Introduction

Home , Crucible steel, Damascus steel, Pig iron

CHAPTER 1 Introduction

Tool steels are the alloys used to manufacture the also used for tool applications. This chapter ex tools, dies, and molds that shape, form, and cut other plores further the philosophies which make tool materials, including steels, nonferrous metals, and steels a very special class of steels, the long histori plastics. This definition of tool steels has been ex cal evolution of iron and steel manufacture, includ panded into the following description in the 'Tools ing steels for tools, and the more recent develop Steels" section of the Steel Products Manual of the ment of tool steels as they emerged from the Iron and Steel Society (Ref 1): development of iron and steel products in general. The classification of the many types of tool steels is Tool steels are either carbon, alloy or described in Chapter 2, and subsequent chapters high-speed steels, capable of being hard describe production, alloy design, heat treatment, ened and tempered. They are usually melted and specific tool steels in detail. in electric furnaces and produced under tool steel practice to meet special requirements. They may be used in certain hand tools or in Tool Steels as Special Alloys mechanical fixtures for cutting, shaping, forming and blanking of materials at either Tool steels have long been considered to be a very ordinary or elevated temperatures. Tool special group of alloys with characteristics similar steels are also used on a wide variety of to but different from those of other steels. Marcus other applications where resistance to wear, Grossmann and Edgar Bain, in their book on tool strength, toughness and other properties are steels (Ref 4), written in 1930 when the physical selected for optimum performance. metallurgy of steel was just beginning to be firmly established, eloquently commented on the relation This description implies that tool steel technol ships been tool and other carbon steels: ogy overlaps the technology of carbon and low-al loy carbon steels, produced in large tonnages, which In some cases, notably in high-speed may be hardened by quench and tempering heat steel, the resulting alloy steel possesses such treatments. Although this association between tool remarkable properties that, in a sense, its steels and other hardenable steels is true, most texts relation to carbon steel is almost unrecog on tool steels exclude treatment of the high-tonnage nizable. Yet it is the authors' opinion that the bar steels that might also be used for tool applica fundamental similarity exists, and that it tions such as hand tools (Ref 1-3). Also, while tool may profitably be recognized. In acquiring steels may be manufactured with properties for use an understanding of alloy steels, great value in nontool applications, such as springs, magnets, attaches in particular to some sort of general bearings, or even structural applications, these uses theory or principle if it is faithfully in accord also are not generally treated in texts that describe with the facts and explains them in terms of the characterization and selection of tool steels. simple fundamental effects. It is believed This edition of Tool Steels, as have previous edi that certain fundamentals of iron metal tions, will also concentrate on those steels that are lurgy are manifest in all steels, modified uniquely manufactured for tool applications, recog in a definite manner by the alloying ele nizing that some more recently developed ultrahigh- ments. strength steels—such as maraging steels, AF1410, and Aeromet 100, developed for structural applica In addition to alloying, tools steels are considered tions that require high toughness—are sometimes special because they are very difficult to manufac- Tool Steels, 5th Edition (#06590G) Copyright © 1998 ASM International ® Author(s):Too G.l Steel Roberts,s G. Krauss, and R. Kennedy All rights reserved. www.asminternational.org

ture, demanding the highest quality in every proc mentation, chance, intuition, and perceptive obser essing step. Peter Payson, in his book The Metal vation. Indeed, the production of hardened tool lurgy of Tool Steels, published in 1962 (Ref 3), steels must be considered truly impressive. It was reflects this consideration in support of separating accomplished without analytical instruments or sci tool steels from more mass-produced steels: entific understanding of chemistry, crystallography, or microstructure. Even tool steel is not perfect, but it is far Iron can be traced to the Egyptians of 5000 to superior to so-called "tonnage" steel in free 6000 years ago, and numerous biblical references dom from internal porosity, sizable undesir confirm this time period for the beginning use of able nonmetallic inclusions, serious chemi cal segregation, and surface defects. Various iron (Ref 6, 7). Widespread replacement of bronze physical methods are used for macroinspec- by iron occurred at about 1200 B.C., perhaps be tion, and metallographic procedures for mi- cause of natural and economic disasters that inter croinspection, to assure that tool steels meet rupted the flow of tin, which was alloyed with cop the minimum requirements set up by the per to make bronze (Ref 7). As a result, the consumer. advantages of iron became known, despite its being an unfamiliar technology, as well as being softer While Payson's statement reflects the manufac and more subject to corrosion than bronze. turing scenario in the mid-20th century, it is far Maddin (Ref 7) explored the early hardening of from accurate today. Carbon and low-alloy bar steel and described a miner's pick—the earliest steels currently are manufactured in high volume to known example of a martensitic steel tool. The pick the highest quality by electric furnace melting, ladle (Fig. 1-1) was found in Galilee and dates from the metallurgy for impurity and inclusion control, and late 13th or early 12th century B.C. Figure 1-2 continuous casting. Nevertheless, tool steels are shows the hardened martensitic microstructure, special and require careful manufacturing. The very which confirms that the pick was hardened by heat high alloy content and microstructure that make ing and quenching. Earlier steel objects have been them desirable for severe applications also make them difficult to manufacture. The contradictory demands of ease in manufacture and high perform ance of tool steel products were early noted by Harry Brearley in the 1916 preface to his book on the heat treatment of tool steels (Ref 5):

The ultimate value of a tool may depend as much on the manner in which it is worked into its finished shape, as on the material from which it is made. The skill and knowl edge of the toolsmith and hardener must Fig.1-1 Miner's pick from Mt. Adir in northern Galilee (13th to 12th century B.C.). Arrow indicates flake, the micro- therefore always be taken into account. If structure of which is shown in Fig. 1-2. Source: Ref 7 for any reason whatever these cannot be re lied upon, the softer steels which are not so readily overheated in forging, or cracked in hardening, are invariably introduced at the cost, and finally to the dissatisfaction, of the tool user.

Historical Evolution of Iron and Steel The earliest uses of steel were for tools and weap ons, and then as now, high hardness and durability were the valued properties for these applications. High hardness was coupled to three factors: the ability to produce iron, the introduction of carbon into the iron to make steel, and the heating and quenching of the steel to produce martensite. The early attainment of each of these factors, and their Fig.1-2 Martensitic microstructure of hardened pick simultaneous incorporation into finished tools and shown in Fig. 1-1. Light micrograph, estimated magnification weapons, must have required considerable experi 500 to 1000X. Source: Ref 7

documented, but their microstructures are pearlitic, example of the type of forge used to produce wootz indicating that quenching was not yet mastered. The steels is shown in Fig. 1-3. following quotation from Homer's Odyssey (Ref 8), Similar processing developed in Europe follow relating to the blinding of the giant Cyclops by ing the Dark Ages when wrought iron was carbu Odysseus and his men, shows that quenching and rized by heating in contact with charcoal to form metalworking must have been well established by what was known as blister steel, from the appear 900 B.C., the approximate date of its writing (Ref 6, ance of surface blisters or scale. The depth of car 7): burization tended to be shallow and nonuniform. Later, short lengths of blister steel were stacked, And as when armorers temper in the ford forged, and welded to produce the product referred The keen-edged pole-axe, or shining sword, to as shear steel. The uniformity of the shear steel The red-hot metal hisses in the lake; structure was improved, but variations in carbon Thus in his eyeball hissed the plunging stake. could not be completely eliminated. Wrought iron production in forges or bloomaries Steel, a workable combination of iron and carbon, directly from ore, similar to those practices de has been historically difficult to produce efficiently scribed for the production of wootz steel, continued and consistently. Two quite different approaches to to be the major production method for iron and steel well into the 19th century. However, blast furnaces, steel production evolved over the millennia. One which converted iron ore by reduction with air, was based on smelted iron, which contained too charcoal, and limestone flux into cast or pig iron, little carbon and required subsequent carburization, eventually developed to produce iron in larger quan while the other was based on the production of pig tities (Ref 6). Pig and cast irons were high in carbon, or cast iron, which contained too much carbon and silicon, and phosphorus and quite brittle. The high required its subsequent removal. The earliest exam carbon content lowered the melting point of iron, ple of the first approach is the production of making it fluid and castable, but the associated iron "wootz" steel in India, which dates back to about carbides or graphite in the microstructure, together 350 B.C. (Ref 6, 7, 9). Iron ore was smelted with with other impurities, prevented working—as is charcoal in forges; air was forced into the charge characteristic of wrought steels with lower carbon through tuyeres with bellows, enabling high tem content. Various low-volume production techniques peratures to be attained. Nevertheless, temperatures to convert cast iron to steel were developed—for were limited, and semisolid sponge iron containing example, the puddling process described by Thack- considerable entrapped slag was produced. The slag ray in his informative account of the history of iron was removed and fragmented into smaller particles and steel (Ref 6) by intense hammering or forging. This wrought iron Modern high-volume liquid steelmaking devel was then carburized in crucibles with charcoal or oped when the high-carbon content and other impu rice husks to produce the wootz or crucible steel. An rities in pig iron were oxidized in Bessemer convert ers and Siemens-Martins open hearth furnaces to create steel (Ref 6). In the Bessemer process, pat ented by Bessemer in 1856, hot air was blown through molten pig iron to reduce carbon and sili con; in the open hearth furnace, first successfully operated by Siemens in a plant in 1868, solid or liquid pig iron and scrap were melted in the furnace, and the oxygen for the conversion of the pig iron to steel was provided by iron ore. Liquid steelmaking has continued to evolve to this day, with the replace ment of Bessemer converters and open hearth fur naces with other types of oxygen converters and electric arc furnaces. The latter developments have been largely applied to large-tonnage, low-alloy, carbon steels, while tool steel production has fol lowed an independent path.

Tool Steel History

Early tool steel history is intimately related to the evolution of steels in general, since the inability to make steel in large quantities tended to concentrate steel applications to tools and weapons instead of Fig.1 -3 Sectional view of a Catalan forge. Source: Ref 10 the structural applications that dominate steel usage

today. Perhaps the best early examples of the skill processes, and powder metallurgy (P/M) production and knowledge required for high-performance steel of tool steel were developed and applied as de manufacture are the swords produced in China, Da scribed in Chapter 3. mascus, and Japan (Ref 9, 11). These swords were Modern tool steels depend strongly on alloying produced by layer welding of high- and low-carbon and heat treatment for their high hardness, and un steel by multiple forging steps. Not only was a derstanding of the interrelationships among carbon functional sword with the high hardness of high- content, alloy composition, and processing came carbon steel and the toughness of low-carbon steel only gradually in the 19th century. The slow pace produced, but the fine layered structure also yielded was caused in part by the necessary concurrent de unique patterns of great beauty. Although pattern- velopment of methods of precise chemical analysis welded steel technologies evolved over many centu and metallography, which could resolve the micro- ries, recorded dates for Damascus blades originate structure of steels. Often, fracture surfaces of steels from A.D. 540 (Ref 9), and Japanese swordmaking were the only measure of steel structure and quality. was clearly documented during A.D. 900 to 1000, Nevertheless, the use of alloying was beginning, when Japanese swordmakers began to inscribe their and Huntsman and others were beginning to use names on the blades (Ref 11). manganese to deoxidize liquid steel (Ref 6). Table 1-1 presents a chronological listing of de Alloying, which initiated the era of modern tool velopments in steels, emphasizing important dates steels, is attributed to Robert Mushet, who in 1868 or periods for tool and high-speed steel technology. added tungsten to a high-carbon steel. Until that The beginning of tool steel history is generally re time, and for 25 years thereafter, other steels for garded as 1740, when Benjamin Huntsman, a clock- tools were made of unalloyed carbon steel. The maker from Sheffield, England, melted pieces of composition of Mushet's steel, marketed as "R. blister steel in a crucible. The melting produced a Mushet's Special Tool Steel" through 1900, con much more homogeneous steel than the blister or sisted of nominally 2% C, 2.5% Mn, 7% W, and shear steels produced by solid-state processing and often 0.50% Cr and 1.10% Si (Ref 13). This steel provided the foundation for producing the high- had the remarkable capacity to harden during air quality steel required for not only Huntsman's clock cooling after forging or heating and is regarded as springs but also the tool steels that were to come. the first high-speed steel. The crucible melting process, typically handling Chromium, as reviewed by R.A. Hadfield (Ref amounts of tool steels about 100 lb in size, contin 14), was investigated as an alloying element in ued to dominate the making of tool steels well into steel, soon after its discovery in 1797. Practical the 20th century (Ref 4), until electric furnace melt benefits of hardening and toughening of steel by ing, with heats of several tons in size, other melting alloying with chromium were first claimed in an American patent filed by Julius Baur in 1865. Henri Brustlein, of Unieux, France, also investigated the Table 1 -1 Important dates in the development production of ferrochrome and its addition to steel of high-speed tool steels (Ref 14, 15), starting in 1876. The benefits of chro Dale Development mium, however, were not always consistently de veloped because the role that carbon played in hard 1200 B.C. First documented hardened steel tool 350 B.C. Wootz steels of India ening and the solid-state phase transformations that A.D. S40 Damascus layered steel blades caused hardening were not clear at the time Had AD 900 Japanese layered steel blades field wrote his review in 1892 (Ref 14). Dark Ages Steel production by carburizing of iron 1740 Crucible melting of steel: Huntsman At the same time, John W. Langley summarized 1868 Air-hardening tungsten alloy steel: Mushet the alloying of steel in a paper delivered to the 1898 High-speed steel high-heat hardening: Taylor/White Society of Civil Engineers (Ref 16). He emphasized 1903 0.70C-14W-4C prototype of modern high-speed steel that, in addition to iron, at least 0.30% C was neces 1904 Alloying with 0.3 % V 1906 Electric furnace melting introduced sary to make hardening possible in steel, and that 1910 18W-4Cr-lV (18-4-1) steel (Tl) introduced manganese was essential to "neutralize the evil ef 1912 3 to 5% C additions for added red hardness fects of sulphur and oxygen." The detrimental effect 1923 12% C for higher-speed machining of phosphorus was also noted, as was the opinion 1939 High C high V superhigh-speed steels (M4 and T15) that only tungsten, chromium, manganese, and 1940 Start of substitution of molybdenum for tungsten 1953 Sulfurized free-machining high-speed steel nickel could be used to make useful alloy steels. 1961 Rockwell C 70 high-speed steel (M40 series) Hardening was associated with rapid cooling "from 1970 Introduction of powdered metal high-speed steels a temperature a little above redness." Otto Thallner, 1973 Higher silicon and nickel contents ofM7 to increase hardness in his book on tool steels, translated from German 1980 Development to cobalt-free superhigh-speed steels 1980 Titanium nitride ceramic coating of tool steels in 1902 (Ref 17), also summarizes the effects of 1982 Aluminum-modified high-speed tool steels manganese, tungsten, chromium, and nickel, the elements most commonly used for alloying tool Source: Modified from previous editions of Tool Steels, with additions from Ref 12 steel at the turn of the century.

A dramatic period in the development of high of the high-carbon, high-chromium die steel com speed steel was precipitated by Fred W. Taylor and positions from England to the United States shortly his colleague Maunsel White at the Bethlehem Steel after World War I, (2) the addition of molybdenum Corporation in the years around 1900. The intrigue, to the lC-5Cr die steels to make them air hardening, technical status, discoveries, players, legal aspects, (3) the development of the special die-casting die and importance of this period have been recorded steels that have found so many other hot-working and analyzed in detail in a very readable paper by applications, (4) the combining of tungsten and Arthur Townsend (Ref 13). Briefly, Taylor and chromium in die steels requiring maximum heat White, in the period from 1894 to 1898, discovered resistance, and (5) the development of various free- that very-high-temperature heating, much higher machining tool steels. In the last category are in than typically applied at that time, and subsequent cluded the graphitic die steels introduced in 1937, air cooling of chromium-tungsten steels, developed the leaded steels first patented in 1939 and applied in that period by others in competition to the Mushet to diemaking in 1946, and the sulfurized tool steels. tungsten steels, could produce and maintain excep New developments in high-speed steels since tional hardness during machining at high speeds, even 1920 have also been impressive. In 1980, there were at red heat. The high-temperature heating, its remark 41 different high-speed steel compositions, of able effects, and the benefits of high-temperature tem which about 12 accounted for the bulk of high pering, were truly innovative contributions by Taylor speed steels used. The large variety is primarily the and White. Prior to 1898, air-hardening tool steels result of the cobalt additions and the substitution of were commonly hardened only by cooling from molybdenum for tungsten due to the availability of relatively low temperatures after forging and were vast resources of molybdenum made possible by the not reheated for hardening. Patents filed by Taylor opening of the Climax Mine in Colorado and the and White were subjected to considerable legal shortage of tungsten because of World War II. These argument because it was not clear whether they developments gave molybdenum a cost advantage, were claiming a new steel or a new heat-treating and molybdenum high-speed steels now occupy a process. major part of the market (a situation that has The intense activity in heat treatment and alloy changed dramatically from 1902, when Thallner development of high-speed steels at the turn of the stated that its high price relative to tungsten prohib century continued and was rapidly joined by great ited its use). advances in metallography, x-ray analysis, and sci The tendency of molybdenum grades to decarbur- entific understanding of phase transformations and ize or lose carbon from surface layers during heat microstructure (Ref 18,19), to make advances in the ing was a major early deterrent to the use of molyb entire field of alloy tool steels. O.M. Becker sum denum steels. This problem was solved in 1930, marizes the state of the art in 1910 and describes all when the Watertown Arsenal discovered that borax the crystalline phases then identified in steels, in coatings greatly reduced decarburization during cluding ferrite, cementite, austenite, and martensite heating for forging or rolling. Also, salt bath and (Ref 10). His descriptions of the transformations controlled-atmosphere furnaces were developed to between the various phases show that this aspect of prevent decarburization and maintain the precise heat treatment was not yet fully understood at that temperature control required for the heat treatment time. Many new compositions were introduced for of high-speed steels. both tool and die applications. Another major high-speed steel development dur By 1920, the number of principal tool steel ing this active period was the discovery that care grades had been extended to about 12; many of fully balanced additions of high vanadium and in these types are still currently produced. Included in creased carbon would form large amounts of very the alloy tool steels developed during this period hard vanadium carbides, which greatly increase were the tungsten-base and chromium-base hot-work abrasion resistance. This discovery led to the die steels, manganese oil-hardening die steel, tungsten so-called superhigh-speed steels that have given finishing steel, tungsten chisel steel, and chromium- excellent service in die applications as well as out vanadium and carbon-vanadium general-purpose standing tools for cutting purposes. High-speed steels. In the high-speed steel field, the improved red steels containing cobalt have been developed with hardness from cobalt additions had been discov hardness close to 70 HRC after heat treatment. ered, and three principal grades were available: 18W- More recently, new methods of manufacture of 4Cr-lV (18-4-1), 14W-4Cr-2V, and 18-4-1 with co tool steels, including electroslag melting, vacuum balt. arc remelting, and powder processing have been Also by 1920, nearly all of the American tool applied to produce uniformity in composition and steel companies had organized metallurgical re structure and high quality. Surface-modification search departments, and the resulting scientific and heat treatments such as gas carburizing and nitrid- technical effort brought additional tremendous pro ing have long been applied to tool steels, and the gress in tool steel development. Included in the most recent developments include carburizing and outstanding developments were (1) the importation nitriding in vacuums and plasmas and the applica-

tion of thin ceramic layers such as titanium nitride 4. M.A. Grossmann and E.C. Bain, High-Speed Steel, by chemical and physical vapor deposition tech John Wiley & Sons, 1931 niques (Ref 20). 5. H. Brearley, The Heat Treatment of Tool Steel, Long mans, Green and Co., London, 1918 6. G.E. Thackray, Notes on the History of Iron and Summary Steel, Trans. Am. Soc. Steel Treat., Vol 6, July-Dec This chapter has introduced the special set of 1924, p 443^190 alloys known as tool steels. The historical account 7. R. Maddin, A History of Martensite: Some shows that the development of tool steels could not Thoughts on the Early Hardening of Iron, Marten- be separated from the evolution of iron and steels in site, G.B. Olson and W.S. Owen, Ed., ASM Inter general until the concepts of alloying and its role in national, 1992, p 11-19 hardening were established in the second half of the 8. Homer, The Odyssey, Book IX:389-394, ca. 19th century. The making of steel, because its suc 900 B.C. cess depended so much on trial and error, was in 9. L.S. Figiel, On Damascus Steel, Atlantis Arts Press, variably shrouded in secrecy. However, with scien 1991 tific enlightenment and rapidly expanding demand 10. O.M. Becker, High-Speed Steel, McGraw-Hill, 1910 for manufactured goods came the need and willing 11. H. Tanimura, Development of the Japanese Sword, J. ness to share information. Met., Feb 1980, p 63-72 Some of the pioneering manuscripts devoted 12. A.M. Bayer and L.R. Walton, Wrought Tool Steels, to the manufacture and application of tool and ASM Handbook, Vol 1, (formerly Metals Handbook, high-speed steels published at the beginning of the Vol 1,10th ed.), ASM International, 1990, p 764 20th century have been noted here, including books 13. A.S. Townsend, Alloy Tool Steels and the Develop by Thallner (1902), Becker (1910), and Brearley ment of High-Speed Steel, Trans. Am. Soc. Steel (1918). From 1920 on, with the scientific principles Treat., Vol 21, Jan-Dec 1933, p 769-795 in place, continual development of instrumentation 14. R.A. Hadfield, Alloys of Iron and Chromium, J. Iron for analysis and control and rapidly expanding tech Steel Inst., Vol 42 (No. 2), 1892, p 49-110 nology and manufacturing capacity, tool steel de 15. M. Brustlein, On Chrome Pig Iron and Steel, J. Iron velopment increased dramatically. The results of Steel Inst., Vol 30 (No. 2), 1886, p 770-778 these efforts are described in detail in the balance of 16. J.W. Langley, Discussion on Structural Steel, Trans. this book. Am. Soc. Civil Eng., Vol 27, July-Dec 1892, p 385- 405 17. O. Thallner, Tool-Steel: A Concise Handbook on Tool-Steel in General, W.T. Brannt, trans., Henry REFERENCES Carey Baird & Co., 1902 18. C.S. Smith, A History of Metallography, University 1. Tool Steels, Steel Products Manual, Iron and Steel of Chicago Press, 1960 Society, April 1988 19. A. Sauveur, The Metallography and Heat Treatment 2. G.A. Roberts and R.A. Cary, Tool Steels, 4th ed., o//ro/ia/u/5tee/, McGraw-Hill, 1912; 2nd ed., 1915; American Society for Metals, 1980 3rded., 1926,4th ed., 1935 3. P. Payson, The Metallurgy of Tool Steels, John Wiley 20. A. Matthews, Titanium Nitride PVD Coating Tech & Sons, 1962 nology, Surf. Eng., Vol 1, (No. 2), 1985, p 93-104