Tool Steels, 5th Edition (#06590G) Copyright © 1998 ASM International ® Author(s): G. Roberts, G. Krauss, and R. Kennedy All rights reserved. www.asminternational.org 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 2 Tool Steels, 5th Edition (#06590G) Copyright © 1998 ASM International ® Author(s): G. Roberts, G. Krauss, and R. Kennedy IntroductioAll rightsn reserved. www.asminternational.org 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).