per square inch absolute. Reference to vapor tables will be necessary here. Was the so called "lever principle"

employed in the above computation? HINT: See Problem 1 above. 9. For copper-silver phase equilibria discuss the transformations taking place upon cooling for the following alloys: 5% copper, 20% copper, and the eutectic composition.

10. Some binary alloys exhibit a characteristic such that there exists a composition that has a lower melting point than cither pure component. Show that this point is a point of tangency for the liquidus and solidus. Would the common tangent to both curves at the above point be a horizontal line on the equilibrium diagram? Substantiate your answer.

1 1. Can water exist in more than one solid phase? Discuss.

12. How many degrees of freedom exist for the following situations: (a) A diatomic gas. (b) A pendulum swung by an elastic and extensible cord. (c) A locomotive making a run between two cities on a specified track. Heat Treatment of Alloys by Precipitation Hardening CHAPTER

Harold Vincent Fairbanks, M.S., Professor of Metallur- gical Engineering, West Virginia University, Morgan- town, W. Va. Arthur R. Foster, M.Eng. Associate Professor of Mechanical Engineering, Northeastern University, Boston, Massachusetts

X RECIPITATION hardening, sometimes called dispersion or age hardening, is the most important method of strengthening nonferrous metals through heat treatment. This treatment has been used very effectively with the light alloys of aluminum and magnesium. It has also been applied to a number of heavy alloys of metals such as copper, iron, nickel, silver, and lead. The susceptibility of an alloy to precipitation hardening depends upon the manner in which its component metals combine rather than upon the properties of the dominant metal.

The process of precipitation hardening consists essentially of two steps.

In the first step, an unstable condition is produced in the alloy structure through the formation of a supersaturated solid solution. This step is called solution heat treatment and produces a comparatively soft, ductile state for the alloy. The second step requires a certain degree of precipitation of the supersaturated phase to impart the increased hardness and strength to the metal. If this step is carried out at room temperature, the process requires several hours or days and is sometimes called age hardening. On the other hand, if an elevated temperature is employed, the process requires much less time and is called the precipitation heat treatment.

7.1. Alloy Requirements

There are two main requirements for alloys to be heat treated by precipitation hardening. The first requirement is that the solubility of the HI .

112 Engineering Metallurgy dissolved constituent decreases with decreasing temperature. The second requirement is that there be some coherency between the precipitating phase and the matrix phase. It is this coherency or lattice conformity between the two phases which brings about the internal crystal stresses required for strengthening of the alloy.

Another influencing factor which governs the degree of hardening is the quantity of available material for precipitation. The slope of the solid solubility curve (solvus line) and the composition of the precipitating phase determines the amount of material available for precipitation.

The closer the precipitating phase is to the solid solubility curve on the phase diagram, the more the material that is available for precipitation. Also, the greater the decrease in the solubility with decreasing temperature, the larger will be the amount of material available for precipitation. The precipitating material may be either a solid solution or some intermediate phase or compound.

However, it must be remembered that a large quantity of material available for precipitation does not insure that heat treatment of the alloy by precipitation hardening is possible. In the Mg-Pb system, magnesium can dissolve 46 percent lead at the eutectic temperature and only 2 percent lead at room temperature. This alloy shows but little strengthening effect of precipitation hardening due to the small amount of coherency of the precipitated phase with the matrix material.

7.2. Step I—Solution Heat Treatment

The process of precipitation hardening is divided into two steps: first, the solution heat treatment, and second, the precipitation heat treatment.

A schematic representation for the process of precipitation hardening is given in Fig. 7. 1 The alloy to be solution heat treated must have a composition that meets two requirements: (1) that at an elevated temperature a solid solution is formed, and (2) that the solid solution can be retained upon rapid cooling to room temperature. A typical alloy used for precipitation hardening is shown as alloy N in Fig. 7.1.

The process for solution heat treatment has two parts: (1) heating, and (2) quenching.

In heating, the alloy is heated to a temperature above the solvus line but below the solidus line. The alloy is held at this temperature for a sufficient period of time to produce a solid solution. For some alloys the temperature range is fairly narrow between the solvus and solidus lines. It is very important not to heat above the solidus line as incipient melting of the alloy will take place destroying the dimensions of the fabricated Heat Treatment of Alloys by Precipitation Hardening 113

PHASE DIAGRAM PRECIPITATION HARDENING

Temp.

Composition Tlmt »-

Kic. 7.1. Schematic representation for the process of precipitation hardening.

part. Also, too high a temperature can produce excessive grain growth. On the other hand, the temperature should be above the solvus line in order to take advantage of the material available lor precipitation hardening and to produce as homogeneous a solid solution as possible.

In quenching, the heated alloy is cooled to room temperature rapidly enough to retain the solid solution. The alloy at room temperature is now in a state of nonequilibrium as only the supersaturated solid solution phase is present.

The retention of the potential precipitating phase in solid solution is the prime requirement of quenching to room temperature. It is important that the alloy be quenched as uniformly as possible as uneven quenching forms areas of high stress in the alloy which will produce soft spots in the subsequent hardening treatment. The stressed areas will harden first and then become softer during the subsequent heat treatment which is necessary for the main body of the material to obtain maximum hardness.

The result of the solution heat treatment is a supersaturated solid solution condition in the alloy. The alloy in this state may be softer than in the annealed condition, but generally it is found to be harder than the annealed state for the alloy, as shown in Fig. 7.6. However, the material after solution heat treatment is still ductile and may be cold worked. Some alloys start to harden upon standing at room temperature. Any deformation required after solution heat treatment for these alloys should be done immediately before the hardening begins. In order to arrest this hardening action, the metal may be placed in a deep freeze which lowers the rate of atomic diffusion due to thermal activity. This fact was made 114 Engineering Metallurgy

use of in the storage of aluminum rivets for assembling of airplane parts during World War II. However, most alloys are fairly stable at room temperature after solution heat treatment.

7.3. Step II—Precipitation Heat Treatment (Aging)

The second step in the process of precipitation hardening brings about

the hardening and strengthening of the alloy. This is achieved by reheating the unstable solution heat treated alloy to some temperature below the solvus line, but sufficiently above room temperature to allow mobility of the atoms held in supcrsaluration.

This precipitation temperature is held for a definite period of time until the hardness of the alloy has reached the maximum or desired value,

and then the alloy is either quenched or air cooled to room temperature.

Once the alloy has reached room temperature, it will retain the desired properties due to the lack of mobility of the atoms in the crystals to migrate.

Attention must be paid to the proper temperature and to the time at temperature in order to insure maximum hardness and strength. Too high a temperature and too long a time will cause softening of the alloy,

and the advantages of this method of heat treatment will be lost.

For some alloys the entire second step is carried out at room tempera-

ture. This is only possible when the mobility of die atoms in the alloys are sufficient at room temperature to cause the desired hardening and strengthening of the alloy.

7.4. Theory of Precipitation Hardening

The heat treatment of alloys by the precipitation hardening process depends upon obtaining an unstable state for the alloy by solution heat treatment, and then controlling the degree of approach toward the stable state for the alloy during the precipitation heat treatment. The maximum hardness and strength are obtained before the equilibrium or stable state for the alloy is reached. The actual hardening and strengthening of the alloy occurs during the precipitation heat treatment step. The mechanism involved may be divided into three stages: (I) the pie-precipitation stage, (2) the inter-

mediate precipitation stage, and (3) the equilibrium precipitate stage.* Fig. 7.2 shows sketches of the various stages in precipitation.

* For an excellent discussion on Precipitation see "Report on Precipitation" by H.

K. Hardy and T. J. Heal in "Progress in Metal Physics", edited by Bruce Charles and R. King, Interscience Publishers, Inc., Vol. 5. pp. 143-278. 1954. o o

Heat Treatment of Alloys by Precipitation Hardening 115

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A. Supsrsoturot. d BOlid _ „ Pit -precipila'e B. c hlttrmtdletc precipitate D. Equilibrium prtcipitote solution

Fig. 7.2. Sketches showing the various stages in precipitation. Open circles represent matrix atoms. Filled-in circles represent solute atoms.

In the pre-precipitation stage, the solute atoms segregate along specific crystallographic planes of the matrix phase in preparation for nucleation. These clusters of atoms are called knots or Guinier-Preston Zones, having been detected by Guinierf and Preston;}; with X-rays.

In the intermediate precipitate stage, nucleation is taking place and

the resultant new lattice of the precipitating phase is present. This new lattice is in the form of thin plates from 2 to 50 atoms thick and 20 to several hundred atoms long or wide. There is a lattice coherency present between the precipitated phase and the matrix phase which results in a highly stressed area in the matrix surrounding the intermediate precipitate.

In the third stage, the precipitate has grown to such a size that the coherency forces between the two adjacent crystal lattice systems are over-

come and the precipitate becomes its own independent crystal entity. This relieves the major stresses in the area and softens the metal.

It is during the second stage in which the intermediate precipitate is formed that maximum hardening and strengthening of the alloy occurs. This is due to the fact that a flexible dislocation cannot move easily through the regions of localized stresses. The greater the number of

these localized stresses per unit volume of material the greater is the strengthening of the metal due to the increased force required to push the dislocations through the many stress barriers.

fCuinier, A., Nature 142 (19S8) 569. j Preston, G. D., Nature 142 (1938) 570. 116 Engineering Metallurgy

This precipitation process is one of nucleation and growth of the solute phase from a supersaturated solution. The driving forces involved

for the diffusion of the solute atoms include: (1) thermal energy, (2) the

supersaturated condition of the matrix phase, and (3) stresses which were developed due to quenching in the solution heat treatment step. The nucleation throughout the alloy does not necessarily start at the same time in the alloy during the precipitation heat treatment. There-

fore, it is the statistical average effect which is produced at a given time for a given temperature. There are undoubtedly particles in all three stages at the same time.

Fie. 7.3. Structure of an alloy containing 3.94 per cent copper, 0.02 per cent silicon, 0.02 per cent iron, and remainder aluminum. (A) as cast, 100x; and (B) quenched, 500x; etched. (Courtesy of Aluminum Research Laboratories)

If the temperature of the alloy is maintained sufficiently low, even the first stage cannot take place. On the other hand, if the temperature for the precipitation heat treatment is too high, only the third stage may prevail. This is called overaging. The alloy becomes softer and loses its strength. It may become as soft as in the annealed state. Photomicro- graphs showing microstructure of an aluminum alloy with various treat- aents are shown in Fig. 7.3. Heat Treatment of Alloys by Precipitation Hardening 117

Optimum tamp.

Strength

-Tim e

Fie. 7.4. Effect of precipitation heat treatment temperature and time on the strength of the alloy.

7.5. The Effect of Time and Temperature During Precipitation Heat Treatment

Both the temperature at which the precipitation heat treatment is carried out and the length of time at temperature are important. As the temperature is increased the mobility of the atoms in the supersaturated lattice structure increases. This means that the growth' of the unstable

submicroscopic platelets in the transition phase is speeded up. This also means that they will attain the critical size at which they shear away from the parent alpha phase sooner. The result of this is that optimum

hardness and strength are attained more quickly. This is shown in Fig. 7.4. Increasing the time of treatment causes the growth of stable precipitated particles of the second phase with marked losses in the strength and hardness of the alloy along with other changes in the properties of the alloy as shown in Fig. 7.5.

If the precipitation temperature is lowered, the diffusion of the solute is atoms impeded. In an extreme case this may mean that it is completely suppressed and no increase in hardness or strength can occur. In other cases the improvement is less than the optimum amount. The temperature which is required to prevent hardening of the solution treated metal varies with the alloy. Some aluminum alloys must be stored at sub-zero temperatures to suppress hardening, but beryllium copper, after solution treatment, remains soft at room temperature indefinitely.

Some data on hardness of 24S aluminum is included in Fig. 7.6. This graph indicates that increased temperatures will speed the rate of hardening. At it 240°F shows that little overaging occurs in a week. It is 118 Engineering Metallurgy

Ten tile Strength

Propertie*

Log Time

Fie. 7.5. Effect of precipitation heat treatment time on alloy properties-

interesting to note that the furnace cooled specimen is much softer than the solution treated material. This indicates that the alloy in the solution treated condition where the solid solution is maintained, is not as soft as the equilibrium structure which contains relatively large particles of hard, brittle second phase. This is because, in the case of 24S, the copper is still in the solid solution with the aluminum in the solution treated specimen. Since the atoms of copper are of a different size than those of aluminum, the movement of dislocations along the various planes in the lattice structure is impeded. This results in greater strength and hardness. Both the temperature and the length of time at the chosen temperature must be specified if satisfactory results are to be obtained in the precipitation hardening process. It may be possible to shorten the time in the heat treating furnace by raising the temperature. Care should be taken that proper temperature and time be selected from published data or, if this information is not available, that they be established experimentally. It can be seen from Fig. 7.4 that although higher than optimum temperatures may reduce the attainable strength slightly, the shortened time may reduce the heat treating costs sufficiently to justify the slight reduction in physical properties. Heat Treatment of Alloys by Precipitation Hardening 119

80 1 240 °F aging temp.

75° F aging temp. J 70 r~~

/8° F agina temp 60 f1

» SO

C0 4Q

10 Fu noce coo led to re*om temp.

I ? 3 4 5 6 7 Time, days

Fie. 7.6. Rockwell B hardness vs. time for 24S. Solution treated 1 hour at 970° K. Furnace cooled specimen cooled very slowly. Other specimens water quenched prior to precipitation hardening.

7.6. Precipitation Hardening of Aluminum Alloys

Extensive use is made of precipitation hardening with many alloys of aluminum. Wrought alloys may be strengthened by either cold work or precipitation hardening. The latter usually results in a smaller decrease in ductility for a similar or even greater increase in strength. In castings, precipitation hardening is especially useful since no cold working can be performed. The binary phase diagrams for Al-Cu, Al-Mg, Al-Si, and Al-Zn all show the characteristic decrease in the solubility of the alloying element in the aluminum rich alpha phase as the temperature decreases. Few commercial alloys are true binary alloys, but the phase diagram for aluminum and the major alloying element usually points the way to possible successful heat treatment. 120 Engineering Metallurgy

700

SO SO 10 Copper, per cent

Fie. 7.7. The aluminum-copper phase diagram: inset, the aluminum-rich portion. {Dix and Richardson)

The Al-Cu phase is shown in Fig. 7.7. Wrought alloys 1 IS, 14S, 17S, and 24S all contain 4.0 to 5.5 percent copper plus minor amounts of silicon, magnesium, manganese, lead, and bismuth, while casting alloys such as No. 108, 112, 142, 195 and 212 contain 4 to 8 percent copper as the major alloying element. In alloys containing up to 5.5 percent copper all the theta phase should go into solid solution with the alpha phase on heating to about 1000°F. A casting alloy such as No. 212 containing

8 percent copper, 1.2 percent silicon, and 1 percent iron cannot dissolve all the theta phase when solution treated as the alpha phase when heated to just below the eutectic temperature can hold only 5.5 percent copper. However, this alloy can be hardened just as the other Al-Cu alloys and the undissolved hard, brittle theta particles will serve to improve the machineability of the alloy. In the presence of magnesium and silicon, other intermetallic compounds which are susceptible to age hardening may be formed such as Mg,Si, AlXuMg, or AlCuMgSi.

The Al-Mg phase diagram shown in Fig. 7.8 is of interest as it shows the type of partial solubility required for precipitation hardening at each end of the diagram. The magnesium rich end of the diagram will be treated in Section 7.7. It can be seen from the phase diagram that aluminum can dissolve 16 percent magnesium at the eutectic temperature of 844°F and very little at room temperature. On slow cooling of the alpha phase alloys to room temperature, the beta phase which is an intermetallic compound of Mg2 Al 3 precipitates. Casting alloy No. 220 containing 10 percent magnesium has the best physical properties of any Heat Treatment of Alloys by Precipitation Hardening 121

of the aluminum base casting alloys. Its major disadvantage is its poor capability which requires special foundry techniques and inhibitors to prevent deep surface defects.

The Al-Si phase diagram shows a decreasing solubility of silicon in

aluminum as the temperature is lowered. However, the amount of the

silicon rich beta phase which is suppressed upon solution treatment is not great enough to produce a commercially important improvement in physical properties. Silicon rich casting alloys are used extensively, but not in the heat treated state. When silicon and magnesium are used together in an aluminum alloy, intermetallic an compound of Mg2Si is formed which allows effective heat treatment. The solubility of this compound in aluminum increases from 0.27 percent at 390°F. to a maximum of 1.85 percent at 1100°F., which is the eutectic temperature. Silicon in excess of the amount form-

ing Mg2Si does not affect the solubility of the compound in aluminum,

but an excess of magnesium does reduce its solubility. Therefore, it is

necessary to be sure no excess magnesium is present.

Wrought alloy SIS (l%Si, .6%Mg, .25%Cr) is of this type. 61S is slightly more complex (.6%Si, l%Mg. .25%Cu, .2r,%Cr) in that it contains copper, which further complicates the mechanism of precipitation hardening by increasing the number of intermetallic compounds which form. The small amount of chromium added to both alloys tends to offset the ill effects of iron which is present as an impurity. Casting alloys 132, 355, and 356 are heat treatable Al-Si-Mg alloys. They have considerably higher silicon contents (5-12%) than the wrought alloys just mentioned. This improves their ratability. Although a high percentage of zinc can be dissolved in aluminum at elevated temperatures (82.8 percent at 720°F. compared with 5 percent at 248°F.) alloys of these elements show only limited response to precipitation hardening. With the addition of magnesium, compounds such as MgZn2> MgZn, and AlMgZn are formed and these alloys will respond very well to precipitation hardening. The wrought alloy 75S containing 5.6 percent zinc, 2.5 percent magnesium, 1.6 percent copper, 0.3 percent chromium and 0.2 percent manganese is essentially a Zn-Mg alloy. It develops the best mechanical properties of any of the commercial aluminum alloys.

7.7. Precipitation Hardening of Magnesium Alloys

Precipitation hardening is especially useful with magnesium base alloys since the close packed hexagonal crystal lattice structure will allow only limited amounts of cold work. The major alloying elements are alu- 122 Engineering Metallurgy

Atomic Per Cent Al 10 20 30 40 50 60 70 80 90

1 1 1 ' i i i 1 1 "C

600

500 463" ./ 451" / „.. ruA *36 * 400 Si 5 6 , Mg/I, \

t.V 1«II

300 /

200

i i i ' _ I .1 -L 1 10 20 H 40 50 60 70 80 to Weight Pet Cent AL

Fie. 7.8. Aluminum— magnesium phase diagram.*

• By permission from Structure 8c Properties of Alloys, Brick & Phillips, McGraw- Hill Book Company, Inc., 1949.

minum, zinc and manganese. The manganese strengthens the alloys by going into solid solution with the magnesium while both the aluminum and the zinc show the decreasing solubility required for successful precipitation hardening. Fig. 7.8 shows that 12.6 percent aluminum is soluble in magnesium at the eutectic temperature of 815°F. while less than 2 percent is soluble at room temperature. Zinc shows virtually no solubility in magnesium at room temperature and has a maximum of 8.4 percent at 646°F. The precipitating phase is an intermetallic compound MgZn. Many alloys contain substantial amounts of both aluminum and zinc so that a ternary diagram is necessary to show the phase relationships in these alloys. The change in solid solubility and the identity of the phase or phases in which precipitation is suppressed by solution treatment is indicated in Fig. 7.9. On this diagram, it can be seen that casting alloy C containing 9 percent aluminum, 2 percent zinc, and 0.2 percent manganese should precipitate only Mgi 7 Al 12> but alloy H containing 6 percent aluminum, 3 percent zinc, and 0.2 percent manganese also precipitates some Mg3Zn2Al 8 . These are the two most commonly used casting alloys. Solution treatment not only improves the strength of the alloys but also Heat Treatment of Alloys by Precipitation Hardening 123

%i<\

Fig. 7.9. Section of ternary diagram for Mg-Al-Zn system. Isothermal lines show the limits of solid solubility at the various temperature levels. To the left of the dashed line the structure consists of solid solution and Mg„Al„. The solid dot shown on the diagram represents the composition of Alloy C which shows this type of microstructure. To the right of the dashed line the structure will consist of solid solution, Al 7.njAlj. Mg1T M , and Mg3 Alloy H whose composition is represented by the small circle on the diagram is an alloy of this type.*

improves the ductility considerably, because the brittle interdendritic eutectic compounds are dissolved. The precipitation treatment reduces the ductility, but leaves enough for most applications. It causes a marked increase in yield strength with little change in tensile strength. Of the wrought alloys only 0-1 containing 8.5 percent aluminum, 0.5 percent zinc, and 0.2 percent manganese, and ZK-60 containing 5 percent zinc, 0.7 percent zirconium have enough alloy content to be effectively precipitation hardened. Extrusions and forgings are usually worked at high enough temperatures and cool fast enough in the atmosphere to be effectively solution treated. Thus, they need only to be precipitation hardened after fabrication is completed.

The precipitation of a second phase with its different crystal lattice dimensions results in the growth of the piece during precipitation hardening. If such dimensional instability cannot be tolerated at the service

• Busk and Miranda Trans. AIME. IG6. 346. 1946. 124 Engineering Metallurgy

temperature, a stabilization heat treatment is required. Stabilizing consists of heating to a temperature somewhat higher than the usual precipitation hardening temperature to allow partial precipitation of the

second phase. The degree of transformation is greater than will occur at

the service temperature, and, therefore, there is no further tendency for

precipitation while the part is in use. This is essentially ovei aging and causes some loss in physical properties. This type of heat treatment may be applied to other precipitation hardening alloys as well as the magnesium base alloys discussed here.

°C Atomic Percentage Beryllium 5, 10 15 SO IS 30 35 40

1000

- (600

700 1200 600

- 1000 500

300, Co I B 3 « 5 6 7 8 9 10 Weight Percentage Beryllium

Fig. 7.10. Copper-rich portion ot the copper-beryllium phase diagram. (H. F. Silliman)

7.8. Precipitation Hardening of Copper Base Alloys

Precipitation hardening is not used as extensively with copper base alloys as it is with the aluminum and magnesium alloys, but there are several alloys with which it may be used very effectively.

Commercial beryllium-copper alloys contain from 1 to 2-\/A percent beryllium plus \/A percent nickel to inhibit excessive grain growth at the solution treatment temperatures. The phase diagram shown in Fig. 7.10 indicates a maximum solubility of 2.1 percent beryllium at 1590T. The figures given in Table 7.1 indicate that precipitation heat treatment is extremely effective with these alloys and that when combined with cold Heat Treatment of Alloys by Precipitation Hardening 125

working, tensile strengths approaching 200,000 psi may be obtained. The usual heat treating cycle consists of solution treatment at 1450°F. followed by a water quench and reheating to 480 to 620°F. for 2 to 3 hours. The high strength, corrosion resistance, and fatigue resistance of these alloys recommend their use for springs, diaphragms, and non-sparking tools. Moncl metal which contains 67 percent nickel and 30 percent copper plus small amounts of iron and manganese forms a single homogeneous solid solution as do all the alloys in the binary Cu-Ni series. However, if 3 percent aluminum is added producing K-Monel or 4 percent silicon is added producing S-Monel, the metal is responsive to precipitation hardening. Tin bronzes show little tendency towards precipitation hardening although the phase diagram indicates that it is possible. The low temperatures at which the alpha phase becomes unstable which is 570°F. for 5 percent tin and 95 percent copper prevents any appreciable precipitation of the delta phase. The addition of up to 8 percent nickel, however, promotes the precipitation of a Ni-Sn rich constituent which improves the physical properties considerably beyond those of the ordinary tin bronzes. The data in Table 7.2 indicates the effectiveness of precipitation hardening on a commercial alloy of this type.

7.9. Precipitation Hardening in Low Carbon Steel

In Chapter 12 where the fundamental structure of carbon steels will be discussed, ferrite is defined as alpha iron holding in solution small amounts of silicon, phosphorus, carbon, and other elements. Of the elements which may be in solution in very small quantities in alpha iron, three: carbon, oxygen, and nitrogen can, under suitable conditions, cause precipitation hardening. Fig. 7.11 shows the approximate solubility of these in iron. It will be noted that the slope of these curves resembles the slope of the solubility curves of the other alloy systems discussed in this chapter. Precipitation hardening in carbon steel is a phenomenon that is common only in the low carbon grades. Since these steels are used rarely in the quenched conditions, precipitation hardening due to iron carbide or iron nitride retained in solution by quenching, followed by slow spontaneous precipitation at room temperature or more rapid precipitation at slightly elevated temperature is seldom encountered. It is, however, possible, by quenching a steel containing 0.04 to 0.10 percent carbon and by precipitation treatment at 140°F. for 10 to 15 hr. to increase the Brinell hardness from 120 to 160. i

126 Engineering Metallurgy

Oxygen, percent .K .04 .os .on .to *— irJBOO / k. \.a*i SolidSol. V — ' soo SB % \l 1 / *IS00 x£4 > D i^ a Sum' / «w j* § , Solution

| | 1 « BOO i a: SoiioSolution "^jC 1 6 g- 400 j { aOMM A B e o .05 ./ .01 .02.05 Carbon, per cent Nitrogen, per cent

Fig. 7.11. Iron-rich portions of the (A) iron-carbon, (15) iron-oxygen, and (C) iron- nitrogen phase diagrams, showing changes in solubility of carbon, oxygen, and nitrogen with temperature. (Epstein)

7.10. Strain Aging and Its Consequences in Low Carbon Steel

There is another variety of precipitation hardening in low carbon

steels known as strain aging. If steels susceptible to aging are strained by cold working, a hardening constituent, probably iron carbide, iron oxide, or iron nitride, precipitates during straining and for some time afterward.

This is accompanied by increased hardness and strength and by decreased impact resistance. The effect of the precipitation can be detected by cold working a rimming steel and a deoxidized steel of the same composition and by testing a sample of each immediately and at intervals of several weeks or months. Strain aging frequently has undesirable consequences. Low carbon sheet for automobile bodies and fenders are given a light final cold working after hot rolling to prevent stretcher strains. This produces conditions ideal for precipitation of the hardening constituent. Unless the sheet is deep drawn immediately, the loss of ductility may be so great that the steel will not withstand the drawing operation successfully. Strain aging has been known to cause failures in boiler plate in the strained areas around rivet holes.

7.11. Precipitation Hardening in Alloy Steels

Of the large number of common low alloy steels, only those containing about 1 percent copper are subject to precipitation hardening. The solubility of copper in alpha iron increases from about 0.4 percent at room temperature to 1.4 percent at 1525°F. Low carbon steels containing 0.7 to 1.5 percent copper can be hardened by a heat treatment causing precipitation of the hardening constitu tent. This treatment consists of heating to If>50°F., cooling to room temperature, and reheating —

Heat Treatment of Alloys by Precipitation Hardening 127 for 3 or 4 hr. at 930°F. Precipitation hardening increases the tensile strength and yield strength about 20,000 psi. Ductility is decreased, but the precipitation hardened copper steels are not brittle. A number of investigations have been concerned with the precipitation hardening of highly alloyed steels which contain 2 or 3 percent titanium and also with stainless steels containing tungsten.

Table 7.1 Typical Properties of Beryllium-Copper Alloy 0.032" Thick Strip 2% Be, 0.25% Co (or 0.35% Ni).*

Tensile Yield Rockwell Condition Strength Strength Elongation Hardness (psi) (psi) (%) Number

Solution treated 72,000 25,000 50 B 60 Precipitation hardened 175,000 80,000 5 C 41 Solution treated and cold rolled 37.1% 107,000 70,000 6 B 97.5 Solution treated cold rolled 37.1 %and precipitation hardened 195,000 110,000 3 C 42.5

* Metals Handbook, 1948 Edition, p. 934.

Table 7.2 Properties of Type A Ni-Vee Bronze (88% Cu, 5% Ni, 5% Sn, 2% Zn)

Property As Cast Heat Treated*

Tensile Strength, psi 50,000 85,000 Yield Strength, psi 22,000 55, 000 Percent Elongation 40 10 Brinell Hardness Number 85 180

* Heated 6-10 hours at 1400°F., water quenched, and reheated 6-10 hours at 500°F.

Courtesy of the International Nickel Co., Inc.

QUESTIONS

1. Define precipitation hardening. Why is it sometimes called age hardening? 2. Discuss the characteristics that an alloy must possess to be precipitation hardened. 3. Discuss what happens if too high a temperature is used during: (a) the solution treatment, (b) the precipitation heat treatment. 4. Discuss what happens if the alloy is held for too long a time at temperature during: (a) the solution treatment, (b) the precipitation heat treatment. 5. Discuss the constitutional changes which take place during precipitation hardening. 128 Engineering Metallurgy

6. How does precipitation hardening effect an alloy in regard to the following properties: (a) tensile strength, (b) yield strength, (c) hardness, (d) elongation, (e) electrical resistance, and (f) corrosion resistance. 7. Discuss the effect of precipitation hardening on the movement of dislocations within the metal. 8. Explain why alloys arc sometimes precipitation hardened at temperatures higher than the temperature at which optimum strength occurs. 9. Explain why wrought magnesium alloys may be precipitation hardened directly after fabrication without a formal solution treatment? 10. What is meant by: (a) overaging, (b) strain aging, (c) stabilizing? What use is made of each? 11. If an alloy were to contain 4% aluminum, 2% zinc and 94% magnesium, what phases would be present in the slowly cooled state at room temperature? 12. What alloying element added to the following materials will make them susceptible to precipitation hardening? (a) tin bronze, (b) monel, (c) low alloy steel. 13. List four elements whose solubility in aluminum decreases with temperature. Can all of them be used effectively as binary precipitation hardening alloys with aluminum? 14. Why is a moderate amount of Mg used with the major alloying element Zn in 75S? 15. What alloying clement produces the strongest commercial casting alloy of aluminum? What is its major drawback?

16. What three alloying elements present in a plain carbon steel may cause it to respond to precipitation hardening? 17. Is precipitation hardening used extensively with low carbon steels? Explain. 18. List some practical applications for precipitation hardening. Light Alloys as Engineering Materials CHAPTER 8

Francis Wii.uam Brown, Ph.D., Associate Professor, Clarkson College of Technology, Potsdam, New York

8.1. History of Aluminum

ALUMINUM was first made in 1825 by a Danish scientist named Oersted by means of a reaction between aluminum chloride with a potassium-mercury amalgam. In 1845 Henri Sainte-Claire Devilie produced globules of aluminum meta] by heating sodium metal with anhydrous aluminum chloride; later this method was improved by using cryolite as the flux. The very nature of these raw materials indicates the large amounts of energy that are required to make aluminum and why this metal under these processing conditions was very costly. Aluminum was produced by Charles M. Hall, in 1886, by the electrolytic decomposition of anhydrous aluminum oxide (alumina) dissolved in molten cryolite. This process has continued since 1886 to the present as the only economical process for the manufacture of aluminum. The first major industrial demand for aluminum was in the aircraft field because of aluminum's unusual combination of lightness and strength. The discovery of the aluminum-copper alloy duralumin and its heat treatment by Wilm in Germany stimulated the use of aluminum. Wilm's early work was done in connection with Zeppelin's development of the dirigible. Prior to the First World War, the Aluminum Company of America supplied the U. S. Navy with duralumin sheet for the Navy's dirigible. The aircraft industry was primarily interested in the strength- weight ratio of the high-strength aluminum alloys; whereas, most other industries were concerned primarily with the cost. It was only about twenty-five years ago that these alloys were considered seriously for automobile, truck, and bus construction and for railroad rolling stock; their consideration for bridge and building construction is even more recent.

129 130 Engineering Metallurgy

8.2. Economics of the Aluminum Industry

For about thirty years after aluminum metal was first produced (1825) , it was a laboratory curiosity worth about $100 per pound. In 1857, Sainte-Claire Deville, as the result of research supported by Napoleon III, succeeded in bringing the price down to S25; two years later, to $17 per pound. Despite the fact that alumina was a very abundant oxide and could be produced economically in pure form, cheap aluminum was impossible because of the high cost of metallic sodium and the aluminum chloride. By 1886, the cost price of aluminum was reduced to $8 per pound, still too high for common use.

The electrolytic reduction process developed by Charles M. Hall in 1886 brought the price down to $1 per pound. In 1943, the price for aluminum ingots reached $.15 per pound. A question that might readily arise here is, "can this cost be cut drastically so that aluminum and steel are on an equal price basis?"

In 1893, less than 100 tons were produced in the United States; ten years later domestic annual production increased to 3,000 tons. In 1915, world production was 85,000 metric tons; in 1939 this had increased to 675,000 tons, of which the United States and Germany each were responsible for 175,000 to 200,000 tons. The peak world production of aluminum during the Second World War, in 1943 was 2,176,000 net tons, of which considerably more than half was produced in the United States. This country's consumption of aluminum in 1952 was distributed over the following fields:

32% Transportation 10% Consumer Durables 15% Building Construction 6% Chemical Uses 14% Miscellaneous 6% Machinery & Equipment 13% Electrical 4% Packaging

It is interesting to note that, despite the fact that during the war the output of aluminum expanded very much more than iron and steel, the total production of primary aluminum in the United States for the year 1943 was approximately equivalent to the average amount of steel ingots produced in 3 working days during the same year. Reasons for this great difference in production and cost may be deduced by comparing the properties of the high grade ores of each of these metals.

Aluminum Iron Name b Source of Main U. S. Ore Bauxite (Arkansas) Hematite (Minn.) Secondary ore Cryolite (Greenland) None Combined water <%) 25% None Combined metal (%) 32% 50% Density of ore 2.55 4.90 Light Alloys as Engineering Materials 131

The transportation costs for the aluminum ores are higher due to the greater distances involved and their higher bulking values. The aluminum ores have specific gravity values near to that of the siliceous gangue (2.2 —2.3); this means gravity concentration of the ore is difficult. The ship- ping costs of bauxite are decreased by calcination of the bauxite to remove water and other volatile matter. The lower percentage of aluminum in the ore is another factor that leads to higher processing costs. A comparison of the Hall Process and the Blast Furnace Process will further bring out the comparative economics of these two commercial metals. A comparison of the costs of aluminum and steel should be made on the basis of the finished article and not on the relative price per pound of ingot metal. Because the specific gravity of aluminum is approximately one- third that of steel, less aluminum by weight will be used. In addition there are economies that result from the unique combination of the properties of aluminum, namely, (1) ease of fabrication, (2) good machinability, (3) good surface color and polish, (4) low maintenance costs, low cost (5) of distribution, and (6) high scrap value.

8.3. Aluminum Ores—Occurrence and Concentration Although aluminum occurs in nature in a higher percentage than iron, the major share exists in the form of clay. Bauxite, the principal ore of alummum, consists of a mixture of the mono- and tri-hydrate of aluminum oxide. Bauxite has been formed in nature by the weathering of clay; bauxite deposits are found primarily in warm countries because high temperatures and rainfall accelerate this hydrolytic degradation of the clay. The main source of supply of high grade ore (50-65% alumina) in this country is from Arkansas; in South America it is found in the Guianas. The impurities in bauxite ore are the oxides of iron, silicon, and titanium. Since these impurities are difficult to separate physically, it is evident that high-grade deposits are very desirable. Aluminum is more chemically reactive than iron and silicon; therefore it is difficult and costly to remove these undesirable impurities from the crude metal. The Bayer process is the most commonly used process for the concentration and purification of bauxite. The fundamental principle involved in this process is "that alumina differs from these other oxides in being amphoteric, i.e., weakly acidic and basic." Alumina will dissolve in hot concentrated alkali but is insoluble in cold dilute alkali. The steps in this process are as follows: (1) high-grade bauxite is finely ground; (2) ore is digested with hot, concentrated sodium hydroxide (caustic soda) to form the soluble sodium aluminate; (3) the insoluble (red mud) oxides 132 Engineering Metallurgy

of iron, silicon, and titanium are filtered off; (4) the hot sodium aluminate is diluted with water, seeded with crystals of alumina hydrate, and allowed to cool; (5) pure hydrated alumina settles out; it is filtered off and calcined to form alumina, (6) the filtrate is concentrated by evapora- tion and used over again.

8.4. Manufacture of Aluminum

The electrolytic reduction process for the manufacture of aluminum is dependent on the fact that anhydrous alumina dissolves in molten cryolite to give an ionic solution of high electrical conductivity. The passage of direct current through this molten electrolyte deposits aluminum at the cathode and oxygen at the anode. Since iron and silicon are objectionable impurities, the iron cell must be properly lined with a material of high purity and of a conducting nature so as to function as the cathode. The anodes must likewise have similar properties. Ordinary carbon fulfills the conductivity requirement but not the purity one. The lining of the cell and the anodes are made from fabricated parts of petroleum coke. The oxygen formed at the anode combines with the carbon to form carbon dioxide gas; therefore, the cost of these electrodes is an important economic factor. When metals are manufactured by an electrolytic process, the weights of the metals deposited are proportional to their equivalent weights. The ratio of weights of some common industrial metals deposited by the same amount of current vary in order listed: aluminum 9, magnesium 12.1, iron 18.6 and copper 31.8. It is evident that the economical manufacture of aluminum necessitates large amounts of low cost electrical energy. Aluminum plants are therefore located near hydroelectric plants. A measure of the relative manufacturing problems of iron and aluminum is indicated by the listed data.

Aluminum Iron (Hall process) (Blast furnace)

Type of operating unit Small, semi-batch large, continuous Capacity of unit, per day 1000 lbs. 700-1500 tons Lining of unit Petroleum coke Refractory bricks Temperature of operation lOOO'C IfiOO'C Weight of metal per lb. ore 50 lb. .63 II). 8"i ll>. Weight of carbon per lb. metal. . . .75 lb.

Value of gaseous by-product . . . none 85 Btu/cu. ft.

Agent used to remove impurities. . . Caustic Soda Air, iron oxide (Bayer Process) Electrical Energy Requirements per lb. metal 12 KWHr. Energy requirements ratio

(theory) 5 1 Light Alloys as Engineering Materials 133

The aluminum produced by the Hall process consists of 99% (minimum) aluminum, the major impurities being iron and silicon. Aluminum of high purity (99.99%) may be obtained by an electro-refining process. The cell used in the Hoopes process consists of three molten layers, the bottom anodic layer of crude aluminum-copper alloy, the intermediate layer of salts, and the top cathodic layer of pure aluminum.

8.5. Physical Properties of Aluminum

The most generally recognized physical characteristic of aluminum is its low density. The lightest metal, magnesium, is two-thirds as dense as aluminum (sp. gr. 2.70); whereas, iron and steel are three times as dense. The density of most aluminum alloys range 2.70-2.80, which means that aluminum and its alloys roughly weighs .1 lb. per cubic inch. The low density of aluminum can be attributed to its low atomic weight (27) and its great chemical reactivity. These two properties in addition generally indicate such thermal properties as low melting point, high specific heat, high heat of fusion, and high coefficient of thermal expansion.

The melting point (1220T.) and heat of fusion (94.6 cal.) are slightly higher than corresponding properties of pure magnesium. Its very high specific heat and its linear coefficient of thermal expansion are second to magnesium. The high solidification shrinkage (6.6%) as well as its "hot shortness" make the casting of pure aluminum rather unsatisfactory.

The electrical conductivity of metals, in decreasing order, are silver, copper, gold, and aluminum. Pure aluminum has 64% of the electrical conductivity of copper on an equal-volume basis but has a 212% value on an equal-weight basis. It is therefore the best commercial electrical conductor on the latter basis. The best thermal conductors, in decreasing order, are silver, copper, gold, and aluminum; aluminum is the best conductor on an equal-weight basis.

This parallelism between electrical and thermal conductivity is expressed by the Wiedemann-Franz law. Another relationship that appears to be indicated is that these two physical properties are related to crystal structure since all four of these listed metals have a face-centered crystalline structure.

The low density and melting point of aluminum are related to its properties of being soft (Brinnell Value 16) and relatively low tensile strength (8,500 psi).

In view of its low melting point and "hot shortness," the tensile properties of aluminum rapidly decrease with a rise of temperature; therefore, aluminum and many of its alloys are not well suited to elevated tempera- 134 Engineering Metallurgy ture applications. Aluminum and all of its alloys become stronger and tougher at subnormal temperatures.

Aluminum is a very malleable and ductile metal; in fact, it is second best of the commercial structural metals. These properties are related to its face-centered cubic crystalline structure, for we note that copper, silver and gold are very malleable and ductile and have face-centered cubic structure. Under standard testing conditions, pure aluminum will stretch 60 per cent of its length before breaking. With the combination of high ductility, malleability, and strength, aluminum and its alloys are adapted to working by most methods of forming and fabrication. Due to the high degree of plasticity and relative chemical inertness to normal conditions, aluminum can be easily finished to a high polish. The brilliance of aluminum paint is due to the high reflectivity of the finely divided flat flakes.

Aluminum is an excellent reflector of radiant energy of all wave lengths. The high durability of aluminum paint is largely due to the efficient reflection of the ultraviolet rays; ultraviolet rays destroy the oil film in paints and lead to loss of adhesion. Aluminum is used as thin films on the reflectors of the most modern telescopes. Aluminum foil is important in the thermal insulation field because of its high reflectivity of infrared radiation. Three important requirements of a molten metal for satisfactory casting arc low surface tension, high fluidity, and relative chemical inertness. Molten aluminum has a surface tension of 520 dynes per centimeter which is one third that of molten iron, both at their respective melting temperatures. Pure molten aluminum has a fluidity 50 per cent that of molten tin; the normal impurities of commercial aluminum increase this value to 70 per cent. Molten aluminum is protected from oxidation by a thin oxide film; excessive agitation must be avoided to prevent the removal of this oxide film. Two other properties of molten aluminum contribute to its poor castability, first—its reactivity with moisture to form aluminum oxide and hydrogen, second—the high selective solvency for hydrogen gas which leads to high-porosity cast metal.

8.6. Chemical Properties of Aluminum

The very high solution potential of aluminum demonstrates that it is the second most chemically active structural metal. In spite of this great activity, aluminum offers great resistance to normal weathering. This relative inertness can be attributed to the presence of a thin protective and adhering oxide film. Other outstanding features of this oxide film Light Alloys as Engineering Materials 135

are its chemical inertness, water insolubility, ability to mend itself, and its electrical and thermal insulation properties.

The chemical inertness of this film is indicated by fact that aluminum tanks are used for storage and shipment of concentrated acetic acid, dilute and concentrated nitric acid, and strong hydrogen peroxide.

When the oxide film, is removed, the active nature of the metal as- serts itself; for example, caustic soda and strongly alkaline solutions readily attack aluminum. When alkaline degreasing solutions are used to

clean aluminum, a corrosion inhibitor such as silicate of soda is added. Aluminum in contact with mercury or its salts will show accelerated corrosion due to the removal of the oxide film. In the melting of aluminum and subsequent handling operations, precautions should be taken to avoid, as much as practicable, contact with moisture, water vapor, or other materials that will react with aluminum to form hydrogen gas. The amount of dissolved gas can be kept at a low amount by using a low temperature, short operating time, minimum agitation, and low absolute humidity of the atmosphere.

The great affinity of aluminum for oxygen is illustrated in its use in the steel industry. The amount of aluminum required for the deoxidation

of molten iron and steel is usually less than 1 lb. 1-1 per ton. When 1/2 lbs. are used, the excess aluminum functions in another manner: it leads to

the subsequent formation of a fine-grained steel.

A similar deoxidation reaction is involved in the thermite welding and

repairing of large ferrous alloy sections, railway and street car rails, etc. Thermite consists of a mixture of granulated aluminum and mill scale. It is ignited by means of flashlight powder or another primer, which leads to localized melting of the aluminum, followed by a violent reaction to form aluminum oxide and molten iron. The excessively high temperature (4500°F.) is due to the great heat of reaction, the low specific heat of iron, and the absence of any volatile product. A modified thermite reaction is often used in the manufacture of pure high-melting metals as vanadium, chromium, molybdenum, and manganese. Atmospheric nitrogen is a very inert gas but will react with aluminum and chromium, at elevated temperatures, to form metallic nitrides. The presence of .85—1.2% aluminum in the nitralloy steel plays an important role when being surface hardened by nitriding. The stainless steels and other chromium alloy steels do not require aluminum for this purpose.

8.7. Aluminum Alloys

The important alloying elements used in the manufacture of aluminum alloys are copper, manganese, magnesium, zinc, nickel, chromium, and 136 Engineering Metallurgy silicon. In special purpose alloys the elements titanium, tin, lead, and bismuth are intentionally added. For example, lead and bismuth are added to the extent of .5% each in the free machining alloy; those metals are insoluble in aluminum and are therefore present as a colloidal dispersion which functions as a lubricant. The total alloy content of the wrought and casting alloys are usually 7% and 16% maximum, respectively. The three main important strength- giving elements in the wrought and cast alloys respectively are copper- magnesium-manganese and copper-magnesium-silicon. Silicon also plays a very important role in the castability of alloys. The individual alloys are designated by numbers, the wrought alloys as a four digit number, and the cast alloys as a two- or three-digit number. Two high-strength wrought alloys are 5056 (5% Mg) and the 7075 (5% castability 356. Zn) ; the alloys that have excellent are 43, 355, There are two classes of wrought aluminum alloys: (1) alloys that are not heat treated and that owe their properties to strain hardening by treatment cold work, (2) alloys whose strength may be improved by heat and whose properties may be altered still more by cold deformation.

8.8. The Wrought Alloys

The various tempers of commercial wrought aluminum alloys until recently have been designated by a system of numbers and letters. The new temper designation consists of a letter indicating the basic temper which may be more specifically defined by the addition of one or more digits. The letter F means "as fabricated," O—"annealed, recrystallized," and H—"strain hardened." The latter may be modified by numbers to give a detailed meaning as per follows: Strain hardened only—HI, plus one or more digits. Strain hardened and then partially annealed— 112, plus one or more

digits. Strain hardened and then stabilized-H3, plus one or more digits. In dealing with the heat-treatable type of alloys, the letter —T following the alloy number signifies that it has been hardened by solution treatment. The specific combination of basic operations is indicated by a number: 6061-T5 means that alloy 60G1 has been hardened by heat treatment and artificially aged. The various basic operations are as follows: —T2 Annealed (cast products only). —T3 Solution heat treated and then cold worked. —T4 Solution heat treated and naturally aged to substantially stable condition. —T5 Artificially aged only. Light Alloys as Engineering Materials 137

Table 8.1 Temper Designations for Strain Hardened Wrought Aluminum Alloys I hat Arc Not Heat Treatable (*)

New Temper Designation({) Strain Hardened Strain Hardened Old Temper Strain Hardened and then and then Dest^iahon only Partially Annealed Stabilized

(Alloys 1100 and 3003) 14 H -H12 -H22 '/2 H -H14 _H24 *A H -H16 -H26 H -H18 -H28 Extra Hard (Not Standard) -H19

(Alloys 3004, 5052, 5056J) #{J -HS2 %™H -H34 ^ -H36 Extra Hard (Not Standard) —H39

# Alloys ( ) listed also available in -O and —F tempers, (t) The tempers shown are standard for the alloys listed. (t) The alloys 3004, 5052 and 5056 may also be obtained in the -HI and -H2 type Conditions. '~

—T6 Solution heat treated and then artificially aged. —T7 Solution heat treated, then stabilized. — Solution T8 heat treated, cold worked and then artificially aged. -T9 Solution heat treated, artificially aged and then cold worked. —T10 Artificially aged and then cold worked. —W Solution heat treated-unstable temper.

In Tables 8.2 and 8.3 there are listed the important wrought alloys in order of their increasing tensile properties, together with other important physical properties. The number of alloying elements and total alloy percentage for the nonheat-treatable are less than the heat-treatable alloys. The principal characteristic of the wrought alloys, especially the heat- treatable type, is their high strength-weight ratio. The typical mechanical properties of the annealed and fully hardened alloys are listed in Tables 8.2 and These 8.3. properties are applicable to room temperature but are satisfactory for the range from about 0° to 150°F. All of the aluminum alloys become stronger and tougher at subnormal temperatures. Due to the fairly rapid decrease in tensile properties, and sometimes the de- 1

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The resistance to corrosion of aluminum alloys is usually very satisfactory, the strain-hardening type alloys being generally superior to the heat-treatable type. The presence of relatively large amounts of coppcr or zinc reduces the over-all resistance to surface attack. The resistance to high temperature oxidation is best insured by the presence of the alloying element nickel.

Maximum resistance to corrosion of the heat-treatable alloys is dependent on the solution heat treatment being followed by a very rapid quench; therefore, thick sections arc not as resistant as thin sections due to their slower rate of quench. Artificial aging usually effects a marked improvement in resistance to corrosion.

Some sheets, plates, and tubes arc produced with a surface coating of pure or low-alloy aluminum that are metallurgically bonded to the core during hot working. These coatings offer electrolytic protection to the core against corrosion. This protection includes cut edges and areas where the coating may be scratched or abraded. 140 Engineering Metallurgy

8.9. Aluminum Casting Alloys

The casting alloys are designated by a number of two or three digits. In some instances the number is preceded by a letter, which indicates a slight variation in composition. For example, the three casting alloys 214, B214, and F214 contain 3.8% magnesium but differ in the silicon content.

Casting alloys may be of two types: (1) the nonheat-trcatable alloys, and

(2) the heat-treatable alloys. The condition of heat treatment is designed by appending some symbol, as 195-T4. The elements used in aluminum casting alloys are primarily copper, silicon, and magnesium. The minor alloying elements are zinc, nickel, manganese, iron, and tin. The requirements for the sand, permanent mold, and die castings arc not necessarily the same, although many of the alloys are used for both sand and die. The 5% binary silicon alloy is the only listed alloy that is used for all three types of castings.

Aluminum finds little use in the production of castings because of its low strength and inferior casting qualities. Two exceptions are the induction motor rotors and cable clamps, which require the higher electrical conductivity of aluminum. The commercial casting aluminum alloys and their important properties are listed in Table 8.4. Note that the total percentage of alloying elements is generally greater than that of the wrought alloys. Some of the important characteristic properties of molten casting alloys such as good fluidity, low surface tension, and low pouring temperature necessitate a high percentage of alloying elements. The low temperature of solidification leads to low gas solubility and porosity, and fine grain structure. The important casting characteristics for a sand and a permanent die casting are (1) freedom from hot shortness, (2) low shrinkage tendencies, and (3) pressure lightness. Low gas absorption is a very important property for a sand-casting alloy. A die-casting alloy should have freedom from hot shortness and mold-filling capacity. Sand castings of almost unlimited size and shape can be made from scores of aluminum alloys, each having unique properties or foundry characteristics. Where only a few castings are required, or where the casting is relatively large, sand castings are indicated. On the other hand, when large numbers of relatively small parts are required, or if a smooth surface and a minimum amount of machining are desired, permanent- mold or die castings are to be preferred. Some typical permanent-mold castings are shown in Table 8.4. The method of casting, too, affects the properties to be expected from a given composition, permanent-mold and die castings usually having a somewhat higher strength than sand castings. In general, when the number and the size of the castings to be made would Light Alloys as Engineering Materials 141

justify the use of any of the three methods, die castings are the cheapest and can be cast to the closest tolerances, while sand castings will cost the most and will require the broadest tolerances. A comparison of the mechanical and tensile properties of the sand permanent mold, and die castings is best made by listing the typical values for alloys 43.

Permanent

Sand Mold r)ie Tensile strength 19,000 23000 30,000 Tensile yield strength 8,000 9 000 16,000 Elongation in 2") ' (% 8 10 Compressive 9 yield strength 9,000 9,000 Brinell hardness •10 45 Shearing strength 14,000 16,000 19,000

Although the casting alloys have a greater alloy content than the wrought, the improvement as compared with pure aluminum is primarily greater in the casting properties rather than the mechanical properties. The maximum tensile strength of the commercial aluminum alloys in the "as-cast" condition for the sand, permanent mold, and die castings are approximately 27,000, 35,000, and 46,000 psi. respectively. The tensile properties of some casting alloys are further improved by solution heat treatment. This type of alloy contains at least one constituent whose solid solubility is substantially greater at elevated temperatures than at room temperatures. The usual amount of this alloying element ,s such that it is completely soluble near the melting point of the alloy. Heat treatment finds limited application with die castings because of blistering and possible loss in dimensional accuracy. The yield strengths in tension and compression for the cast alloys are practically equal. The yield strength in compression is very important in the design of some aircraft structural members. Some special treatments are available that raise the compressive yield strength to a value almost as high as the tensile strength. For example, sand casting alloy T6 3nd T62 condition hav* ^e respective tensile strengths *LZ , 36,000 and 40,000 psi; the compressive yield strengths under these conditions are respectively 25,000 and 38,000 psi. The modulus of elasticity of most aluminum casting alloys is approximately 10,300 kips; therefore this property is about one third that of steel The Brinell hardness of "as-cast" alloys range 40-70 (500-kg. load, 10-mm. ball) whereas, for the ; heat-treated alloys the hardness value is 75-100 The endurance limits for the alloys range 5,500-9,500 psi. This gives an endurance ratio of .25-.38 for cast alloys and .17-28 for the heat-treated 142 Engineering Metallurgy

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alloys, based on a rotating-beam test of 500 million cycles. These ratios are one third to one half of those usual for carbon and low-alloy steels. Inasmuch as the total percentage of alloying elements is usually greater in casting alloys than wrought, the melting point, and the thermal and electrical conductivity, are much lower for the former class. The melting range and the specific gravity range (2.66-2.92) are wider for the casting alloys because of their higher alloy content. The specific gravity and the thermal and electrical conductivity values are lower for the die castings than the sand and permanent mold casting alloys. Centrifugal castings have been very popular for many ferrous and nonferrous alloys because the molten metals and alloys are cleansed of oxides and inclusions by the centrifugal action thereby improving the mechanical properties. The small differences in the specific gravity between the usual oxides and inclusion present in molten aluminum alloys makes this type of casting of a limited advantage.

8.10. Functions of Alloying Elements The advantages and limitations of commercial aluminum as a wrought and cast metal may be summarized as follows. Its advantages as a wrought metal are high electrical and thermal conductivity, excellent malleability and ductility, excellent formability, good machinability, and unusual resistance to normal corrosion. The limitations include low tensile properties, poor high-temperature tensile properties, and high temperature brittleness. Molten aluminum has the desirable properties of low surface tension and good fluidity, the undesirable properties are high selective hydrogen gas solubility, high solidification shrinkage, and great chemical reactivity towards air and moisture. The cast metal has the unsatisfactory properties of high thermal coefficient of expansion, hot shortness, gas porosity, and coarse grain structure. The addition of the alloying elements is primarily to increase the tensile properties and improve certain fabricating qualities of the wrought metal; they function to increase tensile strength and castability of the alloy casting. Such alloying always results in the improvement of some properties and in the further limitation of others. The changes involved depends upon the type and amount of alloying elements, the resulting grain structure, size and distribution of insoluble constituents, homogeneity, etc. Copper. The most important and commonly used alloying element is copper. The intermetallic compound CuAU forms a eutectiferous series with aluminum; the eutectic alloy contains 33% copper and melts at 548°C. The maximum solubility of copper in solid aluminum at elevated 144 Engineering Metallurgy temperatures is 5.65% and less than .25% at room temperature. The copper content of the high-strength wrought alloys usually ranges between 4-4.5% copper. Although tensile properties increase progressively above this range, higher copper alloys are not commercially practicable because of difficulty in working. The high-strength alloys of this type include alloys 2011, 2017, and 2024. The electrode potentials of commercial aluminum and the intei metallic compound CuAl2 are —.84 and —.53 volts respectively. This relatively great difference means that the corrosion resistance of the copper alloys is much less than those of the other alloying elements. Improper heat treatment of the copper-aluminum alloys can further decrease their resistance to corrosion. Copper used as an alloying element in the casting alloys leads to increased fluidity and decreased surface tension of the molten metal. The cast metal is improved by the freedom from hot shortness, decreased porosity, increased tensile properties, and finer grain structure. Silicon. The second most important alloying element in aluminum al-

loy is the metalloid silicon. It differs from the other soluble alloying elements in not forming an intermetallic compound with aluminum. It forms a eutectic having the composition of 11.6% silicon by weight and melts at 577°C, The solubility of silicon in solid aluminum at the eutectic and room temperatures are 1.65% and less than .05%. Although

silicon is a rather hard constituent, it is evident that due to its low solubility and inability to form an intermetallic compound, the binary silicon alloys are not markedly strong. A comparison of the strength

giving properties of silicon and copper is given by the tensile yield strength values of the sand casting alloys #43 (5% Si) and #195 (4.5%

Cu) , namely 8,000 and 34,000 psi respectively. Silicon plays an extremely important role in the casting alloys. Its important contribution to the molten alloy are very high fluidity and low solidification shrinkage. The shrinkage of an 8% silicon-aluminum

binary alloy is 5.6%; the 11.6% silicon-aluminum alloy (eutectic) is 3.8%. The presence of silicon influences the properties of the solid cast metal in having a lower coefficient of thermal expansion, absence of hot shortness, and a lower degree of porosity. An unusual property of silicon

in a casting is to form a sound outer layer or skin which leads to pressure tightness. The silicon alloys 43, 355, and 356 give excellent sand and (>ermanent mold castings. The two die-casting silicon alloys, #13 and #360 are very satisfactory. The ability of silicon alloys to "undercool" leads to the modification of the normal eutectic composition and its properties. In 1921 A. Pacz Light Alloys as Engineering Materials 145 discovered that very rapid cooling or the addition of .05% metallic sodium (or sodium fluoride) results in the formation of eutectic alloys containing as high as 14% silicon, having a freezing point 10-15 degrees lower than normal. This modified alloy has a finer grain structure; it is stronger and more ductile than the eutectic alloy.

The silicon content of wrought alloys is usually less than 1%: one exception is the forging alloy 4032. The presence of silicon in the heat treatable wrought alloys generally increases their response to artificial aging.

The presence of silicon in aluminum alloys does not materially decrease its corrosion resistance, probably due to the formation of a protective siliceous film. The 5% binary casting alloy #43 and wrought alloy 4032 resist atmospheric weathering, chemicals and food acids. Inasmuch as silicon and aluminum are both amphoteric, this type of alloy is not very resistant to alkaline conditions and solutions. The strength and corrosion resistance of silicon-aluminum alloys are markedly increased by the presence of magnesium due to the formation of the intermetallic compound Mg^.Si. Magnesium. The eutectic of magnesium and aluminum contains approximately 33% magnesium and the melting point is 451 °C. The solubility of magnesium in solid aluminum at the eutectic and room temperatures is 14.9% and less than 4% respectively. This variation in the solid solubility with temperature renders some of the alloys susceptible to improvement in tensile properties by heat treatment.

The electrode potential of 4% magnesium-aluminum solid solution is approximately equal to that of aluminum; therefore this type of alloy shows excellent corrosion resistance. In addition, these alloys show the highest resistance to alkaline corrosion, such resistance increasing with magnesium content. Binary alloys containing more than 4.5% magnesium are susceptible to stress-corrosion cracking particularly when they are in the cold worked condition. This corrosion is due to the fact that the insoluble phase is highly anodic to the 4% magnesium-aluminum solid solution. The electrode potentials of the two components are respectively —1.07 and

—.87 volts. This stress corrosion is prevented by the addition of small amounts of manganese and chiomium.

Magnesium is second to silicon in effectively increasing the fluidity of molten alloys. The fluidity of the binary 6% magnesium alloy is approximately the same as the binary 5% silicon alloy. Magnesium is a more reactive metal than aluminum; therefore it reacts more readily with air and moisture. Molten casting alloys of magnesium- 146 Engineering Metallurgy aluminum form considerable dross, a tendency that increases with magnesium content. Alloys containing 5% or more magnesium cast in ordinary green sand molds show surface discoloration and porosity, particularly when thick sections are involved. High magnesium-aluminum should be heat treated in a controlled atmosphere or for a minimum of processing time so as to avoid surface oxidation.

Magnesium ir Silicon. The magnesium silicide forms an eutectic with aluminum containing 13% Mg2Si and melts at 595°C. When an excess of silicon (over that required to form the magnesium silicide) is present, a ternary eutectic of aluminum, MgL.Si and silicon results, melting point 550°C. The composition of this ternary eutectic is approximately 13% silicon, 6% magnesium and 81% aluminum. The Mg,Si constituent in the binary alloy has a maximum solubility in solid aluminum of 1-85% at the eutectic temperature and less than .2% at room temperature. These alloys have the ability to respond to heat treatment and have tensile properties comparable with those of the heat- treated 4% copper alloys.

The corrosion resistance of this type of alloy is very satisfactory because the electrode potential of Mg2Si (—.83 volt) is almost the same as aluminum. Alloys of this type, such as alloys 6053, 6061 and 6151, differ from most other heat treatable alloy in showing no appreciable decrease in corrosion resistance after heat treatment. Manganese. Manganese and aluminum form an eutectic containing 2.2% manganese by weight and melting at 657°C. The intermetallic compound of these metals has the formula MnAle . The solubility of manganese in the solid aluminum at eutectic and room temperatures is .65 and less than .1 respectively. This low solubility indicates that the most common manganese alloy 3003 (1.25% Mn) is a non-heat-treatable alloy. The electrode potential of this intermetallic compound is the same as aluminum, therefore this alloy has exceptional corrosion resistance. Inasmuch as manganese and its compounds are not toxic, this alloy is used in equipment for preparing and processing of foods. It is also used extensively for the manufacture of cooking utensils, conduit and pipes, pressure vessels, architectural applications and builders hardware. Nickel. Nickel forms an eutectic: with aluminum which contains 6.5% nickel and melts at 640°C. The composition of the intermetallic com-

is . solubility of nickel in solid at the pound NiAlt The aluminum eutectic and room temperatures is .06%, and less than .04% respectively. The addition of nickel to the copper-aluminum type alloys definitely improves the high temperature properties and retains the high thermal Light Alloys as Engineering Materials 147

properties. The forging alloys 2018, 2218 and 4032 are ternary alloys of aluminum, copper and nickel. Their uses are for forged cylinder heads and pistons, jet engine impellers and compressor rings and other applica-

tions wherein high strength at elevated temperatures is necessary. Zinc. Aluminum and zinc form an eutectic consisting of 95% zinc and 5% aluminum, melting point 440°C. The solubility of zinc in solid aluminum at the eutectic and room temperatures is 70% and 18% respectively.

Zinc is a low melting and highly volatile metal. One important limita-

tion of binary and complex zinc-aluminum alloys is their great loss in strength at elevated temperatures. For example, castings made from A612 and C612 are not recommended for use at temperatures greatly in excess of 200°F. These casting alloys have the marked advantage that without heat treatment they have tensile properties comparable with the aluminum- copper and aluminum-silicon heat treated alloys. They offer the advantage that they can be heated to the temperature required for brazing and will regain their original properties after a few days. The zinc wrought alloy 7075 has very good strength and good resistance to corrosion. This wrought alloy in the T6 condition in extrusions has a tensile and yield strength of 88,000 and 80,000 respectively.

8.11. Cold and Hot Working of Aluminum Alloys

The purpose of cold working in the fabrication of sheet, rod, wire, tubing, extrusions, and like basic products are (1) increased tensile

properties, (2) grain refinement, (3) improvement in surface appearance, and (4) improvement of dimensional properties. Aluminum and its alloys on cold working undergo strain hardening. The tensile properties increase with the amount of work, in some instances almost proportional to the fractional reduction of the initial

Elongation Alloy Tensile Strength Yield Strength % in 2 in. 3003-O 1G.0OO 6,000 30 3003-H-12 19,000 17,000 10 3003-H-I4 21.000 19,000 8 3003-H-16 25,000 22,000 5 3003-H-18 29,000 26,000 4 Alclad 202-1 -T4 64.000 42.000 19 2024-T3 64.000 44,000 18 2024-T36 67.000 53,000 11 2024-T81 65,000 60,000 6 2024-T86 70,000 66,000 6 148 Engineering Metallurgy

cross section. The elongation decreases with increasing amount of cold work. As a general rule the initial cold work produces a relatively greater effect than subsequent reductions of equal amounts. The effect of cold

work is listed for the two types of wrought alloys.

The degree of strain hardening in aluminum and its alloys has an im-

portant effect on their temperature of recrystallization. It is usual to make 50% or higher reduction in cross-sectional area between annealing or heat treating operations in order to prevent the development of excessively large grain structure.

In the fabrication of the commercial basic products there is an improvement of the surface properties due to tool surface and shape, lubricants used, and the burnishing effect obtained by the cold work. The greater the degree of reduction and the inherent hardness of the alloy, the brighter the surface. Straightening and flattening operations are performed cold and are usually conducted so as to introduce a minimum of strain hardening. Plate, rod and bar are hot worked nearly to the desired thickness, followed by cold work to obtain greater dimensional accuracy. The cast aluminum ingots are hot worked to convert them to wrought structures. Hot working is carried out during the manufacture of extrusions and forgings. The hot working of aluminum alloys require about 30-40% more power than for steel. Extrusions are made in the shape of rods, bars, and complicated cross sectioned materials. The extruded section is highly fibrous in structure which is parallel to its direction of extrusion, therefore the tensile properties are often much higher than other hot- or cold-worked products of same alloy.

Another difference between aluminum and steel is that aluminum does not form scale during heat treatment; therefore a suitable lubricant should be used during hot rolling. Die lubricants are generally used during hammer and pressing forging; extrusions usually do not require lubrication.

8.12. Heat Treatment of Aluminum Alloys

The three basic types of thermal processing that are commonly applied to aluminum alloys are (1) annealing, (2) solution heat treatment, and

(3) precipitation heat treatment. The strain-hardening which results from cold working of the aluminum alloys may be removed by annealing. The annealing process likewise removes most of the temper resulting from solution heat treatment. This thermal treatment permits recrystallization to take place; the higher the temperature used, the greater the rate. The rate of recrystallization is Light Alloys as Engineering Materials 149

also dependent upon the alloy involved and the severity of the cold work. Complete annealing is almost instantaneous for alloys 1100 and 5052 at

650°F., and for alloy 3003 at 750°F. The rate of cooling thereafter is not critically important although rapid cooling may lead to some distortion. The temperature used for annealing the heat-treatable type of aluminum alloy must be carefully controlled and as low as practicable so as to prevent the dissolving of the intermetallic compound. Castings of non heat-treatable alloys are often annealed for the purpose of relieving internal strains, increasing their ductility and shock resistance. Some alloys are improved in strength and hardness by heating in the range of 900-1000°F., given a rapid quench, and then aged at room temperature and/or at slightly elevated temperatures. The alloy after quenching has fair ductility and malleability and is subjected as soon thereafter to forming or other fabricating processing. Some alloys spontaneously increase in tensile properties within a short period of time; other alloys age so slowly that they are artificially aged by heating in the range 250-400°F. Such heat-treatable alloys contain a principal alloying constituent that has a marked increase in solid solubility at elevated temperature. In most instances this alloying constituent is an intermetallic compound. 4i/ The 2 % copper-aluminum alloy 2024 has CuAlj as its hardening agent; its solid solubility at room and elevated temperatures is .25% and 5.65% respectively. All this intermetallic compound goes into solid solution at elevated temperatures and continues to remain in solution after rapid quenching due to the marked decreased mobility of the system. This unstable supersaturated solution has good ductility and malleability; therefore it is formed into proper shape as soon as practicable. During the next few hours fine precipitation of submicroscopic particles of this hard intermetallic compound takes place, progressively increasing the tensile properties.

Other heat treatable alloys such as 6061 undergo a similar transition during heat treatment and quenching, but differ in the respect that the rate of precipitation of submicroscopic particles is extremely slow at room temperature, therefore it is artificially aged by heating in the range of 250-400°F. The alloys normally hardened by room temperature aging may be further increased in tensile properties by this artificial aging.

The time required to reach the maximum strength by artificial aging decreases as the aging temperature is raised; the value of the maximum strength, in turn, decreases. The precipitation of the hardening constituent from solid solution that results from artificial aging is accompanied by small changes in specific gravity and therefore dimensional properties. The 5% and 6% copper-aluminum alloys after artificially 150 Engineering Metallurgy

aging show a decrease in length of approximately .01% and .15% respectively.

The response to artificial aging for alloy 2024 is greatly enhanced if

the material is strain hardened after solution heat treatment and prior to precipitation heat treatment. In this instance the increase in tensile

and yield strengths is more than doubled by a reduction of 6% by cold working prior to artificial aging.

The percentage of alloying elements in casting alloys is higher than for wrought alloys, therefore the solution heat treatment time for casting is much longer. The greater uniformity and the finer grain structure of wrought alloys results in a greater degree of improvement in tensile properties by heat treatment for the wrought alloys.

The solution heat treatment of alloys containing high amounts of magnesium should be done with proper atmosphere control so as to prevent possible oxidation along the grain boundaries. This oxidation condition is manifest by minute surface blisters, decreased tensile properties and embrittlcment. The time of solution heat treatment of clad alloys should be kept at a minimum so as to prevent changes in composition and electrode potential properties of the surface coating due to alloy diffusion from the core.

The cooling rate during quenching is often critical. The alloy 2024, improperly quenched, forms a non-uniform precipitation of microscopic and larger size particles of AIXu which makes the alloy susceptible to localized corrosion.

8.13. Corrosion Resistance of Aluminum Alloys

The excellent corrosion resistance of aluminum and its alloys is due primarily to the adhesion and chemical inertness of the oxide film.

Inasmuch as this aluminum oxide is a nonconductor of electricity it differs from iron rust (cathodic) in not causing galvanic corrosion.

Galvanic or "battery action" type of corrosion is encouraged wherein the alloying metals or their intermetallic compounds have electrode potentials much different than the base metal. The solid solutions and the insoluble intermetallic compounds of manganese, chromium, magnesium and MgoSi have electrode potential values almost equal to aluminum; therefore alloys involving these constituents show good corrosion resistance. The solid solutions of copper, and the insoluble constituents

FeAls , NiAl3 , CuAl2 and Si, have much lower solution potentials than aluminum, therefore alloys of these constituents show only fair resistance to corrosion. Light Alloys as Engineering Materials 151

The poor corrosion resistance of the improperly quenched aluminum copper alloys is due to its heterogeneous nature. Alloy 2024 on solution heat treatment and properly quenched consists of a uniform supersaturated solid solution; its homogeneous nature makes it very resistant to corrosion. The subsequent aging of the alloy leads to the uniform precipitation of submicroscopic intermetallic compound which also shows good resistance to corrosion. When the alloy is quenched slowly, the CuAL precipitates out at the grain boundaries as non-uniform microscopic and larger sized particles. The phase adjacent to the cathodic intermetallic compound consists of areas of depleted solid solution (anodic). The localized corrosion that lakes place in this alloy is termed intergranular corrosion.

When a mild quench is necessary, the resistance to corrosion may be increased if the alloy is subsequently artificially aged. In the case of thick sections, the rate of cooling, even by immersion in cold water, is often not great enough to produce material completely free from susceptibility to this intergranular attack. This condition may also be discouraged by the use of alclad materials. of the One most effective methods for increasing corrosion resistance of an aluminum alloy is to use alclad material or by applying a surface coating of pure or low alloy aluminum. The surface layer must be anodic to the base metal, hence the protection is both mechanical and electrolytic. The present products available with this type of cladding are sheet, tube and wire. One limitation of this type of product is that during heat treatment, temperature and time should be very carefully controlled so as to avoid excessive alloy diffusion into the clad layer, thereby decreasing its effectiveness in protecting the core from corrosion. Alclad 2024 sheet improperly heat treated should be checked by proper tests for excessive "copper diffusion." Another commercial process used to protect aluminum alloys from corrosion is by the surface oxidation by electrolytic means. The process is known as anodization because the aluminum is made the anode and the iron tank the cathode, the processing solution being dilute sulphuric or chromic acid. The anodic film is thicker and more uniform than the natural oxide film; therefore it has increased corrosion resistance, electrical insulation, abrasion hardness and chemical inertness. An unusual property of the anodic film, especially right after its formation, is its great power of adsorption. The excellent anchorage power of anodized aluminum for paint films and dyes is dependent upon this great power of adsorption. 152 Engineering Metallurgy

Chromic acid dip treatment, anodi/ation, painting with zinc chroma te primer, and other like processing and treatments of aluminum alloys are based on the passivation action of chromic acid and chromates, and their adsorption in the oxide film. When an aluminum alloy in use shows excessive corrosion, this condition may be stopped by cleaning area, painting with chromic acid solution, drying and then painting. It is important that aluminum assemblies should consist as much as possible of the same alloy or alloys of like solution electrode potential.

Rivets should be of the same alloy as the base metal, and if different, should be of an alloy that lias a slightly lower potential so that the rivets would be cathodic. Galvanic corrosion can be markedly retarded if the rivets can be electrically insulated by anodic treatment. Welding- rods should likewise be of the same alloy or of slightly lower potential than the base metal. Inasmuch as brazing and soldering involves foreign metals, adjacent areas must be carefully protected from galvanic corrosion. When two metals are involved that have marked difference in potential, it is good practice to coat one with a metal intermediate in electrode potential value. The steel cable used in aluminum cable for transmission of electric current is plated with zinc to discourage galvanic action.

8.14. Joining of Aluminum Alloys

Aluminum and its alloys are usually joined by fusion and electric- resistance welding, brazing and riveting. Brazing differs from welding in that the filler material has a much lower melting point that that of the parent material so that the latter does not melt. The common types of fusion welding are the gas and the arc processes. The three brazing processes are classed by the methods used in applying the brazing heat, namely, furnace, torch, and dip brazing. The electric resistance welding is classified as spot, seam, and flash. The main difference in the welding technique and procedure of aluminum and its alloys as compared to the iron alloys is primarily due to the very high thermal coefficient of expansion and high temperature oxidation of aluminum, and thermal and electrical insulating pro]>erties of its oxide coating.

The high thermal expansion of aluminum and its alloys necessitates the use of properly designed parts and carefully prepared joints so as to minimize buckling and distortion. The filler rod and the base metal are both provided with a flux coating to maintain an oxide-free metal surface during the welding process. The commercial flux materials consist of mixtures of saline chlorides-fluorides. -Proper ventilation must be provided in the welding area because the fumes of the flux are both Light Alloys as Engineering Materials 153

disagreeable and irritating to the welder and highly corrosive to nearby metal structures and materials. The welded part must be given a mechanical cleaning or chemical pickle treatment so as to completely remove residual saline material due to its toxic and corrosive character.

Gas welding involves the use of oxyhydrogen and oxyacetylene torches, the flames in both instances being neutral in character. The gas welding has its main application with materials ranging from .040—1 inch in thickness.

The main arc processes include metal, carbon, atomic hydrogen, and tungsten arcs. The metal-arc is particularly suitable for heavy material; thicknesses up to 21/g inches have been welded satisfactorily. Difficulties are experienced when the metal-arc is used with material less than 5/64 inch thick. The carbon-arc is used for manual and automatic welding; the produced welds are comparable in structure and appearance to gas welds but with less distortion. The atomic hydrogen process is applied in much the same manner as gas welding but differs in the respect that it is possible to control the heated zone by moving the torch away from or towards the work. One limitation of the atomic hydrogen-arc process is the difficulty of removing the residual flux from the welded assembly. The tungsten arc uses an inert gas shield to protect the arc and the weld area; this method has two advantages: (1) satisfactory welds can be made with equal facility in vertical, flat, and overhead positions, and (2) no flux is required. All commercial alloys are not readily adaptable to fusion or electric- resistance welding. The alloys that are commercially fusion welded are 1100, 3003, 5052, 6053, and 6061. In some instances the welding is difficult and/or costly. One limitation of the heat-treatable type of alloys is that the high temperature of welding tends to decrease the mechanical strength and the corrosion resistance. The proper choice of welding rod is very important particularly when dealing with alloys having large amounts of alloying elements. The two most weldable alloys 1100 and 3003 use 1100 welding rod. The high- strength alloys 5052, 6053, and 6061, fairly difficult to weld, are welded with 5% silicon-aluminum alloy filler rod. This 4043 alloy rod has a melting point much lower than pure aluminum, excellent fluidity and wetting properties, and low shrinkage contraction, thereby insuring a minimum of shrinkage stresses and cracks. One of the important applications of fusion welding is the salvaging of aluminum alloy castings that have foundry defects, and the repairing of cracked or broken castings. Castings should be preheated in a furnace to approximately 800°F. This thermal treatment avoids thermal stress, 154 Engineering Metallurgy

reduces the welding time and amount of welding gas required. It is important that careful temperature control is maintained during this preheat treatment so as to avoid "burnt" material and possible collapse of the casting. The areas adjacent to the welded sections of the strain-hardened alloys are annealed by the welding heat. The weld metal has tensile properties almost equal to the annealed metal but lower in ductility. Assemblies of heat-treatable alloys should be welded prior to heating or given a reheat treatment so as to improve the tensile and corrosion resistance properties. Electric-resistance welding has been a large factor in reducing the cost of making joints, improving products, and speeding up production. This type of welding is usually applied to strain-hardened type of alloys, but is often used with the heat-treatable alloys particularly of the alclad type.

The technique and equipment employed for the resistance welding of aluminum and its alloys is considerably different than that used for steel.

In addition the electrical capacity required for aluminum is several times greater. The three types of equipment are classified on the basis of the electrical system used for supplying the welding current, namely,

(1) alternating current, (2) magnetic energy storage and (3) condenser energy storage. The proper choice of electrode shape and its proper maintenance is very important in obtaining high quality and strength spot welds. The welding electrode must have high electrical and thermal conductivity and fairly good strength; electrodes are usually made of copper-silver alloys. Weld pick-up and the softening of the tip are prevented by having cold water circulating through the electrode. Some machines use refrigerated water circulating at a rate of 2 gallons per minute and to within 3/8 inch or closer of the tip face.

The material to be welded must first be freed of oil and grease, dirt, and surface oxidation because these substances have electrical insulating properties. The surface contamination may be removed by an alkaline degreasing bath whereas the oxide coating necessitates mechanical abrasion or chemical etch methods. The metal must be spot welded within a few hours of chemical etch because the oxide film will reform.

Bra/ing has been used in production on certain alloys such as 1 100 and 3003, and to lesser degree on the heat treatable alloys 6053 and 6061. The copper-aluminum alloys and 5056 show poor workability during brazing operations; the resulting joints and adjacent areas have decreased tensile strength and corrosion resistance. The advantages of brazing as compared to gas and arc welding are as follows: (1) it can be used with thinner material, (2) the costs are lower, and (3) the surface appearance Light Alloys as Engineering Materials 155

is superior. The torch-brazed joint has the tensile strength comparable to a weided joint and has almost the same degree of corrosion resistance. Riveting is the most commonly used industrial method of joining the structural aluminum alloys, particularly the heat-treatable type. The rivets are made of commercial aluminum, the magnesium-silicon alloys 6053 and 6061, and the copper alloys 2017, 2024, and 2117. All except the first are received from the manufacturer in the hard, heat-treated condition. Rivets of 2017 and 2024 are solution heat treated, quenched, and driven as quickly as possible; whereas, the other type of rivets are driven "as received."

It is advantageous from a strength point of view to use rivets having about the same mechanical properties as the base metal; whereas, from the driving standpoint it is usually advisable to have the rivets somewhat softer. From the corrosion resistance angle it is advisable to have the rivet of the same composition or electrode potential as the material in which it is driven. For example, alclad sheet alloys are usually riveted with 6053 or 6061 because of their excellent corrosion resistance and nearness to aluminum in their electrode potential value. Rivets that are solution heat treated prior to driving arc generally processed in large quantities, quenched in cold water, washed with alcohol, and placed in refrigerated cabinets. Solution heat treated 2017 and 2024 alloy rivets harden at room temperature over a two-hour period, making them unsuitable for driving thereafter. The rivets will remain soft for 36 hours if kept at 32°F., or lower, after quenching. Small insulated boxes of cold rivets are kept in refrigerated cabinets near the individual work areas. Large rivets of 2017 alloy are usually driven in the hot condition. They are solution heat treated at 930-950°F. for approximately fifteen minutes, inserted in the hole and driven. The relatively cold driving tools and the base metal function as quench. Steel rivets in aluminum-alloy structures are driven hot in the customary fashion; they are heated to I800°F. and driven as quickly as possible. When the service conditions are severe or dissimilar metals are involved, special treatments should be applied to prevent corrosion. Vari- ous methods are used, such as application of zinc chromate primer prior to assembly, use of good grade of joint compound before assembly, use of anodized aluminum alloy rivets; and if steel rivets are involved, they are metallized with aluminum or zinc after driving.

8.15. Magnesium And Its Alloys

Magnesium is a very unusual metal: its properties do not run parallel to any other commercial metal, and its behavior is not as predictable. Mag- 156 Engineering Metallurgy nesiuni holds the unique position among the structural metals in having the lowest density, the highest machinability, and has an inexhaustible source of supply.

The low specific gravity of magnesium (1.74) means that it is 2/3 as dense as aluminum and 1/4 that of iron and steel. The cutting speed for magnesium and its alloys is twice that of free-cutting brass and four times that of cast iron. Seawater contains 1 part in 770 of magnesium therefore one cubic mile of seawater could produce nine million tons of magnesium.

There is also a vast supply of magnesium in magnesite and dolomite.

Magnesium is similar to aluminum in its low melting point, high solidification shrinkage, hot shortness, low tensile strength near the melting temperature, good thermal and electrical conductivity, low density, high strength-weight ratio, and non-magnetic and non-sparking properties. From a chemical point of view these two metals have low atomic and equivalent weights, and high electrode potentials. It markedly

differs from aluminum in its crystal structure, (H. C. P.) , fair malleability and ductility, fair cold workability, poor notch sensitivity, non-protective oxide coating, and lower corrosion resistance. Magnesium is much more chemically reactive than aluminum due to the physical and chemical nature of its oxide coating. The manufacturing procedures and processes of magnesium are very much like that of aluminum in the respect that fl) the impurities must be removed from the ore rather than from the produced metal, (2) requires large amounts of energy, (3) requires high temperatures, and (4) reduction involves nonaqueous reactions.

The most important commercial process for making magnesium is by the electrolytic reduction of molten solutions of magnesium chloride. The formed molten magnesium floats on the surface and must be protected by an inert atmosphere since it chemically absorbs both oxygen and nitrogen. Another limitation of this (Dow) process is that the chlorine constitutes a health and corrosion hazard. Two lesser important processes are the chemical reduction of magnesium oxide by carbon and silicon respectively. In the carbothermic (Hansgirg) process, briquets of magnesium oxide and carbon are heated at 2000°C. to form magnesium vapor and carbon monoxide gas. This reaction mixture on cooling would reverse to the original raw materials; this is prevented by rapidly cooling and diluting with natural gas or other inert gases. The fine magnesium powder that results by this "shock" action must be purified by distillation. The Pidgcon process depends on the fact that silicon in the form of ferrosilicon will remove the oxygen from magnesium, that the formed silicon dioxide is a non- .

Light Alloys as Engineering Materials 157

volatile material, and metallic magnesium under reduced pressure can be distilled at 1150°C. The Hansgirg and Pidgeon processes are more expensive than the Dow process, but have the advantage of not depending on electric power. The improvement of magnesium by addition of alloying elements is not very great. The main alloying elements are aluminum, zinc, and manganese. Zinc is more effective than aluminum in the improvement of mechanical properties but offers the disadvantage of decreasing its corrosion resistance. A compromise is often made by using both. The alloy AZ-63 consists of 6% aluminum and 3% zinc. The main function of manganese is to increase the corrosion resistance. The minor alloying elements are beryllium, calcium, cerium, silicon, tin, and zirconium. Beryllium decreases the tendency of burning during the melting and casting. Calcium reduces oxidation during melting and heat treatment, and increases the reliability and formability of magnesium sheet. Cerium increases the high temperature strength. The main function of silicon is to increase the fluidity of molten magnesium and to decrease the hot shortness and cracking tendencies of the solid metal. Tin increases ductility, and zirconium leads to grain refinement. The very undesirable elements and their approximate maximum tolerance are copper (.1%), nickel (.01%) , and iron (.005%) Magnesium alloys are somewhat difficult to roll; therefore structural members are usually extruded. Cold working, bending, and deep drawing can be satisfactorily accomplished under carefully controlled conditions and with frequent annealing. Most mechanical processing is done at temperatures above 225°C. Pure commercial magnesium sheet in the annealed and hard-rolled conditions have the respective tensile strength values of 27,000 and 33,000 psi. Most magnesium alloys have tensile strength in the range of 40-50 thousand psi, and the maximum of sixty thousand. Special costly alloys containing silver 3% or 6% cerium have tensile strength of sixty-five thousand.

The heat-treatment processes of magnesium alloys is similar to that of aluminum except the temperatures involved are considerably lower. The readiness with which magnesium oxidizes in the air means that special precautions and new techniques are involved. Although aluminum and its alloys may be heat treated in molten sodium nitrate baths, violent ex- plosions may take place with the alloys of magnesium. One outstanding advantage of magnesium and its alloys is its excellent machinability. Cutting fluids are used primarily as a coolant to prevent the magnesium from catching on fire. The turnings from machining .

158 Engineering Metallurgy operations also represent a potential fire hazard. Dry sand should be available in these areas because water or the common fire extinguishers are unsatisfactory for putting out magnesium fires. The casting of magnesium likewise requires special conditions.

Molten magnesium is usually protected by an inert atmosphere of sulphur and sulphur dioxide. Green sand castings can not be used unless inhibitors are present; otherwise the magnesium will react with the moisture. The subsequent finishing of the casting involves metallic dust that represents a fire hazard.

The corrosion resistance of magnesium and its alloys under normal conditions is very satisfactory. Under highly humid conditions and other severe conditions, magnesium and its alloys should have a protective film or coating. Magnesium alloys prior to fabrication are usually shipped with a film of oil or a chrome pickle treatment coating. The chrome pickle is superior to the oil film for protection, but the former is easier to remove prior to spot welding. Magnesium alloys may be joined by gas and arc welding, electric-resistance welding, and by riveting. Most of the wrought alloys can be joined by gas welding using neutral flames of oxyhydrogen, oxyacetylene and oxycarbohydrogen

The filler rod should have the same composition as the parent metal or have a lower melting point. A flux must be used during welding due to the ease of oxidation of magnesium. Magnesium alloy sheet, extrusions, forgings, sand and permanent mold castings are often welded by using an inert gas-shielded arc method. These alloys may be resistance welded after they have been cleaned of surface contamination and oxide coatings. Magnesium may be riveted by the most conventional methods. Magnesium-alloy rivets cannot be used because of their rapid work hardening at room temperature. The rivets generally used are the 5% magnesium-aluminum alloy 5056, which has a like electrode potential. Most processed magnesium alloy parts and assemblies that will be used where corrosion is greater than normal are given a final chromate treatment. The chromate treatment results in the removal or passivation of all foreign metals and produces an adhering protective coating. This coating offers good anchorage for a paint film.

8.16. Beryllium

Beryllium resembles magnesium in its low density (1.84) , its hexagonal close-packed crystalline structure, and its poor cold workability. Its similarity to aluminum is shown by its low equivalent weight, its amphoteric properties, and the refractory and protective nature of its oxide film. The outstanding property of beryllium is its great power of producing .

Light Alloys as Engineering Materials 159 hardness and strength in other metals; it suffers the two great limitations of high cost and toxicity. Its cost is high because (1) its ores are not common, (2) the concentration of beryllium in the ores is often less than 4% average, (3) its manufacture by the modified thermite or Gold- schmidt process is a costly one.

The most important alloy of beryllium is the beryllium-copper alloys, which have high fatigue and corrosion resistance, good electrical and thermal properties, and are virtually non-sparking. The 2% alloy on heat treatment, quenching, and aging develops a hardness equal to Rock- well C-40.

Amounts in the order of .01% beryllium in molten aluminum and magnesium decrease their oxidation and combustibility markedly, and reduce their viscosity, particularly in the case of aluminum. Small amounts of beryllium may be used to case-harden iron and steel.

The vapors and powdered beryllium are very toxic and give rise to lung diseases that are often fatal. Beryllium is available commercially in the form of 12% beryllium-copper alloy. 8.17. Titanium Titanium in one of the most abundant metals in the earth's crust, the ores main being ilmenite (iron titanate) and rutile (titanium oxide) It is made by the modified thermite or Goldschmidt process.* The very important properties of titanium as a structural metal are its high melting point (1820°C.) its , high tensile strength at ordinary and elevated temperatures, its relatively low density (4.5) , and its high resistance to ordinary corrosion. In the annealed and cold-worked (60% reduction) condition the commercial metal has the respective tensile strengths 78,700 and 111,500 psi. Exposure to accelerated corrosion tests using salt spray for 30 days has little effect on commercial titanium. Titanium, like magnesium and beryllium, has the hexagonal close- packed crystalline structure; therefore its workability likewise at ordinary temperatures is poor. When titanium is heated to 875°C, it undergoes a transition to an body-centered cubic structure; hence its workability markedly increases at this temperature. There is a 5.5% volume change during this allotropic transformation. One limitation of titanium is that it will readily unite with and burn in oxygen at 600°C. and with nitrogen at 800°C. At elevated temperatures titanium will likewise unite with carbon and hydrogen.

Titanium is a very important additive element in the steel industry. The great affinity of titanium for carbon is taken advantage of and carbide precipitation avoided in 18-8 stainless steel by the presence of titanium, the amount being about five times that of the carbon content.

* And now produced widely by the Kroll process. 160 Engineering Metallurgy

The presence of .5-1% titanium in 17-7 stainless steel leads to a ferritic structure. The grain refinement of aluminum and its alloys by the presence of small amounts of titanium is due in part to its great affinity for dissolved oxygen, nitrogen and hydrogen. Titanium acts likewise as a scavenger for oxygen and nitrogen in steel. QUESTIONS

1. What is the .most abundant metal in the earth's crust? What is the metal that is inexhaustible in its supply? 2. Magnesium oxide has been reduced industrially by the action of carbon to form metallic magnesium. Why cannot aluminum be economically made in the same manner? 3. What impurity as a metal oxide, if present in bauxite ore, could not be removed in the Bayer Process? i. Aluminum is often melted in iron pots which are usually protected by a coating of whiting or other insulating material. Why is this protective coating important?

5. The general rule regarding gases is "they decrease in solubility in liquids with rise of temperature." How would you explan why the solubility of hydrogen increases with rise of temperature when present in molten aluminum?

6. Accurate control of temperature is very important during the solution heat treatment of aluminum alloys. What are the relative advantages of a molten bath and an air furnace? 7. What test solution would you use on a large assembly to determine whether it was an aluminum alloy or a magnesium alloy? 8. What are the main obstacles of manufacturing aluminum from clay? 9. Excessive rates of heating metal lead to cracking. In view of the fact that aluminum alloys are good conductors of heat and that the nitrate bath is a fair conductor, why is the heating in this bath still rather slow? (What insulation film is involved?)

10. The processing room wherein the Hall process is conducted must be provided with good ventilation. What gas and vapor must be conducted away? 11. Why does the presence of high silicon in aluminum alloys lead to superior fluidity? To lower degree of contraction? 12. How do you explain why magnesium has the highest machinability among the commercial metals? Why arc its turnings so much more combustible than those of aluminum? 13. Why should the heat treatment of magnesium be made in an air furance rather than a molten nitrate bath? 14. Aluminum sheet arrives at the plant with fine (tissue) paper between each sheet. Give three reasons why this has benefical results.

15. After aluminum has been anodized, and then given cold and hot rinses, it should be painted within one hour thereafter. Why should this time be important? 16. What precaution should be made regarding machining or processing of beryllium alloys? 17. How do you account for the fact that titanium has high strength at elevated temperatures?

18. Chlorine gas is passed into molten aluminum on two occasions. What are two conditions that are eliminated by this action. Give reasons for both. Copper and Copper -Base Alloys as Engineering CHAPTER Materials

Waiter R. Hibbard, M.S., Adjunct Associate Professor of Metallurgy, College of Engineering, University of Bridgeport, Bridgeport, Connecticut Roger Greenleaf Stevens, Ph.D., Head, Department of

Chemical Engineering, Southwestern Louisiana Insti- tute, Lafayette, Louisiana

1 HESE metals are such an integral, but commonly unnoticed, part of our daily life that even the housewife in the kitchen and the clerk in the store would find it difficult to establish a new routine of existence if copper suddenly disappeared from the world. Judged solely from the standpoint of absolute value to mankind, copper and its common alloys, the brasses and the bronzes, cannot compete with carbon steels, but each is of vital importance to our civilization and could hardly be eliminated without destroying or at least seriously crippling most branches of industry.

It has been said with justification that electricity is the elixir vitae of machine civilization. It is difficult to conceive of an electrical industry if copper and its ores had never been known, unless nature had provided man with a substitute of equal conductivity, plasticity, and cost. Second only to high conductivity in importance is the resistance of copper and some of the copper-rich alloys to corrosion in certain environments, especially sea air and sea water.

The average automobile uses 45 lb. of copper and its alloys, the average steam locomotive 4,500 lb., and the modern steamship more than 3 million lb. as castings, in pipes and tubes and in electrical equipment. No metal has proved so suitable for steamship propellers as manganese bronze; four of these, weighing 37 tons each, drove the Queen Mary across the Atlantic Ocean in four days.

With two or three exceptions, the copper and copper-rich alloys discussed in this chapter have been important engineering materials for

161 '62 Engineering Metallurgy

years and, unlike some of the light alloys discussed in the previous chapter, are covered by specifications of the American Society for Test- ing Materials.* In addition, most of them also conform to specifications adopted by the Society of Automotive Engineers, the Association of American Railroads, the American Society of Mechanical Engineers

(Boiler Construction Code) , the American Standards Association, the U.S. Navy, and the U.S. Army, and to general federal specifications. These standards—especially those issued by A.S.T.M. -should be consulted for exact chemical composition and for minimum mechanical- property and other requirements which must be met before the material can be considered to be of good commercial quality.

Although copper is present in the earth's crust to the extent of only 0.01 per cent, ore deposits of commercial importance are fairly widespread. Compared with iron ores, copper-ore deposits of commercial quality are lean. The ores of the United States contain on an average somewhat less than 2 per cent copper. In the important ores, copper occurs as the oxide, carbonate, silicate, and sulphide, of which the last named is the most abundant. In some parts of the world, notably in the vicinity of Lake Superior, long weathering of copper ores followed by reduction of the resulting oxide or the carbonate, or the deposition of copper from solution, has produced large deposits of relatively pure native metal, which was distributed far to the south by the glaciers. Native copper was the only metal (except small quantities of meteoric iron) used by the American Indians for tools and weapons until the white man arrived.

The production of high-purity copper from lean sulphide ores is a complex process, but copper exceeding 99.9 per cent purity can be produced cheaply. In brief, the process consists of concentrating and roasting the complex sulphide ore (copper, plus iron, plus other metals) to form oxides and to eliminate some of the sulphur and arsenic as the oxide. The roasted concentrate is smelted with coke and a flux in a reverberatory furnace to remove part of the iron. The copper and iron sulphides fuse into a matte containing about 40 per cent copper. This matte is charged into a Bessemer converter and is blown in a manner similar to the refining of pig iron to produce Bessemer steel. The copper and the remaining iron sulphides are oxidi/ed, and the iron oxide combines with silica to form a silicate slag. The result of these reactions is "blister copper" containing 95 to 97 per cent copper and considerable copper oxide.

'Sec A. S. T. M. Standards, Part 2, 1955. Copper and Copper-Base Alloys 163

In addition to copper oxide, blister copper generally contains small amounts of sulphur, iron, lead, arsenic, antimony, selenium, tellurium,

nickel, and other elements, usually including some gold and silver. In some ores, in fact, the silver and gold are worth so much that the copper could be called a by-product of the refining of the more precious metals. Although the amount varies, a typical anode melted from blister copper frequently contains 30 oz. of silver and nearly i/ 2 oz. of gold per ton. The final step in refining is melting and casting the blister copper into blocks which are electrolyzed. Electrolytic copper contains 99.95 per cent or more copper, with sulphur as the principal impurity. About 25 per cent of the electrolytic copper produced is used for making copper- base alloys; the remainder is melted in a large reverberatory furnace, oxidized to remove sulphur, and then "poled." This consists of inserting green wood poles beneath the surface of the molten metal. In the reaction between the wood and the molten copper oxide enough oxygen is removed so that the remainder, 0.03 to 0.04 per cent, when evolved during solidification, exactly balances the normal shrinkage of the metal. Poled copper is known as "tough-pitch." Its density in the cast condition is approximately 8.5 gm. per cu. cm., compared with 8.94 for the pure metal. Tough-pitch copper is used for wire for electric conductors, for sheets, and for many other purposes. Lake (Superior) copper, which constitutes about 15 per cent of the commercial copper in the United States, is-as previously mentioned-a high-purity native copper and does not usually need electrolytic refining. It differs from electrolytic copper in that it may contain a small amount of silver (which does not affect conductivity) and from 0.002 to 0.04 per cent or even more arsenic, which, in the higher amounts, may reduce relative conductivity as much as 40 per cent. recent development A in copper refining is the production of oxygen- free copper. Oxygen in tough-pitch copper is present as cuprous oxide particles, which are almost completely insoluble in the solid metal. For most purposes this oxide is not harmful, but if extremely high ductility is essential, the copper should be practically free from oxygen. The oxide is also harmful if the copper is heated in an atmosphere containing hydrogen as this gas diffuses into the metal, reacts with the oxide, and causes brittlencss. There are two types of oxygen-free copper. The more common one is produced by deoxidizing the molten metal with phosphorus or some odier element having a high affinity for oxygen. The residual phosphorus, although only about 0.02 per cent is present, reduces the electric conductivity as much as 20 j>er cent. To produce high-conductivity oxygen-free copper (known as OFHC) , very pure copper is 164 Engineering Metallurgy

melted and cast in an atmosphere free from oxidizing gases. A recent innovation consists of granulating brittle copper cathodes, which have been deposited electrolytically and which are free from oxygen, followed by "coalescing" these in a press at high temperatures, which welds the particles together and extrudes the metal as a solid bar.

Copper is malleable and is hot and cold worked into sheet, plates, tubes, and a variety of other shapes. As in the case of most other metals and alloys, cold working increases strength and reduces ductility.

9.1. Properties and Uses of High-Purity Copper

Copper has the highest electric conductivity of all the common metals. On a strength basis, its conductivity is more than twice that of high- purity aluminum. About one half of the 800,000 tons of copper consumed

in the United States in the average peacetime year is used for electrical equipment and for wire in light and power lines. When annealed, high- purity copper has a conductivity of 100 per cent according to a standard set up in 1913 by an international commission. Recently improved methods of purification have resulted in conductivities as high as 102 per cent.

Copper is also the best heat conductor of any of the commercial metals, which makes it (and its alloys) useful as cooking utensils and as radi- ators and other heat-dissipating apparatus. In general, copper and the copper-rich alloys have excellent resistance to corrosion by the atmosphere and by water. Owing to this corrosion resistance and to a reasonable price, copper and some of its alloys are widely used by the building industry for roofing, down-spouts, gutters, screens, drainage and water pipes, and for hardware and interior fixtures. When exposed to the atmosphere for long periods, copper oxidizes and forms a green patina which is ornamental in addition to being adherent and protecting the underlying metal from further attack. Approximately 10 per cent of the copper consumed annually is used by the building industry.

Copper and copper-rich alloys have a wide range of mechanical properties. Annealed high-purity copper has a tensile strength of approximately 30,000 lb. per sq. in., an elongation in 2 in. of about 60 per cent,

and a reduction of area of as much as 90 per cent. It is, therefore, weaker and more ductile than high-purity iron. At the other extreme, precipitation-hardened copper-beryllium alloys may have a tensile strength of 190,000 lb. per sq. in. with accompanying low ductility.

The tensile strength of high-purity copper may be increased to 65,000 or 70,000 lb. per sq. in. by drastic cold working; elongation is lowered Copper and Copper-Base Alloys 165

to about 5 per cent. According to Gillett,* the endurance limit of annealed tough-pitch copper is approximately 10,000 lb. per sq. in., and cold working increases it to as much as 20,000 lb. per sq. in. The modulus of elasticity is approximately 16 million lb per sq. in. The damping capacity is almost as high as that of cast iron. For some applications the low strength of high-purity copper wire is a distinct disadvantage. Unfortunately, most alloying elements added to increase strength reduce conductivity to a fraction of the value for high-purity copper. A satisfactory compromise between increase in strength and decrease in conductivity results from the addition of small amounts of cadmium.

1100

40 50 60 % V 100 Zinc, per cent

Frc. 9.1. The copper-zinc phase diagram. (A. Phillips and R, M. BricKft

9.2. Constitution of the Common Copper-Rich Alloys

None of the four series of important binary copper-rich alloys—namely, copper-zinc, copper-tin, copper-aluminum, and copper-nickel-can be heat treated in the sense that sterling silver or many of the aluminum- rich alloys can be heat treated. Sections of the copper-zinc and the copper-aluminum diagrams arc given in Figs. 9.1 and 9.2. Most of the useful alloys of copper with zinc (brass) and of copper with aluminum

• H. W. Gillett, Metals and Alloys, v. 3, 1932, p. 200. Metals Handbook, \ American Society for Metals, Cleveland, 1939, p. 13G7. 166 Engineering Metallurgy

(aluminum bronze) contain less than 40 per cent zinc and less than 10 per cent aluminum respectively and are, consequently, ductile single- phase alloys consisting of a solid solution of zinc or of aluminum in copper. If the zinc in brass exceeds about 35 per cent, beta phase appears, reducing ductility and impact resistance. Only one such alloy,

Muntz metal, containing -10 ]>er cent zinc, is used widely, and in this the amount of beta is so small that ductility is not seriously affected. Similar to conditions in the copper-zinc system, most of the wrought aluminum bronzes are single-phase alloys containing 5 to 9 per cent aluminum (see Fig. 9.2) . If the alloy contains more than 9.5 per cent aluminum, beta phase appears, reducing the ductility. Although the beta phase cannot be retained in solution by quenching, there is the possibility of securing transition structures by quenching a wrought alloy containing

10.5 to 1 1 per cent aluminum. The result is a tensile strength of 100,000 to 110,000 lb. per sq. in. and an elongation of 10 to 15 per cent, as compared with 65,000 to 85,000 lb. per sq. in. and 35 to 50 per cent elonga-

% Atomic Percentage Aluminum _5 K> JS 20 2S SO

Weight Percentage Aluminum

Fig. 92. The copper-rich portion of the copper-aluminum phase diagram.t (W. L. Fink, L. A. Willey, and C. S. Smith)

ilbid., 1948, p. 1160. Copper and Copper-Base Alloys 167

tion for the alpha-phase alloy. No precipitation hardening is possible in the binary copper-rich copper-aluminum alloys. The principal commercial copper-tin alloys are alpha-phase alloys and contain 1 to 10 per cent tin and usually also zinc, lead, or both. The only important binary alloy is phosphor bronze which contains 4 to 10 per cent tin and which is deoxidized in melting with a small amount (less than 0.5 per cent) of phosphorus. Tin increases the hardness and strength of copper. A cold-rolled phosphor bronze used for springs has a tensile strength of about 100,000 lb. sq. in. with 5 per cent tin; this increases to 120,000 lb. per sq. in. with 10 per cent tin. No heat treatment is possible with commercial copper-tin alloys. Copper and nickel form a continuous series of solid solutions free from any phase change. The strength of the solid solution of nickel in copper increases with the nickel from about 35,000 lb. per sq. in. for very low nickel to 80,000 for 60 per cent nickel; elongation decreases from 70 to 45 per cent in 2 in. There are a large number of important copper-nickel alloys which, owing to their controllable color, to their corrosion resistance, or to their thermoelectric properties, are used for coinage, ornamental metal work, ship construction ( especially in parts exposed to salt water) thermocouples, , and in other special applications. An alloy higher in nickel, Monel metal-containing 65 per cent nickel and 35 per cent copper-is widely used because of its high resistance to many corrosive mediums.

9.3. Nomenclature of the Copper-Rich Alloys

Although the copper industry is no worse than the steel industry in the use of trade names and in confused nomenclature, so many misnamed alloys are commonly used that some attention to this subject seems advisable. The most common examples of confused nomenclature are calling a copper-zinc alloy bronze if it has a color resembling that of the copper-tin alloys and calling a copper-zinc-nickel alloy nickel silver if it has a silvery-white color. This confusion is regrettable, but in most cases nothing can be done about it, as the names are firmly fixed by years of usage. The important fact to remember about the copper-rich alloys is that the name may be more descriptive of color than of composition, and that a "bronze" may contain no tin and may even be a brass; likewise, nickel silver contains no silver. The usual name and the typical compositions of some common copper- rich alloys are given in Table 9.3. It is evident that manganese bronze (item 43) hardware , bronze (item 19) , and commercial bronze (items 9 and 10) are in reality brasses. The only true bronzes given in Table .

168 Engineering Metallurgy

9.3 are items 49 to 59, which are copper-tin alloys deoxidized by phos-

phorus. Nickel silver, formerly known as German silver, is the name of a group of brasses, containing 17 to 42 per cent zinc, in which a considerable part of the copper is replaced by nickel. The composition of four of these alloys is given in Table 9.3. Another example of confused nomenclature is the casting alloy containing 85 per cent copper and 5 per cent each of zinc, tin, and lead. As this alloy contains 5 per cent tin, it is as much of a bronze as it is a brass, but it is called red brass* and is, consequently, sometimes confused with wrought red brass which contains 85 per cent copper and 15 per cent zinc.

9.4. Characteristics and Uses of the High Brasses

As shown by the diagram in Fig. 9.1, 39 per cent zinc dissolves in copper at room temperature. Alloys containing less than this percentage of zinc—when in a state of equilibrium—are, therefore, single-phase solid

solutions and are known as alpha brasses. As shown also by Fig 9.1, alloys containing about 36 to 39 per cent zinc may have a duplex structure if heated to a high temperature but should be single phase at room temperature. Actually, however, owing to a phenomenon known as cor-

ing, cast alloys usually have a duplex structure if the zinc is higher

than 31 or 32 per cent. A zinc content of 35 per cent is about the maximum for an alloy that is to be fabricated as a single-phase material. Although there is a continuous series of alpha brasses containing from 5 to 39 per cent zinc, these alloys are commonly and arbitrarily divided into two classes: high brass, containing 30 per cent or more zinc, and low brass, containing approximately 25 per cent zinc or less (Table 9.3)

Since zinc is cheaper than copper, it follows that high brass is used as widely as possible, and that the zinc content is usually close to the upper limit, which is about 35 per cent.

Although the alpha brasses can be hot worked commercially if they are free from lead, they are not so plastic at elevated temperatures as the beta alloys. In common with most solid solutions they are, however, plastic at room temperature and can be readily drawn, rolled, or spun into a variety of shapes. In fact, these alloys combine a degree of plasticity, strength, ductility, corrosion resistance, pleasing color, and low cost unattainable in any other material of engineering importance. Alloys

13, 14 , 2 9 are used for a large variety of cast and wrought sections, especially hardware and pipe fittings, for tubes and pipe, for ornamental

• The casting alloy containing 85 per cent copper and 5 per cent eacli of tin, zinc, and lead is also known as composition metal and in many foundries as ounce metal because the composition corresponds approximately to 1 lb. of copper. 1 oz. of tin. 1 oz. of zinc, and 1 oz. of lead. Copper and Copper-Base Alloys 169

sections and building trim, for cartridge cases, automobile radiator cores, grillwork, springs, screens, and tor many kinds of nails, screws, and rivets. Many copper-zinc alloys are used because of their color, as stated previously. They change from warm red copper color of copper to commercial bronzes (Tables 9.3 items 9 and 10) which have a bronze color to red brass (item 11) which is reddish hence its name. Some of these alloys in the above range duplicate the color of gold alloys and are used for manufacture of gold filled metals. Then comes low brass (item 12) whose color is pinkish, to cartridge brass (item 13) which is yellow, to yellow brass (items 14 and 15) which are greenish-yellow and finally to Muntz metal (item 16) which has the color of cast statuary bronze.

Muntz metal and its modification, manganese bronze (item 16 and 48, Table 9.3) usually contain , beta phase at room temperature (see Fig. 9.1) which reduces plasticity , when the alloys are cold. These materials are, however, characterized by extreme plasticity at a red heat and can readily be extruded or rolled. Owing to their resistance to atmospheric and water corrosion, these two alloys, especially manganese bronze, are used extensively for extruded or hot-rolled pipes and condenser tubes. Muntz metal is now being replaced, particularly for condenser tubes in marine equipment, by a brass known as admiralty metal. This contains not less than 70 per cent copper, 1 per cent tin, and the remainder zinc. Other than for condenser tubes, admiralty metal, so called because it was developed by the British Admiralty, is used for preheaters, evaporators, air-conditioning equipment, and in other applications where high resistance to salt water, fresh water, oil, or steam is required. Admiralty metal is an alpha-phase alloy and is not so easily hot worked as Muntz metal and manganese bronze, but it is readily worked when cold. In genera], the machinability of the annealed, non-leaded brasses is about the same as that of low-carbon steel, in other words, they are too soft and gummy to be cut readily. Cold working, however, improves machinability, which is also greatly improved by adding lead. Item 29 (Table 9.3) can be machined readily in automatic machines. If this alloy is given a machinability rating of 100, the same brass containing only 1 per cent lead will have a rating of 70 (determined under the same conditions), and item H without lead-will have a rating of only 30. However, lead reduces the yield strength; it also makes the brass more difficult to deform cold and makes it virtually impossible to work the alpha-phase alloys at elevated temperatures. 170 Engineering Metallurgy

9.5. Characteristics of the Low Brasses and the Nickel Silvers

As the amount of zinc in brass is reduced, the yellow color changes to red. Items 8, 9, 10, 11, and 12 (items 9 and 10 are called bronzes because of their color) are used for hardware, ornamental and architectural sections, wire for screens, and for costume jewelry, in addition to being used for tubes, pipe, radiator valves, and other fittings for low-

pressure water and steam. An advantage of the low brasses is that, when

the copper exceeds 80 per cent, the alloy is not subject to "season cracking," a phenomenon prevalent in the high brasses when they are exposed

to a combination of corrosion and stress. Season cracking is the spontaneous failure of the stressed material after a considerable time in a corrosive environment, especially in atmospheres containing ammonia. The time element depends upon the severity of the stress and the severity of the corrosion. Stress caused by cold working, or applied externally, is necessary for failure of this type. The low brasses, like the high brasses, are readily deformed cold provided the lead content is low. Also like the high-zinc alpha brasses, they are difficult to hot work although commercial bronze (item 9, Table 9.3) can be rolled or pierced and, to a limited extent, pressed or extruded, provided that more than traces of lead are absent. Most nickel silvers have a copper content of 45 to 65 per cent. A fairly common alloy is the one containing 75 per cent copper, 20 per cent nickel, and 5 per cent zinc, usually sold under the trade name Ambrac. All the nickel silvers can be classed as low brasses containing nickel. If the copper plus nickel exceeds 63 per cent, the alloys contain only alpha phase. They can be hot worked readily only if the zinc is

about 40 per cent or less than 10 per cent, but they can be cold worked easily. Nickel silvers are valuable because of their white color, a

corrosion resistance that is generally superior to that of the common brasses, good mechanical properties, and moderate cost. The effect of

composition on color is shown f * * by Fig. 9.3. The nickel silvers Nickel. per cent are used chiefly for ornamental work as a base for silver- Fig. 9.3. Effect of composition on the color of nickel silver. (Kihlgren)' and gold-plated ware. .

Copper and Copper-Base Alloys 171

9.6. Properties of the Wrought Brasses

Some typical tensile properties of wrought brass and nickel silver are collected in Table 9.3. As the commercial copper-zinc alloys do not respond to heat treatment, variations in properties are obtained by cold working and annealing which results, as in the case of some of the aluminum alloys, in various tempers, ranging from soft to half-hard, to hard, and to spring temper. With the possible exception of manganese bronze

and nickel silver, the tensile strength of all the annealed brasses is very similar to that of annealed or hot-rolled low- or medium-carbon steel, but the yield strength is much lower. As the brasses cost five to ten times as much as low-carbon steel, the chief justification for the widespread use is, of course, then ease of working cold, their pleasing color, and their corrosion resistance, especially to water and steam. In general, the low brasses are more corrosion-resistant than the higher zinc alloys, especially in impure water and in salt water. The high brasses, in addition to being subject to season cracking are also subject to "dezincification," or removal of the zinc, when the alloy is exposed to impure water or salt water. The action is apparently electrolytic and leaves a spongy mass of copper. Dezincification can cause failure in a relatively short time. Alloys containing 20 per cent or less zinc are not usually subject to this form of corrosion. Manganese bronze has high resistance to salt water but may be subject to season cracking and dezincification.

The endurance limits of the brasses are low, ranging between 10,000 and 20,000 lb. per sq. in. for 100 million cycles, which is equivalent to an endurance ratio of 0.20 to 0.35, or about half that of steel. Annealed brasses have a definite yield point (as evidenced by the drop of the beam) but cold-worked alloys have not. Values for yield strength reported in the literature are usually the stresses causing an extension of 0.5 per cent under load. These values are usually much lower than the yield strength of steel which corresponds to a permanent elongation of 0.2 per cent of the gage length. The modulus of elasticity (secant modulus, see p. 65) varies from 15 million lb. per sq. in. for the copper-zinc alloys and for manganese bronze to about 19 million lb. per sq. in. for nickel silver. The mechanical properties of brass and of nickel silver are affected by grain size which, in the case of worked material, depends on annealing temperature. Material with large grains has lower strength and higher elongation than fine-grained material (except in very thin strip)

• T. E. Kihlgren, Metals Handbook, American Society for Metals, Cleveland 1939 pp. 1443-1445. 172 Engineering Metallurgy

9.7. Cast Brass and Cast Nickel Silver

There are at least a thousand varieties of brass that are melted and poured into sand molds. Most of these, however, fall into the nine general classes of alloys for which specifications have been issued by the American Society for Testing Materials, the Society of Automotive En- gineers, or by the United States government. Some typical compositions and properties of the most commonly used alloys are collected in Table 9.1* The properties of cast brass are determined on test bars cast separately from the same metal (A.S.T.M. specification B208-46T) and tested either with or without machining their surface.

As shown by the data in Table 9.1, the cast brass, with the exception of manganese bronze, are relatively weak and ductile and have a very low yield strength. Nickel silver has better properties than the copper-

zinc alloys containing no nickel, and when the zinc is omitted completely, as in the case of cupronickel, the properties are even better. The nickel silvers are, however, considerably more costly than the other brasses. The cast brasses are easily machined—especially when they contain considerable lead, as most of them do—and have a high resistance to corrosion in many environments. They are, therefore, widely used for architectural and ornamental castings, especially hardware and statuary;

fittings and other parts exposed to sea, air, and water or steam; plumbing fixtures for water; valves and other parts in chemical equipment; laundry and dairy equipment; and many others. Nickel silver is a favorite casting alloy for automobile and household hardware and for industrial use where alkaline waters or chemicals are encountered.

9.8. The Copper-Base Bearing Metals

The principal copper-base bearing metals (bearing metals are discussed more fully in Chapter 10.) are those in Class C of Table 10.2.

On a tonnage basis this group is more widely used than the others.

According to Clamer.t about 10 million lb. of alloy CI (either deoxidized, or not, with less than 1 per cent phosphorus) is used annually in the United States. The leaded bronzes, alloys CI to C5 and C8 and C9, are employed by the railroads and the automotive industry; alloys C2 and C3 are suitable for small bushings and bearings; others are better

• Cast Metals Handbook, American Foundrymen's Association, Chicago. 3rd ed,, 1914, pp. 604-648. t G. H. Clamer, Metals Handbook, American Society for Metals, Cleveland, 1939, pp. 1428-1429. Copper and Copper-Base Alloys 173

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fitted for general-service bearings; alloy C5 is suitable for high speeds and low bearing loads, and CI for moderate speeds and heavy loads. The properties of the copper-tin-lead alloys vary with composition. At room temperature, the property ranges for alloys CI to CIO, inclusive, are as follows:

Property Range of values Tensile strength, lb. per sq. in 28,000 to 40,000 Yield strength, lb. per sq. in 14,000 to 18,000 Elongation in 2 in., per cent 1 to 20 Compression, lb. per sq. in., for a deformation of 0.001 in 10,000 to 13,000 Brinell hardness (500 kg.) 55 to 75

The hard bronzes (alloys C6 and C7, and alloy CI as well) are used in castings for movable bridges and turntables which act as bearings subject to slowly or intermittently applied heavy loads. Most of the alloys in group CI to C7 are used as large bearings ("brasses") in loco- motives. Alloys CI, C8, and C9 are used extensively by the automotive industry. The high-lead alloys C10 and Cll are a fairly recent development and, as steel backed, are used in internal-combustion engines for aircraft and automobiles, trucks, and tractors. These alloys are inferior in tensile properties to the other copper-rich alloys: tensile strength varies from

12,000 to 22,000 lb. per sq. in.; yield strength from 5,000 to 10,000 lb. per sq. in.; elongation from 5 to 12 per cent; and Brinell hardness (500 kg.) from 25 to 40. In general, however, they are superior to the white metals; their melting temperatures are much higher, and their properties are not so much affected by temperature. They are also resistant to repeated flexure which is important with present-day engine speeds. Most important, if lubrication fails momentarily, the heat generated by the friction sweats out a film of lead which acts as a lubricant and prevents seizure.

The railroads were the first to use the high-lead alloys on a general scale, adopting alloy C5 for a large number of bearings. In airplane and automobile engines, these alloys are now used for crankshaft, drive- shaft, and connecting-rod bearings. The most recent development is the addition of 1 to 2 per cent nickel, or up to 1.5 per cent silver, for controlling segregation and sweating of the lead, which is insoluble in copper both in the molten and the solid state.

9.9. The Tin Bronzes

The most important industrial copper-tin alloys are the phosphor bronzes which contain from as little as 1 per cent to as much as II per .

Copper and Copper-Base Alloys 175

cent tin. All wrought phosphor bronzes are alpha-phase alloys, deoxidized by adding enough phosphorus to remove all but a trace of oxygen. The two most commonly used of these alloys (items 51 to 57, Table are 9.3) the ones containing approximately. 5 and 8 per cent tin; another popular alloy contains about 10 per cent tin. Owing to their high re- siliency and endurance limit, these bronzes are used for springs and diaphragms. Phosphor bronze is hot worked with difficulty, but cold working is easy and the alloys are available in six tempers. These, and the range of tensile strengths for each temper and each alloy, are given in Table 9.2. actual The tensile strength depends on the thickness of the sheet. Elongation (in in.) 2 varies from 40 to 60 per cent for soft sheet to less than 10 per cent for the harder tempers. The endurance limit in the hard temper is approximately 25,000 lb. per sq. in., and the modulus of elasticity is 15 million lb. per sq. in., about the same as that of other copper-rich alloys.

Table 9.2. Tensile Strength of Phosphor-Bronzc Sheet

Reduction Tensile-strength range, lb. per sq. in., Temper by cold for bronze containing rolling,

per cent 5% tin 8% tin 10% tin

Soft 40,000 to 55,000 53,000 to 67,000 58,000 to 73,000 Half-hard... 20.7 55,000 to 70,000 69,000 to 84,000 76,000 to 91,000 Hard 37.1 72,000 to 87,000 85,000 to 100,000 94,000 to 109,000

Extra hard . 50.0 84,000 to 98,000 97,000 to 112,000 107,000 to 122,000 Spring 60.4 91,000 to 105,000 105,000 to 118,000 115,000 to 129,000 Extra spring 68.6 96,000 to 109,000 110,000 to 122,000 120,000 to 133,000

Another important copper-tin alloy-which is used in the cast condition-is one containing a small amount of zinc. This alloy, known vari- ously as government bronze, zinc bronze, or more commonly as gun metal (in the late Middle Ages it was used for casting cannons) , contains 88 per cent copper, 10 per cent tin, and 2 per cent zinc-or 88 per cent copper, 8 per cent tin, and 4 per cent zinc. It is cast 'readily in the foundry, has a red color, and combines fine grain with considerable strength, toughness, and resistance to salt-water corrosion. Gun metal is used for joints and fittings, bolts, nuts, valve and pump parts, bushings and bearings for steam lines, and especially in naval construction. The tensde strength of cast and unmachined specimens varies from 30,000 to 45,000 lb. per sq. in., the yield strength from 13,000 to 15,000 lb. per sq. 176 Engineering Metallurgy in., and the elongation (in 2 in.) from 15 to 40 per cent. Impact resistance is high. The modulus of elasticity is 15 million lb. per sq. in.

9.10. Aluminum Bronze and Copper-Silicon Alloys

The copper-rich copper-aluminum alloys combine the strength of a medium-carbon steel with high resistance to corrosion by the atmosphere, by salt water, by a large number of neutral and acid salts of low concentration, and by sulphuric acid. They are more resistant than manganese bronze to salt water and have good strength and ductility. The single-phase commercial alloys contain 10 per cent or less aluminum. It was noted in § 9.2 that, if the aluminum is higher than this, the structure and properties can be changed by thermal treatment. This is also true of the complex alloys containing 5 to 10 per cent aluminum and several per cent of iron, nickel, or manganese. By controlling the composition and by heat treatment the tensile strength of the wrought alloys can be varied from 65,000 to 125,000 lb. per sq. in., with elongations varying inversely with the tensile strength from 5 to 70 per cent. The composition and typical properties of alloys in common use are given in Table 9.3, items 60 to 63.

Aluminum bronze is hard to handle in the foundry. The alpha-phase alloys can be hot and cold worked readily, but they are hard to machine.

Owing to its corrosion resistance, its strength and toughness, and its low coefficient of friction against steel, aluminum bronze is used for gears, valve stems, bolts and nuts for propellers, and for various small parts; as cast, it is used for gun mounts, gears, bearings, propellers, pump parts, and ornamental pieces, especially for service in salt air and salt water. The copper-silicon alloys, usually known as bronzes, of which some 20 to 25 are available under such brand names as Duronze, Olympic, Ever- dur, Herculoy, and others, contain 90 to 98 per cent copper, 1 to 4.5 per cent silicon, and small amounts of zinc, iron, or manganese, or—if easy machinability is desired— 0.25 to 0.50 per cent lead. Strictly speaking, the copper-silicon alloys are copper strengthened by 1 to 5 per cent of other elements. The strength and ductility of these alloys are approximately the same as those of low-carbon steel, and the corrosion resistance and ease of hot and cold working are about the same as those of copper. Strength as well as ductility increases as the silicon increases up to about 4 per cent. This is shown by the typical properties in Table 9.3 (items

66 to 69) . Thermal conductivity and electric conductivity are 8 to 10 per cent of the corresponding values for copper, and the modulus of elasticity is 15 million lb. per sq. in. Copper and Copper-Base Alloys 177

The principal advantage of the copper-silicon alloys is their corrosion resistance in both marine and industrial atmospheres, in sea water and in many industrial waters, in sulphuric and hydrochloric acids, sulphates

and chlorides, and in most alkalies and alkali salts. They are also resistant to alcohol and to many organic acids but are readily attacked by nitric acid and other strong oxidizing agents. The alloys can be used for working stresses up to 10,000 lb. per sq. in. at temperatures of 250°F. (120°C.) or below, and they can be readily welded by oxyacetylene, metallic-arc, and resistance methods.

The copper-silicon alloys are now used extensively for bolts, screws, rivets, boilers, tanks, equipment for laundries and sewage disposal, parts for fans, ducts, and other ventilating and air-conditioning equipment, chemical and electrical equipment, containers for gases and chemicals, and for many other purposes.

9.11. Copper-Beryllium Alloys

Copper-beryllium alloys are a development of the past twenty-five years. Their strength and endurance limit, after age hardening, are high when compared with other copper-rich alloys. This, combined with a corrosion resistance that is practically the same as of high-purity copper, and comparatively high electric conductivity, would be responsible for fairly extensive use if the cost were lower. Beryllium metal, which cost $1,000 a pound in 1927, had been reduced to less than $40 by 1937, and it is estimated that the price could be lowered further to S5 to $10 a pound if there would be more demand for the metal. Assuming, however, that the cost of beryllium is $15 a pound, an alloy of 97.5 per cent copper and 2.5 per cent beryllium would cost between $0.55 and $0.75 a pound in ingot form, which is still too high for extensive commercial use.

At present, copper-beryllium alloys contain 1.85 to 2.25 per cent beryllium. To some 0.35 per cent nickel or cobalt is added to refine the grain. The alloys can be hot and cold worked, but not easily, and are available as sheet, rods, wire, and tubes. As indicated, they respond to precipitation-hardening treatments. If hot worked or welded, they are given a solution treatment by heating to 1425 to 1475°F. (775 to 800°C.) for 30 min., followed by quenching in water. The alloys are usually cold worked after the solution treatment. They are aged at about 600°F. (315°C.) for 1 to 4 hr. depending upon the properties desired. Typical properties are as follows: 9 .

178 Engineering Metallurgy

Table 9.3. Composition, and physical

NOTE: The tabulated values arc average and, because of manufacturing limitations, should not be utei for •petiftcation purpoui. Within manufacturing limitations and when «o tpceifitd, these products will be manufactured to applicable current ASTM Specifications. |s

Nominal Composition, per cent l! Item Name No. Alumi- Phos- Man- < Copper Zinc Tin Lead num phorus ganese Others

C0PPER8 1. Electrolytic Tough Pitch Copper-100 99.9+ 20

99.9+ 0.02 20

3. Silver Bearing Coppcr-112 99.9 10 to IS oz. Silver 20 per Tont 4. Silver Bearing Copper-113 99. 25 to 30 ot. Silver 20 Pit Ton' 6. Phosphorlzed Arsenical Copper-108 99.68 0.02 Arsenic, 0.30 20 8. Tellurium Copper-127 99. JO Tellurium, 0.50. 80 99.00 1.00 80 BRASSES 8. Oilding-4 95.00 5.00 20 90.00 10. 00 20 87.50 12.50 20

85.00 15.00 30

80.00 20.00 10

70.00 30.00 30

14. Yellow Brass-59 66.00 34.00 30 63 00 37 00 40 60.00 40.00 40 LEADED BRASSES 17. Leaded Commercial llronzo-201 90.00 9.50 0.50 50 88.50 9.25 2.25 90 19. Hardware Bronze-267 85.00 13.25 1.75 90 SO. Leaded Tube Brass-220 66.50 33.25 0.25 50 21. Leaded Tube Brass-218 66.50 33.00 0.50 60 22. Fre- Cutting Tube Drass-282 (10.50 31.90 1.60 90 23. Leaded Tube Brass-257 66.00 33.00 1.00 70 24. Threading Brass-223 65.00 34.75 0.25 50 64.50 35.00 0.50 60 20. Medium- Leaded Brass-229 04.50 34.50 1. 00 70 27. High-Leaded Brass-235 64.00 34.00 2.00 80 28. Extra-lllgh-Leaded Hrass-238 62.50 35.00 2.50 100 29. Freo Cutting Brass-271 61.50 35.25 3.25 100 30. High-Leaded Brass-243 61.50 .SvTil 1.80 90 31. Leaded Muntz Metal-274 60.00 39 in 0.60 60 32. Free Cutting Muntz Metal-293 60.00 39.00 1.00 70 33. Forging Brass-2S0 60.00 38.00 2.00 80 34. Architectural Bronce-280 56.00 41.50 2.50 80

1 0.034 to 0.051% Silver. •0.085 to 0.102% Silver. NOTES The values given are to be considered only as a general guide, and variations In properties due to manufacturing variables must bo expected. Higher values for strength and hardness arc obtained by a greater amount of cold working but are limited In general to relatively narrow sheet, small-diameter rod and wire, and smalt-diameter thin-wall tubo. Wide sheet and plate, large-diameter rod, and large sizes of wire and tube have. In general properties between those given for hard and soft tempers, (a) These relative vbIucs are given in comparison with Free Cutting Brass-271, arbitrarily given a value of 100, and are only an approximate Indication, there being no ready means for accurately comparing the machinablllty of metals. They arc a reasonable guide to relative tool life and amount of power required for cutting. 1

Copper and Copper-Base Alloys 179

properties of standard copper alloys

>. •3^- = o Elongation, Yield Strength > . 5 fc Tensile % in 2 In. @ 0.5 % elon- Rockwell r, g ^ 5*> Applicable Strength, psi. ( Unless gation Form under Hardness •ciZ-n ASTM othe wise load, ]>si. No. 1 (b) Specifi- noted) O <5 o cations - -;==£ 9 13 l£8 iz v a. o 5 Hard Soft Hard Soft Hard Soft Hard Soft a i Mis?£ mi (c) (c) (c) (c)

(Sheet B152 4fi. 000 33,000 8 35 40,000 10,000 B51 F35) JRod B124, B133 45.000 32,000 45 40.000 10.000 B50 F35 ! 1083 0.322 .000009b 101. c 226 iWire BI. B2, B3 66,000 35,000 1(e) 35(e) ' B42, B75 ' 1 (Tube B88, Blll.lt 45,000 35,000 10 45 40.000 10,000 B50 F40 1083 0.323 .0000098 B302 85.0 197 Sheet B152 46,000 33,000 5 35 •10,000 10,000 B51 F3 I 1083 0.322 .0000098 100.5 223 Sheet B152 46,000 33,000 5 351 40,000 10.000 B5I F35 1083 0.322 .0000098 100. f 223 Tube B75. Bill 45,000 35,000 10 45 40,000 10,000 B50 F40 1080 0.323 .0000098 45.0 112 Rod B301 45,000 32, 000 18 45 40,000 10.000 B50 F35 1070 Rod 0.323 .0000098 90.0 215 45,000 32,000 12 45 40,000 10.000 B50 F35 1079 0.323 .0000098 99.0 222

Sheet 55,000 B36 35,000 6 38 44,000 11.000 B61 F45 1065 0.320 .0000101 56 139 /Sheet B36, B130 62.000 37.000 6 40 47,000 12,000 B70 B, \WIrc B134 80, 000 38, 000 J(e) 40(e) 1045 0.318 .0000101 43.0 108 Sheet , .| 65.000 38,000 r. 42 50,000 13,000 B73 B3 1035 0.317 .0000102 40.0 100 (Sheet B36 69,000 40,000 7 45 55,000 15.000 B76| B5 | (Wire B134 88,000 42,000 Kc) 42(c) (Tube B-13. Bill, 69,000 1025 0.316 .0000104 37.0 92 40.000 10 50 55,000 15,000 B76 1)5 B135 j (Sheet B36 73,000 43,000 8 50 00,000 16,000 B81 B10 IWiro B134 100,000 47,000 1(e) 45(e) iooo 0.313 .0000106 32.5 81 (Sheet B19, B36 f 76,000 47.000 10 65 62,000 17,000 B83 'B20i (Wire 1)131 110.000 50,000 1(e) 50(e) 1 955 0.308 .ooooiii 27 70 .Tube B135 76,000 47.000 10 60 62,666 17.000 B83 B20 Sheet B36 73,000 45,000 10 60 60,000 17,000 BS0 BIS 930 0.306 .0000112 26.0 69 (Rod 65,000 46,000 20 60 50,000 17,000 B75 B20' (Wire B134 105,000 50,000 1(e) 50(e) 920 0.305 .0000114 26.0 69 Sheet 54.000 45 20,000 B«' 905 0.303 .0000116 28.4 73

Sheet B121 62,000 37,000 6 40 47,000 12,000 B70 Bl 1045 0.318 .0000101 42.0 104 Rod B140 51,000 37.000 IS 40 45,000 12,000 B58 Bl 1040 0.319 . 0000102 42.0 105 Rod BI40 52.000 40,000 20 45 43.000 15,000 B.V, B5 1025 0.317 .0000104 311.0 93 Tube B135 73.000 45,000 10 55 60,000 17.000 B80 B15 935 0.307 .0000112 28.0 68 Tube B135 73,000 45.000 v 10 55 60.000 1)1.' , , 17.000 B80 935 0.307 Q] | g 26.0 68 Tube B135 73.000 45.000 60,000 17,000 B80 B15 935 0.308 0000113 26.0 68 Tubo 73,000 45,000 9 55 60,000 17,000 B80 B15 935 0.307 0000112 26.0 68 Sheet " 73,000 45,000 10 55 60,000 17,000 B80 B15 930 0.306 0000113 20.0 68 Sheet iii2i 73,000 45.000 9 55 60,000 17,000 B80 B15 925 0.300 0000113 26.0 68 Sheet B121 73,000 45,000 S 55 60.000 17.000 B80 B15 923 0.306 on mis 26.0 68 Sheet B121 i3.W»l 45,000 8 52 60.000 17.000 B80 B15 920 0.307 00OO1I3 26.0 68 Sheet 3121 73,000 45,000 / 50 60,000 17,000 B80 B15 905 0.307 0000114 26 68 Rod B16 58,000 47,000 18 60 42,000 18,000 B70 B20 895 0.307 0000114 25.5 62 Sheet B121 73,000 45,000 7 50 60,000 17,000 B80 B15 Sheet 908 0.305 0000113 26.0 68 B171 54,000 „.... 45 20,000 " B45 900 0.304 Tube B13S 0000116 20.0 68 80.666 1 54,000 40 6o,'66o' 20.000 B86 B45 900 0.304 0000116 26.0 68 Rod B124 54,000 45 20.000 J BI5 895 0.305 0000115 26.5 82 Rod Ml rm 25 B65 884 0.305 0000116 28.0 7i

Courtesy American Brass Co. NOTES (Continued)

(b) t ' Sl"* M0 ta " thleki Rod ' '" ia ' dlttraeter: wlro 008° ta - d,ametcr; Tub,, ' "'• SS&r ?SSm ES&VSST " ' ' (c - reduowi about 37% in thioknoss > grants ERR sr^awasss ssr * by »m rm^- and PClrlc 1 conductivity: given for soft annealed metal; in hard temper r< J:! ? conductivity Is slightly»-»-•..lower. (e) Klongation of wire, percent in 10 in. YOUX 1'S MODULUS OF ELASTICITY for copper is about 10,000,000 psl. The addition of zinc to copper reduces the modulus, to about 14,000,000 psl. for Yellow Brass. Pot most bronzes, the value is about f°WOOO0»»•»>">" The to copper ""Edition™"™ ™of ™™>nickel Increases the modulus, Cupro Nickel, 30%-702 showing about 20.000,000 psi. .. —

180 Engineering Metallurgy

Table 9.3 Continued.

The NOTE: tabulated values are average and, because of m«nnhctoring limitations, should not be used tor > specification purposes. Within manufacturing limitations and lehen so specified, these products will be manufactured to applicable current ASTM Specifications.

Nominal Composition, per cent 11 Item No. Name Alumi- Phos- Man- $9 Copper Zinc Tin Lead num phorus ganese Others

SPECIAL BRASSES 94.97 4.00 1.00 0.03 20 36. High Strength Commercial Bronze- 286 90.25 0.90 1.75 0.10 Nickel, 1.00 80 37. Ambronze-421 88.00 10.00 20 38. Manganese Red Brass-507 85.00 14.00 1.00 30 39. Silicon Red Hrass-1027 82.00 17.00 Silicon, 1.00 30 40. Trumpet Brass-435 81.00 18.00 1.00 30 41. Ambraloy-927 (Aluminum -Brass).. . 77.00 20.98 2.00 Arsenic, 0.04 30 42. Arsenical Admlralty-439 71.00 27.96 1.00 30 70.00 29.00 1.00 30 44. Naval Brass-450 60.00 39.25 0.75 30 45. Tobin Bronze"-452 60.00 39.25 0.75 30 60.00 38.55 0.75 6.70 50 47. Leaded Naval Brass-612 60.00 37.60 0.75 1.76 80 58.50 39.25 1.00 0.25 Iron, 1.00 30 PHOSPHOR BRONZES 49. Phosphor Bronze-356 98.70 1.25 0.05 20 50. Phosphor Bronze-361 98.24 1.75 0.01 20 51 . Phosphor Bronze, (A )-302 95.95 4.00 0.05 20 52. Phosphor Bronze, (A j-303 95.75 4.00 0.25 20 S3. Phosphor Bronze-314 95.17 4.00 0.08 0.25 Iron, 0.50 20

54. Phosphor Bronte, (A )-351 94.75 5.00 0.25 20

. 65. Leaded Phosphor Bronze, (B)-379. . 93.90 5.00 1.00 0.10 SO 93.20 6.50 0.30 20

57. Phosphor Bronze, (C)-353 91.75 8.00 0.25 20

58. Phosphor Bronze, (D )-354 89.75 10.00 0.25 20

59. Free Cutting Phosphor Bronze-610. . 87.90 4.00 4.00 4.00 0.10 90 ALUMINUM IBRONZE8,

60. Ambraloy-901 95.00 5.00 20

61. Ambraloy-928 92.00 8.00 20

62. Avlalitc'-915 89.25 0.40 9.26 Nickel, 0.50; Iron, 0.60 20 63. Ambraioy-917 82.00 9.50 1.00 Nickel, 5.00; Iron, 2.50 20

' Trade-Mark Reg. V. S. I'at. Off. t 1.25" and over. NOTES The values given are to be considered only as a general guide, and variations In properties due to manufacturing variables must be expected. Higher values for strength and hardness arc obtained by a greater amount of cold working but arc limited in general to relatively narrow sheet, small-dlamcler rod and wire, and small-diameter thin-wall tub*;. Wide sheet and plate, large-diameter rod, and large sizes of wire and tube have, In general, properties between those given for hard and soft tempers, (a) These relative values are given in comparison with Free Cutting Druss-271, arbitrarily rfiven a value of 100, and are only an approximate Indication, there being no ready means for accurately comparing the machinabllity of metals. They are a reasonable guide to relative tool life and amount of jwwer required for cutting. Copper and Copper-Base Alloys 181

5% = ~Q i?£*^ Elongation, Yield Strength • 3h Tensile % In 2 In. @ 0.5 % elon- Rockwell 5 — Applicable Strength, psl. (Unless gation under Hardness ^S-c Korm ASTM otherwise 1 loud, psl. No. 8*j«n (b) Specifi- noted ii 1 1 J3 £ £ ..a cations a. J. _M R °fi§£ CO i S KM* *3 fcji E = S 1 >S ttti Hard Sort Hard Sod Hard Soft Hard Soft 2 a (0) (e> (c) (o)

Sheet no, ooo 40,000 6 40 50,000 15,000 B70 BS 1060 0.320 .0000101 38.0 95

Rod 70,000 12 60,000 B80 0.320 32.0 81 Tube 46,000 58 20,666 B15 1030 0.316 .0000102 27.0 69

Sheet 69,666 40,000 1 7 46 55,000 15,000 B76 135 1015 0.316 .0000104 22.0 57 Sheet 90,000 55,000 8 60 50,000 20,000 BOO B35 990 0-310 .0000105 14.7 39 Tube 80,000 48,000 10 60 B25 1005. .0.313 . 11000 10S 27.5 70 Tube BUI 85,000 52,000 10 65 60,000 20,000 B85 B30 970 0.301 .0000108 22.6 58 (Sheet B171 48,000 65 18,000 B25\ 935 0.308 .0000112 24.7 \Tube Bill 86, 000 52, 000 io 65 20,000 B30I 64 Sheet B291 7(1,000 47,000 10 65 62,000 17,000 B83 B20 945 0.308 .0000111 16.0 43 1 Sheet B171 56,000 22.000 B501 885 0.304 .0000119 25.8 67 I Rod B21. B124 63.000 56,000 30 40 35.000 22,000 B65 B50f Rod B21, B124 63,000 56.000 35 45 35.000 22.000 B65 BSD 885 0.304 .0000117 25.8 67 Rod B21 63.000 56,000 28 38 35.000 22,000 B6S B50 890 0.305 .0000119 26.0 67 B2I, B124 63,000 Rod 56,000 2i 35 35.000 22,000 B66 B50 l 890 0.305 .0000118 26.0 67 Rod B124, B138 75, 000 60,000 20 30 45,000 30.000 B85 890 0.302 .0000119 23.0 58

Sheet 65,000 40,000 6 48 50.000 14,000 B75 F60 1075 0.321 .0000099 43.0 126 Wire BIOS 105,000 45,000 UO 40(c) 1070 0.321 .0000099 37.8 85 Sheet moo 80,000 48,000 8 SO 65.000 20,000 B86 B28 1050 0.320 .0000099 18.4 47 Tube 70,000 48,000 15 50 58,000 20,000 B80 B2S 1050 0.320 .0000100 18.0 47 Rod B139 (15,000 48,000 30 50 55,000 20,000 B75 B28 1055 0. 322 .000010 16.4 44 [Sheet 11103 80,000 48,000 8 50 65.000 20,000 B86 B281 {Rod B139 65,000 48,000 30 50 55,000 20,000 B75 B28} 1050 0.320 .0000099 18.0 47 (Wire III.'.9 110.000 52,000 1(e) 40(c) 1 Rod B139 65.000 48.000 25 40 55,000 20.000 B75 B25 1050 0.322 .0000099 18.4 48 Wire 120,000 57,000 He) 40(e) 1040 0.319 .0000100 12.0 33 (Sheet B103 93,000 00,000 10 65 68,000 24,000 B94 B50 Rod B139 80,000 60,000 30 AS 24,000 BflOl 1025 0.318 .0000101 13.0 30 iwire B159 130,000 62,000 He) 40(c) Sheet B103 102,000 06. 000 12 65 70,000 28,000 B98 B55 [Rod B139 85,000 65,000 25 65 1000 0.317 .0000102 11.0 29 .Wire B159 145.000 68,000 He) 40(c) Rod B139 60,000 20 45,000 B76 1000 0.320 .000010 12.2 33

Sheet B169 92,000 55,000 7 65 65,000 22,000 B92 B3.il Rod 55,000 65 22,000 B35j 1060 0.295 .0000099 17.0 46 Tube Bill 60,000 60 Sheet BI69 105,000 65.000 7 60 65.000 25,000 BM BSOl 101(1 0.281 .0000099 14.8 Rod BI24, B150 80,000 65,000 30 65 50,000 25.000 B50! 41

Rod 1)124, B150 95,000 80,000 16 22 55.000 40,000 1042 0.274 .0000094 12.8 85 (Plate B171 90,000 12 1 1055 Rod B124, B150 ios.ooo 12 60.000 BIOS } 0.274 .000009 7.S 22

NOTES (Continued)

(b) Form for which properties are given: Sheet, 0.040 in. thick; Rod, 1 in. dlumeter; Wire, 0.080 in. diameter; Tube, 1 In. diameter x 0.065 In. wall thickness. Hani temper: values (c) arc for soft sheet that has been subsequently reduced about 37% In thickness by cold rolling; and for Rod, w ire and Tube of commercial hard drawn temper. (d) Electrical conductivity: given for soft annealed metal; in hard temper conductivity is slightly lower. (e) Elongation of wire, per cent In 10 in. YOUNG S MODULUS OF ELASTICITY for copper is about 16,000,000 psl. The addition of zinc to copper reduces the modulus, to about 14,000,000 psi. for Yellow Brass. For most bronzes, the value is about 15,000,000. Tho addition of nickel to copper Increases the modulus, Cupro Nickel, 30%-702 showing about 20,000,000 psl. — .

182 Engineering Metallurgy

Table 9.3 Continued.

The tabulated NOTE. values are average and, because of manufacturing limitations, should not he met lor specification purposes. Itliin V manufacturing limitations and when so specified, these products will be manufactured to applicable current ASTM Specifications.

•22

Nominal Composition, per cent II Item Name No. Alumi- Man- M Copper Zinc Tin Lead Nickel num Silicon ganese Others CADMIUM BRONZES

64. Hltenso'-901 99.00 Cadmium, 1.00... 20 66. 11 ltenso--965 98.60 0.60 Cadmium, 0.80. 20 COPPER-SILICON ALLOYS

66. Everdur"-1010 95.80 3.10 1.10 30

67. EvordurM012 95.60 0.40 3.00 1.00 60

68. Everdur'-1015 98.25 1.60 0.26 30

69. Evcrdur'-IOM 90.75 7.25 2.00 60 CUPRO NICKELS 70. Cupro Nickel, 10%-755 88.35 10.00 0.40 Iron, 1 25 20

71. Cupro Nickel, 30%-702 68.90 30.00 0.60 Iron. 0.50 20

NICKEL SILVERS 72. Nickel Silver, 10%-76I 65.00 25.00 10.00 20

73. Nickel 8llver. 18%-719 64.50 17.50 18.00 20

74. Nickel Silver, lS%-724 55.00 27.00 18.00 20 7S. Leaded Nickel Sliver, 10%-«25 45.00 42.00 1.00 10.00 2. CO 60 8PECIAL ALLOYS 76. Calsun Bronze*-951 95.50 2.00 2.50 20 77. Chromium Copper-999 99.05 0.10 Chromium, 0.85. 20

• Trade-Mark Reg. \>. 8. Pat. Off. NOTES The values given are to be considered only as a general guide, and variations In properties due to manufacturing variables Higher values for strength and hardness are obtained a greater by amount of cold working but are limited In general to "** ft d srooll-dlamcter thin-wall tube. Wide sheet and plate large^an, eterSSfSRrod, andSfEZilarge SSsizes ofH&t^fffiSwlro and tube ^have.""fIn 1 general, properties between those given for hard and soft tempers (a) These relative values are given in comparison with Free Cutting Brass-271, arbitrarily given a value of 100,' and are only an approximate indication, there being no ready means for accurately comparing the macliinabllity of metals They are a reasonable guide to relative tool life and amount of power required for cutting. Copper and Copper-Base Alloys 183

i "3 § JSd"- Elongation, Yield Strength "31*. Tensile % in 2 in. @ 0.5 % elon- Rockwell o 6 i. * Applicable Strength, psi. (Unless gation under Hardness form 2--- ASTM otherwise loud, psl. N 0. a S. = 2 IS Specifi- o' 3 (1)) noted) o JS SS w; S~ cations & -#=§K _« C6fi >, "3 St"* Hard Soft Hard Soft Hard Soft Hard Soft s Q •<3wS. (c) (c) (c) (o)

IShcet 55.000 37,000 50 48.000 12,000 B65 F471 1076 \Wirc B105 90.000 40.000 1(e) 40(e) 0.321 .0000098 87.0 199 Wire B105 95,000 42. 000 1(c) 40(e) 1070 0.321 .0000098 60.0 13S

Sheet 1)96, B97 95.000 58,000 7 60 60,000 22,000 B92 B35'. Rod B98, B124 90,000 58.000 18 70 60,000 22,000 B90 B35 1019 0.308 .0000100 6.5 19 Wire B99 145.000 60.000 Me) 50(e) Rod B98 I 90.000 18 60,000 1)90 1019 0.308 .0000100 6.5 IS Sheet B97 65,000 40,000 8 46 50.000 15.000 B77 F55 Rod B98 70,000 40.000 IS 50 55,000 15,000 1)80 F55 ' Wire B99, BIOS 122,000 42,000 Ke) 40(c) 1055 0.316 .00000911 11.0 31 Tube 65,000 40.000 8 50 50,000 15,000 B71 F55 Rod BJ24, BlW 95,000 25 53.000 B87 1005 0.278 .000010 9.2 as

Tube Bill 60,000 44,000 15 40 57,000 22,000 BOS B25 U45 0.323 .0000093 9.2 26 1 Sheet B122, B171 77,000 55, 000 5 40 70,000 22,000 B84 B35 Rod 70, 000 55, 000 25 40 60,000 22.000 B80 B35 1 Wire 95.000 58,000 Ke) 35(e) 1225 0.323 .0000090 4.6 17 iTuhe BUI 70.000 55,000 10 45 60,000 22, 000 B80 B35.

Sheet B122 88,000 55,000 7 42 70.000 20,000 B87 B30 1010 0.313 .0000091 8.4 27 Sheet B122 85.000 58,000 4 40 70.000 B85 B40 lll.il Rod 70.000 58,000 20 45 22.000 B40 1110 0.316 .0000090 6.0 IB Wire B206 110.000 60,000 Ke) 40(C) Sheet B122 99,000 00,000 4 45 75,000 22,000 B93 B45 B151 Rod 80,000 60,000 20 45 60. 000 22,000 B85 B45 1055 0.314 .0000093 5.3 17 Wire B206 130, 000 65,000 Ke) 40(0) Rod 70,000 IS 40,666 B70 925 0.303 .000011 7.0 21

Wire B105 135, 000 52, 000 Ke) 40(e) 1054 0.308 .0000099 19.0 50 •Rod 02, 000 35, 000 20 40 57,666 15.000 B70 Fgq 1075 0.321 40.0 u» \Rod (() 72,000 • 03.000 25 25 61,000 45.000 B77 B65 1075 0.321 .0000098 80.0 187

NOTES (Continued)

(b) Form for which properties are given: Sheet, 0.040 in. thick; Rod, 1 in. diameter; Wire, 0.080 in. dia ncter; Tube, 1 In. diameter x 0.065 in. wall thickness. (c) Ilard lemper: values arc for soft sheet that has been subsequently reduced about 37% in thickness by cold rolling; and for Rod, \\ Ire and Tube of commercial hard drawn temper. (d) Electrical conductivity: given for soft annealed metal; In hard tempor conductivity is slightly lower. (e) Elongation of wire, per cent in 10 In. (f) Properties after heat treatment (alloy 77). YOUN'O'S MODULUS OF ELASTICITY for copper is about 16,000,000 psi. The additon of line to copper reduces the modulus, to about 14,000,000 psl. for Yellow Brass. For most bronrts, the value Is about 15,000,000. The addition of nickel to copper increases the modulus, Cupro Nickel, 30%-702 showing about 20,000,000 psi. 184 Engineering Metallurgy

Typical Propcrlics of Copper-Beryllium Alloys

1 Tensile Yield Elongation Brincll Condition strength, lb. strength, lb. in 2 in., hardness per sq. in. per sq. in. per cent

Soft 70,000 31,000 45 110 Average treatment 175,000 134,000 6.3 340 Maximum treatment .... 193,000 138,000 2.0 365

The endurance limit is usually between 35,000 and 45,000 lb. per sq. in. depending on the treatment. The endurance ratio varies from approximately 45 per cent for the unhardened alloy to about 20 per cent for fully hardened material. The modulus of elasticity averages 18 million lb. per sq. in. In these properties the copper-beryllium alloys are far superior to any other copper-rich alloy. They also have relatively high wear resistance and corrosion resistance and are superior to phosphor bronze in resisting metal-to-metal wear when run against steel. At present they are used to a limited extent for springs, gears, diaphragms, bearings, and for other small parts in the electrical and the aircraft industries.

Chromium copper (Table 9.3, item 77) is a heat-treatable alloy which is extensively used for welding electrodes, grid support rods, and for lateral wires in electronic vacuum tubes.

A copper-silver-iron alloy containing 6 per cent silver, 1 per cent iron, copper remainder, is also a heat-treatable alloy used by the Signal

Corps of the U. S. Army for field telephone wire during the Korean war.

9.12. The Copper-Rich Copper-Nickel Alloys

Copper and nickel dissolve in each other in all proportions in the solid state, and their alloys are free from phase changes. Some of the most widely used industrial alloys contain from 2 to 30 per cent nickel (the cupronickels) or 45 per cent nickel (Constantan) . Of the important nickel-rich alloys, the best known is Monel, containing 68 per cent nickel.

This group is discussed briefly in the next chapter. Nickel increases the tensile strength, the yield strength, and the endurance limit of copper, as shown for annealed alloys in Fig. 9.4. Even the strongest alloys are ductile and have an elongation of 45 to 50 per cent in 2 in. As noted in the discussion of nickel silvers, the addition of increasing amounts of nickel to copper changes the red color to white. A 15 per cent nickel alloy has a faint pink tinge, and a 20 per Copper and Copper-Base Alloys 185

cent nickel alloy is practically white. The five-cent coin of the United States, the

"nickel," is an alloy of 75 per cent copper and 25 per cent nickel.

The most common indus-

trial alloys contain 15, 20, and 30 per cent nickel. The 15 and 20 per cent alloys are used for bullet jackets and for a large variety of parts in which the white color, high ductility, and corrosion resistance are of value. They were formerly used for condenser and other tubes for marine engines and oil refineries, but in these applications they have largely been replaced by the 30 per cent alloy. This material can readily be hot and cold 40 SO worked and can be soldered, Nickel, per cent brazed, and welded. It is one of the most resistant of the Fig. 9.4. Effect of nickel on the mechanical copper-base alloys to salt properties of copper-nickel alloys (Wise) water and to many of the corrosive solutions of the chemical industry. The cupronickels are generally resistant to the atmosphere, to industrial waters, and to many acids and alkalies. The 30 per cent alloy, containing also 1 per cent manganese and 1 per cent iron, is cast into a large variety of couplings, ells, tees, and other fittings for valves, pumps, and other similar applications.

The cupronickels do not respond to heat treatment. As in the case of most solid-solution alloys, high strength can be attained only by cold working with concomitant loss of ductility. Cold working with a reduction of 60 per cent increases the tensile strength of a 30 per cent nickel alloy from 50,000 to 85,000 lb. per sq. in. and reduces the elongation from 50 to 4 per cent in 2 in.

Some of the copper-nickel alloys have important electric properties. As shown by Fig 9.5, the electric resistance is at a peak and the tem- .

186 Engineering Metallurgy

perature coefficient of resistance is practically zero in the alloy containing 45 per cent nickel. This alloy, known as Constantan, has high and uniform thermoelectric force against copper and iron and is used extensively for thermocouples for the accurate measurement of temperatures below 1800°F. (985°C.)

40 Nickel, per cent

Fie. 9.5. Effect of nickel on the electrical properties of copper-nickel alloys. (Wise)

It should be evident from the discussion in this chapter that the engineer has available a wide variety of wrought and cast copper-rich alloys for use in his structures and machines. There are so many of these alloys

that an intelligent choice of a specific one is sometimes difficult, especially in view of the fact that a large number of them are proprietary alloys for which extravagant claims are occasionally made.

9.13. Copper and Copper-Base Alloys in Powder Metallurgy

Powder metallurgy is a very old art and a very young science. Sponge iron was formed into weapons a thousand years ago, but when man discovered how to melt steel, he discarded the more cumbersome method of powder metallurgy, and the art was lost until relatively recently. With the exception of filaments for the electric-light bulb, which have been Copper and Copper-Base Alloys 187

made by powder-metallurgy methods for fifty years, and the perfection of hard carbide tools twenty -five years ago, most of the development in powder metallurgy has come in the past fifteen years.*

The process underlying the production of powder compacts is well known and need not be reviewed here. Broadly it consists or preparing metal or nonmetal powders of the proper size and physical characteristics, forming them into the desired shape, compacting under high pressure (10 to 100 tons per sq. in.), and sintering at the proper temperature. Close tolerances can be maintained and no machining of the finished part is ordinarily necessary. Despite the high cost of the equipment for carrying on the various operations, great savings result in many applications, especially for a large number of identical parts that are difficult or expensive to machine from solid metal. The properties of a part made from powder depend on the character of the raw material and upon the pressing and sintering-in other words, on the homogeneity and the absence (or presence) of voids in the finished part-and they range from about 50 per cent to practically the same as for the solid metal or alloy. One of the great advantages of powder metallurgy is that dissimilar metals, or metals and nonmetals, that do not alloy in melting can be readily combined into an article having unique and often very desirable properties. Of the several million pounds of copper, for example, that are compacted by the methods of powder metallurgy a 1 large proportion goes into porous "oilless or "self- lubricating" bearings for electrical machinery, automobiles, machines, and household appliances. More than a billion such bearings are in use. The most common oil less bearingf is made by compacting 87.5 to 90.5 per cent copper powder, about 9 per cent tin powder, and about 1.5 per cent graphite. These bearings are made with a controlled number of voids (by regulating the density) , so that, when installed, they contain 15 to 35 per cent impregnated oil; they are, therefore, both bearing and lubricant and, if properly made, will function as such for the life of the machine. Sometimes 3% lead is added for improved properties.

'The first general technical meetings in the field were held at Massachusetts Inst.ti.te of Icchnology in 1940 ami 1941. The papers presented were published in Urgy'" edited by John Wu,ff Am i( ' an ,L°o !L " Society for Metals, Cleveland 1942, 622 The first conference pp. of the Powder Metallurgy Committee of the In* titute of Mini "S a"* Metallurgical Engineers was held in February, foTT^K1944. The 12 papers presented were published in Trans. Am. Inst. Mining Met Engrs., 161, 1945. 524-634. y. pp. See also ••Seminar on Theory of Sintering" and other papers in rrans. Am Inst. Mining Mel. Engrs., v 166, 1946, pp. 474-587. See also Proceedings of Com. B-9 of A.S.T.M. fA^.T.M. specifications B202-51T. 188 Engineering Metallurgy

Copper and copper-base alloys compacted with graphite are also used successfully for brushes and other parts of motors and generators, to take advantage of the antifrictional character of the graphite and the current-carrying capacity of the copper. Depending on the strength required, the amount of graphite may be 50 per cent or more. Thousands of contacts for making and breaking electrical circuits are produced by compacting copper and refractory metal, such as tungsten or molybdenum or the carbides of these metals. The principles underlying the art of producing powder compacts are

not simple. Not only must such variables as particle size, pressure, and sintering temperature be controlled carefully to secure the density, hard-

ness, and strength desired (all of which arc directly related) , but structural changes resulting from diffusion must also be taken into account. The structure should resemble that of the solid metal or alloy, the grain size should be fine, and the voids (if present) should be uniform in size and distribution. Powders of an alloy may, when compacted, have a structure and properties different from those resulting when the pow-

ders of the metals are used and alloying is accomplished during sintering. On the other hand, prealloying may cause increased resistance to com-

pression and result in less dense compacts. Whether the compact is prepared by hot or cold pressing, whether the powders contain gas, whether they oxidize easily—all of these are important factors and none has been thoroughly studied.

In general the metallurgy of producing metal parts from powder is in its infancy and considerable advance can be expected in the next few years. Other than for the manufacture of hard carbide tools, discussed in a subsequent chapter, the process has been used successfully in producing parts of iron, steel, iron phis graphite, steel plus copper, stainless steel, copper and copper-base alloys, aluminum and aluminum-base alloys, and iron-nickel alloys—and the list is growing rapidly. Where economic factors are favorable the future of powder metallurgy seems secure.

QUESTIONS

1. What are the properties of copper that make it useful commercially? 2. What are the composition and properties of common brass? 3. What is Muntz metal? Commercial bronze? 4. Name two methods for making copper alloys free-machining. 5. Define dezincification. 6. What are nickel silvers and their most valuable property? 7. What is intended when the term "high brass" is used in the brass industry? 8. Give the eutectic composition of the copper-zinc alloy system. Copper and Copper-Base Alloys 189

9. What is red brass? Admiralty metal? 10. What is manganese bronze, and what is its principal use? Is it a true bronze? 11. Compare the composition and properties of cast brasses, cast nickel silvers, and cast manganese bronze. 12. What are the principal metals in the copper-base" bearing alloys? Name some of their principal characteristics and uses. What are the uses for high- lead, copper-base alloys? Mow do those compare as bearings with the white metals? 13. What is the effect of phosphorus on the properties of copper-tin bronzes? What arc the phosphor bronzes? Name the properties and some uses of phosphor bronzes. 14. What is the effect of varying amounts of cold working and varying amounts of tin on the tensile strengths of phosphor bronzes? What is the approximate composition of gun metal? What are the properties and uses of gun metal?

15. is What the general composition of aluminum bronze? Indicate some of its characteristics and uses. 16. Indicate the approximate composition and some of the applications of copper-silicon alloys. 17. Indicate the approximate composition, properties before and after precipitation-hardening, and applications of the copper-beryllium alloys. What is the reason for their limited use? 18. What is the effect of increasing amounts of nickel on the tensile properties and endurance limit of copper? What are the properties of copper-nickel alloys? Name some of the copper-nickel alloys. 19. What general classes of materials may be combined by powder metallurgy methods? Name and give the properties of some articles made by powder metallurgy methods from copper combined with other materials. 20. How are metal-powder compacts made? What variables determine the properties of articles made by the methods of powder metallurgy? Miscellaneous Heavy

CHAPTER Nonferrous Metals and Alloys

10 William H. Tholke, B.S., in Metallurgical Engineering, Instructor of Metallurgy, University of Cincinnati, Cincinnati, Ohio

1 HE preceding chapters on the light metals, copper, and copper base alloys have dealt with the two largest nonferrous metal industries. Before taking up the complex field of iron-carbon alloys, alloy steels, cast irons and the metallurgy associated with the working and manufacturing of these materials, it would be well to look briefly at the remaining commercial nonferrous metals and their alloys, how they are used, and the part they play in the overall metals picture. In terms of density or specific gravity most of these metals are heavier than iron. Table 10.1 lists these metals in alphabetical order with their respective specific gravities. Some categorizing may be done in terms of the physical characteristics of the metals, and the metal elements with which they are alloyed. Some of these metals such as Pb, Sn, Sb, Bi, Cd, and In have low melting temperatures. The alloys of Pb and Sn are often referred to as the white metals. Then, there are those metals which have high melting temperatures as well as good corrosion resistance. These are the metals Ni, Cr, Co, W, Mo, and Re and their alloys. A third group comprises those metals and alloys which by virtue of scarcity of supply are known as the precious and semi-precious metals. These arc the metals Ag, All, Pt, Pd, and Rh. And, lastly, a new. group which has come into prominence through the development of atomic energy. These are the elements Th, Tm, U, and Zr.

190 Miscellaneous Heavy Nonjerrous Metals and Alloys 191

Table 10.1. Miscellaneous Nonfcrrous Heavy Metals and their Specific Gravities

Specific Melting Specific Melting Gravity Temp. °F. Gravity Temp. "F. Antimony (Sb) 6.62 1167 Nickel (Ni) 8.9 2651 Bismuth (Bi) 9.80 520 Palladium (Pd) 12.0 2830 Cadmium (Cd) 8.65 610 Platinum (Pt) 21.5 3224 Chromium (Or) 7.19 3430 Rhenium (Re) 20.0 5740 Cobalt (Co) 8.90 2725 Rhodium (Rh) 12.4 3570 Columbian (Cb) 8.57 4380 Silver (Ag) 10.5 1761 or Niobium (Nb) Copper (Cu) 8.96 1981 Tantalum (Ta) 16.6 5425 Gold (Au) 1 9 . 32 1945 Thorium (Th) 11.5 3300 Indium (In) 7.31 314 Thulium (Tm) 9.35 Iridium (Ir) 22.5 4450 Tin (Sn) 7.30 449 [Iron (1*0 7.87 28021 Tungsten (VV) 19.3 6170 Lead (Pb) 11.34 621 Uranium (U) 18.7 2065 Vanadium (V) 6.0 3150 Mercury (Hg) 13.6 -38 Zinc (Zn) 7.13 787 Molybdenum (Mo) 10.2 4760 Zirconium (Zr) 6.7 3353

LOW MELTING METALS AND ALLOYS

10.1. The While Metals The term "white metals" is usually reserved for alloys of lead and tin in which one or both of these metals may comprise the major percentage of the total alloy content. The remaining alloying elements present may be one or more or the following: antimony, arsenic, bismuth, cadmium, copper, indium, silver, and zinc. The white metal alloys are often more commonly known by their trade names such as pewter, terne plate, babbits, and silver solders. Their applications find greatest usage in the fields of bearing materials, (1) (2) type metals, and (3) low-melting fusible alloys such as the solders and safety fuse plugs. The bearing metals must meet many requirements before they can qualify as a good bearing material. These may be divided into three principal classes. First in importance is proper structure, that is, a soft matrix with sufficient plasticity to conform to slight irregularities in the machining and alignment of the shaft, to allow any abrasive particles in the lubricant to become embedded in the bearing metal (to prevent scoring of steel the journal) , and to retain oil (to prevent mctal-to-metal contact) , combined with a number of uniformly distributed particles which resist wear. Some metallurgists, however, now claim that wear-resisting particles are not necessary for good bearings and that the structure is of minor importance. Such claims 192 Engineering Metallurgy are strongly supported by the successful use of the steel-backed silver bearings and by the cadmium bearings. Second, the mechanical properties must be satisfactorily balanced. The bearing must resist the shock of impact loads, and it must have sufficient strength, ductility, and resistance to compression not to crack or to squeeze out under heavy loads, especially at the operating temperatures, which may be 200 to 300°F. (95 to 150°C.) and in the modern airplane even higher.

The third requirement is easy melting and casting and, most important, good bonding properties. In other words, when cast in a strip as thin as 0.01 in., the metal must adhere firmly to the steel or bronze piece used as a backing and should not spall off or separate from the backing during operation. For some purposes, especially for large railroad bearings, it must be possible to remelt and recast the bearing metal without excessive loss by oxidation. In addition, the bearing metal should have high thermal conductivity, resistance to corrosion by the lubricants used, and it should be cheap. There are four classes of bearing metals. Typical compositions of some common alloys in these different classes, with the specification numbers adopted by the American Society for Testing Materials and the Society of Automotive Engineers, are given in Table 10.2. The white metals include the tin-base (class A, Table 10.2) and the lead-base (class B) alloys. The 14 alloys in these two groups and the numerous unimportant modifications that have been advocated from time to time are old and well known. The tin-base alloys, commonly called babbitts, are widely used in automobile engines and in a large variety of machines where speeds are high and loads are light or at most only fairly heavy. Lead, added (alloys A4 and A5) primarily to reduce the cost, is objectionable if operating temperatures are high as it forms a fusible eutectic with tin and lowers the temperature at which incipient melting takes place. The following illustrates the effect of lead: the temperature at which alloy A5 (18 per cent lead) is completely liquid is about 565°F. (295°C.) ; the corresponding temperatures for alloys Al and A2 (no lead) are 680 and 670°F. (360 and 355 °C). The properties of the tin-base alloys vary somewhat with composition but are usually within the following ranges:

Properly Range of values

Tensile strength, lb. per sq. in., at 70"F. (20°C.) 12.800 to 17,500 Tensile strength, lb. per sq. in., at 200°F. (95°C.) 6,700 to 10,000 Yield strength, lb. per sq. in., 70°F. (20°C.) 4300 to 6500 Yield strength, lb. per sq. in., at 200°F. (95°C.) 2.100 to 3,200 Brincll hardness (500 kg.) at 70°F. (20"C.) 17 to 27 Brincll hardness (500 kg.) at 200-F. (95°C.) 8 to 14 Miscellaneous Heavy Nonferrous Metals and Alloys 193

Table 10.2. Typical Compositions of Some Common Bearing Mclals

Specification Nomina] composition, percent

Class A.S.T.M. S.A.E. Sn Sb Pb Cu Zn Cd Others

Al B23-46T1 10 91 4.5 4.5 2 2 110 89 7.5 3.5 3 3 83.3 8.3 8.3 4 4 75 12 10 3 5 5 65 15 18 2 6 11 87 6.8 5.7

Bl B23-46T6 20 15 63.5 1.5 2 7 14 10 15 75 3 8 5 15 80 4 10 2 15 83 S 11 15 85 6 12 10 90 7 15 15 1 15 84 8 19 5 9 86

CI B144-46TA 792 10 10 80 0.0 to 0.9 P 2 B 7 7 83 3 3 C 5 9 85 1 4 D 7 15 78 5 E 5 25 70 6 B22-46TA 19 80 0.0 to 0.9 P 7 B 16 83 0.0 to 0.9 P 8 791 4 4 89 3 9 793 4 8 85 3 10 48 30 70 n 480 35 65

Dl 17P* 99.75+ Ag 2 17C* 1 98.50+ Ag 3 17R* 99.75+ Ag •1 18 98.5 + 1.2 Ni 5 180 0.6 98.3 + 0.7 Ag 6 98 + t

* 17P is plated, 17C is cast, and 17R is rolled, t 0.5% Ca, 0.5% Na, 0.1 % Li + Al.

In the case of these alloys, the "tensile strength" is the stress producing a deformation of about 25 per cent, and the "yield strength" is the stress producing an elongation of 0.125 per cent of the gage length.

The lead-base white metals (class B, Table 10.2) are cheaper than the tin-base alloys but are not so satisfactory for operation at high speeds and . .

194 Engineering Metallurgy

medium loads. They are used extensively by the railroads for freight- and passenger-car bearings, in electric motors, and in some machines. Alloys B2 and B4 are used in a number of bearings in automobiles. The properties of this group of alloys also vary with composition, usually within the following ranges:

Property Range of values Tensile strength, lb. per sq. in., at 70"F. (20»C.) 12,800 to 15,500 Tensile strength, lb. per sq. in., at 200°F. (95°C). .' 5.00 to 8,000 Yield strength, lb. per sq. in., at 70-F. (20"C.) 2,800 to 3.800 Yield strength, lb. per sq. in., at 200°F. (95-C.) 1,200 to 2.000 Brinell hardness (500 kg.), at 70»F. (20"C.) 15 to 22 Brinell hardness (500 kg.), at 200°F. (95 «C.) 6 to 11

It is evident from these values that the lead-base alloys are inferior in properties to the tin-base metals. As the temperature increases above 200°F., the tensile and the yield strength of the white metals decrease still more; values of 2,000 to 4,500 lb. per sq. in. are common for tensile strengths at 300° F. (150°C.) Elongation and reduction of area increase with temperature as, for example, from 15 to 30 per cent and from 18 to 40 per cent respectively.

The constitution of the white-metal alloys is complex and is imperfectly known. Essentially, the structure (Fig. 10.1) consists of a solid-solution matrix in which needles or. cubes of a hard constituent, probably an intermetallic compound such as SbSn, are embedded. Considerable work has been done recently to determine the effect of small amounts of cadmium on the tin-base alloys. Apparently, 1 per cent cadmium increases the tensile strength and hardness 10 to 20 per cent at normal temperature and at 200 to 300°F. (100 to 150"C.) The copper base bearing materials as listed in class G of Table 10.2 have found wide application particularly in the automotive and other transportation fields. The leaded bronzes (alloys of copper, tin, and lead) in general are slightly superior to the white metals; their melting

temperatures are much higher and their properties less affected by the operating temperature range encountered by bearings of this type. They also have better fatigue life which is affected by repeated flexure. Recent trends in bearing materials have been in the direction of silver or cadmium based alloys, the oilless bearing as made by powder metallurgy, and aluminum base alloys with tin as one of the principle alloying elements. For a more complete picture of bearing alloys, their composi-

tion and properties, the reader is referred to the ASM Metal's Handbook. The type metals are a series of lead based alloys with minor amounts of tin and antimony which arc used in the printing industry. The choice of Miscellaneous Heavy Nonferrous Metals and Alloys 195

Fie 10.1. Structure of tin-base bearing metals (A) containing 86 per cent tin, 6 per cent antimony, and 8 per cent copper; and (B) containing 82 per cent tin, 10 per cent antimony, and 8 per cent copper: etched. 150x- (Courtesy of O. W. Ellis)

alloy depends on the printing method used. In linotype machines it is advantageous to have an alloy that freezes at nearly a constant temperature, hence the use of the ternary eutectic mixture of 84% lead, 12% antimony, and 4% tin. This alloy freezes at 462°F. and makes possible close temperature control as well as good delineation of the type charac- ters in the cast strip. In other printing methods a harder alloy may be necessary and this may be obtained by adding copper in small amounts to the type metal or by solution heat treating and precipitation hardening an alloy which has the necessary amounts of tin and antimony. The artificial aging operation often times can be accomplished in one

hour at 185°F. or 1 minute at 212°F. These alloys are ideal for these applications since they must have low melting temperatures, (1) (2) low cost, and (3) good casting characteristics. Tin imparts to the alloy fluidity which makes for ease of casting, and the antimony increases the hardness and decreases contraction during freezing. Both elements together lower the melting point of the alloy and makes possible precipitation hardening if needed. The fusible alloys are those which are noted principally for their low melting temperatures usually in the range of 125 to 450°F. The solders, which are primarily mixtures of Pb and Sn, could be included 196 Engineering Metallurgy

in this category; although their melting range is usually considered as being from 360 to 570°F. However, when alloying elements such as antimony bismuth, cadmium, and Indium are alloyed with lead and

tin, the melting temperature of the resulting alloy may be as low as 1 I7°F.

These alloys are used in the vital parts of safety mechanisms in fire and explosion safety equipment. They are alloyed to melt or yield at

specified temperatures thus releasing fire doors or sprinkling equipment. However, caution should be exercised in using these alloys where static tensile loads are applied. Since these alloys have low melting temperatures, they similarly have low recrystallization or cold working temperatures. Hence, most of these alloys will under go creep at room temperatures under static loading. Other uses for these alloys include patterns in precision casting methods, forming dies for aluminum and magnesium, filler material inside thin walled tubes during forming operations, cores and forms on which other metals may be electroplated, and safety plugs in tanks and cylinders of compressed gas.

10.2. Lead and Tin as Engineering Materials

While the preceding discussion has dealt with the alloys of lead and tin, both metals are used in considerable quantities in commercially pure form.

Lead is a soft metal (tensile strength of 2,000 psi) which is easily formed by most fabricating methods. Part of this ease of fabrication results from the crystal structure of the metal which is face centered cubic, and part from the fact that it will recrystallize at room temperature. Thus all forming operations at room temperature results in hot working

the lead. Aside from its formability, lead is primarily noted for its corrosion resistance both to atmospheric conditions and to the acids sulphuric and hydrofluoric. As such it is used in relatively pure form as sheet and pipe in plumbing and building applications, in acid plants, and in the

handling of chemicals. Lead or its alloys is rarely used in strength applications where static loads exist, since it will undergo creep at room

temperatures. The principal uses for lead are (1) in storage batteries (2)

as tetraethyl lead in gasolines (3) as a cable sheathing material (4) as alloying element in solders and (5) as a building material. This is in the order of the total yearly amounts used. Lead may be strengthened by alloying with small amounts of cither antimony or calcium which produces a precipitation hardenablc alloy. The resulting room temperature tensile strengths are as high as 12,000 psi. Terne Plate, which is a lead based alloy containing 15 to 25% tin hot dipped onto sheet steel, is a competitor to zinc-coated products. It has Miscellaneous Heavy Nonferrous Metals and Alloys 197

the good corrosion resistance of both lead and tin and the bright appearance and formability of tin plate at a much lower cost than tin plate. Tin in the commercially pure form has essentially only one application, the tin can. This involves depositing a thin layer of tin uniformly over a steel sheet by either hot dipping or electroplating. The latter, while more expensive, has helped conserve tin by producing a thinner coat. Tin has good corrosion resistance particularly to foods and fruit juices hence its use in the packaging and canning industries. However, on steel the coat must be continuous to be protective. Otherwise an electrolytic cell would be established and the steel would corrode preferentially.

10.3. Zinc and Zinc Base Alloys as Engineering Materials Zinc may also be considered as one of the low melting point metals, having a melting temperature of 787°F. This plus its good atmospheric and salt water corrosion resistance are the characteristics which gives zinc its place in the metals industry. The largest use of commercially pure zinc is in the galvanic protection of steel products-ga/wmzzmg sheet, tube, and wire. The protective coat is applied either by hot dipping the steel in molten zinc or by electroplating. The latter produces a thicker coat providing longer protection and without forming a brittle intermetallic compound at the interface between the zinc and the steel which will crack and flake off when the product is bent. In recent years, over of the 40% zinc used in the United States has gone into galvanized products.

Second, in importance in the use of zinc is the die casting industry. Here, zinc alloys containing up to a combined 6% of aluminum, copper, and magnesium are used. An increase in strength is achieved by this alloying and some dimensional stability is attained in those alloys containing copper. The die casting alloys have a characteristic of shrinking in size apparently due to some phase change which occurs upon aging. Approximately two-thirds of the total shrinkage occurs in the first month, and this is often times made to occur at an accelerated rate by heating the parts 3 to 6 hours at 212°F. The automotive industry is probably the largest user of zinc castings. die Many of the accessories, functional parts, and decorative trim are produced by this method. It is an easy and economical way of producing precision parts which need little in the way of finishing operations. Quite often they will be chrome plated to enhance their appearance. The other applications where zinc die castings are used are in household appliances, office equipment, radios, toys, and telephone apparatus. 198 Engineering Metallurgy

The mechanical properties of the zinc and zinc base alloys are similar to those of aluminum and magnesium. Pure zinc has a tensile strength of about 20,000 psi. The high strength die casting alloy has a tensile strength of approximately 52,000 psi. However, this is not meant to imply that zinc may be used for strength purposes. For at room temperature, zinc and its alloys undergoes creep at loads exceeding

6,000 psi. This is as may be expected from its low melting temperature. The fatigue strengths for these metals are of the same order being from 700 to 8500 psi depending upon the alloy. The fabricating characteristics of zinc are good considering this metal has a hexagonal close packed crystal structure. Some of the ease of fabrication arises from the fact that pure zinc is hot worked when fabricated at room temperature. Zinc and its alloys may be fabricated by practically all of the known methods— rolled, drawn, spun, extruded, and cast. Some difficulties are encountered in the joining of zinc, but with proper techniques it may be gas welded or soldered.

Other uses for zinc include (1) use as an alloying element in brasses,

(2) in photoengraving plates, (3) dry batteries, (4) weather stripping and building materials, and (3) more recently as expendable anodes for cathodic protection of underwater steel structures.

HIGH TEMPERATURE METALS AND ALLOYS

The early use of metals for high temperature usage was in steam power generation and in the petroleum industry. In these applications the operating temperatures were in the range of 1000° to 1200°F. Other uses at slightly higher temperatures were in ore roasting furnaces. How- ever, in these applications corrosion resistance and economy are of primary consideration while mechanical strength of secondary importance. More recently with the development of the gas turbine, jet engines, and atomic energy, the operating temperatures have been edging upward and the emphasis is more on mechanical strength along with corrosion resistance. The early high temperature metals and alloys involved the alloy steels and the nickel base alloys. As temperature and strength requirements have increased the trend has been toward nickel, cobalt and chromium based alloys. With the development of the cermets a new approach to high temperature alloys has been made. In cermets, the high temperature strength is attained from an intermctallic compound usually a carbide of tungsten, titanium, chromium, tantalum, or noibium embedded in a matrix of cobalt or nickel. Other examples involve metal oxide embedded in a metallic matrix. The problem here is one of improving the creep resistance of the metal or alloy. To do this it is Miscellaneous Heavy Nonferrous Metals and Alloys 199

necessary to produce a continuous matrix or network which has very good strength and corrosion resistance, hence the carbides or metal oxides. This then is surrounded by a metal which has good oxidation and corrosion resistance.

10.4. Nickel and Nickel Base Alloys

The metal nickel is in many respects similar to iron. Its mechanical properties, modulus of elasticity, and atomic diameter are quite similar.

Hence it will alloy with many of the same elements found in steels. It differs principally in its corrosion characteristics. It resists oxidizing conditions very well but will be susceptible to attack in reducing conditions particularly if sulfur is present. It resists atmospheric corrosion quite well often, and where price will permit it, is used as a cladding material on other metals for this purpose. Nickel is also often used as an intermediate layer in the chrome plating of steels. Nickel Alloys. As aforementioned, nickel will alloy with many of the other metal elements. It forms a continuous series of solid solution alloys with copper. It has a high solubility for such other metals as Co, Cr, Fe, Mn, Mo, Ta, W, and Zn. The elements Al, Be, Si, and Ti have only limited solubility and alloys of Ni containing small quantities of these metals often exhibit precipitation hardening characteristics. Some of the better known nickel base alloys are as follows: Monels are a series of nickel-cop[>er base alloys containing approximately 65% Ni and 30% copper with small amounts of silicon, aluminum, manganese and sulfur. The Monels are primarily noted for their strength and corrosion resistance. The Inconels contain appreciable quantities of chromium and iron with smaller amounts of titanium, aluminum, and niobium. These alloys find application where high temperature corrosion and oxidation resistance is important. The Nimonic alloys were developed as a series which had high strength at high temperatures. An early alloy containing 80% Ni and 20% Cr had been used quite successfully as a heating element in electric furnaces and •appliances. Modifications of this alloy gave rise to the present Nimonic series. The latest developments in this series has large quantities of cobalt in place of the nickel and small amounts of molybdenum, titanium, and aluminum. The Hastelloys are another series of high temperature alloys developed by the Haynes Stellite Company and originally were nickel base alloys containing appreciable quantities of molybdenum and iron. However, the newer alloys showed decreases in the iron content with this being 8

200 Engineering Metallurgy

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replaced by larger amounts of chromium, tungsten, and cobalt. Some precipitation alloys have been made containing small amounts of titanium and aluminum. The Hastelloys are fabricated by the normal working methods but due to their high strengths and good oxidation and corrosion resistance require heavier fabricating equipment or higher temperatures to obtain the desired deformation. Table 10.3 lists some of the typical alloys falling in these various categories. They find application where operating temperatures are in the range of 1300 to 1800°F.

10.5. Cobalt Base Alloys

The element cobalt finds its greatest application in alloy form. It is used in other metal systems to increase strength and corrosion resistance and, combined with chromium forms a series of alloys known as Stellites. The stellites will have cobalt contents ranging from 67% down to 37.5%, and with chromium contents ranging from 20 to 27%. The remaining elements present will consist of varying amounts of the following elements, Ni, W, Mo, Ti, Nb, and Fe. In addition to the above elements, these alloys will all contain carbon percentages from 0.1 to 0.5%, and the carbon will be present in carbide form making these alloys very hard and brittle. As such, with one or two exceptions, these alloys are usually cast directly to their final shape. By virtue of their physical properties they are primarily used where high strength and abrasion resistance is required at high temperatures. Their normal operating temperature range ,s from 1350 to 1800°F. Other uses for cobalt are in magnetic materials where it is alloyed with Al, Fe, and Ni in "Alnico," and Fe rplus V in Vicalloy.

10.6. Other Miscellaneous High Temperature Metals and Alloys Recent experiments have indicated the direction in which future research must be aimed in the development of alloys for future higher temperature applications in the range of 1800 to 2000°F. Some of these metals and alloys will only be mentioned briefly here but will serve to show possibilities and shortcomings. Recent work with alloys of iron, chromium and aluminum; or iron, molybdenum, and aluminum whose alloys are known as "Ironals" have shown promise as good high temperature alloys. It is important that little or no carbon be present. Operating temperatures will be limited to approximately 2100 to 2300°F. Chromium base alloys are also possibilities, but chromium is difficult to melt due to its high vapor pressure and is very reactive with oxygen and nitrogen which makes the alloy very brittle. Experimentation with the metals molybdenum, niobium, rhenium, tantalum and vanadium all of which 202 Engineering Metallurgy have high melting temperatures, as can be seen in Table 10.1, are for the most part limited in use at high temperatures due to their oxidation reactions. The oxides of Mo, V, and Re all volatilize at temperatures below 1200°F., while Ta and Nb form spongy oxides quite rapidly at or above this temperature. It therefore appears that if these are to be used for high temperature applications they will have to be clad with another metal or ceramic which will keep oxygen away from the base metal.

OTHER METALS AND ALLOYS

10.7. The Precious and Semi-Precious Melals and Alloys

The metals and alloys which find themselves in this group are here by virtue of their scarcity in nature and their usage as coinage and jewelry. They are the metals silver, gold, platium, and the allied platium group- palladium, iridium, osmium, rhodium, and ruthenium. Most of the metals are found in nature in the metallic form or alloyed with the ores of copper, lead, nickel, or zinc. Usually they are present in very minor amounts and thus are obtained as by-products and only when the economics of the operation make it worth while.

Silver. The element silver in the pure form is a soft white metal with good corrosion resistance and high electrical conductivity. Since it is quite soft, it is rarely used as a pure metal except perhaps as a plating on jewelry items. When used as a base metal, it is alloyed with copper, the common alloys being with 10% copper in coinage, and with 7.5% copper as sterling silver. The silver solders have still higher copper contents. Silver is also used in an amalgam in dental alloys and is combined with tungsten and molybdenum by powder metallurgy for use as heavy duty electrical contacts. As aforementioned, it is used rather widely in electroplating of hardware, light reflectors and costume jewelry. The amounts used here are quite small and their cost is negligible. However, they add materially to the eye appeal and reflectivity of the base metal and improve the oxidation resistance of the part.

Gold, like silver, is quite soft and corrosion resistant. Similarly, it is rarely used in pure form due to its softness, the one exception being as gold leaf in sheet form with a thickness of approximately 0.00005 inches. In this form it can be used in lettering, display, and decorative applications. More often it is alloyed with copper, silver, or platium for jewelry applications. When alloyed with platinum, palladium, and nickel it takes on the characteristic white color and becomes white gold. Gold can also be plated on jewelry either by dipping on the molten metal or by electroplating. The latter yields heavier coats. Still thicker coats are Miscellaneous Heavy Nonferrous Metals and Alloys 203

obtained by cladding with sheet material by rolling, soldering or brazing

the sheet to the base metal. The other application for gold is as a

medium of exchange on an international basis, for it is accepted throughout the world.

The platinum metals while better known for their jewelry applications, in actuality, find greater application in commercial uses such as cata- lysts, high temperature corrosion resistant materials, electronics, and the chemical industry. With the exception of palladium, the other metals —platinum, iridium, osmium, rhodium, and ruthenium—all cost more per ounce than gold. Of this group, only platinum is used in pure form. The others being used as alloying elements in platinum. Platinum is used in making chemical and medical apparatus usually as an alloy containing

iridium. In pure form, platinum wire is used for a platinum-rhodium alloy wire as a high temperature thermocouple. This couple readily resists the high temperatures and oxidizing atmospheres that cause other metals to fail rapidly. Rhodium is sometimes used as a replacement for silver on plated articles which will be subjected to sulfur atmospheres. Rhodium has a silvery appearance but will not tarnish under these conditions.

10.8. Metals and Alloys in Atomic Power Applications While considerable interest has been shown in recent years in the application of atomic energy for power purposes, much of the information has of necessity been withheld due to security reasons. However, the part metals play in this new industry can be broken down into four fields: (1) as fuel elements, (2) as structural materials or moderators, (3) as control materials, and (4) materials which can be irradiated and ber come gamma ray emitters.

In the first category, as fuel elements, it is necessary that the material undergo atomic fission. While many of the elements high in the period table in terms of atomic number are radioactive, only uranium and perhaps thorium may be used as fuel elements. It is necessary that the fuel material be rather plentiful in nature and undergo fission at a rapid enough rate to make it economically feasible as a source of energy. In the second category, the structural materials in an atomic reactor should be able to withstand the temperatures involved, should not trap or absorb neutrons, and should have good corrosion resistance. The metals which meet these requirements are aluminum, beryllium, and zirconium. Zirconium seems to be the preference particularly where high temperatures are encountered. 204 Engineering Metallurgy

In the third category are those metals which actually control the fission rates in a reactor. The boron steels are the materials commonly used by virtue of availability and ease of fabrication. Hafnium, which has a greater capture cross section than the steels, is not as prevalent in nature and hence is not as widely used. However, this is a field which is being thoroughly investigated and other alloys will in all probability be developed. In the fourth category are the metals cobalt, cesium, thulium, and iridium. These metals when irradiated become radioactive and isotopes of these metals are used as gamma ray emitters. The metal isotopes and their halflife periods are listed in Table 10.4.

Tablc 10.4 Radioactive Isotopes

Isotope Av. Energy* Radiation Output mr/hr* Half Life

Cesium 137 670 KV 3750 27 yrs.

Cobalt 60 1200 KV 14000 5.3 yrs.

Iridium 192 375 KV 5950 75 days •Approximate **Milli-roentgcns per hour at a distance of 12 inches.

These metals find wide usage in radiographic examination of metal parts for internal defects. In many applications they have advantages over x-rays. In general they are less costly, have greater portability, and can be used in smaller places. The gramma ray source usually is smaller than a cube 14 inch on each side. Exposure times for the radiographic films are about the same as for high voltage x-rays.

QUESTIONS

1. List five metal elements which are noted primarily for their low melting temperatures? 2. What are the "white metals," and for what are they principally used? 3. What are the principal requirements of a bearing alloy? 4. Why is a eutectic alloy used in linotype machines? 5. How does tin and antimony affect the casting characteristics of alloys containing these elements? 6. What are the principal uses of lead, tin, and zinc? 7. Which of the metal elements do we consider as being high-temperature materials 8. What is a cermet? How is strength at high temperatures attained in a cermet? Miscellaneous Heavy Nonferrous Metals and Alloys 205

9. What are the better known nickel-base alloys, and what are the principal alloying elements in them? 10. In what ways is the element cobalt like nickel? What other uses is made of cobalt? 11. Many of the elements have high-melting temperature; why can they not be used as high temperature metals or alloys? 12. Why are the metals gold, silver, platinum, palladium, etc. so expensive? 13. What are the four categories for metals with respect to atomic energy? 14. What are the requirements of metals that arc used as fuel elements in atomic reactors?

15. Why is it important that the structural materials in atomic reactors not absorb neutrons? How do they differ from the control materials? 16. Which of the elements are commonly used as gamma ray emitters in radiographic examination? 17. How do gamma rays differ from x-rays? 206 Engineering Metallurgy Manufacture of Metals 207 The Manufacture and Composition of Carbon Steels CHAPTER and Alloy 11 Howard P. Davis, M.S., Associate Professor, Depart- ment of Mechanical Engineering, University of Wyoming, Laramie, Wyoming Floyd Sheldon Smith, M.S., Associate Professor of Me- chanical Engineering, Alabama Polytechnic Institute, Auburn, Alabama

MAN has been using iron for at least 5,000 years. For almost half of this time he used it as a precious metal which came from heaven— as meteorites—and which was seized when found and laboriously worked into jewelry, charms, and amulets. These he wore for personal adornment or to ward off the evil spirits that harassed him daily. For the last 2,500 years, he has had terrestrial iron, reduced from its ores, and it is characteristic that the first use he made of it was to fashion weapons. Damascus swords, made by the Moslems in the Middle Ages from steel imported into the Near East from India, and by the famous families of Japanese sword makers between a.d. 500 and 1500, are the finest specimens of the early steelmakers' art which have come down to us. They were made by welding alternate strips of high- and low-carbon steel into a bar, forging out this composite material, doubling it over and re- forging again and again until a laminated strip of the finest quality resulted. This was drawn out into a sword or dagger blade, heat treated, sharpened, and finally etched with a dilute acid to bring out the lamina- tions. Such a blade is shown in Fig. 11.1, in which the blade and scab- bard are photographed against a background of the damask pattern near the dagger point, magnified eight times. Man's next use of iron was to fashion tools, so that he could make life easier for himself. Since the beginning of large-scale manufacture about 1870, steel production has increased greatly each decade; by 1953 the world was using over 258 million tons a year. Of this the steel industry of the United States has been producing about 100 million tons annually.

208 The Manufacture and Composition 209

Fie. 11.1. Damascus steel dagger. The background is a photograph of the etched damask pattern of the blade. 8x- (Ziegler, Mining and Metallurgy, v. 20, 1939, p. 69)

The use of chemical analysis for determining the composition of steel and cast iron did not become general until about seventy years ago, more than a decade after large-scale steelmaking by the Bessemer and the open-hearth processes had been introduced. For many years the analysis of steel and cast iron consisted of the determination of carbon, manganese, sulfur and phosphorus, silicon, if present, alloying and elements when these were specially added. Only in 210 Engineering Metallurgy

the past few years has it been realized that these common elements tell only a part of the story of the composition and that in addition some ten or twelve other metals, metalloids, and nonmetals may be present. As a

result, the chemical composition of unalloyed steel is no longer as simple

as it seemed thirty years ago; on the contrary, it is very complex and worthy of considerable attention.

Despite the fact that commercial carbon steels and cast irons contain many other elements in addition to carbon, metallurgists look upon them as alloys of the metal iron and the metalloid carbon because carbon has the greatest effect on their structure and properties.

Commercial carbon steels contain, in addition to carbon, varying amounts of manganese, silicon, sulfur and phosphorus, and small percentages of oxygen, nitrogen, and hydrogen. Occasionally they also contain small amounts of copper, nickel, chromium, molybdenum, lead, arsenic, aluminum, and tin. Some of these are added intentionally to improve the quality, but most of them are picked up from the raw materials used in the various processes. Steel men commonly divide these elements into three classes: harmful, beneficial, and neutral.

11.1. Definitions of Ferrous Engineering Materials

For many years committees of metallurgists in English-speaking

countries have been trying to define carbon steel, alloy steel, cast iron, pig iron, wrought iron, and other products of the industry precisely and concisely, but with little success.

It would be a waste of space, and probably even confusing, if the complex definitions for the numerous products of the iron and steel industry were all set down at once.

Carbon steel may be defined as a commercial alloy of iron and carbon containing less than 2.0 per cent carbon, less than about 1.30 per cent manganese, less than 0.5 to 0.6 per cent silicon, small amounts of adventitious elements, and no intentionally added special element. If the

alloy contains more than 2.0 per cent carbon and is poured direct from

the blast furnace into cast iron molds of uniform size, or is used in the

molten condition as an intermediate product, it is called pig iron; and if

this pig iron (usually with the addition of scrap) is remelted and poured into a sand mold, thereby assuming the desired shape of the finished section, it is known as cast iron. This means that whether an iron-carbon alloy containing more than 2.0 per cent carbon is called pig iron or cast iron depends upon whether the material is an intermediate product or a finished product. The Manufacture and Composition 211

Pig iron and cast iron have been differentiated here chiefly to indicate some of the necessary distinctions in formulating definitions of ferrous

products. Pig iron is of little interest to the engineer; cast iron is an

important engineering material and is discussed in some detail later.

Ingot iron is a commercially pure iron. It contains between 99.75 and 99.90 per cent iron. The common elements, carbon, manganese, silicon, sulfur, and phosphorus, account for about half the impurities; the occasional elements make up the other half. Ingot iron resembles carbon steel in that it is melted and refined by the basic open-hearth

process, but it is lower in carbon (about 0.02 per cent) than the lowest carbon steels made industrially, which range from 0.04 to 0.08 per cent. Ingot iron is used chiefly in the form of galvanized and enameled sheet.

Wrought iron is a low-carbon material and is no longer widely used.

The nature of the refining is such that this material contains a larger proportion of solid nonmetallic inclusions than low-carbon steel made by the usual processes. Owing to the nature of the process and the composition of the inclusions, wrought iron when rolled has relatively low

strength but is ductile and tough, and is easily welded. It is used for staybolts, rivets, some grades of pipe, boiler tubes, and especially for heavy chains and hooks. Wrought iron has well-marked directional properties.

There is no precise and concise definition of alloy steel, but practically there is seldom any difficulty in distinguishing between carbon and alloy steels. The practical criterion is: if an element, not used primarily for deoxidation or degasification, is added to carbon steel or is present ad- ventitiously in the amount required to produce a desired specific effect, the resulting material is an alloy steel.

It may be asked legitimately why the dividing line between carbon steel and cast iron is placed at exactly 2.0 per cent carbon. Although this

percentage is fixed primarily for convenience, it has considerable scientific justification. The 2.0 per cent carbon concentration marks the maximum solid solubility of carbon in iron at very high temperature, and below

this concentration graphite—a common form of carbon in cast iron— is not ordinarily present under normal conditions of treatment.

For convenience, the commercial iron-carbon alloys may be divided into four classes:

Low-carbon steel Less than 0.25 per cent carbon Medium-carbon steel 0.25 to 0.65 per cent carbon High-carbon steel 0.65 to 1.70 per cent carbon Cast iron and pig iron 2.50 to 4.50 per cent carbon .

212 Engineering Metallurgy

11.2. Iron Ore and the Manufacture of Pig Iron

The four essential raw materials of the iron and steel industry arc iron ore, coke and coal, limestone, and scrap steel and scrap cast iron. The iron ore used in the United States is chiefly hematite, an impure ferrous oxide

(Fe2 Og) containing between 45 and 55 per cent metallic iron and be-

tween 20 and 35 per cent earthy matter, known as gangue, which is made up of silica, alumina, calcium oxide, magnesium oxide, and water, plus varying small amounts of phosphorus and occasionally sulfur. Hematites and other usable iron ores are widespread throughout the world; the commercial ores in the United States are estimated at more than 10 billion tons,

of which 90 per cent is located near Lake Superior and in Alabama. Iron ores are graded by phosphorus content into Bessemer and non-Bessemer (basic)

ores. As phosphorus is not removed in the blast furnace, the ore must not con- WOT tain more than approximately half the amount of this element permitted in 800'f Bessemer steel.

Iron ore is reduced by coke in the blast furnace, limestone being used to flux the gangue and the ash of the coke. The raw materials, in the approximate ratio of 2 tons of ore (frequently includ-

ing some scrap steel or cast iron) to 1 ton of coke i/ and 2 ton of limestone, are charged into the top of the furnace, and air, preheated to 1000° F. (540°C.) or

above, is blown into furnace through a number of nozzles (called tuyeres) near the bottom. The blast furnace,

shown in cross section in Fig. 11.2,* is a steel shell lined with firebrick; 90 to

100 ft. high, 17 to 20 ft. in diameter at

the top, and 24 to 28 ft. in diameter Fig. 11.2. Cross section of a at the bosh modern blast furnace showing tem- (just above the tuyeres) perature and chemical reaction Operation is entirely continuous: The levels. (Based on and Camp Fran- raw materials, carefully weighed and cis) analyzed, are charged into the top at * Based on J. M. Camp and C. B. Francis, The Making, Shaping and Treating of Steel, Carnegie Steel Co., Pittsburgh, 5th cd., 1940, p. 288. The Manufacture and Composition 213

Limestone 774

't5fi«8 Coke 1954 Row , materia!^ B^^3gfi Ore, elc. 3925

Air 8500+

5000 10000

Sasesl2,000± _]

Products <

Kj Slag 867

Ftc. 11.3. Relative proportions by weight of the raw materials and products of the modern blast furnace. (Boylston)

frequent intervals and slowly settle to the bottom, the downward pressure being slightly greater than the upward blast pressure plus the friction of the burden on the walls of the furnace.

As the charge settles, the temperature increases (Fig. 11.2) until near

the bosh the iron ore is practically all reduced and the spongy iron is heated to incandescence. Here it starts absorbing carbon, which lowers the melting point. The iron then becomes pasty, absorbs carbon more rapidly, and finally melts, trickling down over the remaining unburned incandescent coke into the hearth. The calcium oxide in the limestone forms a fusible slag that is much lower in specific gravity than the iron

and that floats on top of the molten metal in the hearth from where it is

drained at regular intervals. The molten high-carbon iron is drained from

the hearth into ladles every 5 or 6 hr. and then is either poured into molds to solidify as pig iron or used in the molten condition in the manufacture of Bessemer or open-hearth steel. A modern blast furnace produces between 800 and 1 ,200 tons of pig iron every 24 hr. and, barring shutdowns for economic conditions, will operate continuously for years. The average proportion of the raw materials entering the blast furnace and of the resulting products is shown graphically in Fig. 11.3.* The slag is mostly waste and must be disposed of, a considerable problem in some plants. 1 As Fig. 1.3 shows, an enormous volume of gas is given off. This gas, which totals more than 100 million cu. ft. a day for a single furnace, contains 20 to 25 per cent carbon monoxide and has a heating value of

• H. Boylston, Iron M. and Steel, John Wiley & Sons, Inc., New York, 1935, p. 71. 214 Engineering Metallurgy

90 to 100 B.t.u. per cu. ft. It is, consequently, used to heat the blast and to generate power. Thermally the iron blast furnace is not efficient. Owing, however, to the value of the gas as a by-product and to the operation of large units, the over-all efficiency is relatively high, and pig iron is made cheaply.

Chemistry of the Blast Furnace. The preheated air, which is blown into the furnace under a pressure of 15 to 20 lb. per sq. in., comes into immediate contact with incandescent coke forming large volumes of carbon monoxide gas. This gas, together with the carbon in the coke, reduces the iron oxide by two general reactions:

Fe O 3 *± 2 s + CO 2 Fe -f 3 C02 , (1)

Fe2Os + 3 C *± 2 Fe + 3 CO. (2)

Both reactions are reversible and may proceed in either direction depending upon the temperature and other conditions prevailing at the various levels in the furnace. The net result, however, is reduction of the oxide to metallic iron. reactions Both are progressive: the Fe2 O s is reduced first to Fe 3 4 , then to FeO, and finally to Fe. Reaction (1) predominates in the upper part of the furnace (see Fig. 11.2) , beginning at a temperature of about 400°F. (210°C.) ; reaction (2) begins at a temperature of approximately 1200°F. (650°C.) and continues until the iron oxide is completely reduced in the zone of the bosh.

The chemical reactions in the blast furnace are essentially reducing so that any easily reducible oxides in the iron ore or the coke (in addition to the iron oxide) are also reduced. Thus, all the phosphorus and most of the manganese will be found in the iron. Oxides of silicon and sulfur are more stable but are partially reduced. The remaining oxides—of calcium, magnesium, and aluminum—are very refractory and are not acted upon by the carbon or carbon monoxide even at the highest temperature of the furnace. To flux these refractory oxides, the necessary amount of limestone (CaCO.,) must be charged. The limestone calcines in the upper part of the furnace and, together with the calcium and magnesium oxides in the ore, combines chemically with the alumina and with part of the silica to form a fusible slag composed of calcium, magnesium, and aluminum silicates.

The pig iron, tapped from the furnace, contains about 4 per cent carbon, all the phosphorus, and most of the manganese in the raw materials. The amounts of silicon and sulfur in the iron are controlled to some extent by a careful selection of raw materials but chiefly by regulating the chemical characteristics of the slag, that is, by controlling the proportion of calcium oxide to silica, and by controlling the tempera- ,

The Manufacture and Composition 215

ture. The latter depends upon the amount of coke, the temperature of the blast, the amount of water vapor in the blast (the humidity) and other variables. The operation of a modern blast furnace and the production of cheap pig iron of satisfactory quality is an art which demands long experience and skill of a high order.

Operation and Use of the Cupola. The cupola is a vertical, cylindrical type of furnace, consisting of a steel shell lined with firebrick. Usually the charging door is located 10 to 25 feet above the bottom of the furnace. Air enters the cupola through the tuyeres, located near the lower end of the cupola. Fuel, metal, and flux enter the cupola through the charging door. The initial charge of coke is known as the "bed charge." The metal is deposited on the coke. Then alternate layers of coke and iron are added. Generally a limestone flux is charged on the coke. The purpose of the flux is to form a slag with the dirt, impurities in the metal, and the coke ash.

The metal charge ordinarily consists of local gray iron scrap, purchased gray iron scrap, pig iron, and steel scrap, where increased strength is desired.

The proportions of various metals in the charge are governed by the type of castings desired. The base of the charge is pig' iron. Since the pig iron is of known composition, the greater the amount of pig iron and the smaller the amount of scrap, the more uniform will be the composition of the resulting castings. For high grade castings, such as automobile cylinders, the percentage of pig iron will range from 25 to 50 per cent of the charge, while for low grade castings, almost no pig iron will be used. Fluorspar, soda ash, and a number of proprietary fluxes are sometimes used in combination with the limestone to thin the slag and also to aid in the removal of sulphur.

Air is introduced into the cupola to aid in the combustion of the coke, which in turn melts the metal. Under ordinary conditions the blast pressure will range from 8 to 20 ounces per square inch, depending on the height of the cupola and the number and character of the metal charges. The ratio of the coke and metal charges are ordinarily one pound of coke to eight pounds of iron. The charges are very carefully weighed. As the metal charge becomes molten it trickles down through the bed charge of incandescent coke and is collected in the crucible zone of the furnace. The liquid slag which collects above the molten metal is drawn off through the slag notch located just below the tuyeres. The molten iron is drawn off through the tap hole, located at the bottom of the cupola, into crucibles and poured into sand molds to form gray cast iron castings. 216 Engineering Metallurgy

11.3. Acid and Basic Processes

The reduction of iron ores and the manufacture of steel are chemical processes and are unique in large-scale industrial operations in that the chemical reactions involved take place at high temperatures, nearly always above 2000°F. (1100°C.) and usually in the range 2500 to 3500°F. (1370 to 1925°C). Few refractory materials withstand such temperatures. The only ones that are available cheaply arc the oxides of silicon, calcium, and magnesium. These are relatively inert at normal temperatures but become active chemically at steelmaking temperatures: silica (Si02) becomes an active acid, and calcium and magnesium oxides become active bases, reacting at high temperatures; thus,

-» CaO + SiOs CaSiOs , (3)

MgO + Si0 2 -> MgSi03 . (4)

It is characteristic of silicates such as these that they fuse at temperatures much lower than the fusing temperatures of the oxides. Molten metals are refined by slags which are essentially silicates of calcium, magnesium, and iron and are formed by a reaction between definite amounts of a basic oxide and the acid oxide silica. Depending on whether the basic or the acid constituent is in excess, the slag will have a basic or an acid reaction at steelmaking temperatures. The character of the refractories used in the furnace—which is decided by economic considerations—determines the character of the slag and thus determines whether the process is basic or acid.

It is obvious that, if a furnace has an acid lining, the slag must be acid in character or it would attack and destroy the furnace lining. Thus, in an acid process the slag must contain an excess of silica, and in a basic process, where the lining is magnesium oxide, the slag must contain an excess of calcium oxide. The advantages of acid and basic steelmaking processes may be summed up as follows:

Advantages of Acid Process. Acid slags contain less free oxide than basic slags. Consequently, if all other things are equal, acid steels will be cleaner and of higher quality than basic steel. Acid refractories are much cheaper than basic refractories.

Advantages of Basic Process. The basic slags will remove phosphorus and sulfur from the steel. Therefore, lower priced scrap and pig iron can be used since it is not necessary to avoid small amounts of phosphorus and sulfur. Fluxing materials used in the steelmaking process are classified as basic, acid, or neutral according to their chemical activity at elevated temperatures. Silica is the only substance that is classified as a strictlv The Manufacture and Composition 217

acid fluxing material. Sand, gravel, and quartz are available in large amounts and in sufficiently pure state to be used as acid fluxing materials, but normally they are not used because the lining of the furnace will

furnish all that is required. In basic processes, silica might be used as a

flux if excess lime has been charged or where the raw materials are too low in silicon. Iron and manganese oxides act as basic materials when present in acid processes, but they are not normally added for this purpose and therefore are not classified as fluxing materials. The most common basic fluxing materials are limestone and dolomite, limestone being mostly CaO, while

dolomite is a double oxide of calcium and magnesium. "Burnt" lime,

a calcined limestone, is usually added in the basic open hearth when more

than the amount furnished by charged limestone is found to be needed at a later stage of the heat. In order to make the slags less viscous without changing their acid, basic, or oxidizing properties, a neutral substance with a low melting

point such as flourspar (mostly CaF2) is most commonly used.

11.4. Bessemer Processes

Forty years ago, 25 to 30 per cent of the so-called tonnage steels were made in the Bessemer converter. The price differential in favor of Bessemer steel, averaging S2 to $4 a ton, disappeared some years ago owing to the increasing scarcity and higher cost of low-phosphorus ores. This condition, coupled with the disadvantage that Bessemer steel containing

more than about 0.20 per cent carbon is considered by many metallurgists to be inferior in quality to steel of comparable carbon content made by the basic open-hearth process, has resulted in the decline of Bessemer- steel production to about 3 to 4 per cent of the total. During the past 15 years it has leveled off to between 3 to 6 million tons per year.

The Acid Bessemer Process. The acid Bessemer converter is a pear- shaped steel vessel lined with silica brick and mounted on trunions by which it can be tilted 180 degrees. The bottom contains a large number of tuyeres to admit the blast. In making steel by the Bessemer process the converter is tilted forward, and 15 to 25 tons of molten pig iron are poured into the belly of the vessel. It is then turned upright, and at the same moment the blast is turned on. Refining begins immediately. No

external source of heat is needed as sufficient is generated, by the reaction of oxygen with the carbon and other elements in the pig iron, to raise the temperature several hundred degrees. The blast of air entering the converter forms large volumes of iron oxide as soon as it strikes the molten metal, which immediately oxidize 218 Engineering Metallurgy

Fie. 11.4. Bessemer converter in operation. (Courtesy o\ Jones and Laughlin Steel Corporation)

the manganese and silicon. The phosphorus is oxidized; but as there is no excess of lime, stable phosphate is not formed, and none of this element is removed from the metal. Sulfur is also unaffected.

The progress of the chemical reactions is followed by observing the color and length of the flame shooting from the mouth of the vessel.

Refining is complete in 10 to 15 minutes when the manganese, silicon, and carbon in the pig have been reduced to a low percentage, after which the converter is tilted forward and the metal is poured into a ladle from The Manufacture and Composition 219

which it is poured into ingot molds. Harmful oxides are eliminated as far as possible, and the proper amounts of carbon and manganese are secured by adding an alloy of iron, manganese, and carbon to the ladle as the contents of the converter are transfered to the ladle.

No slag-making materials are added; the slag formed is composed of compound silicates of iron and manganese—oxidized from the pig

iron—plus some silica eroded from the lining. It is acid in character. Owing to the rapidity with which die reactions occur and to the large volume of iron oxide formed, acid Bessemer steel usually contains more dissolved iron oxide than basic open-hearth steel. The production from a Bessemer plant of three vessels is 5,000 to 6,000 tons in 24 hours. To produce the same tonnage of basic open-hearth steel, a battery of

twenty 100-ton furnaces is needed.

For many years, the Bessemer process has been important in the steel industry, but, in contrast to the open-hearth process, little attention has been paid to the chemistry or metallurgy of refining. In 1940, however, some of the larger plants making Bessemer steel instituted a study of the process, and some noteworthy developments resulted.* A photo-electric cell is now being used to determine precisely the end of the blow, and a process has been developed for removing phosphorus by treating the molten metal in the ladle. Very recently a method has been worked out for deoxidizing Bessemer steel, so that it is apparently equal in quality to open-hearth steel of approximately the same composition, with the object of using this deoxidized material for seamless tubes and pipes.

The Basic Bessemer Process. This process is not used in the United States, mainly because the iron ore being used, and consequently the pig iron being produced from it, is not suitable for this process. It is necessary that there be at least 2% phosphorus present in the pig iron in order that the oxidation of the phosphorus during the afterblow can maintain the temperature of the bath high enough to prevent solidification. The silicon content must be low in order that the expensive basic lining last as long as possible. The operation of the basic process is similar to that of the acid process except that lime is added, so the slag formed is basic and the phosphorus will be removed. After the manganese, silicon, and carbon have been oxidized out there is the additional afterblow during which the phosphorus is oxidized out of the steel.

• These developments in Bessemer steel arc described in papers by Graham. Work. McGinley and Woodworth, Yocum. lulton. Bowman, Wright, and Dunkle in Transac- tions, Iron and Steel Division, American Institute ot Mining and Metallurgical En- gineers, 1941, 1942, 1944. 1945. 220 Engineering Metallurgy

The cost of the basic process is higher than the acid process due to the higher cost and shorter life of the basic refractory lining, the longer blowing time and the fact that lime has to be added to the process. In Europe, where this process has been used with success, the high phosphorus ores necessary are available, and there is also a market for the slag as a fertilizer, which helps overcome the higher cost of the process.

Fie. 11.4A Cut-away model of a modern open-hearth furnace showing roof construction (upper left), bottom and port construction (lower left), cutaway checker chamber showing brick construction (lower center), and stack (right). (Courtesy H. W. Graham, Jones and LttUgMitt Sled Corporation)

11.5. Open-Hearth Processes

Description of the Modern Plant. The typical open-hearth furnace has a capacity of 100-150 tons, with the largest furnace in the United States having a capacity of 350 tons. The furnace is a chamber about 80 ft. long and 18 to 20 ft. wide. The hearth is shaped like an elongated saucer, the inside dimensions being about 40 ft. long, 14 to 18 ft. wide, and 20 to 30 in. deep at the center. The hearth lining consists of a mixture of burned magnesite and basic slag sintered into place. The furnace is fired The Manufacture and Composition 221

ftC, I1.4B Pouring molten blast-furnace iron into the open-hearth furnace. (Coxr- tesy\y of Carnegie-Illinois Steel Corporation)

with gas, oil, tar, or powdered coal, using preheated air, and if a lean gas is used, this is also pre-heated. At the center of the back wall is the tapping hole, in. 8 in diameter, in such a position that when opened it will drain all the steel from the hearth into the ladle. There is a hole for the slag runoff at the top level of the hearth and toward one end of the furnace. The walls are made of silica 13i/£ brick, in. thick, and extend 8 ft. above the charging floor level. The charging doors make up most of the front wall of the furnace and are lined with silica brick and have water cooled 222 Engineering Metallurgy frames, the bottom of the doors being just a few inches above the slag line.

The arched roof is 12 in. thick, made of silica brick, and is built independent of the walls so that it can be replaced without completely rebuilding the furnace.

To attain the high temperature necessary to make steel, the regenerative principle is used. White-hot exhausi gases from the furnace are led through a series of brick checker-work chambers on the way to the stack. After about 15 minutes, these exhaust gases are diverted to another set of checker chambers at the other end of the furnace, and the cold air is led through the heated checkers on the way to the furnace. The direction of the exhaust and incoming gases is reversed every 15 minutes initially, and the time interval is reduced as the temperature of the furnace approaches the tapping temperature. The reversal of the gases is automatically controlled depending on the temperature of the checkers and the furnace. The regenerative process makes it possible for the gases to attain a temperature of approximately 3500°F. (1925°C.) as they sweep across the hearth, in addition to creating a saving in the amount of fuel used in the furnace.

A hot metal mixer is usually located in the same area with the bank of open-hearth furnaces. It is a large refractory-lined vessel capable of storing as much as 1500 tons of molten metal from the various blast furnaces until it is needed in the open hearth. Some are equipped with heating devices but mostly they depend on the heat from the hot iron as it comes from the blast furnace to maintain a molten condition in the mixer. The mixer makes it possible to have a molten metal with a more consistent chemical composition available to the open hearth, so that a wide variation in the charge is not necessary, and also to decrease the time from charge to tap of the furnace, because of the retained heat. This also tends to cut down on the fuel used. Operation of the Process: Charging. The order of charging the materials into the furnace is quite important. The limestone is charged first in order that it will not enter into the slag reaction until the desired time. Any ore charged into the furnace is placed on top of the limestone. The limestone and ore protect the hearth from damage during the charging of the scrap iron which follows immediately. II the scrap is light and bulky it may be necessary to melt part of it down in order to charge the rest into the furnace. When the furnace reaches a temperature somewhat higher than the melting point of the pig iron, the molten iron is poured into the furnace from a ladle, using a spout placed in one of the charging doors. The ratio of the amount of steel scrap to pig iron in the charge varies according to the price of the scrap. When the price of scrap is low, The Manufacture and Composition 223

the charge might consist of 75% or more scrap and 25% or less pig iron, but if the price of scrap is high, then a charge of 25% scrap and 75% pig iron is possible.

Melting the Charge. As soon as the steel scrap is charged into the furnace, it is heated with a hot oxidizing flame that will cause the scrap to melt and become oxidized. This oxidized material later becomes in- strumental in the purification of the pig iron by oxidizing out the impurities. If enough oxides are not provided by the oxidation of the

scrap, additional oxidation is obtained by the ore that is charged in on top of the limestone in the charge. This oxidation usually takes place for about two hours after the start of charging, when at that time the hot metal is added to the furnace.

The Ore Boil. As soon as the hot pig iron is added to the furnace the oxides present begin to react with the impurities in the iron. First to be removed are silicon and manganese, forming SiOa and MnC) which, along with FeO, combine to form the early slag. At the same time the oxygen is combining with the carbon in the pig iron to form CO which, when escaping, causes the slag to "froth" and increase in volume to such a degree that a considerable amount will overflow through the slag runoff hole. In this manner much of the silica that would have to be neutralized by the limestone is eliminated, requiring less limestone to be charged into the furnace. It is interesting to note that a good deal of iron is lost here (as in the FeO slag) , and that sometimes this slag is re- charged into the blast furnace to recover both the iron and the manganese from their oxides.

The Lime Boil. After the ore boil has subsided, the temperature of the furnace has reached the point where limestone will calcine, giving off large

volumes of . This C0 2 characterizes the "lime boil," which is much more violent a boil than the "ore boil." At the same time the lime formed by calcination rises from the bottom of the hearth and replaces the iron and manganese oxides in the slag, with any excess of lime tending to make the slag more basic. The basic property of the slag makes it more capable of retaining phosphorus in the slag as a phosphate. This lime boil also tends to mix thoroughly the contents of the hearth so that the chemical composition and the temperature are more uniform from top to bottom. The Working Period. During the working period there are several specific aims. These include: (a) lowering of the phosphorus and sulfur content below the maximum values specified, (b) eliminating the silicon and manganese residue not oxidized out during the melting period, (c) eliminating the excess carbon as quickly as possible, (d) conditioning the slag so that it will be of the proper viscosity and have a basic chemical 224 Engineering Metallurgy composition, and (e) raising the temperature of the bath to approximately 150°F. above the melting point of the steel to be tapped. Tapping the Heat. When the carbon content has been reduced to the desired amount and the temperature of the steel has been raised to the tapping temperature, the heat is ready to tap. The clay plug and a portion of the dolomite that fills the taphole is removed. The rest of the dolomite is burned out with an oxygen torch, and the taphole is cleared by using tapping rods from the front of the furnace. The center of the ladle is placed off center in order to cause a swirling action and to insure rapid melting and good mixing of the ladle additions.

Ladle Additions. Usually included in the ladle additions are alloys of manganese to remove oxygen and sulfur, alloys of silicon that also remove oxygen, and aluminum which is added to further the deoxidizing process and also to control the grain size. In order to increase the carbon content of the steel after tapping, some carbon is recovered from the manganese alloys, and if more is needed it can be obtained by adding ground anthracite coal to the ladle. Other intentionally added elements may be added either in the furnace or in the ladle. All ladle additions should be added before any slag comes from the furnace, otherwise they become mixed with the slag causing high alloy losses and a possible phosphorus reversion. By using varying amounts of these alloying elements, high quality steel of any desired carbon and manganese content can be made.

Chemistry of the Basic Process. The basic open-hearth is an oxidation process. Oxygen in the air or in some form of iron oxide (iron ore) is used to oxidize the carbon, silicon, manganese, phosphorus, and part of the sulfur in the charge.

In the basic open-hearth process limestone (CaCOs) is charged with the scrap and pig iron; and when the metal is melted, this limestone, calcined to lime (CaO) by the heat, rises to the top of the molten bath to form slag. Iron oxide in the form of iron ore is added to augment the iron oxide which, in the form of rust, was on the scrap and pig iron. The iron oxide oxidizes the silicon and manganese first, according to the reactions '-* Si + 2 FeO SiO a + 2 Fe, (5)

Mn + FeO -> MnO + Fe, (6)

MnO + Si02 -» MnSiOg, (7a) FeO + SiO, -* FeSiO.,. (7b)

The manganese and iron silicates, of low melting point and low specific gravity, rise to join the slag. The phosphorus is then oxidized: The Manufacture and Composition 225

2 P 4- 5 FeO -» Pa 8 + 5 Fe, (8a) 3 -» (FeO) (8b) Pa 5 + FeO aP3 B ,

(FeO) ,PaOB + 3 CaO -» (CaO)3 PsO., + 3 FeO. (8c)

The iron phosphate formed by reaction (8b) is unstable, but in the presence of an excess of lime, reaction (8c) takes place, and the more stable calcium phosphate is formed and also leaves the metal and joins the slag.

Reactions (5) to (8) are exothermic and take place at relatively low temperatures, as the charge is melting or just after melting is complete. The oxidation of carbon,

C + FeO - CO + Fe, (9) by reaction (9) is endothermic and takes place only at high temperatures. When all the impurities have been eliminated as far as possible or desirable, the molten metal usually contains less than 0.10 per cent carbon, less than 0.20 per cent manganese, less than 0.01 per cent silicon and usually less than 0.03 per cent phosphorus. Sulfur drops from above 0.04 or 0.05 to 0.025 or 0.035 per cent. As the result of these oxidizing reactions the molten metal also contains a relatively large quantity of dissolved iron oxide which, as discussed in Section 11.10, is harmful. It also contains too little manganese to combine with the sulfur. To remove harmful oxide and to ensure the presence of sulfur as manganese sulfide, manganese is added to the molten metal in the furnace or as it is tapped from the furnace. This reacts with dissolved iron oxide,

Mn + FeO -» Fe + MnO, (10) to form manganese oxide which is insoluble in the molten metal and which rises to the top to join the slag. The Acid Open Hearth Process. The open-hearth used in the acid process is similar to that used in the basic process except that it is usually smaller, and since the slag is acid, the lining of the furnace hearth is made of acid material also. The initial charge consists of cold pig iron and scrap. No ore is added because the iron oxide is basic and would react with the lining of the hearth. For this same reason scrap alone is not used as a charge, for it would oxidize and have the same detrimental effect on the furnace lining. Only manganese, silicon, and carbon can be removed readily; therefore a premium grade of pig iron and scrap that is low in both phosphorus and sulfur is necessary. This tends to increase the cost of acid steel over the basic type. The acid slag tends to be less oxidizing than the basic slag, and for this reason there are fewer oxide inclusions in the steel when it is poured, resulting in a "cleaner" steel. The acid steel is usually tapped at a higher temperature, in order that the steel may stand in the ladle for 30 minutes or more, thus 226 Engineering Metallurgy

Fig. 11.5. Heroult electric furnace tilted for tapping, showing arrangement of electrodes (Courtesy of American Bridge Company)

giving what inclusions are present a chance to rise to the surface and enter the slag.

11.6. Manufacture of High-Quality Steels by the Electric Processes

High-quality carbon and alloy steels are made by the acid open-hearth and the acid and basic electric process (Table 11.1). Most carbon tool steels and special alloy steels are made in the basic-lined electric furnace. Selected steel scrap is melted in an arc or induction furnace. The arc furnace, shown in Fig. 11.5, produces most of these materials in lots of The Manufacture and Composition 227

5 to 70 tons; the induction furnace, with a capacity up to 5 tons, is used chiefly to melt high-alloy steel scrap without loss of costly alloying metals. The large arc furnaces are used chiefly for the production of low-alloy steels by duplexing, that is, by refining further a charge of molten steel that has been partly refined by the acid Bessemer or the basic open-hearth process.

If the scrap contains too much carbon and phosphorus, these elements are oxidized together with the silicon and manganese, under conditions similar to those outlined for the basic open-hearth process on p. 224.

When oxidation is completed, the slag is removed and a new one is added, composed of lime, fluorspar, and silica sand or crushed ferrosilicon. This slag, which consists principally of calcium silicate when melted, is deoxidized, and strongly reducing conditions are established in the furnace by scattering powdered coke or ferrosilicon over the surface. Since slag and metal tend to be in equilibrium, eliminating iron oxide from the slag also deoxidizes the metal. Furthermore, a strongly reducing slag will remove sulphur almost completely from the metal. Owing to the complete deoxidation of the slag and to the absence of oxidizing gases— a condition attained only in the basic electric process- much sounder steel can be made by this process than by either the acid Bessemer or the basic open-hearth process where gases and slag are strongly oxidizing at all times. The acid electric process combines the nonoxidizing character of arc heating with the lining and slag of an acid process. Acid electric steel is usually not so well deoxidized as basic electric steel, but it is cheaper.

The small acid electric furnace is especially suitable for intermittent operation and, owing to the high temperatures of the arc, is widely used for melting steel for small high-grade castings.

The acid process is also used to produce ingots, high-quality forgings, ordnance and alloy steel. The furnace charge is usually all scrap, and practically the same purifying procedure is used as in the acid open-hearth. The steel can be more thoroughly deoxidized since the oxidizing effect of the furnace gases on the slag is eliminated. The process is used mainly to melt and refine carefully selected scrap low in phosphorus and sulfur, as the process will remove neither. The general features of the various steelmaking processes arc summarized in Table 11.1.

11.7. Wrought Iron

Wrought iron is a ferrous material of highly refined iron incorporated with 1 to 4 percent of a ferrous silicate slag. Two primary methods are 228 Engineering Metallurgy The Manufacture and Composition 229

used in its manufacture: (1) the puddling process, and (2) the Aston or Byers process.

In the puddling process, a charge, weighing about 600 pounds, of iron scrap, pig iron, iron ore, and mill scale are melted and refined in the puddling furnace. The fuel used for heating and refining the charge is bituminous coal burned on grates at one end of the furnace. After the charge is molten, it is mixed by the operator, using the puddle bar. Dur- ing the refining period, the oxidization of the carbon, silicon, and manganese is brought about by reaction with the iron oxide. As the refining progresses, the melting temperature rises, so the temperature within the furnace is not sufficiently high to keep it molten, and it becomes a plastic mass known as a puddle ball. The puddle ball is then removed from the furnace, squeezed and rolled into bars to eliminate much of the en- trained slag. The bars, known as "muck" bars, are cut into short lengths, fastened together in piles, reheated to a welding temperature and rolled.

If this process is repeated a second time, the product is known as double-refined iron.

The Aston or Byers process consists of melting Bessemer grade pig iron in a cupola and refining it in a Bessemer convenor until the carbon, silicon and manganese are removed. At the same time, an iron silicate slag of the same composition as that produced in the puddling operation is prepared in an open-hearth furnace.

The refined metal is poured at a predetermined rate into a ladle, called a thimble, containing the molten slag. The temperature of the metal is higher than that of the slag and the former rapidly solidifies. Gases are liberated during the solidification period and the metal disintegrates into small globules sinking to the bottom of the thimble to form a spongy mass. This operation is known as shotting. The excess slag is then poured off and the solidified ball of metal, containing slag, is transferred to the squeezer and rolls where the same procedure follows as in the case of puddled iron. Wrought iron is a highly ductile material and is used for pipe and pipe fittings, bolts, sheets, bars, and plate.

11.8. Special Steel-making Processes

The Duplex Process. The acid Bessemer and the basic open-hearth are used in succession. Most of the carbon and all of the silicon and manganese are oxidized in the converter and then the metal is transferred to the basic open-hearth where the phosphorus, sulfur, and the remainder of the carbon are removed. Under good conditions an open-hearth may produce up to twice the tonnage per week by the duplex process. 230 Engineering Metallurgy

The Super-refining Process. This is a special duplex process in which

purified basic open-hearth metal is transferred to a basic electric furnace,

deoxidized, and finished into high-quality electric steel. Care is taken to have the residual manganese as high as possible in the open-hearth

product. Carbon is added to bring this component near the required

finishing range. Since no further oxidation is required the basic carbide slag is immediately charged, and dcoxidation and de-sulfurization are carried out by the carbide and the residual manganese in the metal.

The Triplex Process. This process is carried out in two ways: (1) a combination of the acid Bessemer and two open-hearths in succession, or

(2) a combination of the duplex and super-refining process. The first

is used when pig iron is very high in phosphorus. The open-hearth is

tapped and charged into another open-hearth where a new slag is built up and the rest of the phosphorus and carbon removed.

The Talbot Process. This is a process that results in a very large tonnage of metal in a very short period of time. The increase in capacity is obtained by using a bath of metal forty or more inches in depth. The

first charge, usually consisting of pig and scrap, is worked down to the

desired composition, and about one third of the metal is tapped. Ore and limestone are added to the bath to produce a very basic oxidizing

slag. Molten pig iron, equal to the metal tapped, is poured through the

slag. The reaction is so rapid that the silicon, manganese, and most of the

phosphorus is removed. The charge is then worked down to the required

carbon and phosphorous content as the temperature is raised. For a period

of from 3 to 6 hours, the bath is purified, and another heat is tapped The slag is reoxidized, more pig added, and the procedure repeated. At the end of a week the furnace must be drained and the lining patched, making the process very expensive.

The Duplex Talbot Process. This process is a combination of the Talbot and Duplex processes. Metal from the Bessemer converter, free of carbon, silicon and manganese, is poured through the oxidized slag, the phosphorus being removed almost immediately. The addition of pig iron raises the carbon content and aids in deoxidation.

The Monell Process. Limestone, ore, and sometimes steel scrap, are

charged upon a basic hearth and heated until pasty. Molten pig iron is then added. The low temperature will remove the phosphrous very rapidly with a violent reaction. The silicon, manganese, and carbon is worked down in the usual manner.

The Campbell Process. A charge of pig and scrap is melted in a Camp- bell tilting basic furnace and the phosphorus, silicon, manganese and part of the sulfur are removed. The furnace is tapped and the metal The Manufacture and Composition 231 charged into an acid open-hearth furnace. An acid slag is built up and the heat is finished by the usual acid procedure. The Crucible Process. The crucible process consists of melting properly proportioned scrap, fcrro-manganese and charcoal in a closed crucible.

This is primarily a mixing and refining process. Each crucible holds 50 to 100 pounds of metal, which is heated in a gas fired regenerative furnace for approximately four hours for complete refining.

11.9. Mechanical Treatment of Steel

Ingot molds: Types, Sizes, Shapes. Most of the steel made in the

United States is cast into ingot molds of one type or another, irrespective of the type of steel, for further fabrication into desired shapes. This is done, rather than casting into desired shapes directly, because of the better physical properties obtained, and because it can be done much more cheaply. Two general types of ingot molds are used in the United States: the big-end-up and the big-end-down. As their names imply, the big-end-up mold is larger at the top, and the big-end-down is larger at the bottom.

Most steel is solidified in the big-end-down type mold, since it is suitable for the cheaper semi-killed steels and for the rimming-type steels. These steels avoid shrinkage or actually increase in volume in the molds by the evolution of gases on cooling, thereby avoiding the formation of "pipe." By the use of a "hot-top," a refractory collar added to the mold, killed steels may be poured into big-end-down molds because the steel in the hot-top section is kept molten and will fill any cavity produced by shrinkage.

Big-end-up molds tend to produce less pipe since there is a larger volume of steel at the top of the ingot, and it remains molten longer.

However, when used with killed steels, a hot top is generally used. Big-end-up ingot molds are more costly and are generally reserved for high-quality and alloy steels.

Bottom-poured ingots are those in which the metal is introduced from the bottom, usually into big-end-up molds with refractory hot-tops. Due to the complicated arrangement for pouring, they are more expensive and are usually reserved for high-quality and high-alloy killed steels.

Hot Working. After the ingot has solidified, it is transferred to a soaking pit or heating furnace to equalize the temperature and to heat it to a temperature at which the steel is very plastic. It is then hot worked by rolling, pressing, or forging it into a finished or semifinished product. A few special carbon and alloy steels used in very heavy sec- 232 Engineering Metallurgy

tions, such as large guns, axle shafts, armor plate, and the like, and a few tonnage products are worked from the ingot directly into the finished section. It is, however, considered better practice, whenever the size of the finished product permits, to reduce the ingot to a section known as a bloom, slab, or billet, which is reheated, either with or without intermediate cooling and inspection, and is rolled into finished products. These may be rails, structural shapes, plates, sheets, strip, pipe and tubes, bars, or rods of a wide variety of sizes and forms. Finished sections from the large rolling mill, especially bars and rods, may be fabricated further into a multitude of small articles.

Hot working has two primary objects: (1) to produce various shapes and sizes economically, and (2) to improve the structure and properties by breaking up the coarse crystal structure of the ingot. Cold Working. A large tonnage of hot rolled bars, rods, sheets, pierced tubing, and other hot rolled products are worked at room temperature.

This operation is known as cold working. Cold working is a finishing operation and the articles coming to this stage have been roughly shaped by hot working. The general methods of cold working are cold rolling, cold drawing, cold pressing, or stamping. Hot rolled shapes are finished by cold rolling and cold drawing processes to produce smooth surfaces, accurate dimensions, and increased strength. The drawing of hot rolled rods through dies is used principally in making wire and in finishing seamless steel tubing. In the cold pressing or stamping process, plate and sheet for various purposes are often shaped between dies by means of a heavy hydraulic press.

Cold working also has two primary objects: (1) to produce sections, sizes, and shapes that cannot be produced economically by any other method, and (2) to produce certain combinations of properties, especially very high strength accompanied by considerable ductility, that also cannot be secured economically by any other process.

The mechanical treatment of steel is an art that has been perfected mechanically until it is now in a relatively high state of development. Hundreds of millions of dollars have been invested in rolling mills and forging plants that are marvels of mechanical efficiency and low-cost production. Modern rolling mills, such as the recently developed continuous strip mill, are, in fact, such marvels of costly perfection. Although the mechanical equipment used and the methods of hot and cold working are a fascinating field for discussion, they are of little concern to the engineer who uses steel and who does not care whether strip is rolled on a continuous or a hand mill so long as it has the properties he requires and is cheap enough for him to use economically. The effect The Manufacture and Composition 233 of hot and cold working on the structure and properties, a subject of great interest to most engineers, is given more consideration elsewhere in this book. Steel and other metallic materials that have been hot or cold worked (or, in some instances, cast) frequently must be joined together to form the finished structure. The principal methods of joining are welding and riveting. The equipment and the methods used for joining are of much interest to engineers but are hardly wilhin the scope of a book on elementary metallurgy and are, therefore, not discussed further here.

11.10. Harmful Elements in Carbon and Alloys Steels

The five so-called harmful elements present in all steels in varying amounts, but mostly in small fractions of 1 per cent, are phosphorus, sulfur, oxygen, nitrogen, and hydrogen.

Phosphorus. In the amounts usually present in carbon and alloy steels phosphorus is combined with the iron as iron phosphide (Fe3 P) , which dissolves in solid iron as sugar dissolves in coffee. Although metallurgical opinion is not wholly unanimous, most available data, plus many years of experience with acid steels containing 0.07 to 0.12 per cent phosphorus, indicate that this element makes steel cold short, in other words, brittle when cold, particularly in its resistance to impact. This brittleness is more marked in high-carbon steels than in low-carbon grades and is apparent if the amount of carbon plus phosphorus is above 0.30 per cent. The static ductility is not appreciably affected. Because of the brittleness caused by phosphorus, the maximum amount permitted is usually given in engineering specifications. This varies from a maximum of 0.10 to 0.12 per cent in low-carbon acid Bessemer steels to a maximum of 0.045 per cent in rails and railway materials, structural shapes, sheet, strip, and other products of the basic open-hearth process. In tool steels and other high-grade high-carbon and alloy steels, the maximum permitted is usually 0.03 and occasionally even 0.02 per cent. The average amount of phosphorus in various grades is shown in Table 11.1.

Despite the fact that, in general, phosphorus has a bad name and for nearly a hundred years has been considered a nuisance that should be kept as low as possible, it has been discovered lately that it has its good points. It raises the tensile and the yield strength and improves other proj>erties, including the resistance of steel to some varieties of corrosive attack. Some of the new low-alloy high-strength steels contain about 0.10 per cent phosphorus used together with small amounts of copper, nickel, or chromium. The carbon, however, is low. 234 Engineering Metallurgy

Sulfur. In a manner analogous to that of phosphorus, sulfur will combine with iron to form iron sulfide (FeS), which dissolves in molten iron.

However, manganese, if present, having an affinity for sulfur which it does not have for phosphorus, will rob the iron of its sulfur to form manganese sulphide (MnS) . This compound is almost completely insoluble in solid iron. Consequently, when the iron solidifies, manganese sulfide is present in the mass of metal as discrete particles. These particles, together with oxides and silicates, are known to the metallurgist as solid nonmetallic inclusions or, more simple, as inclusions, and to the man in the mill by the more expressive term dirt. If insufficient manganese is present, some of the sulfur remains as iron sulfide. This compound melts at a temperature lower than the usual temperatures for rolling or forging, with the result that the steel is likely to crack during hot working. This brittleness or fragility at elevated temperatures is known as hot shortness.

Practically all commercial steels contain plenty of manganese, so that engineers who use steel at elevated temperatures need not worry about hot shortness caused by sulfur. Of much more interest to engineers is the fact that manganese sulfide particles are always present and that they may be present in large amounts if the sulfur is high. If these particles segregate in vital areas, they may have a deleterious effect on the ductility of the steel; in fact, impact resistance may be greatly reduced.

Any particle of dirt in the metal, if large enough, if the shape is favorable, and if it is strategically located, may act as a stress raiser and cause failure, especially by fatigue, much sooner than would normally be expected.

Sulfur has few virtues when present in steel. Compared with phos-

phorus, it is a real nuisance and is, therefore, kept as low as possible.

Its only benefit, so far as is known now, is that, if the manganese sulfide

is present in the proper amount and is well distributed, it makes the steel easier to machine. Steels that are machined automatically at high speeds and that are used for parts that are not subject to high impact stresses contain 0.080 to 0.150 per cent, or even more, manganese sulfide.

With higher sulfur percentages the manganese is frequently increased from the usual amount in low-carbon steels, namely 0.30 to 0.60 per cent,

to O.fiO to 1.65 per cent to ensure that no iron sulfide is present. The average sulfur percentages for various grades of steels made by the

different processes are shown in Table II. 1. In general, the higher

amount of the range is the maximum permitted by the usual specifications.

Oxygen. All steel, except that made by some special variants of the basic

electric process, is melted and refined by gases and slags that are essen- The Manufacture and Composition 235 tially oxidizing. At the high temperatures used, oxygen combines avidly with iron, carbon, manganese, silicon, and some other elements that may be present, forming a variety of gaseous or liquid oxides. Some of these dissolve in molten steel and are thrown out of solution when the steel solidifies, others are deoxidized by later slags or by alloys added especially for this purpose. The result of these deoxidizing reactions, which are more effective in some processes than in others (see Table 11.1), is to produce a steel which is more or less free from oxygen and its reaction products. Even under the best conditions, however, the steel is never wholly cleansed; hence, all steels when solid contain a larger or smaller quantity of gas cavities or solid inclusions bearing oxygen in some form. These oxides or combinations of oxides (silicates) form, together with sulphides, the most common source of the solid nonmetallic inclusions or dirt found in commercial' steels. Like sulfide inclusions, oxides and silicates, if segregated or if present in large particles, may act as stress raisers or as loci of weakness where, under favorable conditions, failure may start.

Inclusions that are entrapped in the steel ingot (or casting) when it solidifies arc usually in the form of small rounded particles varying in size from those which are submicroscopic to those which can be seen on a polished surface with the unaided eye. At high temperatures most of the inclusions are plastic. Consequently, when the ingot is rolled or forged, they elongate into stringers or threads. It should be emphasized that inclusions, once they are entrapped in the steel, may be distorted, elongated, or otherwise changed in form by hot or cold work, but they cannot be removed or even diminished in amount by any treatment. No commercial carbon or alloy steel is wholly free from dirt. The amount present and the degree of dispersion of the particles depend to a considerable extent upon the nature of the melting process. As indicated in Table 11.1, basic electric steel is inherently the cleanest, acid electric steel ranks next, and acid Bessemer steel ranks lowest. The process itself is, however, not the only factor. Also important are quality of the raw materials used and the skill of the man who makes the steel.

It has been well known for many years that most ferrous materials exhibit directional properties, that is, some properties, notably impact resistance, are lower if determined on a specimen cut at a right angle to the direction of rolling than if determined on a specimen taken longitudinally. Elongated inclusions are an important factor in causing this difference in properties.

Owing to the impure raw materials available to the iron and steel industry and to the necessity of using-except in a few restricted cases- 236 Engineering Metallurgy

oxidizing reactions to get rid of these impurities economically, one of the chief problems of steel men has been to eliminate the harmful effects of oxygen and its compounds by deoxidizing as thoroughly as possible. For more than sixty years one of the principal tenets of metallurgists has been "the more complete the deoxidation, the better the steel." Within the

past twenty or twenty-five years, however, it has been found that the phrase "for certain steels" should preface this doctrine of deoxidation. McQuaid, Bain, and other investigators in this field have shown that a certain degree of oxidation may be very desirable in some carbon and

alloy steels in which a control of the grain size is advantageous. The amount of oxygen in solid steel varies from a trace to as much as 0.02 per cent: few steels contain less than 0.005 per cent; the average

amount in commercial carbon steels is probably between 0.01 and 0.015 per cent, with the higher amounts found normally in the lower carbon

grades. Most of the oxygen—probably all of it in most steels— is present as an oxide or silicate. Small amounts of oxygen are difficult to determine accurately by chemical analysis, and it is even more difficult to determine in what form it exists in the metal.

Nitrogen and Hydrogen. Although the mechanism is not well understood, it is generally conceded that the presence of either nitrogen or hydrogen in carbon and alloy steels will tend to increase their brittleness.

11.11. Manganese in Carbon and Alloy Steels

Manganese is an essential and a beneficial element in nearly all

grades of steel because it performs a vital dual role: It combines with sulfur to form manganese sulfide, a less obnoxious impurity than iron

sulfide; and, as noted previously, it deoxidizes the .metal by reacting

with oxygen to form an oxide which is less soluble in molten steel than iron oxide and which, therefore, will leave the metal more readily. The amount used varies with the grade of steel and with the amount of

oxidation in melting; enough is added to most steels so that, after its purifying action has been completed, 0.30 to about 0.75 per cent, occasionally as much as 1.00 per cent, will be left in the metal. A small amount of manganese also increases the strength of steel. Either alone or, preferably, in combination with relatively small amounts of other alloying elements, it produces such a favorable combination of properties that the so-called intermediate-manganese alloy steels are

finding increasing use in some fields of engineering, especially for railroad rolling stock and in bridges. They were also widely used during the war as a substitute for low-alloy steels containing considerable nickel. The Manufacture and Composition 237

The amount of manganese in these steels ranges from about 1.00 to 1.90 per cent. There is considerable confusion, even among metallurgists, about the dividing line between these steels and carbon steels as some steels that are made and used as intermediate-manganese alloy materials

may contain 1.00 per cent or even less, and a few steels that have for years been classed as plain carbon steels— rails are an example—contain as much as 1.00 per cent or even more manganese. If carbon steel and intermediate-manganese steel cannot be differentiated by their manganese content, they can sometimes be classed according to the industrial

application for which they were made. This is a glaring example of the slipshod terminology that has grown up over the years in the iron and steel industry and that confuses maker and user, metallurgist and engineer, alike.

11.12. Carbon Monoxide, and Rimming and Killed Steels

As a result of the reaction between oxygen and carbon in the molten metal large volumes of carbon monoxide gas are formed. Most of this gas is off given during refining; some of it, however, remains entrapped and, unless removed by silicon or other degasifier as described in § 11.13, will be still in the metal when it starts to solidify. Most of this remaining carbon monoxide escapes during solidification. Owing, however, to the viscosity of the metal just before it solidifies, some of the gas remains entrapped to form cavities of varying size in the ingot or casting. These cavities-blow holes as they are commonly called-will usually weld in rolling or forging, especially in low- and medium-carbon steels, and will disappear. If, however, the inner surface of the cavity becomes oxidized, which frequently happens in heating for rolling when the cavity is near the surface or if the steel is high in carbon, the surfaces of the cavity do

not weld and the result is a defect

known as a seam (Fig. 1 1 .6). Seams are usually located on the surface of the rolled section where they can be detected and removed by machining, chipping, or grinding; but they may be internal, and if so they may, like inclusions, act as loci where premature failure will start.

Carbon steels are frequently classi-

fied according to the method of removing gas cavities or rendering them Kic. 11.6. Scams on the surface of a innocuous, as killed, or rimming shaft. 2x- types. 238 Engineering Metallurgy

Killed steels, which nearly always contain more than 0.25 or 0.30 per cent carbon, are those which must be thoroughly sound and free from gas cavities. The descriptive adjective comes from the action of the

molten metal when poured into the ingot mold: it lies perfectly quiet with no evolution of gas; there is neither bubbling nor churning of the upper surface of the metal. Killed steels are degasified by silicon, alumi-

num, or other degasifiers. All plain carbon forging steels, all rails, all

high-carbon tool and spring steels, and all alloy steels are thoroughly killed. Rimming steel—so called from the rim of solid metal next to the mold wall

(Fig. 11.7)—is low-carbon basic open-

hearth steel in which deoxidation is partly completed in the furnace or in the ladle

but which is not degasified. When the

steel is poured into the ingot mold and

begins to solidify, there is a brisk but controlled evolution of gas which results in an ingot having a sound gasfrec surface and in locating the blow holes so far be-

low the surface that there is no danger of

their becoming oxidized when the ingot is heated for rolling (Fig. 11.7). The blow holes will weld completely in rolling, and

the result is a material with a clean surface free from seams. Rimming steels are especially suitable for sheet and strip, notably for thin, deep-drawing stock used in large tonnages for automobile bodies and fenders. Close control of the slag composition (particularly the amount of

iron oxide) , slag viscosity, and the pouring temperature, is necessary to produce a

steel that rims properly when it solidifies. Most basic open-hearth steels containing less than 0.15 per cent carbon are made so that they rim; steels containing Fig. 11.7. Cross section of an in- 0.15 to about 0.25 per cent carbon, used got of rimming steel about one- jd , f structural shapes and plate, ' " twentieth natural size. (Courtesy of < ' American Rolling Mill Company) are killed or are partly killed. The Manufacture and Composition 239

11.13. Silicon and Other Degasifiers

Like manganese, silicon is beneficial and is added to carbon steel as a purifier and, in larger amounts, as both purifier and alloying element.

It is effective in removing oxygen and is, therefore, added to those grades of steels (usually containing more than 0.30 per cent carbon) in which

gas cavities do not weld readily or are otherwise harmful. Silicon is so used in a fe"' medium-carbon and high-carbon basic open-hearth steels

(for example, rails) , in practically all acid open-hearth and acid electric

steels, and in all basic electric steels. It is added rarely to such low- carbon basic open-hearth steels as structural material, wire, sheet, plate, and strip or to those low-carbon Bessemer steels which are used for pipe and tinplate. The reaction product of silicon with carbon monoxide

gas is silica (Si0 2), which reacts readily with manganese oxide and iron oxide, forming silicates. These are insoluble in the molten metal and are of such low specific gravity that they readily leave the steel. Silicon thus

carries deoxidation farther than manganese and is in addition an effective degasifier. Enough is added to complete the purifying reactions and leave 0.10 to 0.40 per cent in the finished steel.

There is occasionally some confusion—but not so much as in the case of manganese— in distinguishing between a silicon-treated carbon steel

and a silicon alloy steel. Some specially deoxidized open-hearth steels used in bridge construction, containing around 0.25 per cent silicon, have

been and still are termed silicon steels by civil engineers. This is un- fortunate, and it is hoped that the tendency to call a material silicon steel only if the silicon is 0.50 per cent or more will become more widespread. Steels containing 1.50 to 2.25 per cent silicon and about 0.75 per cent manganese are used widely for springs, and steels containing 0.5 to 5.0 per cent silicon and small amounts of other elements are used in the construction of dynamos and other electric apparatus. Small amounts of aluminum and titanium are added to some grades of carbon steel as final deoxidizers and degasifiers. It has been discovered recently that aluminum, probably through the agency of minute aluminum oxide particles, can be used to control the grain size of carbon and alloy steels. As this is one of the major recent developments in ferrous metallurgy, it receives detailed attention elsewhere in this book. Both aluminum and titanium are also used, in much larger amounts than is necessary for deoxidation and degasification, as alloying elements.

11.14. Other Elements

Since the introduction of alloy steels some fifty years ago there has been a gradually increasing contamination of carbon steel by nickel. 240 Engineering Metallurgy

copper, and other alloying elements owing to the inadvertent mixture of alloy-steel scrap with the other scrap. In the case of elements that oxidize readily, for example, chromium, vanadium, aluminum, titanium, and a

few others, the contamination is not serious, as they are largely oxidized and removed during melting and refining, leaving usually not more than traces in the steel. In. the case of such alloys as nickel and copper, which do not oxidize, the amounts present will normally increase slowly over a

period of years. The amount of these elements in the scrap is not of such serious consequences for steels made by a process that uses also a large proportion of pig iron, which is normally free from nickel and :opper, as it is in steel made using a large percentage of scrap (Table 11.1). In the latter the scrap may contain an unsuspected 0.10 to 0.25 per cent nickel and 0.05 to 0.10 per cent copper, sometimes even enough to affect the properties materially.

A recent development in free-machining steels is the addition of lead

to low-carbon material. This element is insoluble in carbon steel, but by

special methods of addition to the molten metal it may be incorporated

as a suspension so finely disseminated throughout the metal that it is not easily visible in an unetched section with the microscope. These submicroscopic particles of lead apparently act as an internal lubricant and in addition cause the chips to break up readily. Recently reported data indicate that a lead content of 0.10 to 0.25 percent greatly increases ease of machining but has no appreciable effect on mechanical properties. Carbon steel may be contaminated with small amounts of arsenic, tin, and antimony, traces of which may be present in iron ores and may persist through melting and refining. These metals may also be picked up from scrap containing bearing metals or tin cans. The amounts present are small, rarely exceeding 0.05 per cent. Very little is known about the effect of small amounts of most of these adventitious elements. According to the present state of our knowledge, their effect, with the possible exception of that of antimony, which is assumed to be harmful, may be called neutral. During the war, when some detinned scrap was used in the manufacture of open-hearth steel, considerable attention was given to the effect to tin. It was found that small amounts of this metal do not cause hot shortness in rolling; they do, however, increase hardness and reduce toughness. Tin, therefore, may be considered definitely harmful in some grades of steel, especially in those used for deep drawing.

11.15. Low-Alloy Steels A large number of metals (and some metalloids and nonmetals) have been alloyed with carbon steel, either alone or in various combinations, and the resulting properties have been studied more or less completely. The Manufacture and Composition 241

These include manganese, silicon, nickel, chromium, vanadium, tungsten, molybdenum, copper, and phosphorus as the more common alloying elements and cobalt, aluminum, zirconium, titanium, nitrogen, lead, boron, and selenium as the less frequent additions. Steel may also be coated or plated with zinc (galvanizing) , tin, lead, chromium, or nickel, but such products are not classed as alloy steels.

There are two general classes of alloy steels: (1) the low -alloy engineering steels and (2) the high-alloy tool and die steels, corrosion- and heat-resistant steels and special-purpose alloys. In tonnage, the first class is the more important; in value to man, both are probably of equal importance, for both have played a vital role in the development of our present-day machine-age civilization. Before World War II there were several hundred low-alloy engineering steels, of which some 70 to 80 were considered as standard by the Society of Automotive Engineers. These steels were divided into eight main classes: intermediate-manganese, silicon-manganese, nickel, nickel-chromium, molybdenum (including chromium-molybdenum and nickel-molybdenum), chromium, chromium-vanadium, and tungsten steels. There were also a large number of low-alloy structural steels, most of them developed between 1933 and 1939, that contained small amounts (usually less than 1 to 1.5 per cent) of nickel, copper, chromium, manganese, silicon, phosphorus, and molybdenum in various proportions and combinations. These were developed to satisfy a demand for a material of better properties than carbon steels but cheaper than the S.A.E. grades. These have been nicknamed "Irish stew steels" by Gillett, a happy and accurate designation.

11.16. High-Alloy Steels

There are almost as many varieties of the high-alloy steels as there are of the low-alloy grades. These special materials can be divided into three classes. The first includes the well-known high-speed steels and the closely allied die and valve steels. The primary requirement is hardness, especially at elevated temperatures, which is attained by large amounts (10 to 25 per cent) of tungsten, chromium, and occasionally cobalt, plus smaller amounts of vanadium, molybdenum, or silicon, together with relatively high carbon percentages.

The second class includes all the highly alloyed steels used primarily because of their corrosion resistance or scale resistance at normal and at high temperatures. The basic element in these steels is chromium, in amounts ranging from 10 to 35 per cent, either alone or together with varying amounts of silicon, manganese, nickel, copper, or molybdenum. The best known steels of this group are the cutlery steels, containing 242 Engineering Metallurgy about 0.35 per cent carbon and 14 per cent chromium, and the soft stainless steels, containing low carbon, 15 to 25 per cent chromium, and 6 to 20 per cent nickel, which are used widely for building trim, hardware and fixtures on buildings and automobiles, various kinds of utensils and more recently, in the construction of railroad equipment and aircraft. Not so well known but just as important are the highly alloyed steels containing nickel, chromium, molybdenum, cobalt, and other alloying metals, used in steam plants, oil refineries, gas turbines, including jet engines, and in other applications where high stresses and high temperatures are encountered. Also in this second class are the alloys containing up to 65 per cent nickel, 15 to 20 per cent or more chromium, the remainder being iron. These are the heat-resistant alloys of high electric resistivity that are used for the heating elements of all our domestic and industrial heating appliances.

The third class includes a large number of alloys of highly specialized but important uses. They arc the steels with special magnetic and electric properties, alloys with special expansion characteristics, steels with exceptional wear resistance, and many others. Although these materials are not manufactured in large tonnages, some are of such importance that by means of them the communication and other industries have experienced a veritable revolution in the past twenty or twenty-five years. Nearly all the high-alloy materials are made in electric arc or induction furnaces. Alloy steels are much more costly than carbon steels so that care is exercised to keep the quality as high as possible. Selected scrap is used, deoxidation is carried farther, and sulfur and phosphorus are usually required to be lower than in carbon steels.

QUESTIONS

1. Name the four principal raw materials of the iron and steel industry. Name the two main classes of final products. 2. What are the general principles underlying the reduction of iron ore in the blast furnace? Name the two principal reactions in the blast furnace, and give the approximate temperatures at which each reaction takes place most readily.

3. In the manufacture of pig iron, describe how the amounts of phosphorous, sulfur, and silicon are controlled in the product.

4. What is blast furnace slag, and where does it come from? What are its principal (unctions? 5. Describe the acid Bessemer process. What are die raw materials used? De- scribe the principal oxidizing reactions. How does the cost and quality of the Bessemer process compare with the basic open hearth? The Manufacture and Composition 243

6. Sketch briefly the general features of the basic open-hearth process, and

compare it with the acid open-hearth process in (a) raw materials, (b) slags, (c) chemical reactions, (d) general quality of the product. 7. Give two advantages of acid open-hearth steel production over basic open- hearth production. Give two advantages of basic open-hearth production over acid open-hearth production. 8. Describe the regenerative principle of the basic open hearth. Give two reasons for its use. 9. Differentiate between the "ore boil" and the "lime boil" by describing the reactions in each. 10. Describe how the impurities, such as phosphorous, sulfur, manganese, silicon, and carbon are eliminated in the basic open hearth. 11. Give four advantages of the electric processes for making steel. Do they have any disadvantages?

12. Describe the manufacture of wrought iron. What are its uses? How does wrought iron differ from ingot iron? 13. What stcelmaking process would you recommend for the production of steel for the following: (a) wire for suspension cables? (b) sheet steel for tin cans? (c) alloy-steel crankshafts for airplane engines? (d) rails for mainline railway track? (e) armor plate for a battleship? (f) small high-

strength castings? (g) tools for machining automobile connecting rods? 14. Compare the usual amounts and the principal effects of sulfur and phosphorous in carbon and alloy steels. 15. Describe the principal functions of manganese and silicon when added to carbon steel. Why is manganese a valuable alloying element? 16. Describe the essential differences between rimming and killed steels. How do these two classes ordinarily differ in carbon content? 17. Give a practical definition of low-alloy steels. Define high-alloy steels and give the characteristic property of the classes of high-alloy materials. The Constitution of S tee I CHAPTER

12 Hollis Philip I.eighly, Jr., Ph.D., Chairman, Depart- ment of Metallurgy, University of Denver, Denver, Colorado John Stanton Winston, M.A., M.S., Chairman, Depart- ment of Metallurgy, Mackay School of Mines, Univer- sity of Nevada, Reno, Nevada

1 HE most common group of alloys in use today are those referred to as steels. These alloys consist primarily of iron plus carbon in varying amounts, to which may be added various alloying elements to achieve the desired properties. Steel is used in tremendous quantities to produce necessities and luxuries; without steel our civiliza-

tion would be quite primitive. It is not intended to indicate, historically speaking, that steel is a new material. On the contrary, steel was produced as early as 1200 B.C. However, it is only within the past century that methods have been developed to produce steel in large quantities cheaply enough to make it a material of very common usage.

12.1. The Allotropy of Iron

The effects of carbon additions to iron are intimately associated with the allotropy of iron. In the process of freezing and cooling to atmospheric temperature, iron undergoes two allotropic transformations. Upon solidification at 2802°F. (1539°C.) iron assumes a body-centered cubic struc- C ture (8 Fe). At 2552 F. (I400°C.) this structure transforms into a face- centered cubic structure (y Fe). This structure persists clown to 1670°F. (910°C.) where it transforms back to the body-centered cubic structure (a Fe). Actually, 8 and a iron are the same form of iron. They are often regarded, however, as distinct modifications because of the two ranges in temperature in which the body-centered cubic structure is stable. The transformations of iron from one crystal structure to another are accompanied by changes in various properties; such as, density, electrical conductivity, and thermal change. The thermal change consists of an

244 .

The Constitution of Steel 245 1

Time

Fig. 12.1. Cooling curve for pure iron.

absorption of heat during heating and an evolution of heat during cooling. Fig. 12.1, the cooling curve for pure iron, illustrates how the measurement of the changes in a given property at transformation may be used for determining the temperature of transformation.

12.2. Iron-Carbon Phase Diagram

In the previous chapter it was shown that iron is reduced from its ore by means of carbon and that the resulting product, pig iron, contains approximately 4% carbon. Two effects of the carbon immediately evident are that it lowers the melting point of the iron and makes the resulting alloy brittle. It was also shown diat the chief function of the steel making process is to lower the carbon content to the proper level in order to obtain a certain combination of desired properties. The effects of carbon are so strong that its content is specified in hundreths of a per cent. The best approach to understanding the nature of the iron-carbon alloys is to study the phase diagram of the system shown in Fig. 12.2. In pure iron-carbon alloys, the carbon not in solid solution usually occurs as cementite (Fe C) which 3 , contains 6.68 per cent carbon by weight. Although cementite is metastable and decomposes, given sufficient time at elevated temperature, into iron plus graphite, conditions are generally such that in steels and pure iron-carbon alloys the carbon is present as cementite. Therefore the phase diagram chosen for study concerns the equilibria between iron and cementite; and thus, in reality, is the iron- iron carbide diagram. Although this diagram covers a small proportion of the possible combinations of iron and carbon, it includes the entire range of commercial iron-carbon alloys (plain carbon steels and cast irons) 246 Engineering Metallurgy

Atoms per cent, carbon

4 6 8 10 12 14 16 IS 20

\"Hypoeutectoid\Hyptreutectoid /rons-r* Stee/s

Fie. 125. Iron-carbon equilibrium diagram.

Carbon is soluble to a limited extent interstitially in each of the allotropic forms of iron. Inspection of the iron-iron carbide diagram shows that a-iron can dissolve a maximum of 0.025 per cent carbon, y-iron up to 2 per cent and 8-iron up to 0.1 per cent. These solid solutions of carbon in alpha, gamma, and delta iron are named ferrite, austenite and l-ferrite, respectively. The greater solubility of carbon in face-centered gamma iron than in body-centered alpha or delta iron is probably due to the larger size and more favorable location of the interstices in the face-centered structure.

Since delta iron has little to do with the final microstructure of iron- carbon alloys, transforming as it does to austenite, it will not be considered further. It should be noted, however, that in alloys between 0.1 and 0.5 per cent carbon content austenite is formed by a periiectic reaction. Alloys whose compositions fall between 0.5 and 2 per cent carbon solidify as austenite. For those alloys containing more than 2 per cent carbon a eutectic reaction occurs producing austenite and cementite.

The typical eutectic structure which forms is known as iedeburite. Alloys containing this brittle structure cannot be hot- or cold-worked but must be cast, and are therefore known as cast irons. The Constitution of Steel 247

All alloys with compositions between 0.025 and 6.68 per cent carbon undergo a eutectoid reaction at 1333°F. (723°C.) when austenite containing 0.8 per cent carbon decomposes, at constant temperature, into pearlite, an intimate mixture of ferrite and cemcntite (Figs. 12.3F and

12.4) . A plain carbon steel of this composition is called a eutectoid steel. Steels having less carbon or more carbon than this are called hypoeute- rtoid and hypereutectoid respectively.

From the cooling curve for pure iron (Fig. 12.1) it is evident that arrests are experienced when the transformations occur. Such arrests are also present in the cooling curves for iron-carbon alloys. The temperatures at which these transformations in the solid state occur are called critical temperatures or critical points. The loci of these points (lines) on the phase diagram are given symbols. In hypoeutectoid alloys, A3 and Aj are the upper and lower critical temperatures (lines GS and PS Fig. in the eutectoid 12.2); alloy and hypereutectoid alloys, A.t and A, coincide (line SK Fig. 12.2) and, therefore, is designated A ll3 . The zone between A3 and A, is known as the critical range. Some of the other lines appearing on the phase diagram have also been given symbols. The curve separating the fields austenite and austenite plus cementite (line SE) is designated A,.„, and the line of temperatures of transformation of B-iron to y-iron (line AHJB) is called A^. An understanding of the phase relationships shown by the iron-iron carbide diagram and the final microstructures attained is best obtained by following the process of solidification and subsequent cooling of several representative alloys of the system.

12.3. Phase Changes of Slowly Cooled Plain Carbon Steels

At the completion of solidification all steels consist of austenite while at atmospheric temperature they consist of ferrite and cemcntite. Their microstructure depends upon the relative amounts of ferrite and cementite and the way in which these are arranged. This, in turn, depends upon the carbon content. The phase changes in carbon steels cooled slowly from a high temperature can be illustrated by three steels containing (A) 0.30 per cent carbon, (B) 0.80 per cent carbon, and (C) 1.30 per cent carbon. These are indicated in Fig. 12.2 by lines xx', yy' and zz'.

At 2820°F. (1550°C), the first steel (xx') is a solution of carbon in molten iron. Upon slow cooling, there is no change until the solidification temperature 2768°F. (1520°C.) is reached. Here solidification of S iron begins and progresses with further cooling until at 2720°F. the peritectic reaction of S + liquid -» austenite occurs with an excess of liquid in this case. Further cooling causes ihe liquid to disappear so that at 2687°F. 248 Engineering Metallurgy

Fig. 12.3. Microstructures of slowly cooled iron-carbon alloys: (A) low-carbon iron; (B) 0.20 per cent carbon steel; (C) 0.40 per cent carbon steel; (D) 0.80 per cent carbon steel; (E) 1.3 per cent carbon steel; (F) pearlite at high magnification; etched. A to E, lOOx: F, 1.000X- .

The Constitution of Steel 249

(1475°C.) solidification of austenite is complete. The white-hot steel, which consists wholly of a solid solution of carbon in gamma iron, cools unchanged until a temperature of 1512°F. (822°C.) is reached. At this point on line A 3 (GOS) , the austenite is saturated in iron (ferrite) . If the temperature falls slightly, the excess iron is precipated in the form of alpha phase. Since the austenite loses no carbon during the process, the loss of some iron as alpha phase by the austenite is equivalent to increasing the carbon content of the austenite. As the temperature continues to fall, precipitation of alpha iron and increase of carbon content of the austenite (since there is progressively less austenite for a constant amount of carbon) occur continuously. When the temperature reaches that of the

A i (PSK) , the austenite is saturated with carbon. Further cooling results in simultaneous precipitation of alpha iron and carbon (as iron carbide)

This process goes to completion, which is to say that the austenite dis- appears completely.

If the cooling is very slow through the temperature interval from line

is A s (GOS) to just below line A x (PSK), the excess ferrite which first formed assumes the shape of discrete polyhedral grains, and the remaining austenite, containing 0.80 per cent carbon, transforms to the pearlite aggregate in which the ferrite and cementite particles are in the form of relatively large laminated plates. The structure of the slowly cooled steel consists of 36 per cent pearlite and 64 per cent excess ferrite.*

If a piece of slowly cooled carbon steel is polished and etched with dilute acid and examined with the microscope, the relation between the carbon content and the amount of free ferrite (etches light) and pearlite

(etches dark) is clearly evident. This relation is shown in Fig. 12.3. Micrograph A shows the structure of iron containing practically no carbon; it consists wholly of polyhedral grains of ferrite. In B, the steel containing 0.20 per cent carbon, the dark grains of pearlite are numerous, and in C, the material containing 0.40 per cent carbon, they are still more numerous. The parallel plates of cementite and ferrite making up the pearlite aggregate are shown clearly at high magnification in micrograph

F, and when viewed by the electron microscope, as is evident from Fig. 12.4.

Referring again to Fig. 12.2 (line yy'), at 2820°F. (1550°C.) a steel containing 0.80 per cent carbon is a solution of iron carbide in molten iron. As the molten metal cools, it is unchanged until a temperature of 2670°F. (1468°C.) is reached where solidification of the austenite begins. This is not complete until a temperature of 2520°F. (1380°C.) is

• 0.30 per cent carbon is equivalent to 4.5 per rent Fe3C. Since 1 part of Kea C is associated with 7 parts of ferric to form pearlite. 4.5 per cent Fe,C plus 31.5 per cent ferrite form 36 per cent pearlile. ,

250 Engineering Metallurgy reached. It will be noted that with higher carbon the solidification range is wider.

After the steel has solidified completely, the solution of cementite in gamma iron cools unchanged until a temperature of 1333° (723°C.) is reached. Owing to the fact that the steel contains 0.80 per cent carbon and, therefore, the exact amounts of cementite (12 per cent) and iron (88 per cent) to form the aggregate pearlite, there will be no preliminary precipitation of alpha iron from the saturated austenite, as was the case with the 0.30 per cent carbon steel described in the previous section. As the steel cools through the A x temperature of 1333°F. (723°C.) the austenite undergoes precisely the same change as was described for the last stage of transformation of the 0.30 per cent carbon steel, that is, simultaneous precipitation of alpha iron and iron carbide. There is no further structural as steel change the cools from A x to room temperature, and if cooling through the A x temperature is slow, the cementite and ferrite form well-defined plates as shown in Fig. 12. 3D and, at high magnification, in Figs. 12.3F and 12.4.

In the case of a steel containing 1.3 per cent carbon (19.5 per cent cementite), represented by line zz' in Fig. 12.2, the changes taking place during slow cooling are fundamentally analogous to those taking place in a 0.30 per cent carbon steel. The essential difference is (in addition to a wider solidification range) that the A cm line marks the preliminary separation of excess iron carbide rather than excess alpha iron. When a steel containing 1.3 per cent carbon cools slowly, at 1640°F (893°C.) the austenite is saturated with carbide. Further cooling results in precipitation of this excess carbide from the saturated austenite, which continues as the temperature falls from 1640°F. to 1333°F. (the A x temperature) . The remaining austenite, which now contains 0.80 per cent carbon (12 per cent cementite) , here transforms to pearlite, as in the preceding case. At room temperature, the structure is made up of grains of pearlite (etches dark) with a network (or needles) of excess carbide

(light) . This structure is shown in Fig. 12.3E.

The discussion to this point has all been concerned with slow cooling. In heating carbon steels, the reverse changes take place. For example, a 0.30 per cent carbon steel that has been cooled slowly consists at room temperature of a structure of 36 per cent pearlite and (>4 per cent excess ferrite. If this steel is heated slowly, no change takes place until a temperature of 1333°F. (723°C.) is just exceeded. Here, the alpha iron of the pearlite transforms to gamma iron and the cementite goes into solution. As heating is continued, the excess alpha iron changes to gamma The Constitution of Steel 251

Fie. 12.4. Electron micrograph of pearlite in eutectoid steel. Polystyrene-silica rep- lica. etched. Approximately i/ 20,000x. A in. on the micrograph = i micron = 0001 mm. (Courtesy of C. S. Barrett)

iron and is absorbed by the austenite. At 1512°F. (822°C.) , these changes are complete, and the steel, now a solid solution of 4.5 per cent cementite in gamma iron, remains unchanged to the melting point.

12.4. Isothermal Transformation in Steel

Considerable work has been done in studying the effect of continuous cooling upon the structure and properties of steels. The effect of the various alloying elements was known in a rather general way, however, no good explanation was available. In 1930, Davenport and Bain # presented a paper describing the results of a study of the isothermal transformation of steel. This paper resulted in a further extensive study of many iron-carbon alloys, which gave a better understanding of the mechanism of austenite decomposition and the part played by the alloying elements. results The of these studies are published in the Atlas of Isothermal Transformation Diagrams.^

• E. S. Davenport and E. C. Bain, Trans. Amer. Inst. Min. & Met. Eng V 90 1930 p. 117-154. Atlas of t Isothermal Transformation Diagrams, U. S. Steel Corp., 1951. 252 Engineering Metallurgy

1 1 1 1

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Fig. 12.5. Schematic Drawing of a Typical Isothermal Transformation Curve Re- vealing Significant Features.

The results of isothermal transformation studies are presented in the form of diagrams such as the one in Fig. 12.5. These are usually referred to as TTT curves (time-temperature-transformation) with time as the

abscissa and temperature as the ordinate. The left curve indicates the beginning of transformation with the right curve showing the completion. In the case of high or low carbon steels, the formation of the pro-eutectoid cementite or ferrite is usually indicated on the diagram as an additional feature.

To construct a TTT curve, it is necessary to heat thin specimens of the desired steel into the austenite region for sufficient time to obtain a homogeneous structure. The specimens are then quenched into a pot of

molten lead or salt which is maintained at some constant temperature, i.e., isothermal, below the austeniti/.ing temperature. These specimens are

kept in the molten bath for varying time intervals from 1 second to as much as 105 seconds or longer. At the end of the desired time interval, the specimen is quenched into cold water causing the isothermal transformation to be- terminated. Any austenite not transformed isothermally is transformed by quenching to martensite. The specimens quenched isothermally at a particular temperature are prepared metallographically and etched. Microscopic examination will reveal that the isothermal transformation product appears as dark areas in a white field. The The Constitution of Steel 253

Fie. 12.6. Micrograph of coarse pcarlitc formed by isothermal transformation at I275°F. 600X. (left)

Ftc. 12.7. Fine pcarlitc formed by isothermal transformation at 1200°F. 600X. (right)

Fig. 12.8. Nodular pearlitc formed by isothermal transformation at 1100°F. This specimen is only partially transformed. 600X. (left)

Fig. 12.9. Bainite formed by isothermal transformation at 850°F. This specimen is only partially transformed. 600 X. (right) 254 Engineering Metallurgy white field represents the untransformed austenite prior to water quenching. The time associated with the specimen showing the smallest amount of dark area is assumed to be the time lor the beginning of isothermal transformation at the particular temperature while the end of transformation is determined by the specimen showing a completely darkened surface. By repeating the experiment at several different temperatures, enough data are obtained to make possible the construction of a diagram similar to that shown in Fig. 12.5. The transformation products obtained arc a function of the transformation temperature. Near the eutectoid temperature very coarse pearlite forms as is shown in Fig. 12.6 while finer pearlite forms as the temperature is reduced (see Fig. 12.7) until at the "nose" of the curve a very fine pearlite forms in nodules giving nodular pearlite, Fig. 12.8. Below the temperature of the nose, a product forms, which only occurs during isothermal transformation, bainite, Fig. 12.9. The exact nature of this material is not well understood but it appears to be some sort of very fine mixture of fcrrite and cemeiuite. The above description applies to those alloys which do not yield a pro-cutectoid constituent. Pro-eutectoid ferrite or pro-eutectoid cementite forms before pearlite is formed.

12.5. The Effect of Cooling Rale Upon the Resulting Structure

In the previous section, the isothermal transformation of steel was discussed. However, during the heat treatment of most steel parts, it is not possible to remove the heat within the extremely short time as is done for obtaining a TTT diagram. As a result, the surface may be very severely quenched whereas the interior may cool very slowly. Such variation in cooling rate will certainly result in different structures. It was observed that isothermal transformation just below the austenitizing temperature caused the austenite to transform very slowly into coarse pearlite. As the temperature was reduced, the transformation was more rapid and the pearlite became finer. In Fig. 12.10 there is shown a schematic drawing of a portion of an isothermal diagram with some typical continuous cooling curves. Curve A shows very slow cooling, such as might be expected in furnace cooling. The austenite exists until the curve reaches the beginning of transformation at which lime coarse pearlite begins to form. It continues to form until the transformation is complete. Now curve B shows moderately slow cooling that might be expected in still air cooling of steel. Austenite exists until the curve meets the beginning of the transformation curve at which time medium pearlite begins to form and continues to form until the transformation is complete. The specimen cooled as shown by curve C would have still finer The Constitution of Steel 255

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pearlite. The critical cooling rate as shown by curve D shows that the austenite would transform into martensite without any pearlite forming. Martensite begins to at form the temperature shown as M 3 and increases as the temperature is reduced. Any cooling rates more rapid than D will result in complete transformation into martensite, Fig. 12.11.

- w '

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Fie. 12.11. Martensite formed by water quenching after austenitizing. 600x- 256 Engineering Metallurgy

12.6. Effect of Alloying Elements Upon the Iron-Carbon Diagram

In Section 12.2, the iron-carbon phase diagram was discussed in detail. Now, the effect of the common alloying elements upon the eutectoid temperature and composition will be discussed as well as the effect of the alloying elements upon the physical properties of steel. Alloying elements tend to divide themselves into two groups: those that widen the range of gamma stability by increasing the S-y transformation temperature and lowering the y« transformation temperature and those that narrow the range of gamma stability by lowering the 8-y transformation temperature and raising the y« transformation temperature. The common gamma stabilizers are manganese, nickel, cobalt, copper, nitrogen and carbon, while the common alpha stabilizers are silicon, chromium, tungsten, molybdenum, phosphorus, vanadium, titanium, aluminum, boron and sulfur. Nickel and manganese are such strong gamma stabilizers that it is possible to have stable austenite at room temperature such as occurs in the austenitic stainless steels and Hadfield steel respectively. Certain of the alpha stabilizers reduce the gamma region to a narrow loop and may result in ferrite existing continuously from the liquidus temperature to room temperature.

By an element being an alpha or gamma stabilizer means that it either raises or lowers the eutectoid temperature respectively. As was indicated previously, nickel and manganese are gamma stabilizers and thus reduce the eutectoid temperature, nickel being a bit more effective than manganese. The other gamma stabilizers have a similar effect however rather reduced. By the same token the alpha stabilizers raise the eutectoid temperature, thereby reducing the region of gamma stability. Titanium has the greatest effect in raising the eutectoid temperature followed by molybdenum, tungsten, silicon and chromium in decreasing order. Most alloying elements have an effect in reducing the carbon content necessary for the eutectoid composition. This results in a completely pearlitic alloy with less carbon than indicated by the iron-carbon diagram. Again, titanium has the greatest effect in reducing the eutectoid carbon content with molybdenum, tungsten, silicon, manganese, chromium, and nickel in descending order. This effect reduces the amount of cementite and causes the steel to be softer and less brittle. Most alloying elements have a useful effect upon the heat treating of steel; certain alloying elements have other useful properties. One of the properties is the ability of strengthening the ferrite to obtain sheet and plate materials for the fabrication of high strength, lightweight objects such as pressure vessels, railroad cars, truck bodies, etc. These are The Constitution of Steel 257 usually referred to as high-strength, low-alloy steels being sold under several trade names. They rely on the fact that phosphorus, manganese, copper, titanium and silicon form solid solutions with ferrite and thereby

strengthen it. Nickel, molybdenum, and aluminum are mild strengthened.

Another property that alloying elements have is their ability to form carbides. These carbides may have a grain refining action on the austenite during heat treatment to prevent excess grain growth. They also may help block creep in high temperature applications. In the case of certain types of stainless steel, columbium and titanium are used to form a more stable carbide than chromium and thus enhance the corrosion resistance of the steel. Columbium, titanium, zirconium, vanadium, tungsten, molybdenum, and chromium are considered as strong

carbide formers while manganese is only moderate. Phosphorus, nickel, sulfur, cobalt, and copper have no effect in the formation of carbides while aluminum and silicon tend to cause the carbides to decompose, precipitating graphite.

12.7. Effect of Alloying Elements on the Isothermal Transformation of Steel

Referring back to Fig. 12.5, we see that there are regions on the diagram

labeled pearlite and bainite. By comparing this figure with Fig. 12.10, it

is apparent that if thick sections are to be completely hardened, it will be necessary to displace the curve to the right, thus requiring a longer time

to elapse before the beginning of transformation. This is precisely the effect of alloying elements on the TTT curve. By suitable selection of alloying elements, it is possible to displace the curve enough to permit complete hardening of quite large pieces by air cooling.

Usually the effect of an alloying element is determined as a function of its ability to displace the pearlite or bainite region to the right. In the case of the pearlite region, manganese, molybdenum, phosphorus, and chromium have the greatest effect in that order, while silicon, nickel, and copper have much smaller effects. The displacement of the bainite region to the right is greatest with manganese, phosphorus, and chromium. Silicon, nickel, and copper have reduced ability while molybdenum has no effect on the bainite region. It is interesting to note that sulfur has a slight tendency to displace the curve to the left. By careful selection of proper alloying elements, the beginning curve for transformation can be displaced considerably to the right. It should be noted that small additions of several alloying elements are more effective than one large addi- 2 J ° Engineering Metallurgy tion. This is evident in the alloys that have been developed recently with their multi-element additions.

The addition of alloying elements has another effect in the thermal treatment of steel. In Fig. 12.5 it can be seen that there is a horizontal line labeled M s . This is the temperature at which martensitc begins to form. It continues to form as the temperature is reduced. Most of the common alloying elements tend to reduce the M, temperature, carbon being quite strong in its ability. The other elements reduce the M, in the following order; manganese, vanadium, chromium, nickel, copper, molybdenum, tungsten. Silicon has no effect while aluminum and cobalt tend

raise . to the M5 Alloy additions may lower the M, to such an extent that complete transformation of austenite to martensite mav not be possible by quenching to room temperature. It may require subatmospheric cooling in order to form martensite.

12.8. Effect of Hot Working on Structure

So far, the discussion in this chapter has been confined to the phase and structural changes in carbon steels when they are heated or, more important, cooled at varying rates between the temperature where austenite is stable and room temperature. It was stated that there is no change in the austenite if it is heated or cooled between the transformation temperature and the melting point. This is true. There is no change in the internal structure of the austenite grains, but there may be a great change in the size of the grains. If carbon steel is heated above the transformation temperature, the austenite grains grow, and the higher the temperature the larger the grains. Small-grained steels are generally considered to have better properties than coarse-grained material of the same composition. Since grain size at room temperature depends largely upon the grain size of austenite when it is ready to transform, the grain size of austenite should be small. One of the important factors affecting the grain size of austenite, and of pearlite and ferrite, is mechanical working. When molten steel cools, solidification usually begins by crystallization at the wall of the mold, and if the temperature of the steel is high when it is poured, and if the ingot or casting is large and of favorable shape, these crystals may become very large (Fig. 12.12). Hot rolling or forging, which is ordinarily carried out at temperatures at which the metal is plastic, breaks up the large crystals and elongates the fragments in the direction in which the work is applied. These elongated grains, together with elongated inclusions, give carbon and many of the alloy steels their directional properties. The Constitution of Steel 259

FIG. 12.12. Large columnar crystals in a steel ingot, natural size. (Hanemann and Schroder, Atlas Metallographiats, Berlin, 1927)

Austenitc grains, even when plastically broken and deformed, have the tendency to grow together again. This tendency is the greater the higher the temperature. Hence, if the finishing temperature of hot working is much above the transformation temperature, grain growth may occur while the steel is cooling to the transformation temperature, especially if the section is so large that cooling is slow. For this reason, if hot- rolled or forged steels are to be used without subsequent heat treatment, and if a fine grain is desirable, the hot-working temperatures and the amounts of reduction are controlled so that the finishing temperature will not be much above the transformation temperature. Thus, the steel will cool below this teni]>erature before appreciable grain growth has taken place. The effect of hot working in breaking up the coarse grain structure of cast 0.20 per cent carbon steel is shown in Fig. 12.13B.

12.9. Effect of Cold Working on Structure

Cold working is usually defined as mechanical deformation of steel at temperatures below the transformation temperature. In practice it is 260 Engineering Metallurgy

Fig. 12.13. Structure of a 0.20 per cent carbon steel: (A) as cast, and (B) as hot rolled; etched. 100x-

generally carried out at room temperature. If steel is plastically deformed at these low temperatures—as in drawing wire, cold rolling or extruding bars, or cold rolling sheet— the pearlite and ferrite grains are elongated in the direction in which work is applied. This mechanical distortion results in rapid increase of hardness and brittleness.

At atmospheric temperature, the rigidity of the steel is so great that the distorted grain fragments cannot rearrange themselves by recrystal- lizing into their normal equiaxed shape. Hence, if cold working is continued, the hardness and brittleness accompanying the fragmentation of the grains will increase with the amount of deformation until soon, unless the steel is annealed, it will fail. The amount of cold deformation that steel will stand depends upon its ductility, which, in turn, is related directly to chemical composition and prior heat treatment. Thus, annealed low-carbon wire will withstand more reduction by drawing before failure, or before reheating for recrystallization becomes necessary, than annealed medium- or high-carbon wire.

The unstable structure of cold-worked steel* and its accompanying hardness and brittleness are caused by the distortion of the ferrite and pearlite grains and not, as in quenched steel, by the presence of submicroscopic particles of carbon in a distorted alpha-iron lattice. But, as

• The structure of severely cold-worked steel is unstable in that the fragmented grains will recrystallize if given the opportunity; that is, if the steel is heated to the proper temperature, which is considerably below the A transformation. Actually, x however, owing to the rigidity of the material at room temperature, the fragmented grain structure will persist indefinitely. The Constitution of Steel 261

Fig. 12.14. Structure of a 0.20 per cent carbon steel (A) as cold-worked and (B) as cold- worked and annealed; etched. 500 X-

the stability of quenched steel can be increased by tempering (see Chap. 13), so can the stability of structure of cold-worked steel be increased by reheating below the transformation temperature. This is known as process annealing. Structural changes which occur when cold-worked steel is reheated are not gradual. Cold-worked steel is softened by heating to 800 to 1200°F. (425 to 650°C), the exact temperature depending on the composition, si/e of the section, and the amount of cold work. Reheating at the proper temperature effects complete recrystallization and restoration of the original structure and properties. The material can then be cold worked again until grain distortion and its accompanying brittleness have again become so serious that another reheating is necessary. The effect of cold working on the structure of a 0.20 per cent carbon steel (whose structure as hot-worked is shown in Fig. 12. ISA) is illustrated in Fig. 12.14A. The effect of reheating on the distorted grain structure is shown in Fig. I2.14B. Note that after annealing the grains are equiaxed, and all indication of distortion has disappeared.

QUESTIONS

1. Describe the changes taking place in cooling from 3,000°F., iron-carbon alloys having the following carbon contents, 0.3%, 0.8%, 1.5%, 3.0%, 4.3% and 5.0%. When crossing a two phase field, indicate how each phase changes in quantity and composition. 2. Using the lever rule, give the composition and amounts of the phases present in the above alloys at the following temperatures, 2,500°F 2 000°F 1,500°F., and 1,000°F. 3. Describe the crystal structures of delta iron, austenite, and ferrite. What is the crystal structure of pearl ite? How does it differ from the previous mentioned phases? 262 Modern Metlallurgy jor Engineers

4. Give the range of carbon contents for steel and cast iron. From the iron- carbon diagram, give one reason that cast irons frequently have limited ductility.

5. Describe in detail, the method of constructing a TTT diagram. What is the

white constituent which is present in specimens which are only partially

transformed? Where does it originate? 6. Sketch the TTT curve from Fig. 12.5. Superimpose on this diagram curves representing your opinion of the transformation curves for alloys containing manganese, silicon, phosphorus, nickel, copper, chromuim, and molybdenum. Sketch the position of the M, also.

7. Sketcli the iron-carbon diagram from Fig. 12.2. Superimpose on this diagram, the position of the eutectoid temperature and composition for alloys containing, titanium, molybdenum, tungsten, silicon, manganese, chromium, and nickel.

8. Define the term carbide former. Give several examples of carbide formers. Define alpha and gamma stabilizer. Give examples, why gamma stabilizers are important. 9. What are columnar crystals and how are they formed? What is the effect of hot working on the crystals? Why should the finishing temperature of hot working be just above the transformation temperature? 10. What is cold working? How docs it affect the structure and properties of hot worked steel? What happens to die structure and properties when cold

worked steel is reheated to 800 to 1,200°F.? 11. Define the terms, hypoeutectoid, hypereutectoid, and eutectoid as used in steel. Describe the difference that will occur when such steels are slow cooled from the austenite region. 12. Sketch the unit lattice cells for austenite and ferritc. Point out the differ-

ences. Which is the closest packed cell? Since carbon is an interstitial atom,

indicate where it fits in the austenite unit cell. 13. From the iron-carbon diagram in Fig. 12.2, point out the various reactions which occur. Indicate the reactants and products. 14. From the iron-carbon diagram, indicate the amount of pearlite present in alloys containg 0.3%, 0.8%, 1.2%, and 1.9% carbon. What is the other constituent in each of the alloys? 15. Cast iron is quite extensively used and is rather inexpensive as compared with cast steel. From the material presented in this chapter, give reasons to explain this situation. 10. Describe the changes that take place during the solidification and subsequent cooling of molten pure iron. Compare these charges with those that occur in copper. 17. Define the term ferrite strcngthener. Why are these important? Give examples of ferrite strengtheners. 18. Describe the formation of martensite. Upon what variables is martensite formation dependent?

19. When steel is cold worked, how arc the mechanical properties affected? De- scribe applications where cold worked steel is superior to annealed steel. 20. Describe the solidification of steel in a large ingot mold. Consider the differences which would occur at the surface and interior of the ingot. Fundamentals of Heat Treatment Steel CHAPTER of 13 Abram Eldrkd HoSTETTER, Ph.D., Professor of Metal- lurgy, Kansas State College, Manhattan, Kansas

JL HE process of quenching and tempering steel to improve its hardness and wear resistance has been used for at least

3,000 years. Some of the early tools and weapons testify to the skill of the early metal smiths. Little was known, however, regarding the nature of the changes taking place in the steel during heal treatment, and the suc-

cess of these early metal workers is a tribute to their patience and persist- ence in developing the art of heat treating. Medieval literature is filled with the magic heat-treating processes developed by these early iron workers, especially for the treatment of swords. The Spanish swords of Toledo, for example, gained world renown because of the supposedly mysterious properties of the water in which they were quenched; the famous swords of Damascus were heated to the color of the rising sun and were quenched by plunging them into the belly of a fat Nubian slave.

•A favorite quenching medium was urine, and this is understandable since we know that chloride brines and other salt solutions extract heat from red-hot steel 10 to 20 percent faster than water alone.

Today our approach to the heat treatment of steel is somewhat more scientific. Much has been learned about the complex reactions taking place during the heat treatment of steel and various factors such as alloy content, cooling rate, tempering temperature, etc. are much better understood today. Our knowledge of the structural changes taking place in metals is by no means complete, but with the knowledge we have, heat treatment processes can be prescribed to give desired properties with a minimum of "trial and error" procedure.

263 F .

264 Engineering Metallurgy

1650'

1530° K

0.6 0.8 Per cent carbon

Fig. 13.1. Schematic representation of the grain size of carbon steel as affected by heating. (Stoughton)

13.1. Grain Size and Grain Growth

There are two means by which the grain size of carbon steel can be changed. The first is by cold working the metal and annealing it at a temperature below the critical range. This is called process annealing. The grain size thus produced depends mainly on the extent of cold working and the annealing temperature. The second method is by heating the metal above the critical range where, as described in Chapter 12, austenite is produced. With the formation of austenite, recrystallization takes place, resulting in a fine grain structure (Fig. 13.1) In Fig. 13.1 the change in grain size with raising temperature is indicated by varying sizes of circles. If a coarse-grained hypoeutectoid steel is slowly heated, grain refinement starts at the lower critical range (1330°F.) and gradually proceeds throughout the critical range. Grain refinement is then complete at the upper critical temperature. This is not a reversible process however. That is, on cooling, the grain size does not revert back to the original large grain size, but remains the maximum size reached while above the critical range. On cooling, the austenite grains convert to ferrite and pearlite or martensite, depending on the cooling rate. In either case the grain size observed at room temperature reflects the prior austenitic grain size. In the case of eutectoid and hypereutectoid steels, grain refinement takes place at the eutectoid temperature as shown in Fig. 13.1. In case a steel has a fine grain at room Fundamentals of Heat Treatment of Steel 265

temperature there will be little or no change in grain size when it is heated through the critical range.

If a steel is heated to produce fine-grain austenite and then heating

is continued several hundred degrees higher, the fine grains of austenite grow to a larger size as shown in Fig. 13.1. Time also plays a part in determining the final grain size. At higher temperatures the grain growth is rapid, while at temperatures just above the transformation

range, there is little tendency for grain growth. The eventual grain size

of steel therefore depends on both the temperature to which it is heated

and the time it is held at this temperature.

The effect of temperature on grain growth is shown in Fig. 13.2. Two pieces of steel containing .40% carbon were heated above the critical range and cooled slowly. Sample A was then heated to 1550°F. (in the fine grain range Fig. 13.1) and held there for 30 minutes, and sample B was heated to 1900°F for 30 minutes. The samples were cooled slowly, well below the critical cooling rate, and as a result, both samples are soft and consist of grains of ferrite and pearlite. The grain size of sample B, however, is much larger then that of sample A because of the grain growth at 1900°F. When austenite is cooled to room temperature, the hardness and the resulting structure is dependent on the cooling rate, as indicated in Chapter 12. If cooling is slower than the critical cooling rate, pearlite would be produced, and if quenched faster than the critical cooling rate, martensite is produced. The grain size in both cases, however, is the same if they were heated to the same temperature. Thus the grain size depends on the temperature to which a steel is heated, but the hardness depends on the cooling rate and is quite independent of the grain size. Steel may be hard or soft, depending on the cooling rate, and be fine or coarse grained, depending on the temperature to which the steel was heated.

13.2. Controlling and Classifying Grain Size

When steel is gradually heated above the critical range, there is a certain range of temperature (Fig. 13.1) where there is little tendency for gram growth. Above this range, grain growth becomes more and more apparent as the temperature rises. It has been found, however, that by adding definite small amounts of aluminum or other deoxidizers to carbon and alloy steels, the tendency for grain growth at elevated temperatures can be controlled. That is, the fine-grain temperature range (Fig. 13.1) is broadened so that a carbon steel containing the proper amount of aluminum can be heated to 1830°F. and still remain fine 266 Engineering Metallurgy grained, whereas a similar steel without the aluminum would quickly become coarse grained at this temperature. The research work of McQuaid and Elm, along with other investigators, has served to emphasize the importance of grain size in the heat treatment of steel. For many years a coarse-grained structure was considered to be undesirable and every precaution was taken in hot working and heat treatment to avoid it as far as possible. This idea is now known to be wrong in some cases. Coarse-grained steels machine more easily and harden more deeply than fine-grained steels. On the other hand, fine- grained steels do not crack so readily when quenched and in general have greater toughness at low temperatures. The grain size of steel can be classified in terms of a standard scale established by the American Society for Testing Materials. The grain- per size class are numbered 1 to 8. In this system the number of grains square inch at 100X is 2"—1 where n is the grain-size numbers. Photo- graphs of each of the grain-size classes are available in metallurgical handbooks, and the grain size of a sample is readily obtained by comparing the grain size of these photographs with that of the sample when viewed or photographed at 100X. Table 13.1 lists the ASTM grain-size numbers and the corresponding grains per square inch. Three different grain sizes are shown in Fig. 13.3 as they appear at 100X.

Table 13.1. A.S.T.M. Grain-Size Numbers

Grains per Sq. In. at 100X A.S.T.M. Grain-Size Number Average Range

less 1 1 1 J/£ or 2 2 lj^to 3 3 4 3 to 6 4 8 6 to 12 5 16 12 to 24 6 32 24 to 48 7 64 48 to 96 8 128 96 or more

Another system for classifying grain size is the Shepherd fracture standards. This set consists of ten broken steel specimens graded from fine to coarse grain. The grain size of a sample is then determined by break- specimens. ing it and visually matching its grain size with one of standard facilities are not This method is especially useful where metallographic Fundamentals of Heat Treatment of Steel 267 MM

5&isi§k;i"i^jj

Fie. 135. Structure of 0.40 per cent carbon steel (A) heated just above the transformation temperature and slowly cooled and (B) overheated and slowly cooled; etched. IOOx-

available. The fracture standards are prepared by hardening and then breaking them. The sample being tested or compared to the standards must also be in the hardened condition to get a valid comparison. It must be remembered that the grain size referred to above the transformation temperature is austcnitic grain si/e. In most carbon steels, however, little austenite is retained at room temperature, except in highly alloyed steels, and at room temperature we are concerned with grain size of the transformation product on cooling (martensite, ferrite, pearlite,

bainite etc.) . By proper polishing and etching of martensite, the previous austenitic grain size is revealed since the grain size of the martensite follows that of the previous austenitic grain size. Steel in the martensitic

condition may also be notched and broken to display the grain size. The broken surface consists of many small facets. The facets follow the fracture planes through the grains and thus their size gives an indication of the martensitic grain size. On slow cooling hypoeutectoid steel, the austenitic grain size is reflected in the grain size of the ferrite and pearlite since the proeutectoid ferrite forms at the austenitic grain boundaries and the pearlite forms in the body of the parent austenitic grain as shown in Fig. 13.2b.

13.3. Effects of Hot Working on Grain Size

When steel is hot worked at temperatures around 2000°F., the grains are broken up into finer grains. This fragmenting process keeps the grain 268 Engineering Metallurgy

Fig. 13.3. Typical structure of slowly cooled carbon steel having (A) grain size 2: (B) grain size 5; and (C) grain size 8; etched. 100X-

size fine even though there is a great tendency for grain growth at this temperature. The two factors thus offset each other, and the grain structure remains fine during the hot working. If, however, the hot working

is stopped and the metal held at the hot working temperature for a short time, grain growth takes place rather fast, and after cooling to room temperature, the grain would be coarse. In this case the steel should be reheated to a temperature above the critical range in order to refine the grain. Hot working may be continued as the temperature falls to a point just above the critical range. In this case there is little chance for grain growth, and the grain remains fine. When steel is cast as an ingot, the grain size is very large and columnar. Hot working is employed to break up this coarse condition and produce a fine grain structure and thus greatly improve the physical properties. , ' T

Fundamentals of Heat Treatment of Steel 269

1600 1600. K 1400 1400 \

1200 vV" (200 \ f 1000 V K 1000 \CV si 800 K^ -T 800 \ Q- o. E E 1° 600 V * 600 \ x\ *) MS l) Ms\ N 400 X \ 400 \ \ \ \ 200 200 ;

Mf .— Mf

Time Time A B

Fig. 13.4. A. The TTT curve for a typical carbon steel. B. The TTT curve for an alloy steel. Cooling curve O represents the cooling rate on the outside of a bar. Cool- ing curve C represents the cooling rate in the center of a bar.

13.4. Hardenability

If a one-inch bar of carbon steel containing .70% carbon is quenched in brine, the outside of the bar will be hard (close to Rc65) while the center of the bar %vill be only Rc30 to 35. Such a steel is said to have low hardenability. The structure on the outside would be martensite because the cooling rate on the outside portion of the bar was faster than the critical cooling rate as shown in curve O, Fig. 13.4A. The structure of the center of the bar would be pearlite since here the cooling rate is much slower than the critical cooling rate as shown in curve C, Fig. 13.4A. How deep the outside

hardened shell will be depends upon the critical cooling rate since all of the steel that is cooled at the critical cooling rate or faster will be hard. If a one-inch bar of an alloy steel is quenched, as just described for a carbon steel, the bar will be hardened to the center, and only martensite would be observed. Such a steel has high hardenability (hardens deeply). The actual cooling rate of the two steels at a given distance from the surface is essentially the same, but the TTT curve for the alloy steel plots to the right as shown in Fig. 13.4B. In this case the cooling rate represented by curves O and C are both faster than the critical cooling rate because the TTT curves plots sufficiently far to the right. Thus a comparison of the relative position of the TTT curve for various steels on the time scale affords a quick qualitative means of comparing their 270 Engineering Metallurgy relative hardenability. The role of alloying elements in determining the position of the TTT curve has been covered in Chapter 12. There are applications in industry where only surface hardness is needed and high hardenability is not important. For such applications it usually is good economy to use carbon steels. For certain uses, however, such as aircraft parts, uniformly high strength and hardness is generally required through the part. In these cases it becomes necessary to use an alloy steel such as S.A.E. 4140 in order to have sufficient hardenability. When a steel has high hardenability, a slower quench (oil quench) can be used to obtain full hardness in a rather large section. Since this is

the case, it is unwise to use a brine quench because such a quench is much faster than necessary and greatly increases the chances of cracking the part. Such steels are referred to as oil hardening steels, and steels with low hardenability arc called water hardening steels.

13.5 Grain Size and Hardenability

When carbon steel is heated to a temperature just above the critical range and quenched in brine, very shallow hardness is obtained. If, how-

ever, the same steel is heated high enough to produce a coarse grain before

quenching, the hardness is much deeper. The increased depth of hardness indicates a slower critical cooling rate as explained earlier. 1 his would mean that an increase in grain sizes makes the transformation more slug-

gish, and transformation is slower in starting at a given temperature. The TTT curve is thus moved to the right when the grain size is increased,

which is the same effect observed when certain alloying elements are added to the steel. Because of the shallow hardening feature of fine-grained carbon steel, a somewhat coarser grain may be desirable even though coarsening of the grain may induce some brittleness. The actual grain size desired in carbon steels may therefore be a compromise of moderately fine grain in order to avoid the severe brittleness associated with a very coarse grain and not have the low hardenability associated with the very fine grain. In

alloy steels, however, this dilemna is not encountered since hardenability

is afforded by the proper alloying elements.

13.6. Quenching and Properties of Martensite

As discussed in chapter 12, increasing progressively the speed of cooling, up to a certain rate, lowers the A, transformation temperature progres-

sively from 1333° F. (723°C.) to approximately 1020°F. (550°C.) . While this increase in rate affects the speed with which the gamma iron trans-

forms, it does not affect the completeness of the transformation. The Fundamentals of Heat Treatment of Steel 271

increased cooling rate causes the pearlite lamella to become finer. If,

however, the cooling rate is increased still more, an important phenom-

enon occurs. The transformation temperature is suddenly depressed

to 480°F. (250°C.) or lower, and the steel, especially if it contains more than about 0.30 per cent carbon, becomes very hard and brittle. The rate of cooling that is just fast enough to suddenly depress the A^ tempera-

ture to 480°F. is known as the critical cooling rate and is attained by

quenching a high-carbon steel in water or an alloy steel in oil.

The changes taking place when high-carbon steel is cooled at the critical rate or faster are in general simple, although some of the details are not wholly understood. In brief the mechanism of quenching is as follows: At a temperature above 1333°F. (723°C.) , the structure is austenite, the solid solution of carbon in gamma iron. In cooling more slowly than the critical rate, the face-centered gamma iron changes to body-centered alpha iron and the carbon is thrown out of solution as iron carbide. If cooling is faster than the critical rate, the change of face- centered gamma iron to body-centered alpha iron is arrested and an intermediate crystal structure, the body-centered tetragonal, is formed. The carbon, instead of being wholly expelled from solution, is entrapped in the tetragonal lattice as a supersaturated solution. The result is a highly strained, very unstable crystal structure containing a multitude of entrapped submicroscopic particles. Steel fails under stress by slipping or gliding along crystallographic planes of relative weakness; these submicroscopic particles act as keys and help to prevent easy slip (Fig. 13.5). The result is much higher strength and hardness, accompanied, however, by increased brittleness. The sudden lowering of the transformation temperature to about 480°F. (250°C.) and the increased hardness and strength are accompanied by the formation of a structural constituent known as martensite, characterized by an angular, needlelike appearance (Fig. 13.6). The A B n -^CF :cc C^ <) J3T ±h =D= \=Q-r^^^6\ -Or Q.0=

Fie. 13.5. Schematic representation of a crystal containing (A) no hard panicles. (B) one large particle, and (C) many small particles. 272 Engineering Metallurgy needlelike formation, while common in rapidly cooled high-carbon steels of small cross section, is by no means universal. Martensite is a transition constituent; its composition, hardness, and appearance under the microscope depend primarily upon the amount of carbon, the size of the section, and the quenching treatment. Depending upon the carbon content, the strength of steels which are chiefly martensitic in structure may vary from 150,000 to more than 350,000 lb. per sq. in., and such steels may be as brittle as glass or may show considerable toughness. Metallurgists are generally agreed that the primary cause of the hardness of martensite (and drastically quenched medium- and high-carbon steels) is the precipitation of submicroscopic particles of carbon or iron carbide from the gamma solid solution and the retention of these particles as a supersaturated solution in the tetragonal alpha-iron lattice, where they act as a multidude of keys effectively preventing slip. Some authori- ties, thinking that this explanation does not account for all the hardness, have postulated lattice distortion, internal strain, and the fine grain size as important contributing causes.* The internal structure of martensite and

Fig. 13.6. Structure of martensite in high-carbon steel quenched in iced brine. (A) 500x: (B) 3,000x; etched.

• A complete and wholly satisfactory explanation of the hardness of martensite will not be available until more is known about the atomic forces that hold the crystal lattice together and about the nature and magnitude of the strains produced by an entrapped stranger atom in a lattice. Fundamentals of Heat Treatment of Steel 273 the causes of its hardness are of much interest to the physical metallurgist but are of only passing interest to the engineer who wants a basic knowledge of heat treatment. Far more important is a realization of the instability of structure that results from quenching.

13.7. The Instability of Quenched Carbon Steels

When carbon steel is cooled slowly through the transformation tem-

perature, all phase changes take place and the carbon is completely

precipitated as large particles of iron carbide (cementite) , resulting in a structure, laminated pearlite, that is very stable. If, however, the steel

is cooled very rapidly, phase changes are arrested before completion, the carbide is entrapped in the crystal lattice as exceedingly minute particles,

and the lattice is strained, resulting in a structure, martensite, which is very unstable.

As is characteristic of an extremely unstable condition, there is a very strong tendency for quenched steel to become more stable under the slightest provocation. Re- heating to 500°F. (260°C.) or above will give the atoms

enough mobility so that instability and the strained structural condition are relieved more or less completely, depending on the temperature and the time. The instability after

quenching is accompanied by high internal stresses, so high that frequently a small piece of water-quenched high-carbon steel, though undisturbed, will crack (Fig. 13.8) or will, occasionally, go to pieces with explosive violence hours or even days after quenching. To relieve these stresses and to produce at the same time a more Stable «™«ural condition, F.G. 13.7. Quenching cracks in the spline end of a shaft, natural size, etched. quenched steel is always re- ;

274 Engineering Metallurgy heated. This Is known as tempering or, colloquially and somewhat in- accurately, as drawing*

13.8. Retained Austenite and Cold Treatment

steel is When quenched, the cooling must be continued to the M a temperature before austenite will start to transform to martensite and before complete conversion takes place the temperature must lower to or below the M f temperature (Fig. 13.4). If cooling stops between these temperatures only part of the austenite transforms giving rise to what is called retained austenite. temperature The level of the M, and M t are both lowered by the addition of alloying elements and increase of carbon content as indicated in Chapter 12. If the M f temperature is below room temperature then quenching to room temperature will result in only part of the austenite being transformed and the remainder being retained as austenite. If however the steel is quenched and then cooled by refrigeration well below the M temperature the transformation of the austenite martensite can r to be made fairly complete.

Since martensite is much harder than austenite the hardness, strength and wear resistance of steel can be improved by eliminating the retained austenite by cooling the steel down to about minus 100°F. This can easily be done by placing dry ice in a container of acetone and using this as the cooling medium.

This kind of cold treatment is often used to reduce the retained austenite particularly in the case of tool steels where alloy content encourages retained austenite and hardness and wear resistance must be especially high or in steel gages where dimensional stability is important.

13.9. Structural and Other Changes in Tempering

When a drastically quenched high-carbon steel is reheated, two things happen: The unstable tetragonal crystal structure of the martensite transforms to the more stable body-centered cubic lattice of the ferrite, and at the same time internal stresses are relieved. To produce such changes, it is only necessary to reheat to about 200 to 400°F. (100 to 200°C.) for some steels the temperature of boiling water is sufficient. Tempering at these low temperatures produces only a very slight mobility of the

iron and carbon atoms, and it is likely that the only effect, in addition

to relief of stresses, is the escape of the carbon atoms from the tetragonal lattice as it changes to body-centered cubic. As a result low-temperature

• The terra drawing for tempering originated from the old blacksmith's expression "drawing the temper." Drawing in its correct sense means pulling a wire through a die to reduce the cross section. .

Fundamentals of Heat Treatment of Steel 275 tempering produces no appreciable change in hardness. Tempering at

400°F. (200°C.) or lower is, therefore, commonly used for quenched high-carbon tool steels which must be very hard but should be free from internal stresses. When tempering medium and high carbon steels in this low temperature range of 180 to 480°F., it has been established that some sort of transition carbide (not Fe3 C) is precipitated. The composition of this carbide seems to have the composition approaching Fe2C or Fe^C.,. As the tempering temperature goes higher this first carbide is absorbed and re- precipitated as cementitc (Fe3 C)

When drastically quenched carbon steel is tempered at 500°F. (260°C.) or higher, the particle size of the iron carbide increases. How far this return to stability proceeds depends upon the tempering temperature and to a lesser extent upon time. Most of the growth of carbide particles at any definite temperature occurs in the first few minutes; the reaction then slows down but continues for some time. In practical heat treatment the time for tempering is rarely less than 30 min. for each inch of cross section.

As the carbide particles grow larger, it follows that there will be fewer

of them to act as minute keys to prevent slip (see Fig. 13.4) . Hence, hardness and strength decrease, and ductility, as measured by elongation and reduction of area, increases. The higher the tempering temperature, the larger the carbide particles and the more stable the resulting structure.

Finally, when the tempering temperature is within a few degrees of the A, transformation temperature, the cementite has grown to particles so

Fie. 1S.8. Structure of high-carbon steel quenched in ice water and tempered (A) at 575-F. (300°C.) and (B) at 1100 o F. (595°C); etched 500x- 276 Engineering Metallurgy large that they are readily visible with the microscope at low magnifica- tions, and the properties are practically the same as those of a steel of corresponding carbon content cooled slowly through the A x temperature. Structural changes in tempering can be followed by microscopic examination of polished and etched specimens but not so closely as by dimensional changes and other methods. Martensite which has a struc-

ture similar to that shown in Fig. 13.6 is white after etching, that is, etches slowly. Tempering at 575°F. (300°C.) produces, for the same degree of etching, a dark, almost black, structure indicating precipitation and considerable growth of the carbide particles. After tempering at

1100°F. (595°C.) , the particles have increased in size so that they are visible in the microstrucfure. These two stages are shown in Fig. 13.8.

Tf quenched steel contains retained austenite and is not cold treated to transform the retained austenite to martensite, tempering is somewhat more complex. The martensite will go through the various stages of tempering just described. However, the retained austenite will, at about

450°F. to 500°F., transform to a dark etching bainite, %vhich is quite hard, and may result in a little increase in hardness.

QUESTIONS

1. What happens to the structure of a small piece of .8% carbon steel con-

taining a mixture of large and small grains when it is subjected to the following treatments: (a) heated slowly to about 1400°F., held 80 min., and cooled slowly to room temperature; (b) heated slowly to 2000°F., held 30 min., and cooled slowly to room temperature; and (c) heated slowly to 2000°F., held 2 hr., and cooled slowly to room temperature? 2. Define grain-growth tendency. Compare grain-sue control by submicroscopic particles and by controlling the heating above the critical. Why is the former method preferable?

3. What ,is critical cooling rate? What is its effect on the austenite-pearlite transformation and on the resulting structure of high-carbon steel? Is a cooling rate of 600°F. per sec. more or less than the critical cooling rate for eutectoid steel.

4. When a high-carbon steel is cooled faster than at the critical rate, what

is the effect on the crystal structure, the appearance under the microscope,

and the properties? What is martensite, and why is it hard? 5. Why is high-carbon steel that is drastically quenched unstable? What undesirable conditions may accompany this instability and how may they be avoided? 6. Describe the changes in structure and properties that take place when

drastically quenched steel is reheated to (a) 350°F.; (b) 600°F.; and (c) 1200°F. 7. What effects have the alloy elements on the temperature at which austenite transforms to pearlite? How are these effects related to heat-treatment? Fundamentals of Heat Treatment of Steel 277

8. Name the three principal variables affecting the hardness of quenched steel. If other factors are constant, which will be the harder one as quenched: (a) steel containing 0.30 or 0.60 per cent carbon? (b) steel containing 0.60 or 0.80 per cent carbon? 9. Define hardenability. What is its relation to grain size and to the alloying elements present? Distinguish between shallow-hardening and deep-hardening steels. 10. In what classes of steel is deep hardening important? Why? 11. Describe process annealing. 12. How does grain size effect hardenability? 15. What factors determine the grain size of steel after process annealing? 14. What is the relationship between grain size and toughness? 15. What are two standards for classifying grain size? 16. Describe the effects of hot working on grain size. 17. What factors determine the amount of retained austenite after quenching? The Operations of Heat Treatment CHAPTER

14 Omar C. Moore, M.S., Associate Professor of Chemical Engineering, Alabama Polytechnic Institute, Auburn, Alabama Robert E. SHAFFER, M.S., Associate Professor of En- gineering, University of Buffalo, Buffalo, New York

1 HE value of heat treatment to mankind, emphasized in previous chapters, has little relation to the proportion of the world's steel tonnage that is treated. This tonnage amounts, in the United States, to possibly two or three million tons annually, certainly less than 5 per cent of the total amount of steel used. Heat treatment is most valuable for tools and dies, which are in turn a very small proportion of heat-treated steel.

Heat treatment is ordinarily an expensive operation, because costly furnaces, quenching baths, apparatus for controlling the atmosphere in the furnace, and temperature-recording and -controlling equipment are necessary. Some treatments cost as much or more than the steel itself, but this is no criterion of the value of the operation. Moreover, the cost of steel and of treatment is usually a very small part of the cost of the finished article and is even a smaller proportion of the value of the article to industry. One example will indicate this. The steel in a die may cost $2 and the heat treatment may increase this to $4. On this $4 die, the machine work frequently costs $100, but the completed die will turn out thousands of pieces of a finished product at a cost which is a very small fraction per piece of the cost of a similar product turned out, one by one, by hand. Owing to the economic value of a properly heat-treated article and to the customary expense of the machine work to prepare this article for use, it is poor economy on the part of the engineer to take chances on the quality of the steel or the heat treatment so that a few cents may be

278 The Operation of Heat Treatment 279

saved. It is, furthermore, poor practice for the engineer to design expensive tools, dies, or other parts without requesting the metallurgist to advise whether the finished article will or will not survive the heat treatment satisfactorily. Thousands of dollars are wasted every year because tools and other articles, which cost much to machine, crack in quenching. Such waste can usually be prevented by the intelligent selection of the proper steel and by avoiding, in the design, sharp corners or reentrant

angles where stresses concentrate. This factor is discussed in some detail later in this chapter.

The operations of heat treatment discussed in this chapter are those used for carbon steels, most alloy steels, and some nonferrous alloys. Modifications of these operations—used for a few high-alloy steels, for certain other iron alloys, and for a number of nonferrous alloys—are described in the chapters where these materials are discussed or in the chapter on precipitation hardening.

14.1. Heating Cycle

Uniform and consistent results from any cooling cycle can be obtained if the proper care has been taken in the heating of the steel prior to the method of cooling. The steel must be heated slowly to the proper temperature range and held at this temperature long enough for homogenization of the austenitic structure. The temperature used and time required are functions of the chemistry of the Color Orange- steel, section size and shape, prior grain Salmon 900 size, original condition of the steel, prior Bright- 850 heat treatments, mechanical working. Red or boo The usual time requirement at heat for Cherry — 750 homogenization of the austenite for a TOO Medium\ forging or casting, is one hour per inch Cherry ] 650 of section thickness. This is not a firm Dork l Cherry ) 0.40 0.80 120 1.60 rule and is frequently neglected. How- Percentage Carbon ever, adequate time at heat must be

FlC. 14,1. Temperature ranges provided for solution of carbides, and for the heal, treatment of carbon also, if uniformity of product is to be steels. (Metah Handbook) achieved upon cooling for the diffusion of carbon and alloying elements from areas of high concentration into the lean areas.

Modern heat treating equipment should provide some means of surface protection for the steel during the heating cycle. The use of con-

trolled atmospheres in heat treating furnaces is becoming standard prac- 280 Engineering Metallurgy

tice. If during heating the steel is exposed to combustion gases or air in the furnace, it will oxidi/e, dccarburize, or both. An oxidized surface is easily detected and can be removed by machining or grinding. Decarburi- zation does not show and will result in low surface hardness and poor mechanical properties if not removed. Protective atmospheres are provided by several different methods and a number of different gases are used, including hydrogen, nitrogen, carbon dioxide, and sulfur dioxide.* With the proper selection and composition control of the furnace atmosphere it is possible to produce heat treated parts free of scale and decarburization.

14.2. Annealing

Process annealing is the recrystallization of a cold-worked steel, by heating below the A t transformation temperature, usually in the range between 800° to 1200°F., the exact temperature being governed by the amount of cold working, composition, grain size, and time held at heat. Heating to the proper temperature affects complete recrystallization and restores the original structure and properties. Too high a temperature or too long time at heat may produce a coarsening of the grain size with a subsequent loss of toughness, hardness, and strength.

For full annealing, the steel is heated slowly to from 50° to 100°F. above the A3 transformation (Fig. 14.1) , held at this temperature until uniformly heated, and then slow cooled to room temperature. The cooling may be done in the furnace, or in an outside pit covering the steel with asbestos, sand, lime or cinders. The rate of cooling is in the range of 5° to 20°F. drop per minute. The objects of annealing are to soften the steel for machining, to refine the grain structure, to relieve stresses from cooling or machining, and to increase ductility. In hypoeutectoid steels it produces the poorest strength and wear resistance of any structure except spheroidizing. An- nealing is commonly used for castings and forgings to reduce the coarse grain structure prior to machining.

14.3. Normalizing

steel is The heated slowly at least 100°F. above the As or Acm transformation (Fig. 14.1) for the proper time, and then cooled in still air. The cooling rate can be varied by changing volume of air or air

• Discussion of controlled atmospheres and equipment is outside the scope of this book. For a more complete description see: American Society for Metals, Metals Hand- book. The Society Cleveland, 1948. The Operation of Heat Treatment 281

temperature. The normal rate of cooling is the range of 120° to 200°F. per minute, depending on section size and air conditions.

Normalizing of low carbon content steels is usually done for stress relief, grain size refinement, to improve toughness and to increase strength and ductility. Steels containing over 1% carbon, slowly cooled from high temperatures, may have the excess carbides forming in a network around the grains of pearlite. This carbide network is stable and unaffected by reheating to the annealing and hardening temperatures (Fig. 14.1). This carbide formation will increase machining costs and produces an undesirable condition in the heat treated part. By heating above the Acm temperature and normalizing, insufficient time is available for the excess carbide to form a network around the grains or to form large plates in the pearlitic structure. Some finished parts, castings, forgings, and rolled shapes to be used after a normalizing treatment should be given a temper to relieve cooling stresses resulting from non-uniformity of cross section. This temper will usually give a noticeable improvement in ductility with little or no change in hardness or strength.

14.4. Spheroidizing

The spheroidized structure, ferrite and carbides, is usually produced by prolonged heating, 3 to 8 hours at a temperature slightly above or below the A! or A3 2 , transformation. The prolonged heating causes the carbides to coalesce into spheres, completely destroying the pearlitic formation and resulting in a matrix of ferrite with the carbon in the form of spheroidal carbides. The time for spheroidizing is usually lessened by a normalizing treatment prior to spheroidizing. In medium and high carbon steels the spheroidized structure has the lowest hardness, strength and wear resistance, but the best machineability, toughness, and ductility.

Though the spheroidized structure is desirable for machining and forming, it may cause some difficulty in heat treatment. The carbide material, if in large spheres, may not go into solution when a simple heating and quenching cycle is used, resulting in spotty and low hardness. It is desirable to give steels of this type a normalizing treatment prior to the hardening cycle to help break up and to dissolve the excess carbide areas. A similar structure of a ferritic matrix with spheres of undissolved carbides will result if a martensitic structure is tempered at a high temperature 1200— 1250°F. This method of producing the spheroidized structure is not recommended because of the problems involved in quenching higher carbon steels. 282 Engineering Metallurgy

14.5. Quenching for Hardening

For hardening, the sieel is slowly heated above the A. t transformation

range (Fig. 14.1). The hardening temperature range for carbon steels is

the same as the full annealing range. The steel is usually held at heat

long enough to insure uniform heat distribution in all sections. It is then removed from the furnace and immediately quenched in the cooling medium.

In carbon steels it is generally true that the faster the heat is extracted, the harder, stronger, and more wear resistant the finished part will be. High hardness is frequently accompanied by excessive brittleness. Water quenched high-carbon steels will often crack or shatter if dropped, or if allowed to stand overnight before tempering.

The common quenching media are air, oil, water, and brine solutions.

Selection of the proper quenching medium is based on the chemistry of the steel, section size and shape, and the final mechanical and physical properties required. Air has the slowest cooling rate, and brine solutions the fastest rate. The cooling rate of a quenching medium will vary with the amount of agitation, temperature, and composition. Water at 65°F. is considered to have a cooling rate of one; brine solutions have as much as twice the cooling rate of water; oils vary from 15% to 50% of the cool- in rate of water.*

Hardening of a steel is usually required because of the demand for the maximum of one or more of the mechanical or physical properties that the steel is capable of assuming. Sometimes because of size, shape, variation in cross section, or some production problem, a compromise must be maele. If this must be done, a careful study of the different cooling media should be made in order to select the one best suited to produce the best compromise of mechanical and physical properties.

For lower carbon steels, water is usually used because of its low cost, easiness of handling, and availability. In medium and high carbon steels, oil is often used because it produces less warpagc, distortion, and cracking than with a water quench. However, by using an oil quench instead of water, maximum hardness, strength, and wear resistance may not be obtained.

When the heated steel is quenched in a liquid, there are two distinct steps or stages of heat removal that are passed through before reaching the temperature of the quenching medium. Step One: The vaporization period. In this period of the cooling cycle the quenching medium will vaporize when it comes in contact with the

• American Society for Metals Handbook. The Society, Cleveland, 1948. pp. 616-619. The Operation of Heat Treatment 283

steel. If the vapor is allowed to form a film around the part being quenched, the cooling rate is slow because the vapor film insulates and blocks off contact between the cooling liquid and the steel. By breaking up the vapor film and allowing the liquid to vaporize and be carried away,

the fastest cooling rate is achieved. The breaking up of this vapor film is accomplished by proper agitation or circulation of the quenching liquid or by agitation of the parts being quenched, or both.

Step Two: The liquid period. After the quenched pari has lost suffi-

cient heat so that the liquid is no longer vaporized upon contact, the

removal of heat is now a simple problem of heat transfer and the rate of

heat loss is relatively fast as compared with the vapor cycle.

The difficult and critical period in quenching is the elimination of the vapor film. Prolonging this step may result in unsatisfactory hardening. The part may have areas which arc low in mechanical properties, or

the entire microstructure may be unsatisfactory and fail to produce the required mechanical properties.

The internal stresses introduced by quenching are the result of (1) dimensional changes in cooling, (2) large temperature gradients between

surface and center, and (3) the change in volume accompanying the

change from gamma to alpha iron. If the steel is ductile, these changes

cause slight plastic flow of the grains which relieves the stress. In high- carbon steels where no plastic deformation takes place these stresses may build up to a high value.

Stress distribution in quenching is complex and difficult to analyze

accurately. In many instances, it is certain that the sum of the induced

stress and the working stress is near or above the tensile strength of the steel. Thus, it was found* that in hollow cylinders the stress that was introduced by quenching at the inner surface of the hole was about 150,000 lb. per sq. in. As the working stress on these cylinders was 100,000 lb. per sq. in., the total stress was 250,000 lb. per sq. in. or close to the actual tensile strength of the steel.

Internal stresses may affect the strength and ductility and frequently

cause warping and cracking. The magnitude of these stresses is dependent on the size and shape of the piece, the kind of steel, the quenching temperature, and the coolant. When the stress builds up to a value higher than the tensile strength, the steel cracks. There are two types

of such defects: (1) local cracks, which are usually external and start

at sharp comers or deep tool marks, and (2) internal cracks, which

follow the major axis of the piece. Quenching in water is more likely to

• O. V. Greene, Trans. Am Soc. Steel Treat., v. 18, 1930, pp. 369-403. 284 Engineering Metallurgy cause cracking than quenching in oil or air. Coarse-grained steels crack more readily than fine-grained material. Localized heating in grinding frequently causes cracking, as does increasing the temperature from which the steel is quenched.

The best procedures in avoiding quenching cracks are as follows: (1) use generous fillets where the contour changes and avoid tool marks which may act as a notch; (2) avoid rapid heating, especially through the transformation range, which may cause cracking because of the uneven expansion; (3) temper immediately after quenching; and (4) quench in oil instead of water wherever possible.

14.6. Tempering

The previous discussions of the principles underlying quenching included details of structural changes and related changes in properties. Some of these changes in physical properties are often desirable, particularly where hardness and wear-resistance are of importance. However, this increase in hardness, strength and wear-resistance is at the expense of ductility because, in most instances, the brittleness has also increased. The extent of brittleness, coupled with hardness, etc., depends on the severity of quench and the percentage of carbon in the steel. Tempering lowers the hardness and strength and decreases wear-resistance; it also increases ductility.

The purpose of tempering is not to decrease hardness but rather to restore ductility and improve toughness. A way of doing this and at the same time retaining the original hardness and strength would be desirable. However, since tempering does decrease the hardness and strength, but at the same time restores ductility, it would be well to consider what is desirable in the way of strength. Generally a structural part is not subjected to deformation until it breaks; it is the stress capacity without permanent deformation which is important. Tempering should be carried out at the highest temperature that will insure retention of sufficient strength so that the part can withstand loads comparable to its yield strength. The higher the temperature, the tougher the product, but this will necessarily be at the expense of hardness and strength.

For tempering, the hardened steel is reheated in a furnace, in heavy oil, in molten salts, or in molten lead, and held long enough for the heat to penetrate to the center. For most steels, the speed ol cooling after tempering is of little consequence, provided the steel was not heated to the transformation temperature. In practice, the cooling after tempering is usually in the air. .

The Operation of Heat Treatment 285

The metallographic structure of tempered steel will be further discussed in Section 14.7, where tempering is compared to other heat treatment processes.

14.7. Isothermal Treatments

One result of the study of the subcritical transformation of austenite, which also led to the S. curve, by Bain and his associates* was the discovery of austempering, a process of interrupted quenching whereby high-carbon and low-alloy steels of suitable composition and cross section can be hardened without cooling to atmospheric temperature, thus eliminating the possibility of introducing high quenching stresses with possible distortion or cracking. The process cannot be used if a very hard martensitic structure, as obtained by water quenching high-carbon steels and

tempering at 200 to 300°F. (100 to 150°C), is desired; but it may be used to obtain a structure corresponding to tempered martensite of any required degree of fineness, such as is usually attained by cooling rapidly to room temperature followed by reheating to 400°F. (200°C.) or above. At any temperature below the A\ transformation and above the temperature—about 400°F. (200°C.) —where martensite is formed, the austenite transforms to pearlitej of definite structure at a definite rate. If, therefore, small specimens of high-carbon steel are heated above the transformation temperature and are quenched very rapidly into a bath maintained at any intermediate temperature, for example, 500 to 800°F.

(260 to 425°C.) , and if the specimen is held in this bath for a definite time, the transformation of the retained austenite and the resulting structure may be controlled closely. Steels so treated are more ductile than similar steels cooled rapidly to room temperature and tempered at a temperature that produces a corresponding structure. The hardness and strength of the austempered steel are about the same as if the steel had been quenched and tempered in the usual manner, but elongation, reduction of area, and impact resistance are usually higher. The explanation advanced for the higher ductility in the austempered steel is its freedom from submicroscopic cracks that are caused by drastic quenching (to form martensite) and not subsequently healed by tempering.

A schematic representation of austempering is shown in Fig. 14. 2. t To prevent any transformation of the unstable austenite to pearlite,. the specimen must be small enough to cool past the nose of the S curve very rapidly; for the steel whose diagram is given in Fig. 14.2, it must cool

•E. C. Bain and E. S. Davenport, U.S. Patent 1.924,099, Aug. 29, 1933. f Based on diagram by F. J. McMulkin, Iron Age, v. 157, June 27, 1946, p. 58. % (or Bainite) 286 Engineering Metallurgy

1400

I30C

1200

MOO

-11000

900,

800"°.

700 | "E 600

500

400

300

200 Mostly \morfensife

I5min.30min. Ihr. Bhr. Icty . 100

I I 1000 10,000 100.000 Time, sec.

Fig. 14.2. Schematic T-T-T diagram for carbon steel showing critical-cooling-rate curve, MB temperature, and temperatures for austempering and martcrnpering. (Mc- Mulkin)

through the temperature range of 900 to 1000°F. (480 to 540°C.) in about

1 sec. The specimen is therefore quenched rapidly into a bath maintained at the desired temperature-600°F. (315°C.) in Fig. 14.2-and held in the bath until the transformation to bainite (emulsified ferrite and cementite) is complete, that is, for about 45 min. Structural changes are now complete, and the specimen may be cooled, at any rate, to room temperature. A process that carries the principles underlying the subcritical decomposition of austenite farther than austempering is martempering, which was developed a few years ago by Shepherd.* In this process the

* B. F. Shepherd has described martempering in a number of papers. A lucid description, which also includes an elementary discussion of the transformation of austenite to martensite in quenching and isothermally, as mapped by the S curve, is contained in two arUcles by Shepherd in Production Eng., July, 1945, p. 438, and Aug., 1945, p. 515. The Operation of Heat Treatment 287

steel is quenched at its critical rate or faster—so that it will pass the nose of the S curve without any transformation of the austenite—into a bath maintained at a temperature slightly above the Ms point, in other words just above the temperature where martensite starts to form. This temperature varies with carbon content and slightly with the amount of alloying elements present, but for high-carbon and many alloy steels it is approximately 400°F. (200°C.) . The steel is held in this bath just long enough for the temperature to become uniform throughout the cross section, after which it is removed and cooled in air. A schematic representation of martempering is shown in Fig. 14.2. The specimen is quenched at the critical cooling rate or faster into a bath maintained at about 450°F. (230°C.) and held in this bath for a sufficient time to be heated through (about 1 min. for a small specimen of the steel shown

in Fig. 14.2) . It should be noted that the specimen should not be heated so long that the line where bainite begins to form is crossed by the time axis at this tempering.

Martcm|>ering is based on the principle that, if austenite is retained by rapid cooling down to the temperature where martensite begins to form, the austenite-martensite transformation will proceed, regardless of the cooling rate from this temperature down to atmospheric temperature; and that on cooling slowly, as in air, from about 400°F. full hardness will result without the concomitant high internal quenching stresses which are characteristic of drastically quenched high-carbon or alloy steels. Ac- cording to Shepherd, this treatment improves the mechanical properties, especially ductility and toughness. The method can be used for heavy sections and for pieces of irregular shape which often crack in the usual hardening operation.

Martempering resembles austempering only in that the quenching is

interrupted at an intermediate temperature; but the steel is not held at this temperature long enough for any appreciable amount of the austenite to transform. In martempering, therefore, the austenite transforms to hard martensite, while the steel is cooling slowly from this intermediate

temperature to atmospheric temperature; in austempering the steel is held at the intermediate temperature long enough for the austenite to transform to a ferrite-carbide aggregate of desired structure and properties.

Patenting is a very old process used to heat treat medium- and high- carbon rods which are to be cold drawn into high-strength wire. The rods or the wire (if patenting is used as an intermediate stage in drawing coarse wire to finer sizes) are heated to a temperature considerably above

the A 3 transformation temperature, so that some grain growth takes place, 288 Engineering Metallurgy and cooled in air or quenched in a bath of molten lead maintained at

850 to 950°F. (455 to 510°C). The object of patenting is to produce a fairly coarse austenitic grain that transforms in the air or at the lead-bath temperature to large pearlite grains which are made up of fine ferrite and cementite. Such a combination of controlled overheating and finely dispersed carbide and ferrite in the pearlite is essential for the steel to withstand the cold work to which the wire is subjected in drafting and is also essential if satisfactory quality is to be secured. Patented and cold-drawn wire for use in cables in suspension bridges is fairly high in carbon and has a final strength of around 225,000 lb per sq. in. and an elongation of about 2 to 6 per cent in 10 in. Patented and cold-drawn wire is the only suitable material found thus far for suspension-bridge cables. A high-carbon wire, cold drawn and then quenched and tempered so that it has the same mechanical properties, is unsatisfactory. The reason for this is not clear, but the first attempt to use heat-treated wire instead of patented and cold-drawn material ended so disastrously that it is doubtful whether it will ever be tried again." Patented and cold-drawn wire has been used for suspension- bridge cables for many years, and failures are unknown. The most that can be said is that apparently the equiaxed structure of the heat-treated wire does not resist so well as the elongated "fibrous" structure of the cold-drawn material the propagation of minute cracks on the surface, formed when the wire is bent around the anchorage shoes. Some emphasis should be given to the advantages of austempering and martempering as compared to tempering a martensitic steel. A tougher, more shock-resistant steel is obtained in austempering or martempering. Tempering involves the decomposition of martensite to granular mixtures of cementite and ferrite in which the cementite particles tend to be round rather than lamellar as in pearlite. Temperature and time,, primarily temperature, are important factors, along with the amounts of alloying elements in the case of alloy steel, in determining the size of the separated carbide particles. At very low tempering temperatures the resulting decomposition products are hardly discernible under the microscope. Tempering at progressively higher temperatures results in microstructures that can be resolved under the microscope; the carbide par-

• The wire used in the construction of the Mt. Hope Bridge, at Providence, and the Ambassador Bridge, at Detroit, both ot which were suspension bridges, contained 0.80 per cent carbon and was oil quenched and tempered. Although the mechanical properties were satisfactory, the wires began to fail soon after the cables were spun; the bridges had to be dismantled and rebuilt with the usual patented and cold-drawn wires in the cables. See E. E. Thum, Metal Progress, v. 21, 1932, No. 6; v. 22, 1932, No*. 1, 2, and 3. The Operation of Heal Treatment 289 tides are progressively larger, and eacli micro-structure is softer and tougher than the preceding one. The structure in which the divorced cementite, embedded in the ferrite matrix, can be seen with moderate magnification is termed spheroidite. In the practice of austempering, the formation of martensite structure is avoided by suddenly cooling the heated steel to a predetermined temperature at which only bainite forms and by holding it there for sufficient time to complete the transformation of austentite to bainite. Thus a product having ductility (toughness) comparable to that of a tempered steel is obtained in a single operation and without subjecting it to the quenching necessary to produce martensite. Martempering, which produces a hard martensitic structure, avoids the high internal quenching stresses produced in a fully hardened steel by drastic quenching. Attention should be called to the commercial limitations of austempering and martempering. In practice, small section sizes (plain steel) inch respond best to the above method because of the necessity up to y4 of suppressing the austenite-to-pearlite transformation. The maximum size that can be effectively treated, though, is determined by the composition and characteristic S-curve of the alloy.

14.8. Case Hardening Processes

There are many important industrial products, such as gears, camshafts, piston pins, and the like, which should have a hard, wear-resistant surface and a tough core. Carburizing is the most widely used method of securing such a combination of properties. In this process, unalloyed low-carbon or low-carbon alloy steel is heated in contact with a carbon- aceous material, from which the steel absorbs carbon. The depth of absorption depends on temperature, time, the alloying element present

(if any) , and the carburizing compound. With a commercial compound, a case %„ in. deep is obtained by heating for 8 hr. at 1700°F. (925°C). Two general methods are in use: pack carburizing and gas carburizing. In the former, the steel article, fully machined and finished except for a small allowance for grinding, is packed, together with the compound, in a heat-resistant alloy box. The carburi/ing compound is coke or charcoal mixed with an "energizer," which is usually barium carbonate. At the carburizing temperature, the barium carbonate dissociates into barium oxide and carbon dioxide; the latter reacts with the carbon in the charcoal to form carbon monoxide gas (CO a + C = 2CX)) . This gas reacts with the iron at the surface of the steel to form iron carbide. By diffusion, the carbon then penetrates below the surface. In the other method, used 290 Engineering Metallurgy

Fie. 14.3. Structure of slowly cooled carburized steel (A) at the surface and (B) in the transition zone between surface and core. The structure of the low-carbon core is shown at the bottom of B; etched. 100x-

chiefly for small articles, the steel is heated in a retort furnace into which a gas composed of hydrocarbons or carbon monoxide is introduced. This method is more rapid than pack carburizing, and the case depth can be more closely controlled. After carburizing for a sufficient time at the proper temperature (both of which have been standardized) , the carbon at the surface varies from about 0.8 to about 1.1 per cent. This grades off uniformly with increasing distance from the surface, s/ until at a depth of >/4 or A in. the structure is unaffected. Typical structures of slowly cooled carburized steel, at the surface (A) and in the transition zone (B) , are shown in Fig. 14.3.

Carburized steel is always heat treated to produce the required hardness of the case and usually to improve the ductility of the core, which may be too coarse-grained from the long heating at high temperature.*

After heat treatment, the case of carburized steel articles is martensitic in structure and very hard, and the core has the ductility of low-carbon

•Sec Metals Handbook, 1948, pp. 681-G85, for a detailed description. See also S. Epstein, The Alloys of Iron and Carbon, Vol. I, Constitution, McGraw-Hill Book Company, Inc., New York, pp. 335-340, for a discussion of principles. The Operation of Heat Treatment 291 steels. Even greater toughness and shock resistance, combined with a hard surface, can be secured by carburizing a low-carbon alloy steel, especially one containing 3 to 5 per cent nickel. carburiz- Austenite grain size is important in carburizing steels. After ing and heat treatment, a coarse-grained material has a uniformly hard surface, but the core may not have such high impact resistance as is generally desired. Fine-grained steels, on the contrary, are prone to carburize irregularly and develop soft spots on the surface after heat treatment. These conditions can frequently be avoided by special quenching techniques, most of which use a spray of water under pressure, or by the use of alloy steel. Nitriding, an established process for producing a hard surface on steel, uses nitrogen as the hardening agent instead of carbon. It was in- vented by Adolph Fry in Germany about thirty years ago and is now used widely in applications where a superhard surface combined with considerable corrosion resistance and resistance to softening at elevated temperatures is desired. It is used by the automotive and aircraft industries for valve seats and for guides, gears, and piston pins of internal-combustion engines and in a number of applications in steam plants and oil refineries.

The process consists of heating the finish-machined article in a closed

container into which ammonia gas is forced under pressure. At a temperature of 850 to 1000°F. (450 to 540°G.) the ammonia is mostly dissociated into nitrogen and hydrogen. The former combines with the iron and other elements in the steel to form complex nitrides which diffuse into

the surface layers. This diffusion is slow; a case varying in depth from

0.0005 to 0.02 in. is obtained after 5 to 25 hr.

Unalloyed carbon steels cannot be nitrided successfully. Some alloying element which forms a stable nitride must be present. The steels used for nitriding, therefore, contain chromium, aluminum, molybdenum, and vanadium in various combinations. Aluminum and chromium are the effective elements in producing high hardness, while molybdenum and vanadium increase the toughness of the steel and also increase the depth of penetration of the nitrogen. Nitriding steels are usually heat treated before nitriding to obtain the desired mechanical properties, which are not affected by the treatment with ammonia, provided that the steels were tempered prior to nitriding at the same or higher temperatures than used during the treatment with ammonia.

The outstanding advantage of nitriding is high surface hardness and wear resistance. This surface hardness cannot be equaled by any other method known to ferrous metallurgy. It averages a Vickers hardness 292 Engineering Metallurgy

number of 1200 as compared with 650 to 700 for quenched high-carbon steel of maximum hardness.

Another advantage is that no subsequent heat treatment is necessary, as it is for carburized steel. Nitriding is, however, accompanied by an increase in the size of the section. This growth is dependent upon the time and temperature of the operation but is constant for a given set of conditions. It averages 0.001 to 0.002 in. for the usual temperatures and for nitriding periods of 15 to 20 hr. To allow for this growth, parts that are to be nitrided are machined so that they are undersize by the amount of the expected growth. If it is desired to prevent the absorption of nitrogen in certain areas, these are plated with nickel or coated with tin. Nitriding is a relatively costly process. In the first place, special alloy steels are necessary; in the second, nitriding equipment is expensive. A nitrided camshaft, for example, costs from 5 to 15 times as much as a comparable article which has been carburized and heat treated. Since the cost depends largely upon the quantity treated, an appreciable reduction in cost would result if the demand for nitrided steel would permit large- scale production. Cyaniding is another method used to produce case hardening. This process uses molten sodium cyanide, sodium chloride, and sodium carbonate as the bath in which the steel is immersed. The case produced is very thin, seldom over 0.002 of an inch, and the temperature required is from 1300 to 1600°F. (700 to 870°C). modification A of the cyaniding process is to bubble ammonia gas through the molten salt bath, thus liberating nitrogen faster and increasing the nitrogen content of the case. This modified process is termed chapmanizing.

Carbonitriding involves both carburizing and nitriding in a single operation by heating the steel (1 200-1 600°F) in a carburizing gas (CH GjH etc.) 4 , 8 , containing ammonia. This process has gained wide acclaim and many steel parts are being case-hardened by this method today. Lower temperature processing results in a case similar to a nitrided steel. Higher temperatures produce a case like a carburized steel. Nitrided cases are very stable; the hardness is unaffected by long heating at temperatures as high as 750 to 850°F. (400 to 455°C.) and is not affected by heating for short periods at temperatures as high as 1000 to C 1 100°F. (540 to 595 C.) . The surface is resistant to such corrosive agents as the atmosphere, alkalies, crude oil, tap water, salt water except when it is moving, ethyl gasoline, and many others. It is, however, not resistant to mineral acids. It has the highest wear resistance of any ferrous material The Operation of Heat Treatment 293

and is finding increasing use in applications where this property is of prime importance.

14.9. Flame Hardening and Induction Hardening

There are a number of applications for steel articles that should be hard only in a relatively small area on the surface. For such uses, it is not economical to heat treat a section weighing 100 lb. or more by the usual method of heating in a furnace followed by quenching and tempering, or by carburizing to harden the whole surface. In some cases, heat treating a complex section by the usual method causes too much distortion. Also, it is not generally economical to operate a furnace for the treatment of a single piece. For heat treatment under such conditions as these, flame hardening and induction hardening have been developed recently.

Flame hardening consists of heating above the Aa transformation temperature any desired external spot of the finished steel article, as for example the teeth of a gear (Fig. 14.4),* with an oxyacetylene torch. This is adjusted for flame composition so that the steel does not oxidize. The distance of the torch from the work and the time of heating are also controlled so that the steel does not become overheated to the point where grain growth is serious. As soon as the surface area is at the proper temperature, it is quenched and immediately tempered to relieve quenching stresses.

Two general methods are used. In the first, the steel piece is fixed and the torch moves across the area to be heated at a predetermined rate. This must be controlled carefully as the temperature of the oxyacetylene flame is approximately 6300°F. (3500°C.) , but the piece to be hardened should not be heated higher than 1475 to 1550°F. (800 to 850°C.) . In the second method, the torch is stationary, and the steel piece moves slowly under the flame. The hot area is usually quenched in water, which is conveniently conveyed through one tube of the torch and flows out of an orifice under pressure just behind the flame.

In addition to economy under certain conditions, flame hardening has other advantages. It does not scale or pit the surface; it has no effect on the chemical composition; it hardens only at the surface and to a depth of not more than \/4 in., and distortion is minimized. Its disadvantages are that it is more costly than the usual methods if large numbers of identical pieces are to be treated, and that high-carbon steels and very coarse grained steels cannot be treated, as they arc likely to crack at the surface.

• R. L. Rolf, Trans. Am. Soc. Metals, v. 27, 1939, pp. 43-00. 294 Engineering Metallurgy

Fie. 14.4. Flame hardening the teeth of a large gear. (Rolf)

Steels most suitable for flame hardening are carbon steels containing 0.35 to 0.60 per cent carbon and low-alloy steels containing 0.25 to 0.50 per cent carbon. Such articles as alloy-steel gears which have been heat treated so that they have optimum strength and toughness can be effectively hardened on the surface of the teeth by this method.

Differential hardening by induction is a recently developed method for

securing high hardness on a relatively small bearing surface and is especially applicable to heavy-duty crankshafts* and to gear teeth, the ends of shafts, pins, hubs, and similar sections that act as thrust bearings.

The surface to be hardened is enclosed in a perforated induction block

that surrounds the steel but does not touch it. A high-frequency current of 2,000 to 200,000 cycles (from 25 to 50,000,000 may be used) is passed

through the block, inducing eddy currents which rapidly (in I to 5 sec.)

* The method is described in detail by M. A. Tran and W. E. BcnninghofT in Trans. Am. Soc. Metals, v. 25, 1937, p. 935. Sec also Induction Hardening, American Society for Metals, Cleveland, 1946, 172 pp. This is a series of lectures covering theory and practice. The Operation of Heat Treatment 295

Fie. 145. Cross section of a crankshaft showing hardened (while) bearing surfaces, etched. (Tran and BenninghofJ)

heat the surface to be hardened to a temperature above the upper transformation temperature. As soon as this occurs, water under pressure is sprayed onto the hot surface through the holes in the induction block. The heating and quenching cycle and other conditions are controlled, so that if steel of the proper composition and grain size is used, the bearing surface will have a hardness of about 60 Rockwell C (600 Brinell) to a depth of i/jj in. or more, and decarburization, grain growth, and distortion will be prevented. One of the most difficult problems to solve in the manufacture of internal-combustion engine crankshafts is how to secure a combination of high hardness and wear resistance on the bearing surfaces and ample toughness and relatively easy machinability for the rest of the shaft. This problem has been solved by induction hardening. The shaft can be heat treated by quenching and tempering at a high temperature, or even by normalizing, to obtain a structure of fine pearlite, which has satisfactory toughness and ductility and can be machined fairly easily. Hardening by induction heating produces a bearing surface with ample hardness and wear resistance and leaves the fillets, where the stress concentrates, ductile and relatively stress-free. An etched cross section of a heavy-duty induction-hardened crankshaft is shown in Fig. 14.5. The hard bearing surfaces are readily apparent. Heat treatment by induction heating has expanded rapidly in the past few years; at present approximately one third of all the quenched and tempered parts used in the United States are hardened by this method.*

• E. L. Cady, Materials and Methods, v. 24, Aug., 1946. pp. 400-410. This is an excellent summary with emphasis on the economic aspects. 296 Engineering Metallurgy

QUESTIONS

1. Why is complete Austenization of hypoeutectoid steels essential for full hardening? Why is only partial austenization sufficient for hypereutectoid steel to be hardened? 2. What is process annealing? What is full annealing? Explain the difference between these two annealing processes on the basis of prior processing, temperature required, changes in micro constituents, cooling rates and general usage.

3. What are the objects of normalizing low carbon steels? Of normalizing high carbon spheroidized steels? 4. Compare air, oil, water, and brine as quenching media. What is the mass effect in quenching? What other conditions affect the selection of the proper quenching medium? 5. What causes internal stresses in quenched steels? Why should these internal stresses be relieved? 6. How is spheroidized structure obtained? What are the reasons for spheroidizing? What problems arise in the heat treatment of a spheroidized structure for full hardness? 7. What is the overall objective of tempering? Correlate the effects that tempering has on the hardness, brittleness. strength, and toughness properties.

8. List the metallographical structures produced in tempering a steel when martensite decomposes at progressively increased temperatures. Compare the physical properties of these different structures. 9. What is meant by austempering, and what are its advantages and limitations? 10. Compare martempering with austempering. 11. Why is carburizing limited to low-carbon or low-carbon alloy steel, and how is carburizing accomplished? 12. Compare the processes: cyaniding, nitriding, and carbo-nitriding. Carbon Steel as an Engineering Material CHAPTER 15 Arthur C. Forsyth, Ph.D., Associate Professor of Metal- lurgical Engineering, University of Illinois, Urbana, Illinois

J. Edward Krai.ss, M.S., Head, Department of Mechani- cal Technology, New York City Community College, Brooklyn, New York

Jf OR a thousand years before 1850 man's entire stock of ferrous metals consisted of cast iron, wrought iron laboriously made in small quantities at high cost, and a few high-carbon steel tools made even more laboriously and at still higher cost. These were valuable materials and represented a long step forward from the metals of the bronze age, but they were too costly or too low in quality to be used in building railroads, bridges, buildings, and ships. Between 1850 and 1860 Henry Bessemer in England and William Kelly in the United States made discoveries which led to the development of a process for making carbon steel in quantities large enough and at a cost low enough to spark a century of industrial progress. A reasonably accurate understanding of this progress can be obtained from the following production figures: 800,000 tons in 1870, 28 million tons* in 1900, 119 million tonsf average world production between 1935

and 1939 (42 million tons in the U. S.) , and 253 million tonsj between

1951 and 1955 (103 million tons in the U. S.) . This shows an increase of over 100% in the last twenty years. Before World War II approximately 95 per cent of the steel produced in the world was carbon steel. This per-

• Estimate by Sir Robert Hadfield, The (London) Times Trade and Engineering Supplement, Jan. 28, 1953. f Steel, Jan. 4. 1943. p. 357, Average Production of Steel (1935-1939) for Ingots and Steel for Castings, 119,454,600 toss. Same period for the United States, 42.176,600 tons. t Steel, Vol. 140, No. 1, p. 172, Average World Production of Steel (1951-1955) for Ingots and Steel for Castings, 253,032,163 tons. Same period for the United States, 103,065,068 tons.

297 298 Engineering Metallurgy

centage has now dropped to about 90.8 per cent (in the United States) •,

but carbon steel is still the most important metallic material on a tonnage basis.

About 1.3 per cent of the carbon steel produced annually is poured into

- castings )- which are used in the form the metal assumed in the mold with no further mechanical treatment. The remaining 98.7 per cent is poured into ingots that are hot worked by rolling, forging, or pressing into a large variety of finished or semifinished products. Most finished wrought sections receive no further treatment, but a few are subjected to some form of heat treatment before use. Some semifinished wrought products are processed further by additional hot working; the rest are fabricated

into the desired form by cold working. (See Section 1 1 .9.) Cold-worked steels may or may not be heat treated before being used.

There are, therefore, four major classes of finished products of carbon steel: castings, hot-worked products, cold-worked products, and heat-

treated products. It is impossible to decide which of these four classes

is the most important in present-day economic life, since each plays a

definite role for which there is no understudy. Without hot-rolled

I beams, there would be no skyscrapers; without heat-treated tools they could not be built; without cold-drawn wire for elevator cables, they could not be used; and without castings for steam and water lines,

they would not be habitable. It is the same in most other industries: each

class of carbon steel is indispensable, and the cost or tonnage is no

criterion of its value to man.

15.1. Carbon-Steel Castings as Engineering Materials

Unalloyed steel castings have many industrial uses. Despite the fact that their mechanical properties are generally inferior to those of hot-

worked steel of the same composition, the proportion of castings to rolled or forged material remains substantially unchanged year after year. The

reasons for this are twofold: (1) Sections that are so complex as to be difficult or even impossible to fabricate by rolling or forging can be pro-

duced readily at low cost by casting; (2) castings are cheaper than

forgings if a few pieces of a fairly intricate shape are desired. The eco-

nomical production of such articles by forging is possible only when a

• Steel, Vol. 140, No. 1, p. 174, Average U. S. Alloy Steel Production, Total Ingots and Castings, 9,488,097 tons (1951-1955). + Steel, Vol. 140, Xo. 1, p. 178, Average U. S. Production of Carbon Steel Castings, (1951-1955) 1,216,948 tons. Carbon Steel as an Engineering Material 299

Table 15.1. Chemical Composition and Minimum Mechanical Properties for Medium Strength and High Strength Steel Castings *

Chemical Composition (per cent) Tensile Yield Elonga- Reduc-

(maximum) t Strength Point tion in tion in Grade (lb. per (lb. per 2 in. Area sq. in.) sq. in.) (per cent) (per cent) C Mn Si

Nl 0.25 0.75 0.80 N2 0.35 0.60 0.80 N 3 1.00 U. 60-30 0.25 0.75 0.80 60,000 30,000 22 30 60-30 0.30 0.60 0.80 60,000 30,000 24 35 65-30 65,000 30,000 20 30

65-35 0.30 0.70 0.80 65,000 35,000 J 24 35 70-36 0.35 0.70 0.80 70,000 36,000 22 30 80-40 80,000 40,000 18 30 80-50 80,000 50,000 22 35 90-60 90,000 60,000 20 40 105-85 105,000 85,000 17 35 120-95 120,000 95,000 14 30 150-125 150,000 125,000 9 22 175-145 175,000 145,000 6 12

• A.S.T.M 1955 Standards, Part One, Ferrous Metals, 27-55, and A148-55, pp. 1026 1027 and 1035-1036.

f Maximum sulphur 0.06 per cent; maximum phosphorus 0.05 per cent. For each reduction of 0.01 per cent carbon below the maximum specified, an increase of 0.04 per cent manganese above the maximum specified will be permitted to a maximum of 1.00 per cent.

X When agreed upon by the manufacturer and the purchaser, and when full annealing is required by the purchaser, the yield point value of the 65-35 class shall be 33,000 psi instead of 35,000 psi.

large number of pieces of a simple shape are produced thus spreading thinly the cost of the expensive forging dies. Steel for castings may be melted by any of the processes enumerated

in Table 11.1. Most of it is made by the basic or acid open -hearth or the

acid electric process. The open-hearth process is used for melting steel for large castings; the acid electric process, owing to its flexibility and

to the high temperatures obtainable, is favored for small castings, especially those of intricate shape. Steel castings are used for railroad equipment, especially for underframes of cars, for agricultural and excavating equipment, various parts of machines, electric equipment, and a large variety of small parts and fittings. There are several grades of steel castings for which specifications have been issued by the American Railway Association, the United States 300 Engineering Metallurgy

government, and a number of technical societies. Typical specifications for chemical composition and minimum tensile properties are given in Table 15.1.

For high-strength castings (above 80,000 lb per sq. in. tensile strength), the chemical composition, with the exception of maximum

percentages of sulphur and phosphorus, is not specified; the foundryman is permitted to adjust the percentages of carbon and manganese and to

use alloying elements, if necessary, which, when combined with a suitable heat treatment, will result in the desired properties. (See Table 15.2.)

15.2. Factors Affecting the Properties of Carbon-Steel Castings

Since the properties of carbon-steel castings are greatly improved by heat treatment, many castings are now annealed, normalized and annealed, or normalized and spheroidized. Quenching and tempering of

castings of such shape that distortion is not likely to be serious, are used more and more frequently.

Table 15.2. Mechanical Properties of Steel Castings*

Steel castings are generally specified by mechanical properties, leaving to the producer control of the chemistry within certain limits, t

Structural Grades—Carbon Steel

Tensile Strength psi 60,000 70,000 85,000 100,000

Mechanical Properties J 6 Mod of Blast in Tension, psi: 30.1 x 10° 30.0 x 106 29.9 x 10" 29.7 xlO Tensile Strength, 1000 psi: 60(a) 70(b) 85(c) 100(d) Yield Point, 1000 psi: 30 38 50 70 Elong in 2 in., %: 32 28 24 20 Reduction of Area, %: 55 50 40 46 Hardness, Bhn: 120 140 175 200 Impact, Izod, Ft-Lb: (70° F) 30 30 20 30 (-50° F) 8 10 10 15 Fatigue Str, (End Limit), 1000 Psi: 25 31 38 47

* Selected data from "Materials Engineering File Facts." Materials and Methods vol. 38, No. 4, 1953, pp. 153. Data sheets include current specifications, physical and mechanical properties, thermal treatment etc. for carbon and alloy steel castings. found in Materials f "Summary of Standard Specifications for Steel Castings" may be and Methods, Vol. 36, No. 1, 1952, pp. 121 and 123. Minimum mechanical properties and maximum chemical compositions are listed. X Normally expected values in the production of steel castings for the tensile strength values listed. (a) Annealed, (b) Normalized, (c) Normalized and Tempered, (d) Quenched and Tempered. Carbon Steel as an Engineering Material 301

V SxSJC

^ V '&N*S? ££

Fie. 15.1. Photomicrographs of Typical Microstructures o£ Small Steel Castings containing 054% C, 0.60% Mn., 0.84% Si., 0.032% P. and 0.036% S. Notice the change in grain size due to the following heat treatments. A-as cast, B-normalized 1700°F. (926°C). C-annealed 1700°F. (926°C), and D-quenched from 1700'F. (926°C.) tempered at 1200°F. (651 °C). Nital etch lOOx. Courtesy of the Mining and Metallurgy Depart- ment, University of Illinois.

Large castings cool so slowly in a sand mold that a very course angular structure results. If the casting is not too large, simple annealing will refine the grain and improve the properties (Fig. 15.1C) , but such treatment does not effect complete homogenization in large castings. These are subjected to high-temperature normalizing followed by a grain- refining treatment. Quenching and tempering produce the finest structure (Fig. 15. ID illustrates the structure for small castings) and the best combination of properties.*

Grain size is directly related to the rate of cooling from the solidification temperature to the A 3 transformation temperature. It follows, there-

•Table 152 shows the normally expected values for mechanical properties in the production of steel castings for the tensile strength values listed. ,.

302 Engineering Metallurgy

fore, that a small casting usually has smaller grain size and better proper-

ties than a large casting; moreover, if the casting is not heat treated, it is also true that in large castings the properties are better near the surface than at the center, f The factor of greatest importance to the properties of cast carbon steel is carbon content. If other things are equal, increasing the carbon increases the tensile and the yield strength and decreases elongation, re-

duction of area, and impact resistance. When the carbon is higher than

0.50 per cent, the casting is usually so brittle, even after heat treatment,

that it is used only for a few specific applications—for example, rolls and dies—where hardness and wear resistance are the most important properties. In general, carbon-steel castings of satisfactory mechanical properties can be made readily by regulating the carbon content and by using a simple annealing or normalizing treatment. The minimum properties given for medium strength castings are conservative and are easily attained in commercial production (compare properties in Table 15.1 and 15.2) Cast carbon steels containing 0.20 to 0.40 per cent carbon have an endurance ratio between 0.40 and 0.50. In the cast condition the ratio

is usually between 0.40 and 0.43. Heat treatment (that is, annealing,

normalizing, or quenching and tempering) improves the ratio, but it is usually somewhat lower than the endurance ratio of a comparable steel that has been hot worked. Heat treatment also improves the yield ratio, elongation, and reduction of area of cast carbon steel and increases the resistance of the material to single-blow impact. The added cost of heat

treatment is usually justified if castings are to be subjected in service to stresses of considerable magnitude (Table 15.2).

15.3. Hot-Worked Carbon Steels as Engineering Materials

Most hot-rolled or forged carbon steels contain between 0.05 and 0.30 per cent carbon and are used for sheet, plate, strip, tubes, pipe, various structural sections, tinplate and other coated sheets, and a large number of semifinished sections that are hot or cold worked into bars, wire, sheet,

and tubes. Also included in this class is the well-known "machine steel" (containing about 0.20 per cent carbon and 0.40 per cent manganese)

which is used for a wide variety of low-stressed machined parts and is the mainstay of every crossroads blacksmith shop in the world. Medium- carbon steels containing between 0.30 and 0.70 per cent carbon are used for railway materials, especially rails, for a large number of forgings, and

fOwing to the mass elfect and lo segregation, it is frequently a serious problem to cast a test specimen that has properties truly representative of ihe casting. Carbon Steel as an Engineering Material 303

for high-strength wire. The higher carbon grades, containing 0.70 to 1.30 per cent carbon, are used largely for tools and cutlery and to a lesser extent for springs and wire. Low-carbon steels are rarely heat treated; the medium-carbon grades may or may not be heat treated, depending upon their use; the high-carbon steels are almost always used in the hardened and tempered condition.

The properties of hot-worked carbon steel are affected by (1) the

composition, (2) the several variables present in hot working, and (3) the rate of cooling from the rolling temperature. Except for deep-drawing

sheet, this third factor is not important for low-carbon steels; in the case of small sections of medium- and high-carbon steels, air cooling may cause enough hardening to mask any effect of moderate changes in composition or of variations in the hot-working operation. The effect of composition is discussed in the next section. Of the three variables inherent in hot working—direction of work, amount of work, and finishing

temperature— the first is the most important. In general, specimens cut longitudinally and transversely to the direction of hot working have about the same tensile strength and yield strength, but the elongation, reduction of area, and impact resistance of the transverse specimens are lower than those of the longitudinal specimens. This difference in directional

properties is present even in clean steels, but it is accentuated by inclusions which are plastic at hot-working temperatures and which are thus elongated into fibers by the mechanical work.

15.4. Effect of Composition on Static Properties of Hot-Worked Carbon Steels

The effect of carbon on the tensile properties of hot-worked basic open-hearth steels containing between 0.30 and 0.60 per cent manganese

and between 0.02 and 0.04 per cent phosphorus is shown in Fig. 15.2.* The center line of the hatched area represents the properties to be expected most frequently. The top and bottom boundaries of the hatched area give the limits of the properties to be expected at least 95 per cent of the time in the testing of commercial rolled or forged basic open-hearth steel of these carbon percentages. The data used for plotting Fig. 15.2

were obtained mostly by statistical analysis. It is safe, therefore, to conclude that of 1000 specimens of 0.20 percent carbon steel about 700 will have a tensile strength of between (53,000 and 65,000 lb. per sq. in., and

•The values in Fig. 15.2 are a summary of all the data given by F, T. Sisco, in The Alloys of Iron and Carbon, Vol. II, Properties, Chap. 4, § A. anil Include not only laboratory investigations and other data but also the data obtained in Germany by statistical analysis of more than 100,000 individual tests. 304 Engineering Metallurgy

0.1 0.2 0.3 0.4 0.5 Carbon, per cent Fie. 15.2. Effect of carbon on tensile properties of hot-worked carbon steels.

about 970 will have a tensile strength (60,000 to 68,000 lb. per sq. in.) which falls within the hatched area for steel of this carbon content. There will, however, be about 30 specimens, or 3 per cent of the whole, with a strength falling above or below the limits shown. These proportions should hold for steel of any carbon content between 0.05 and 0.70 per cent.

The elongation values used for Fig. 15.2, if not originally determined on a 2-in. gage length, were converted to this gage. In connection with

the elongation it should be noted that the usual flat specimen for plate, sheets, and similar products has a gage length of 8 in.; hence, values given in Fig. 15.2 are some 20 to 30 per cent higher than would normally be obtained on an 8-in. gage* The increase in Brinell hardness with in-

•If standard flat specimens of basic steel of about 60,000 lb. per sq. in. tensile strength and with gage lengths of 2, 4, and 8 in. are tested, the respective elongation values will be approximately 45, 35, and 30 per cent, respectively. Carbon Steel as an Engineering Material 305 creasing carbon approximately parallels the increase in tensile strength: each 0.10 per cent carbon raises the hardness 20 numbers, from about 100 for 0.10 per cent carbon steel to 220 for 0.70 per cent carbon steel. Although both manganese and phosphorus affect the tensile strength, the influence of the former is unimportant in most carbon steels. For example, if a basic open-hearth steel containing 0.20 per cent carbon and 0.50 per cent manganese has a strength of 65,000 lb. per sq. in., increasing the manganese to 0.80 will increase the strength only 3,000 to

4,000 lb. per sq. in. Phosphorus increases the tensile and the yield strength about 1,000 lb. per sq. in. for each 0.01 per cent present. In basic open-hearth steels, this element is commonly so low that it has little effect; in acid steels, however, there is usually enough present so that it exerts considerable strengthening action. Thus, acid steel is usually stronger, with lower elongation and reduction of area, than basic steel of the same carbon content.

15.5. Effect of Composition on Other Properties Increasing the carbon of hot-rolled steel decreases the impact resistance; low-carbon basic open-hearth steels have an Izod value of 50 to 60 ft.-lb.; for hot-rolled steels containing 0.50 to 0.80 per cent carbon, it is usually less than 20 ft.-lb. Increasing the phosphorus decreases the impact resistance; an acid Bessemer steel is not so tough as basic open- hearth steel of otherwise identical composition. The brittleness induced by phosphorus is discussed in § 11.10. In connection with impact resistance it should be emphasized that this property is greatly affected by grain size, direction of hot working, amount and distribution of inclusions, and other factors (aside from chemical composition) related to the structure.* These factors may have so much influence on the impact resistance that the effect of variations in composition may be completely obscured. The endurance limit of hot-worked carbon steels increases with the tensile strength. Owing, however, to the fact that high-carbon steels have less plasticity under load than low-carbon steels, and are more likely to contain internal stresses, the endurance ratio is usually between 0.35 and 0.42, compared with 0.50 to 0.60 for wrought iron, ingot iron, and steels containing less than 0.35 per cent carbon.

The modulus of elasticity of carbon steels is between 29 and 30 million lb. per sq. in and is not greatly affected by composition and heat treatment. Some of the other physical constants and most of the electric and magnetic properties are affected, more or less, by changes in composition.

•Also Libsch, see: Temper Kmbrittlement in Plain Carbon Steels, by J. F. A. E. Powers and G. Bhat, Trans. A.S.M. Vol. 44, 1952, pp. 1058. 306 Engineering Metallurgy

Since these properties are not of great importance to most engineers,

discussion of them is omitted here.

15.6. Cold-Worked Carbon Steels as Engineering Materials

Cold working has two advantages. First, sections of certain sizes, shapes, or surface finishes can be produced more readily and more economically by this method than by any other. Some important industrial

products, in fact, cannot be produced at all except by cold working; it would be economically impracticable to produce by any other method wire small enough to be woven into screens or in quantities large enough for modern telegraph and telephone systems. In addition, cold working is the most effective and the cheapest method of securing a smooth surface and accurate size. Second, certain combinations of properties characteristic of cold-worked material cannot be secured by any other process; for example a tensile strength of 300,000 to 400,000 lb. per sq. in., combined with considerable ductility, can be attained readily.

The economic value of cold-worked carbon steel is attested by the fact thai in an average year about 3 million tons of wire* 12 million tons of sheet* and strip, and 1.8 million tons of bars* and other cold-worked products are made in the United States. Sheet, strip, tubes, and bars are cold worked primarily to obtain certain sizes, shapes, and surface finishes

economically. High strength is usually of secondary importance. Further- more, most cold-rolled sheet and strip, and some cold-drawn wire, especially the low-carbon grades which are used for screens, fences, barbed wire, nails, screws and nuts, telegraph and telephone lines, and for many other common applications, are annealed before use. Because annealing completely destroys the effect of cold work, the properties of such material need not be considered further here.

Carbon steels that are cold worked to secure high strength as well as to produce special shapes and sizes and are not subsequently annealed comprise an important class of engineering materials. The properties of

these steels and how cold working affects them are, therefore, worthy of considerable attention. This class of material includes some low-carbon sheet, strip, and wire, but the most important is wire containing about 0.25 to 1.00 per cent carbon used for needles, surgical and dental instruments, music (piano) wire, springs, rope and cable wire, and numerous other purposes where high quality and reliability combined with high tensile strength and considerable ductility are required.

•Estimated from -Shipments of Steel Products". Steel Vol. 140, No. 1, 1957, pp. 176. Carbon Steel as an Engineering Material 307

10 20 JO 40 SO 60 70 SO SO 92 Reduction by cold working, percent

Fig. 15.3. Stress-strain curves of cold-worked low-carbon steel (Kenyan and Burns)

15.7. The Important Properties of Cold-Worked Steel

The static properties ordinarily determined on cold-worked steels are tensile strength and elongation, usually with a gage length of 10 in. for wire and of 2 or 8 in. for sheet and strip. Owing to the small cross section of most cold-worked materials, reduction of area, which is a valuable

measure of ductility, cannot be determined accurately. Yield strength is rarely determined on cold-worked wire and sheet or strip; but for cold- rolled bars of large enough cross section so that accurate measurements are possible, yield strength and reduction of area are frequently reported. Cold working increases the yield strength; with reductions in cross sec-

tion of 30 to 70 per cent it is at least 90 per cent of the tensile strength, and with higher reductions it may for practical purposes be the same as the tensile strength. As shown by Fig. 15.3,* the stress-strain curve of

severely cold-worked steel is curved from the origin; the elastic limit is, therefore, very low and may be zero. Several unstandardized tests are used to determine the ductility of cold-worked wire and sheet. Some of these are crude, but in experienced

hands they give reliable indications of excessive brittleness or local ir-

regularities. The most common of these is the bend test in which a specimen of definite size, clamped in a vise and bent 180 deg. over a definite radius, must not crack. In another test, a specimen is bent back and

•R. L. Kenyan and R. S. Burns, Trans. Am. Soc. Metals, v. 21, 1933. p. 595. 308 Engineering Metallurgy forth 90 deg. over a mandrel until failure occurs. The number of 90-deg. bends before failure is a measure of the ductility.

Another test of the ductility of wire is the twist. A specimen of suitable length is twisted in a machine, which has one revolving head, until it fails by shear. The number of 360-deg. revolutions of the movable head of the machine is the number of twists. Some metallurgists consider that the twist is a good measure of ductility; others believe that its principal value is to detect local irregularities or flaws in the wire.

10 20 30 40 50 60 70 80 90 100 Reduction of area in drafting,percent

Fie. 15.4. Effect of cold working on ihe tensile strength of carbon steel, (l.egge)

15.8. General Effects of Cold Working on Strength and Ductility These particular properties are markedly affected by cold work and the effect it produces on grain structures. Cold work rapidly increases the tensile strength while decreasing the ductility. The latter is evident by the decrease in elongation. The change in these properties is directly Carbon Steel as an Engineering Material 309

10 20 30 40 50 60 70 80 90 100 Reduction ofarea in drafting .percent

WW. 15.5. Effect of cold working on the elongation of carbon steel. (Legge)

related to the amount of cold work. Figure 15.4* gives an indication of the effect of cold working on the tensile strength. The effect of cold work is directly related to the carbon content of the steel. As the carbon content increases, the tensile strength increases more rapidly and to a greater degree for each increment in the reduction in area.

The converse effect is evident when the elongation is considered. This value reduces sharply with small amounts of cold work and does not vary greatly with the carbon content (Figure 15.5). As the reduction in area* is increased, the effect on the elongation is very much smaller. One should bear in mind that the figures referred to only approximate the properties resulting from cold work. While the amount of cold work is the prime varible having an effect on strength and ductility, it is not the

•The term reduction of area is used in cold working to signify the amount that the cross section has been reduced by the cold working operation. It should not be confused with the same term used in tensile testing. 310 Engineering Metallurgy sole factor. Composition is important, and the structure of the material has an effect, particularly for medium and high carbon steels.

Another factor that causes variation in total effect is the cross-sectional area. Table 15.3 indicates the variations that exist as a result of this factor. Obviously a 1" bar is less affected at the center than a wire would be.

Table 15.3. Effect of Cold Rolling on the Tensile Properties of a Steel Containing 0.14 Per Cent Carbon.

Reduced 30 Reduced 60 Property Annealed per cent by per cent by cold rolling cold rolling

58,800 80,600 98,100 34,040 75,300 t)e> inn 27,500 15 600 -ininn 41.7 22 10 5 65.8 58 43

Since the degree of cold work cannot be equal in all directions, the grain shape is changed, mechanical twinning occurs, and preferred orientation of the grains also results. While cold working generally has the same effect on the properties of sheet and strip as it has on wire, mechanical directionality shows up in sheet and is usually undesirable.

Tensile Yield strength, strength, Elongation Condition lb. per lb. per in 2 in., sq. in. sq. in. per cent

Hot-rolled strip 40,000 28 Cold rolled 50 per cent 96,000 2 Annealed 33,000 38

The effect on the properties varies with the carbon content. A reduction by cold rolling of 60 per cent has the following effect on strip:

Carbon, Increase in tensile Decrease in per cent strength, per cent elongation, per cent

0.10 65 70 0.20 63 70 0.30 61 68 0.50 57 67 Carbon Steel as an Engineering Material 311

15.9. Variables Affecting the Properties of Cold-Worked Wire

There are many variables which affect the properties of cold-drawn carbon steel wire. Only a few may be referred to at this point. Approxi- mately 85 per cent of all wire used today is drawn from either acid Besse- mer or basic open-hearth low-carbon steels. Hard-drawn Bessemer wire is from 10 to 15 per cent stronger than the hard-drawn open-hearth wire. The tensile strength of Bessemer varies from 90,000 to 150,000 lb. per square in. while the open-hearth varies from 80,000 to 125,000 lb per sq. in. for similar sizes. A slightly increased elongation and a greater ability to withstand twist is characteristic of the basic open-hearth wire.

The effect of cold work on the grain structure is particularly noticeable in wire. The fragmentation and distortion of the grains occurs throughout the material since we are dealing with small cross sections.

The effect of galvanizing low-carbon cold-drawn wire is a reduction in tensile strength, an increase in elongation, but this is complicated by the induced brittleness as shown in a twist test. This brittleness is probably caused by the formation of a brittle iron-zinc compound at the interface between the steel and the zinc coating. Here failure starts easily under the torsional stresses.

High-strength wire for cables, ropes, and springs and for music wire is usually made from high-grade deoxidized medium and high-carbon steel. The steel is rolled into rods which are patented. The best combination of strength and ductility results from drawing a lead-patented rod.

Air patenting is cheaper, but the properties of the wire are not as good.

Wire drawn from either lead- or air-patented rod is, however, much superior to that drawn from a hot-rolled rod. Hard-drawn music and cable wire drawn from lead-patented rod has a strength of 160,000 to 220,000 lb. per sq. in. in sizes of 0.2 to 0.3 in. and of 280,000 to 400,000 lb. per sq. in. in sizes smaller than 0.1 in. in diameter. Despite the high strength and low elongation (0.5 to 1.5 per cent in 10 in.), the wire is very ductile and withstands many 90° bends and can be wrapped around itself or rolled into long springs of small diameter without cracking.

The wire used for the cables of large suspension bridges is an acid open- hearth steel containing about 0.80 per cent carbon, 0.50 percent manganese, and low sulfur and phorphorus. The wire is drawn from patented rods, and the usual size is 6 gage (0.192 in. in diameter) . It has an average tensile strength of 215,000 to 225,000 lb. per sq. in., a yield strength for a permanent set of 0.75 in. in 10 in. of 160,000 to 175,000 lb. per sq. in., an elongation of 3 to 7 percent in 10 in., and a reduction of area of 25 to 30 percent. Rope or cable formed from high-strength wire has, owing to its 312 Engineering Metallurgy

construction, a breaking strength which is never more than 90 per cent, and usually only 70 to 80 per cent, of the sum of the breaking strength of the individual wires.

15.10. Effect of Cold Working on Dynamic Properties As has been pointed out previously the transition temperature has an effect on these properties. Although cold working increases the endurance limit by about the same percentage as it increases the tensile strength, this means relatively little because most cold-worked material has such a small cross section that standard highly polished specimens usually cannot be tested. For specimens of wire tested with the surface produced by the die, the endurance ratio varies from about 0.50, for material containing 0.05 per cent carbon, to 0.25 to 0.35, for high-carbon wire having a tensile strength of 275,000 to 350,000 lb. per sq. in. The low value for hard-drawn high-carbon wire is largely due to high internal stresses as reheating the wire to effect a partial or complete recrystallization of the grains raises the ratio.

Cold working also decreases the resistance of carbon steel to impact. The magnitude of this decrease depends primarily upon the amount of cold work and the carbon content—high-carbon steels are embrittled to a greater degree than low-carbon materials—and upon other variables.

The actual Izod impact value may be as low as 5 to 10 ft. -lb. In general, however, notched-bar impact resistance is not an important property of cold-worked material.

15.11. Heat-Treated Carbon Steels as Engineering Materials

As we approach the problem of heat treatment, it would be well to review a few definitions. The two treatments that we are particularly concerned about are annealing and normalizing. Annealing is a process involving heating and cooling, usually applied to induce softening.

Normalizing is a process in which steel is heated to a suitable temperature above the transformation range and is cooled in still air at room temperature. With the exception of carburizing and annealing of cold-worked material, low-carbon steels are seldom heat treated. Quenching generally improves the properties but hardly enough to justify the cost. Medium- carbon steels are frequently heat treated; the general improvement in strength and ductility, in machinability, or in some other property due to thermal treatment is usually well worth while. Since high-carbon steels are used primarily for tools where high hardness is the chief requirement, these materials are always quenched and tempered. Frequently they are Carbon Steel as an Engineering Material 313 also annealed or normalized to improve machinability or the structural condition prior to quenching.

The effect of heat treatment on the properties of cast steels having been treated earlier in this chapter and the characteristics of treated high- carbon steels being more easily presented in the chapter on tool steels, it remains only necessary to briefly cover the properties of medium-carbon steels after heat treatment.

Referrring to our definitions as a basis, it is easy to point out that annealing is used primarily to improve the machinability of medium- carbon steels. Comparing steel in the 0.30-0.60 per cent carbon range, we find very small differences in the physical properties. The tensile and yield strengths are 6,000 to 10,000 lb. per sq. in. lower while the elongation and reduction in area are a bit higher. While no effect is evident on the endurance ratio, slight increases in impact strength are apparent. Hot-rolled and normalized medium-carbon steels have practically the same tensile properties. This can be attributed to the fact that most carbon steels after hot working are air cooled from temperatures very close to the normalizing temperatures. Since normalizing produces a homogeneous structure, it can be justified, economically, for large cross sections after forging or rolling. In addition to producing a finely divided evenly distributed carbide, it eliminates directionality due to mechanical fibering of inclusions such as oxide, slag, etc. Owing to the residual stresses introduced by quenching, medium-carbon steels are seldom used in the quenched and untempered condition. The properties of such materials are therefore of little interest. If, however, the internal stresses are relieved by tempering, quenched medium-carbon steels become valuable structural materials.

If quenched steel is tempered at a temperature high enough so that the particle size of the carbide changes, strength and hardness decreases and ductility increases almost uniformly as the tempering temperature increases. This is shown in Fig. 15.6, which gives the ranges of properties (included within the hatched bands) that result 95 per cent of the time from tempering water- and oil-quenched 0.40 to 0.50 per cent carbon steels at increasing temperatures.

Charts showing the effect of tempering on the tensile properties of quenched carbon and alloy steels of varying carbon content are widely available in the literature and in a number of handbooks. It should be emphasized, however, that these charts, while undoubtedly of value in indicating the properties to be expected most of the time, should never be used for specification purposes as individual specimens frequently vary considerably from the average shown on the chart. 314 Engineering Metallurgy

Tempering temperature, deg. F. 600 800 1000 1200 600 800 _l_

500 600 700 300 400 500 600 700 Tempering temperature, deg. C.

Fic. 15.6. Effect of tempering on the properties of 05 in. bars of water-and oil-quenched 0.40 to 0.50 per cent carbon steel (S.A.E. 1045). (Sisco, The Alloys of Iron and Carbon, Vol. II, Properties, McGraw-Hill Book Company, Inc., New York, 1937, p. 200.)

Carbon steels containing the usual amounts of manganese (0.30 to 0.90 per cent) are shallow hardening. The time available to quench a carbon steel past the nose of the S-curve is usually less than a second, with the result that even on severe quenching martensite forms only in very small pieces or on the surface of larger sections. Consequently, the structure of a carbon steel quenched in oil or even in water generally consists of a little martensite mixed with a relatively large amount of a ferrite-

cementite transition structure. It will be shown in the chapters on hardenability that the mechanical properties of a quenched and tempered steel

are in general better if the structure before tempering was almost wholly martensite rather than a mixture of martensite and one or more transition constituents. The primary reason why low-alloy steels are favored for

quenched and tempered parts is that the alloying elements increase hardenability and thus increase the likelihood that martensite will be

formed in large quantities when the steel is quenched in oil. Carbon Steed as an Engineering Material 315

15.12. Effect of Section Size on the Properties of Heat-Treated Medium-Carbon Steels

It is well known by metallurgists generally, but by surprisingly few engineers, that the properties of steel determined on a small heat-treated bar of steel may not be representative of the average properties on a

large bar similarly treated. The effect of mass on properties is important.

Since in many industrial applications large sections are used, it may be more essential for the engineer to know the average properties or, in some cases, the minimum properties than to know the properties at the

surface. It is not always practicable to test full-sized I beams or locomotive axles; hence, data that indicate the effect of increasing size of section are of considerable value.

A heavy section cannot be cooled as rapidly as a thin section; the interior of a heavy section, cooling more slowly than the surface, transforms not only at a later time but at a higher temperature and therefore to a softer product than the surface. This results in differences in properties between the exterior and the interior, possible distortion, residual stresses, etc. If we were to heat and quench a series of bars of SAE 1045 steel of different diameters " 5", the from \/2 to smallest piece would cool most rapidly and each succeeding piece more slowly

Size of Bar Surface Hardness 0.5" 59 1.0 58 2.0 41 3.0 35 4.0 30 5.0 24

A study of the surface micros tructu re reveals that for the hardest

material the microstructure is predominately martensitc, and as hardness decreases, the amount of martensite decreases rapidly and pearlite in varying degrees of coarseness appears. If the bars are cut and a hardness

survey taken of the cross section, it would show the variation in cooling rate from outside edge to center by the variation in hardness. (Fig. 15.7).

In most large sections the cooling rate upon quenching decreases as

the distance from the surface increases and, if other variables do not enter importantly, hardness and strength decrease and ductility increases slightly. Data for medium-carbon steel are given in Fig. 15.7. The effect

of mass is more pronounced in water-quenched than in oil-quenched

material, and for the same quenching treatment the effect of mass is less pronounced with high tempering temperatures than with lower temperatures. 316 Engineering Metallurgy

Diameter

Fie. 15.7. Hardness of series of SAE 1045 steel bars heated and quenched.

The endurance ratio of quenched and tempered carbon steels is 0.50 ± 0.05 unless the quenching treatment has introduced stresses which were not wholly relieved by tempering. When such stresses are present the ratio may be as low as 0.30 or 0.40. Quenching and tempering usually have a favorable effect on impact resistance, which, in general, is much higher than that of a rolled steel of corresponding composition. Temper- ing increases the impact resistance of a quenched steel: if the Izod value of a water-quenched 0.30 per cent carbon steel is 10 ft.-lb., it will be 20 to 25 ft.-lb. after tempering at 700°F. (400°C.) and 45 to 65 ft-lb. after tempering at 1100 to 1200°F. (595 to 650°C.) . The modulus of elasticity is, of course, unaffected by thermal treatment.*

QUESTIONS

1. By what per cent has the steel production in the United States increased in the last twenty years? 2. Why should the per cent production of carbon steel in the United States be less now than it was before World War II?

* Sec footnote, p. 305. Carbon Steel as an Engineering Material 317

3. Give two reasons why a large tonnage of steel is melted and poured into castings every year. How is steel for castings usually melted? What is the general relation between structure, heat treatment, and properties of cast carbon steel? Does carbon steel have a low or high hardenability? 4. How does the per cent of carbon in steel castings affect the hardness of the martensite produced when the castings are quenched? 5. What carbon content in steel castings would be most likely to cause cracking during the quenching cycle? If cracking during quenching were a real possibility, what heat-treating cycle could be used to prevent cracking? 6. How would the rate of cooling from die rolling temperature affect the properties of hot worked carbon steel? 7. Name the three general classes of hot-worked carbon steel, and give the principal industrial uses of each class. Which of these three classes leads in tonnage? 8. List the principal variables that affect the properties of hot-rolled carbon steel. What tensile properties would be expected 95 per cent of the time from hot-rolled steel containing 0.15 per cent, 0.30 per cent carbon? How do manganese and phosphorus affect the tensile strength of hot-rolled carbon steel? 9. What is the effect of carbon on notched-bar impact resistance? on endurance limit? on modulus of elasticity? 50. Why are steels cold worked? What properties are ordinarily determined on cold-drawn wire and on cold-rolled bars? What is the effect of cold working

on the yield point? How is the ductility of cold-worked wire usually determined?

11. What is the general efTect of cold working on tensile strength and elongation? If a hot-rolled rod containing 0.15% carbon has a tensile strength of 60,000 lb. per sq. in. and an elongation of 25% in 10 in., what strength and elongation would be expected in drawing to wire with (a) a reduction of 40%? (b) a reduction of 70%?

12. What percentage increase in tensile strength can normally be expected if

automobile fender steel is cold rolled 50%? What is the effect of this reduction on the elongation? Give typcial tensile properties (a) of small sizes of music wire, and (b) of wire for suspension bridges.

13. What is the general effect of cold working on the impact resistance and on the endurance limit of carbon steel?

14. What general classes of carbon steel arc heat-treated? What is the advantage of either annealing, or normalizing, or quenching and tempering such

steels? What is the general effect of tempering on the tensile properties? For a tempering temperature of about 1000°F., what would be the expected properties of a 1-in. round bar of 0.45% carbon steel (a) when quenched in water? (b) when quenched in oil?

15. If a small specimen of hot-rolled carbon steel containing 0.45% carbon has a tensile strength of 93,000 lb. per sq. in., a yield strength of 54,000 lb, per sq. in. an elongation of 20% in 2 in., and a reduction of area of 85%. what treatment can be used to increase these values to 103,000: 70,000; 25; and 60; respectively? With these latter tensile properties for a heat-treated small section, what would be the corresponding properties at the center of a 4-in. bar? Low - A Hoy Steels as Engineering CHAPTER Materials

16 Robert E. Bannon, S.M., Professor of Metallurgy, Newark College of Engineering, Newark, New Jersey Sic>fUND Lever.v Smith, M.Met.E., Professor of Metal- lurgy, College of Mines, University of Arizona, Tucson, Arizona

Al.I.OY steels are about seventy years old. The pioneer investigations of the effect of the various alloying elements on carbon steel were made between 1875 and 1890 in England, Germany, France, and, to a lesser extent, the United States. Owing to the high price of alloying metals, the only use for these materials on a relatively large scale was in armor plate. By 1910, alloying metals were much cheaper, and from then on industrial applications multiplied rapidly, especially between 1915 and 1918 when there was an abnormal demand for high-quality alloy steel for war materials. This demand, together with the approximately simultaneous development of the first "stainless" steel, stimulated metallurgists all over the world to investigate the effect on carbon steel of all possible combinations of many alloying elements. The result has been alloy steels by the hundreds— steels containing one alloying element in addition to carbon, and steels containing half a dozen. Before World War II much of the research work on alloy steels was of the hit-or-miss type or was carried out to develop a steel that would duplicate the properties of a patented composition without actual in- fringement. One result of this frenzied research between 1910 and 1950 was a phenomenal growth of metallurgical literature; another was the development of many unnecessary alloy steels which, with slightly different compositions, duplicate the characteristics of some of the older and better-known steels. However, some remarkable developments have resulted from this mass of hit-or-miss research, including several new steels and special ferrous alloys with splendid properties and of much value to industry. It has also shown to engineers that alloy steels make

318 Low-Alloy Steels as Engineering Materials 319 possible the design of certain structures that would be impossible with unalloyed carbon steels and cast irons. Without alloy steels the modern airplane, the streamlined railroad train, the modern automobile, and many other important developments would never have been brought to their present efficiency. The most characteristic indication of the increasing value of alloy steels to the engineering professions is that their production has increased at a greater rate than total steel production. The output in 1910 was

600,000 tons, or 2 per cent of the total; in 1939 it was more than 3,000,000 tons, or about (i per cent of the total output of the steel industry. During 1944, when total steel production in the United States reached 89,642,000 net tons of ingots, alloy-steel ingot production was 10,630,000 net tons, which is 12 per cent of the total. The demand for alloy steels in the average prosperous postwar year, for example 1960, cannot be foreseen now; probably it will average about 8 or 10 per cent of the total.

The amount of published data on alloy steels and cast irons is so large that it is now impossible for a metallurgist to know everything that is going on in this field; it is a hard job even to keep up with developments in a single branch.* To summarize adequately, in a single book, the present status of knowledge of alloy steels is a difficult task, and it is, of course, quite impossible to give many details of their properties in three short chapters. Consequently, the discussion in this and the next

two chapters is restricted to a brief description of the characteristics of these materials and to a concise outline of the metallurgical fundamentals in the relation of the common alloying elements to carbon steels.

16.1. Balanced Compositions in Low-Alloy Steels

A workable definition of alloy steels and a general classification of these materials into low- and high-alloy steels, including common compositions for the low-alloy grades, are given in Chap. 11. Many low-alloy steels, especially the S.A.E. grades, are purchased under specifications that give desired ranges for the various alloying elements and for carbon and manganese and usually maximum percentages for sulphur and phosphorus. Widely available in the literature and in handbooks are charts of average, typical, or minimum mechanical properties of the various steels after definite heat treatments. Such specifications and charts are the result of long experience and arc of unquestioned value to engineers.

• Alloys of Iron Research, founded in 1930 by The Engineering Foundation of New York, is reviewing this literature and correlating and summarizing the most important of the world's research on alloy steels and cast irons. Results are published in a series of monographs. ,

320 Engineering Metallurgy

In the past few years, however, there has been a trend toward making use of steels of "balanced composition" or of definite hardenability rather than attaching too much importance to rigidly specified compositions and the necessity of accepting the data in mechanical-property charts. Steels of "balanced composition" are those in which the percentages of carbon and alloying elements, as well as the heat treatment (if any) are varied to produce a desired combination of properties for a particular application. Assume, for example, that for a structural application a steel

must have a minimum yield strength of 50,000 lb. per sq. in. as rolled, must weld readily, and must not air harden after welding. A steel containing 0.30 per cent carbon, 0.75 per cent of alloy metal A, and 0.5 per cent of alloy metal B has the required strength and welds readily, but it air hardens unless specially treated during or after welding. Because this composition is not balanced for this particular application, the carbon is reduced to 0.15 per cent, and the amounts of the alloying metals are increased to 1.0 and 0.75 percent, respectively. The yield strength is still well above the required minimum, the material still welds readily, but owing to a better balance of the carbon and metals A and B the steel does not harden in air after welding.

Producing steel with a definite hardenability is primarily a problem in balancing the composition correctly. The effect of carbon and of the various alloying metals on hardenability is known with considerable accuracy (this is discussed in detail in the next two chapters) , with the result that steels having a definite response to quenching can be readily produced by balancing the amounts of carbon and of the one or more alloying metals which are readily available. The final criterion in the selection of an alloy steel for a specific application is economy. Any alloy steel is more costly than plain steel of corresponding carbon content and should not be used for a structure or a machine merely because it contains one or more special metals, or because the steelmaker says it is better than some other steel. Alloy steel should be used only if considerable weight can be saved, or if the design can be materially simplified, and only if its total cost—meaning the cost of the steel, of its treatment (if any) , and of fabrication—is less, when spread over the expected life, than the total cost of any competing material.

16.2. General Effects of the Alloying Elements on Carbon Steel

The general effects of the common alloying, elements on the structure and, consequently, on the properties of carbon steel are discussed in

Chaps. 12 and 13. It is the purpose of the discussion in this and the next Low-Alloy Steels as Engineering Materials 321

two chapters to expand this and to show how these alloying elements are used to produce steels which are more satisfactory for certain applications than the unalloyed carbon steels.

Hot-rolled carbon steel is low in cost and has excellent properties, and with certain carbon percentages these properties can be still further improved by heat treatment. Carbon steel also has several disadvantages

as an engineering material. An important disadvantage is the rapid decrease of ductility as the carbon (and the strength) increases. Another

is that, because it is shallow hardening, optimum combinations of properties in heat-treated material can usually be attained only in small sec-

tions. A third disadvantage is that carbon steels, whether hot rolled or heat treated, suffer marked deterioration of properties when used at temperatures considerably below or above normal. Alloying elements are added to carbon steels to overcome, partly at least, these disadvantages. The principal effect of the alloying elements in low-alloy steels is to

increase hardcnability by heat treatment. In addition, all the common

alloying elements dissolve in the ferrite and strengthen it to some extent. Five of these elements also form carbides, but of these only molybdenum, vanadium, and tungsten have a strong carbide-forming tendency. Chang- ing the properties of carbon steel materially by the formation of such

carbides is important chiefly in the high-alloy and the tool steels, which are discussed in later chapters. The ferrite-hardening (and -strengthening) potency of the elements forming solid solutions with iron varies (increasing from chromium to

2 4 6 e 10 12 It 16 IB 20 22 Alloying element in alpha iron, per cent

Fie. 16.1. Effect of the common alloying elements, when dissolved in iron, on its hardness. (Bain) •-

322 Engineering Metallurgy phosphorus), as shown in Fig. 16.1.* None of these, with the possible exception of phosphorus, is so potent as carbon. In fact, the strengthening effect is small, provided that the carbon content and the structure are held constant. This is shown by the curves for the furnace-cooled specimens plotted in Fig. 16.2.f If, however, the structure is altered by chang-

ing the treatment, that is, 240 by taking advantage of the 230 effect of the element on har- 220 denability, small amounts

210 of an alloying clement may, */ ft as shown in Fig. 16.2, have 200 o\ yV a great effect. In fact. Fig. 190 f 0/ 16.2 shows that chromium 180 c 1 increases the hardenability i 4 no f>V enough so that high tensile- 160 / strength values result from 1 / I / / moderately (air) £150 ( slow / / cooling. §140 * 8 £.130 / -n / 16.3. Effects of Phospho- tiro10tr

100 Of the alloying elements J 90 which dissolve in the fer- 0.30 %c __ __ rite, phosphorus, as just 80 -- ~OJ0 __ — noted, is the most potent in 70 its strengthening action . — 60 (Fig. 16.1) . The addition 50 of 0.01 per cent of phos- 12 3 4 5 6 7 Chromium, percent phorus increases the tensile and the yield strength ap- Fie. 165. Effect of carbon, chromium, and cool- proximately 1,000 lb. per ing rate on the tensile strength of carbon steels. (Wright and Mumtna) sq. in., that is, by about as

• E. C. Bain, Functions of the Alloying Elements in Steel, American Society for Metals, Cleveland, 1939, p. 66. Data reported by C. R. Austin (Trans. Am. Soc. Metals, v. 31, 1943, p. 331) indicate that the strengthening effect of manganese and molybdenum in small percentages is greater, and that of chromium is slightly less, than those shown by Bain's data in Fig. 16.1. Neither Bain's nor Austin's data arc very precise, because the iron used was not of high purity and the slope of the curves shown in Fig. 16.1 may be altered by future work. The ferrite-strengthening cirect of the alloying elements is, however, not of great importance. t Plotted from data by E. C. Wright and P. F. Mumma, Trans. Am. Inst. Mining Met. Engrs., v. 105, 1933, pp. 77-87. Low-A Hoy Steels as Engineering Materials 323 much as these properties are increased by carbon. This strengthening is, moreover, attained with less sacrifice of elongation and reduction of area than in the case of carbon addition. f Another advantage is that phosphorus, either alone or more intensely if copper is present, increases the resistance of the steel to atmospheric corrosion. Phosphorus is favored as an alloying element chiefly in low-carbon steels that are used in the rolled or normalized condition.

Manganese, added to steel in amounts in excess of that necessary to combine with the sulphur and to deoxidize the metal—0.1 to 0.25 per cent for most steels— partly dissolves in the ferrite and partly replaces iron in iron carbide (Fea C) to form a carbide that may be represented by

(Fe,Mn)3C. If the manganese is below 2 per cent, the amount of this carbide formed is small. Moreover, this substance is so much like iron carbide that it has in itself no appreciable effect on structure and properties. The effect of manganese in solid solution is less than that of phosphorus; the amount by which 0.01 per cent manganese increases the tensile strength varies with the carbon content from about 100 to 500 lb. per sq. in.; it increases the yield strength somewhat more than this. A yield strength of at least 50,000 lb. per sq. in., together with good ductility, can be readily attained in hot-rolled structural shapes of 0.20 to 0.30 per cent carbon steel by adding 1.00 to 1.75 per cent manganese. Manganese in amounts exceeding about 0.75 per cent has a strong effect on the hardenability of carbon steel. In steels that are heat treated, this effect is an advantage, since it permits the use of less drastic quenching mediums, which means less distortion or cracking. In hot-rolled structural steel increased hardenability may be a disadvantage, unless the carbon is very low, because air hardening may occur in cooling from the rolling temperature with serious loss of ductility. In general, manganese reduces the ductility of untreated carbon steel, but this is not serious if the carbon is low.

Silicon in the amounts present in low-alloy steels dissolves in the ferrite.

Its effect on tensile strength is somewhat greater than that of manganese

(Fig. 16.1) ; it raises the yield strength and yield ratio and does not appreciably reduce ductility. Its effect on hardenability is strong but not so strong as that of manganese. It has no appreciable effect on corrosion resistance when present in small amounts. In other than the low-alloy structural steels discussed later in this chapter, silicon is used as an alloying element in one important group of spring steels that contain about 2 per cent silicon, 0.60 to 0.90 per cent manganese, and 0.50 to

f Phosphorus is likely to make steel cold short, that is, brittle when cold, unless the carbon percentage is low. 324 Engineering Metallurgy

140 70 ,Reducfion ofarea_ Tensile strength.,

per cent I | Reduction ofarea. 120 percent - 601

^ 100 50 „- 5?

=9 80 40 c a c c

8 60 30-

c o 40

(A) Nickel (B)Chromium (with carbon 0.20%) (with carbon 0.20%) 20

I 2 3 4 I 2 3 Alloying element, per cent

Fig. 16.S. Effect of (A) nickel and (B) chromium on the tensile properties of rolled carbon steels. (Bain and Llewellyn)

0.65 per cent carbon (the S.A.E. 9200 series) and in certain steels for electric apparatus.

16.4. Effects of Nickel and Chromium

Nickel dissolves in the ferrite of carbon steel and exerts a strengthen-

ing effect that is somewhat less than that due to phosphorus, silicon, or properties of a manganese (Fig. 16.1) . The effect of nickel on the tensile

0.20 per cent carbon steel is plotted in Fig. 16.3A,* which shows that in the hot-worked (unhardened) condition nickel has little effect on elon-

gation and reduction of area; it does, however, raise tensile and yield strengths considerably. In the heat-treated condition, nickel steels are characterized by high impact resistance, which remains high at low temperatures. Nickel does not form a carbide. It increases the hardenability of carbon steels mildly, being less effective in producing deep hardening than molybdenum, manganese, or chromium. It also increases the resistance

• Figures 16.3A and B are based on data quoted by E. C. Bain and F. T. Llewellyn, Trans. Am. Soc. Civil Engrs., v. 102, 1937, p. 1249. Low-Alloy Steels as Engineering Materials 325 of carbon steel to some forms of corrosion, especially in combination with copper. Chromium dissolves in the ferrite and also forms carbides if the carbon is high. As indicated by Figs. 16.1 and 16.2 its ferrite-strengthening effect when in solid solution is weaker than that of any of the common alloying elements, but it increases hardenability greatly, as shown by the curves for the air-cooled specimens in Fig. 16.2.* The effect of chromium on the tensile properties of small air-cooled (hot-rolled) sections of a 0.20 per cent carbon steel is plotted in Fig. 16. 3B. In such sections, the increase in tensile and yield strengths and the decrease in elongation and reduction of area are much greater than the corresponding effects of nickel. Although chromium is a toughening element and increases the impact resistance of very low carbon steels, its primary function is to increase hardenability, decrease the magnitude of the mass effect, and, of course, increase corrosion resistance, especially if more than

3 per cent is present. Owing to its effect on hardenability, chromium is a common addition to the S.A.E. nickel steels in the proportion of about half as much chromium as nickel.

Chromium is used widely as an alloying element in high-carbon steels, where it forms a carbide and also dissolves in the ferrite. When these materials are quenched and tempered at a low temperature, they are very hard and have good wear resistance. They contain about 1.0 per cent carbon and 0.75 to 1.50 per cent chromium and are used for tools, dies, and ball bearings.

16.5. Effects of the Other Common Alloying Elements

The other alloying elements used in low-alloy steels are copper, molybdenum, vanadium, and, to a very limited extent, tungsten. Copper, in the small amounts added to carbon and low alloy steels dissolves in the ferrite, but its effect on tensile and impact properties is small. If more than 0.6 per cent is present, the steels, as noted in another chapter, may be precipitation hardened. Copper is most useful as an alloying clement for its effect on atmospheric corrosion. The addition of 0.10 to 0.40 per cent reduces this variety of corrosion so much that a large proportion of low- carbon sheets, especially when very thin, and many of the new high- strength structural steels now contain copper.

* If the carbon is low (below 0.08 or 0.09 per ceni), chromium up to approximately 3.0 per cent has only a small effect on the hardness and strength when steel is air cooled. For example. Franks (R. Franks. Trans, Am. Soc. Metals, v. 35, 1945, p. 616) reported an increase in tensile strength from 55,000 to 70,000 lb. per sq. in. in 0.08 per cent carbon steel when chromium was increased from to 3 per cent. With 6 per cent chromium, the strength was 197,000 lb. per sq. in. 326 Engineering Metallurgy

Molybdenum, vanadium, and tungsten are strong carbide formers. In the amounts present in low-alloy steels a portion of these three elements

dissolves in the ferrite and probably strengthens it moderately (Fig.

16.1). Vanadium is valuable chiefly for its effect on austenite grain size. Steels containing 0.10 to 0.20 per cent are usually fine-grained and are not so readily coarsened by overheating as some of the other low-alloy steels. Molybdenum, vanadium, and tungsten all contribute to deep hardening, and the first two are very effective in small amounts. Molybdenum

or vanadium is used in a number of low-alloy steels, especially the S.A.E. grades, usually together with another alloying element, such as

chromium, nickel, or manganese. Owing to its strong effect on hardenability and to the fact that the world's largest deposit of molybdenum ores is located in the United States, the use of molybdenum as an alloying element has increased greatly in the last few years. The most im-

portant function of tungsten and, to a lesser extent, of molybdenum is to increase the hardness and stability of the structure of alloy steels, especially at elevated temperature. For this purpose, relatively large amounts are required, usually in conjunction with chromium. This is discussed in a later chapter.

16.6. Low-Alloy Structural Steels as Engineering Materials

As noted previously, there are two broad classes of low-alloy engineering steels: the structural steels and the S.A.E. steels. The structural steels have a higher yield strength than carbon steels; they are usually of carefully balanced composition and are used for structural purposes without heat treatment other than natural or controlled cooling after hot rolling. The S.A.E. steels are, in general, much more costly than the low-alloy structural steels and should be heat treated to obtain optimum properties. In contrast to the structural steels, in which the function of the alloying elements is primarily to strengthen the ferrite and to produce a fine

pearlite when the material is air cooled, in the S.A.E. steels the alloying elements also affect the response of the steel to thermal treatment and, in a few cases, harden it by forming carbides. Low-alloy steels of high yield strength are forty years old. One of the first contained about 0.25 per cent carbon and 1.50 per cent manganese and was used between 1900 and 1915 to reduce dead loads in long-span bridges and steamship construction in the United States and in England. A little later, a deoxidized carbon steel (misnamed silicon steel) was developed in the United States. This material contained 0.25 to 0.35 per cent carbon, 0.70 to 1.00 per cent manganese, and 0.20 to 0.40 per cent silicon. In the hot rolled condition the yield strength of these two steels Low-Alloy Steels as Engineering Materials 327 was 45,000 lb. per sq. in. or higher, and a considerable tonnage was used for bridges between 1915 and 1928. A steel o£ similar yield strength, containing 0.12 per cent carbon, 1 per cent silicon, and 0.70 per cent manganese, was developed in Germany in 1925 but, owing to its variable quality, it was not used long. This German steel is important because it was the first attempt to produce a steel that had high yield strength but was low enough in carbon to be welded readily without the troublesome brittleness in and around the weld that frequently results from welding a steel containing more than 0.20 per cent carbon and 0.75 per cent manganese.

About fifteen years ago, the depression stimulated steel manufacturers to investigate the possibility of producing a low-cost, easily weldable, tailormade steel that would have strength properties and resistance to atmospheric corrosion much superior to those of ordinary structural steel. The depression also focused the attention of engineers upon the desir- ability of reducing cost in certain structures by the use of smaller sections of a steel of higher strength, and by welding instead of riveting. The result was that the steelmakers produced the steels and the engineers used them for railway rolling stock, streetcars, trucks, buses, cranes, steam shovels, and similar structures. One of the essential factors in this advance was the realization that tensile strength is not always the best criterion of the suitability of a steel for certain structural uses, that resistance to buckling or crumpling, which is related to yield strength, is essential, and that increased corrosion resistance is an economic necessity in many engineering structures.

The low-alloy structural steels have, as rolled, approximately twice the yield strength of low-carbon steel, combined with high ductility, easy weldability, and a resistance to atmospheric corrosion which is much higher than that of carbon steel. In tonnage lots, they cost 15 to 50 per cent more than comparable sections of plain carbon steel. Owing primarily to their high yield strength, it is possible to reduce the size of a section 25 to 35 per cent and still design a structure that has ample stiffness. Despite somewhat higher fabricating and machining costs, it is frequently possible to save 10 to 15 per cent of the cost of a structure by the use of the properly selected low-alloy steel.

16.7. Composition and Properties of the Low-Alloy Structural Steels

The first of the present crop of some thirty of these steels was made in 1929 and contained 0.30 per cent or less carbon, 1.05 to 1.40 per cent manganese, 0.60 to 0.90 per cent silicon, and 0.30 to 0.60 per cent chromium. Known under the trade name of Cromansil, this steel attracted 328 Engineering Metallurgy

considerable attention, and a number of others followed it quickly under such trade names as Cor-Ten, Man-Ten, Hi-Steel, Jal-Ten, Konik, Yoloy, Otiscoloy, N-A-X, Tri-Ten, Carilloy T,, and others. As a group, the low-alloy structural steels are characterized by low carbon (to improve ductility and weldability, and to prevent air hardening) and by the presence of at least two, and sometimes as many as six, of the following alloying elements: manganese, silicon, nickel, copper, chromium, molybdenum, phosphorus, boron, and vanadium. During the war titanium was an important minor constituent in these steels. The percentage of alloying elements is usually low. Manganese is 1.70 per cent or less, with an average of about 1.25 per cent. Silicon is 1 per cent or less, the usual amount being between 0.20 and 0.70 per cent. Nickel varies from 0.3 to 2.0 per cent, and copper, owing to its favorable effect on resistance to atmospheric corrosion, is usually at least 0.10 per cent and may be as high as 1.50 per cent. Molybdenum, if present, is less than 0.40 per cent, and chromium is less than 1 .5 per cent. Phosphorus, if used as an alloying element, is between 0.05 and 0.20 per cent, vanadium up to about 0.1, and boron around 0.003 per cent.

In all the low-alloy structural steels the composition is balanced to ensure for hot-rolled sections a minimum yield strength of 50,000 lb. per sq. in., a tensile strength of 70,000 to 105,000 lb. per sq. in., and an elongation of 18 to 30 |>er cent in 8 in., depending upon composition. The impact resistance of most of these steels is satisfactory; usually it is higher than 25 ft. -lb. Claims have been made that the endurance ratio is higher than for carbon steels, but not enough data have accumulated yet to accept this as generally true. As a group, the steels have a resistance to atmospheric corrosion considerably superior to that of carbon steels. These steels are outstanding in the relation of properties to cost. There are too many of them now; probably there will be some casualties in the next few years, but it seems certain that some will survive and that the survivors will be widely used as structural materials where a saving in weight is important.

16.8. The S.A.E. Low-Alloy Steels

Of the alloying elements in low-alloy steels, nickel and chromium were the first used commercially. Steels containing about 3 per cent nickel were forged into armor plate between 1885 and 1890, causing a revolution in naval warfare. Chromium was used even earlier: steels containing 0.60 per cent were installed as main members of the Eads bridge over the Mississippi River at St. Louis in 1874. Extensive use of the steels included in the S.A.E. grades began about 1910. It has been claimed with Low-Alloy Steels as Engineering Materials 329 considerable justification that the availability of these steels at a fairly reasonable cost was the chief factor in the development of the automotive and aircraft industries, and that without them the weight per horsepower of internal-combustion engines would be at least twice, and the efficiency less than half, what it is today.

Nearly fifty years ago, the Society of Automotive Engineers proposed specifications for the composition of a number of low-alloy steels that were being used at that time by the automotive industry. These specifications were adopted in 1911, and since that time the compositions recommended in the S.A.E. specifications have been used widely in all branches of mechanical manufacturing in the United States and abroad. Revision and extension of the specifications have kept pace with progress in the automotive and steel industries, with the result that engineers generally look upon the S.A.E. steels as the most satisfactory low-cost, high-quality steels commercially available for the production of automotive and aircraft equipment and machine tools, and they know that these steels will give satisfactory service when properly treated. and used.

in 1941, the Society of Automotive Engineers in collaboration with the American Iron and Steel Institute made major changes in the composition ranges of their standard steels. These changes consisted primarily of adopting narrower ranges for chemical analysis as made on a specimen representing a whole heat, plus an allowance for check analyses on individual specimens from the same heat. At the same time the American Iron and Steel Institute issued standard analysis specifications for low- alloy steels, which are known as A. I.S.I, specifications and which, in general, follow the same system of classification as is used by the S.A.E.

The S.A.E. specifications for low-alloy steels for 1956 cover 114 steels divided into eight series. These are shown in Table 16.1.* The A. I.S.I, specifications cover 170 steels divided into 13 classes, eight of which are the same and have the same series numbers as the S.A.E. grades.

The system of classifying these low-alloy steels is evident from Table 16.1. Series 1300 comprises the medium-manganese steels; 2300 contains 3.50 per cent nickel; and 2500 contains 5.00 per cent nickel. The 3100 series contains nickel and chromium but the amounts are different in

• In addition to specifications for low-alloy steels, both the Society of Automotive Engineers and the American Iron and Steel Institute have standardized unalloyed carbon steels. The S.A.E. specifications cover 22 carbon steels (series 10XX) ranging from less than 0.10 to 1.05 per cent carbon, and 10 high-sulphur free-machining steels (series 11XX) containing up to 0.15 per cent sulphur and as much as 1.65 per cent manganese. These specifications are given in the S.A.E. Handbook, 1956 edition, pp. 51-55, and in Steel Products Manual, Sec. 2. Carbon Steel, Semi-finished Products, published by the American Iron and Steel Institute, New York, 3d rev., June, 1943. 5

330 Engineering Metallurgy

the 3100, and 3300 series. The 4000 series is made up of plain molybdenum steels, the 4100 series of chromium-molybdenum, the 4600 and

4800 series are nickel-molybdenum, and the 4300 series which is popular in the aircraft industry contains nickel, chromium, and molybdenum. The last two digits of an S.A.E. or an A. I.S.I, number indicate the middle of the carbon range; thus, a 2330 steel contains 3.25 to 3.75 per cent nickel and 0.28 to 0.33 per cent carbon. The larger number of A.I.S.I. steels is due chiefly to more detailed specifications according to carbon

Table 16.1. Basic Numbering System and Composition Ranges for S.A.E. and A.I.S.I. Low-Alloy Steels *

Composition range, per cent Series S.A.E. or A.I.S.I. Molyb- Manganese Nickel Chromium denum Other

1300 1.60 to 1.90

2300 3.25 to 3.75 2500 4.75 to 5.25

3100 1.10 to 1.40 0.55 to 0.75 3300 3.25 to 3.75 1.40 to 1.75

4000 0.20 to 0.30 4100 0.80 to 1.10 0.15 to 0.25 4300 1.65 to 2.00 0.40 to 0.90 0.20 to 0.30 4600 1.65 to 2.00 0.20 to 0.30 4800 3.25 to 3.75 0.20 to 0.30

5100 0.70 to 0.90 5200 1.20 to 1.50

6100 0.80 to 1.10 0.15 min. V

8600 0.40 to 0.70 0.40 to 0.60 0.15 to 0.25 8700 0.40 to 0.70 0.40 to 0.60 0.20 to 0.30

9260 0.70 to 1.00 1.80 to 2.00 Si 9300 3.00 to 3.50 1.00 to 1.40 0.08 to 0.1 9800 0.85 to 1.15 0.70 to 0.90 0.20 to 0.30

t86BO0 0.40 to 0.70 0.40 to 0.60 0.15 to 0.25 Boron, .0015 to .0020

'S.A.E. Handbook, Society of Automotive Engineers, New York, 1956, pp. 50-55; Steel Products Manual, Sec. 10, Alloy Steels, American Iron and Steel Institute, New York, 3d rev., June 1945, pp. 13-30. t xxBxx, B denotes Boron Steel. ,

Low-Alloy Steels as Engineering Materials 331

percentage. For example, in the S.A.E. series 4100 there are 10 steels, covering a carbon range from 0.17 to 0.53 per cent; in the corresponding A. I.S.I, series there are 18 steels to cover the same range of carbon. In addition to carbon and alloying elements, these specifications give ranges for manganese (regardless of whether it is an alloying element) silicon, sulphur, and phosphorus. The A.I.S.I. specifications also carry letters that identify the process by which the steel was made: A for the basic open-hearth, D for the acid open-hearth, and E for the electric- furnace process. Some of the low-carbon S.A.E. alloy steels do not conform exactly to the ranges shown in Table 16.1. For example, in the low-carbon 4100 steels, the chromium is lower (0.40 to 0.60 per cent) and the molyb-

denum is higher (0.20 to 0.30 per cent) than in the steels containing more than 0.30 per cent carbon. The manganese in most standard S.A.E. steels varies widely. The 2300 series is typical; manganese is 0.40 to 0.60 per cent in the low-carbon grades, and 0.60 to 0.80 or 0.70 to 0.90 per cent in the higher carbon steels. The standard S.A.E. and A.I.S.I. steels are only a small fraction of the low-alloy steels used commercially. Some of these other steels vary slightly, but many vary greatly, from the standard compositions given in Table 16.1. A few of them, which are widely used and which vary slightly, usually in only one element, are given S.A.E. numbers prefixed

by the letter X. An example is X3140, the nominal composition of which is given in Table 16.2. In this steel the chromium is 0.70 to 0.90 instead of 0.55 to 0.75 per cent.

16.9. The S.A.E. Low-Alloy Steels as Engineering Materials Composition, series number, and recommended heat treatment for all the S.A.E. steels are given in the S.A.E. Handbook and in many other readily accessible sources and need not be considered here. These steels are more expensive than the low-alloy structural steels, and, furthermore, they must be heat treated to be of the greatest value. They are widely used in the construction of automobiles and aircraft— especially engines—and for machine tools, particularly for highly stressed parts such as crankshafts, camshafts, axle and other shafts, gears, bolts, studs, steering knuckles, spindels, collets, and valves. A few of the S.A.E. low-alloy steels are used in the rolled and air cooled conditions; there is one series, discontinued as standard by S.A.E. a few years ago, which contained less than 2 per cent nickel, that was widely used for engine bolts and boiler plate and, to a lesser extent, for bridge and ship construction. The medium-manganese series 1300 is also used occasionally as rolled and air cooled, and a few of the low-alloy 332 Engineering Metallurgy

structural steels discussed in § 16.7 are essentially S.A.E. 1320 containing a small amount of copper to increase resistance to atmospheric corrosion. Most of the S.A.E. steels are used in applications, especially in moving parts, where stresses are relatively high, and they must be quenched (usually in oil) and tempered to secure the maximum combination of strength and ductility. When properly heat treated, they combine high ductility with high strength. Compared with carbon steels of the same tensile strength and hardness, heat-treated low-alloy steels have 30 to 40 per cent higher yield strength, 10 to 20 per cent higher elongation, and 35 to 40 per cent higher reduction of area. The impact resistance of the

S.A.E. steels is frequently twice that of a carbon steel of the same tensile strength. The most important characteristic of the S.A.E. low-alloy steels as a class is that they are deeper hardening than carbon steels; in other words, the mass effect is smaller. It is easier to harden large sections of low- alloy steels throughout than is the case with carbon steels. Moreover, as the critical cooling rate is much lower, oil quenching can be used to attain high strength and hardness with less possibility of distortion or the introduction of high internal stresses.

Tempering temperature, deg. C.

700 900 1I0O 1300 800 1000 1200 1400 Tempenng temperature, deg. F. Fig. 16.4. Elfect of tempering on the tensile properties of oil-quenched (A) nickel- chromium (S.A.E. 3140) and (B) nickcl-chroniium-molybdenum (S.A.E. 4S40) steels. {S.AM. Handbook, 1941) Low-Alloy Steels as Engineering Materials 333

16.10. The New Metallurgy of Low-Alloy Steels

For many years charts of average or minimum tensile properties and hardness (occasionally impact resistance as well) of S.A.E. and other low-alloy steels, as quenched and tempered at various temperatures, were published in the S.A.E. Handbook and in other readily available sources. The properties indicated by these charts were, moreover, commonly used by engineers in the design of machines and other important equipment. Figure 16.4 shows two such charts. These charts of properties were considered invaluable because of the commonly held assumption that each alloy steel had a set of mechanical properties after heat treatment that was characteristic of, and due to, its chemical composition, and that, in general, these properties could be reproduced at will by subjecting any steel of the same specified composition range to the same thermal treatment. This assumption is statistically valid, but the accompanying and widely held assumption that certain low-alloy steels have inherently a better combination of strength and ductility than others (Fig. 16.4) is not valid, at least insofar as the assumption is based on charts of mechanical properties.

Table 16.2. Typical Tensile Properties of S.A.E. Low-Alloy Steels (0.35 to 0.45 * Per Cent Carbon) Quenched in Oil and Tempered at 1000° F. (540° C.)

Elon- Reduc- Nominal chemical Tensile Yield gation tion Steel composition, per cent strength, strength, in of No. lb. per lb. per 2 in., area, sq. in. sq. in. per per C Mn Ni Cr Mo cent cent

1340 0.40 1.75 141,000 120,000 18 52

2340 0.40 0.75 ! 3.50 137,000 120,000 21 60 3140 0.40 0.75 1.25 0.60 143,000 119,000 17 57 X3140 0.40 0.75 1.25 0.75 155,000 1 30,000 17 55 4140 0.40 0.75 1.00 0.20 170,000 147,000 16 56 4340 0.40 0.65 1.75 0.65 0.35 184,000 160,000 15 52 4640 0.40 0.65 1.80 0.25 151,000 131,000 19 56

* Data obtained on standard test specimens machined from bars less than 1 .5 in. in diameter.

Some typical tensile properties of S.A.E. low-alloy steels containing 0.35 to 0.45 per cent carbon as quenched in oil and tempered at 1000°F. (540°C.) are given in Table 16.2. These properties were collected by Hoyt* and are undoubtedly representative. For a definite heat treatment. 334 Engineering Metallurgy

tensile and yield strengths vary as much as 40,000 lb. per sq. in., and the variation in ductility as measured by elongation and reduction of area is considerable. The existence of such lists of properties (and published

charts of properties) makes it understandable that engineers designing a crankshaft for an internal-combustion engine, for instance, believed

that there is a marked inherent difference in the mechanical properties of heat-treated low-alloy steels. Two relatively recent events in ferrous metallurgy were mainly responsible for the development of what might be called a new philosophy of low-alloy steels and for changing greatly our ideas about the relation between composition and properties. They were also primarily responsible for the disappearance of mechanical-property charts from the S.A.E. Handbook (in 1942) and from many other publications and for the almost overnight development during World War II of the National Emergency steels, an accomplishment in which American metallurgists can take much justifiable pride. These two events were the work of Janitzky and Baeyertzf on the similarity in mechanical properties of heat- treated low-alloy steels, and the practical application of the principles underlying the relation of carbon, alloying metals, and hardenability. The relation among carbon percentage, the amount of alloying elements present, hardenability and other variables, and the properties of

steel is discussed in detail in the next two chapters. In brief, this relation

is as follows: If M per cent of nickel and N per cent of chromium pro duce a structure of 50 per cent martensite at the center of an oil-quenched

2-in. bar of steel containing 0.45 per cent carbon; and if X per cent of manganese and Y per cent of molybdenum produce the same structure in an oil-quenched bar of the same size and approximately the same carbon content, the tensile properties of the two steels after tempering to the same hardness will for all practical purposes be identical.

16.11. Similarity of Properties of Heat-Treated S.A.E. Low-Alloy Steels

Some of the data obtained by Janitzky and Baeyertz are reproduced in Figs. 16.5 and 16.6. These investigators quenched 1-in. round bars of

• S. I.. Hoyt, Metals and Alloys Data Book, Reinhold Publishing Corporation, New York, 1943, 334 pp. Metals Handbook, American Society for Metals, + E. J. Janitzky and M. Baeyertz, in Cleveland, 1939, pp. 515-518. The work of Janitzky and Baeyertz on the similarity of properties of heat-treated low-alloy steels was antedated by several years by work done but not published by the metallurgical laboratory of General Motors Corpora- tion which resulted in the removal of the old-style property chart from General Motors Standards several years previously. The first publication of a part of this work was by A. L. Boegehold, Metal Progress, Feb., 1937, pp. 147-152. ,

Low-Alloy Steels as Engineering Materials 335

260 o

1 240 y. ^ |220 b 9

"7 200 Yields*-en -jtn- A , G

\ 180 «7 t*~ 1*160 7 • 1 Wc ter quenched 140 1 « S. A. €.1330 & * I 2330. & 3130 120 v 4130— &9 a 5130A ° 613 100

80 120 160 200 240 20 40 60 Yield strength Elongation and reduction thousand lb.pcrsq.in. of area, per cent

Fig. 165. Tensile properties of water-quenched and tempered S.A.E. low-alloy steels containing about 0.30 per cent carbon. (Janitzky and Baeyertz)

a number of S.A.E. alloy steels in water and oil and tempered them at various temperatures in accord with recommendations given in the S.A.E. Handbook. Yield strength, elongation, and reduction of area were plotted against tensile strength with the result shown in Figs. 16.5 and 16.6. For tensile-strength values of less than 200,000 lb. per sq. in., there is little difference in the steels. At high strength values considerable scatter is evident; this is undoubtedly due to internal stresses caused by quenching rather than to any inherent difference in the steels. The data reported by Janiuky and Baeyertz in 1939 have been confirmed since by a number of other investigators. Patton,* for example, plotted tensile properties and Izod impact resistance for 409 tests on 180 heats of 37 grades of carbon and low-alloy steels. The steels used were mostly N.E. and S.A.E. grades, but a considerable number of others were included; all of the steels, however, were of such a composition that it was reasonably certain that the test specimen hardened completely in quenching. For tensile-strength values of 100,000 to 200,000 lb. per sq.

• W. G. Patton, Metal Progress, v. 43, 1943, pp. 726-733. — ' .

336 Engineering Metallurgy

~»

280 » 1 •» -•r 9 *H » 260 r « A 1

a. Yiela'strength—>. 6* * _220 A A A "2

§200 PI £ _._L--_ (* 3 3 A— -a- //I ?/n

€l80 c © © c Oil quenched 5 I / * S.AE.234S i L "160 — « 3115- © • 3240 . •, r .s ^Reduction - ow— 140 f- a 4310- • —-> Of area . J © 4fi«- 1/ S/15 h 120 ' • V [a 1 I 1 1 J 80 120 160 200 240 20 40 60 reduction Yield strength, Elongation and area, per cent thousand lb. per sq. in. of

S.A.E. low-alloy steels Fie. 16.6. Tensile properties of oil-quenched and tempered containing about 0.40 to 0.45 per cent carbon. (Janitzky and Baeyertz)

reduction-of-area values in., yield-strength, elongation (in 2 in.), and least two-thirds of fell close to a line representing expected values in at that varied the tests, and 90 per cent of the values fell within a band not more than ± 5 per cent from the expected values. expected Of the values that varied more than ± 5 per cent from the values the variation was such that no single steel and no group of steels could be considered superior to any other steel or group of steels. It was above 200,000 also evident that steels heat treated to a tensile strength responsible for con- lb. per sq. in. contained internal stresses that were than siderable scatter in properties. Impact resistance was more erratic other property values, varying 20 per cent or more from the line of expected values. The most recent correlation of mechanical properties, for low-alloy 16.7.* steels containing 0.30 to 0.50 per cent carbon, is shown in Fig.

• Courtesy American Society for Metals. Low-Alloy Steels as Engineering Materials 337

100 120 140 160 180 200 Tensile Strength, 1000 psi

FlC. 16.7. Normally expected mechanical properties (heavy line) and average variation (hatched band) of quenched and tempered low-alloy steels containing 0.30 to 0.50 per cent carbon (Metals Handbook, 1948, page 458).

16.12. The Engineering Properties of the S.A.E. Low-Alloy Steels

Some typical tensile properties of S.A.E. carbon and low-alloy steels, including a few triple-alloy (N.E.) steels, collected from various sources are given in Table 16.3. All the steels whose properties are given in this tabulation were heat treated to a tensile strength of 150,000 lb. per sq. in.

(Brinell hardness number approximately 300) . The similiarity in properties of the low-alloy steels is evident; also evident is the inferior yield 338 Engineering Metallurgy strength and ductility of the carbon steels which are inherently shallow hardening in the sizes treated, namely, 1- to li^-in. bars.

The effect of tempering on the tensile properties of 1-in. round bars of S.A.E. low-alloy steels containing 0.30 to 0.35 and 0.40 to 0.45 per cent carbon is shown by the charts of Fig. 16.8. These are the properties that can be expected 90 per cent of the time if an S.A.E. low-alloy steel of a size that will harden with a structure of approximately 50 per cent martensite at the center when quenched in oil is tempered at the temperatures shown.

Table 16.3. Typical Tensile Properties of S.A.E. and N.E. Carbon and Low-Alloy * Steels Heat Treated to a Tensile Strength of 150,000 lb. per sq. in.

Elon- Reduc- Nominal chemica Tempering Yield gation tion Steel composition, per cent temperature strength, in 2 of No. lb. per in., area. sq. in, per per C Mn N1 Cr Mo V °F. °c. cent cent

1045 0.45 0.75 600 315 114,000 10 35 1060 0.60 0.75 750 400 113,000 15 45 1080 0.80 0.40 950 510 111,000 16 47

1340 0.40 1.75 950 510 128,000 16 50 2340 0.40 0.75 3.50 950 510 132,000 20 59 3130 0.30 0.65 1.25 0.60 850 455 128,000 17 57 3140 0.40 0.75 1.25 0.60 950 510 127,000 17 56 X3140 0.40 0.75 1.25 0.75 1000 540 130,000 17 55

4065 0.65 0.80 0.20 1100 595 130,000 16 47 4130 0.30 0.65 0.65 0.20 1050 565 131,000 20 61

X4130 0.30 0.50 0.95 0.20 1050 565 130,000 20 57 4140 0.40 0.75 1.00 0.20 1100 595 131,000 18 60 4340 0.40 0.65 1.75 0.65 0.35 1200 650 130,000 19 57 4640 0.40 0.65 1.80 0.25 1000 540 131,000 19 56 5140 0.40 0.75 1.00 1000 540 130,000 18 55 6150 0.50 0.75 1.00 0.18 1200 650 135,000 18 54

8640 0.40 0.90 0.50 0.50 0.20 1050 565 133,000 17 53 8740 0.40 0.90 0.50 0.50 0.25 1100 595 130,000 18 56 8750 0.50 0.90 0.50 0.50 0.25 1200 650 129,000 18 54 8950 0.50 1.00 0.50 0.50 0.40 il200 650 133,000 18 57 9440 0.40 1.10 0.45 0.40 0.12 1050 565 130,000 18 57

* All test data obtained on standard specimens machined from bars 0.75 to 1.5 in. in diameter. All quenched in oil except 1045, which was quenched in water. Low-Alloy Steels as Engineering Materials 339

Tempering temperature .deg.C. 400 500 600 300 400

600 800 1000 1200 60O 800 1000 Tempering temperature, deg.F.

Fie. 16.8. Effect of tempering on the normally expected tensile properties of low- alloy steels containing (A) 0.30 to 0.35 per cent carbon, and (B) 0.40 to 0.45 per cent carbon.

160

ISO

140

•E 130 It" ||II0

3100 | B. 8b30" A. X3I40 Carbon 0.30°, "6 J -6 90 ' Carbon 0.407. Mawnese 0.807. - c

, Manganese 015V Nickel. 0.657. - 1.25% Nickel Chromium 0.507. if TO • Chromium 0.157.- Molybdenum 0.257." Reduction ofarea si £e

I 2 3 I 2 3 Diameier of section, in. Diameter of section.in.

Fig. 16.9. Effect of mass on the normally expected tensile properties of (A) a through-hardening (X3140) steel, and (B) a semithrough-hardening (8630) steel, quenched in oil and tempered at 1000°F. 340 Engineering Metallurgy

The mechanical properties of heavy sections o£ heat-treated low-alloy steels depend on how deeply the material hardens in quenching and upon several other variables such as grain size, quenching tenqjerature, and quenching medium. The higher the carbon (up to about 0.60 per cent) , the harder and stronger is the material at and near the surface; and, in general, the higher the percentage of alloying metals promoting hardenability, the harder and stronger is the steel both at the surface and throughout the cross section.

The effect of section size on two low-alloy steels is shown in Fig. 16.9.* The nickel-chromium steel X3140 could be classed as a through-hardening material, and the triple alloy steel 8630 is undoubtedly semithrough- hardening material. Both steels have a relatively high strength (after oil quenching and tempering at 1000°F.) in small sections; and in both steels the strength decreases as the size of section increases, but the tensile and yield strengths of 8630 decreases to a greater extent and more rapidly with increasing section size than those of X3140. The impact resistance of the low-alloy steels depends on a number of variables including composition, grain size, thermal treatment, and others. Patton, whose data were discussed in the previous section, plotted the Izod impact values given for the 400 specimens and found that the normal expected value decreased from 85 or 90 ft.-lb. for steels with

100,000 to 120,000 lb. per sq. in. to about 20 or 25 ft.-lb. for steels or

200,000 lb. per sq. in., leveling off at this value for higher tensile strengths.

Because impact resistance is very sensitive to slight variations in structure, Patton found much more scatter than in any of the other properties.

The endurance ratio of polished specimens of low-alloy steels is approximately the same as, or perhaps slightly lower than, for carbon steels. Available data in the literature give ratios of 45 to 50 per cent with the average a little below 50. Because of internal stresses, the endurance limits of heat-treated low-alloy steels with a tensile strength of 250,000 lb. per sq. in. or higher do not exceed approximately 100,000 lb. per sq. in.

QUESTIONS

1. Why should low-alloy steels be balanced? 2. Give as many advantages as you can as to why low-alloy steels arc becoming so popular. 8. Explain the effects of adding small amounts of nickel to steels. 4. Explain the effects of adding small amounts of chromium to steels.

• Data on X3140 from International Nickel Co., Handbook on Nickel Steels, § 2, 1934; data on 8630 from Republic Steel Corp., Handbook on N£. Steels, 1943. Low-Alloy Steels as Engineering Materials 341

5. Explain the effects of adding small amounts of molybdenum to steels. 6. Explain the effects of adding small amounts of vanadium to steels. 7. Explain the effects of adding small amounts of tungsten to steels. 8. Explain the effects of adding phosphorus to steels. 9. What arc the disadvantages of adding phosphorus to steels? 10. Name three alloying elements that arc carbide formers. 11. Why is the carbon content low for low -alloy steels? 12. What advantages do low-alloy steels have compared to plain carbon steels when quenched? IS. Name twenty specific uses for low-alloy steels. 14. Compare in chart form the physical properties of low-alloy steels containing 0.40 per cent to a plain carbon steel containing 0.4 per cent carbon. 15. Explain why silicon is not used to any great extent in low-alloy steels. 16. List various sources where physical properties of low-alloy steels may be obtained. 17. What do the letters S.A.E. stand for? 18. What do the letters A.I.S.I. stand for? Hardenability CHAPTER

17 Walter M. Hirthe, M.S.M.E., Assistant Professor of Mechanical Engineering, College of Engineering Marquette University, Milwaukee, Wisconsin Richard O. Powell, College of Engineering, Tulane University, New Orleans, Louisiana

r OR more than a thousand years, hardness has been the most highly valued attribute of iron-carbon alloys; in fact, the word "steel" has long been an accepted synonym for hardness, durability, and reliability. This is natural; to early social groups, with limited knowledge of the properties of the metals then available, unhardenable wrought iron was no better, and in many instances worse, than properly made and fabricated bronze. When wrought iron was carburized and heat treated, however, the resulting steel was far superior to anything previously available, not only for weapons, but also for plowshares and other tools essential to an agricultural and pastoral society. It is not surprising, therefore, that hardness became a supreme and lasting virtue in metals, and the medieval metal worker who knew how to produce hardened steel was for centuries considered something of a magician. Today, as well as a thousand years ago, no one questions the importance of hardness and its accompanying high strength; in fact, the ease with which hardness and its related properties can be controlled by thermal treatment makes steel man's most important metallic material. Owing, however, to inherent limitations in our present methods of thermal treatment, hardness may be only superficial. Metallurgists have realized for many years that hardenability, which, briefly, is the depth to which martensite will form in definite sections under definite cooling conditions, is often as important as actual hardness, and there are many applications for which steel should be practically as hard and strong at or near the center as at the surface. Surface-hardened steels are valuable for many engineering uses, but if weight is to be saved and high stresses

342 Hardenability 343 withstood, steels hardened throughout are necessary. In addition—and this is equally important—a steel hardened throughout by quenching has properties, as tempered to a definite hardness, that are superior to the properties of a steel tempered to the same hardness but not hardened throughout.

17.1. Hardness and Hardenability in Carbon Steels

Among engineers who use heat-treated steel there is considerable confusion concerning the difference between hardness and hardenability, that is, between the maximum hardness that a steel attains in quenching and the depth to which it hardens. There is no direct and close relation between the two, despite the fact that hardness is used to measure hardenability. An example or two should make this clear. Suppose that two 1-in. bars of carbon steel containing, respectively, 0.30 and 0.70 per cent carbon are quenched from above the transformation temperature into ice water or iced brine. The surface hardness of the first will be approximately 55 Rockwell C and that of the other one about 65 Rockwell C. Both bars will harden to a depth of about \/A in. The hardness values of the two steels differ (at the surface), but the hardenability is the same.

If small specimens of either carbon or low-alloy steels are cooled from the proper quenching temperature at the critical cooling rate or faster, so that no transformation occurs before martensite begins to form, the

0.40 0.5O 0.60 0.70 0.80 0.90 Carbon, percent

Fjc. 17.1. Relation between carbon content and maximum martensitic hardness (upper curve), and maximum hardness usually attained (hatched band). {Based on data by Burns, Moore, and Archer, and by Boegehold) 344 Engineering Metallurgy

maximum hardness attainable depends on the carbon content. This is shown by Fig. 17.1.* It should be emphasized that the upper curve shows the maximum hardness obtained when the specimen is entirely martensitic in structure. In the commercial heat treatment of carbon and low- alloy steels this is rarely realized, except with very small sections. Ordi- narily the martensite is contaminated by varying small amounts of transition structures that reduce the hardness appreciably. In consequence, many metallurgists put the highest practical hardness obtainable in the commercial quenching of these steels at 5 to 10 Rockwell C numbers below the maximum, as is shown by the hatched area in Fig. 17.1.

17.2. Hardness and Hardenability in Low-Alloy Steels

The difference between hardness and hardenability is also well illustrated by Fig. 17.2,* which shows hardness values over the entire cross section of quenched bars of various diameters of S.A.E. 1045 (0.45 per cent carbon) and S.A.E. 4140 (0.40 per cent carbon, 1.0 per cent chromium, 0.20 per cent molybdenum) steels. If 1-in. bars of these two steels are quenched in water from above the transformation temperature

(Fig. 17.2), their hardness at the surface is practically the same; at the center, however, S.A.E. 4140 has a hardness of 59 Rockwell C, whereas S.A.E. 1045 has a hardness of only 34 Rockwell C.

This illustration shows clearly that two steels of about the same carbon content may have the same hardness at the surface but a different hardenability. They may also have a very different hardness at the surface and a different hardenability, depending primarily upon the relation of the cooling rate to the location of the nose of the S curve and, consequently, upon whether the rate of cooling is sufficient to prevent much pearlite from being formed during quenching.

17.3. Cooling Rate and Hardenability

In many ways steel is a very cooperative metal and responds readily to the treatments man gives it, no matter how unreasonable or illogical these may be. In other ways, however, it is as stubborn as the traditional army mule, and, like it or not, we can do little about it. Heat extraction is typical of this characteristic. We can put heat into a piece of steel by

Fig. 17.1 is I.. L. •The upper curve of based on data collected by J. Burns, T. Moore, and R. S. Archer, Trans. Am. Soc. Metals, v. 26, 1938, pp. 1-36. The hatched band is based primarily on data given by A. L. Boegehold, S. A. E. Trans., v. 52, 1944, pp. 472-485. * Based on diagrams published in U. S. S. Carillory Steels, Carnegie-Illinois Steel Corporation. Pittsburgh., 1938, 214 pp. Hardenability 345

1 i S.A.E. 04! ,QUEN :hed in WAid

i i

1 f 1 I

V /

\ - J / \ ) / \\S ' / V \ / s \ \ -:: ;>

H * : *-- "::z. A ~* m — ^ --» --H Diameter of bar.in. Diameter of bar, in.

Fig. 17.2. Hardness over entire cross section of bars of various diameters of S.A.E. 1045 and S.A.E. 4140 steels quenched in water. (U. S. S. Carilloy Steels) electrical or other methods at almost any rate we choose, but we cannot extract the heat from the interior any faster than thermal conductivity will permit. No matter how fast we chill the surface, the heat will flow

from the center to the surface of a large section at a slow rate, which is

largely unaffected by the composition of the material. This process is complicated by the fact that during quenching the steel conforms approximately with Newton's law of cooling—that heat loss is proportional to the temperature difference between the bar and the quenching medium —and also by the fact that, quenching mediums vary greatly in their ability to extract heat from the surface of hot steel.

And it must be remembered that it is not always desirable to quench steels drastically in water, as this treatment is likely to result in high internal stresses which may cause severe warping, cracking, or some other unhappy consequence. 346 Engineering Metallurgy

17.4. Time Delay and Hardenability

The relatively slow heat transfer from the center to the surface of a

section 1 in. or larger severely limits our ability to cool steel rapidly

enough to secure deep hardening, if the critical cooling rate is high. There is consequently only one method of securing deep hardening:

reduce the critical cooling rate. In other words, what we do is to decrease the tendency of austenite to transform to pearlite in the temperature range of the nose of the S curve, or to bainite at lower temperatures; this is another way of saying that we push the line of the S curve that marks the beginning of austenite transformation below the critical tem-

perature to the right so that more time is available for the steel to cool to 600°F. (315°C.) or below without such transformation.

Below the A t transformation temperature, austenite is, of course, unstable; and the more the temperature is lowered, the greater becomes the instability. The tendency of unstable austenite to transform increases with lowered temperatures; but this is counteracted to a degree by the fact that as the steel gets colder it gets more rigid; that is, its ability to transform decreases.

The forces involved in the tendency of unstable austenite to transform to pearlite, bainite, or martensite are complex, and at present they are not well understood. The net result, however, is that in all steels there is a time interval, commonly known as time delay, which elapses before the unstable austenite begins to decompose. This interval may be very short or very long, depending upon a number of factors— in addition to temperature, which is most important— that include among others homogeneity of austenite, grain size, and chemical composition. Small differences in time delay are important in hardenability; but even so, all carbon steels and many low-alloy steels have a time delay at the nose of the S curve that is rarely more than 3 to 5 sec. (for continuous cooling), which means that few of them harden to the center except in very small sections. In other words, changes in the S curve due to continuous cooling may be important in the heat treatment of a razor blade or a small tool but not of a 2-in. shaft or a 55-mm. gun tube.

17.5. Variables Affecting Hardenability

There are a number of variables that affect time delay and thus affect hardenability. Homogeneity of the austenite increases its sluggishness and thus increases time delay. Heating long enough or at a high enough temperature to ensure homogeneity is also likely to change the grain size, and since grain size greatly affects hardenability it is difficult to untangle the two variables. It is, however, a fact that, if one or more areas in the Hardenability 347 austenite are high or low in carbon or in alloying elements, transformation during cooling will be accelerated. Premature transformation in a segregated area has a "trigger action" that sets off transformation over the whole section, thus decreasing time delay and hardenability. Since homogeneity depends on diffusion and diffusion takes time, one of the common but insufficiently recognized causes of a variation in hardenability in steels of the same composition is insufficient time at the quenching temperature for the austenite to become homogeneous. This is also the reason why the hardenability of a steel as determined in the laboratory, where the homogeneity factor is likely to be taken care of, may differ from the hardenability of the same steel heat treated in large lots in the plant.

Another and even more important factor is austenite grain size. Trans- formation begins principally at the grain boundaries; hence, as the number of grains increases, the likelihood that transformation will start quickly is increased. Small grain size thus decreases time delay and lowers hardenability. On the other hand, increasing grain size decreases the chances for transformation to begin quickly; it lengthens the total time for the reaction to start and increases hardenability.

Much work has been done in this field, which can be summarized

briefly by stating that if other factors do not enter, increasing the grain size from about 8 (A.S.T.M. rating) to 5 or 6 will double the time available for the austenite to cool to a low temperature without transforming to pearlite; and increasing the grain size to 2 or 3 will more

than double the time and it may even triple it.

Grain-size control during melting is relatively simple; it would, therefore, be easy to increase the hardenability of shallow-hardening grades by using a steel that would be coarse grained just before quenching. Unfortunately, however, the consequences might not be pleasant. Coarse-

grained steels frequently crack if cooling is drastic, and they are likely to be sadly lacking in toughness (impact resistance) at room temperature and below. In fact, this lack of toughness is usually so dangerous that most steels that are to be used in highly stressed parts are deliberately made fine grained, and deep hardening is secured by the use of alloying elements.

By a proper balance of carbon content and the various alloying elements, the critical cooling rate can be lowered so much (that is, the

time delay is greatly increased) that the steel will harden deeply when large sections are quenched in oil or even on cooling in air. This, and the methods used for measuring hardenability, are the subjects covered in the following sections. 348 Engineering Metallurgy

17.6. Methods of Determining Hardenability

The first of several tests proposed to measure hardenability directly is the penetration-fracture (P-F) test developed by Shepherd.* This involves quenching short bars, % in. in diameter, in a brine solution after homogenizing for 30 min. at 1450, 1500, 1550, and 1600°F. (790, 815,

845, and 870°C). The bars are fractured and the fracture is compared with a standard. One half is polished on the end and etched with acid to determine the depth of hardening. The steel is rated according to the numerical values of the fracture (grain size) and the penetration (hardenability). The test demands considerable skill and experience. It has never been used widely except by companies making or using large quantities of high-carbon tool steel. It is possible by this test to classify tool steels as sensitive or insensitive to grain growth with increasing quenching temperature and as deep or shallow hardening.

Unquestionably, the most reliable test for hardenability is made simply by quenching bars of varying sizes from the proper temperature, cutting the bars in half, and determining Rockwell C hardness from center to surface. These values are then plotted as a U-shaped diagram similar to that shown in Fig. 17.2. For a study of the hardenability complete enough to rate a steel accurately it is necessary to prepare, quench under controlled conditions, and measure the hardness of, the transverse sections of five to eight or more bars varying in diameter from i/, to 4 in. or more, which takes 15 to 20 hr. This is the only disadvantage of the test, but it is a major one.

Twenty years ago, when hardenability was attracting wide attention, one of the important problems in the field was to devise a simple, readily reproducible, comparatively accurate test to determine the relative response of the commonly used medium-carbon and low-alloy steels to quenching.

It had been apparent for some time that, if the hardness values at the center of quenched bars are plotted against the diameter, depth-hardening curves result. Four such curves are shown in Fig. 17.3. The two curves in Fig. 17.3A are plotted from the data given in Fig. 17.2, and the difference between the shallow-hardening carbon steel (1045) and the deep-hardening chromium-molybdenum steel is clear. The curves plotted in Fig. 17.3B are from hardness data on two low-alloy steels which differ slightly in response to quenching. The hardenability of these two steels is practically the same for \/r and s^-in. bars and for bars larger than H/£ in., but it varies considerably with intermediate sizes.

• B. F. Shepherd, Trans. Am. Soc. Metals, v. 22, 1934, pp. 979-1016. Hardenability 349

3 4 5 'h V4 I Diameter of bar. in.

Fig. 17.3. Depth-hardening curves for (A) water-quenched shallow-hardening (1045) and deep-hardening (4140) steels and (B) two low-alloy steels differing in hardenability in intermediate sizes.

It was, of course, immediately recognized that any test that could be used to determine hardenability rapidly and cheaply must distinguish not only between deep- and shallow-hardening steels where differences are great (Fig. 17.3A) but also between the hardenability of two steels whose depth-hardenability curves resemble those plotted in Fig. 17.3B. Such a test was developed by Jominy and his coworkers at the research laboratory of General Motors Corporation and has now been standardized and is used for specification and control purposes.*

17.7. The Jominy End-Quench Hardenability Test

The Jominy test consists of quenching the end of a 1-in. round bar under controlled conditions and determining the hardness (Rockwell C) at intervals along the bar, beginning with the quenched end. The fixture and the test bar are shown in Fig. 1 7.4. The test bar is rough-machined and normalized by heating to approximately 150°F. (85°C.) above the

A 3 transformation. After holding at this temperature for 30 min. the bar is cooled in air and finish-machined to the dimensions shown in Fig. 14.6.

It is then heated in a nonscaling atmosphere to about 75°F. (40°C.) above A z and held 20 min., after which it is rapidly withdrawn from the furnace by the collar, placed in the fixture, and a column of cold

• The method has been tentatively standardized by American Society for Testing Materials (A. S. T. M. Standards, Part 1A, 1955, A255-48T, pp. 636-642) and by Society of Automotive Engineers (Handbook, 1953, pp. 112-120). I

350 Engineering Metallurgy

water passing through a i/£-in. pipe and under pressure sufficient to rise

2i/£ in. above the opening is directed against the red-hot end until the

specimen is cold.

Two flat surfaces 1 80 deg. apart and 0.015 in. deep are ground longitudinally on the bar, and Rockwell C hardness measurements are taken at 45'-. -in. intervals, \Unimpe

1 r l-in. round about 2 in. The hardness values are specimen plotted against distance from the quenched end, which results in curves + Waterat 7tttS' similar to those in Fig. 17.5. 'i-in. inside diam orifice Owing to the method of quenching From quick- and to the laws of heat flow, the opening valve .._' Jominy bar when cold has a structure and hardness that result from cooling Fig. 17.4. The Jominy standard rates which vary from about 600°F. end-quench hardcnability test. 8 (335 C.) per sec. on the quenched face and about 420°F. (235°C.) per sec. at % e in. from the quenched end to less than 10°F. (5°C.) per sec. at the other end of the bar. These are equivalent to the center cooling rate of a i/g-in round bar quenched in agitated cold water as one extreme and a 4-in. round bar quenched in oil as the other.

Some typical curves obtained by the Jominy test arc reproduced in

Fig. 17.5.* In these the distance is shown as inches from the quenched end; in many curves it is shown as the number of sixteenths of an inch.

The horizontal hardness value indicated by the short dashed lines is the average maxima obtained by quenching unalloyed steels of the median carbon content indicated.

17.8. Relation of the End-Quench Test to Actual Cooling Rates and the Selection of Steel by Hardenability

Jominy and a number of others, including a Society of Automotive Engineers' committee on hardenability tests/ have done much work in

•W. K. Jominy, Metal Progress, v. 38. 940. pp. 685-690. fS. A. E. Handbook, 1953, "Recommended Practice for Determining Harden- ability," pp. 112-120 Hardenability 351

Rate of cooling at I300°R, °F. per sec. 420190 100 75 55 45

'/4 3/8 '/2 5/8 3/4 »/« Distance from water cooled end, in.

Fig. 175. Jominy end-quench curves for carbon and low-alloy sleels containing 0.40 per cent carbon. (Jominy) recent years in correlating Jominy distance and hardness with cooling rates. Such a correlation is shown in Fig. 17.6, in which is plotted the cooling rate at center and surface and at various radii, bar size, and the position on the Jominy bar. By using the values given in Fig. 17.6, we can readily tell what the hardness should be at the center, at one-half radius, at three-fourths radius, and at the surface of any steel whose end- quench curve has been determined. Suppose, for example, that we want to select a steel for a li^-in. round shaft that should be quenched in oil and tempered at a fairly high temperature and that should have a minimum Rockwell C hardness of about 35 at the center and 40 at the surface. Tempering at 1000°F. (540°C.) causes an average reduction in hardness of 15 to 20 numbers at the surface and 7 to 10 numbers at the center; so the hardness of the steel as quenched in oil should be 55 to 60 Rockwell C at the surface and 42 to 45 Rockwell C at the center. According to Fig. 17.6, the center cooling rate in oil for a li^j-in. round bar is 25°F. per sec. and the corresponding distance on the Jominy bar is % 6 in.; the surface cooling rate is about 90°F. per sec, the distance is slightly in. and Jominy more than % 6 It is evident from the curves shown in Fig. 17.5 that our requirements can readily be met by S.A.E. X3140 but not by the other four 0.40 per cent carbon steels whose curves arc reproduced in Fig. 17.5. There are, of course, many other low-alloy steels that will serve equally well for this application. 352 Engineering Metallurgy

Goolinq rate.deg. F. per sec. at 1300'F. § 8 » ssssa ^350=93^3355* 4 lllll 1 § ? i i i i i i / / / / A / / / / /.., 3 i/r / .c / \1 42 12 f Ml ^i s m 1 Water -quenched rounds

2 J 4 5 6 1 8 9 10 12 14 16 18 20 24 32 48 Distance from quenched end, sixteenths of on inch

FlG. 17.6. Correlation of surface, center, and intermediate cooling rates, bar sizes, and distance from quenched end of Jominy bar, for oil and water quenching round bars. (Boegehold)

17.9. Virtues and Shortcomings of the Jominy End-Quench Test

The Jominy test can be made relatively quickly and cheaply, and the curves for a particular lot of steel obtained in one laboratory check closely with results from other laboratories. This agreement is fortunate because even before a tentative standard was worked out by the American

Society for Testing Materials in 1942, it was put to good use in the war- time development of the low triple-alloy steels. In brief, the method used when the common alloying elements became short and had to be conserved, and when insufficient time was available for extensive mechanical and service testing, was to make up experimental heats of the new triple-alloy steels, adjusting the composition according to the expected effects of carbon and the alloying elements, and to determine the end-quench hardness characteristics. If the end-quench curve of Hardenability 353

standard the new steel coincided closely with that of one or more of the higher alloy steels, it was reasonably certain that the new steel could be substituted for these standard steels with considerable assurance that it would have the mechanical properties desired after heat treating to obtain a definite hardness at a definite distance on the Jominy bar. The Jominy test will tell with accuracy whether the nose of the S curve has been missed in quenching and that there is no pearlite in the microstructure of quenched bars of certain diameters. It will not tell, however, whether the structure as quenched is contaminated by bainite. acicular There is little difference in the hardness of martensite and that of marked bainite which forms just above the M R temperature, but there is a difference in the properties of tempered martensite and those of tempered bainite of about the same hardness. Consequently, because the end- quench test does not make this distinction, there is no guarantee that structure and properties will be exactly what is required. There are other shortcomings of the end-quench test. These are not inherent in the test itself; rather, they arc precautions that should be observed in applying the results too widely. If a steel is segregated or otherwise heterogeneous (and many large heals are), Jominy curves of a specimen cut from a segregated bar will be different from the curves of specimens cut from a bar that is not segregated, even though the two bars were rolled from the same heat. Another shortcoming is that the harden- test tells only the hardenability of a round bar. To determine ability of other shapes, and especially of odd-shaped sections, such as a gear blank, a rather complicated procedure must be used.

17.10. Hardenability Bands

About ten years ago, the steel industry, represented by the American Iron and Steel Institute, and the major consumers of heat-treatable carbon and low-alloy steels, represented by the Society of Automotive Engineers, worked out hardenability bands for a few steels, to be used for specification purposes. The composition limits of three of these steels, designated by the letter H (Table 17.1), are somewhat wider than ordinarily permitted. Three such bands for the steels given in Table 17.1 are shown in Fig. 17.7. The steelmaker must produce a material whose

hardenability is within the specified limits, and he is, therefore, given slightly more latitude in chemical composition so that he can make adjustments of the analysis while the steel is still in the furnace. Thus, if

the chromium is on the low side and cannot be increased readily, he can keep manganese, or molybdenum, high to ensure that the hardenability will be within the required limits. 354 Engineering Metallurgy

Table 17.1 Composition Limits (1955) for Three Low-Alloy Steels for Which Hardcnabihty (H) Is and Is Not Specified

Chemical composition, per cent Steel No. C Mn Si Ni Cr Mo

4140 0.38-0.43 0.75-1.00 0.20-0.35 0.80-1.10 0.15-0.25 4140H 0.37-0.44 0.65-1.10 0.20-0.35 0.75-1.20 0.15-0.25

8640 0.38-0.43 0.75-1.00 0.20-0.35 0.40-0.70 0.40-0.60 0.15-0.25 8640H 0.37-0.44 0.70-1.05 0.20-0.35 0.35-0.75 0.35-0.65 0.15-0.25

8740 0.38-0.43 0.75-1.00 0.20-0.35 0.40-0.70 0.40-0.60 0.20-0.30 8740H 0.37-0.44 0.70-1.05 0.20-0.35 0.35-0.75 0.35-0.65 0.20-0.30

Incidentally all H steels are made by a practice that ensures fine grain, which means that the hardenability is obtained solely by adjusting the chemical composition.

17.11. Relation of Hardenability to Engineering Properties

If steels two are treated so that their structure is the same, their important engineering properties will be the same most of the time.» The work of a number of investigators has established this beyond question, and it is universally accepted as fundamental in ferrous metallurgy. For steels used in highly stressed engineering structures and machines, the primary object of heat treatment is to produce optimum properties by controlling the size, character, and distribution of the carbide, and the chemical composition is the principal variable by which this control can be exercised.

Thus, the amount of carbon is controlled to ensure that the martensite or other phase formed by thermal ueatment-and the structure resulting from a change in these constituents by tempering—will have the required hardness and strength; and one or several alloying elements are added, if necessary, to ensure that the desired structure and properties will be attained at the depth beneath the surface that is required for a specific application. Optimum properties result from a structure that is mostly martensitic before tempering.

• Theoretically two steels having the same structure should have the same properties all the time. This, however, is an ideal to he approached but hardly attained. There are a number of factors other than structure that may affect properties. The principal one is internal stress, which may be produced by any of a number of manufacturing operations. Another is gas, taken up by the metal during melting, refining, or fabrication. These, and possibly others, are undoubtedly responsible for the fact that the properties of steels having the same structure vary considerably from the normal in about one case in ten. Hardcnability 355

4 8 12 16 20 24 28 32 Distance from quenched end of specimen.sixteenths of an inch

Fig. 17.7. Hardenability bands for three 0.40 per cent carbon low-alloy steels. (Society of Automotive Engineers and American Iron and Steel Institute)

In ordinary heat treatment it is therefore desirable to have martensite before tempering and, if possible, to avoid cooling at a rate slightly lower than the critical (commonly known as slack quenching), in which case bainite or a mixture of martensite and bainite will be formed. In all but the very shallow-hardening steels, it is relatively easy to obtain martensite upon quenching sections less than about 1 in. in cross section; but if optimum properties are required in heavy sections, a steel with high hardenability should be used. Slack quenching results in a low yield ratio (low yield strength as compared with tensile strength) and frequently lower elongation, reduction of area, and impact resistance than are desired. In general, a deep-hardening steel in which martensite will form in quantity even in large sections will, after tempering, have a yield ratio of 92 to 88 per cent; a shallow-hardening steel in which the structure is mostly bainite will, after tempering to the same hardness, have a ratio of 85 to 75 per cent or even lower, depending on how much bainite is formed. The effects of slack quenching are especially apparent in the notch toughness of low-alloy steel at or below room temperature. Fre- quently the impact resistance of a steel containing pearlite or bainite as

quenched is only a fraction of the impact value of the same steel when 356 Engineering Metallurgy

tempered to the same hardness but quenched to ensure a structure that was almost entirely martensitic before tempering. Tempering naturally reduces hardness and strength, but for a given degree of tempering the reduction is greater if the quenched structure contains much bainite than if it is primarily martensite. As a result, the center portion of heat-treated heavy sections may be too soft and weak.

17.12. Relation of Tempering to Hardenability

Fundamentally, tempering accomplishes two objectives: First, it relieves dangerous internal stresses, which may cause brittleness or even warping or cracking; and second, it improves ductility and toughness by causing transformation of any hard, brittle martensite present to a more ductile aggregate of ferrite and cementite. Concomitantly with the increase in ductility, hardness and strength are decreased. Since quenched steel is almost never used in engineering structures and machines without tempering, it may be justifiably asked why end- quench data are obtained on a quenched specimen instead of on one that is quenched and tempered. The answer is that, if the Jominy bar were quenched and tempered, we would not get the information we want and, further, that such information as we would get might be misleading. The final structure and properties of a quenched and tempered carbon or low-alloy steel depend to a large degree on the quenched structure, and

the end-quench test gives a fairly accurate picture of the hardness and its accompanying structure (and the corresponding cooling rates) for sections of various sizes quenched in various mediums. For that reason it is the as-quenched structure that is the essential variable in determining the final properties of a heat-treated steel, and it is very important that we know how this structure may be controlled.

17.13. Fundamentals of Calculated Hardenability

In 1942 Grossmann, a pioneer for many years in exploring the fundamentals ol quenching and tempering, presented his classic paper on calculated hardenability.* The data reported in this paper showed that the hardenability of most carbon and low-allow steels could be predicted within a limit of error of ± 15 to 20 per cent, provided the complete chemical composition and grain size were known and thai the austenite before quenching was homogeneous and free from undissolved carbides. The work was based on the concept that "pure" steel has a fundamental hardenability due to carbon alone, and that the final hardenability can

• M. A. Grossmann, Trans. Am. Inst. Mining Met. Engrs., v. 150, 1942. pp. 227-259. Hardenability 357

"0 0.10 0.20 0.30 0.40 0.50 0.60 0.10 0.80 0.90 1.00 Carbon, per cent

Fie. 17.8. Relation among ideal diameter, D„ carbon content, and grain size. (Grossmann)

be calculated by multiplying this basic hardenability by a factor for each chemical element present. Grain size may be taken into account either in the fundamental hardenability of "pure" steel or in the final product of multiplication. In working out his method Grossmann had to make another and, as

it turned out later, considerably more questionable assumption. This was that the hardenability should be based on a bar of ideal diameter (Di), defined as the diameter (in inches) of a bar that would show no unhardened core in an ideal quench, which was in turn defined as a quench in which the outside of the bar is instantly cooled to a temperature of the quenching medium. This, of course, is a condition that is never attained in practice, but Grossmann believed that it could be used as a reference point. 358 Engineering Metallurgy

3.80 8.80

i / 8.40 3.60 Cr / **"/ / 3.40 J 8.00 / j— 7.60 3.20 r 1 / / J / 3.00 7.20 \i *. / / 2.80 ^ Mn J right, g2-60 / scale a* k° S< / 6 00 c e*2.40 ' / , ,' / °- 1.2.20 / S / 560 ji I / / "I" 2.00 520^ 3 a i y 4 2 1.80 80 4 1.60 M 40 /e x tension 1.40 4 00 /of Mn curve 1.20 3.60 0.80 1.20 1.60 2.00 u i i i 1.00 3.20 1." c. 0.! 10 l.i 2. )0 2.40 2.80 3.20 3.60 £1 mi- nt, pei cent

FlG. 17.9. Multiplying factors Cor the live common alloying elements. (Boyd and Field)

Using the assumption of ideal diameter and ideal quench, Grossmann determined experimentally the hardenability factors for all of the elements commonly found in steel either as residuals or purposely added.

By using these factors it is easy to calculate the ideal diameter (D,) of a bar that will just harden in an ideal quench, and by applying the proper

correction it is easy to calculate the diameter (/)) of a bar Uiat will just harden in any of the commonly used quenching mediums. Basic data for the hardenability in terms of D, of "pure" steel, as dependent on carbon content and grain size, are given in Fig. 17.8. The multiplying factors for a variety of chemical elements are given in Fig. 17.9.* The factors shown in Fig. 17.9 arc those determined by Gross- mann, modified in some cases by later data.f Using the factors in Fig. 17.9, the ideal diameter D, can be readily calculated as indicated by the data in Table 17.2. It is important to remember that all elements present (except the gases) should be determined and taken into account in making the calculation. Thus, if only carbon, manganese, and silicon were considered for steel 1040 in Table 17.2, the ideal diameter would be 0.81 in., instead

• L. H. Boyd and J. Held. "Calculation of the Standard End-Quench Hardenability Curve from Chemical Composition and Grain Size," Contributions to the Metallurgy of Steel, No. 12, American Iron and Steel Institute, New York, Feb., 1946, 25 pp.

|- See, for instance, Trans. Am. Inst. Mining Met. Engrs., v. 154, 1943, pp. 386-394: or "Symposium on Calculated Hardenability," ibid., v. 158, 1944, pp. 125-182; or "Sym- posium on Hardenability," ibid., v. 167, 1946, pp. 559-734. Hardenability 359

in >r. m oin | in i--_ CO re S.S in q S 3 o t-i fi tr

la _.c » -r, o - 00 o o 05 ss c; r- CN M- c # SO Q sir o (H «tf* a- E sO * c OS 3 b o in o so C 2 r* •- *" — Crt

'•J _>. CM oo o to 'J~, o - < CM CM "o B§ o O d | 2 d < ON CM CM CO E b o CM NO 3 3 ** pi CO c-i 3 b a 1 c u "* CM m CO in f- re 1 G 65 o o c u (J d d - • d c -O .a be in c> in re _C b © 5 so U ">- s 3 a. 1 O fe eg 0 -r to 3 2 m O t- t- &s E d d d "" £ "0 >- q ~ re b oX p* o CN — c c - - * re 8 o s V o g "2 " CO o CO m a. 6« CM CM s S3 d d d d

n b c- to r» r~ M 8 cm 00 CM 3 1 2 to * tO to u a £ IS re o o CO * SO CO Q i ON 00 SO 2 * d d d d CM X t-i »-« to o in CM Bl b CM CM CM CM - c o 2 O o d d X < P c> CM c> CM CJ «o * to » 1 fiS o O d d X

•a cc r- r~ I* Tc to t >. o g o o 1 (0 CO o 2 2 oU a- O z 360 Engineering Metallurgy

of 0.98 in., an error of 17 per cent, which would increase if the amounts of residual nickel, chromium, and molybdenum were higher than shown in the table. Sulphur and phosphorus need not be considered if these impurities are low, and no factors are given for them in Fig. 17.9. The

factor for 0.02 per cent phosphorus is 1.05 and the factor for about 0.03

per cent sulphur is 0.97 (sulphur decreases hardenability); consequently, in most high-grade carbon and low-alloy steels the effects of these two elements cancel out.

17.14. The Accuracy of Calculated Hardenability

Continued work over the past ten years has demonstrated that attempts to calculate hardenability by the use of multiplying factors represent an oversimplification of the problem. We now know that additional variables are involved, variables that are difficult to determine. Nevertheless, the calculation of hardenability has been used with considerable success in practice where its limitations are understood. Recent work indicates that there probably are two hardenabilities, one affecting the transformation of austenite to i>earlite and the other the transformation of austenite to bainite, but there are few data which make it possible to differentiate the two. For example, it has been shown that nickel has more effect on the austenite-bainite than on the austenite- pearlite transformation, so that the curve for the multiplying factor for nickel may have a different location from that shoivn in Fig. 17.9, depending on the structure that results from the transformation. This undoubtedly explains the discrepancies found by several investigators in attempting to confirm Grossmann's multiplying factors for nickel and for some of the other alloying elements.

The most important fact 'that was unrecognized by Grossmann and some of the other early workers in the field of calculated hardenability is the intensifying effect that one alloying element may have upon another. For example, if manganese, chromium, or molybdenum is present, the effect of nickel on hardenability is much greater than its effect as a single alloying element. Similarly molybdenum is more effective in increasing hardenability in a steel containing nickel than in one containing chromium as the second alloying element.

As a result of these recent discoveries it can be concluded that no line, either curved or straight (see Fig. 17.9), can be drawn that will show precisely the effect of an alloying clement on hardenability in all types of low-alloy steels. The calculation of hardenability can be used most successfully and reliably on alloy steels containing only one alloying element. Hardenability 361

Distance from quenched end, in.'1

l'"ic. 17.10. Kffect of boron on the end-quench characteristics of some carbon and low-alloy steels. (American Iron and Steel Institute)

17.15. The Effect of Boron on Hardenability

One of the most intriguing metallurgical discoveries of the past ten years or so is that very small amounts of boron greatly increase hardenability in some carbon and low-alloy steels. The increase results from the addition of Q.002 to 0.003 per cent, as ferroboron, or as a complex alloy containing 0.5 to 10 per cent boron and iron, aluminum, silicon, titanium, and other elements in various combinations, or even as boric acid or borax. Some typical end-quench curves for untreated and boron-treated steels are reproduced in Fig. 17.10.*

• American Iron and Steel Institute. "Report on Special Alloy Addition Agents," Contributions to the Metallurgy of Steel, No. 9, The Institute, New York, 1942, 30 pp. 362 Engineering Metallurgy

The steel to which boron is added should be fine-grained and thor-

oughly deoxidized as boron is an efficient deoxidizer, and trouble is

occasionally encountered in assuring that the boron is uniformly distributed in the metal. Very small amounts of boron are difficult to determine by chemical analysis, and it is not certain what residual percentage is the most effective, nor have we any definite knowledge of the mechanism by which this element affects hardenability. We do know, how-

ever, that an addition of less than 0.001 per cent is ineffective, and that

if more than 0.003 or 0.004 per cent is added there may be a decided decrease in hardenability.

The only definite knowledge to date is that boron increases hardenability by increasing the time available for the steel to cool past the nose

of the S curve—it pushes the nose to the right—but even here its effect may not be enough to increase the hardenability of shallow-hardening

steels materially as Fig. 17.10 shows. Why it should increase the harden-

ability of 4023 but not of 4042 (Fig. 17.10) is not easily explained.

Data on the effect of boron on mechanical properties are erratic and no definite conclusions can be drawn yet. It has no perceptible effect on the properties of the steel as rolled, normalized, or annealed, nor on the maximum hardness after quenching. In heat-treated steels it frequently

is responsible for improved yield ratio and ductility, but whether this is

a direct effect or a by-product of its effect on hardenability is impossible to judge at present.

QUESTIONS

1. Explain the difference between "hardenability" and "maximum hardness" of a steel.

2. Why is the "maximum hardness" of a steel less than the hardness usually obtained in commercial quenching for a given carbon content?

3. Compare the hardness at the center of a 5 in. round of S.A.E. 4140 with the hardness at the center of a 1 in. round of S.A.E. 1045 when both are quenched in water.

4. How does the "delay time" vary with temperature in the isothermal transformation of austenite?

5. Define "critical cooling rate."

6. What is the effect of a variation of grain size on hardenability? 7. What is the effect of a variation in the homogeniety of the austenite on formation diagram and hardenability? 8. How does deoxidation practice influence hardenability?

9. What is the relationship between the "C-curves" of an isothermal transformation diagram and hardenability? 10. Discuss several methods of determining hardenability. Hardenability 363

11. What is the hardness at the % radius of a 2 in. round of S.A.E. 4640 quenched in oil?

12. If a steel specification is based on a hardness of 50 Re for an equivalent Jominy distance of six-sixteenths, which of the steels in Fig. 17.6 would be acceptable?

13. What are the advantages and disadvantages of the Jominy hardenability test?

14. What are "hardenability bands" and "hardenability steels?"

15. What are the advantages and disadvantages of "slack quenching?"

16. Define "ideal critical diameter" and "ideal quench."

17. What is the effect of a proeutectoid phase on hardenability?

18. Calculate the ideal critical diameter for the following composition: C = 0.40 Grain size No. 5 Mn = 0.60 Si = 0.30

19. Calculate the ideal critical diameter for the following steel: C = 0.40 Grain size No. 8 Mn = 0.60 Cr = 0.50 Si = 0.30 Mo = 0.20

20. What is the effect of boron on hardenability? Special Purpose Steels CHAPTER 18 Frederick Leo Coonan, D.Sc, Professor and Chairman, Department of Metallurgy and Chemistry, U. S. Naval Postgraduate School, Monterey, California Jules Washington Lindau, III, M.E., Associate Professor of Mechanical Engineering, University of South Carolina, Columbia, South Carolina

oOME types of service in industry require unusual properties in metals. Many of the alloys discussed in the previous chapters are not entirely satisfactory for these specialized requirements; many alloys that are satisfactory do not conveniently fall into the established patterns developed in the earlier chapters. Included in this group are alloys particularly suitable for (1) tools and dies, (2) corrosion and heat resistance, (3) magnetic applications, (4) wear resistance, (5) low temperature, and (6) ultra high-strength service. The alloys in two of these categories will be treated in separate chapters.

Chapter 19 is concerned with tools and dies, and Chapter 21 with wear resistance. Most of the discussion here will be concerned with steels developed for resistance to corrosion and, to a lesser extent, with alloys utilized in service involving magnetic effects.

18.1. Classes of Stainless Steels

The element chromium combines readily with oxygen forming an oxide that is thin, tightly adherent, self-healing, and that protects the metal itself from further attack. When dissolved in iron, it forms an alloy that has similar stainless and oxidation-resisting properties.

The chromium bearing steels may be divided into three classes: (1) special purpose steels containing 3.5 to 10 per cent chromium with or without 0.5 to 4 per cent silicon, nickel, molybdenum, or tungsten as additional alloying elements; (2) corrosion- and heat-resisting steels containing more than 10.0 per cent chromium and not more than 2 per cent

364 Special Purpose Steels 365

of another alloying element; and (3) corrosion- and heat-resisting steels containing more than 10 per cent chromium and more than 2 per cent of one or more other alloying metals.

Chromium steels containing 3.5 to 10% chromium have better corrosion and heat resistance than the S.A.E. low-alloy steels containing chromium (see Fig. 18.3) and have excellent properties at elevated temperatures. Steel with about 0.10 per cent carbon, 4 to 6 per cent chromium, 0.40 to 0.75 per cent silicon, and about 0.50 per cent molybdenum

is used widely in oil-refining equipment. Another steel in this class is silchrome, a material commonly used for automobile- and airplane-engine valves and containing about 0.50 per cent carbon, 7 to 10 per cent chromium, and 1.00 to 3.75 per cent silicon. Silchrome is resistant to attack by the exhaust gases of internal-combustion engines and has satisfactory strength and hardness at a dull red heat. Moreover, it costs considerably less than the highly alloyed valve materials containing nickel, chromium, and silicon.

The most important high-alloy steels in which chromium is the only alloying element in large amounts are those included in class 2. These may be divided into four groups:

2/1. Low-carbon, high

2B. Cutlery steels containing 12 to 16 per cent chromium and 0.20 to 0.60 per cent carbon.

2C. Tool and die steels containing 12 to 18 percent chromium and 0.70 to 2 per cent carbon. 2D. Heat-resisting steels containing 20 to 30 per cent chromium, with

less than 0.50 per cent carbon for hot-worked material, and with 0.50 to 2.5 per cent carbon for castings.

Small amounts—generally less than 2 per cent— of nickel or other metals are occasionally added to the plain chromium steels. With the exception, however, of 0.15 to 0.50 per cent sulphur to improve machinability, the addition of small percentages of a second alloying element is not common.

The principal corrosion- and heat-resisting steel of class 3 is the familiar low-carbon 18 per cent chromium, 8 per cent nickel alloy known to metallurgists and engineers as 18—8. This is a very important high- alloy engineering steel. Despite the cost, the amount of 18—8 used per year has increased enormously. Most of the 18—8 steels contain no other alloying element. A few that must have exceptional stability at elevated temperatures contain larger percentages of chromium nickel, 1 1

366 Engineering Metallurgy

plus cobalt, tungsten, or molyb-

-hromium.ato •nic per cent denum, and some contain ti- 10 20 30 40 H tanium or columbium, added 2000 | 1 1 to stabilize the carbon and pre-

1800 vent intergranular corrosion. -3200 This is discussed later. ,w elf 1600 In the past several years new -2800 manganese bearing and disper- 1400 sion hardening types of austeni- -2400 tic steels have been developed. /*/p/ a <->l200 UJ These will also be discussed in cr T3 h20Q0-§ a later section of this chapter. 3a. nmcfW u" 1000 Up to + im T7« 1- gc 3 ! 18.2. Constitution of High- -1600 I 800 ^** V Chromium Steels Q. *. / 3fc_» T] E '"Sw ^_ i -1200 Chromium in amounts less £ 600 \ | V 1 than 30 per cent dissolves com- — | i| pletely in carbonless iron. It 400 1 -800

1 j 1 has marked effects on the tem-

y 4// ha h> perature of the /f transforma- 200 1 -400 3 1 B tion, where alpha changes to Alpha 1 iron, or vice versa, n*> + -0 gamma and • "n ' Sigma , | on A 4 , where gamma changes

-200 1 to the high-temperature modi- if 2C 3C * ) sc I fication of alpha (delta) iron, Zhrom um pet ce -.f or vice versa. These effects are Fro. 18.1. Iron-rich portion of the iron- important in commercial high- chromium phase diagram. (Kinzel and Crafts) chromium steels and should re- receive some attention.

A portion of the iron-chromium diagram for alloys containing less than

50 per cent chromium is shown in Fig. 18.1.+ From this diagram it is evident that adding 8 per cent chromium to carbonless iron lowers the point slightly At and lowers the A 4 point nearly 275°F. (15()°C.) . Further additions of chromium raise /J., and at the same time lower A 4 until, with the addition of about 12 per cent chromium, these two points merge. The result is what is known as the gamma loop and is a characteristic of several binary alloy systems of which iron is one component. The significance of the gamma loop is plain. If an alloy containing 10 per cent chromium and 90 per cent iron is heated slowly, no change in

+ A. B. Kinzel and Walter Crafts, The Alloys of Iron and Chromium, Vol. I, Low- Chromium Alloys, McGraw-Hill Book Company, Inc., New York, 1937. p. 43. Special Purpose Steels 367 structure occurs until a temperature of 1650°F. (900°C.) is reached. Here the alpha solid solutions changes to gamma solid solution. If heating is continued, the gamma remains unchanged until a temperature of

2350°F. (1200°C.) is reached, where it changes to alpha (delta) solid solution. On slow cooling, the reverse changes take place. If, now, an alloy containing 15 per cent chromium and 85 per cent iron is heated, no change (except grain growth) takes place between room temperature and the melting point, and, conversely, no structural change occurs in cooling. Alloys containing more than 12 per cent (and less than 30 per cent) chromium and very little or no carbon have no allotropic transformations.

The concentration limits of the gamma loop are changed by carbon, which moves the extreme limit of the loop to the right and at the same time increases the width of the duplex alpha-plus-gamma field. Thus, the loop, which with negligible carbon extends to 12 per cent chromium, extends to about 18 per cent chromium with 0.40 per cent carbon. These relationships are complicated further by the carbides which are formed

by chromium and which together with iron carbide (Fe3C) form

(Cr.Fc) 7 C3 and (Cr.Fe) 4C in the commercial high-chromium steels. The iron-chromium carbides are partly soluble in the gamma solid solution but not in the alpha solid solution and may be retained in supersaturated solid solution by quenching, in a manner analogous to the retention of iron carbide in the gamma iron of unalloyed high-carbon steel.

18.3. Relation of the Constitution of High-Chromium Steels to Their Heat Treatment

From the foregoing discussion it is evident that we should know from the composition whether high-chromium steels will or will not respond

to heat treatment. Thus, a steel containing 12 to 18 per cent chromium

and less than 0.13 per cent carbon (class 2A, § 18.1) will or will not harden depending upon the relative amounts of chromium and carbon.

If the steel contains 0.12 per cent carbon and 13 per cent chromium,

some austenite is formed if the steel is heated to 180()°F. (980°C.) , and this transforms to martensite upon quenching. If the steel contains 0.06 per cent carbon and 16 per cent chromium, no phase changes take place, and the steel does not harden when quenched from 1800°F. (980°C). Low-carbon high-chromium steels in which no phase changes take place

remain ferritic in structure at all temperatures. They are relatively soft

and ductile, and the only change caused by heating is grain growth which reduces the ductility. ,

368 Engineering Metallurgy

Fig. 18.2. Structure of (A) oil-quenched cutlery steel containing 0.30 per cent carbon and 12.7 per cent chromium, and (B) oil-quenched high-carbon chromium steel containing 0.70 per cent carbon and 17.5 per cent chromium; etched. 500 X- (Kinzel and Forgeng)

Cutlery steels containing 12 to 16 per cent chromium and 0.20 to 0.60

per cent carbon (class 2B, § 18.1) undergo a phase change: when they are heated to about 1800°F. (980°C.) , the alpha solid solution changes to gamma and the carbide goes into solution. These steels respond to heat treatment very much like unalloyed high-carbon steels. In a steel containing 12 per cent chromium the structure is entirely pearlitic with

only 0.3 per cent carbon when the steel is cooled slowly; it is, conse-

quently, martensitic when the steel is cooled rapidly (Fig. 18.2) .* These cutlery steels differ from unalloyed steel containing 0.80 to 0.85 per cent carbon chiefly in that the chromium lowers the critical cooling rate, that

is, they become as hard when quenched in oil, or even in air, as an unalloyed high-carbon steel when quenched in water. Since steels containing 12 to 14 per cent chromium and 0.30 to 0.40 per cent carbon correspond to unalloyed "eutectoid" steels, it follows that high-chromium steels containing about 1 per cent carbon (class 2C,

§ 18.1) are hypereutectoid and, when quenched and tempered, have numerous excess carbide particles in the microstructure (Fig. 18.2B) which increase the wear resistance and make the steels useful for tools and dies.

Courtesy of A. B. Kinzel and W. D. Forgeng. Special Purpose Steels 369

The heat-resisting steels containing 20 to 30 per cent chromium (class treatment, depending upon 2D, § 18.1) may or may not respond to heat the carbon content. The carbon range for this class is 0.5 per cent or less cent for steels which are hot or cold worked, and from 0.5 to 2.5 per for those which are used in the form of castings. At room temperature the steels containing about 25 per cent chromium and less than 0.70 per cent carbon are ferritic; their structure consists of alpha solid solution take place in heating. and the carbide (Cr,Fe) 4C, and no phase changes These steels cannot be hardened; they can, of course, be annealed if cold worked. The higher carbon grades respond to quenching, but this treatment containing is seldom used as no advantage results. High-carbon castings 20 to 30 per cent chromium are very hard as cast and must be annealed

if machining is necessary. The price of ferrochromium (70 per cent chromium) used in the manufacture of high-chromium steel varies inversely with the carbon percentage. Ferrochromium containing less than 0.5 per cent carbon, which must be used to produce high-chromium steels with low carbon per-

centages, is much more costly than ferrochromium containing 4 to 6 per

cent carbon. The economic factor is, therefore, important in selecting a high-chromium steel for a certain application; the advantages resulting from the use of a steel containing less than 0.12 per cent carbon, instead of one containing 0.40 per cent, must be balanced against the higher cost of this material.

18.4. Mechanical Properties of High-Chromium Steels

As remarked in a previous chapter (§16.4), chromium has little

solid-solution effect, that is, when dissolved in alpha iron, it strengthens

the iron relatively little. This is shown by the data in Table 18.1 for annealed steels. If the composition is such that the steel undergoes a phase change, the principal effect of chromium is to increase the hardenability, thus producing marked changes in properties with relatively slow cooling rates. The tensile properties of high-chromium steels cooled in air or oil depend obviously upon how much carbon is present and upon what proportion of the carbides dissolves in the gamma solid solution at high temperatures. Some typical values for tensile properties of low-carbon 12 to 15 per cent chromium steel and for cutlery steel are given in Table 18.2. The high-chromium steels are more resistant to tempering than carbon and low-alloy steels. Increasing the tempering temperature has

little effect on the properties until a temperature of 950 to 1050°F. (510 370 Engineering Metallurgy

to 565°C.) is reached when there is a sudden decrease in tensile strength, yield strength, and hardness and a moderate increase in ductility. The impact resistance of the quenched and tempered low-carbon and medium-carbon (cutlery) steels is in general satisfactory, except when the steels are tempered at, or cooled slowly through, the temperature range 875 to 1000°F. (470 to 540°C.) . For reasons yet unknown this treatment causes brittleness, which is manifested by very low notched- bar impact resistance. The endurance limit of both the hardenable and the unhardenable high-chromium steels is about one half of the tensile strength or approximately the same as for carbon and low-alloy steels. The modulus of elasticity, however, seems to be somewhat higher; most available data indicate that it is between 31 and 32 million lb. per sq. in.

18.5. Corrosion and Oxidation Resistance of High-Chromium Steels

Although high-chromium steels are more resistant to certain kinds of corrosion than any other ferrous material, they are not corrosion resistant in the broadest sense of the term. Neither are they always stainless. In fact, to some forms of attack, they are no more resistant than the unalloyed carbon steels.

Steels containing 10 per cent or more chromium are resistant to oxidizing mediums and to corrosion caused by fruit juices, food products, and beverages. The low-carbon 12 to 18 per cent chromium steels are resistant to a large number of organic acids and to all concentrations of nitric acid, except 65 per cent boiling nitric acid. They are also resistant to strong alkalies and to many salt solutions but are readily attacked by chlorides and other halides. Sulphuric acid, except when concentrated and cold or when containing nitric acid, and hydrochloric acid of all concentrations attack these steels readily.

In general, corrosion resistance is a function of the amount of chromium dissolved in the iron, but the relation is not direct. Corrosion resistance is not important with low percentages of chromium; it becomes more pronounced as the chromium increases from 4 to 10 per cent and increases very rapidly with slightly more than 10 per cent. The corrosion resistance is thought to be due to the formation of a thin, tight, adherent film of chromium-iron oxide which, if broken, is self-healing unless the corrosive agent (as, for example, hydrochloric acid or chlorides) dissolves the film as rapidly as it is formed. It is generally assumed that in order to produce a self-healing film there should be at least one atom of chromium to seven atoms of iron. Increasing the chromium from 10 to

30 per cent increases the corrosion resistance, but the increase is not proportional to that of the chromium. Special Purpose Steels 371

Table 18.1. Effect of Chromium on the Tensile Properties of Annealed Steels Containing 0.10 Per Cent Carbon*

Chromium, Tensile strength, Yield strength, Elongation in Brinell per cent lb. per sq. in. lb. per sq. in. 2 in., per cent hardness

50,000 38,000 44 90 5 65,000 26,000 38 135 12 72,000 35,000 36 140 15.5 75,500 42,500 33 148 17 80,000 47,000 30 155 27 84,500 55,000 26 160

• Based on E. E. Thum, Metal Progress, v. 29, No. 6, 1936, pp. 49-57. 104.

TABU 18.2. Typical Tensile Properties of High-Chromium Steels*

Composition, Quenched and Tensile Yield Reduc- per cent tempered at strength, strength, Elongation tion of lb. per lb. per in 2 in., area, C Cr °F. °C. sq. in. sq. in. per cent per cent

400 205 146,000 135,000 15 38 600 315 145,000 134,000 15 40 0.09 12.2 800 425 143,000 134,000 16 46 1000 540 135,000 120,000 20 52 1200 650 95,000 79,000 29 60

400 205 170,000 160,000 8 18 600 315 168,000 160,000 10 20 0.09 800 425 166,000 160,000 15 45 1000 540 160,000 148,000 18 46 1200 650 90,000 83,000 21 50

600 315 240,000 208,000 3 5 800 425 245,000 210,000 5 20 0.30 13.0 1000 540 250,000 195,000 7 20 1200 650 160,000 138,000 12 38 1400 760 140,000 105,000 14 45

• Stainless Steels, American Society Based on data by 1). J. Giles, The Book of for Metals, Cleveland, 1935, pp. 267-276; and O. K. Parmiter, Trans. Am. Soc. Steel Treat., v. 6, 1924, pp. 315-340. 372 Engineering Metallurgy

Carbides, if not in solution, have a deleterous effect on corrosion resistance. For this reason, the cutlery steels should be heat treated if op-

timum corrosion and stain resistance is desired. The alloys containing carbon in excess of that which dissolves in the gamma solid solution at high temperatures are not so resistant to oxidizing mediums as the lower carbon steels. The high-chromium steels rust slowly when exposed to the atmosphere, but the thin coating can readily be wiped off, leaving the surface under- neath unaffected. Steels containing 12 to 14 per cent chromium and less than 0.12 per cent carbon are, therefore, widely used for automobile and building trim. In industrial at- 45 mospheres, especially those con- ~bon taining sulphur, the high-chro- 40 usfe e/5.»/c — mium steels may pit badly. The resistance of the high- 35 chromium steels to oxidation at

JO elevated temperatures increases with the chromium but, as Chro miurr stee Is 25 shown in Fig. 18.3, the increase is not important until more than 8 20 | 10 per cent chromium is present. With higher chromium percent- 'faint ess sree/s V ages the increase in oxidation re- \ sistance is fairly uniform until about 20 per cent chromium is \ present, when the resistance is Vj Chrome ron very high. The data shown in 1 1 — Fig. 18.3 were obtained* by heat- 12 16 20 24 28 J2 sur- Ch romi um, per cent ing 0.5-in. cubes with ground faces for 48 hr. at 1830°F.

Fie. 18.3. Effect of chromium the re- on (100()°C.) . With the exception sistance of chromium steels to oxidation at of a few iron-nickel-chromium 1830°F. (lOOO-C). (MacQuigg) and iron-chromium-aluminum

alloys, steel containing 20 to 35 per cent chromium is the most resistant of the ferrous alloys to high-temperature oxidation.

18.6. The Constitution of 18-8

Nickel (as well as chromium) is soluble in both alpha and gamma iron. When in solution it (1) lowers the A 3 point, where alpha iron

• C. E. MacQuigg. The Book oj Stainless Steels, American Society for Metals. Cleve- land, 1935, pp. 351-368. Special Purpose Steels 373

j-2800uJ -2400 i -2000 u -1600"° -1200 J -800 L

•2800 v

-2400 g -2000 «

5 10 150 5 10 150 5 10 15 N ickel .per cent

Fig. 18.4. Effect of chromium on phase changes in iron-nickel alloys. (Bain and 4 born)

transforms to gamma on heating and where gamma transforms to alpha on cooling, and (2) makes these allotropic changes, especially the one on cooling, very sluggish. A small portion of the iron-nickel phase diagram is shown in the upper left corner in Fig. 18.4.+ As noted on a previous page (p. 366) , chromium restricts the temperature-concentration region where the gamma solid solution is stable, that is, it produces a gamma loop; nickel, on the contrary, increases the limits of this area.

When chromium is added to the iron-nickel solid solution it dissolves.

The first effect is a lowering of the delta- (alpha-) gamma transformation temperature and a widening of the field where delta (alpha) is stable. This is apparent from the sections of Fig. 18.4 showing phase relations for alloys containing S and 6 per cent chromium. As the chromium is increased, the area of the delta (alpha) region is increased

Society for Metals, f E. C. Bain and R. H. Aborn, in Metals Handbook, American Cleveland, 1939, pp. 418-422. 374 Engineering Metallurgy

further, and with 9 per cent or more the low-temperature area where alpha is stable is also increased. Increasing the chromium thus moves the boundaries of the gamma and the alpha-plus-gamma phase regions to the right.

Consider now the small diagram at the lower left in Fig. 18.4 for 18

per cent chromium: if 3 per cent nickel is added and the alloy is heated,

no phase changes take place at any temperature. If, however, 8 per cent nickel is present and the alloy is heated, a phase change occurs at about

660°F. (350°C.) , and some of the alpha solid solution changes to gamma solid solution. If the alloy is heated still further, this allotropic change continues until at 1200°F. (650° C.) all the alpha has changed to gamma. There is no further change if the heating is continued almost to the melting point. Now, if the alloy is cooled, the reverse changes should take place, that is, gamma should start to transform to alpha at 1200°F. and should be completely transformed at 660°F. Owing, however, to the

sluggishness of the reactions, only a small amount of alpha is actually

formed in slow cooling, and it is relatively easy to suppress by rapid cooling the formation of any and thus to obtain at room temperature an alloy that is entirely austenitic.

Austenitic steels are strong and ductile and are readily deformed cold into a variety of useful products. For a combination of maximum resistance to corrosion by certain mediums, austenitic structure at room temperature, and other useful properties the most economical composition is 18 per cent chromium and 8 per cent nickel, with carbon as low as

is commercially possible. With this composition it is advisable to guard against the presence of any alpha by heating the material to 1830 to

2010°F. (1000 to 1100°C.) and quenching it in water or in an air blast. Increasing the nickel to 10 per cent or more increases the stability of the austenite but also increases the cost.

18.7. The Role of Carbon in 18-8

If 18—8 could be made free from carbon, many of the troubles of the manufacturers and users of this steel would be over. Unfortunately, however, this is difficult, and even if made by the best commercial practice, 18—8 contains 0.06 and frequently as much as 0.12 per cent of this element. • Unfortunately also, only about 0.01 or 0.02 per cent carbon is soluble in the iron-chromium-nickel austenite at room temperature. This solubility increases with increasing temperature as shown in Fig.

• Carnegie- Illinois Steel Corporation has produced 18-8 with carbon well under 0.06 per cent by feeding oxygen into the molten bath during refining. See Electric Furnace Steel Proceedings, Am. Inst. Mining Met. Engrs., v. 3, 1945, pp. 107, 108, 127. Special Purpose Steels 375

IOUU — 1 — -- CL+Ls -- d£?. -- ~- 3*y+l ,- ^ r^ ,_ -2400 — -- __ -- ~~ -r -— - — T ,' ____ -- y+L+carbide -?fififlCUUU Y °I000 LL H600 *?

' ? ' ""• i(a;/* A> *'-r D 7 ""!

-1200-^ CO D. "!

. a+vtcarb ide | 500 E -800," r

i n tearbi'a e -400

q i

1 i C i 2 0.3 0.4 :c«r pon pe- cent

Fig. 185. Effect of temperature on the solubility of carbon in iron-chromium- nickel austenite. (Kinzel and Franks)

Fig. 18.6. Structure of (A) quenched 18-8 and (B) quenched and reheated 18-8 showing carbide precipitation at grain boundaries; etched. 250 X- (Kinzel and Forgeng) 376 Engineering Metallurgy

18.5,f until at 1830°F. (1000°C.) 0.14 per cent, and at 2010°F. (1 100°C.) 0.22 per cent is soluble. One advantage of quenching 18—8 is that the carbon is held in supersaturated solution at room temperature. This carbon, however, will precipitate if conditions are favorable. Severe cold working may cause some precipitation; in addition—and this is most important—on heating to a high temperature, as in welding or in service in superheaters or other equipment operating at elevated temperatures, some carbon precipitates from solution.

If this were the only thing that happened, it would not be serious.

However, when the carbon precipitates, it is apparently thrown out of solution at the grain boundaries as a chromium carbide, thus impover- ishing the austenite grains adjacent to the boundaries in chromium and making them susceptible to corrosion. The usual structure of quenched

18—8 showing the polyhedral grains of austenite is pictured in Fig. 18.6A,* the carbides which have precipitated at the grain boundaries are shown in Fig. 18.6B.

Carbide precipitation occurs frequently if 18—8 is heated in the range

570 to 1470°F. (300 to 800°C.) , making the material very susceptible to a form of attack known as intergranular corrosion or intergranular disintegration. It is very hard to detect and was the cause of a number of disastrous failures before a cure was found.

Carbide precipitation can be prevented by reducing the carbon to

0.02 per cent or less, which is not easily attained commercially, and it can be reduced innocuous by adding titanium equivalent to 5 times the carbon content, or columbium equivalent to 10 times the carbon content, to the steel when it is made. Both of these elements combine with the carbon to form a stable carbide and prevent the carbon from combining with the chromium. When 18—8 is "stabilized" with titanium or columbium it can be used at elevated temperatures or can be welded without danger of premature failure.

18.8. Properties of 18-8

The austenitic chromium-nickel steels can readily be cold rolled into sheet or strip or cold drawn into wire. They can also be riveted, soldered, or welded. Welding is now commonly used for fabricating 18—8. Because of its high electric resistivity and low thermal conductivity, spot welding has been especially successful. This method of joining is used in the

f A.. B. Kinzcl and R. Franks, The Alloys of Iron and Chromium, Vol. II, High- Chromium Alloys, McGraw-Hill Book Company, Inc., New York, 1940, p. 275. • Courtesy of A. B. Kinzel and W. D. Forgeng. Special Purpose Steels 377 construction of the stainless-steel streamlined trains which have attracted much attention recently. Austenitic chromium-nickel steel has, as quenched, a fairly high tensile strength with high ductility and resistance to impact (Table 18.3). It hardens rapidly by cold work (Table 18.3) . The increase in tensile and in yield strength is greater and the decrease in ductility is considerably less than that caused by comparable cold working of low-carbon steel. ratio of Carbon increases the strength (Table 18.3) . The endurance quenched 18—8 is 0.40 to 0.45, that is, somewhat lower than the usual ratio for carbon and low-alloy steels. Peculiarly, the endurance limit is usually higher than the proportional limit.

Table 18.3. Typical Mechanical Properties of Austenitic Chromium-Nickel Steel

Composition, per cent Tensile Yield Elonga- Reduc- Condition of specimen strength, strength, tion in tion of Izod lb. per lb. per 2 in., area, impact. C Ct Ni sq. in. sq. in. per cent per cent ft-Ib.

0.16 18.0 8.0 Water-quenched bars 90,000 37,500 68 78 Ill 0.17 18.0 8.0 Watcr-qucnchcd bars 96,500 40,000 65 75 103 0.07 17.9 8.5 Cold-rolled bars, 15% 142,700 117,800 30 61 88 0.07 17.9 8.5 Cold-rolled bars, 40% 227,600 201,000 11 43 17 0.07 18.0 9.5 Cold-drawn wire, 60% 263,000 4 0.16 16.0 10.2 Cold-drawn wire, 85% 273,000 3 0.07 18.0 8.0 Water-quenched sheet 86,400 33,500 63 0.16 18.0 8.0 Watcr-qucnchcd sheet 93,300 46,600 57 0.06 18.0 14.0 Watcr-qucnchcd bars 80,000 63 75 0.14 18.0 14.0 Water-quenched bars 90,000 53 70 0.14 25.0 20.0 Watcr-qucnchcd bars 90,500 50,000 47 63 90

The coefficient of expansion of 18-8 is about 50 per cent higher than that of carbon steels, and the thermal conductivity at normal temperatures

is about 0.04 cal. per sec. per sq. cm. as compared with 0.16 cal. per

sec. per sq. cm. for low-carbon steel. The stress-strain curve of 18—8 is curved from the origin. This steel has, therefore, a low proportional limit, no yield point, and no true modulus of elasticity. The secant modulus varies from 26 to 28 million lb. per sq. in. The austenitic chromium-nickel steels have exceptionally high resistance to creep. This is discussed in a later chapter. Nickel increases the resistance of low-carbon 18 per cent chromium steels to corrosion by nonoxidi/.ing mediums, permitting these materials to be used economically in environments where the high-chromium steels containing no nickel would be attacked rapidly. These austenitic steels 378 Engineering Metallurgy

are resistant to many organic acids, organic solvents, and oils which

attack the high-chromium steels. Their resistance to staining in the atmosphere is also higher than that of the plain chromium steels. Their

oxidation resistance is high, especially at 1650°F. (900°C.) , and if the chromium and nickel are both increased, they may, if not highly stressed, be used at temperatures as high as 2010°F. (1100°C). The corrosion and oxidation resistance and the strength and stability at high temperature can be increased by the addition of silicon, tungsten, molybdenum, cobalt, or columbium.

18.9. Recent Developments in Stainless Steels

There are a number of elements—for example, carbon, nitrogen, and titanium— that affect the phase boundaries of the 18 per cent chromium section of the ternary iron-chromium-nickel diagram. As a result, the 18—8 alloy may have a duplex structure of ferrite and austenitc unless precautions are taken to prevent it. It is also evident that lowering the nickel slightly (see the 18 per cent chromium section in Fig. 18.4) will make it impossible to produce a structure consisting entirely of austenite even by quenching. In fact, the 18—8 composition is so critical that some metallurgists believe that with very low carbon and 8 per cent or slightly less nickel, an austenitic structure is possible chiefly because the small percentage of nitrogen introduced during metling prevents normal phase changes. In practice, an austenitic structure is assured by rapid cooling

(Sect. 18.6) and by keeping the nickel well over 8 per cent, especially if the carbon is less than about 0.08 per cent.

The ferrite chromium-nickel steels, which are magnetic, are much stronger and less ductile than the austenitic material. They have not been used industrially, as their properties, including corrosion resistance, are not enough better than those of low-carbon 16 to 18 per cent chro- alloys mium ( which are also ferritic) to justify the much higher cost.

One of the disadvantages of the austenitic chromium-nickel steels is that high strength can be attained only by cold working—heat treatment is used solely to ensure the proper structure. This limits the application of such cold-worked material as sheet, tubes, strip, and wire products. Some years ago, Smith, Wyche, and Gorr* discovered that by adding 0.50 to 1.00 per cent titanium, which is a ferrite former, to a steel containing about 0.07 per cent carbon, 17 per cent chromium, and 7 per cent nickel a ferritic structure can be readily produced; this structure can

• R. Smith, E. H. Wyche, and W. W. Gorr. Trans. Am. Inst. Mining Met. Encrs., v. 1G7, 1946, pp. 313-345. Special Purpose Steels 379 be hardened by precipitation of a carbide that is retained in supersaturated solution in the ferrite matrix by a suitable heat treatment. The heat treatment consists of heating the alloy to temperatures between about 1400 and 2000°F. (760 and 1095°C.) to produce a homogeneous austenitic structure, followed by cooling to room temperature is then reheated (at any rate faster than slow furnace cooling) . The alloy to temperatures between 950 and 1050°F. (510 and 565°C.) to precipitate the hardening constituent. This steel, known as stainless W, has many desirable properties. Repre- sentative mechanical properties are given in Table 18.5. In addition to the high tensile strength, it has a high yield strength and a definite

modulus of elasticity, which is about 28 million lb. per sq. in. The endurance limit on polished specimens varies from 80,000 to 90,000 lb. per

sq. in., depending on the treatment; for specimens of sheet having a

normal rolled surface it is about 30 per cent of the tensile strength. Stainless W has much lower ductility than the austenitic material, which in view of its strength and structure is not unexpected. The steel

is readily welded. Its corrosion resistance is apparently the same as that of

austenitic 18—8, and it is not subject to intergranular embrittlement. This steel has been the forerunner of a group developed to meet a need in industry for austenitic alloys that respond to a hardening heat treatment. Approximate compositions of representative members are given in the table below.

Table 18.4. Nominal Composition of Prccipitation-Hardenahle Grades

Designation C Cr Ni Other

Stainless W 0.07 17 7 0.70Ti, 0.20A1 17-4PH 0.04 16.5 4 4.0Cu, 0.35Cb + Ta

17-7PH 0.07 17 7 I.IOAI AM350 0.07 17 4 2.75Mo

17-10P 0.12 17 10.25 0.25P 17-14 Cu-Mo 0.12 16 14 2.5Mo, 3Cu, 0.45Cb, 0.25Ti HNM 0.30 19 9.5 3.5Mn, 0.30P

Data from Roach and Hall, Materials and Methods, April 1956, p. 144.

The steels, except Am350, noted in the table, contain elements forming compounds that are soluble in austenite at elevated temperatures but that may be retained in a supersaturated condition in the phase existing 380 Engineering Metallurgy

Table 18.5. Typical Room-Temperature Mechanical Properties of Precipitation- Hardcnable Grades

Ten Yld. Elong. Hard- Charpy Endur. Str. Str. in ness, Im- Limit Designation Condition 1000 1000 2 in. Rock- pact 1000 psi psi % well C Ft-lb psi

Stainless W Annealed 135 95 7 25 _ Aged 'A hr. at 950° F. 210 195 7 43 — 80 Aged Vi hr. at 1050° F. 190 170 8 39 — — 17-4PH Annealed 150 110 12 — — — Aged 1 hr. at 900° F. 195 180 13 43 19 90

17-7PH Annealed 130 40 30 R b85 ^_ ^_ Treated at 1400° F. 145 100 9 31 — — Aged at 950° F. 215 200 8 45 6 78 AM350 Annealed 186 52 22 R b94 100 — Subzero cooled 197 110 12 40 — — Tempered at 750° F. 199 159 11 41 51 80

17-10P Annealed 89 37.5 70 Rb82 120' Aged 137 88 25 30 40' — 17-14 Cu-Mo Annealed at 2250° F. and aged 86 42 45 R b80 110' — HNM Annealed at 2000° F. aged at 1200° F. 153 110 26.5 34 18 —

Data from Roach and Hall, Materials and Methods, April 1956, p. 145. at room temperature. Subsequent heating to temperatures in the vicinity of 1000°F. will produce a finely dispersed precipitate that is accompanied by an increase in hardness and strength. The treatments producing this hardened condition have been described for stainless W. Alloy 17-4 PH responds in a similar fashion. Mechanical properties are given in the table. The alloy 17-7 PH on annealing remains austenitic. It is however unstable and may be converted to martensite on cold work or by cold treatment. Subsequent heating to about 1000°F. will produce a precipitate in a state of dispersion that will cause a marked increase in hardness and strength. The alloys with appreciably higher nickel, 17-10P, 17-14, HNM, will remain austenitic on rapid cooling from about 1900°F. and retain dispersion hardening compounds in solution, on aging at about 1300°F. the compounds precipitate. The presence of phosphorus will intensify the hardening effect. The continuing expanding use of nickel has led to a shortage of the available supply of this metal. In an effort to alleviate to some extent this Special Purpose Steels 381 situation, several low-nickel, high-manganese austenitic stainless steels have been developed as well as nickel-free, heat-resisting austenitic alloys. Approximate compositions are given in the table below.

Table 18.6. Composition of New High Manganese Grades

1 Designation C Mn Cr Ni N

A1S1' 201 0.15 max. 5.5/7.5 16.0/18.0 3.5/5.5 0.25 max. A1S1' 202 0.15 max. 7.5/10.0 17.0/19.0 4.0/6.0 0.25 max. CMN* 0.65 12 25 — 0.45 G-1928 0.55/0.65 8.0/9.0 21.25/22.75 — 0.30/0.40

1 Designation of the American Iron & Steel Institute. 2 Crucible Steel Co. of America. 8 Allegheny Ludlum Steel Corp. Data from Roach and Hall, Material/! arts Methods, April 1956, p. 138.

Table 18.7. Typical Tensile Properties of High-Manganese Grades

Yld. Str. Designation Form Condition Test Ten Str. (0.2%) Elong. Temp. °F 1000 psi 1000 psi %

A1S1 201 Sheet Annealed Room 115 55 55 Sheet \i Hard Room 150 105 25

A1S1 202 Sheet Annealed Room 105 55 55

CMN Sheet Ann. & Aged 1200 76 — 10

G-192 Bar Hot worked Room 149 84 11

Data from Roach and Hall, Materials and Methods, April 1956, p. 139.

The mechanical properties of types 201 and 202 are essentially the same as A1S1 301 (17 Cr-7Ni) and A1S1 302. Corrosion tests seem to indicate that type 201 is comparable in corrosion resistance to 301, and type 202 resembles 304.

18.10. Superstainless Steels

During the early years of World War II, American metallurgists were faced with an urgent and difficult problem in the metallurgy of high-alloy steels: to develop a superstainless steel for use in turbosu- 382 Engineering Metallurgy

perchargers and gas turbines (jet engines) . In addition to its great importance for military aircraft and propelled missiles, solution of the problem would probably have far-reaching peacetime ramifications, be-

cause it has been known for some time that a gas turbine has a high over-all efficiency for power generation in marine and other forms of transportation.

The problem was to develop an alloy that would have a relatively long and useful life as highly stressed wheels, buckets, and other parts of turbines, when operated at very high temperatures and when exposed to highly corrosive hot gases. Two methods of attack were used: to produce a superstainless steel based on the 18—8 analysis—which has excellent elevated-temperature properties—and to develop further existing nickel-rich and cobalt-rich alloys of the Inconel, Stellite, and Hastelloy types, which are extremely heat- and corrosion-resistant. Some of the principal alloys developed and used during the war are given in Table 18.8.

Early in World War II, German engineers were ahead of ours in the design of jet engines, but the lack of suitable alloying elements was responsible for an average life for these power units of much less than 100 hr. After designs were perfected in the United States, the following typical requirements for alloys for superchargers were set up: they should withstand a stress of at least 12,000 lb. per sq. in. at 1500°F. (815°C.) for at least 1,000 hr. without failure and be resistant to exhaust gases containing tetraethyl lead. For gas turbines, temperature and stress conditions were somewhat less severe. It is to the credit of American metallurgists that most of the alloys listed in Table 18.8 meet the requirements satisfactorily.

For industrial gas turbines, economics will be more important than it was for military aircraft during the war. Some of the elements used in large quantities during the past three or four years in alloys for turbosuperchargers and jet engines—cobalt is an example-will be too scarce or too costly to be used in turbines which must develop power economically; further, the life of the alloys must be much longer, up to at least 100,000 hr., for example, for a marine turbine. Turbine efficiency increases rapidly with increasing tempera lure so that postwar superstainless alloys must withstand very high temperatures for long periods of time. None of the alloys available now will meet this requirement unless stresses are much below those characteristic of superchargers and jet engines. If high stresses must be withstood at very high temperatures and for long periods of time, new alloys must be developed. Tt is to be expected, therefore, that some of the alloys listed in Table 18.8 will .

Special Purpose Steels 383

Table 18.8. Chemical Composition of Supcrstainless Steels and Heavy Nonferrous Alloys for Turbosuperchargers and Gas Turbines

Nominal composition, per cent Alloy designation

' Cb Ti C Mn Si Cr Ni j Co Mo W

Supcrstainless steels: 19-9 DL 0.30 1.00 0.55 19.00 9.00 1.25 1.25 0.40 0.30 19-9 W-Mo 0.10 0.60 0.50 19.00 9.00 0.40 1.26 0.40 0.35 S 495 0.45 1.00 0.50 14.00 20.00 4.00 4.00 4.00 S 590 0.45 1.00 0.60 20.00 20.00 20.00 4.00 4.00 4.00 0.40 0.60 0.60 20.00 20.00 44.00 4.00 4.00 4.00 0.80 6.25 1 6-25-6 f 0.08 1.50 16.50 25.00 0.30 0.60 1.00 13.00 20.00 0.60 2.25 0.40 1.00 1.00 15.00 25.00 3.00 4.00 0.35 1.25 1.00 15.00 27.00 4.00 0.35 4.00 0.50 18.50 4.50 1.35 1.35 0.60 0.10 0.75 0.50 19.00 12.00 3.25 1.25 N 155 MultimctJ.. 0.15 1.00 0.50 20.00 20.00 20.00 3.00 2.00 1.00 0.04 0.50 0.50 13.00 24.50 3.00 2.25 Rcfractalloy 0.03 0.70 0.65 18.00 37.00 20.00 3.00 3.00 25-20-2 Si 0.10 0.50 2.00 25.00 20.00

Heavy nonferrous alloys: 0.15 0.75 0.25 13.00 78.00 Inconcl, modified § 0.10 0.60 0.60 15.00 74.00 0.25 0.60 0.60 27.00 2.00 65.00 5.00 Stellite 6059 0.40 0.60 0.60 26.00 32.00 35.00 5.00 Stellite 30-422 0.40 0.30 0.25 27.00 16.00 50.00 6.00 Hastelloy B 0.05 0.75 0.75 65.00 29.00 Hastelloy C 0.15 1.00 1.00 16.00 55.00 17.00 5.00

* Contains only 4 per cent iron.

t Contains 0.10 per cent nitrogen. nitrogen. X In development stage, contains 0.15 per cent § Contains 0.70 per cent aluminum.

disappear, except for military use, in the next few years, and that other and more economical or better alloys for industrial gas turbines will take their place. The supcrstainless alloys are important almost wholly for their strength and corrosion resistance at elevated temperatures, and further discussion

is, therefore, deferred to a later chapter. The following data give a general idea of the overall improvement in these alloys during the last few years: The stress which causes failure in 1,000 hr. at 1200°F. (650°C.)

is 1,000 lb. per sq. in. for 3 per cent nickel (S.A.E. 2340) steel; 4,000 lb. 384 Engineering Metallurgy

l'i

I 12 IDO S / *I0 *-, / c o / 1 / b 1 /

°6 J

"o £4 o 2 \J o ) ) C I 9 2() 30 4( 5 60 70 80 90 100 Nickel, per cent

Fie. 18.7. Coefficient of expansion of iron-nickel alloys between and 200° F. {Marsh)

for the silchrome used before the war; 19,000 lb. for 17W (Table 16.4) used at the start of the war; 30,000 lb. for gamma Cb used in 1944; and 36,000 lb. for 16-25—6 used at the end of the war.

18.11. High-Nickel Steels and Special Iron-Nickel Alloys

Nickel dissolves in gamma and in alpha iron and has a strong effect

it to or on the position of A 3 , not only lowering atmospheric temperature below but, in addition, slowing greatly the rate of the phase change and producing some anomalies in properties which are of great value to industry. Alloys of iron with 20 to 30 per cent nickel may easily be made nonmagentic (despite the fact that both iron and nickel are magnetic). Alloys of iron with 30 to 60 per cent nickel have variable and controllable expansion characteristics which were discovered late in the nineteenth century in France. The fact that alloys of iron with 45 to 85 per cent nickel can be treated to produce high magnetic permeability and low hysteresis loss has led to wide use of some of them in telephone, telegraph, and radio equipment.

As shown in. Fig. 18.7,* the coefficient of expansion between and C 200° F. (— 18 and 100 C.) decreases to nearly zero as nickel approaches

* Based on data in J. S. Marsh, The Alloys of Iron and Nickel, Vol. I, Special- Purpose Alloys, McGraw-Hill Book Company, Inc., New York, 1938, p. 160. Special Purpose Steels 385

possible 36 per cent. Increasing the nickel from 36 to 60 per cent makes it "6 10 - to have any desired coefficient of expansion between 1 and 12 X measur- As the result, a number of alloys have been developed for use in ing tapes, watch parts, and parts of machines and instruments which must remain constant in dimension despite temperature changes. The use of invar struts in aluminum-alloy pistons is well known. Some of the commercial alloys with controlled expansion characteristics are given in Table 18.9.

If chromium and tungsten are added to an alloy containing about 32 per cent nickel, the affect of temperature (in the vicinity of room temperature) on the modulus of elasticity is zero. This alloy is known as elinvar and is used for hairsprings of watches and for springs of other precision measuring instruments. The invars and other high-nickel alloys have the additional advantage that they have excellent corrosion resistance.

The alloys known as permalloy, hipernik, and perminvar are used in applications where their high or constant permeability produces marked economics or increased efficiencies in communication. Because under weak magnetizing forces permalloy has a permeability many times greater than that of all other ferrous materials except iron of the highest purity, permalloy is a commercially feasible high-permeability material. The development of this alloy has made modern long-distance telephony

possible, and its use in the loading of submarine cables has speeded up the transmission of messages several hundred per cent. Hipernik has initial and maximum permeabilities much greater than those of commercial grades of high-purity iron and the advantage of relatively high maximum induction, which makes it valuable for use in radio transformers. The efficiency of radio loud speakers and other apparatus requiring powerful permanent magnets has been greatly increased by the recent development of the iron-nickel-aluminum-cobalt permanent-magnet ma- alloys are terials (see Table 18.9) . In addition to alnico, two Japanese noteworthy: the M.K., containing 25 to 30 per cent nickel, 20 per cent cobalt, and 12 per cent aluminum; and the K.S., containing 10 to 25 per cent nickel, 15 to 36 per cent cobalt, and 8 to 25 per cent titanium.

18.12. Austenitic Manganese Steel

Of the large number of high-alloy steels that have not been discussed in this chapter, austenitic manganese steel is of sufficient industrial importance to deserve mention. This steel was discovered a number of years 386 Engineering Metallurgy

Table 18.9. Commercial High-Nickel Iron-Nickel Alloys

Composition,* per cent Name Characteristic properties

Ni Co Cr W Al Cu

36 Low expansions from to 200° F. (-18 to 100° C.)

Supcrinvar . . 31 5 Zero expansion near room temperature 32 5 2 Low thcrmoclastic coefficient 42 For sealing in soft glass Kovar 29 17 For sealing in hard glass Platinilc .... 46 Expansion same as platinum

Permalloy . . . 78.5 High permeability at low field strengths Hipernik .... 50 High permeability at higher field strengths

Perminvar . . 45 25 Constant permeability over a range of flux densities

Alnico I . . . . 20 5 12 Permanent magnet of high magnetic hardness

Alnico . II . . 17 12.5 10 6 Same as above Alnico III... 25 12 Same as above Alnico IV... 28 5 12 Same as above

* Balance of composition is mostly iron.

ago by Hadfield in England during his pioneer researches on alloys of iron with manganese, silicon, nickel, chromium, and other elements. The

annual tonnage of high-manganese steel used is small, but, as is the case

with many ferrous materials, production is no criterion of its usefulness to industry. High-manganese steel contains 1.00 to 1.30 per cent carbon

and 11 to 14 per cent manganese and is made by the basic open-hearth, Bessemer, or electric process. It may be poured into ingots and rolled into

a variety of sections, or it may be cast in sand molds. Owing to its work-

hardening capacity when cold, it cannot be machined readily; hence,

if shaping is necessary, it usually must be ground. Economically this is a great disadvantage. In amounts under 50 per cent, manganese dissolves in alpha and gamma iron. Like it nickel lowers the A 3 temperatures where alpha transforms to gamma in heating and where the reverse change occurs in cooling, and slows the transformation rate. As noted in a previous chapter, manganese also forms a carbide, (Fe.Mn) 3C, which is closely allied

to Fe8C in structure and properties. Alloys containing 1.00 to 1.30 per Special Purpose Steels 387 cent carbon and 13 per cent manganese, if cooled very slowly so that all phase changes take place, contain at room temperature alpha solid solution plus excess carbide. In the usual cooling after hot working, or in a mold in the case of a casting, these phase changes, owing to the low reaction rates, do not take place, and the structure consists of the gamma solid solution plus, possibly, a little alpha, plus excess carbide as massive particles or as a network around the grains. With this structure the steel 2010°F. is hard and brittle. If, however, the steel is heated to 1830 to (1000 to 1100°C), the carbide goes into solution in the gamma, and if the steel is now quenched in water, a structure consisting wholly of austenite with no free carbide is obtained. Unfortunately, tempering to relieve quenching stresses is impossible because reheating in the temperature range 930 to 1290°F. (500 to 700°C.) or even lower may cause precipitation of carbide or change of some gamma to alpha and thus may cause brittlencss. As water quenched, rolled austenitic manganese steel has the following properties:

Tensile strength, lb. per sq. in 130,000 to 160,000 Proportional limit, lb. per sq. in 40,000 to 60,000 Elongation in 2 in., per cent 60 to 70 Reduction o£ area, per cent 40 to 60 Brinell hardness 180 to 220

As is characteristic of austenitic alloys, the high-manganese steels have no well-defined yield point and are nonmagnetic.

High-manganese steel is important industrially chiefly because of its high abrasion resistance. Although the Brinell hardness of the heat-

treated steel is low, cold working causes a rapid increase to as much as 550 Brinell. As abrasion consists largely of cold working the surface, this capacity for work hardening has important ramifications. Thus, in steam- shovel buckets, in crushers, grinders, and other machines, and in rails, cross-overs, switches, frogs and such sections where pounding, pressure, or abrasion by rock and sand or by other metals are encountered, high- manganese steel outwears high-carbon steel by several hundred per cent.

QUESTIONS

1. Name the three classes of high-alloy steels containing chromium as the principal alloying element. Give the approximate composition of six common high-chromium steels, and name the class to which they belong.

2. What is the effect of up to about 15 per cent chromium on the transformation temperatures in high purity iron? Describe the phase changes that take place if an alloy containing (a) 10 per cent chromium and 90 per cent iron, 388 Engineering Metallurgy

and (b) 20 per cent chromium and 80 per cent iron, is heated slowly to 2,500°F. (1,370°C) and cooled again. 3. What is the effect of carbon on the phase changes that take place when alloys of iron and chromium containing up to about 20 per cent chromium are heated to a high temperature and cooled again? What effect has carbon on the response of these alloys to heat treatment? 4. What structure would be expected if the following steels were quenched in oil or water: (a) 0.35 per cent carbon and 14.50 per cent chromium? (b) 0.75 per cent carbon and 16.75 per cent chromium? (c) 0.07 per cent carbon and 15.80 per cent chromium? How does the hardenability of a steel containing 0.35 per cent carbon and 14 per cent chromium compare with the hardenability of an unalloyed high-carbon steel? 5. What is the relation between the tensile properties and the carbon content of heat-treated 14 to 18 per cent chromium steels? Give two peculiarities in tempering quenched cutlery steels and the effect of these on tensile strength and impact resistance. What is the relation between the chromium content and the corrosion (and oxidation) resistance of chromium steels? What is the effect of carbides on corrosion resistance? To what corrosive media are the 12 to 18 per cent chromium steels especially resistant? Why? 6. What is the general effect of nickel on the alpha-gamma transformations

in iron-chromium alloys? What is the effect of adding 5, 8, or 10 per cent nickel on phase changes in alloys containing 18 per cent chromium and approximately 82 per cent iron when they are slowly heated and cooled between room temperature and 2,400°F. (1,315°C)? Why are alloys containing 18 per cent chromium. 8 per cent nickel, and low carbon usually austenitic at room temperature?

7. What is intergranular corrosion in 18-8? What causes it? What is the under-

lying mechanism of the phenomenon? How is it prevented? 8. What are the characteristic mechanical properties of 18-8, and what effect has heat-treatment on these properties? How does the resistance of 18-8 to corrosion and oxidation compare with that of the high chromium steels? fcrritic What are chromium -nickel steels and what is stainless W? 9. What are superstainless steels? What alloying elements in addition to chromium and nickel are used in these materials? What are the general characteristics of superstainless steels, and what are they used for? 10. Outline the main points of difference between the well-established 18-8 grade of stainless steel and the new group of these materials. Tool Steels, Die Steels, and Cemented Hard Carbides CHAPTER

19 JOSEPH Gi.rland, Sc.D., Assistant Professor, Division of Engineering, Brown University, Providence, Rhode Island Instructor, Metallurgy Department, Joseph J. Kaufman, Academy of Aeronautics, Flushing, New York

XHE steady, if slow, advance of civilization over the past ten or twelve thousand years has been largely the result of prog- they are ress in the design of tools and in the material from which fashioned: from stone to copper, then to bronze, and then to iron, and after a long period to carburized iron (steel)-which was the most important step of all—and finally to high-speed and other alloy- steel tools and sintered hard carbides, which made possible great reductions in the cost of finished structures and machines. All tools can be divided broadly into two general classes: (1) cutting and should and shearing tools and (2) forming tools. Requirements differ be considered before the proper steel is selected. Thus, for cutting and shearing, high hardness, ability to withstand wear, stability of structure to withstand the heat generated by the friction of the cutting edge against the work, and a substantial amount of toughness to prevent chipping and breakage are important. For shearing tools, a lack of deformation or warping in heat treatment is usually also important. Forming tools consist chiefly of dies, which are used to transfer their impression to either hot or cold solid metal, or to molten metal. In addition to pos- sessing the qualities necessary in a cutting or shearing tool, forming tools must be resistant to distortion in heat treatment and to cracking caused by sudden changes in temperature during use. The representative types of tool steels and their principal characteristics are given in Tables 19.1 and 19.2. The recently adopted AISI-SAE classi-

fication is used. As Table 19.2 indicates, tool steels vary widely in characteristics.

389 390 Engineering Metallurgy

The selection of the proper tool steel for a certain machining operation

is difficult and is a problem in the solution of which the advice of the

tool-steel metallurgist is very helpful.

An accurate evaluation of a steel that is to be used for a specific tool

in a specific machining operation is practically impossible, and as a result, large quantities of tool steel are, and always have been, purchased by brand name.

The only reliable performance test of a tool steel is to use it in actual production and to use enough similar tools of the same steel to make the results convincing. Large users of tool steel frequently distribute their yearly or half-yearly requirements among a number of tool-steel manufacturers and keep accurate records of the performance of each brand. At the end of this production test the company whose brand makes the best showing receives 50 per cent or more of the business for the next period.

19.1. High-Carbon Tool Steels

The steels of class W, Table 19.1, are the cheapest and, despite high distortion in hardening, low resistance to softening at elevated temperature, and shallow hardening, the most widely used of the tool steels. There are four classes, graded according to carbon content as follows: 1. Carbon 0.60 to 0.75 per cent; used for machinery parts, hot-forging dies, rivet sets, battering tools, large chisels, and set screws. 2. Carbon 0.75 to 0.90 per cent; used for forging dies, boilermakers' tools, hammers, sledges, mining tools, and miscellaneous blacksmiths' tools.

3. Carbon 0.90 to 1.10 per cent; used for drills, saws, cutters, taps, small shear blades and dies, anvils, wood- and stone-working tools of various kinds, punches, and axes. 4. Carbon 1.10 to 1.40 per cent; used for small drills, taps and lathe tools, files, cutlery and small edge tools, jewelers' tools and dies, brass- and copper-working tools, wire-drawing dies, and razors. The hardness and toughness of the high-carbon tool steels vary with

carbon content; they also vary with heat treatment, that is, with tempering temperature. Since high hardness and considerable toughness are

incompatible in an unalloyed high-carbon steel, it is necessary to sacrifice

part of one or the other depending upon the use for which the tool is intended. Thus, for razors, which must be very hard and which are not ordinarily subjected to shock, a 1.20 per cent carbon steel, treated to give a hardness of 60 to 65 Rockwell C, is used; but for a hammer or a chisel, which must withstand impact, a steel containing 0.70 to 0.90 •

Tool Steels, Die Steels, and Cemented Hard Carbides 391

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392 Engineering Metallurgy

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§ o-s h b 2 "O < r3 2S3--H-S — BUM* * * o 1 1 o o 6 £ o £ 6 c < < 6 < < a--- _- ^ * ^Jt^JL o o o §2 be o u

Tempering femperature.deg.C. 100 300 500

"As 200 400 600 800 1000 I20C quenched Tempering temperature, deg.F.

steel. Fic. 19.1. Effect of tempering on the hardness of water-quenched high-carbon (GUI) per cent carbon, treated to about 55 Rockwell C and with more toughness, will be chosen. Wear resistance, which usually is not so high in carbon steels as in some of the alloy steels, is secured by the use of a 1.20 to 1.40 per cent carbon steel, heat treated so that there will be numerous well-distributed excess carbide particles in a martensitic matrix. Such a structure (Fig.

19.2) is important for cutting tools and wire-drawing dies. Under the quenching conditions necessary to obtain high hardness, high-carbon steels warp considerably. In tools of simple section this may be minimized by quenching in special fixtures; for dies and some tools which must be finish-machined before heat treatment and for which stability of dimensions is necessary, special nondeforming alloy steels, which harden in oil or air, are used. 394 Engineering Metallurgy

Fig 19.2. Structure of water-quenched high-carbon tool steel: small carbide particles in martensitic matrix, etched. 500X-

High-carbon steels, even though of practically the same composition, may differ in their response to quenching (hardenability). If the steel has an inherent tendency to become coarse-grained when heated considerably above the transformation temperature, it hardens relatively

deeply and uniformly when quenched but is likely to crack. If it is fine- grained after such treatment, it is shallow hardening and may have soft

spots on the surface, but it is not so likely to crack in quenching. Un- alloyed high-carbon steels are all shallow hardening compared with tool steels containing considerable molybdenum or chromium, but the hardenability can be controlled to some extent by controlling the grain-growth tendency during manufacture. The difference in hardenability between

two lots of high-carbon steels of identical chemical composition is shown in Fig. 19.3.* Bars % in. in diameter were quenched in brine from the temperatures noted and were not tempered. The fractures of the "fine- grained" bars are shown at the top, and those of the "coarse-grained" bars at the bottom. The coarser grain and the greater depth of hardening for the latter are clearly evident. Unalloyed high-carbon steels arc apt to be inherently erratic in their response to heat treatment. Even steels of the same composition, made by the same manufacturer, may differ in their tendency toward grain growth and in depth of hardening. To render the response to quenching more uniform, small percentages of chromium or vanadium are frequently

* Shepherd P-F hardenability test specimens. Tool Steels, Die Steels, and Cemented Hard Carbides 395

tool steel. Top, Fie. 19.3. Fracture of quenched and untempered high-carbon hardening. (Gill) fine-grained, shallow hardening; bottom, coarse-grained, fairly deep

and chro- added to steels of class W. Vanadium inhibits grain growth prob- mium increases hardenability; both have a favorable effect which ably compensates for the increased cost. high hard- As a class, the high-carbon tool steels are characterized by sections. ness at the surface and a relatively soft core in all but very small Owing to this soft core, Gill* rates these steels as tough and resistant to dis- shock. They are, as previously mentioned, subject to considerable tortion in quenching—due to the formation of martensite at the surface and of bainite or pearlite in the center, and to the stresses caused by differential cooling of surface and center—which means that they are not suitable for tools of complex or irregular shape. Small amounts of nickel, chromium, molybdenum, or tungsten are added to some varieties to increase hardenability and to permit quenching in oil instead of water.

19.2. Low Alloy Tool Steels

Low alloy steels of class L, which include the well-known ball- and 1.5 roller-bearing material containing about 1 per cent carbon and 1.2 to dies, rivet per cent chromium (S.A.E. 52100) , are used to some extent for sets, small rolls, and for a few cutting and shock-resisting tools. They are relatively cheap, at least as compared with some of the highly alloyed tool steels, and have exceptionally high strength combined with relatively good toughness. They are also much deeper hardening than the high- carbon steels. Whether the steels are water or oil hardening depends chiefly on the manganese content, which in the oil-hardening grades is 0.50 to 0.60 per cent.

• Tool Steels, American Society for Metals, Cleveland, 1944. .

396 Engineering Metallurgy

The carbon content of these steels depends on the service requirements; if toughness is important as in pneumatic hammer pistons or

chisels, it is 0.40 to 0.70 per cent; but if maximum hardness and wear resistance are necessary as in files, drills, and cutlery, it is between 0.90 and 1.10 per cent. The amount of chromium in the steel depends primarily on the depth of hardening required, which in turn depends on

the size of the tool that is to be treated. Increasing the chromium from 0.50 to 1.50 per cent more than doubles the time available for the steel to cool past the pearlite nose of the S curve without transforming. Also in this class are high-carbon variants of some of the S.A.E. engineering steels. Most of the medium-carbon S.A.E. nickel-chromium or chromium- molybdenum steels are fairly deep hardening;* increasing the carbon to 0.60 to 0.70 per cent and maintaining the chromium at well over 1.0 per cent and the molybdenum at 0.30 per cent or more increases the quenched hardness and the depth of hardening; at the same time the steel retains considerable toughness. These steels are, therefore, finding relatively wide use in many machine parts and especially in die blocks for hot forging. These steels are generally quenched in oil.

19.3. Medium Alloy Tool and Die Steels

Advantage is taken of the effect of manganese or chromium when dissolved in alpha iron of decreasing the critical cooling rate and of increasing deep hardening to produce a valuable and extensively used class

of oil-hardening "nondeforming" tool and die steels (class O) . These

steels are used when the design of the tool or die is such that distortion or cracking is likely to occur in water quenching. In this group there are

steels containing 0.90 to 1.45 per cent carbon. If the manganese is 1.60 per cent, the steel usually contains no other alloying element although the addition of 0.10 to 0.25 per cent vanadium to reduce grain growth is optional. To increase wear resistance the manganese may be lowered to 1.20 per cent, and about 0.5 per cent chromium plus 0.5 per cent tungsten may be added; or the manganese may be reduced to 0.25 per cent, and 0.75 per cent chromium and 1.75 per cent tungsten may be added. These modified steels are not so likely to crack in quenching as the higher manganese steels, but, on the other hand, they do not become so hard. The steels of class O are quenched in oil from 1400 to 1500°F. (765 to 815°C.) and are tempered at 325 to 500°F. (160 to 260°C.) They are used as stamping dies, thread-rolling dies, dies for molding plastics, punches, broaches, blanking dies, and especially as master tools and gages.

• Kor example, the SAE 3100 series and 4100 series. . .

Tool Steels, Die Sleeb, and Cemented Hard Carbides 397

Silicon-manganese steel (S.A.E. 9255 and 9260) containing 0.50 to 0.60 per cent carbon, 1.80 to 2.20 per cent silicon, and 0.60 to 0.90 per cent as manganese is widely used for springs. Recently it has become popular a tool steel, especially for punches, chisels, and shear blades, owing to its low cost and is fairly high wear resistance and toughness (class S-5) Strictly speaking, silicon-manganese steel is a misnomer, as the amount of manganese is not greater than that present in many of the S.A.E. carbon and low-alloy steels. Distortion in quenching is high, but it is somewhat less than for plain high-carbon steels, and wear resistance is higher. The steel hardens more deeply than high-carbon steel and is tougher. To increase deep hardening, 0.20 to 0.40 per cent chromium or 0.40 to 0.60 per cent molybdenum is occasionally added. Steels of class S are usually quenched in water (or oil, if enough chromium and molybdenum are present) from 1575 to 1625°F. (860 to 885°C.) and are tempered at 350 to 600°F. (175 to 315°C.) The medium-chromium and low-tungsten tool and die steels are fairly or very deep hardening and, generally, can be hardened with little distortion. They have higher resistance to wear than unalloyed high- carbon steels and they have satisfactory toughness.

Steels of class S of low carbon content and therefore of relatively high toughness, are used for chisels, punches, shear blades, and battering tools and occasionally for dies. These steels do not have high resistance to tempering. According to Gill, chisels of this steel have three times the

life of unalloyed chisel steel.

The steels in class A contain 1.0 per cent carbon and 1.0 per cent

molybdenum. Chromium varies from 1 .0 to 5.0 per cent, and manganese varies inversely with the chromium from 3.0 to about 0.50 per cent. These steels are air hardening and are widely used for intricate dies that must maintain their dimensions accurately after heat treatment. They are used

for- blanking and forming dies, rolls, punches, and drawing dies but not for dies that operate at high temperatures.

19.4. High-Alloy Tool and Die Steels

Dies that are to be used in applications where they are heated to temperatures 600°F. (315°C.) or above are usually rather highly alloyed, (class H). Steels in these classes are characterized by high hardenability (the pearlite nose of the S curve of some of these steels is far to the right of the time axis), very low distortion in heat treatment—which is natural in view of the fact that most of the steels can be hardened in air —high hardness at elevated temperatures, and fairly good toughness. Most .

398 Engineering Metallurgy

Fie. 19.4. Structure of high-chromium die steels (A) forged and annealed, 750X". and (B) oil quenched and tempered at 400°F. (200°C), 500x: etched. (Kinzel and Forgeng)

will withstand temperatures of 1 100°F. (595°C.) or even slightly higher.

This is to be expected as some of them approach high-speed steel in composition and in many characteristics. The structure pf the high-chromium die steels (class D) as forged and quenched and tempered is shown in Fig. 19.4. Although, after annealing, the Brinell hardness of these steels is 200 to 230, they are difficult to machine. Steels of this type harden deeply with little distortion and do not soften appreciably when tempered below about 900°F. (480°C.) Their wear resistance is high. Despite high cost and difficulty of machining, these steels are among the best as dies for forming and trimming, blanking, thread rolling, and wire drawing, as shear blades, bushings, and rolls, and as small machine parts where resistance to abrasive wear is important. They are seldom used as cutting tools.

19.5. High Speed Steels

The majority of today's tool steel cutting tools are made of high speed steel. The principal types are given in Table 19.3. .

Tool Steels, Die Steels, and Cemented Hard Carbides 399

Table 19.3. Classification of High-Speed Steel by Chemical Composition

AISI- Nominal chemical composition, per cent SAE Type Class C Mn Si W Cr V Mo Co

T-1 18-4-1 0.70 0.25 0.25 18.0 4.0 1.0 T-2 18-4-2 0.85 0.25 0.25 18.0 4.0 2.0 T-7 14-4-2 0.80 0.25 0.25 14.0 4.0 2.0 T-3 1.00 0.25 0.25 18.0 4.0 3.0

T-4 18-4-1 Co 0.75 0.25 0.25 18.0 4.0 1.0 5.0 T-8 14-4-2 Co 0,80 0.25 0.25 14.0 4.0 2.0 5,0 T-6 20-4-2 Co 0.80 0.25 0.25 20.0 4.5 1.5 12.0 M-1 Molybdenum W 0.80 0.25 0.25 1.5 4.0 1.0 8.0 8.0 M-10 Molybdenum . . 0.85 0.25 0.25 4.0 2.0

M-30 Mo WCo 0.85 0.25 0.25 2.0 4.0 1.3 8.0 5.0 M-34 Mo W Co 0.85 0.25 0.25 2.0 4.0 2.0 8.0 8.0 M-2 Mo W 0.85 0.25 0.25 6.0 4.0 2.0 5.0 M-4 Mo WV 1.30 0.25 0.25 5.5 4.0 4.0 4.5 M-35 Mo WCo 0.85 0.25 0.25 6.0 4.0 2.0 5.0 5.0

The familiar 18-4-1 composition has long been the basic type. But since

World War II, molybdenum is replacing tungsten in these alloys. Today, 80% of all shipments of high speed steel are of types M-1, M-2, and M-10; whereas type T-1 represents only 10% of all high speed steel being used. In general, cutting performance increases with tungsten or molybdenum. Vanadium is necessary for satisfactory performance and approximately 1 per cent or more is always present. Chromium is held constant at about 4 per cent, with carbon varying from 0.50 per cent (if toughness at the expense of some hardness is desired) to as much as 1.50 per cent. Cobalt has been used in the regular grades of high-speed steel with considerable success. It increases red hardness and the efficiency of the tool where temperatures are high as during heavy roughing cuts.

High-speed steel is hard, strong, and brittle at room temperature and retains its hardness and strength at temperatures up to 1100 to 1200°F. (595 to 650°C.)

The cutting performance of high-speed steel, which is from three to fifty times that of unalloyed high-carbon steel of the same hardness, is largely dependent upon the heat treatment. The optimum quenching temperature depends upon composition, and a variation of ±50°F. (±30°C.) from the optimum will affect the performance of the tool. ,,.

400 Engineering Metallurgy

Engineers owe a large debt to two Americans, F. W. Taylor and Maunsel White, who, while working at Bethlehem Steel Company in October, 1898, discovered the "weird and novel" heat treatment which gives high tungsten-chromium steel its property of red hardness. Taylor and White found that a steel containing 1.14 per cent carbon, 0.18 per cent manganese, 7.72 per cent tungsten, and 1.83 per cent chromium performed poorly in rapid machining when heat treated in the usual way. After considerable investigation the present-day treatment was developed. This consists of heating the tool slowly to 1500°F. (815°C.) then rapidly to a temperature slightly below the melting temperature— with modern high-speed steels this is 2200 to 2350°F. (1205 to 1285°C.) which would ruin carbon or low-alloy steel—and cooling it rapidly in oil, or, sometimes, in air. The tool is then tempered immediately by reheating to 1000 to 1150°F (540 to 620°C.) . When heat treated in this way, high-speed steel machines at high speeds and with such heavy cuts that the point of the tool becomes red hot, without losing any hardness. Although considerable work has been done on the constitution of iron-tungsten-carbon alloys, little is known about the effect of 4 per cent alloys. chromium and 1 to 2 per cent vanadium on these Studies of microstructure indicate that high-speed steel when annealed consists of a relatively soft (probably ferritic) matrix and a large number of particles of a complex carbide. When the steel is heated to a high temperature, the matrix changes to austenite and some of the carbides go into solution. To produce red hardness and satisfactory cutting performance, the temperature to which the steel is heated must be as high as possible without causing incipient melting. When the steel is quenched from this temperature, a structure such as is shown in Fig. 19.5 A* results. It

is thought that the polygonal grains of the matrix are composed of austenite plus untempered martensite. Reheating to 1()50°F. (565°C.)

tempers the martensite slightly so that it etches rapidly (Fig. 19.5B), and upon cooling from the tempering temperature the retained austenite transforms to more martensite. The secondary hardening that results from tempering is shown in Fig. 19.8 The cause of red hardness, the most important property of high-speed steel, has not yet been explained satisfactorily. It is usually considered to be related to the high stability of the complex carbide particles and to the fact that the most favorable size of the particles for stability of

structure is not altered by temperatures as high as 1100°F. (595 ° C.)

• I.. The Alloys iron and Tungsten, McGraw-Hill Book Company, Inc., J. Gregg, of New York, 1934, pp. 294, 295. Carbides 401 Tool Steels, Die Steels, and Cemented Hard

&4&&L *3£ L ,CK-^^^ <8f>*

(A) quenched and (B) quenched and FIO. 19.5. Structure of high-speed steel; tempered; etched. 750x- (Gregg)

the substitution The most recent development in high-speed steels is Table 19.3) This was of molybdenum for most of the tungsten (see . Emmons.t who pioneered by Richie* at Watertown Arsenal and by has three advantages developed the steels commercially. Molybdenum as effective as tungsten in pro- in high-speed steel: (1) Since it is twice Table 19.3 ducing red hardness, only 6 to 9 per cent is necessary, as for pound than tungsten; and (3) it is shows- (2) it is cheaper pound domestic ores are available. not a strategic material, as ample supplies of molybdenum grades have been Initial difficulties in the heat treatment of between molybdenum solved, so that, at present, there is little difference performance. 1 he in- and tungsten base alloys as to fabrication and tool is due mainly to creased consumption of molybdenum high-speed steels their lower price. more expen- Compared to carbon tool steels, the high-speed steels are used in the right kind sive and arc not as tough. However, properly out-perform tools made from any of equipment, the high-speed steels will which will stand up other type of steel. It is the only type of steel edge by modern, high- to the high temperatures generated at the cuttting speed, production machines.

1930. • S. B. Richie, Army Ordnance, V. 11, July, 193. Emmons, Trans. Am. Soc. Steel Treat., V. 21, 193S, p. fj. V. 402 Engineering Metallurgy Temperinq temperature , deq.C. SOO 400 soo ' 60

Tempering temperature, deq. F.

Fig. 19.6. Effect of. chromium on the resistance of 0.35 per cent carbon steel to tempering. (Bain)

19.6. Function of Alloy Additions in Tool and Die Steels

The structure of tool steels consists essentially of a dispersion of hard carbide particles in a matrix of steel. The purpose of small alloy additions is to improve the properties of the matrix by increasing the hardenability and refining the grain si/e. The most common alloy additions to low alloy steels are chromium, vanadium and manganese. Chromium increases the depth of hardening and adds some wear-resistance; vanadium increases toughness and limits grain growth; manganese permits oil hardening with much less distortion than water quench. To further reduce the likelihood of cracking during cooling, about 1% molybdenum makes a steel air hardened. Larger amounts of alloy elements, together with increased carbon contents, form hard alloy carbides which are responsible for the superior hot hardness and wear-resistance of alloy tool steels. The amount and the dispersion of the carbide particles determine the hardness; the greater the relative volume of the hard phase, the harder and more brittle the steel. The principal function of large alloy additions to tool steels is to increase the amount of the carbide phase. Tool Steels, Die Steels, and Cemented Hard Carbides 403

Tempering temperature, deg. C.

O 400 600 600 1000 1200 Tempering temperature, deg. F.

Fig. 19.7. Effect of molybdenum on the resistance of 0.35 per cent carbon steel to tempering. {Bain)

As is evident from Table 19.1, tungsten, molybdenum, and chromium are the most important alloying elements in medium and high alloy tool steels, and together with vanadium they are more imi>ortant in high- speed steel than in the ordinary tool and die steels. All three are soluble to a considerable amount in gamma and in alpha iron and, more important, all three form hard and stable carbides. All three, when dissolved in gamma iron, slow the reaction rate for austenite to transform to pearlite—chromium and molybdenum strongly and tungsten weakly. The carbides formed by these three metals dissolve to some extent (depending on the amount of carbon and the temperature) in austenite and can be retained in supersaturated solution by quenching. This increases hardness and wear resistance. In addition, the carbides in excess of those dissolved, if favorably distributed, greatly increase the resistance of the steel to abrasive wear. In contrast to tungsten, which has a minor cflect on hardenability, chromium and molybdenum have a powerful effect. This was discussed in detail in earlier chapters. The most valuable property of tungsten, molybdenum, and chromium when present in relatively large amounts r

404 Engineering Metallurgy

Tempering tempcroture.deg. C. 100 200 300 400 500 600 703 J 1 ' ' - ' i I i —

48l ~_ As J00 500 700 900 UOO quenched Tempering femperalure^d

Fic. 19.8. Effect of tempering on the hardness of high-speed. (Emmons)

in steel containing sufficient carbon is that they make the steel "red hard"; that is, the steel retains its hardness at elevated temperatures, in some cases so high that the steel becomes red in color. Tungsten, molybdenum, and chromium also increase the resistance to tempering. This is shown by Fig. 19.6* for steel with 0.35 per cent carbon and increasing chromium, by Fig. 19.7* for steel with 0.35 per cent carbon and increasing molybdenum, and by Fig. 19.8f for high-speed steel containing 0.70 per cent carbon, 18 per cent tungsten, 4 per cent chromium, and 1 per cent vanadium. The increase in hardness of these steels when tempered between 900 and 1100°F. (480 and 595°C.) is due primarily to the transformation to martensite of some austenite retained by the quenching treatment.

19.7. Cast Alloys

Alloys rich in cobalt, chromium and tungsten were developed and placed on the market in 1907 under the name of Stellite. They contain a high proportion of a hard constituent (complex chromium-rich carbide) within a cobalt rich matrix phase. Cutting tools of this type usually contain from 45 to 55% cobalt, 25 to chromium, 35% 12 to 17% tungsten, 1 to 3% carbon. The properties cannot be varied by heat treatment, but are controlled by the composition, the softer alloys having a higher strength. The tool grades range in hardness from 40 to 64 Rockwell C. Considerable hardness and

•E. C. Bain, Functions of the Alloying Elements in Steel, American Society fo* Metals. Cleveland, 1939. t J. V. Emmons, Trans. Am. Soc. Steel Treat., v. 19, 1932, pp. 289-332. .

405 Tool Steels, Die Steels, and Cemented Hard Carbides

Tungsten carbide with Fie 19.9. Typical structures of sintered hard carbides. (A) carbides with 4 per cent cobalt. (B) Tungsten, titanium, tantalum and columbium with alkali ferricyanide. about 8 per cent cobalt. Polished with diamond dust and etched 1.500X- (Courtesy Kennametal Inc.)

strength are maintained at high temperatures, their "red hardness" making these alloys superior to high speed steels in resisting wear at elevated temperatures. At the same time they are appreciably weaker and somewhat more brittle. The alloys are sold commercially under a variety of trade names. They are used as cutting tools and wear resisting parts. Since they are expensive and have relatively low strength they are usually cast into small pieces of the desired size and shape which are used as inserts supported by a stronger backing material. Or, thin layers can be welded as a hard surface coating over steel parts.

19.8. Cemented Carbide Tools

Cemented carbides, also called sintered carbides, are used mainly for tools and dies. They are the hardest of all metallic tool materials, consisting largely of finely divided carbide particles embedded in small amounts of a soft ductile metal, usually cobalt (see Fig. 19.9) The hard, abrasive carbides of tungsten, molybdenum, titanium, tantalum, zirconium, and other metals have been known for nearly seventy-

five years, but their use for tools is much more recent. It began about twenty-five years ago in Germany and about twenty years ago in the United States. All cemented carbide tools contain tungsten carbide (WC) as the essential constituent. The tungsten carbide may be used alone for the machining of cast iron, nonferrous metals and plastics. But steel-cutting grades, in addition, must contain titanium carbide. The carbides of titanium and tungsten form a solid solution which has the outstanding characteristic of resisting the abrasion of the steel chip as it glides over .

406 Engineering Metallurgy

the tool. Smaller quantities of tantalum, columbium and vanadium carbides are also added to some grades. Cemented carbide tools are made by the methods of powder metallurgy. The finely ground carbide powder is mixed with cobalt, pressed into the desired shape, and sintered at 2450°F (1343°C) or above. The sintering temperature is above the liquidus of the alloy system and densification occurs in the presence of a liquid phase. The resulting product is substantially free from porosity. The properties of sintered carbides vary with composition, particularly with the proportion of cobalt. In general, hardness is increased but toughness or shock resistance is decreased by smaller binder contents. Tungsten and titanium carbides are among the hardest materials known; although less hard than diamond, their hardness is of the same order of magnitude as that of aluminum oxide, boron carbide and silicon carbide. Depending upon the amount of cobalt used (which varies from 3 to about 20 per cent), the Rockwell A hardness (diamond indenter, 60 kg. load) ranges from 88 to 93, corresponding approximately to a Rockwell

C hardness of 72 to 76. A good portion of this hardness is retained at the high temperatures encountered during metal cutting, permitting a cutting speed three or four times greater than that of high speed steels.

The material has little ductility, strength is determined by a transverse rupture test and varies from 150,000 to 400,000 lb. per sq. in. The cemented carbides have very high compressive strengths (500,000 to

900,000 lb. per sq. in.) . They also have some of the highest elastic moduli known (up to 90,000,000 lb. per sq. in.) Cemented hard carbides are widely used for wire-drawing dies, extruding dies, and numerous other dies for burnishing or spinning metal sections, such as metal radio tubes, shell cases, projectile jackets, eyelets, and many others, and also for gages, thread guides, valve parts for erosive materials, and the like. The most extensive application of these materials is in mass-production machining of cast iron, steel (with Brinell hardness values as high as 500) , and most of the other metals, and also of many difficultly machined nonmetallic materials including plastics, rubber, resins, glass, asbestos, and others. Cemented carbides arc costly and the tool generally consists of a carbide tip or insert brazed to a steel holder or shank, but great economies frequently result from the use of these tools. The greater machining speeds and increased tool life permit production increases between 30 and several hundred per cent while saving approximately one third of the production costs. Tool Steels, Die Steels, and Cemented Hard Carbides 407

QUESTIONS

1. Name the two main classes of tool steels. What properties are important in each class? Give eight essential characteristics that tool steel should have, and rate these in the order of their importance.

2. How are tool steels usually purchased? Why is it difficult to devise a performance test to evaluate tool steels? How important is die cost of the steel

from which a tool is made? 3. Give the characteristic properties of high-carbon tool steel. What is the approximate carbon content of the four main classes of high-carbon tool

steels, and whv is the carbon percentage adjusted according to the use to which the steel is to be put? What carbon percentage would you specify for the following: razor blade, hammer, drill, anvil, rivet set, small shear blade, ax, and file? 4. What are the outstanding characteristics of the steels of class L? How is the hardenability of these steels increased? Compare these steels with the S.A.E. low nickel-chromium and chromium-molybdenum steels. 5. What are the principal alloying elements in the oil-hardening, nondefonn- ing tool steels of class 0? Why are these steels widely used? What are they used for? Give the important properties of the silicon-manganese steels when used as tools. 6. What are the general effect of (a) chromium, (b) molybdenum, and (c) tungsten on hardenability, wear resistance, tempering, and hardness at high temperature?

7. What is the chief advantage of the medium-chromium, low-tungsten tool and die steels as compared with high-carbon tool steels, and how do the two groups compare in wear resistance, red hardness, and toughness? What are the outstanding characteristics of the high-alloy tool and die materials? What are the main alloying elements used in these high-alloy steels?

8. What is the approximate composition of the principal types of high-speed

steel? How is high-speed steel heat treated? Why is cobalt added to some high-speed steels? What are the advantages and disadvantages of molybdenum high-speed steel as compared with the usual 18-4-1 material?

9. What is the most valuable property of high-speed steel, and what is the usual explanation for this property? What variables affect the cutting performance of high-speed steel? 10. What hard carbides are used as tool and die materials? How does the hardness of these carbides compare with that of quenched tool steel? Describe the structure of these tools. 11. Compare and discuss the compositions, microstructures and properties of a typical stainless steel and a typical tool steel. Relate the microstructures to the desired properties. (Note particularly the difference in carbon contents.) . .. .

Cast Iron CHAPTER

20 CHARLES Arthur Naglkr, Ph.D., Associate Professor, De- partment of Chemical and Metallurgical Engineering, Wayne State University Detroit, Michigan Kenneth E. Rose, M.S., Professor of Metallurgical En- gineering, University of Kansas, Lawrence, Kansas

v*AST irons are alloys of iron and carbon containing more than 2 per cent carbon. Other elements, notably silicon, are generally present also. Metallurgically, cast irons are characterized by

(1) an eutectic reaction during freezing, and (2) more carbon than can be dissolved in austenite at any temperature.

In steel, carbon is rarely present in any form other than combined as a carbide. However, iron carbide, or cementite, is a metastable compound which can be decomposed to iron and carbon (as graphite) . Because of the embrittling effect of cementite, most of the carbon in commercially useful cast iron is converted to graphite unless the hardness and wear resistance of the cementite is of primary importance. Factors tending to encourage the decomposition of cementite are:

1. High temperature. 2. Time (especially at elevated temperature)

3. Slow cooling rates during freezing (a corollary of 1 and 2) 4. High carbon content. 5. Presence of graphite.

6. Presence of graphitizing agents (including silicon) Cast irons may be classified roughly according to the percentage of carbon, but because of the various ways in which the carbon is associated with the iron, it is usually more convenient to make the classification according to the form of the carbon, rather than the amount. The following outline suggests a procedure for such a classification. A. Carbon combined with iron as cementite. 1. White cast iron (usually about 2% carbon). 2. Chilled cast iron (usually 3% or more carbon) 408 Cast Iron 409

Fie. 20.1A. Flake graph- Fig. 20.1B. Spheroidal Fig. 20.2. Temper car- ite in gray cast iron. graphite in nodular, 01 hon (graphite) in mallc- ductile cast iron. able cast iron.

B. Carbon both in combined and elemental (graphite) form.

1. Mottled cast iron— a. mixture of white cast iron with graphite, generally undesirable.

2. Gray cast iron—graphite as thin, interdendritic flakes which form

during freezing. Cementite is present in pearlite in the matrix. Gray cast iron is sometimes described as "steel plus graphite." (See Fig. 20.1 A.) 3. Nodular or ductile cast iron—graphite as spherulites which form during freezing of specially treated irons. Massive cementite as well as pearlite may be present in the as-cast condition. (See Fig. 20. IB.) 1 Pig iron—a product of smelting in the iron blast furnace, used primarily as a raw material. Large, smooth, star-like flakes of

primary graphite or "Kish" are embedded in a matrix which is essentially gray cast iron. C. Graphite formed by heat treatment, some cementite remaining. 1. Pearlitic malleable cast iron—made by annealing white cast iron to decompose the excess cementite, leaving a matrix of pearlite. D. All carbon present as graphite in a matrix of ferrite. 1. Ferritic gray iron—generally undesirable for strength and wear resistance, but may be made by annealing ordinary gray iron to achieve the maximum machinability. 2. Ferritic malleable cast iron—made by heat treatment of white cast iron to decompose both the excess cementite and the cementite that would otherwise be in pearlite. (See Fig. 20.2.) 3. Ductile cast iron—made by annealing nodular cast iron to attain

a high degree of ductility. The carbide in such iron is decomposed as in malleable cast iron. 410 Engineering Metallurgy

20.1. White Cast Iron as an Engineering Material

Cast iron containing all or nearly all the carbon in the combined form has a silvery-white fracture and is known as white cast iron. Very hard and brittle and practically unmachinable, it is important chiefly (1) as an intermediate product in the production of malleable iron castings and (2) when it is produced, by regulating the composition and by cooling rapidly, as a thin, hard layer on the surface of a softer iron casting. The

latter product is known as chilled iron and is used where high surface hardness and wear resistance are important. The microstructure of white cast iron can be predicted from the iron

iron-carbide phase diagram used for steel; that is, it consists of cementite and pearlite.

20.2. Malleable Cast Iron as an Engineering Material

Malleable cast iron is a valuable engineering material and is widely used for machinery, railroad equipment and automobiles, agricultural machinery, pipe fittings, hardware, household appliances, and in many other applications. Intricate castings are more readily melted and poured with white cast iron than with carbon steel and, despite the cost of malleablizing, are cheaper than carbon-steel castings. Malleable cast iron compares favorably with gray cast iron in machinability and with low-carbon steel in properties.

To produce malleable castings, cast iron solidfying with a white fracture is poured into a sand mold; the hard, brittle casting is then annealed (malleablized) to dissociate the cementite. Malleable cast iron contains

2.0 to 2.5 per cent carbon, practically all of which is graphite (usually called temper carbon) , 0.7 to 1.2 per cent silicon, 0.40 to 0.60 per cent manganese, less than 0.20 per cent sulfur, and less than 0.10 per cent phosphorus. During the first stage of malleablization, at 1600° to 1700°F., cementite which is not soluble in the austenite is converted to ragged loosely formed nodules of "temper carbon." Second-stage malleablization takes place at or slightly below the lower critical (A,) temperature after cooling from the first stage treatment. This causes the remaining cementite, including that which normally would be in pearlite, to gi aphitize. Second-stage malleablization is complete when the structure consists entirely of temper carbon in a matrix of ferrite.

20.3. Engineering Properties of Malleable Cast Iron

White cast iron for malleablizing is melted chiefly in an air furnace; some, however, is melted in a cupola. The tensile strength and elongation Cast Iron 411

(as measured on a test bar) of cupola malleable iron are lower than those of air-furnace malleable, but the bursting strength of castings is usually considerably higher. Cupola malleable, therefore, is widely used for pipe fittings. Most malleable iron is purchased under specifications issued by the railroads or by the valve- and pipe-fittings industry. Most of these specifications give the following as minimum properties:

Tensile strength, Yield strength, Elongation in 2 in., Kind of malleable iron lb. per sq. in. lb. per sq. in. per cent

50,000 32,000 10 40,000 30,000 5

Reduction of area is usually about 20 per cent and is seldom determined.

Malleable cast iron is unique among ferrous materials in that the elongation increases as the tensile strength increases. This is shown by the data for air-furnace malleable given in Table 20.1. Malleable cast iron, with a notched-bar impact resistance of 7 to 9 ft-lb., as compared with

1 ft.-lb. or less for gray cast iron, is much tougher than gray cast iron.

The modulus of elasticity is 25,000,000 lb. per sq..in., that is, slightly lower than that of carbon steel. Malleable cast iron has an endurance

ratio of about 0.5, that is, the same as carbon steel. It is considerably more sensitive to notches than gray cast iron.

Table 20.1. Tensile Properties of Air- Furnace Malleable Cast Iron*

Tensile strength, Elongation in 2 in., Tensile strength, Elongation in 2 in., lb. per sq. in. per cent lb. per sq. in. per cent

51,000 16.5 55,000 19.0 52,000 17.0 56,000 19.6 53,000 17.6 57,000 20.3 54,000 18.3 58,000 21.0

In malleable iron, as in carbon steel, the tensile strength decreases with increasing size of section; a typical change is from 55,000 lb. per

• Proc. Am. Soc. Testing Materials, v. 31. 1931, Part II. p. 317. For ranges of composition and properties of American malleable iron, see Cast Metals Handbook, American Foundrymcn's Society, Chicago, 3d ed., 1944, pp. 308-314. 412 Engineering Metallurgy sq. in. for sections 0.25 to 0.50 in. in thickness to 45,000 lb. per sq. in. for sections of 1.25 to 1.50 in. The yield strength, however, is practically unaffected by section size. In contrast to steel, in which elongation usually increases with mass, the elongation of malleable iron decreases; at the center of a 2- or 3-in. section, for example, the elongation may be only about half what it is at the surface.

Malleable cast iron has poor wear resistance, but in machinability it ranks about the same as gray cast iron. It is easier to plane, drill, or mill than carbon or alloy steels, including those containing lead or high sulphur.

Recently, for a few special applications, the graphitizing operation in the production of malleable has been modified, or alloying elements have been added and special treatments used so that the annealed material contains some pearlite. The properties of pearlitic malleable cast iron depend upon the amount and distribution of the iron carbide; usually it is stronger and less ductile than fully graphitized material. A unique and now well-known example of a specially treated pearlitic alloy malleable cast iron is in the crankshafts of some automotive engines.

The alloy is melted and cast under conditions that produce white cast iron. It is malleablized by a treatment that produces small graphite nodules in a matrix consisting almost entirely of pearlite. This treatment is followed by a complex normalizing and annealing treatment. The composition is approximately as follows: 1.5 per cent carbon, 0.7 per cent manganese, 0.9 per cent silicon, 1.75 per cent copper, and 0.5 per cent chromium.

20.4. Gray Cast Iron as an Engineering Material

Cast iron in which most of the iron carbide has dissociated into iron and graphite is usually soft and readily machinable. It has a gray, almost black, fracture and is thus known as gray iron. More than 90 per cent of all the iron castings produced are of gray iron.

Gray cast iron is a low-cost material which is easily melted and cast, and is economically machined. These are the primary reasons for its wide use. Of the hundreds of engineering applications for gray cast iron, some of the most important ones, together with the usual compositions, are given in Table 20.2.*

The properties of gray cast iron that determine its suitability for engineering uses are the result of controlling four variables: (1) chemi-

• Trans. Am. t'oundrymen's Assoc, v. 39, 1932. pp. 56-64. For a complete list of specific applications ot plain and alloy cast irons, see Cast Metals Handbook, Ameri- can Foundrymen's Society, Chicago, 3d ed., 1944, pp. 335-594. — —

Cast Iron 413

Table 20.2. Recommended Analyses of Plain Cast Irons for Common Engineering Applications

Composition, per cent Typical charge, per cent

Use Cast- Total Si Mn P S Pig iron Steel C scrap scrap

3.25 2.25 0.65 0.15 0.10 40 40 20 Automobile cylinders 3.35 2.25 0.65 0.15 0.10 40 40 20 Automobile pistons 3.40 2.60 0.65 0.30 0.10 50 45 5 Automobilccas tings, general 3.50 2.90 0.65 0.50 0.06 60 40 Automobile piston rings individually cast 3.40 2.10 0.60 0.50 0.10 35 55 10 Agricultural implements medium sections 3.50 2.20 0.55 0.70 0.10 40 60 Agricultural implements light sections 3.25 2.25 0.50 0.50 0.09 50 50 Machinery —sections not over 1 in. 3.25 1.75 0.50 0.50 0.10 50 40 10 Machinery— 1.5-in sections 3.23 1.25 0.50 0.50 0.10 50 25 25 Machinery—2-in. sections 3.25 1.25 0.65 0.20 0.10 50 25 25 Pressure castings—air cylinders 3.40 1.75 0.80 0.35 0.09 55 20 25 Gas-engine cylinders—light 3.40 1.50 0.80 0.35 0.09 55 20 25 Gas-engine cylinders—medium 3.40 1.25 0.80 0.35 0.09 55 20 25 Gas-engine cylinders 3.30 2.00 0.50 0.60 0.10 50 40 10 heavy Valves and fittings 3.50 1.15 0.80 0.10 0.07 70 30 Firepots and ketUes 3.50 1.00 0.90 0.20 0.07 90 10 Ingot molds 3.60 1.00 0.75 0.20 0.07 60 40 Pots for caustic soda 3.60 1.75 0.50 0.80 0.08 70 25 5 Light and medium sand- cast water pipe 3.40 1.40 0.50 0.80 0.08 60 25 15 Heavy sand-cast water pipe 3.40 1.40 0.50 0.80 0.08 70 30 Soil pipe 3.35 0.65 0.60 0.35 0.12 12.5 80 7.5 Car wheels (0.90 per cent combined C) 3.60 1.25 0.55 0.40 0.10 45 40 15 Chilled plow iron

3.75 0.85 0.50 1 0.40 0.10 45 40 15 Plow moldboards

cal composition, (2) rate of cooling from the solidification temperature, (3) special methods of melting, including superheating and ladle addi-

tions, and (4) heat treatment. In addition to low cost and easy machinability, gray cast iron has a number of properties—notably notch insensitivity, high damping capacity,

and high compressive strength—which make it especially valuable in some applications. In tensile strength, ductility, and toughness, however, ,

414 Engineering Metallurgy

it is much inferior to steel. In addition, the absence of a well-defined yield point and modulus of elasticity makes it unsuitable for many engineering structures. Despite its disadvantages, however, the production of gray-iron castings is around 5 or 6 million tons a year in the United States.

20.5. Structure of Gray Cast Iron

Considering gray cast iron as a carbon steel containing graphite, the three primary metallographic constituents present at normal temperatures are alpha iron (ferrite) , iron carbide (cementite) , and free carbon

(graphite). The cementite is usually associated with ferrite as pearlite— the same constituent found in carbon steel— and the graphite is distributed through the ground mass as irregularly shaped plates or flakes of varying sizes. However, very rapid cooling from the melt may suppress the formation of graphite and produce chilled iron, a type of white iron. Very slow cooling increases the proportion of ferrite to pearlite. The ferrite of gray cast iron, unlike the ferrite of low-carbon steel which is relatively pure, contains all the silicon and a little of the phosphorus in solid solution. Of the elements in gray cast iron, carbon has the strongest effect on structure and properties. This effect depends on the amount of total carbon as well as on the relative amounts of combined carbon and of graphite present. This in turn depends upon the silicon which decreases the stability of the iron carbide, thus promoting graphitization. Next in importance is phosphorus, which combines with the iron to form iron phosphide (Fe3 P) . A small amount of this compound dissolves in the ferrite, the rest forms a eutectic (Fe—Fes P) with iron. When the

phosphorus is 0.1 per cent or above, the eutectic (known as steadite) is visible as a distinct constituent in the microstructure (Fig. 20.3.) The eutectic melts at a temperature of 1750° to 1800°F. (955° to 980 ° C.) much below the melting point of gray cast iron. If about 0.5 per cent

phosphorus or more is present, there is enough eutectic to make the iron

more fluid. High-phosphorus iron is, therefore, used for ornamental castings and other thin, intricate sections. Owing to the hardness of the phosphide eutectic, cast irons containing considerable phosphorus may be harder to machine than low-phosphorus material. In the amounts usual for American gray cast irons (Table 20.2) phosphorus has little effect on the strength, but it may increase the brittleness. For cylinders

and various kinds of pressure castings 0.5 per cent is, therefore, usually

the maximum permitted (Table 20.2) . The effect of phosphorus on

graphitization is negligible. Cast Iron 415

Fie. 20.3. Phosphide cutcctic in gTay cast iron. (A) 100x; (B) l.OOOx- The cutectic its is the irregular-shaped light-colored constituent shown in A. At high magnification duplex character is apparent. (Kunkele, Giesserci, v. IS, 19)1, p. 73)

Commercial gray cast iron always contains enough manganese to insure that the sulphur is present as manganese sulphide. Owing to the notch insensitivity of gray cast iron, more sulphur is tolerated than in steel because manganese sulphide inclusions are not so likely to be loci of weakness where failure will start under repeated stress. Manganese in excess of the amount necessary to combine with the sulphur forms iron- manganese carbide. This compound is more stable than iron carbide; hence, manganese inhibits graphitization. In the amount usually present

(less than 1 per cent) the stabilizing effect of manganese is much less important than the strong graphitizing effect of silicon.

20.6. Relation between Properties and Structure of Gray Cast Iron

There is a close relation between properties and structure of gray cast iron. The properties are primarily dependent upon the characteristics of the "steel" matrix and upon the amount and distribution of the graphite. These in turn are dependent upon the variables already men- 416 Engineering Metallurgy tioned, namely, chemical composition, rate of cooling from the solidification temperature, special methods of melting, and heat treatment. The amount of iron carbide to be decomposed into iron and graphite or to be left undecomposed depends (if the effect of cooling rate is ig- nored) upon the amounts of total carbon and silicon. For small specimens cast in sand molds the relation between total carbon, silicon, and structure is shown in Fig. 20.4.*

?.b J.O i.l> 4.0 4.5 5.0 5.5 6.5 7. Silicon, per cent

Fic. 20.4. The Maurer cast-iron diagram.

A modification of the Maurer diagram by Coylet is shown in Fig. 20.5 in which approximate tensile strengths are plotted against composition. This diagram shows that for small specimens cooled in sand molds the

highest strengths are attained if the total carbon is 2.00 to 3.25 per cent

and if the silicon is 1 .00 to 2.25 per cent.

20.7. Effect of Cooling Rate

The relation among properties, structure, and composition is affected greatly by varying die rate of cooling from the solidification temperature to the gamma-alpha transformation temperature. As the mass is increased, the cooling rate is decreased, and the dissociation of the iron carbide is increased. Thus if a 1-in, sand-cast bar has a matrix corresponding to a

• E. Maurer, Kruppsche Monatshelte, v. 5, 1924, pp. 115-122. + F. B. Coyle, Trans. Am. Soc. Steel Treat., v. 12, 1927, pp. 146-465. Cast Iron 417

Z.50 3.00 3.50 4.0C "..50 Silicon. per cent

Fie. 20.5. Cast-iron diagram showing cltect of carbon and silicon on tensile strength. (Coyle)

0.50 per cent carbon steel, the same iron if cast as a 6-in. bar in sand will ordinarily have a matrix that corresponds to a 0.10 per cent carbon steel. On the other hand an iron of such composition that, when poured into a sand mold it will have a matrix of pearlite, will, if poured into a cast- iron mold, form white iron on the surface next to the cold mold but will be pearlitic in the interior of the casting. Thus, the lines of the Maurer diagram (Fig. 20.4) are moved to the right if the cooling rate is increased, and to the left if the cooling rate is decreased. The approximate relation among structure, cooling rate, and properties is shown in Fig. 20.6.*

It is evident from the foregoing that the relation among the composition, cooling rate, structure, and properties of gray cast iron is not simple. Another important factor, which adds to the complexity, is the size and form of the graphite flakes.

20.8. Effect of Graphite Size on Structure and Properties

The strength of slowly cooled carbon-steel castings increases as the carbon (cementitc) increases. The strength of slowly cooled gray cast iron would also increase as the cementite increases if no graphite were present. Graphite destroys the continuity of the grains of the "steel"

matrix and makes it weak and brittle. As the si/.e of the graphite particles increases and as they become more flaky and platelike, the iron becomes progressively weaker and more brittle.

* Cast Metals Handbook. Am. Foundrymen's Soc, Third Edition, 1944, p. 360. 418 Engineering Metallurgy

Strength increases t r r 350 S u c t u e Hard and brittle *\»hite iron. Cemenlite andai/stenife

300 Mottlediron. Cementite,grapliiler andpearlitc

250 w Greatest strength. Hard to machine VW |

°zoo — Best nigh-test irons - Close gray iron. Graphite, andpeartite

/air strength. Easy machining ,. . 4 -^-p^Med^mgroy.ron.eraph,^yf, , and„ndZrf,w„lfair finish co 150 peartite ana fernte

100 .Low strength. Open grain. Open gray iron. Graphite and ferrite Machines soft "-§

FJO. 20.6. Relation of hardness and strength o£ gray cast iron to structure and section size. (American Foundrymen's Society)

High-strength cast iron must have a "steel" matrix containing considerable pearlite, and the graphite flakes should be small; there must, how-

ever, be 2.0 to 2.5 per cent graphite present if easy machinability is

required. The control of the graphite size is accomplished by five meth-

ods. The first consists of melting an iron of such composition that it would normally solidfy with a white fracture and then treating it by adding a special graphitizer, such as powdered silicon or calcium silicide, just before the castings are poured. The second consists of melting an iron that would normally solidify with a white fracture and mixing with this a definite amount of a soft gray iron melted in another cupola. The

third method consists of melting gray cast iron and superheating it considerably above the usual casting temperature. This decreases the size of the graphite flakes and increases the amount of pearlite in the casting. The fourth, used considerably in Europe, consists of pouring a low-silicon iron into a preheated mold. The fifth method consists of adding alloying elements. To produce high-strength iron by any of these methods the several variables, including the percentages of carbon and silicon, must be controlled carefully.

20.9. Evaluation of Gray Cast Iron for Engineering Applications

The suitability of gray cast iron for engineering structures and machines is judged chiefly by tensile, transverse, and compressive strengths. Cast Iron 419

properties may Hardness, endurance limit, wear resistance, and other Most metal- be valuable criteria of its value for specific applications. transverse test as the lurgists experienced with cast iron consider the best single source of information on the quality of cast iron. at the center of a cast The transverse test is made by applying a load (unmachined) round bar, supported at each end, until fracture occurs. casting has been The size of the bar depends upon the size of the and 18, standardized* at 0.875, 1.20, or 2 in. in diameter with a span of 12, fracture the bar and or 24 in. between supports. The load necessary to fracture are the amount of deflection at the center at the moment of load noted. In the United States transverse strength is reported as the in pounds; in Europe the load is converted by the formula Pic MR =-u breaking to a value called modulus of rupture. In this formula P is the the neutral load, I the distance between supports, c the distance from axis of the bar to the extreme fiber, and / the moment of inertia. The relation of modulus of rupture to tensile strength varies with the quality of the iron from 2.5 times the tensile strength for low-strength high-strength irons to approximately the same as the tensile strength for in design if irons. Transverse strength or modulus of rupture is of value to heavy cast iron is used as a beam and especially in pipe subjected earth loads. thou- Gray cast irons are graded by (minimum) tensile strength (in sands of pounds per square inch) into seven classes: 20, 25, 35, 40, gray 50, and 60. As discussed before, since the stress-strain curves of cast iron are curved from the origin, this material has no proportional limit nor modulus of elasticity. The secant modulus, representing a point on the stress-strain curve of 25 per cent of the tensile strength, varies between 12 and 20 million lb. per sq. in. In many engineering applications, for example machine beds, columns, pipes used as structural members, and various other supports, cast iron by subjecting, is used for its strength in compression. This is determined sand-cast bars 0.75 to 0.80 in. in diameter and 1 to 3 in. long to a compressive load until failure occurs. The stress-strain curve in compression strength is than tensile is also curved from the origin. Compressive higher low-strength strength; it varies from about 4 times the tensile strength for irons to about 3 times the tensile strength for strong irons. The other properties of gray cast iron can be summarized in a series of short sentences. Endurance limits are given in Table 20.2. The en-

•A. S. T. M. Standards, Pari 1, 1955, A48-40, pp. 1848-1854. 420 Engineering Metallurgy

durance ratio, determined on highly polished specimens, is more erratic than for steel. Values between 0.30 and 0.60 have been reported. A very important characteristic of gray cast iron is the almost negligible effect of surface on the endurance limit; the same notch which reduces the endurance limit of steel as much as 75 per cent reduces the endurance of soft cast iron less than 15 per cent. The impact resistance of cast iron is low and is usually unimportant. Notched-bar tests are meaningless, and drop tests are frequently erratic. The damping capacity of gray cast iron is higher than for any other ferrous material. Gray cast iron is readily machinable unless the surface is chilled; conversely, if the surface is chilled, cast iron is very wear-resistant and is machined with great difficulty. It is relatively easy to produce gray-iron castings having a surface layer in which the carbon is entirely combined as cementite. Chilled gray cast iron is, therefore, widely used for large gears, for a variety of rolls, for wheels of railway cars, and for many other articles where high surface hardness and wear resistance are necessary. Machining the surface, if necessary, is, however, a slow and costly operation.

20.10. Nodular or Ductile Cast Iron as an Engineering Material The size, shape, and distribution of graphite in cast iron has a pronounced effect upon strength and ductility. This may be illustrated by comparing these properties in malleable cast iron with the same properties in gray cast iron. The higher ductility in malleable cast iron is related to the rounded, compact form of the graphite (temper carbon) , which causes less disruption of the matrix than do the long slender flakes of graphite in gray cast iron. Until recently, the rounded graphite form could be achieved only by heat treatment of solid metal, but spherulitic or nodular graphite can now be made to form during the freezing interval by means of special treatments and additions developed and patented in the United States and in several other countries. High strength and elongation of as much as 20 to 25 percent are reported in cast iron made by these processes.

The total carbon in nodular cast iron is about the same as in gray iron. Magnesium or cerium are most widely used to promote the formation of nodular graphite. Only a few hundrcths of a percent of these elements is needed, but the addition must be made shortly before the metal is cast, and the melt must be very low in sulphur. Metallurgists do not clearly understand the mechanism of graphitization in nodular cast iron. The high shrinkage in nodular iron castings suggests that little, if any, formation of graphite occurs during freezing. Furthermore, massive cementite is not uncommon in the as-cast metal, and for maximum ductility this must be eliminated by a separate annealing heat treatment. Cast Iron 421

z so " rn CM in cm cm g m m in o mhOr-^soo cn tj- r- m 1 r-r-fSMNNtn ^ ^- ^- CM S3 s

o L* S .13- 8-2 :, A in in «o m r*. cm in r- o O fO * 6 Nl co ^- ui oo — n© r- a « o Sim" .-T e| cm" ofm" rn" to eg oo c 3 C Q •O 3 So «2 wC If *-

*— to ' ifi NO Cn CO U1 • 0. s NO O O in — o u-j so \o so !* co co M g a J* o ;= o O O O o "3.S r3 H CN NO CM CO -*• CM CO Q-5 ss h cn * u*i in \o r*- h* °d >> o

1 co ~ 41 03VNO m C J= CM C rr <*• no r-^ o cm co O O o O o ,« «,-,-, CM CM CM a c - NO en « O O O O M"" rOO CM B 3 c MCN S 6 omouiowr iq "8 H c-rn VnOCOO oi « ^-~- — — eg a 6S .2s —a9 mm a a o o J3 £ S Go 5 o o o o o o o c o o o o o o o o o o o u .- o o o o o o o m o o »n S « o o o o " " co co w m O Hi no cm o" oo r-^* 0- O O a. Nn* vmh-co T3 NO -* SO CN a S3 o f2 t» -3 sO -O >- c w |>>

o o o J3 o o o V \ii o m o > bo „ oo o ooo o c OO OO 8 o o o o o o o t.- o o o o W bO'1 o o o o o o m o o o o a •"* o"o"in"o" a m r- o o * r- CO CN CM T NO CO „a CO NO CO CN O-— -— — IPm — — " ^< M 3 O a a p o a ,u coo 3 H a si & li XI "O > s s L t» tq rt 1 oxx a - •u„«^« o rt m o-= o o « t> a I ooo oooooo o o o o in m in in i 5 111 CnT n6" --T no"

Engineering Properties. Specifications of the ASTM (A 399-55) recog-

nize two commercial grades designated as 80-60-03 and 60-45-10. The first

of these is in the as cast condition; the second is an annealed grade. In each, the first two sets of numbers indicate minimum tensile and yield strengths in 1000 psi, and the third set indicates minimum percentage elongation.

Nodular iron is more readily machinable than gray irons of similar hardness. It is somewhat stiffer than gray cast iron, having a modulus of

elasticity of 21 to 25 million psi. The damping capacity is between that of gray iron and cast steel. Nodular iron has somewhat better resistance

to oxidation, but the resistance to atmospheric corrosion is about the same as for gray iron.*

20.11. Melting and Casting of Irons

Different methods of melting are used in order to control the chemistry and microstructure of cast iron.

The greatest tonnage of iron is melted in a cupola furnace, Fig. 20.7, a cylindrical type of furnace mounted vertically on legs and lined with refractory brick. Depending on the type of foundry operated, the furnaces can be lined with either basic or acid brick. The lining of the cupola with basic brick is a relatively new innovation. Cupolas are charged some distance above the floor of the furnace, and depending on the capacity, the charging door is located 15 to 30 feet above the tuyeres.

On the lower portion of the furnace is a group of tuyeres attached to a wind box that supplies the necessary oxygen for combustion. The area

below the tuyeres makes up the molten metal reservoir, which is periodically tapped. Above the metal tap hole and usually on the opposite side is the slap tap hole. Materials charged to the cupola are pig iron, scrap iron or steel, coke, and flux. The materials are weighed accurately and charged in alternate layers. The function of the cupola furnace is to melt the various components of the charge, and the molten mass of metal becomes carburized by being in contact with the hot coke. The carbon solubility in the iron will depend on the temperature of operation of the cupola furnace. The carbon solubility will reach an equilibrium value after a given length of time. Cupola furnaces are not operated under equilibrium conditions. However, the time that the molten metal is allowed to remain in contact with the coke is carefully controlled. When the metal is tapped from the cupola furnace, the air is shut off, and

• A. S. M. Metals Handbook. Supplement-Metal Progress—Vol. 66, No. 1-A, July 15, 1954. Cast Iron 423

FlG. 20.7. Section through a cupola furnace. (Whiting Corp.)

the tap hole is opened to allow the molten metal to flow from the reservoir in the bottom of the cylindrical shaft furnace. After the tap is completed, the tap hole is plugged and the air is again turned on and melting continues until the next tap. Raw materials that are charged into the cupola furnace ultimately determine the chemistry of the melt that will be cast. The size of scrap and pig charged depends on the size of the cupola furnace. Coke, which is the fuel, is carefully selected on the basis of size, heating value, chemistry, porosity, and strength, and must necessarily be low in sulfur and phosphorus content. The molten iron in the cupola furnace has a high affinity for sulfur and phosphorus, and invariably the sulfur 424 Engineering Metallurgy

and phosphorus that are charged with the raw materials will be totally accounted for in the molten iron. The slag produced in a silica brick- lined cupola will not effect the removal of sulfur or phosphorus. In the case of the basic lined cupola, the product will show a considerable decrease in sulfur content due to the molten iron being in contact with the basic slag. By far the best method of producing a low sulfur and phosphorus iron is to minimize initially the charging of the two elements

into the cupola. The melting rate in the cupola furnace is controlled

by the amount of air that is admitted through the wind box into the tuyeres. The air causes the combustion of the coke which in turn supplies the heat for fusion. In controlling closely the operation of the cupola with respect to air, it is not uncommon to find that the air is con- ditioned to a constant humidity. Research work on the effect of moisture on quality of iron has indicated the need for the control of moisture to insure constant quality of iron. A study on the effect of moisture in the air feed of a cupola furnace has indicated a definite relationship between hard spots in cast iron and a critical amount of moisture in the air.

The cupola furnace is considered to be an intermittent type of furnace. On the basis of 24 hours of foundry operation, the cupola may be considered to operate 18 hours, and the remaining time allotted to repair of the lining and to general maintenance on the furnace.

The melting of malleable iron is usually carried out in a reverberatory or air furnace, Fig. 20.8. A relatively small tonnage is melted in electric furnaces. The fuel used in the reverberatory furnace is usually powdered coal or fuel oil. The raw materials added to the air furnace are scrap iron, pig iron, and flux. Iron of various analyses are selected so that when fusion and oxidation are completed the resulting melt is close to the desired analysis. Some adjustments are made in final analysis by addition of ferro alloys containing desired elements. It is well to mention that the carbon content of the metal melted in the air furnace is considerably lower than that produced in the cupola furnace and necessarily has a higher melting and casting temperature. The feature that differentiates the product of the cupola furnace from that of the air furnace is that the fracture of the latter is white and that of the former is gray. The gray fracture is caused by free carbon being in the form of graphite, and the white fracture is caused by carbon being in solution. The molten iron is cast into suitable molds for ultimate solidification into the desired shape of ultization.

Prior to the casting of the metal from the melting furnace, various tests are carried out on the molten metal to determine its suitability and quality for casting. A chill test is carried out on gray cast iron to de- Engineering Metallurgy 425

Flo. 20.8. Section through a reverberator^ or air furnace. [Whiting Corp.)

termine if the metal when cast will be free from a white cast-iron outer layer. The test block is fractured and the depth of white layer measured. A fluidity test is carried out to determine the flow characteristics of the metal under certain standardized conditions. The test is carried out in a spiral fluidity mold, the flow of the metal in filling the spiral being a measure of the viscosity and tendency to produce useful casting. To determine the quality of the iron after casting, physical tests such as tensile strength, transverse breaking strength, and hardness tests are carried out. The alloys are studied to determine the chemical analysis with specific interest in such elements as carbon, silicon, manganese, phosphorus, sulfur and others that may be present. Tests on iron that are cast from the air or reverbratory furnace are as follows: study of fracture to determine how much white cast iron is formed; chemical analysis to determine the presence of such elements such as carbon, silicon, manganese, sulphur, phosphorus, and others. In the case of white cast iron the chemical analysis will predetermine whether the alloy will satisfactorily be annealed to malleable iron. On a tonnage basis the

largest amount of iron cast as white cast iron is ultimately annealed into malleable iron.

20.12. Ternary System of Iron, Carbon, and Silicon

Cast irons can be considered on a simplified basis as alloys of iron, carbon, and silicon. In reality, the alloys contain, in addition to the alloying elements mentioned, small amounts of phosphorus, sulfur, manganese, and other elements. In order that we be able to interpret the various changes that take place in the cooling of a cast iron from its

liquidus temperature to room temperature, it is imperative that we become familiar with the ternary system of iron, carbon, and silicon. The ternary system allows the plotting of die composition on the basal temperature plane in three dimensions of carbon, silicon, and iron; the

vertical axis is temperature. Drawn in Fig. 20.9 is a corner of the ternary system iron-carbon-silicon. The familiar iron-graphite constitu- 426 Engineering Metallurgy

Fie. 20.9. Iron-rich corner of the ternary system of iron, carbon, and silicon.

tion system is recognized; the other section is for the iron-rich corner of the iron-silicon system. The iron-silicon system contains a gamma loop, which terminates at the transformation temperature from alpha to gamma, and gamma to delta iron. A section taken through the system at constant amounts of silicon illustrates the effect of silicon on the temperature and composition of the eutectic and eutectoid of the system.

In Fig. 20.10 is illustrated sections taken through the system at constant silicon content of per cent, 2 per cent, 3.8 per cent and (i.5 per cent.

It is readily noted in Fig. 20.10 that the transformation which takes place at the solidus temperature line for the 2 per cent carbon alloy in the iron-carbon system is simply a transformation of austenite-plus- liquid to austenite-plus-cementite. In the case of the 2 per cent silicon section of the iron, carbon, and silicon system, the transformation at 2 per cent carbon involves a three-phase region changing from liquid-plus- austenite to austenite-plus-graphite. A region of austenite, cementite, J —

Cast Iron 427

1600 a- — [ OX Si 7*0 # L J> \ 7' L ^ /IsCa i 1200 -^ r- o — -/ « 1000 .^ r+Ca «V> 800 v a + ca 600 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Carbon, percent

600 0.5 1.0 1.5 2.0 2.5 3.0 35 4.0 45 5.0 Carbon, per cent

600 05 1.0 15 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Carbon, per cent

6.5% Si 1400 zl o — "-^/ *Ca °.I200 a\ ^Sfcs^p ^p - ' \- a- 3 t •L-a? -a ^ *tc*L >Ca a 1000 71 ££ -

\ 800 a*L a

600 0.6 1,0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Carbon, per cent

cent silicon, Fig. 20.10. Sections through the iron, carbon, and silicon system at per {Alloys Iron and 2 per cent silicon, 3.8 per cent silicon, and 6.5 per cent silicon. of Co., 1935. Silicon, Greiner, Marsh, and Stoughton, New York: McGraw-Hill Book 428 Engineering Metallurgy

3% Total Carbon. 2% Si Liquid A*L

c L.Liquid A, Austenite A*C A+C+G A+G P, Pearlite C.Cementite G G, Graphite -A,Pt C AiP,C*G A r P-,G A.t>+$+F F. Ferrite P*C P*C*G P*G P.G* C F.G (Whili (Mottled Iron) Iron) K

Moderately Moderate', Foil Moderate Foil Siow S'o* Cooling Cooling Cooling Cooing Co:'-ng

Fie. 20.11. Transformation occurring during the solidification and the cooling of cast iron. (Lorig)

and graphite is passed through prior to the formation of the solid phase.

This region is used in explanation of the formation of graphite from the melt on solidification. The explanation of the freezing of iron, carbon, and silicon alloys is complicated by the presence of two systems—the stable system of iron and graphite and the metastable system of iron and cementite. In both systems prior to reaching the liquids temperature, the carbon is in solution in the iron. Considerable changes occur during the solidification of alloys according to the stable and metastable system. Al- loys that freeze according to the stable system will give structures that contain graphite plus transformation products of austenite upon cooling. In the case of the metastable system, the products of transformation on cooling contain iron carbide (cementite) plus transformation products of the austenite. Hence, it is evident that an alloy has different microcon- stituents formed, depending on the rate of cooling from above the liquidus to room temperature. Fig. 20.11 above illustrates the relationship of cooling to microstructure for a 3 percent carbon, 2 percent silicon cast iron. It is noted that on fast cooling the microstructure is basically pearlite and cementite, and on slow cooling the microstructure is ferrite and graphite. At an intermediate cooling rate, it is possible to form a structure of cementite, pearlite, and graphite which is referred to as mottled iron. In Fig. 20.11 notice that the rate of cooling determines whether the stable or metastable constitution system is applicable in considering the process of freezing. When alloys of iron, carbon, and Cast Iron 429

(Tentative recommended Fig. 20.12. Types of graphite flakes in gray cast iron. designation practice for evaluating the microslructure of graphite in gray iron, ASTM A247-41T).

present in the silicon solidify according to the stable system, the carbon is uncombined form of graphite. Depending on the conditions of cooling, of sizes nucleation, and other factors, the graphite will assume a variety and shapes. The flake-type chart for graphite is reproduced in Fig. 20.12. Depending on the mode of formation, any one of five different types of graphite patterns can be formed, namely, uniform distribution random orientation, rosette grouping random orientation, superimposed flake orientation, size random orientation, interdenderitic segregation random and interdendritic segregation preferred orientation. The size of graphite magnifica- flakes is usually rated on the basis of length measured at 100 of tions. All types of graphite flake patterns may be present in a section gray iron cast, with the exception of the superimposed flake size, which is found typically in hypereutectic irons. It is generally agreed that the flake type of graphite forms directly from the melt on cooling below the region of austenite plus liquid or austenite plus carbide of the stable system of iron and carbon. The formation of graphite from the melt is explained on the basis of the binary system of iron and carbon for simplification purposes only. In explanation of the formation of the interdendritic graphite pattern, which contains fine graphite, it is assumed that the eutectic transformation takes place at a lower temperature than that of the large graphite flakes. This tends to indicate that the fine graphite structure is formed during the undercooling of the melt (and the absence of residual nuclei in the melt to initiate the formation that all of graphite of the graphite) . It can be generally concluded forms found in cast irons form directly from the melt on solidification. The innoculation of cast irons is practiced in many foundries to control the size of graphite flake formed in the cooling of various section sizes. The amount of innoculant added will control the size and distribution of the graphite flakes. Any one of a number of innoculants can be used, such as ferrosilicon, calcium silicide, combinations of titanium, aluminum 430 Engineering Metallurgy

and silicon, mill scale, and others. In specially controlled heats of cast iron it is possible by innoculation to alter the shape of the graphite. The graphite formed on cooling after innoculation assumes the shape of spheres. The chemical composition of the cast iron must be carefully controlled with respect to sulfur, which must be present in an amount less than 0.02 per cent. The spheriodizing additive may be cerium or magnesium in alloyed forms. The addition of the innoculants results in exothermic reactions, and care must be taken in making the alloy addition. The mechanism of the formation of the nodular graphite particle on the cooling of the cast iron takes place in the following manner. The addition of the cerium or magnesium to the cast iron, low in sulphur, causes the graphite to crystallize in the form of nodules. The constitution diagram of iron, carbon, and 2 per cent silicon can be used to explain the formation of temper graphite during the long- time annealing of white cast iron to form malleable cast iron. The formation of temper carbon takes place in two stages during the mal-

leablizing of white cast iron. The first stage is carried out at a relatively high temperature of 1600° to 1700°F. by slowly heating to this temperature and holding until the cementite has decomposed into gamma iron and temper carbon. The gamma iron dissolves in the austenite and the carbon will be uncombined. The amount of carbon that remains in solution can be determined from the solubility of carbon in iron as determined in the constitution system of iron, carbon, and 2 per cent silicon.

Prior to the second stage of malleablization the alloy is slowly cooled from 1600° - 1700°F. to 1200° - 1300°F. and held at this temperature until the austenite that had transformed on cooling into pearlite decomposes into ferrite and temper carbon. The amount of carbon that the austenite will yield can be determined again from the carbon solubility

line. At approximately 1200°F. the solubility of carbon in iron is less than 0.10 per cent. The alpha iron formed dissolves in the ferrite, and the uncombined carbon deposits on the temper carbon that was initially rejected during first stage of malleablization.

20.13. Heat Treatment of Cast Iron

There are a number of methods whereby the heat treatment of cast iron can be carried out. Briefly we can summarize the various type of heat treatment under the headings of stress relief and growth, annealing and normalizing, hardening and tempering, isothermal heat treatment, flame hardening and induction hardening. Stress Relief and Growth. Cast irons unless given a stabilizing heat treatment are susceptible to growth on remaining at room temperature Cast Iron 431

for extended lengths of time. It is common practice when time permits to store gray cast-iron parts for a length of time of six months or more to allow the natural growth to occur on aging at room temperature. Growth in cast irons can be accelerated by heat treatment, but the heat-treating temperature is kept at a minimum to lessen graphitization of the ccmentite phase. Cementite is unstable in the presence of silicon and when held at slightly elevated temperatures, tends to break down into graphite and ferrite. The breakdown results in a decrease in mechanical properties of the iron. The room temperature aging of the gray cast iron is most desirable in that no conditions are offered for the graphitizing action. The temperature range used for the aging to promote growth is between 700° and 1300°F. It is well to mention that all gray cast irons do not show growth on aging, and this problem has been investigated by metallurgists for a number of years. There has evolved no acceptable answer as to why some compositions of irons show the growth phenomena and others do not. Suggestions have been made as to mechanism of growth, and they embody some of the following concepts: due to graphitization of some of the cementite, a volume expansion takes place; penetration of oxygen at the grain boundaries and internal oxidation occurs with an increase in volume; thermal gradients and volume changes during heating and cooling. Depending on the type of heat treatment cycle given the gray cast iron for dimensional stabilization, any or all of the mentioned factors may apply. Oxidation is a growth factor from room temperature up to about 800°F. Cementite break-down is a factor in temperature ranges from 800° to 1300°F. Thermal gradients and volume changes occur over all of the heat-treating ranges. Annealing and Normalizing. The heat treatments of annealing and normalizing are usually applied to gray cast irons for the purpose of increasing machinability. The cooling rate from above the critical temperature will affect the coarseness or fineness of the pearlite—ferrite— cementite aggregate associated with the graphite. As the coarseness of the pearlite of the gray cast iron increases, the hardness of the material decreases. It is possible to over-anneal a gray cast iron. During machining of such material, tearing takes place, resulting in a poor surface finish. During the annealing or normalizing cycles, graphitization may take place with a lowering of the combined carbon content of the cast iron, resulting in a decrease in strength. In determining the temperature above which the cast irons must be heated to promote austenization, the usual iron, iron-carbide diagram cannot be used. The proper section through the ternary diagram of iron, carbon, and silicon should be consulted. 432 Engineering Metallurgy

Hardening and Tempering. The gray cast iron, the pearlitic malleable irons, and the pearlitic nodular cast irons respond to heat treatment much in the same manner as do plain carbon steels. The ability to respond to heat treatment stems from the presence of carbon in the combined state. The carbon present in the combined state causes the formation of austenite when the ferrite-carbide aggregate is heated above the upper critical temperature. Some graphitization may take place during the heating to the austenitizing temperature. The cast iron is held at the proper austenizing temperature until the desired degree of solubility of carbon has taken place, and the alloy is then quenched. The usual precautions taken in the heat treatment of cast iron are the same as those for steels. To cause the formation of a completely martensitic matrix from the austenite, it is necessary that the section be quenched at, or faster than, the critical cooling rate. If the cooling is somewhat slower than the critical cooling rate, some of the soft pearlitic constituents are formed and associated with the martensite. When conditions of service require properties other than high hardness, the irons are given a tempering treatment. It is quite common to give the heat treated cast iron, to be used in the martensitic condition, a stress-relief heat treatment to minimize cracking during grinding and to minimize the brittleness of the martensite. The tempering curves of hardness versus time are similar to that obtained for steels. The hardness of the martensite decreases as the tempering temperature is increased. The resulting microstructure formed by the tempering of the martensite is similar to those produced in hardening and tempering of steels. In studying the hardenability response of cast iron, the Jominy hardcnability type of bar can be used, the hardness being determined along the edge of the bar in a manner similar to that for low alloy steels and plain carbon steels. As one may anticipate, the hardness of the fully heat- treated cast iron quenched at or faster than the critical rate is somewhat less than a steel of the same combined carbon content, due to the presence of graphite. The presence of graphite in the martensite matrix decreases the hardness considerably. The use of a light-load hardness tester will verify the fact that the hardness of the martensite between the graphite flakes is as great as the martensite in a lull heat-treated steel. The presence of graphite in the matrix of the cast iron during heat treatment to full hardness acts as a cushion and tends to minimize the formation of quench cracks that many occur during the considerable volume changes that accompany the transformation of austenite to martensite. —

Cast Iron 433

7.--"

»T 00M1 IIIT tiktiuti If ^ « it no in noo

•1 JOOH Ui 1 MM • s.ai \ ^ZUC GRAY CAST IRON LTNI oo« J ^s. I ^- * C - 3. 14 too 1 i riao 1 Mn - 0. 81 M P - 0.219 5 TOO S - 0. 064 N Si - 2. 15 I 000 N f, p

iac ioo — if *N V"s 400 V N rl , n,

IOO

too

•

cast iron. Fig. 20.13. Time. Temperature. Transformation (TTT) diagram for a (Nagler and Dondelt).

Isothermal Heat Treatment. The various types of cast irons such as gray cast iron, pearlitic malleable iron, and nodular cast irons can be heat treated by any of the known isothermal transformation methods. The TTT curve for cast irons does not differ greatly from that of low alloy, high alloy, or plain carbon steels. Fig. 20.13 is a typical TTT diagram for a cast iron. The combined carbon in the cast iron is an influencing factor in determing the position of the upper nose of the TTT diagram. As the carbon content increases, the upper nose of the TTT diagram is shifted to the right. The maximum shift to the right is produced by a eutectoid composition of combined carbon. Amounts of carbon above or below the euctectoid composition tend to shift the nose of the TTT curve to the left. The cementite or the ferrite, whatever the excess proeutectoid constituent may be, nucleates the transformation of austenite to pearlite at the position of the upper nose of the curve. The products of transformation produced isothermally are identical to those produced 434 Engineering Metallurgy in steels. Starting at a temperature just below the lower critical temperature (Aij), coarse pearlite is formed; fine pearlite is formed at the upper nose of the TTT curve; below the upper nose the upper bainite is formed; at the lower range the lower bainites are formed; at the M, temperature manensite is formed from the austenite on cooling to room temperature. The isothermal method of heat treating cast irons produces increased toughness of the matrix of cast irons.

Flame Hardening and Induction Hardening. The gray cast iron, pearlitic malleable cast irons, and the nodular cast irons respond to induction and flame hardening. The principle of this process of hardening is to heat rapidly above the upper critical temperature the surface of the parts to the desired depth and immediately quench. The heated portion of the iron is changed to an austenitic condition initially, and when quenched, the austenite is converted to a martensite. Depending on the thickness of the section size, there may be no alteration in the properties of the core. The intent of this form of heat treatment is to produce a marked increase in hardness on the surface of the part with retention of un- altered core properties. The hardened portion of the cast irons illustrates the presence of a martensite matrix with graphite. This heat treatment produces a hard-wearing case on the tough shock-damping core of the cast iron, and the depth of hardening can be controlled very closely.

20.14. Normal and Alloy Elements in Cast Iron

The normal elements in cast iron include such elements as carbon, silicon, manganese, phosphorus, and sulfur. The commonly used alloying elements include copper, chromium, molybdenum, vanadium, and nickel.

Carbon. Carbon in cast iron is present in the uncombined and combined forms. In the uncombined form the carbon can assume the shape of flakes or nodules. In the combined form the carbon can be present as a simple iron carbide (cementite) , binary carbide, and other complex alloy iron carbide forms- The presence of the carbon in the uncombined state is controlled by such factors as chemical analysis of the iron and the cooling rate from the casting temperature. In the malleablizing of white cast iron, an additional type of carbon is formed called tempered carbon.

Silicon. Silicon is generally considered as a graphitizing element. The amount of silicon present in a cast iron determines whether the carbon is present in the combined or uncombined state for a given cooling condition. The graphitizing effect of the silicon can be somewhat offset by the addition of carbide forming elements. Silicon dissolves in the ferrite and acts to toughen the iron. Cast Iron 435

Manganese. Manganese combines with the sulfur and forms manganese inclusions. Some sulfide, which is insoluble in iron and forms non-metallic of the manganese may form manganese silicate, also present in the iron the as a nonmetallic inclusion. The manganese remaining is dissolved in ferrite and acts as a ferrite toughener. steadite, Phosphorus. Phosphorus is present in cast iron in the form of a ternary eutectic of iron, carbon, and phosphorus. Steadite is a low melting constituent contributing to the fluidity of cast iron. The steadite melts at about 1750°F. The presence of phosphorus adds to the brittleness of cast iron and tends to lower the impact strength of it.

Sulfur. Sulfur is present in cast iron in the combined form of manganese sulphide or iron sulphide. Iron sulphide, when form usually occurs in the grain boundaries and reduces the impact toughness of the iron. Chromium. Chromium has a two-fold effect on cast iron: it dissolves in in the the ferrite and is a carbide former. The chromium that dissolves ferrite acts as a toughener. Chromium that is present in an amount over that necessary to saturate the ferrite occurs in the pearlite as a massive iron-chromium carbide. Chromium has a tendency to produce a uniform fine-fracture grain size. As the per cent of chromium added to the

cast iron is increased, there is a tendency for the carbon to remain in a combined form, and the fracture will be white. Chromium in the amounts of from 0.15 to 2.00 per cent may be added to cast iron and still provide gray fractures if suitable graphitizing elements are present. Additions of chromium to the cast iron will increase tensile strength, hardness, wear resistance and depth of chill, and corrosion and oxidization resistance. Chromium will delay the isothermal transformation of cast iron during heat treatment. Molybdenum. Molybdenum dissolves in the ferrite and, to a limited

degree, is a carbide former. Molybdenum does not act as a graphitizing agent. The addition of molybdenum to cast iron increases tensile strength, hardness, transverse strength, and resistance to deflection on transverse loading. The addition of molybdenum to cast iron when cast into large section sizes tends to promote uniform structures. This results in an iron of uniform strength, hardness, and density throughout the

large section size. Molybdenum additions to cast iron promote machinability resulting from structural uniformity and a minimum occurrence of large carbides. Molybdenum has little or no effect on retarding isothermal transformation over the range of baintic transformation products. The isothermal transformation to pearlite products is retarded by the addition of molybdenum. 436 Engineering Metallurgy

Copper. Copper dissolves to a limited degree in the ferrite and acts

as a toughener, and is a mild promoter of the formation of graphite. Copper tends to increase strength, hardness, wear resistance, and corrosion resistance. The addition of copper to cast iron reduces the tendency

to form a chill layer, hence increases machinability. Copper is not used

as an individual additive element but is usually added with such elements as chromium, molybdenum, nickel, and vanadium.

Nickel. Nickel dissolves in ferrite and is a graphitizer. Nickel addi-

tions reduce depth of chill and tend to refine the grain size. Nickel is effective in retarding the transformation of austenite over the whole

range of transformation. If the graphitizing effect of nickel is disregarded, nickel is considered to increase the strength, hardness, resistance to deflection, toughness, corrosion resistance, and machinability. In addition

to using the nickel in gray cast iron, it is utilized in martensitic cast iron containing comparatively large percentages of nickel and chromium. The alloy has superior wear and corrosion resistance. A grade of cast iron termed austenitic is cast with the addition of nickel and chromium for service conditions requiring lo wexpansion and stainless qualities. Vanadium. Vanadium when added to cast iron performs the role of a strong carbide former. Vanadium retards graphitization and stabilizes cementite and has a high affinity for carbon and combines to form vanadium carbides. The addition of vanadium to cast iron tends to produce small uniformly distributed graphite flakes and decreases fracture grain size and increases the tensile strength, transverse strength, hardness,

and machinability. Vanadium is alloyed in cast iron with elements such as nickel, copper, chromium, and molybdenum. Vanadium, when added to cast iron, acts as an effective deoxidizing agent in the removal of dissolved oxygen from the iron. Little to no effect can be found on the isothermal transformation of cast irons because at ausieniting tempera-

tures the vanadium carbide is quite stable.

QUESTIONS

1. What is the general appearance of the fracture of gray, mottled, and white cast iron? What is responsible for the different fractures of these three

classes of cast iron? What is their relative industrial importance?

2. What is the relation of gray cast iron to steel? In what respects are the properties of gray cast iron inferior to those of steel? Give die variables that affect the properties of gray cast iron. What characteristics make gray cast iron a valuable engineering material?

3. How is a gray cast iron melted, and what raw materials are used? How is the desired composition attained? How much carbon, silicon, and phosphorus are usually present? Cast Iron 437

constituents in gray cast iron? Describe 4. Name the primary metallographic the structure as seen the role played by carbon and silicon in iniluencing with the microscope. What is steadite? Describe its appearance. silicon, and the structure 5. What is the general relation between carbon and poured and appearance of the fracture? If a small specimen of cast iron, the structure of the in a sand mold, contains 3.5 per cent carbon, what is 1.5 cent? (c) 2.25 per cent? matrix if the silicon is (a) 1.0 per cent? (b) per would you For small castings, what percentages of total carbon and silicon 38,000 specify to obtain a tensile strength of (a) 33,000 lb. per sq. in.? (6) lb. per sq. in.? structure of gray cast iron? If 6. What is the effect of cooling rate on the steel the matrix of a small sand-cast bar has a structure corresponding to a containing 0.60 per cent carbon, what would the matrix correspond to if the the casting was a block 6 to 8 in. square? What is the general effect of control of graphite- size of the graphite flakes on the properties? How is flake size accomplished? Why is cast iron frequently annealed? gray cast iron for engineering 7. What tests are commonly used to evaluate are applications? Which is the best of these tests, and how is it made? How cast iron, results reported? What is the approximate secant modulus of gray strength? and how is it determined? What is the average compressive representative of (a) a high- 8. What properties would you consider to be gray test gray cast iron? (b) a low-strength iron? (c) an average American cast iron? How do the endurance limits, impact resistance, and damping capacity of gray cast iron compare with those of carbon steel? cast iron, and when is die addi- 9. Why are alloying elements added to gray tional cost of these elements justified? What are the general effects of (a) nickel? (b) chromium? (c) molybdenum? for? Give the approxi- 10. How is malleable cast iron produced? What is it used mate tensile strength and elongation of American malleable cast iron. What ratio, impact re- is the relation between the two? Compare endurance sistance, and machinability of malleable cast iron with those of gray cast iron. gray cast iron? 11. What are the significant differences between pig iron and In what way are they similar? In what respect does mottled iron resemble un-annealed, nodular iron? gray cast iron 12. How does the development, or formation, of graphite in differ from that in malleable cast iron? Give at least one reason for using a lower carbon content in malleable iron than in gray iron. 13. In Table 20.2, study the percentages of carbon and silicon recommended for various types of castings and satisfy yourself why these analyses arc good for the particular application. in producing high 14. Why is closer control of chemical composition necessary strength gray iron? parts. 15. Draw a section through a cupola and label the operating cupola. 16. List the requirements of coke that would be suitable for use in a in cast iron? 17. Why are large amounts of sulfur and phosphorus undesirable 18. Differentiate between a graphitizing element and a carbide-forming element. irons? 19. What roles are performed by alloying elements when added to cast 20. What different types of graphite patterns are formed in gray cast iron. Mach inability, We ar Resistance ,

and Deep - rawing CHAPTER D Properties

21 Irvinc J. Lkvinson, M.S., Professor of Mechanical En- gineering, Lawrence Institute of Technology, Detroit, Michigan

oINCE complex metallurgical, mechanical, and environmental factors are involved, a precise evaluation of machinability, wear resistance, and deep-drawing is extremely difficult. This problem of evaluation has been attacked from many fronts, but to date only gener- alities have been formulated—no hard and fast rules exist.

21.1. Variables Affecting Machinability

To remove metal by machining, the tool must first penetrate the surface. The ease with which this occurs is a function of the static- and dynamic hardness of the cutter and the metal to be machined, the rate of penetration, the depth of penetration, the frictional and shear forces involved, and the relative rigidity of the system. Once the tool has penetrated the surface the chip formed should readily break. While spectacular, the long coiled chip is not the desired one. Breaking the chip mechanically involves a loss of power and thus is not the most desirable answer. The ideal method of breaking the chip is to provide a built-in breaker in the form of a weak and brittle constituent. It is wrong to assume that because a metal is soft it is easy to machine. Toughness is usually a corollary of softness, and, as a consequence, the chips formed separate only with difficulty. In steel, a completely spheroidized structure is considered the easiest to machine, although the presence of some small pearlite islands does promote chip breakage.

21.2. Evaluation of Metallic Materials for Machinability

Experience has shown that a correlation exists between the basic physical properties of the metals and their machinability. Figure 21.1,

438 —

Machinability, Wear Resistance 439 Tensile strength, thousand lb.persg.in. 100 120 140 ) 20 40 60 80 i 1 i_ _l , 80 r \ 70 Feed S *lmm. , _ per revolution 60 \ Depth t=2mm.

50 V/oy.t/eeb feels .40 truckirali c o£ astsfeels 'E30 ~X^ hfc^ '-'ft; £.20

E 10 20

> O Heeds -Imm. . per revolution Depth t-4mm. > o-50 c V £4< 15 *s. 30

30 40 50 60 10 60 90 100 , |0 20 Tensile strength, kg. per sq.mm. for tool life of 1 hr. to tensile strength Fig. 21.1. Relation of cutting speed for a Dabringhaus, Maschmenbau, v. 9, cuts of 0.08 and 0.16 in. (A. Wallichs and H. 1910, p. 251)

increasing tensile for example, shows that machinability decreases with probably a good strength. This property, together with hardness, is controlled condi- criterion of machinability. It is also true that, under of cast and hot- tions, there is very little difference in the machinability acid and basic steels, rolled steels, of carbon and low-alloy steels, or of provided tensile strength and hardness are the same. difficult to machine Cast steels and nonferrous alloys may be more if the than wrought material of the same composition and structure it, the sand skin of the casting has cooled rapidly enough to harden or if has not been removed thoroughly from the surface. indicate Many efforts have been made to devise a laboratory test to been accurately relative machinability in service. Recent attempts have 5

440 Engineering Metallurgy

concerned with the work-hardening capacity of the material cut, ihe object being to use the maximum hardness so induced as a measure of machinability.

21.3. Free Machining Steels

Despite the impossibility of evaluating machinability accurately, experience has shown that the machinability of some materials can be improved by certain treatments and, further, that there are alloys of certain compositions which can be machined more easily at high speed than other alloys of the same general type. Such materials are, therefore, used widely in automatic machines for mass production of machined parts. The machinability of those low-carbon steels which arc too soft and

tough to machine easily is improved by heat treatment and, especially by cold working. Quenching a low-carbon steel to increase the Brinell

hardness number from 90 or 100 to 170 to 210 improves its machinability

(provided, of course, that no martensite is formed by quenching) . Cold working is even more effective in increasing the hardness and at the same time decreasing the toughness; hence, most low-carbon steels that must be machined economically are cold drawn or cold rolled. This operation also has the advantage of producing a smooth surface. The best and cheapest method of improving the machinability of low- carbon steels is to add sulfur, or lead, or both. However, owing to the

Table 21.1. Relative Machinability Rating of Cold-Worked Free-Cutting Steels

Composition, per cent Machinabil- S.A.E. ity rating, No. per cent C Mn S P

X1112 0.08 to 0.16 0.60 to 0.90 0.20 to 0.30 0.09 to 0.13 140 1112L* 0.08 to 0.16 0.60 to 0.90 0.10 to 0.20 0.09 to 0.13 140 1112 0.08 to 0.16 0.60 to 0.90 0.10 to 0.20 0.09 to 0.13 100 X1314 0.10 to 0.20 1.00 to 1.30 0.075 to 0.150 0.045 max. 90 XI 31 0.10 to 0.20 1.30 to 1.60 0.075 to 0.150 0.045 max. 87 1115 0.10 to 0.20 0.75 to 1.00 0.075 to 0.150 0.045 max. 81 1120 0.15 to 0.25 0.60 to 0.90 0.075 to 0.150 0.045 max. 78 XI 330 0.25 to 0.35 1.35 to 1.65 0.075 to 0.150 0.045 max. 76 X1335 0.30 to 0.40 1.35 to 1.65 0.075 to 0.150 0.045 max. 72 XI 340 0.35 to 0.45 1.35 to 1.65 0.075 to 0.150 0.045 max. 70 1020 0.15 to 0.25 0.30 to 0.60 0.055 max. 0.045 max. 63 1040 0.35 to 0.45 0.60 to 0.90 0.055 max. 0.045 max. 60 1010 0.05 to 0.15 0.30 to 0.60 0.055 max. 0.045 max. 53

* Contains 0.20 to 0.25 per cent lead. Machinability, Wear Resistance 441 high sulfur (plus high phosphorus for some of the steels), these "free- machining" or "free-cutting" steels have much lower impact resistance than low-sulfur steels of the same carbon content and should be employed, therefore, only where low impact resistance is not a handicap.

Leaded steel is a development of the past few years. The addition of lead improves the machinability of low- as well as of high-sulphur steel.

Leaded steel is also more resistant to impact than high-sulphur material. A somewhat similar effect is obtained in stainless steel with the addition of Selenium. The relative machinability of the free-machining steels com-

pared with three low-sulphur steels is given in Table 21.1.*

Iron carbide particles, if favorably located and of favorable shape, act as chip breakers; thus, unalloyed carbon steels containing 0.20 to 0.40 per cent carbon, when properly annealed, machine much more easily than those containing carbon below 0.15 per cent (Table 21.1). The

best chip breaker is manganese sulphide (MnS) . In high-sulphur steel, inclusions of MnS, when well distributed as small particles, break up the chips very effectively and, as Table 21.1 shows, greatly improve machinability. Despite their low impact resistance large tonnages of high- sulphur steels are used.

21,4. Relative Machinability of Steel and Nonferrous Alloys

An approximate evaluation of the relative machinability of some common ferrous and nonferrous materials can be made by averaging the data re|>orted by Bostont for drilling, milling, and planning tests. They were reported as the horsepower required to remove one cubic inch of metal

per minute. The order is as follows: Horsepower per cu. in. Material per min. Magnesium alloys 0.30 Bearing bronze 0.35 Aluminum alloy, 8% Cu 0.35 Free-cutting brass 0.38 Manganese bronze 0.60 Hard cast iron 0.60 Malleable iron 0.75 Yellow brass, unleaded 0.85 Free-cutting, steel, cold drawn 0.90 Forged carbon steel, 020% C 1.12 Nickel steel, 0.40% C 1 .20

Annealed copper 1 .35

Tool steel, high carbon 1 .60

Low-carbon, high-chromium steel 1 .70 Monel metal 1.70

• Adapted from a tabulation by H. W. Graham, Metal Progress, July, 1939, p. 53. tO. W. Boston, Proc. Am. Soc. Testing Materials, v. 31, 1931, part II, pp. 388-421. 442 Engineering Metallurgy

This is not an infallible evaluation, and the order might be changed for some of the alloys if the conditions of the tests were changed slightly.

It does, however, show that it requires much more power to machine the free-cutting steels than is needed for some of the magnesium and aluminum alloys, some of the free-cutting brasses and bronzes, or cast or malleable iron. Soft gray cast iron, containing no hard excess carbide or phosphide particles, and malleable iron are the most readily machinable of all ferrous materials, usually ranking higher than high-sulphur steel. The ease with which these materials are cut is due primarily to the softness of the ferrite plus the numerous small particles of graphite which act as chip breakers and as a lubricant. In general, copper- and aluminum-base alloys are easy to machine.

There is, however, enough difficulty with these alloys so that special compositions are used for material that is cut in automatic machines.

Free-cutting brass and bronze contain 1 to 4 per cent lead; their machinability is 70 to 100 as compared to 20 to 30 for the same alloys containing no lead.

21.5. Types of Wear

The importance of wear resistance needs no amplification. Wear occurs in machines when shafts rotate; when pistons and valves operate; when a gear meshes and drives another; when steel or cast-iron wheels

start, run, or stop on rails; when brakes are set; when excavating, con- veying, crushing, or mixing machines handle sand, clay, rock, coal, or minerals; when agricultural implements arc used; and in a multitude of other engineering applications. Some of the most destructive wear of metals is caused by rapidly moving soft materials; for example, the wearing of hardened steel parts by cotton or silk thread in textile machinery and the wear of turbine blades by high-velocity gases. The wear of metals may be divided broadly into metal-to-metal wear

and abrasive wear. Another division is possible: into wear under rolling friction or under sliding friction and, further, according to whether

lubrication can or cannot be used. The latter is, of course, most important in the metal-to-metal contact of machine parts. Lubrication may be

omitted from detailed consideration because, although it greatly influ-

ences the rate of wear, it probably has little or no effect on the type of

wear. Wear resistance is an important property of metals used in ma-

chines even if lubrication is of the best, because perfect lubrication is

more of an ideal than a reality. Moreover, even if lubrication is so

thorough that it prevents a metal-to-metal wear, it frequently happens Machinabiiity, Wear Resistance 443 that extraneous grit in the lubricant causes severe abraisive wear. Another important factor entering into the wear of machines is corrosion of the wearing surfaces caused by condensed moisture or acids in gases and lubricants.

Wear involving a single type is rare; in most machinery, both abrasive and metal-to-metal wear occur. This is the case in such parts as shafts revolving in bearings, where abrasive wear occurs until the shaft and bearing are "run in" and worn to a mirror finish, after which metal-to- metal wear predominates. Since in such service wear can rarely be avoided completely, even with the best lubrication, it is common to use a hard metal and a relatively soft one together, the softer material being used (as in a bearing) for the part which is most economical to replace. In some kinds of excavating and material-handling equipment, abrasive wear is the principal type involved.

21.6. Variables Affecting Wear Resistance Like machinabiiity, wear resistance depends upon two factors, the metallurgical and the mechanical. The latter has been discussed briefly in the previous paragraphs and may, consequently, be dismissed with the statement that wear of metal depends as much, or even more, on the conditions of service as upon the metal used.

From the metallurgical standpoint, wear resistance is inversely proportional to machinabiiity and, in general, directly proportional to hard-

ness. Hardness, however, is not the only metallurgical factor of importance. Wear by abrasion, like machinabiiity, involves tearing off small particles of the metal. Consequently, strength and toughness are important factors in resisting wear, The wear resistance of the various carbon and alloy steels receives

attention elsewhere; it can be dismissed here with a brief mention of

the general types of ferrous material used for wear resistance. The first

and most common, especially for metal-to-metal wear, is a cast iron or a steel with a surface of high hardness. Usually, in an application require- ing high surface hardness, considerable toughness, especially of the core,

is also required. This combination of properties is attained by quenching and tempering an alloy steel or by case carburizing or nitriding a carbon or alloy steel and heat treating to produce high surface hardness combined with considerable ductility in the core.

For abrasion resistance, two kinds of materials are used: (1) an aus-

tenitic steel, which is relatively soft but which, under the severe cold

working of the abrasion, transforms to martensitic; and (2) a carbon or alloy steel, on which a hard surface layer of some special alloy is deposited by electric arc or other method. o —>

444 Engineering Metallurgy

80 130

70 120 — c{ ocw/C" | . ' < - .*60 3 110 r '/,- — c r. x a /f _ U-^x-f ?S0 :;.: B a _x--"" 90 / X"* , S* =3 1 ' ps- B scale -i s ] |30 60 f^•y* ! 4 *20 O 70 yj r~ tt at io 50 Ck^Jr 50

o o Hardened 0.16 * » Normalized ° D Annealed 0.15 d \. g.gO.14 » jS l| an \ X ) %.% 0.12 k ^n > x 1 ^. S., .-» a" \ ^r»~ |1 ™H 0.10 \ (,_ a 1 X ""« •*x. f-8* 0.09 \, S" \ § £0.08 >V — -o 0.07 -' ^ — 0.06 0.2. 0.4 0.6 0.8 1.0 1.2 1.4 Carbon, per cent

Fig. 21.2. Effect of carbon content and heat-treatment on metal-to-metal wear of carbon steels. (Rosenberg)

21.7. Evaluation of Steel for Wear Resistance

The evaluation of the wear resistance of steels is one of the most difficult problems a metallurgist is called upon to solve. Simulating actual service conditions by a laboratory test— the most important phase of this

problem—can be accomplished only rarely and then only if wear in service occurs rapidly and if it is possible to design a machine which duplicates service conditions exactly. Wear is usually a slow process.

Shidle* notes that, when a 5-ton truck has finally worn out, it has lost only 5 lb. of metal. The useful life of the average automobile engine is probably 100,000 miles; hence the difficulty of devising a test which will simulate such service conditions accurately. Several types of laboratory machines have been developed that determine the wear resistance of a material fairly rapidly and under relatively closely controlled conditions. For determining metal-to-metal wear, the

N. G. Shidle, Automotive Ind., v. 66, Mar. 19, 1932, p. 449. Machinability, Wear Resistance 445

most common is the Amsler machine, which tests a specimen under pure sliding or pure rolling friction, or a combination of the two. Pressures can be varied at will, and wear can be determined at any temperature, with or without lubrication. Some data on metal-to-metal wear at room temperature of carbon steels, together with Rockwell hardness, are shown in Fig. 21.2.* For the determination of abrasive wear, two machines are commonly employed: the Brinell and the Spindel (used widely in Germany) , which use sand or emery as the abrasive. Much care is necessary in the selection of the abrasives as these may not be uniform. The order of merit of a series of steels and alloys with one abrasive may change completely if another abrasive or a different lot of the same abrasive is used. It has been said with considerable justification that machines for determining resistance to abrasive wear are a better test of the abrasive than of the metal.

The important fact to be recognized is that any wear-testing machine determines the relative resistance of a series of metals or alloys under a few carefully regulated conditions, but that the results obtained, while valuable for comparing the materials, cannot be translated quantitatively

into probable life in service unless the test used duplicates in all respects, including time, the conditions of service.

21.8. Importance of Deep-Drawing Properties

Practically all steels and most of the nonferrous alloys can be deformed cold, but how much deformation they will withstand without failure, or without annealing to restore ductility, varies greatly, depending upon composition, structure, and other factors. In general, the higher the ductility as shown by the tensile test, and the lower the hardness, the more a material can be deformed without fracture. Thus, low-carbon wire can be drawn through a die to a smaller size than high-carbon wire without intermediate annealing to restore the original ductility and softness. As cold forming by pressing or stamping is cheap and readily adapted to large-scale production, this method is used widely, especially by those industries producing automobile fenders and bodies and a variety of small stamped utensils for domestic and other uses. With the exception of aluminum and stainless steels, practically all deep-drawn products are coated—galvanized, tinned, painted, enameled, lacquered, or plated.

High tensile strength is usually not important; consequently, the ma-

• 1366, 1367. S. J. Rosenberg, Iron Age, v. 128, 1931, pp. 446 Engineering Metallurgy

terials most widely used are those with the least tendency to harden when cold worked. The principal ferrous materials are, therefore, ingot iron, unalloyed basic open-hearth steels containing 0.04 to 0.10 per cent

carbon, and the soft stainless steels, all in the form of thin-annealed or annealed and slightly cold-worked sheets.

As the manufacture of deep-drawn products, such as of fenders and

bodies by the automotive industry, is a continuous low-cost operation,

it is common practice for fabricators of the grade of sheet to arrange

their various operations so that the material is worked until it strain

hardens almost to the breaking point before annealing is necessary. Steel with a ductility just slightly below the standard will, therefore, show excessive breakage.

Consumers' requirements have become more severe year by year. It is only necessary to compare the automobile fender of the last five years with those of fifteen years ago to prove how the steel industry has kept up with this demand by raising the quality of its deep-drawing stock.

There is still much room for improvement, both on the part of the steel maker and on the part of the consumer of deep-drawing sheets. What is

needed most urgently is a test giving a reliable indication of drawability and, even more importantly, a standardization of consumers' requirements.

21.9. Evaluation of Steel for Deep Drawing

Drawability is a weighted average of a large number of components, including, among others, yield point and yield ratio in tension, hardness, elongation, reduction of area, and possibly impact. Drawing different shapes involves a different relation of the components; hence, even if all the components were known and could be weighted correctly, it would be necessary to change the weighting for different drawing operations. The difficulty of weighing even the known components accurately is increased by the virtual impossibility of determining accurately reduction of area—probably the most important component—on thin test specimens Some users of deep-drawing stock have arrived at certain combinations of tensile strength, elongation, and hardness for different thicknesses of sheet to indicate satisfactory drawing properties, but such correlation

is worthless to others who draw their product under slightly different conditions. To overcome these difficulties, and in the hope that a single test could be devised to measure drawability, much time and effort have been devoted to the cupping test. Several varieties have been developed, chief of which arc the Erichsen, used in Europe, and the Olsen, used in Machinability, Wear Resistance 447

• ';

Fie. 21.8. Structure and Erichsen cupping tests o[ (A) fine-grained and (B) coarse-grained low-carbon sheet steel. (F. Kbrber, Stahl u. Eisen, v. 47, 1927, p. 1158) the United States.* Essentially, the test consists of supporting a specimen of the sheet between two dies, while a ball or a plunger is brought down against the specimen forcing it into the shape of a cup. The flow of the metal can be followed in a mirror, and the first fracture can be observed. The depth of the cup, or the pressure necessary to cause failure, is measured. The test gives some information on how the material deforms but not enough invariably to predict how it will deform in service. Metallurgical opinion on the value of cupping tests is divided; some metallurgists consider them not only unreliable, but actually misleading. Because the structure and the hardness of steel are important factors in deep drawing, great care is exercised in the melting, rolling, annealing, and finishing of the sheet so that the grain size will be controlled ac-

Sisco. • Cupping tests and their significance are discussed by S. Epstein and F. T. in The Alloys of Iron and Carbon, Vol. I, pp. 272-278, and Vol. II, pp. 646-650 respectively. 448 Engineering Metallurgy

Strain-

Fic. 21.4. Stress-strain curves of low-carbon deep-drawing steel showing upper yield point A, lower yield point B, and yield-point elongation E. Slight cold rolling reduces the amount of yield-point elongation (second curve from right), and further cold working eliminates it entirely (right).

curately and the hardness will be within certain limits. A fine-grained steel can be deformed to a greater extent and has a much better surface after deforming than coarse-grained material. This is shown in Fig. 21.3.

The surface of the fine-grained steel is smooth after cupping; the coarse- grained material, on the contrary, has a crinkled "orange peel" surface.

21.10. Yield-Point Elongation, Stretcher Strains, and Deep-Drawing Properties

Most annealed low-carbon steel used for deep drawing (0.04 to 0.10 per cent carbon) has two yield points, an upper and a lower one. The upper yield point (A in Fig. 21.4.) is the stress at which yield begins, and its location is dependent to a considerable extent upon testing conditions.

The lower yield point (B in Fig. 2 1 .4) represents the lowest stress to which the applied load sinks during stretching; this value is probably more characteristic of the material than the upper point. The distance that the material stretches while passing through the yield point is known as the yield-point elongation (E in Fig. 21.4) , and this value is an important one in determining whether the sheet will have satisfactory deep-drawing properties.

When annealed low-carbon sheet is deep drawn, strain lines—known as stretcher strains—frequently appear on the surface (Fig. 21.5). This defect is analogous to the orange-peel surface caused by large grains (Fig. 21.3). These same stretcher strains appear on the surface of a tensile-test specimen which has a double yield point and considerable yield-point elongation, and the severity of the defect apparently is related directly to the amount that the specimen stretches between the upper Machinability, Wear Resistance 449

Fie. 21.5. Stretcher strains in cold-drawn low-carbon sheet. (J. Winlock and G. L. Keltey, Trans. Am. Soc. Steel Treat., v. 18, 1930, p. 241) and lower yield points. By subjecting deep-drawing sheet to a slight amount (0.5 to 2 per cent) of cold rolling on a "temper mill," the yield-

point elongation is eliminated and stretcher straining is prevented. The stress-strain curve for a steel in this structural condition is shown at the right in Fig. 21.4.

There is still much to be learned about deep-drawing steels and their behavior under extreme cold deformation. In general, microscopic examination, Rockwell B hardness, yield ratio, and yield-point elongation, plus some form of cupping test, give about as much information on drawability as can be secured at present.

21.11. Deep-Drawing Properties of INonferrous Alloys

In general, the working of metals and alloys of suitable properties by

deep drawing is an economical operation, and a considerable tonnage of nonfcrrous materials is fabricated into finished and semifinished articles by this process. Second only to deep-drawing low-carbon steel in

tonnage is alpha brass, containing more than 62 per cent copper and

less than about 38 per cent /inc. The variables affecting the deep-drawing properties of alpha brass are very important and have, as a result, been subjected to more study than similar properties for any other deep- drawing material with the possible exception of steel for automotive fender and body stock. The principal variables that affect the cold-working characteristics of alpha brass are composition, hardness, and grain size. Ductility is lowered 450 Engineering Metallurgy by small amounts of tin, lead, and iron, and the presence of much beta constituent should be avoided. Beta is a hard constituent that is likely to occur if the copper is less than 62 or 63 per cent and may be present even if the copper is 65 or 66 per cent. Considerable brass of this composition is deep drawn. To be readily subjected to a large amount of cold deformation, the grain size should be small; the optimum size depends on the nature of the drawing operation and to some extent on the degree of smoothness desired on the surface. An average grain size of from 0.035 to 0.10 mm. is about right. Brass strain hardens readily and must be annealed at intervals if the cold working is severe. The annealing temperature varies from 900 to 1100°F. (480 to 590°C.) and care is exercised to prevent grain growth during annealing. Deep-drawn brass will occasionally have an orange-peel surface, similar to that found in deep-drawn steel (Fig. 21.3), caused usually by large grain size. The principal defect to which deep-drawn alpha brass is subject is season cracking, a variety of stress-corrosion cracking that is encountered in a large number of nonferrous alloys and in some steels, especially the stainless grades. Season cracking is due to a combination of high internal stress and some forms of corrosion and has been especially troublesome in deep-drawn cartridge cases. It can be prevented by annealing to relieve residual stresses.

Most of the other soft metals, especially aluminum, copper, lead, tin, nickel, and zinc, and some of their alloys are deep drawn commercially into a variety of useful shapes. In most cases, special techniques and equipment—for example, impact extrusion for collapsible tubes used widely for drugs, cosmetics, and toilet articles—must be used, and for each metal there are special problems to be solved, many of which are metallurgical. For example, one of the principal problems in deep drawing aluminum is to prevent the abnormal grain growth which is likely to occur in annealing, and which may cause a rough surface and premature breakage of the deep-drawn part. In deep drawing high-purity copper, abnormal grain growth is also a problem.

QUESTIONS

1. Calculate the theoretical horsepower required to machine a hollow monel metal shaft having an outside diameter of 3.250 inches, an inside diameter of 1.625 inches, and a length of 36.00 inches, from a blank 36>/j inches long, having a outside diameter of 3% inches. 2. Design a dynamometer to determine the power required to turn a given material in a lathe. Make use of the spring characteristic s of the tool holder, and show how the power could be calculated on the basis of the data obtained. Machinability, Wear Resistance 451

those affecting wear re- 3. Compare the variables affecting machineability with sistance. What conclusions can you draw?

4. Discuss the effect of wear on fatigue life. of wear. 5. Differentiate between erosion, galling, and corrosion as a mode List some examples of cadi. friction as a 6. Discuss the difference between rolling friction and sliding mode of wear. study the rate of 7. Discuss the possibility of using radioactive materials to wear. are these 8. List the environmental factors that effect wear resistance. Why as important as the physical properties of the material itself? 9. Describe the cupping test and discuss its limitations. effect the deep- 10. What is meant by "double-yield point" and how does it drawability? 11. What benefits are obtained in having a homogeneous structure in a material

which is to undergo deep drawing? 12. Discuss the effects of lattice configuration on deep drawing properties. Corrosion and Oxidation CHAPTER 22 Mars G. Fontana, Ph.D., Professor and Chairman, De- partment of Metallurgical Engineering, The Ohio State University, Columbus, Ohio Richard Edward Grace, Ph.D., Associate Professor of Metallurgical Engineering, Purdue University, Lafay- ette, Indiana

L4ORROSION causes destruction or deterioration of metals and alloys because of electrochemical or chemical reaction with the environment. Corrosion can be considered as metallurgy in reverse. For example, iron and steel are made from iron-oxide ores, and they revert to oxides when rusting occurs.

The annual cost of corrosion in the United States is estimated at §6 billion. Practically all environments—water, soils, acids, alkalies, gases, oils, etc.—are corrosive to some degree. Corrosion of fuel systems in automobiles alone costs $100 million per year. One plant spends $2 million annually for painting to prevent external rusting of steel equipment. Some of the results of corrosion are poor appearance, high operation and maintenance costs, plant shutdowns, contamination of products, loss of valuable products, and safety hazards.

Corrosion can be classified in two general ways, namely (1) wet and

dry or (2) direct combination and electrochemical. Wet corrosion occurs when liquids are present, usually electrolytes. Dry corrosion occurs in.

vapors and gases: a liquid phase is not present. Dry corrosion is usually associated with high temperatures. Direct combination usually involves reaction between a metal and nonmetallic elements or compounds such as steam, oxygen, sulfur dioxide, and chlorine at high temperatures similar to dry corrosion. This subject is discussed later under high-temperature oxidation.

22.1. Electrochemical Corrosion

The electrochemical theory of corrosion is the accepted theory. There

are two basic requirements: (1) anodes and cathodes must be present to

452 Corrosion and Oxidation 453

anodes and cathodes form a cell, and (2) direct current must flow. The for may be very close together (local cells) or they may be far apart, example, two sections of pipe. The current may be self-induced or it may be impressed on the system from an outside source. current The anode, or anodic area, is where corrosion occurs and where leaves the metal. The cathode, or cathodic area, does not corrode; current enters the metal here. Anodes or cathodes can form on a single piece of metal because of local differences in the metal or in the environment. The metal at the anode dissolves, loses electrons, is oxidized, and becomes an ion. This situation is shown schematically in Fig. 22.1. The

Fie. 22.1. Formation of Ferrous Ions in the Corrosion of Iron.

electrons (e) are left on the metal and travel through it to the cathode. Current flows in the opposite direction. Immersion or contact with an

electrolyte is required to complete the circuit or to carry current from the anode to the cathode. Corrosion in pure water (or acids) occurs as follows: Fe — 2e = Fe++ (Anode reaction)

2H+ + 2e = H2 (Reduction at cathode) The overall reaction is:

Fe + 2H+ = Fe+++ H 2 . The quantity of current which passes through this cell is directly proportional to the amount of metal that corrodes. For example, one ampere per year is equal to about 20 lbs. of steel. Anything that changes these reactions or the cell circuit could reduce or increase corrosion. If the 454 Engineering Metallurgy hydrogen bubbles collect on the cathode and thus form an insulating blanket, current flow is reduced and corrosion is practically stopped.

This is called polarization and, in this case, cathodic polarization because it occurs on the cathode. Most waters contain dissolved oxygen which combines with the hydrogen as shown in Fig. 22.2. This is cathodic depolarization and corrosion proceeds. This is the basis for one of the earliest forms of corrosion control, namely deaeration of boiler water.

Steel Wafer

+ 2 -<-2H2

Fig 22.2. Corrosion of Steel by Ordinary Water Containing Oxygen. Depolarization.

In neutral or alkaline waters the anode reaction is the same, but the cathode reaction is: i/ H 2 + 2 2 + 2e = 2 OH-. The overall reaction is

i/ . Fe + H2 + 2 = Fe (OH) 2 ? Fig 22.3 is a basic diagram which summarizes the requirements for corrosion of metals by electrolytes. Differences in the metal surface, or lack of homogeneity, may be due to impurities, grain boundaries, orientation of grains, differences in composition of the microstructure, differences in stress, scratches, nicks, and holes. It might be said that nature abhors a vacuum—corrosion abhors homogeneity.

22.2. EMF Series and Passivity

Different metals have different tendencies to corrode or vary in reactivity. The familiar Electromotive Force Series, shown in Table 22.1, Corrosion and Oxidation 455

Hydrogen is lists elements in order of decreasing corrosion activity. listed and shown as zero because a hydrogen electrode is used as the reference electrode or half cell to measure the electrode potentials of the other elements. The more negative the potential of an element is, the greater the tendency to corrode.

Table 22.1

Electromotive Force Series Galvanic Series—Sea Water

Cation Volts Magnesium and magnesium alloys* Zinc Na + -2.71* 2S Al (Comm. pure) Mg+ + -2.40 Cadmium A1 + + + -1.70 24 ST Al (4.5% Cu, 1.5% Mg, 0.6% Mn) Be + + -1.69 ( Steel or iron U + + + + -1.40 [Cast iron Mn + + -1.10 13% Cr iron (active) Zn ++ -0.76 Ni-Resist (High Ni cast iron) Cr + + -0.56 (l8-8 (active) Cr+ + + -0.50 U8-8-M0 (active) Fe + + -0.44 Lead-tin solders Cd + + -0.40 Lead Co + + -0.28 Tin Ni + + -0.23 [Nickel (active) Sn + + -0.14 llnconcl(active) (80% Ni, 13% Cr, 7% Fe) Pb+ + -0.12 fHastelloy B (60% Ni, 30% Mo, 6% Fe, 1% Mn) H+ 0.00 Ichlorimct 2 (66% Ni, 32% Mo, 1% Fe) Sb+ ++ +0.10 Brasses (Cu-Zn) Bi + + + +0.23 Copper Cu + + +0.34 Bronzes (Cu-Sn) Cu + +0.47 Cupro-nickcU (60-90% Cu, 40-10%Ni) Ag + +0.80 Moncl (707c Ni, 30% Cu) Pb+ + + + +0.80 Silver solder Pt + + + + +0.86 (Nickel (passive) Au + + + +1.36 l,Inconel (passive) Au + + 1.50t rChromium steel (passive) (11 to 30% Cr) 18-8 (passive) 18-8-Mo (passive) fHastelloy C (62% Ni, 17% Cr, 15% Mo) ichlorimct 3 (62% Ni, 18% Cr, 18% Mo) Silver Titanium Graphite Gold Platinum t

* Anodic end

t Cathodic end 456 Engineering Metallurgy

According to the EMF Series, the addition of chromium to iron should increase corrosion; yet this mixture results in stainless steels. Aluminum is active hut it is widely used for corrosion resistance. This effective increase in nobility or corrosion resistance is due to passivity, which can be simply defined as "a condition that causes the metal not to corrode when it should." Passivity is generally due to surface films which act as barriers between the metal and its environment. These films could be absorbed gases or adherent corrosion products. A series for commercial metals and alloys in a given environment could show different positions than indicated by the EMF Series. Table 22.1 gives the order for the Galvanic Series (sea water) . Chromium, aluminum, and silicon as elements or as alloy additions to steel are the three most important from the passivity standpoint. WET CORROSION The Eight Forms of Corrosion

Corrosion may be classified into the eight forms in which it manifests itself. The basis for this classification is the appearance of the corroded metal. Some of these forms are unique, but all are interrelated. This classification is arbitrary, but it covers practically all corrosion failures and problems. The eight forms are (1) uniform corrosion, (2) galvanic or two-metal corrosion, (3) concentration cell corrosion, (4) pitting, (5) intergranular corrosion, (6) stress corrosion, (7) dezincification, and (8) erosion-corrosion.

22.3. Uniform Corrosion Uniform corrosion or overall general attack occurs when anodic and cathodic areas keep shifting, and corrosion takes place more or less uniformly over the entire exposed surface. The metal becomes thinner and eventually fails. This form of corrosion accounts for most of the destruction of metals on a tonnage basis. From the technical standpoint, however, uniform attack causes the least concern largely because service life can often be quite accurately estimated on the basis of relatively simple corrosion tests. It is the localized form of corrosion that often results in an unexpected failure.

22.4. Galvanic or Two-Metal Corrosion When two dissimilar metals are in contact, or otherwise connected electrically, and exposed to a corrosive electrolyte, coirosion of the less resistant metal is increased and corrosion of the more corrosion-resistant

metal is decreased. All of the former usually becomes the anode, and the latter the cathode. Current flows here similar to the situation shown Corrosion and Oxidatio7i 457 Environment

Current Current enters leaves ./y

Fie. 22.3. Basic Diagram Showing Requirements for the Corrosion of Metals by Electrolytes.

in Fig. 22.3. The driving force for the current is the potential generated by the electrodes. This, of course, is the principle of the so-called dry battery. Table 22.1 shows a practical galvanic series for metals and alloys exposed to actual sea water. The farther apart two metals are in this series the greater the potential when they are coupled. The material higher in the series becomes anodic to one below it. Magnesium and copper would form a bad galvanic couple as far as corrosion is concerned. In many cases, the accelerated attack occurs on the anodic metal near the junction with the cathode. The effect spreads farther in a linear direction as the conductivity of the environment increases. An important effect that is often overlooked, and rapid failures result,

is the area effect, or the ratio of cathodic area to anodic area. A large cathode and small anode accelerate further the attack on the anode. For example, a metal by itself in a given environment may corrode at X mils per year; the rate may be lOx when coupled to a more noble metal; this but it may be 1000X when the area ratio is large. Fig. 22.4 illustrates effect. Note that the steel rivets (small anodic area) are completely corroded; whereas, the other specimen with a large anodic area still represents a strong joint. An excellent practical example of rapid failure because of an unfavorable area ratio concerns large tanks handling a solution mildly corrosive 458 Engineering Metallurgy

; *V ' *&*:

-&&- COPPER RIVETS STEEL RIVETS

IN STEEL PLATE IN COPPER PLATE LARGE ANODE LARGE CATHODE SMALL CATHODE SMALL ANODE

Fie. 22.4. Effect of Area Relationship on Corrosion of Rivets in Sea Water (15 Months). [Courtesy The International Nickel Co.)

to steel. The bottoms were made of 18-8 stainless steel, and they were welded to ordinary carbon steel sides. The steel was covered with a baked coating. Small defects in the coating exposed tiny anodic areas which were in electrical contact through the welds with the larger bottom area of stainless steel. The steel in the sidewalls near the welds failed by perforation in several weeks.

22.5. Concentration Cell Corrosion

Anodes and cathodes can form because of differences in the environment. For example, a potential difference can be measured between two identical copper electrodes immersed in different concentrations of hydrochloric acid. The cells formed because of differences in the environment are called concentration cells. This form of corrosion is also designated crevice corrosion, gasket corrosion, and deposit corrosion, and is usually associated with stagnant conditions. Corrosion and Oxidation 459

Low Metal Ion Concentration

High Metal Ion Concentration

Joint Metal Ion Concentration Cell Riveted Lap

High Oxygen Concentration

Low Oxygen Concentration

Oxygen Concentration Cell

Fig. 22.5. Typical Examples of Concentration Cell Corrosion. (Courtesy The Inter- national Nickel Co.)

The most common forms of concentration cells are oxygen cells and ion cells as shown in Fig. 22.5. Fig. 22.6 shows attack on the face of the flange of a pipe because of concentration cell corrosion. The inside surface of the pipe is in good condition.

22.6. Pitting

Pitting corrosion is readily recognized because of pits or holes. This is one of the most vicious forms of corrosion and one of the hardest to predict. The anodic area remains stationary and corrosion progresses in- wardly on one spot. Pitting can be considered as the intermediate situation between no corrosion (complete passivity) and uniform corrosion (entirely active) in that the surface breaks down and corrodes only in relatively small areas. 460 Engineering Metallurgy

Fig. 22.6. Gasket Corrosion Caused by Concentration Cells.

Pitting often starts because of concentration cell effects such as under a permeable deposit. The environment under the deposit becomes ex- hausted of oxygen, or increases in ion concentration; whereas, the surrounding metal away from the deposit is exposed to essentially a constant concentration of oxygen or ions. The rate of penetration often accelerates because the pit acts as a crevice and thereby increases the concentration cell effect. An unfavorable ratio of cathode to anode also exists.

This is one reason why the stainless steels often show severe pitting in some environments with most of the surface showing no attack.

22.7. Intergranular Corrosion

Intergranular corrosion consists of selective or localized attack at the grain boundaries of a metal or alloy. In some cases, complete disintegration of the metal results even though a relatively small portion of the total weight is dissolved. The austenitic stainless steels, such as 18-8, are particularly susceptible when they are not properly heat treated. If this steel is heated in the range of about 950° to 1400°F. for an appreciable time, it becomes sensitized or susceptible to intergranular attack in Corrosion and Oxidation 461 environments where high chromium content is needed for corrosion resistance. The commonly accepted explanation for this phenomenon is boundaries. This that chromium carbides (Cr23C„) precipitate in grain removes chromium from solid solution and lowers or depletes the chromium in areas adjacent to the grain boundaries. This lowers the corrosion resistance of these areas, and corrosion proceeds aided by galvanic corrosion and a large cathode area represented by the grains, which are cathodic. Fig. 22.7 shows intergranular attack, or weld decay as it is often called.

Fife 22.7. Intergranular Corrosion of Welded 18-8 Stainless Steel.

The plate to the left of the weld in Fig. 22.7 contains about 0.60% titanium. These steels are stabilized with niobium (columbium), titanium, and tantalum, which are strong carbide formers and thus preferentially tie up the carbon so that chromium carbides cannot form. The relatively new, very low carbon steels (0.03% max.) can stand much more abuse from this standpoint. Another procedure to minimize inter-

granular corrosion is to finally heat the part or equipment to about 1950°F. at which point the chromium carbides dissolve, and then rapidly cool by quenching through the sensitizing temperature zone. Sometimes the stabilizing element fails to do its duty, as shown by Fig. 22.8. In this case, intergranular corrosion occurs in a narrow zone in the parent metal immediately adjacent to the weld or melted metal. 462 Engineering Metallurgy

I"ic. 22.8. Knife-Line Attack of Stabilized 18-8 Stainless Steel.

The attacked zone is usually a few grains wide and hence has been christened knife line attack. The mechanism is briefly as follows. The metal next to the weld is heated to a very high temperature, and when rapidly cooled, the niobium stays in solution because niobium carbide does not have time to form. If this material is then heated in the sensitizing zone, chromium carbides form as if no stabilizing element were present. Experiments have shown the niobium carbides dissolve at temperatures over 2250°F. One recommended cure for this situation is to heat the material to around 1950°F, where chromium carbides dissolve and niobium carbides form. Other alloys are also sometimes attacked intergranularly, as shown in

Figure 22.9, which is typical of attack on nickel and high-nickel alloys when exposed to sulfur-bearing atmospheres at high temperatures.

22.8. Stress Corrosion

Stress corrosion could be defined in a general way as the acceleration of corrosion by stress. In some cases, more or less over-all corrosion is increased because of stresses in the metal. Stress corrosion is more generally interpreted as the type of attack that causes cracking, and it is this

form of failure that is under discussion here. In most cases, the magnitude of the stresses required to cause stress

corrosion is high. These stresses could be due to applied loads or to residual stresses, although failures because of the former are relatively rare because most structures are over-designed. The most common of the residual stresses that cause stress corrosion are those resulting from cold working or cold forming and also the "locked-in" stresses resulting from Corrosion and Oxidalion 463

Fig. 22.9. Intcrgranular Corrosion of Nickel at High Temperatures. Top SOX. Bottom 500X.

welding. In a few cases, plant failures have occurred because of severe thermal gradients in the metal, which cause high stresses. Tensile stresses are required for cracking.

Stress corrosion usually occurs when over-all or uniform corrosion is

low and often negligible. It is somewhat similar to pitting in this respect

in that most of the metal is passive but active sites develop. The presence of stress often accelerates this localized attack. A crack is initiated and propagates to failure. Two of the earliest and classic examples of stress corrosion arc the "season cracking" of brass cartridge cases and the "caustic embrittlement" of riveted steel locomotive steam boilers. Am- munition became worthless during the wet seasons. Boilers exploded because of cracks starting near the rivets or stressed areas. Statements have been made to the effect that all metals and alloys could be made to crack under selected conditions of stress and corrosion. Fortunately stress corrosion happens much less frequently than other forms of corrosion but cracking failures are often serious and unexpected.

Table 22.2 lists a number of metals and alloys and the environments in which stress corrosion may occur. This does not mean that failure would always develop in these combinations, but one should be aware of the

possibilities. For example, nickel is widely used in caustic services, and

cracking is difficult to develop unless severely cold-worked metal is used. On the other hand, brass cracks readily in ammonia, and "as welded" 464 Engineering Metallurgy

2 50

.200 —>• g 150

100

10 20 30 40 50 % CONCENTRATION

Fig. 22.10. Effect! of Temperatures and Concentration on Cracking of As-Welded Steel Equipment in Sodium Hydroxide Based on Service Experience.

steel will fail quite readily in strong sodium hydroxide at elevated temperatures. Fig. 22.10 shows the relation between temperatures and concentration of caustic on the cracking of as-welded steel. These data are based on actual service performance.* A number of theories have been proposed for the mechanisms of stress corrosion. The Electrochemical Theory calls for susceptibility to selective corrosion along more or less continuous paths and a high stress tending to pull the metal apart along these paths. These continuous paths could be along grain boundaries where a precipitated anodic phase, or an anodic depleted zone resulting from precipitation, may exist. Trenching or pitting results causing stress raisers or notches where cracks begin to form and then propagate because of corrosion and stress at the point of the crack. Differences in potential between the grain boundary area and the grains have been observed. This theory explains the intergranular cracking observed in some systems. The Mechanical Theory states briefly that localized corrosion forms

the notch or stress raiser, but the propagation of the crack is then due to mechanical effects, and corrosion docs not play a continuous role. According to the Strain Accelerated Decomposition Theory an anodic

phase is formed because of the presence of stress. In other words a site

that is not susceptible to attack becomes susceptible because of a phase decomposition or formation caused by stress.

'Corrosion, 7, 295-302 (1951), H. Vv". Schmidt, P. J. Gegner, G. Heincman. Corrosion and Oxidation 465

Metals and Alloy's Tabu: 222. Environments That May Cause Stress Corrosion of

Material Environment Aluminum alloys NaCl-HjOj solutions NaCl solutions Sea water Air, water vapor solutions Copper alloys Ammonia vapors and Amines Water, water vapor Cold alloys FeCh solutions Acetic acid—salt solutions Inconel Caustic soda solutions Lead Lead acetate solutions Magnesium alloys NaCl-KsCrO-i solutions Rural and coastal atmospheres Distilled water Monel Fused caustic soda Hydrofluoric acid Hydrofluosilicic acid Nickel Fused caustic soda Ordinary steels NaOH solutions NaOH-NaaSiO* solutions Calcium, ammonium, and sodium nitrate solutions Mixed acids (HsSOi-HNOs) HCN solutions Acidic HiS solutions Sea water Molten Na-PI» alloys and Stainless steels Acid chloride solutions such as MgClz BaCh NaCl-H202 solutions Sea water HjS NaOH-H2S solutions Titanium Red fuming nitric acid

of the crack by The Anodic Shift Theory explains the propagation by cor- stating that when a certain stress concentration is brought about plastic deformation takes place rosive attack (pitting or trenching) , some anodic direction and the electrode potential of the metal is shifted in an the deforma- at the crack base because of the latent energy produced by attacked, and the crack tion. The anodic area produced in this manner is behavior. This again produces stress concentration sufficient for plastic and simul- process can be visualized as a continuous shifting in potential taneous metal removal. theory of The Film Theory is another form of the electrochemical than composition stress corrosion but suggests a different driving force electrochemical differences. Stress corrosion is started by the usual 466 Engineering Metallurgy

attack, and stress concentration at the base of the incipient crack is sufficient to tear or break any protective film that may be present. The crack is propagated by the simultaneous corrosion of the exposed metal and tearing of any film that may form. The fact that potential differences do exist between filmed and film-free metal surfaces has been confirmed many times. In one case, the potential at the base of notches in alloys subject to tensile stretching was found to be as much as 0.7 volt anodic to the remainder of the specimen. Attack in these bare areas becomes permanently faster than on the filmed surface, and preferential corrosion occurs.

One of the mysteries of stress corrosion concerns an explanation of why, in some cases, intergranular cracking occurs while in others the cracks are transgranular or across the grains.

The work of Priest, Beck, and Fontana* shows that corrosion and stress must be present simultaneously for cracking to occur. Motion pictures of crack propagation at actual speed show that cracking stops when cathodic

protection is applied, and propagation resumes when it is removed. This

indicates that the mechanical theory is not correct. Failure is called corrosion fatigue when cyclic stresses are involved.

22.9. Dezinci fixation

As this term implies, the phenomenon of dezincification was first observed on brasses. The zinc is selectively leached out of the alloy leaving a brittle, weak, porous mass consisting predominantly of copper plus copper oxides. The obvious mechanism is solution or corrosion of the brass; then the zinc stays in solution, and the copper plates back on. Dezincification can be readily observed because the attacked areas show the color of copper as compared to the yellow brass. Brasses with 15 per cent or less zinc are practically immune, and additions of tin, arsenic, phosphorous, and antimony increase the resistance of brasses to dezincification.

De/incification can occur uniformly, as shown in Fig. 22.11, or as the plug type, or attack in spots. Fig. 22.12 shows a section through a dezincified plug and also the porous nature of a dezincified area.

The selective removal of one of the constituents of any alloy falls into the category described here. The so-called graphitization of cast iron is a misnomer. What actually occurs is the removal or corrosion of iron leaving the graphite network. Selective removal of cobalt in high-cobalt alloys, and silicon in copper-silicon alloys have been observed.

•Trans. ASM. 47, 473-492 (1955). Corrosion and Oxidation 467

:-.'.. A ..

Fie. 22.12. Section Through Dezincified Plug.

Fie. 22.11. Uniform Dezincification of Brass.

5000

45 50 55 DEGREES CENTIGRADE

Erosion- Fie. 22.13. Inhibiting Effect of Copper Ion in a Sulfuric Acid Slurry on Corrosion of I8-8SM0 Stainless Steel. 468 Engineering Metallurgy

22.10. Erosion-Corrosion

Many metals and alloys depend upon surface films or corrosion products for corrosion resistance. If these surface layers are removed by mechanical wear effects, active and rapid corrosion occurs. Erosion- corrosion is often encountered under conditions of high velocity, tur- bulence, impingement, and solids in suspension. Valves, pumps, elbows, centrifugals, agitators, and heat exchange tubes often fail because of erosion-corrosion. Fig. 22.13 shows the effect of high velocity on corrosion of I8-8SM0 stainless steel by a sulfuric acid slurry. The x on the abscissa shows no attack under static conditions. The curve marked "no copper" shows the rapid increase in corrosion with temperature under high velocity conditions. The addition of cupric sulfate reduced attack under these conditions. Fig. 22.14 shows the typical wavy appearance due to erosion- corrosion.

Fig. 22.14. Erosion-Corrosion of a Pump Impeller.

Cavitation may be considered as one form of erosion-corrosion. Rough- ened surfaces or deep craters are often found on the trailing faces of ship propellers and pump impellers. This is cavitation and is largely due to the mechanical effect or pounding caused by the formation and collapse of air or vapor bubbles in the liquid on the metal surface. Fretting corrosion is the attack that occurs at contact areas between metals under load and subject to vibration and actual slip at these surfaces. It is sometimes called chafing corrosion, friction oxidation, and false Brinelling. The latter because the attacked areas often look like mechanical indentations. The corrosion product on steel is usually a Corrosion and Oxidation 469

tearing away of ferric oxide debris. Two proposed mechanisms involve these metal particles because of seizing or galling and then oxidation of Fretting particles, or oxidation first and then wearing away of the oxides. could be considered as one type of erosion-corrosion.

22.11. Methods for Combating Corrosion

Many corrosion problems can be solved by more than one method the or means. Sometimes a combination is used. In any case, most economical solution to the problem is usually adopted. Methods for combating corrosion are arbitrarily classified into eight general categories that cover essentially all corrosion problems. Alloying, or better corrosion resistance, involves using materials with better resistance to corrosion. If steel corrodes, copper, stainless steel, or high-silicon iron may do the job. Proper heat treatment for optimum corrosion resistance is also in this classification. Cathodic protection involves making the structure to be protected namely, the cathode of a cell. This can be done by two general methods use of sacrificial anodes or impressed currents. The most widely known is the sacrificial anode system is galvanized steel where the zinc coating and anode and corrodes preferentially, thereby protecting the steel. Zinc magnesium anodes are also widely used. The important point is to have direct current entering the entire surface of the structure to be protected. Impressed currents involve passing a direct current from an external source through an anode, through the corrosive environment, and then "permanent" to the structure. In this case, a very corrosion-resistant or are used for anode is needed. Carbon and 14.5% silicon iron (Duriron) impressed current anodes. Underground pipe lines, water tanks, sea-going vessels and piers are examples of structures using cathodic protection. Metallic and inorganic coatings are typified by nickel, chromium, and install tin electroplates, and glass and enamels. The principle here is to a barrier to separate the metal from the environment. Organic coatings involve paint-type materials, which act as a barrier. Proper metal surface preparation, such as sandblasting, is the most important factor, followed by a proper primer for good adhesion and com- patability with over-coats, and then by good top coats themselves. The will use of an expensive paint does not guarantee that good protection obtain. problems. Metal purification is not often utilized, but it does solve some High-purity aluminum, which is available at reasonable cost, is a good example. Contamination and catalytic effects are sometimes involved. 470 Engineering Metallurgy

^.-4.3-rs--?

Fig. 22.15. Pump Casing Lined with F.puxy Resin. (Courtesy, The Duriron Co., Inc.)

Alteration of the environment often concerns addition or removal of an ingredient in the environment. Deaeration is an example of the former and inhibitors of the latter. An inhibitor is anything added to an environment to decrease corrosion. In many cases, reduction of temperature or concentration, without substantially affecting the process, solves a corrosion problem or permits the use of less expensive materials such as steel. Corrosion and Oxidation 471

Nonmetallic materials involve integral or monolithic construction or thick coverings such as sheet linings. These materials can be classed into stone- five general types: (a) carbon and graphite; (b) ceramics, such as ware, porcelain, fused silica, glass, brick, mortars, and cement; (c) plastics, such as phenolics, vinyls, polyethylenes, acrylics, epoxies, styrenes, nylon, silicones, and fluorinatcd polymers (d) natural and synthetic rubbers; and (e) wood. Fig. 22.15 shows a cast iron pump casing lined with a plastic. Design involves the shape of a structure. Equipment life can be prolonged or corrosion costs reduced through the use of bottom outlets designed to drain completely, readily replaceable or interchangeable parts, standard lengths of tubing, increased thickness in more vulnerable areas, designing to prevent crevices or stagnant areas, and the use of butt-welded instead of riveted joints.

22.12. Corrosion Testing

Corrosion testing is conducted to evaluate or select a metal or alloy for a specific environment or definite application, to evaluate a given material to determine environments where it is suitable or attacked, for controlling corrosion resistance of a material or corrosiveness of an environment, or for corrosion research purposes. These tests include laboratory, pilot plant, large scale plant, or field tests.

The most important point in corrosion testing is to duplicate as closely as possible the actual conditions to be encountered. Factors such as temperature, concentration, velocity, aeration, time, galvanic effects, stress, and intermittent or continuous operation must be considered. Chemical and metallurgical history of the specimens must be known and the speciments identified. A clean metal surface is preferred. Most corrosion tests involve determination of loss in weight of the specimen and conversion to a rate of linear penetration for prediction

of life expectancy. An expression for corrosion rates that is widely used

by engineers is "mils per year," and its formula is as follows: 534 W = mils per year DAT W = Weight loss in milligrams D = Density in grams per cubic centimeter A = Area in square inches T = Time in hours. 472 Engineering Metallurgy

22.13. Liquid-Metal Corrosion*

The use of atomic energy for commercial power has increased interest in liquid metals such as sodium, lead, and lithium, as heat-transfer media. In addition to the usual forms of corrosion, another important factor enters the picture. This is mass transfer or the solution of the equipment in a hot zone and deposition of this metal or alloy in a colder area. This is due to marked change in solubility of one metal in another with change in temperature. In addition, a more careful selection of materials of construction must be made because equipment is not as readily accessible for repair as in more conventional systems.

HIGH-TEMPERATURE OXIDATION

22.14. High-Temperature Oxidation

High-temperature oxidation of metals and alloys, commonly called dry corrosion, can be described as the chemical reaction of a metal or alloy with oxygen, sulfur, nitrogen or any halogen. Generally, however, the agents involved are solid alloys and oxygen gas. The resulting oxide is generally less dense than the metal from which it was formed, and it has poor light reflectivity, electrical conductivity, and mechanical strength. While the term oxidation can apply to many different kinds of reactions, the common chemical change is the valence increase of the metal involved to a higher positive value. In the subsequent paragraphs, the effects of oxygen pressure, temperature, and substrate material are discussed to point out the salient features of all oxidation processes. Reference to different metals and alloys will be made, but much practical information can be gained from the metallurgical literature. Little is known about the nuclcation of oxide phases on metal surfaces. On the other hand, the growth of oxides has been studied extensively; various theories and experimental techniques have progressed well enough so that theoretical predictions can often be checked quite accurately. Unlike simple diffusion studies in solid solutions, oxidation measurements call for boundary conditions fulfilled at interphase interfaces. The gas- solid oxide interface is the external boundary while the solid-oxide, solid- metal interface is the usual internal boundary. In rate studies of oxide growth it is usually desirable to know the oxygen pressure in the gas phase and the oxygen solubility in the metal phase. These quantities fix the concentration gradient of cations and anions in the oxide phase, but

• Certain of the material presented in Sections 21.1 to 21.13 of this chapter appeared in a monthly column on Corrosion by M. G. Fontana for Industrial and Engineering Chemistry, who gave permission to reproduce it in this textbook. Corrosion and Oxidation 473

mechanism. Oxygen pres- they do not give information about the growth oxide growth, but the effect is sure is known to influence the rate of the oxidation rather small. Increased temperature generally increases yield ambiguous re- rate; however, the criterion of oxidation rate may simply grams of sults. For example, if the criterion for oxidation were and the oxide gained per unit area and unit time on a metal substrate could result oxide had a very high vapor pressure, temperature increases proceed rapidly. in vaporization or loss of oxide, yet oxidation would O, system at temperatures An example of this behavior is the Mo, Mo() 3 , above 600°C. Internal oxidation occurs in many copper and silver-base characterized by the alloys and in certain steels. This kind of oxidation is below the formation of small particles of oxides of the alloying elements external scale. engineers Since scaling or oxidation is primarily a deleterious effect, used may be are concerned with design problems in which the part to be important design subject to hot gases. Size and shape retention may be of the requirements, but frequently the electrical and optical properties in relays, oxide phase are considered. In electrical contacts, for example because the the choice of an oxidation-resistant alloy is imperative, con- formation of a poorly conducting oxide scale can prevent electrical aluminum alloys tact from being made. The choice of stainless steels and evident when it is for kitchen utensils which should have eye appeal is and do known that the oxides of these alloys are nearly transparent temperatures. not materially affect the light reflectivity of surfaces at low reduction in area of any Lastly, it is obvious that through oxidation the part unfit structural member designed to carry a given load may make the when parts are used for service. This effect on strength can be magnified cracked off and in fatigue-loaded operations where the oxide is continually fresh oxide forms at the expense of base metal.

22.15. Formation of Oxides The most common method for obtaining quantitative oxidation data unit time and on metals and alloys is by measuring the weight gain per condensation reaction unit area. In vapor-deposition processes such as the gas phase is at the cool end of a zinc retort, the species which exists in the frequently the same species that exists on the substrate and the reaction in metal progresses because the substrate material is cold. Similarly, cladding by vapor-phase deposition, the vapor may be a different atomic: formation is not species than the substrate, but intermetallic compound reaction. The important in determining the rate of the condensation by a driving force for these reactions is a supersaturation brought about 474 Engineering Metallurgy

temperature distribution. Furthermore, the nucleation of new grains in these deposition processes occurs in the same way that nucleus formation proceeds in the water-to-ice transformation, the growth of rock candy, the freezing of metals, and the electrodeposition of metals.

The problem of nucleation of an oxide on a metal substrate is complicated by the fact that the gas species chemically reacts with the substrate form a structure of the type . Mx O v The following single steps may occur to a greater or lesser extent in the formation of any oxide. As is evident from the listing, most of the steps are likely to affect the nucleation rate and the growth rate of an oxide grain. The rate of any oxidation reaction is governed by the single slowest step of the type listed below:

1. Surface adsorption of an O t molecule on metal.

2. Dissociation of 2 to O atoms. 3. Ionization of O to 0=. 4. Ionization of M to M+x and electrons. 5. Surface x migration of O, 0—, M or M+ .

6. Diffusion of O in the metal phase until the solubility limit is reached. 7. Nucleation of the oxide phase. Once an initial oxide layer has been formed, several other possible steps may occur. There are:

1. Diffusion of 2 gas within cracks in oxide film.

2. Surface adsorption of an 2 molecule on metal oxide.

3. Dissociation of 2 to O atoms. 4. Diffusion of 0= through oxide layer. 5. Ionization of M to M+ x and electrons at the metal-metal oxide surface. 6. Diffusion of M+ x through oxide layer. Fig. 22.16 shows a sequence of microscopic steps that may be operative when a bare metal surface is oxidized. Since the view has been taken

that the rate of oxidation is governed by the single slowest step, any experimental work necessitates the reduction of a system of many factors to a system in which a single variable can be studied to yield meaningful results.

22.16. Dependence of Oxide Growth on Gas Pressure

The effect of pressure on oxidation rate has been successfully investigated for very few metals. Among these are iron, copper, zinc, zirconium, titanium, vanadium, and cobalt, reacting with oxygen. At various pressures the dependence of a reaction rate of a solid with oxygen is intimately related to the mechanism of the reaction. The conditions for growth are related to the nonstoichiometry of the oxide phases developed, and the Corrosion and Oxidation 475

/ J~— LINES Of

a-adsorbed oxygen Fic. 22.16. Gulbransen's postulation of oxidation mechanism, large crystals, d-uniform film, b-fine mosaic structure of crystals, c-nucleation of and K. F. Andrew, oxide layer appears. Taken from E. Gulbransen, W. R. McMillian Trans. A. I. M. E. 200, 1027 (1954).

controlled within the mechanism for growth is assumed to be diffusion are copper oxide phase. Two metals which have been studied extensively information which and zinc. The following paragraphs present the oxidation predicts and supports the pressure dependence of the observed rate. observed to increase with 1. The electrical conductivity of Cm2 was oxygen pressure according to the relation.^

Vs a = constant PQ 148. K. Hauffe. and C. Wagner, Zeit. phys. Chem., B37 (1937). •J. Gunderman, conductivity are Since the three variables that make up electrical varying concentration, valence, and mobility of the conducting species, for oxygen pressure must change one or more of the variables to account is viewed as cation the pressure dependence. If the structure of the oxide of 0. deficient, the following model could represent the structure Cu2

Cu" Cu-> Cu* O o- 0= o-

Cu* Cu^ Cu< Of

by the Then, one oxygen molecule may form two molecules of CutO following relationship.

Ot + 4«- -* 4Cu +D + 4*"D + 2 c"*° 476 Engineering Metallurgy

Applying the law of mass action and realizing that the concentrations of Cu+ vacancies and Cu++ions are identical, one finds

C8 defects — = constant

If the oxidation rate of copper is determined by the concentration of defects operative in the oxide phase near the gas-oxide interface, then

oxidation rate a defects cc PjJ C Oj

Wagner and Grunewald* report that the oxidation rate of copper at 1000°C. is a linear function of the seventh root of the oxygen pressure. This agreement supports the theory reasonably well because such measurements are difficult to make when the pressure dependence is so small. Oxides which behave similarly are FeO, NiO, CoO, and Ag^O. 2. On the other hand, a class of oxides exists in which the electrical conductivity decreases with increasing oxygen pressure. A typical example of this type of oxide is ZnO. The following structure shows a typical cation excess oxide.

+ Zn* 0- &++ 0- Z"*+ 0-

0- ++ * + Z" o- Zn o- fir* dB> Zn + * 0- Zn ** 0- Zn+ * 0- CD 0- ++ + + Zn 0- Z" * 0- Z"*

If the number of interstitial ion and electron defects were decreased by an increased oxygen pressure according to the reaction,

i/aO, + <^f) + 2 (P) -> ZnO Electrical conductivity or Oxidation rate a C defects a ^o Vt

•C. Wagner, and K. Grunewald, Zeit. phys. Chem., B40 (1938), 455. Corrosion and Oxidation 477

Kubachewski and B. FIG. 22.17. The oxidation-time relationships. Taken from O. Publications, London Hopkins. Oxidation of Metals and Alloys. Butterworth's Scientific (1953).

Various measurements have shown that electrical conductivity varies agreement with the negative fifth root of the oxygen pressure, and the cation excess with theory is relatively good. Other oxides which possess structures are BeO, Fet O s , MoO s and SnO t .

22.17. Dependence of Oxide Growth Upon Time Various weight change versus time relationships exist for oxidation oxidation-time relation- rates. Fig. 22.17 shows a composite graph of the are the linear ships. The two most important of these empirical relations and parabolic equations. found The linear relationship is the simplest equation that has been to express experimental data. The equation Am = kt linear relates the change in mass per unit area with time; k, is called the rate constant. No other constant is necessary in this linear equation because at t= o, [\m = o. Another form of this relationship is

Ax = k,*

layer. These relations where Ax is the change in thickness of the oxide have been found to apply when the molar volume of the oxide phase is less than the molar volume of the parent metal which reacted. Physically, these oxides cannot cover the entire metal surface; fresh metal surface is 478 Engineering Metallurgy

continually exposed and the oxidation rate is quite constant. Common metals which conform to this type of oxidation are lithium, sodium and magnesium. Pilling and Bedworth expressed their criterion of linear vs parabolic oxidation rate by the following relation:

Mi >. Dm

where M = molecular weight of oxide phase, D = density of oxide phase, m = atomic weight of metal phase, d = density of metal phase.

When the above ratio is less than unity, the linear rate law is obeyed;

but when the ratio is greater than unity, the parabolic rate law is called for.

That is to say, if the molar volume of the oxide is greater than the metal which reacted (but not too great to crack off under stress during growth), a protective film would be expected. A simple statement of this situation would be that the increase in film thickness would be inversely

proportional to the actual film thickness at any time. For example, if the following equation were integrated

d(Ax) = kp 1_ dt Ax to Ax* = 2k p t we have a statement of the parabolic expression for rate of oxidation. This "law" was enuciated by Pilling and Bedworth and frequently bears their name. A similar expression for the change of mass of an oxide film can be written by substituting for the Am Ax and changing units kp accordingly. Fig. 22.18 shows typical oxidation data which follows the parabolic equations. Protective films are found on many metals and alloys and are highly desirable. Examples of these are aluminum, chromium, copper, beryllium, titanium, and iron. Lastly, some oxidation rates have been observed that are faster than the parabolic rate at first but tend to protect the parent metal very well. Examples of these rates are the cubic and logarithmic expressions which follow:

3 (Ax) = k c t (cubic) and

Ax = A'log t -+- b (logarithmic) Corrosion and Oxidation 479

16-

* 6

•00'

10 12

Aija Fig. 22.18. Oxidation of oxygen-saturated titanium. Taken from M. Simnad, Spilners and 0. Katz, Trans. A. I. M. E. 203, 645 (1955).

These expressions are commonly used to represent data on very thin oxide film formation at relatively low temperatures. At intermediate temperatures most metals tend to behave parabolically, and at high temperatures the linear rate is often observed. Engineering choices of metals for a particular operation often necessitate the gathering of data on oxidation resistance in the temperature interval desired and under the exact service conditions of gas composition, moisture, pressure, and surface preparation.

22.18. Dependence of Oxide Growth Upon Temperature

The temperature dependence of oxidation rate is generally the same form as that for diffusion and ionic conductivity. The following ex- ponential relation applies for a single oxide in a given temperature interval.

k = l-^r'l

In this expression, k is the experimental rate constant (linear, parabolic,

cubic, etc.) and C is a constant having the same units as k. A term called

/ 480 Engineering Metallurgy

the experimental activation energy is given by A£„, while R is the gas constant (1.986 calories/mole degree) and T is the temperature in absolute units (Kelvin) . Information about the mechanism of oxidation and electrical conductivity can often be gained by comparing the experimental activation energy for diffusion of a known species with the activation energies for oxidation or electrical conduction. If the values for are A£„„ identical, one may generally assume that a diffusing species accounts for a given oxidation rate or an experimental specific conductivity. Many accurate data are needed in this particular method of de- ducing the mechanism of metallurgical reactions. the On other hand, some metals and alloys tend to oxidize very rapidly upon reaching certain temperatures. It has been observed that the temperature at which "catastrophic" oxidation occurs frequently coin- cides with the formation of liquid oxide phase, generally a binary or ternary eutectic. Severe attack of this type may actually result in a weight loss of the oxidized specimen, because the oxide may volatilize appreciably.

22.19. Oxidation Prevention

Gold and platinum are the only metals which will not form oxides in air at room temperature. Silver, palladium and rhodium oxides thermally decompose to the metals and oxygen gas at 190, 800 and" 1 100°C, respectively. In some instances it is possible to use one of the noble metals at a temperature where its oxide simply cannot form. However, high costs prohibit such applications. Cladding base metals with precious metals has been attempted, but oxygen diffusion through the precious metal coating is fast enough at elevated temperatures to oxidize the base metal internally. This kind of oxidation results in an oxide layer formed between the base and the precious metal.

One way to retard elevated-temperature oxidation of carbon steels and low-alloy steels is to heat them in a neutral or reducing gas. Such precautions, although expensive, are used in annealing low-carbon sheet and wire to prevent discoloration or the formation of an oxide coating of appreciable thickness. If it is necessary to prevent oxidation of medium- and high-carbon steels by heating in a neutral or reducing atmosphere— as is sometimes the case in heating treating tools—precautions must be taken to prevent decarburization of the surface layers. Removal of carbon from the surface with the production of a soft outer layer is sometimes more of an evil than scale formation.

The common method of protection of a metal or alloy is by the formation of a protective oxide film which inhibits further oxidation. In Corrosion and Oxidation 481

possible to select designing a part for a particular operation it may be after a thin film of oxide has a particular material that will protect itself India. This formed. The classic example of this is the Delhi pillar of about 6.5 tons, was mass of wrought iron, some 24 ft. high and weighing about A.D. 300. made by hand welding small blooms. It was erected atmosphere, it had Owing to a "tight" oxide coating and a pure, dry years, but when Had- resisted corrosion remarkably well for about 1600 moistened the surface, field brought a piece of the pillar to England and

it rusted badly overnight. and Oxides formed on pure metals are relatively simple in structure have composition when compared with the oxides formed on alloys which micro- many elements in solid solution or have several phases in their oxide closest structure. When several oxide phases form, as on iron, the while the oxide which to the metal, has the highest per cent of the metal of metal. Thus, forms at the external surface has the lowest per cent layer of FeO, when iron oxidizes at elevated temperatures, an innermost outermost layer of Fe2 3 appear. and intermediate layer of Fe3 4 , and an separately When alloying elements are present, their oxides may appear denote metals and or as special oxides of the type A„B,0, where A and B Oxidation resistance of a pure metal x, y and z are small whole numbers. alloying elements. can be increased or decreased by the addition of steel Additions of chromium, nickel, aluminum, and silicon to iron and oxides protect the serve to increase the oxidation resistance, because the be parabolic. base metal. In these cases the oxidation rate tends to Boron, molybdenum, and vanadium markely decrease oxidation resistance In dilute copper of steels because the resulting oxides are nonprotective. resistance to alloys beryllium, aluminum, magnesium, and silicon increase oxidation while titanium, cobalt, nickel, and manganese seem to have

little effect. High chromium, usually together with high nickel or high cobalt, is Typical necessary for resistance to oxidation at elevated temperatures. nickel costly alloys include low-carbon 18 per cent chromium, 8 per cent per cent steel; 10 to 35 per cent chromium steel; 35 per cent nickel, 15 alloy; a 62 chromium steel; an 80 per cent nickel, 20 per cent chromium the per cent nickel, 15 per cent chromium, 23 per cent iron alloy; nickel-molybdenum alloys of the Hastelloy type; the cobalt-chromium life at alloys of the Stellite type, and a number of others. All have long temperatures of 1300 to 1800°F. (700 to 980°C.) or even higher and plants. are used in oil refineries, power plants, furnaces, and chemical Type A typical oxidation study yielded the results shown in Fig. 22.19. Cr, were 410and 430 stainless steels, having 11 per cent and 16 per cent 482 Engineering Metallurgy

too TME M HOURS

Fig. 22.19. Oxidation of types 410 and 430 stainless (electropolishcd) at 1600"F in moist and dry air. Taken from D. Caplan and M. Cohen, Trans. A. I. M. E. 194 1057 (1952).

oxidized at 1600°F. in wet and dry air. The wet air caused rapid attack on both steels in comparison with the dry air, and in both cases the oxide formed on the 16 per cent Cr steel was more protective than that on the

1 1 per cent Cr steel. One general feature in tailor-making an oxidation-resistant alloy is to add an alloying element which has a very high affinity for oxygen and whose oxide has very low electrical conductivity. Such oxides either have few ions free or able to migrate or have structures which hinder diffusion. In either case, the oxides formed tend to be protective.

Another way of protecting a base metal is by cladding it with another metal which oxidizes very slowly. Cladding techniques include:

1. Metal spraying. 2. Hot dipping. 3. Roll cladding. 4. Casting. 5. Electroplating. 6. Cementation. Corrosion and Oxidation 483

application While all of these methods are not generally suitable for the are of a single metal on a base metal substrate, frequently two or more used with success. The use of nickel clad steel results in almost mirror- like surface finishes after a year or longer of operation near room temperature. Studies on the oxidation of nickel-clad steel showed that the oxide formed was invisible to the naked eye. Chromium plating is as effective as nickel, which is generally used as an undercoat for chromium plated objects. Aluminum-coated irons have been shown to be 30 times more resistant to oxidation than noncoated irons, and aluminum is one of the most versatile metals to clad with. Every clading technique is Copper, used to a greater or lesser extent with aluminum-clad products. extensively zinc, lead, brass, bronze, palladium, and rhodium are all used for cladding to hinder surface reactions.

QUESTIONS

1. Why do metals corrode in electrolytes?

2. What is meant by anodic polarization? corrosion 3. Describe an example where two of the eight methods of combating arc used simultaneously. in a plant consists 4. A common piece of equipment used for heating water the of a shell-and-tube exchanger with water inside the tubes and steam in iron sheets are used. shell. In many cases copper tubes with thick cast tube Why? prevent inter- 5. Describe in detail procedures and equipment needed to granular corrosion of a large tank (for a railroad tank car) fabricated by welding 18% Cr-8% Ni stainless steel (not stabilized). or not a given 6. Describe a test you would devise to determine whether material would be susceptible to stress-corrosion (cracking) in a given environment. indicating difficulty 7. Sketch a heat exchange system using liquid sodium a that may arise because of "mass transfer." involving heat and 8. Why are the stainless steels often used for applications oxidation resistance? "graphitiation" 9. Based on the electrochemical theory of corrosion, why docs of grey cast iron occur? stabilized stainless 10. Describe the mechanism of ••knife-line" attack of the steels. structure, (b) cation 11. Describe a defect lattice which is a (a) cation deficient excess structure. their oxides. these 12. The following data arc given for several metals and Do metals oxidize linearly or parabolically? 484 Engineering Metallurgy

(A) Metal Atomic 3 Weight Density, g/rm , 25°C Al 26.97 2.70 Be 9.02 4. SO Cu 63.57 8.92 Pl> 207.21 11.34 Mg 24.32 1.75 Na 22.99 0.97 Zn 65.38 7.H (B) Metal Oxide Molecular Weight Density, g/cmS, 25°C A1L,03 101.94 3.7 BeO 25.02 3.0 CiigO 143.14 6.0 CuO 79.57 6.4 PbO 223.21 9.5 MgO 40.32 3.7 Na,0 61.99 2.3 ZnO 81.38 5.5

13. Calculate the mobility of electrons in a mole of pure solid copper at 25°C, if the specific conductivity a = cze^, where a = 10» ohm-' cm-*, c = concentration of carriers, z = valence of carriers, e = electronic charge and /i = electron mobility.

14. Test the following data for "fit" to the parabolic, linear, or cubic rate laws for the oxidation of metal A.

Oxide Thickness, A time, minutes

500 10 1000 40 2000 160 15. If the experimental activation energy for oxidation of Zn to ZnO is 28.5 Kcal/mole and the frequency factor, C, is 2.8 x 10-» cmVsec, what is the value of the oxidation rate constant at 400°C?

16. Describe in your own words the Pilling and Bedworth rule of oxidation. 17. Are metal oxides better or poorer electrical conductors than metals? Explain your reasoning.

18. What is catastrophic oxidation? 19. If a first order interference color of yellow is seen on a slightly oxidized tool steel, estimate the oxide thickness. Assume a representative blue radiation of 4700 A, and that first order extinction of blue occurs when

-. . when is A" = — A the wave length being destroyed and v is the index

of refraction of the oxide film (^ ~ 3). Effect of Temperature on MechanicalProperties of Metals CHAPTER

James R. Macdonai.d, Ph.D., Chairman, Department of 23 Mechanical Engineering, School of Engineering, The University of Mississippi, University, Mississippi George V. Smith, Ph.D., Assistant Director for Metal- lurgical Engineering, School of Chemical and Metal- lurgical Engineering, College of Engineering, Cornell University, Ithaca, New York

FOR centuries man's only interest in the reactions of metals to stresses at elevated temperatures was related to the reactions of the metals to stresses used to form them into usable shapes. subjected to He learned that at elevated temperatures metals could be repeated instantaneous stresses of forging, rolling, or drawing without fracturing easily. Interest in metals for extended service at elevated temperatures had its short- beginning in 1924. Prior to that time most studies were confined to to study time high-temperature tests, for at that time there was no need increased the effects of long-time stress at high temperatures. But with the temperatures and pressures being employed in the steam-power generating industry, problems that were not understood began to develop. Equip- ment operating at high temperatures and presumably designed conserva- carbon tively with an ample factor of safety warped and even failed; the deforma- and low-alloy steels being used, instead of exhibiting only elastic tion as well-behaved steels should, deformed plastically under surprisingly low loads. At normal temperatures, most high-melting point metals and alloys plastic deform elastically under working loads. There is no appreciable exceed deformation (permanent set) so long as the applied load does not subjected at the elastic limit. Furthermore, most metals and alloys when normal temperature to stresses above the elastic limit—as in cold working —strain harden; in other words, permanent deformation distorts the grains and increases strength and hardness. This also holds true at elevated temperatures so long as the temperature is below that at which rapid recrystallization takes place. 485 486 Engineering Metallurgy

As is evident from this introduction, the problem facing the metallurgist and engineer was a phenomenon involving small amounts of plastic deformation over long periods of time at elevated temperatures. This phenomenon is called creep. Entering importantly into the problem of

creep is strain hardening, which is opposed to the softening that occurs during recovery and recrystallization. Recovery and recrystallization depend upon time, temperature, prior plastic strain and the previous structural condition of the material. For steel, the recrystallization temperature ranges from about 700°F. to the lower critical temperature.

23.1, The Importance of Creep

Dickinson demonstrated in 1922 that steel deformed slowly at a red heat under stresses which were much lower than the values used in what was then considered good design. The connection between this slow deformation at high temperature and the failure of certain power- generating equipment was established, and the importance of creep was recognized. As a result, the American Society of Mechanical Engineers and the American Society for Testing Materials set up a joint committee in 1924 to study the phenomenon, to devise a method for determining slow deformation, and to accumulate useful data on slow deformation of industrially important metallic materials. This work has spread from metals to ceramics and has develoj>ed considerable interest in a new series of materials which are produced from mixtures of metals and ceramics known as cermets. Laboratories are continuing to study creep, and although many data have been accumulated, much more needs to be done.

23.2. The Engineering Significance of Creep

In the selection of a steel or other alloy for high-temperature use the mechanical properties of the material as determined at room temperature are of little importance. The requirements for metallic material which is to be used in various kinds of equipment operating at high tem[>eratures vary widely, but in general the following are important: (1) The metal should be resistant to oxidation or other attack by the hot gases (including air) which are present; (2) it should have a stable structure which will remain relatively unchanged under the stress and temperature conditions imposed; (3) it should withstand the stress imposed with a negligible or at least with a harmless amount of distortion for the useful life of the equipment; and (4) it should withstand momentary overloads at the usual operating temperature, or short periods of fluctuating temperature at Effect of Temperature 487 normal loads. There are few metallic materials which meet all these requirements. It is, therefore, not easy for the metallurgist to supply the engineer with materials suitable for equipment operating at high temperatures and fairly high loads. This difficulty is not lessened by the fact that engineers are usually uncertain about the exact loads and the exact temperatures in the equipment they design and use.

The steam-power generating industry is an excellent example of the trend to higher temperatures during the past years. In 1905, the initial temperature of commercially used steam turbines was only 500°F. In a two-year period from 1906 to 1908 it was raised 100°F. and it remained at 600°F. for nine years. In the next three years (1917 to 1920) it was raised another 100°F., but it was not until 1926 that the temperature attained 750°F. Making use of the accumulated high-temperature data manufacturers increased the steam temperature to 950° by 1938. During World War II there was no change because of the scarcity of critical materials, but shortly thereafter the temperature was increased to 1 100°F.

Certain units in the petroleum industry have always been operated at metal temperatures in excess of those employed by the steampower industry. The ability to do this is based on shorter expected metal life, shorter operating cycles, and more frequent inspection, with opportunities of replacement prior to fracture of the parts operating at the higher temperatures. In fact, prior to 1930, operation of the thermal cracking units was generally of the intermittent-coking type, so that the cycles were very short, often only 40 to 50 hours. Today the length of the operating cycle is often measured in months, or years, rather than in hours. Develop- ments in high-temperature alloys have contributed to this ability to use longer operating-time periods, which in turn have greatly increased production because of less down-time of the units.

General conclusions with respect to temperature trends within the chemical industry cannot be drawn because of the wide variety of products manufactured by the different companies, and often within the same organization. The requirements of this industry do extend from moderate temperatures and extremely high pressures to very high metal temperatures (1800 to 2000°F. (980 to 1095°C.)) with low pressures.

During World War II there was an ever-increasing demand for stronger and stronger alloys that could be used at higher and higher temperatures. The exhaust turbosupercharger for aircraft engines involved rotor tem- pertures up to 1350°F. (730°C.) and blade temperatures to 1500°F. (815°C.) or even higher. The requirements for engines in present-day jet planes are somewhat lower. The expected life in these applications is 488 Engineering Metallurgy

currently relatively short, a fact which helps to simplify the metallurgical asjjects of the problem. The temperatures in gas turbines for central sta- tion power generation are somewhat lower and the expected life very much longer. Another application, just beginning to acquire importance, that

may require alloys that can be employed at very high temperatures is the commercial use of nuclear energy for generating power.

c /\ ,s z ELEVATED D r EMPERJ TURE < J_ // Uip ^Xi/ TE

( 1

0-B ELASTIC DEFORMATION AT EM 'ERATURE ' A ROOM A-C PLASTIC DEFORMATION. OR CREEP A-0+ CREEP CURVE A-D FIRST-STAGE CREEP F FRACTURE - D-E SECOND-STAG-E CREEP E-F THIRD-STAGE CREEP u i i i i . i i 1 1 1 1 1 1 TIME OF STRESS APPLICATION

Fic. 23.1. Diagrammatic sketch of elastic versus plastic properties.

23.3. The Creep to Rupture Curve

From Fig. 23.1 the complete creep curve under constant temperature

and constant load is seen to consist of three stages. The portion of the

curve AD, known as the first stage, represents a condition of continually decreasing creep rate; section DE, the second stage, is one of substantially

constant creep rate; and EF, called the third stage, is one of continually

increasing creep rate and ends in fracture (rupture) . The total deformation to fracture in this case would be represented by OA, the elastic deformation, plus AC, the plastic deformation. When tests are made at other loads (holding temperature constant) or at other temperatures (holding load constant) , a family of curves, depicted schematically in Fig. 23.2, is obtained. At relatively high temperature and load, the period of first-stage primary creep may be virtually absent; whereas, at relatively low temperature and load, creep may cease, for practical purposes. The characteristic form of the creep curve (Fig. 23.1) led early investigators to suggest an interplay between the forces of strain hardening Effect of Temperature 489

Increasing temperature (constant load) Increasing load (constant temperature)

Time

Fic. 235. Schematic illustration of creep at low and high temperatures and loads. and strain softening, the latter arising from crystal recovery or in occasional instances from recrystallization. According to this view, hardening predominates during first stage or primary creep, and the creep rate diminishes with time; whereas, during second stage creep, the forces of hardening and softening balance one another, and the rate of creep is essentially constant.

This conception of the creep curve is, in principle, still currently accepted, with refinements, however, as regards the nature of the plastic flow processes and crystal recovery. The tertiary period of accelerating creep (third stage) is clearly associated with impending fracture. The mode of fracture at elevated tem-

peratures may be either transgranular, i.e., through the grains, or inter-

granular, i.e., between the grains. Transgranular fracture, which is the predominant mode of fracture of metals at ordinary temperatures, is less likely to occur, the higher the test temperature and the slower the rate

of deformation (or the longer the time for rupture) . The intergranular

mode of fracture is characterized by equiaxed rather than elongated grains, by intensive intergranular voiding, by a minimization of local "necking" deformation, and often by relatively low over-all strain at frac-

ture. Hence, little or no advance warning of impending fracture is afforded.

23.4. Determination of Creep The knowledge of what a metal might do over a short period of time cannot be projected to cover a long period of time. Much effort was wasted in the early work on determining the reactions of alloys to stresses at elevated temperatures for longer periods by attempts to find a rapid

method. It took considerable time and effort to discover there was no easy short cut. However, determination of short-time, high-tempcrature 490 Engineering Metallurgy

Kir.. 23.3. Modern creep-testing equipment. The electric furnaces for heating the specimens arc shown at the left (white): the lever system for loading the specimens is above the furnaces. (Batlelle Memorial Institute)

tensile strength, which is a simpler test than the creep test, and of the time to rupture in relatively short times, as determined in creep to rupture tests at high stresses are usually undertaken, for the data are useful in deciding whether an alloy is suitable for parts, as in rockets, that would be subjected to high temperatures for short periods, or for parts that would be under maximum stress or maximum temperature only during the event of failure of other parts. Such data also point out which alloys justify further investigation where the maintaining of stresses for longer periods at elevated temperatures is required. The American Society for Testing Materials has adopted recommended practices for conducting creep tests, E22-41, and time for rupture tests, E85-50T. These standards place restrictions on temperature distribution and variation, on the sensitivity of the extensometer system, and on the accuracy of the loading mechanism. Specimens are usually maintained under constant load for at least 1000 hours, and the amount of creep is determined periodically during the test. The difference in deformation between any two points divided by the difference in time of the two points represents the creep rate at the Effect of Temperature 491 average of the periods of time. The stress that will result in an average creep rate equal to an assigned creep rate is known as the creep strength for the alloy at the specified temperature. The stress causing rupture at a given temperature in a specific time is known as the rupture strength. The equipment necessary consists of an electric furnace capable of close temperature control and a suitable device for holding the specimen in exact alignment in the furnace. The load is applied by dead weights through a system of levers. A battery of modern creep-testing furnaces and their auxiliary equipment is shown in Fig. 23.3. Test specimens are usually 0.505 in. in diameter (cross section 0.2 sq. in.) and have a gage length of at least 2 in. From the data accumlated in a series of creep and rupture tests at different stresses and temperatures, the information necessary for selection of working stresses is obtained. Fig. 23.4 shows the temperature variation of the stresses necessary for 1 per cent creep at several time intervals in comparison with the temperature variation of the yield and tensile strengths determined in short-time tensile tests.

23.5. Effect of Variables on Creep

Creep is one of the most structure-sensitive properties of metals and

is, therefore, greatly influenced by many factors that have little or no influence on the other characteristics. For example, a series of steels having practically the same tensile properties and hardness at room temperature may shoiv wide differences in their creep resistance at higher temperatures.

Owing to the complexity of the subject, information is not avail-

able concerning all the possible variables and it is probable that, as yet, not all of these are known. Certain information is available regarding many of them and among those to be considered are chemical composition, heat treatment, grain size, method of manufacture, previous deformation, and structural instability. Chemical composition. In general, creep and rupture strengths of metals increase with increase of the melting point. Thus, lead creeps significantly at room temperature, iron at perhaps 800°F., and molybdenum at perhaps 1500°F. In any base, alloying additions are effective in further strengthening a metal. The specific effect of an alloying addition

depends on whether it enters into solid solution or results in the appearance of an additional phase. In general, greater strengthening can be effected by dispersed secondary phases than by formation of a solid solution. 492 Engineering Metallurgy

Temperature, deg. C.

POO 800 900 1000 1100 isoo Temperature, deg. F.

Fig. 23.1. Effect of temperature on short-time tensile properties and on creep stress of cast carbon steel. (Kanter)

Low alloy steels, when appropriately heat treated, are in general most effectively strengthened by alloying additions of so-called strong carbide- forming elements, like molybdenum, vanadium, and tungsten, the first two of which are extensively used for this purpose. Chromium is widely employed to improve scaling resistance, but it is deleterious to creep strength beyond about 1.5 per cent. Molybdenum is also an effective strengthener of the austenitic chromium-nickel stainless steels. The ultra- high strength alloys currently in use for high-temperature applications often comprise a solid-solution base composition, containing, for example, cobalt, nickel, chromium, and perhaps iron, with a dispersal phase or phases resulting from alloying additions of such elements as columbium, vanadium, molybdenum, and tungsten. Heat Treatment and Microstructure. The effects of alloying additions cannot be considered without reference to heat treatment, whereby con-

trol is exercised over the grain size and the nature and dispersion of the precipitate phases. At the more elevated temperatures, creep and rupture strengths are greater, the coarser the structural grain size. However, there

is some evidence that an optimum grain size exists for each temperature and strain rate or rupture time, and it seems unwise to generalize too broadly. Similarly, fine dispersions generally appear stronger at relatively low temperature, and coarser dispersions stronger at higher teni]>eratures, but exceptions exist. Effect of Temperature 493

70 000

60 000

10' 10" Stress Cycles -

Fig. 23.5. Effect of temperature on fatigue of 12 Cr steel. (Welch and Wilson)

Other Variables. The methods employed in manufacturing a metal may exercise indirect but important effects. Included in this category are melting process, deoxidation practice, casting temperatures, etc., variations of which result in variations of chemical composition and microstructure. For example, a higher casting temperature is conducive to coarser grain size. Strain hardening by prior cold working or by warm working may be useful for increasing creep and rupture strengths for service temperatures below the temperature range of recrystalli/ation. The microstructure of metals and alloys used at elevated temperatures often changes during service with consequent effect on properties and be-

havior (see Section 23.7) .

23.6. Fatigue and Combined Fatigue and Creep

In general, as temperature increases, fatigue strength progressively decreases. At the same time, the slope of the curve of stress vs. number of cycles becomes steeper, and, even for metals like steel, the curve does not exhibit a horizontal portion corresponding to the "endurance limit," Fig. 23.5. Moreover, all environments, including air, become corrosive for the usual structural metals and lower the fatigue strength accordingly. An exception may occur to the general rule of decrease in fatigue strength with increasing temperature when the temperature range is one in which metallurgical changes occur that strengthen the metal. Such may be the case for certain steels which undergo strain-aging at tempera- 494 Engineering Metallurgy

Sites* to Mean Stress

5 10 15 20 25 Mean Stress. 1000 psi

Fie. 23.6. Stress combinations Tor 0.2% and .5% creep and for rupture of 24S-T4 aluminum at 500°F.; cycle stress applied at 3600 times per minute. (ASM Metals Hand- bonk, 1955 Supplement, p. 184.)

tures up to 600-700°F., so that the fatigue strength of such steels in this

temperature range is not much different than at room temperature. At temperatures sufficiently high that creep occurs under static loads, superimposed cyclic stressing results in complex behavior involving

both creep and fatigue phenomena. Whereas there is an apparent absence of plastic deformation in fatigue fractures at ordinary temperatures, fracture at elevated temperatures under cyclic stress may show both creep deformation and the conchoidal markings that characterize fatigue. Behavior under combined fatigue and creep conditions cannot be predicted other than approximately from separately obtained creep and fatigue data. Moreover, since creep and fatigue properties vary differently with temperature, behavior under combined conditions cannot be predicted for other temperatures from data obtained at one temperature. Hence, it is necessary to conduct tests involving combined fatigue and creep). Owing to the elevated temperatures of testing, certain variables become more important than in fatigue at low temperatures. These include the effect of corrosive agents in the environment, the changes in microstructure that may occur {see Sect. 23.8) , and since creep is time- dependent, the frequency of cycling.

Typical results of combined fatigue-creep tests, are illustrated in Fig. 23.6 for the aluminum alloy 24S-T4 tested at 500°F. The ordinate scale Effect of Temperature 495 charts the alternating stress in completely reversed loading, that is for zero mean stress, whereas the abscissae axis corresponds to static loading, are both that is, for zero alternating stress. Within the quadrant, there relatively a steady stress and a superimposed cycling or fatigue stress. At will low mean stress, fatigue conditions predominate and fatigue failure tend to determine working stress, whereas at low alternating stress, behavior corresponds more to creep conditions, and working stress will be prescribed by excessive creep or creep-rupture. Fig. 28.6 indicates by the perpendicular approach of the data curves to the axes that the super- imposition of a small alternating stress on a given mean stress or of a small steady stress on an alternating stress does not appreciably lower the stress that can be supported.

23.7. Structural Changes During Creep

Most metals and alloys undergo changes in microstructure during heating at elevated temperatures. The changes cause changes in properties, ¥9

' " ' • • ^ \ -• v v * •

AS POLLED AFTER JQOOOHR. SERVICE C-A40 STEEL IOOOX

Fig. 23.7. Carbide spheroidizalion in steel containing 05% molybdenum, (U.S. Steel Corp. Research Laboratory.) 496 Engineering Metallurgy

ORIGINAL HOOF jgOOF I500F ' m?m ' m i

XIOOO

' L • i: 1 ty\ ORIGINAL HOOF I300F I500F

Fig. 23.8. Microstrucniral changes in Cr-Ni-Mo austenitic siainless steel during 3000 hours creep tcsi. (U. S. Steel Corp.)

and hence, the material has different properties at different times after being put in service. The rate at which the changes develop is principally dependent upon the temperature and the time, but may also be influenced by the imposed stress and the resulting strain. Large stresses and plastic strains may greatly accelerate microstructural changes, whereas small stresses and strains may not cause detectable differences from unstressed material. The kinds of readily observable changes in microstructurc which may occur are varied, and include carbide spheroidization or graphiti/ation in steels, recrystallization, grain growth and transformation or precipitation processes of various kinds. Other changes such as strain-aging, temper embrittlement of steels, 885°F. embrittlement of high-chromium ferritic steels are not readily, if at all, detectable in the microstructure.

In ferritic steels, carbide spherodization during service is widespread as the temperature exceeds perhaps 900° F. A well-advanced example is shown in Fig. 23.7 representing the microstructures of i/£ per cent molybdenum—0.15 per cent carbon steel before and after 30,000 hours of service at about 1 100°F. in a petroleum refinery. Clearly the properties of the steel before and after service would be different. In some steels the carbide phase may break down to elemental carbon (graphite) and iron. Microstructural changes during creep test of an austenitic Cr-Ni-Mo stainless steel are shown in Fig. 23.8. In this material carbide and chi phases, not differentiated in the illustration, precipitate. In other Effect of Temperature 497

dependence. (Hay- Fie. 23.9. Thermal conductivity of various metals— temperature thorne)

COEFFICIENTS OF THERMAL EXPANSION

COEFFICIENTS ARE MEAN VALUES FROM 70T TO .000012 . INOICATEO TEMPERATURE i

1 .00000;

0O0OOT

.000004

.000005 JOO 400 800 MO «O0 BOO 1400 TEwpepaTum - -f

Fie. 23.10. Coefficients of thermal expansion-temperature dependence. (Michel)

austenitic stainless steels, or in high chromium ferritic steels, sigma phase may precipitate. The microstructural changes that occur during exposure at elevated temperatures generally, but not necessarily, cause a deterioration in strength as measured at elevated temperatures. Often the most profound 498 Engineering Metallurgy

effects are observed in loss of toughness at room or lower temperature, and as a result the material may fail during shut-down periods rather than at elevated temperatures. In a particularly severe instance the notched-bar impact energy absorption of austenitic stainless steel exposed

at 1500°F. (Fig. 23.8) , was lowered to 10-15 foot lbs.

23.8. Variation of Other Properties with Temperature

A number of properties of metals other than strength may be of interest in the design and operation of equipment for service at elevated temperatures. Among such properties are thermal conductivity, thermal expansivity, clastic modulus and thermal shock or thermal fatigue.

AUSTENITIC STEELS FERRITIC STEELS leCi-BNI 13041 0.10 CARBON 3CY-0.5Mo-l.5S l8Cr-8Ni-Mo<3l6> 1, 00 CARBON 3Cf-0.5Mp-l.5Si iec«-aNi.Ti (321) 0.SM0 5Cr-0.5Mo-Cb I8C'-8N1-CM347] 0.8C'-0.3MO 5C'-0.SMO-TI 25Cr-l2NI 13091 ICr-0.5Me 9Cr -IMo 25O-20NU3I0) 2Cr-0.»M0 l2Cf (410) 2 25Cr-IMo ITCr (430)

AOSTEMTiC STEELS"

600 800 1000 1200 TEMPERATURE - F

Fig. 23.11. Young's modulus E of various materials—temperature dependence. (Michel)

Although thermal conductivity and expansivity vary with variation in chemical composition, they are relatively unaffected by other metallurgical variables such as heat treatment and microstructure. The variation with temperature of thermal conductivity of a representative group of metals

and alloys is shown in Fig. 23.9. Fig. 23.10 shows a corresponding chart for thermal expansivity. Of particular interest from the practical point

of view is the disparity between the expansion coefficients of carbon steel and austenitic stainless steel, which leads to "thermal" stresses when the materials are joined and heated, as they sometimes are. The austenitic

steels also have significantly poorer thermal conductivity at relatively low Effect of Temperature 499 temperature; at relatively high temperatures different steels possess approximately the same conductivity. The elastic moduli at elevated temperatures are sometimes needed for computing elastic strains. These constants may change gradually with composition, but are relatively insensitive to metallurgical structure. With increasing temperature, both the tensile modulus (Young's) and shear modulus decrease. The temperature variation of Young's modulus of for a representative group of metals is shown in Fig. 23.11. The rate level decrease is approximately linear up to a certain temperature which depends upon the material, beyond which a more rapid decrease occurs. Poisson's ratio does not change significantly with temperature. Thermal shock and thermal fatigue represent a common mode of failure in applications involving relatively rapid changes in temperature. Thermal shock and thermal fatigue may be differentiated depending on whether failure occurs after relatively few or many cycles, respectively, of heating and cooling. There are no standardized tests by which to measure these properties, and resort is had to special tests which attempt to simulate actual service conditions. The stresses giving rise to thermal shock or fatigue are influenced by such properties as thermal expansivity, thermal conductivity, strength and toughness and by the rate of change of temperature and the geometrical shape of the part.

23.9. Design for Elevated Temperature Service

Although a number of properties will be of interest in the design of equipment for successful operation at elevated temperatures, the govern- ing criteria, at least where corrosive conditions are not severe, will generally be the strength. Owing to creep phenomena, however, strength

at elevated temperatures is not a simple property. From the very fact equip- that creep is occurring, the design engineer recognizes that in time, ment will either distort beyond a tolerable limit or will fracture or both.

It is necessary therefore to choose working stresses such that neither of these events occurs. The amount of deformation that may be tolerated depends on the specific application; in a steam turbine it may be only a fraction of one per cent in 100,000 hours, whereas in steam piping, several per cent deformation in the same interval may be tolerated. In all applications, of course, the working stress must not be large enough to cause fracture. In all equipment designed to operate under creep conditions it is necessary to design for a specific life, not for unlimited life. The basic information relating to the amount of deformation as a function of lime comes from creep tests, which must be made under dynamic conditions if 500 Engineering Metallurgy

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Fic. 23.12. Design data for alloy containing 20 Cr-20 Ni-20 Co-3 Mo-2W-l Cb-Bal. le, 1200°F. (ASM Handbook, 1955 Supplement) service involves superimposed cyclic stresses. The basic data for design against fracture comes from creep to rupture tests, from which it is possible to chart time for rupture as a function of stress. Although there is a general relation between the stress for a specific amount of creep in a certain time and the stress for rupture in a specific time, it is not always possible to insure against fracture by insuring against excessive deformation, because the amount of deformation at fracture at elevated temperatures may be less than that which the designer may be willing to accept in a specific application. Moreover, design of equipment for long-life service necessarily involves extrapolations, with attendant uncertainties as to when creep will enter the third or accelerating-rate state. A useful form in which to summarize creep and rupture data for the design engineer is in the form of "design charts," an example of which is given in Fig. 23.12. Such charts show the variation with stress of the time for several specific amounts of creep, the time for transition to accelerating-rate creep and the time for fracture, and from them it should be possible to select the working stress for specific applications. For certain general classes of construction, permissable working stresses may be set by Code bodies, an important one of which is the Boiler and Pressure Vessel Code Committee of the American Society for Mechanical Engineers.

23.10. Variation in Mechanical Properties at Reduced Temperatures

In addition to knowing the properties of metallic materials at elevated temperatures, the metallurgist and engineer should know something of the 501 Effect of Temperature

northern behavior o£ metals and alloys at reduced temperatures. In automobiles latitudes, the metals used for rails and railroad equipment, to and other machines are subject at times to temperatures as low as 50 merchant ships 60°F below zero. During World War II a number of sea or air temperatures. split into two sections while subject to low also encounter low Aircraft or guided missies operating at high altitudes importance also temperatures, and low temperatures are of increasing in the chemical process industry. know the properties In a number of fields, therefore, it is necessary to metals at these tempera- of metals at reduced temperatures. The testing of and techniques tures involves experimental difficulties, but equipment refrigerants have been developed for obtaining reliable test data, using nitrogen, liquid air or such as ice, dry ice (solid carbon dioxide) , liquid liquid helium. at room temp- Tensile test properties of a number of metals and alloys data show that erature and at -300°F are given on page 504. These decreases. The tensile and yield strengths increase as the temperature depends upon the nature effect of temperature on ductility, however, others it increases. The of the metal. For some, ductility decreases, for the Cr iron decrease in ductility of the carbon and nickel steels and 16% characteristic of ferritic are noteworthy and a manifestation of a general a transition in the steels to undergo, with decreasing temperature, ductile to one that is character of the fracture from one that is relatively relatively brittle. The temperature range over which the ductile to brittle transition state of stress and occurs for a specific ferritic steel depends upon the (acting across a refer- the speed of testing. The greater the normal stress in the reference plane) the ence plane) relative to the shear stress (acting , Consequently, in higher the temperature at which the transition occurs. have a high notched bar specimens which, compared to the tension test, sharpness of the notch, ratio of normal to shear stress, increasing with the higher temperature than low ductility will be experienced at appreciably ductile to brittle behavior in the tension test and, in fact, transition from increased speed of test- may occur at room or higher temperature. With brittle behavior occurs at higher tem- ing, the transition from ductile to perature. speed of testing, "impact- In line with the effects of stress state and notches has come into testing of specimens having "keyhole" or "vee" metals. Fig. 23.13 widespread use for evaluating the "toughness" of notched-bar impact energy to shows the variation with temperature of for an austenitic chromium- fracture for a number of ferritic steels and 502 Engineering Metallurgy

nickel steel which does not exhibit a transition from ductile to brittle fracture.

23.11. Effects of Metallurgical Variables

The transition from ductile to brittle fracture depends upon a number of metallurgical variables. The effects of these variables moreover may not be revealed by the simple tension test unless made at extremely low temperatures. It is possible therefore to have two heats of steel which are nominally identical as judged by chemical analysis or by tensile

properties, in tests at room temperature, but one of which is tough in

notch-bar impact test at room temperature, the other brittle. It is also possible to have two heats which exhibit similar notched-bar toughness at room temperature, one of which undergoes transition to brittle behavior at 50°F, the other at —50°F. It should be clear therefore that this charac-

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Fir. 23.13. Effect of low temperatures on the impact resistance of (A) water-quenched low-carbon 8 per cent chromium, 8 per cent nickel steel; (B) annealed low-carbon 3.5 per cent nickel; (C) annealed low-carbon 13 per cent chromium steel; (D) normalized 0.35 per cent carbon, 1.5 per cent nickel, 0.0 per cent chromium steel; (K) annealed

0.35 per cent carbon steel; and (!•') normalized 0.35 per cent carbon steel. Specimens for steels B and E had keyhole notch; others had V notch. (The International Nickel Company) 503 Effect of Temperature

can be teristic of metals and the effects of metallurgical variables thereon temperatures. properly studied only if tests are made over a range of to brittle The specific metallurgical variables that affect the ductile transition in ferritic steels include chemical composition, deoxidation practice, finishing temperature of hot working, heat treatment and the strain- resulting grain size and microstructurc, degree of cold work, and the same, aging or teni|>er embrittlement, if any. Other factors being de- transition temperature of steels increases with carbon content and generally creases with manganese up to one per cent or more. Nickel is low-carbon credited with being beneficial to low-temperature toughness of (Fig. steel; commercial steels are available containing 3.5% 23.13), 5% lower the and 9% of nickel with transition temperatures progressively greater the nickel. lower The transition temperature of deoxidized ("killed") steels is is the most than for undeoxidized (rimmed or capped). Aluminum but silicon is effective of the elements usually used for deoxidation, working, the lower beneficial. The lower the finishing temperature of hot working is bene- the transition temperature, and normalizing after hot decrease in grain size. Cold ficial. Transition temperature decreases with induce strain-aging which may work is detrimental in itself and may also the various micro- raise the transition temperature substantially. Of that of tempered structures which may be imparted to ferritic steel, tempering treat- martensite exhibits optimum toughness, provided the which has a deleterious ment is such as to avoid temper embrittlement, play a prominent role in effect on toughness. Since alloying elements can be ob- determining to what extent the fully martensitic structure toughness. 'tained, they thus may exercise an indirect effect on temperature is Transition from ductile to brittle fracture with lowered Although the an- by no means a characteristic solely of ferritic steels. -300°F sub- nealed, wrought austenitic steels exhibit toughness at a number of other stantially equivalent to that at room temperature, as do it is now recog- metals having the face-centered-cubic crystal structure, having this crystal structure, nized that most and perhaps all metals not tungsten do exhibit transition including zinc, titanium, molybdenum, and behavior.

23.12. Design for Low-Temperature Service temperatures, except for The problem of design for service at reduced essentially one applications in which corrosive conditions are severe, is not to say that other prop- of insuring against brittle fracture. This is not of interest, and in some cases erties, such as thermal expansivity, are controlling factors. 504 Engineering Metallurgy

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I 505 Effect of Temperature

tem- Since metals are stronger at reduced temperatures than at room data are perature, working stresses based on room temperature test adequate readily supported at low temperatures, if the material possesses concen- toughness to preclude brittle behavior in the presence of stress trations and fast speeds of loading. tensile stresses, Bi iuie behavior in metals depends on the presence of not be concerned with the and if these are negligible, the designer need are significant, problem of toughness. If, however, the tensile stresses transition from and the material of construction is one that undergoes designed so that its ductile to brittle behavior, the equipment should be lowest service transition temperature under service loads is below the since the transi- temperature. This is more easily specified than effected metal, and the range of tion behavior is not a simple property of a geometry and size of temperature over which it occurs depends upon the data obtained on the test specimen and speed of loading. Consequently, application to engi- small, laboratory test specimens are of questionable fracture has neering structures and the problem of design against brittle economical not been satisfactorily resolved insofar as choosing the most considera- material for a specific application. If cost is not an important against by selecting high-nickel, tion, brittle fracture can be easily guarded ferrous group, or ferritic steel or chromium-nickel austenitic steel in the assuming adequate one of the face-centered cubic non-ferrous metals, strength. QUESTIONS

to engineers? What are the principal What is creep, and why is it important 1. equipment requirements for a metallic material to be used satisfactorily in exposed to high temperatures? most important variables in creep 2. How is creep determined? What are the testing? How much time is necessary for a creep test? more than the maximum allow- 3. How is the creep stress, which produces no determined? How able distortion of the material over the expected life, relation of creep to other mechani- accurate is such a procedure? What is the cal properties? stages of creep. What is the effect 4. Describe what happens during the three - creep? of strain hardening and recrystallization on second-stage deoxidation? (c) precipita- 5. What is the effect on creep of (a) grain size? (b) tion hardening? (d) condition of the iron carbide? (at constant temperature) of carbon, 6. What is the relative creep strength high-chromium, and aus- plain molybdenum, chromium-molybdenum, plain strength of these steels tenitic chromium-nickel steels? How does the creep compare with the creep strength of the supcrstainless steeb? brass, admiralty metal, bronze, Monel 7. Compare the suitability of carbon steel, at temperatures o£ metal, duralumin, and the magnesium-base alloys for use 506 Engineering Metallurgy

500 to 800°F. (260 to 425°C), Compare the creep strength of nickel and Monel with Inconel and with the Hastclloy- and Stellite-type alloys.

8. What is the general effect of elevated temperature on the tensile strength, yield strength, elongation, reduction of area, and endurance limit of metallic materials? If a steel has a tensile strength of 120,000 lb. per sq. in. at room temperature, what will be the approximate strength at I200°F. (G50°C.)? at I100°F. (760°C.)?

9. What are stress-rupture tests? What is the significance of these tests? How does the rupture strength at I200°F. (fi50°C.) of austenitic chromium-nickel steel compare with that of superstainless steel and of the cobalt-rich nonferrous alloys?

10. Why is it important to know the effect of low temperatures on the properties of metallic materials? What is the usual effect of these temperatures on tensile and impact properties? What is primarily responsible for low-temperature brittleness in carbon steels, and what alloys reduce this brittleness? Index

147 Acid process, 216, 217, 225 cold and hot working, treatment, 147 Activation energy for oxidation, 480, 484 cold working vs. heat Afterblow, 219 composition, 138, 142 Ag-Cu system, 106 corrosion resistance, 150 free-machining, 136 Age hardening, 51, 52, 111 143-147 Aging, 107 function of alloy, Air cooling, 254 joining, 152 Air furnace, 424 number code, 136, 137 149 Allotropic transformation, 29, 51-52 precipitation heat treatment, against corrosion, 155 Alloy steels, protection classification, 241 riveting, 155 castings, 141 composition, 242 sand, mold and die high-quality production of, 226-227 solution heat treatment, 149 low-alloy composition, 319-326 tensile properties, 138 low-alloy production, 319 welding, 153 low-alloy S. A. E., 328-332 wrought, 136 Institute steel low-alloy similarity of properties, 332-339 American Iron and Steel 329-331 low-alloy structural, 326-328 specifications, precipitation hardening, 126-127 Amslcr test, 445 Alloys, controlled expansion, 385-386 Annealing of cast iron, 431 areas, 453 Alloys, for high temperatures, 198-202 Anodes and anodic 453-454 Alloying effects, 47-51 Anode reactions, Alloying elements, effect on carbon steel. Anodization, 151 320-322 Area effect. 457 vessel code Alnico, 201 A.S.M.E. boiler and pressure Annealing, alter cold work, 42-44 committee, 500 Alpha Stabilizers, 256 A.S.T.M. standards, 299, 300, Aluminum. carbon steel castings, 299, 300 chemical properties. 134 Atomic imperfections, 14, 15 deoxidation agent, 135 Atomic structure, 5 economics. 130 Aiistempering, history, 129 commercial limitations, 289 manufacture, 132 critical cooling rate curve. 286 nitriding agent, 135 defined. 285 strength of austempercd ores, 131 hardness and outstanding properties, 131 steel, 285 physical properties, 133 process. 285 purification, 133 to obtain greater ductility, 285 thermite welding, 135 Austenite, defined, 246 steel, 385 vs. steel, 130, 132 Austenitic manganese Austcnilic stainless steel. 256 Aluminum alloys, 1 17-123, alloying elements, 135 Austenitic steel, 374, annealing, 1-18 expansion of, 377 brazing, 152 mechanical properties. 377 casting, 140 sensitization, 376 507 508 Index

Babbit. 192 Carlion (Cont.) Bainitc, 253. 289, in cast iron, 408. 109 hardcnability, 257 solubility in iron, 216 Bars, carbon steel cold worked, 306 Carbon monoxide, in steel, 237-238 Basic process, 216, 219 Carbon steel, Bayer process, 131 castings, 298, 302 Bearing metals, 191 composition, 233-242 composition, 193 effect of alloying elements, 318-326 mechanical properties, 192, 194 clfect of carbon, 247-250 Beginning of transformation, 252 elFect of chromium and nickel, 323 Bend tests, low alloy steels, 77 elfect of copper, 325 Beryllium, effect of molybdenum. 325 copper, 117, 124, 125, 127 effect of manganese and silicon, 322 manufacture. 157 elfect of phosphorous, 322 physical properties, 158 effect of section si/c on, 315 toxicity, 159 effect of tempering, 338 use, 159 cutectoid, 247 Bessemer process, 217 heat treated, 312 Binary equilibrium diagrams, 102 hypercutectoid, 247 Binary system, 101 hypocutcctoid, 247 Blast furnace, silicon in, 239 chemistry, 214 tensile properties, 322, 324 description, 212 tonnage production, 297, 306 efficiency, 214 Carbontriding, 292 products of, 213 Carburizing, defined, 289 Blow holes, in steel, 237-238 Cast iron, Boron, effect on hardenabilily, 361 definition. -108 Brasses, 160, 168, 178, 179, effect of section size, 415-417 additions to. 178-180 machinabiliiy, 409, 412, 418, 420 cartridge, 170, 178 relation of properties to cooling rate, high, 168, 178 416-417 leaded, 178-179 Cast tool alloy, 404, 405 low, 170 Casting process, 25-28 Muni/, metal, 169, 178 Catastrophic oxidation, 480 naval, 180 Cathodes and cathodic areas, 453 phase diagram. 165 Calhodic protection, 409 red. 170, 178 Cathode reactions, 453, 454 special 180-181 Cavitation, 468 yellow, 169, 178 Cemented carbide tools, 405, 406 Brinel] hardness, 70, Ccmentitc, defined, 245 conversion tables 75, 76 Ccmcntitc. stability in cast iron, 408 Brittle fracture, 32, 82 Cermets. 198 Building-up principle, 9 Chapmani/ing, 292 Charging. 222 Charpy test, 83 Calculated hardenabilily, 356, Chemical interaction in metal solutions, 18 accuracy, 360 Chilled iron, 408, 410, 420 table, S59 Chromium, Campbell process, 230 elfccl on tensile properties, 371 Carbide formers, 257 ferritcs, 369 Carbide, precipitation, 37(5, 461 in stainless steels, 364 Carbide stabilizing, 257 in tool steels, 395, 398, 403 Carbide tools, 405, 406 Chromium steel, Carbon, low-chromium, effect of chromium, amount and importance in steel, 210-211 323, 325 Index 509

(Cont.) Chromium steel (Cont.) Corrosion engineering importance, 326-329 testing, 471 452, 456 tensile properties, 371 wet, Covalent Bond, 10 heat treatment, 367 mechanical properties, 369 Creep, 488-490 resistance to oxidation and corrosion, curve, 370-372 definition, 486 of, 489 Cladding techniques, 482 determination chemical composition, 491 Cobalt. 190-191, effect of effect of Cr, Mo, 492 alloys of. 201 effect of micro-structure, 492 uses', 201, 201 of variables, 491 Coefficient of mass diffnsivity, 99 effect importance of, 486 Coherent precipitate, 21 489 Cold treatment, 274 intergranular, Cold-worked carbon steel, 306 rale, 490 significance of, 486 Cold- worked wire, 311 488 Cold-worked bars, sheet. 306 stages, strength, 491-493 Cold working. 28, 40-42, 259, 306, transgranular, 489 effects on strength and ductility, 308 Critical cooling rate, 255 effect on dynamic properties, 312 process, 230 purpose, 232 Crucible systems, 11 Columnar grains, 27 Crystal fibering, 38, 39 Combating corrosion, 469 Crystallographic Completion of transformation, 252 Cubic system, 12 system, 108 Composition, effect of, 303-305 Cu-Al '379-380 Compression tests, 77 Cu-Mo, Cu-N'i system, 102 Concentration cell corrosion, 458 215 Continuous cooling, 254 Cupola, use, test, 447 Cooling curve, 98 Cupping Curie temperature, 59 Cooling rate, 344. 350 368 Cupola- furnace, 422 Cutlery steel. 365, 292 Copper, 162-164, 178-9, Cyaniding, Cvclic stresses, at elevated temperatures, arsenical, 178 494 lake, 163 leaded, 178 OFHC, 163 phosphorized, 178 Damping capacity, 94, production, 162 of cast iron, 420. 122 properties and uses, 164 DeBtoglie'a equation, 6 silver-bearing, 178 Decarburi/ation, 91 tellurium, 178 Deep drawing. of steel for, 446 Copper alloys. 165-189, evaluation alloys, 449 constitution, 165-178 to 181 properties of nonferrous nomenclature, 167 Defect lattice, 475 precipitation hardening, 117, 124-125 Defects in crystals, 14, 127 atomic imperfections, 14, 15 constitution. 165, 178-182 importance of defects, 15 phase diagram, 165 line imperfections, 14 Corrosion, surface imperfections, 14 cost, 452 Deformation, 32 definition. 452 elastic, 488 metal, 37-42 dry, 452 of polycrystallinc electrochemical, 452 plastic, 488 of 100, fatigue, 92 Degrees freedom. 26 rate, 471 Dendrites, 510 Index

Design. 471 Fatigue, 493, chan, 500 combined with creep, 493-495 for elevated temperatures service, failure, 86 499-500 Eerrite, for low temperature service, 504-505 defined, 246 Dczincification, 406 strengthen, 256 Die steels, 396, 397 Kick's law, 99 Flame hardening, Differential hardening, 294 293, of cast iron, 434 Diffusion. 99 Fleming, Sir Ambrose, 3 Dislocations 14, 15, 23 Free-machining steels, 440 Dispersion hardening. 111 Fretting, 91 Domain, 58 Fretting corrosion, 468 Double yield point, 448 Ftigacity, 98 Ductile cast iron, 409, 420, 421 Full annealing of steels. 280 Duplex process, 229 Furnace cooling, 254 Duplex Talbot process, 230 corrosion, Dynamic Properties, 81 Galvanic 456 Galvanic series, 455 Gamma loop, 256, 366 Effect, Gamma rays, source materials, 204 carbon in cast iron, 434 Gamma stabilizers, 256 Chromium in cast iron, 435 Gas carburizing, 289 composition, 302, 303 Gibbs, J. Willard, 100 copper in cast iron, 436 Gold, 190-191, 202 gTain size, 301 Grain boundaries, 17 molybdenum in cast iron, 435 Grain growth. 44, nickel in cast iron, 430 and hot working, 267 phosphorus in cast iron, 435 control of, 265 silicon in cast iron, 434 in steel, above critical temperature, 264 sulphur in cast iron, 435 soaking time and grain growth, 264 vanadium in cast iron, 436 tendency for, 265 Elastic limit 63, 64, 485 Grain refining, 257 Elastic Modulus, 498, 499 Grain size, 15, Electrical conductivity of oxides, 475 and properties, 29-31 Electrical resistivity, 58 control, 24-29 Electromotive force series, 454, 455 In relation to hardenability, 270 Electric furnace, 226-227 in relation to toughness and macliin- Electron cloud diagrams, 6 ability, 266 Electron compounds, 21 measurement of, 266 Elongation 67, 68, process annealing and grain size, 264 method of measuring, 67 Grains in metals, 15 yield-point elongation, 63 Graphite, Energy level diagrams, 7 comparison of forms in cast irons, End-quench test, 349, 350, 352 408-409 Endurance limit, 87 control in gray cast iron, 418 Endurance strength, 88 in malleable cast iron, 410 Engineering, relation to metallurgy, 3, 4 in gray cast iron, 412, 417 Epoxy resin, 470 in ductile or nodular cast iron, 420 Equi-axed grains, 27 Graphite flake size, 429 Equilibrium, 97 Graphite precipitation, 257 Erichscn test, 446 Graphitizing agents in cast iron, 408 Erosion-corrosion, 468 Gray cast iron, 409, 412-420, Eutetic, 105 properties, 412-415, 417-421 Eutcctoid, iron-carbon, defined, 247 structure, 409, 414, 415. 418 Index 511

199-200 Grossman, 356 Inconel, Indices of direction, 13 Hadfielcl steel, 256 Induction hardening, 293 Hall process, 132 iron, 434 Hansgirg process, 156 of cast definition, 211 Hardenabilily, Ingot iron, Inhibitors, 470 and alloying elements, 269 iron, 429 and cooling rate, 344 Inoculants in cast and engineering properties, 354 Inoculation agents, 28 and tempering, 356 Insulators, II 484 and time delay, 346 Interference colors, corrosion, 151, 376, 460 bands, 353 Intergranular phases in alloy systems, 20, calculated, 356 Intermediate 21 determining, 348 electron compounds, intermediate solid solutions, 20 effect of boron on, 361 interstitial compounds, 21 factors affecting, 346 laves phase, 21 in carbon steels, 343 intermetallic compounds, 21 in low-alloy steels, 320. 344 normal phases, 21 in relation to grain size, 269 sigma solid solution, 20 in relation to quenching medium, 270 Intermediate compound, 107 Hardness, Intermetallic stresses, 283 conversion scales, 75, 76 Internal 21 conversion to tensile strength, 75 Interstitial compound, solid solution, 20, 101 definition, 69 Interstitial determination, 69-77 Interstitial solutes, 20 nonferrous alloys, 76 Ionic bond, 10 numbers. 75 Iron, allotropy, 244-45 tests compared, 72 alpha, 244 tool steels, 392, 399 244, 245 Hardening of cast iron, 432 cooling curve, alloys, Hardening spheroidized steel, 281 Iron-carbon temperature, 247 Hastelloy, 199-200 critical slowly cooled alloys, Heat treatment, 300-302 phase changes, 247-250 Heat treating carburized steel, 289 245-247 Heisenberg's uncertainly principle, 6 phase diagram, phase diagram, 366 Hexagonal system, 12 Iron-chromium Iron-nickel alloys, 384 High manganese steel, 381 201 High nickel steels, 384 Ironal, Irreversibility. 99 High speed steels, Isothermal heat treatment of cast iron, classification, 399 composition, 399 433 Isothermal transformation, 251 heat treatment, 400 structure, 400 lzod test, 83 Homogcnizalion of auslcnite, 279 test, 349 Hoopes process, 133 Jominy Hot metal mixer, 222 Hot rolled, 303 Killed steel, 237-238 Hot worked carbon steels, 302, 303 Knife-line attack, 462 Hot working, 28, 38-40, purpose, 231 l.aves phases, 21 Lead, 190-191, 193-195 Ideal diameter, 357 alloys of. metals, 194 Ideal quench, 357 in type Impact specimens, 83 uses, 197 Lever principle, 104 Impact strength. 81, boil, 223 low-alloy steels, 328. 336, 340 Lime 1

512 Index

Line imperfections, 14 Manufacture of steel (Cont.) Liquid metal corrosion, 472 Talbot process, 230 Liquid solutions, 19 triplex process, 230 Liquids, 102 Martcmpering compared to austempering, Low alloy steels, 257, 287, A.I.S.I. steels, 329-331 commercial limitations, 289 chromium as an alloying element, 324, described. 286 325 underlying principle, 287 copper in, 325 Martens. 2 engineering importance, 318-340 Marlensite. 255 general effects of alloying elements Mass transfer, 99 320-322 Mechanical fibering, 39 manganese and molybdenum in, 322, Mechanical properties, 299-300 326 in cast irons, 41 1, 421 nickel and silicon in, 323, 324 Mechanical working, 28-29, prosphorus as an alloying clement, 322 and engineering properties 32-47 properties, heat treated, 332, 340 Melting the charge, 223 S.A.E. steels, 328-332 Metal purification, 469

structural, composition and properties, Metallic bond, 1 326-328 Metallurgy, tungsten and vanadium in, 325, 326 as a science, 2

Low carbon steel, 125-126, as an art, 1 strain aging, 126 importance to engineers, 3, 4 Low Temperature, 84 Metals, static properties, significance, 57-79 Machinability, effect of micro-structure, 59 evaluation of metallic materials for, Metals in atomic reactors,

138, 441 control materials, 20-1 ratings for free-cutting steels, 440 fuel elements, 203 relation of cutting speed to tensile gamma-ray source, 204 strength, 438 structural materials, 203 variables affecting, 438 Mg-Ca system, 107 Malleable cast iron, 409-412, 421 Mg-I.i system, 109 Magnesium, Miller indices, 13 manufacture, 156 Miscibility of metals, 18 properties, 155 Modulus of elasticity, 63-65 vs. aluminum, 156 Modulus of rigidity, 64 Magnesium alloys, Modulus of rupture, in cast iron, casting, 158 419 Molybdenum in tool and steels, corrosion resistance, 158 die 396, 399, 403 heat treatment, 157 Molybdenum precipitation hardening, 121-124 in steel, effect on properties, 325, 326 Magnetic properties, ferromagnetic ma- Moncl, 199-200, terials, 58, 59 K-Monel, 125 Mallcablizing cycle, 430 Metal, 102 Malleablizing white cast iron, 410 Process. 230 Manufacture of iron, 212-214 S-Monel, 125 Manufacture of steel, Mottled cast iron, Campbell process, 230 409 crucible process, 231 duplex process, 229 Newton's law of cooling, 345 duplex Talbot process, 230 Nickel, 190-191, 199, electric- furnace process, 226-227 alloys of, 199-200 Monell process, 230 in cermets, 198 super-refining process, 230 physical properties, 199 Index 513

Nickel in steel, Pearlite, 253, defined. 247 effect on properties, 323, 324 257 Nimonic, 199 hardcnability, iron, 412 Nitriding, malleable cast advantages, 291 Perfect crystal, 23 108 defined, 291 Pcritectic reaction, limited to alloy steels, 291 Pitting. 459 process, 291 Pewter, 191 Ni-Vee Bronze, 127 Phase, 97 in metal systems, 19 Nodular cast iron, 409, 420, 421 Phase diagrams Nodular graphite formation, 430 Phase rule, 100 Nodular pearlite, 253 Phosphorus, iron, 413-415 Normalizing of steels, 280, 281 in gray cast on properties, 322 Nose of the curve, 254 in steel elfect testing, 59 Notch sensitivity in cast iron, 413-420 Physical 156 Nondcforming tool and die steel, 396 Pidgeon process, Normal intermetallic compounds, 21 Pig iron, 409 Normalizing of cast iron, 431 classed, Notched-bar impact, 502 composition, 211 and Bedworth rule, 478, 484 Notched-bar test, 85 Pilling 6 Notch brittlencss, 81 Planck's constant, constraint, 85 Notch constraint, 85 Plastic Plastic design, background for, 63 Platinum, 190-191, 202, Notch sensitivity, 89 metals, 203 Objectives of annealing, 280 Polarization, 454 Olsen test, 446 Polycrystalline metals, 16 Open-hearth processes, 220 Polyphase structures, 49-51 Orange peel, 30 Powder metallurgy, 29 Precious metals, 190, 202 Ore boil, 223 Precipitates, 21, Overaging, 52, 116-117, coherent, 21 precipitation hardening, 116 non-coherent, 21 Oxidation, Precipitation hardening, 52, 53, 107, high-temperature, 472 alloy requirements. 111 internal, 473 alloy steel, 126-127 of copper, 475 aluminum alloys, 117-123 of stainless steel, 482 carbon steel, 125-126 of titanium, 479 copper base alloys, 117, 124, 125, 127 of zinc, 476 definition, 111 prevention, 480 delayed precipitation, 113 rate, dislocations, 115-120 linear. 477 elfect of temperature, 117 parabolic, 478, 484 effect of time, 117 cubic, 478 Guinier- Preston zones. 115 logarithmic, 478 lattice conformity, 112 resistant alloys, 481 magnesium base alloys. 120-124 formation, 473 Oxide overaging, 116 Oxygen pressure, 475 process for, 1 12 theory of, 114 Pack carburizing, 289 time and temperature effects, 114, Passivity, 454, 455 119-120 in metals, 15 Patenting, defined, 287, Prior history elfect of steels, 280 use of patented wire, 288 Process annealing cementite, 254 Paul! exclusion principle, 7 Proeulecloid 514 Index

Procutectoid fcrritc, 254 Solute atoms, 18, Properties of a spheroidized steel, 281 clfcct of solute atoms on properties, 20 Properties of solid solutions, 20 formation of intermediate phases, 20 Proportional limit, 63, 64 interstitial solid solutions, 20 Protective atmospheres, 280 substitutional solid solutions, 20 Protective oxidation, 478 Solution heat treatment, 111, 112, Pumps, 470 alloy requirements, 112 Purpose of hardening, 282 elfect of quenching, 113 elfect of temperature, 113 Solutions in metal systems, 18, Quality factors, 23, 24 factors controlling extent of solubility, Quantum numbers. 6 18 Quench cracks, 284 liquid solutions, 19 Quenching, solid solutions, 20 mechanism of, 270 Sonic testing, 78, 79 media, 282 Sorby, 2 stages, 282, 283 Space group, 12 Space lattice, 12 Rate of growth, 27-28 Specific gravity, of the heavy metals, 191 Rate of nucleation, 27-28 Spheroidite, 289 Radiation damage, 53-54 Sphcroidization. 495 Recovery, 42 Spheroidizing steels, 281 Recrystallization, 28, 43, 45-46 Stabilizing, 123-124 Reduced temperatures, 500-505 Stainless iron, 365 stresses, Residual 93 Stainless steels, classes of, 364 Retained austenite, 274 Stainless W. 379-380 Reverberatory furnace, 424 Static mechanical properties, 60 Rockwell hardness, described, 70, 71, Steel, relation to other hardness values, 75 hardening of, early study. 2 Rotating-beam test, 87 mechanical treatment, 231-232 Rupture strength, 491 precipitation hardening alloy steels, 126-127 S.A.E. steels, low carbon steels, 125-126 low-alloy, 328-332 stainless steel, 127 properties, 333-340 properties, charts, value, 333-334 specifications for, 329, 331 strain aging, 126 Scaling, 473 world production, 208 Sensitization of stainless steel, 460 Steel making processes, classification, 208 Service factors, 54 Stellite, 404, 405 Shear modulus, 64 Strain aging, 126 Shear tests, 77 Strain hardening, 35-36, 493 Shot pecning, 92 Strain rate, 82 Sigma phase, 21 Strain softening. 489 Silchrome, 365 Strengthening nonferrous metals, Silicon in steel, precipitation hardening. 111 effect on properties, 239, 323 Stress concentration, 89, Silver, 190-191, 202 locating and magnitude, 60 Size factor, in forming metal solutions. Stress corrosion, 462, 464, 465 18 Stress raisers, 60 Slip. 33-36 S-N diagram, 88 Stress relief in cast iron, 430 Solid solutions, 20, Stress-strain curves. 307 alloys, 102 Stress-strain curves, cast iron, 65, effects, 47-49 low carbon, cold-worked steel, 62 Solidus, 102 common metals, 64 Index 515

Stretcher strains, 30, 448 Tool material, Structural changes, 495-498 cast alloys, 404, 405 Structure of carburized steel, 290 Cemented carbides, 405 Structure of steel, tool steels, 389-404 effect of cold working, 259 Tool steels, Subatmospheric cooling, 258 characteristics, 389, 391, 392 Substitutional solid solutions, 101 chromium in, 395, 398, 403 Substitutional solutes, 20 classification, 391 Sulphur in cast iron, 413-415, 420 composition, 391 Super refining process, 230 high alloy. 397 Super saturated solid solution, 101 high carbon, 390 Super stainless steels, 382-383 high speed, 398-401 Surface finish, 89, 91 low alloy, 395 Surface imperfections, 14 manganese in, 396 Surface protection during heating, 279, medium alloy, 396 280 molybdenum in, 396, 399, 403 Surface-rolling, 92 properties, 392 System, 97 silicon in, 397 structure, 394, 398, 401 tempering, 393, 402, 403 process, Talbot 230 tungsten in. 399, 403 embrittlement, 305 Temper vanadium in, 399 graphite, 430 Temper Torsion tests, 77 Temperature, effects of, 485-506 Toughness, 502 martensile, 285 Tempered Transformation on cooling of cast iron, Tempering, 428 and hardenability, 356 Transition elements, 9 change in toughness, 275 Transition temperature, 83, 502 effect physical properties of steel, on 284 Transverse test, cast iron, 77 method, 284 Triaxial stress, 85 of cast iron, 132 Triple point, 101 purpose of, 284 Triplex process, 230 retained austenite and quenching, 274 'ITT curve. 252 Tensile strength, metals 61-69, Tungsten carbide, 405, 406 standard specimens, 67, 68 Tungsten, in tool and die steels, 399, 403 standardization, 61 Twinning, 36-37 Ternary-system iron, carbon, silicon, 425, Two-metal corrosion, 456 427 Type metals, 191-194 Terne plate, 191, 196 Types of graphite flakes, 429 Thermal conductivity, 497, 498 Thermal expansivity, 497, 498 Understressing, 93 Thermal fatigue, 499 Uniform corrosion, 456 Thermal shock, 499 Unit cell, 12 Thermodynamics, second law of, 98 Uses of carbon steels. 302 Thermostatics, 97 Time delay, 346 Vanadium, Tin, 190, 191, In steel, 325 alloys of, 192-193 in tool steel, 399 in type metals, 194-195 Van dcr Waals' bond, 10 plating, 197 Vicalloy, 201 Tin bronze, 125 Titanium carbide, 405, Wave mechanics, 6 manufacture, 159 Wear resistance, properties, 159 elfect of carbon content and heat treat- use. 159, 160 ment, 444 1

516 Index

Wear resistance (Conl.) Yield point, 06, 67 evaluation of, 444 Yield strength, 66 variables affecting, 443 Wear-types, 442 Zinc, 191, 197 White cast iron, 408, 410 crystal structure, 198 White metals, 190-191 die castings, 197 Wire, carbon steel cold rolled, 30G fabricating methods, 198 Working stresses, 499 mechanical properties, 198 plating, Wrough t iron, 2 1 197 Wrought iron, manufacture, 227, 229 uses, 198