Surface Hardening of Steels: Understanding the Basics (#06952G)

Surface Hardening of Steels Understanding the Basics

Edited by

J.R. Davis Davis & Associates

ASM International Materials Park, OH 44073-0002

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First printing, November 2002

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Library of Congress Cataloging-in-Publication Data

Surface hardening of steels : understanding the basics / edited by J.R. Davis p. c.m. Includes bibliographical references and index. 1. Steel—Metallurgy. I. Title: Surface treatment of steels. II. Davis, J.R. (Joseph R.)

TN731.S85 2002 672.3′6—dc21 2002034248

ISBN: 0-87170-764-0 SAN: 204-7586

ASM International® Materials Park, OH 44073-0002 www.asminternational.org

Contents

Introduction Chapter 1 Process Selection Guide ...... 1 Diffusion Methods of Surface Hardening ...... 1 Surface Hardening by Applied Energy ...... 5 Surface Hardening by Coating or Surface Modiﬁcation ...... 7 Important Considerations for Process Selection ...... 11 Process Comparisons ...... 12

Surface Hardening by Thermochemical Diffusion Treatments Chapter 2 Gas Carburizing ...... 17 Gas Curburizing Practices ...... 17 Characteristic Features of Carburized Cases ...... 17 Carbon Sources ...... 18 Carburizing Equipment ...... 19 Preparation of Parts for Carburizing ...... 21 Carburizing Process Variables ...... 21 Gas Carburizing Atmospheres ...... 25 Carbon Concentration Gradients and Surface Carbon Content ...... 29 Process Planning ...... 39 Selective Carburizing ...... 42 Dimensional Control ...... 43 Case Depth Measurement ...... 44 Microstructures of Carburized Steels ...... 46 Martensite ...... 46 Austenite ...... 49 Carbides ...... 53 Alloying Effects ...... 55 Intergranular Fracture at Austenite Grain Boundaries ...... 57 Microcracking in Carburized Steels ...... 60 Excessive Retained Austenite and Massive Carbides ...... 60 Residual Stresses ...... 62 Surface and Internal Oxidation ...... 64 Properties of Carburized Steels ...... 65 Bending Fatigue Strength ...... 66 Other Properties of Interest ...... 79

Chapter 3 Vacuum and Plasma Carburizing ...... 91 Vacuum Carburizing ...... 91 Plasma Carburizing ...... 105

Chapter 4 Pack and Liquid Carburizing ...... 115 Pack Carburizing ...... 115 Liquid Carburizing and Cyaniding ...... 118

Chapter 5 Carbonitriding ...... 127 Applicable Steels and Applications ...... 127 Composition of Case ...... 128 Depth of Case ...... 128 Hardenability of Case ...... 131 Hardness Gradients ...... 132 Void Formation ...... 133 Furnaces and Furnace Atmospheres ...... 133 Control of Retained Austenite ...... 134 Quenching Media and Practices ...... 135 Tempering ...... 136 Carbonitriding of Powder Metallurgy Parts ...... 137 Combined Carburizing/Carbonitriding Treatments ...... 138

Chapter 6 Nitriding ...... 141 Gas Nitriding ...... 141 Modiﬁed Gas Nitriding Processes ...... 165 Pack Nitriding ...... 167 Liquid Nitriding ...... 167 Plasma (or Ion) Nitriding ...... 177 Wear Resistance of Nitrided Steels ...... 187

Chapter 7 Nitrocarburizing ...... 195 The Compound Layer and Diffusion Zone ...... 195 Liquid Nitrocarburizing ...... 195 Gas Ferritic Nitrocarburizing ...... 204 Plasma Nitrocarburizing ...... 208

Chapter 8 Boriding ...... 213 Advantages and Disadvantages of Boriding ...... 213 Boride Layer Characteristics ...... 214 Steel Selection ...... 215 Effects of Alloying Elements ...... 215 Boriding Processes ...... 217 Properties of Borided Steels ...... 221 Applications for Pack Boriding ...... 223

Chapter 9 Thermal Diffusion Process ...... 227 Process Characteristics ...... 227 Characteristics of TD-Processed Materials ...... 232 Applications ...... 233

Surface Hardening by Non-Diffusion Methods Chapter 10 Surface Hardening by Applied Energy ...... 237 Flame Hardening ...... 237 Induction Surface Hardening ...... 242 Laser Surface Hardening ...... 264 Electron-Beam Surface Hardening ...... 267

Chapter 11 Surface Hardening by Coating or Surface Modiﬁcation ...... 275 Hard Chromium Plating ...... 275 Electroless Nickel Plating ...... 282 Hardfacing ...... 286 Thermal Spraying ...... 293 Chemical and Physical Vapor Deposition Processing ...... 297 Ion Implantation ...... 301 Laser Surface Processing ...... 302

Appendix

Appendix 1 Iron-Carbon Phase Diagram ...... 311 Appendix 2 Austenitizing Temperatures for Steels ...... 315 Appendix 3 Hardness Conversion Tables ...... 317 Subject Index ...... 321 Alloy Index ...... 349

Preface

Surface hardening is a generic term covering a number of processes used to improve the wear resistance of ferrous parts without affecting the more soft, tough interior of the part. The combination of a hard surface and softer interior, made possible, for example, by case hardening methods, is of inestimable value in modern engineering practice. By the use of high-quality alloy steels, great strength and toughness in the core can be combined with extreme surface hardness, resulting in a composite structure capable of with- standing certain kinds of stress to a high degree. For less exacting requirements, there are many applications where low or moderate core properties, together with a high degree of surface hardness, can be obtained with cheaply fabricated, low-priced carbon steel. Three of the major types of treatments that are conventionally used to enhance surface wear resistance are classified as thermochemical, thermal, and coating or plating. Examples of thermochemical treatments include carburizing, nitriding, and carbonitriding (case hardening methods). Examples of thermal or applied energy treatments include flame and induction hardening. Coating or plating processes include hard chromium plating, electroless nickel plating, and various hardfacing methods. These long established surface hardening technologies are continually being improved and remain among the most widely used. However, today completely different surface hardening technologies are being applied to and developed for steels. The objective remains the same, that is, enhanced surface performance, but technologies that incorporate high- energy beams, plasmas, magnetic and electrical fields, and vacuums are being applied. Some of these technologies have been used for some time in the electronics industry to fabricate thin film devices and circuits and therefore cannot be considered new technologies. Their application to steel, however, is relatively new, and they elevate the level of sophistication and control of surface hardening. Surface Hardening of Steels: Understanding the Basics is a practical selection guide to help engineers and technicians choose the optimum surface hardening treatment for a given application from an ever increasing number of options. Emphasis is placed on characteristics such as processing temperature, case/coating thickness, bond strength, and hardness level obtained. The advantages and limitations of the various surface mod- ification technologies are compared. Recent developments in the understanding of the relationships between microstructure and fatigue and wear performance are reviewed, as are more recently introduced surface hardening processes such as vacuum/plasma- related technologies, laser processing, chemical vapor deposition/physical vapor deposition, and ion implantation. Methods for evaluating hardness patterns and depths of hardness for quality control and failure analysis are described. Metallurgical comparisons are made between those processes that offer rapid heating and rapid cooling (self- quenching) characteristics—for example, induction hardening—and conventional furnace hardening. Metallurgical characteristics and properties obtained by atmosphere and vacuum carburizing are also compared. Wear and corrosion data are also provided to demonstrate the benefits of each process.

The successful completion of this book would not have been possible without the generous assistance of the ASM staff. In particular I would like to thank Scott Henry, Assistant Director, Technical Publications; Don Baxter, Managing Editor, Advanced Materials & Processes and Heat Treating Progress; and Eleanor Baldwin from the ASM Library. I called on them often for help, perhaps too often, but they always came through. Thanks again my fellow colleagues and friends.

Joseph R. Davis Davis & Associates Chagrin Falls, Ohio

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