Tool steels are the alloys used to manufacture the tools, dies, and molds that shape, form, and cut other materials, including steels, nonferrous metals, and plastics.

Tool steels are either carbon, alloy or high-speed steels, capable of being hardened and tempered. They are usually melted in electric furnaces and produced under tool steel practice to meet special requirements. They may be used in certain hand tools or in mechanical fixtures for cutting, shaping, forming and blanking of materials at either ordinary or elevated temperatures. Tool steels are also used on a wide variety of other applications where resistance to wear, strength, toughness and other properties are selected for optimum performance.

Also, while tool steels may be manufactured with properties for use in nontool applications, such as springs, magnets, bearings, or even structural applications. Tool Steels that are uniquely manufactured for tool applications, recognizing that some more recently developed ultrahigh strength steels—such as maraging steels, AF1410, and Aeromet 100, developed for structural applications that require high toughness—are sometimes also used for tool applications.

Classification of tool steels

The very large number of tool steels is effectively classified by the widely used system developed by the American Iron and Steel Institute (AISI). This system is the starting point for the selection of the proper steel for a given function from the large number of steels available.

The AISI classification system arranges tool steels into groups that are based on prominent characteristics such as alloying (for example, tungsten or molybdenum high-speed steels), application (for example, cold-work or hot-work tool steels), or heat treatment (for example, water-hardening or oil-hardening tool steels). Table 2-1 lists nine main groups of tool steels and their identifying letter symbols.

In addition to the AISI classification, the steels are identified by designations in the Unified Numbering System (UNS) for Metals and Alloys, established in 1975 by the Society of Automotive Engineers (SAE) and the American Society for Testing and Materials (ASTM).

The combinations of the properties of the various groups of tool steels are strong functions of heat treatment processing as determined by the alloying. The final heat treatment of tool steels invariably consists of a three-stage process: heating for austenite formation, cooling to transform the austenite to martensite, and heating or tempering to eliminate retained austenite and form carbides within the martensite. Several very general alloying and heat treatment principles are introduced at this point to provide a base for comparing the various groups of tool steels:

• The hardened microstructure of a typical tool steel consists of a matrix of tempered martensite containing various dispersions of iron and alloy carbides.
• High carbon and high alloy content promote hardenability or the ability to form martensite on cooling.
• The higher the carbon and alloy content in supersaturation in the martensite, as inherited from the parent austenite, the higher the density of carbides that can be formed on tempering.
• The higher the content of strong carbide-forming elements, the higher the density of carbides that are stable in austenite during hot work and austenitizing. These carbides are retained as components of the microstructure in addition to those formed in martensite during tempering.
• The higher the carbon content of the martensite and the higher the density of carbides, the higher the hardness and wear resistance but the lower the toughness of a tool steel microstructure.

WATER-HARDENING TOOL STEELS

These kinds of tool steels have the lowest alloy content of all the tool steels and are essentially carbon steels. Therefore, their hardenability is low, and water quenching is required to form martensite. Even with water quenching, only the surface of a tool may harden. However, the high carbon content of the water-hardening tool steels ensures that the martensite will be of high hardness where it forms. Consistent with the low-alloy content, only iron carbides are produced by heat treatment of the water-hardening tool steels.

SHOCK-RESISTING TOOL STEELS

These kinds of tool steels have been developed to provide high toughness and fracture resistance in combination with high strength and wear resistance under conditions of impact loading. The high toughness is achieved by maintaining carbon content at moderate levels, thereby keeping the carbon content of the martensite low and carbide dispersions fine. The alloy content of the shock-resisting tool steels is higher than that of the water-hardening tool steels; accordingly, the hardenability also is higher. The alloying elements also promote beneficial dispersions of carbides.

OIL-HARDENING, COLD-WORK TOOL STEELS

These kinds of tool steels have been developed to provide very high wear resistance under cold-working conditions. High hardness is provided by high-carbon martensite tempered at low temperatures to provide very fine dispersions of carbides. Hardenability, because of high carbon and moderate alloy content, is sufficient to provide good depth of hardening by oil quenching. The alloying and very high carbon content of type O7 steel promote graphite formation, which enhances machinability and die life.

AIR-HARDENING, MEDIUM-ALLOY COLD-WORK TOOL STEELS

These kinds of tool steels also have high wear resistance under cold-working conditions. The various grades, due to ranges of carbon and alloy content, have various combinations of hardness and toughness. Similar to the oil-hardening steels, the wear resistance is provided by high-carbon martensite and fine carbide dispersions. However, because of the high alloy content, hardenability is high enough to permit martensite formation on air cooling. The very slow cooling minimizes distortion and promotes good dimensional stability during heat treatment. This group of tool steels also has a grade, A10, which contains graphite in its microstructure.

HIGH-CARBON, HIGH-CHROMIUM COLD-WORK TOOL

These kinds of tool steels have extremely high wear and abrasion resistance. Not only is tempered high-carbon martensite an important component of the microstructure, but large volume fractions of alloy carbides also play an important role in achieving high wear resistance. Some of the alloy carbides are produced by solidification and coexist with austenite during hot working and austenitizing, and some are produced by tempering. Again, the high alloy content provides excellent hardenability and makes possible martensite formation on air cooling with attendant benefits for dimensional control. Although the high abrasion resistance of the D-type tool steels is desirable for cold-work applications, the machining and grindingoperations during manufacture of finished dies and molds are difficult.

LOW-ALLOY, SPECIAL-PURPOSE TOOL STEELS

These kinds of tool steels can be used in a variety of applications with characteristics overlapping those of water- and oil-hardening grades. The alloying provides moderate hardenability, in some cases suitable for oil hardening, but the availability of medium-carbon grades makes possible higher toughness compared to the high-carbon oil-hardening tool steels.

MOLD STEELS

These kinds of tool steels have low-carbon contents relative to other tool steels in order to permit the shaping, by hubbing or machining, of cavities for plastic molding or metal die casting while the steels are in a soft annealed condition. The molds or dies are then carburized and hardened to obtain high surface hardness and wear resistance. A key requirement for molds and dies is good polishability and excellent surface finish. Sometimes martensitic stainless steels are used for plastic molding dies if corrosion resistance is a factor in performance of the lower-alloy P steels.

CHROMIUM HOT-WORK TOOL STEELS

These kinds of tool steels must have excellent resistance to high-temperature impact loading, to softening during high-temperature exposure, and to thermal fatigue. This demanding set of requirements—typical of forging, many other types of hot working, and die casting—is accomplished by the use of medium-carbon contents and relatively high concentrations of chromium and other strong carbide-forming elements. The medium-carbon content promotes toughness by limiting the carbon concentration of the martensite and by limiting the size of alloy carbide particles. Good high-temperature strength is achieved by tempering at high temperatures where fine, stable dispersions of chromium and vanadium alloy carbides precipitate. These carbides coarsen only slowly in service. The high-alloy content of the H steels also provides excellent hardenability and permits the hardening of heavy sections by air cooling.

TUNGSTEN HOT-WORK TOOL STEELS

These kinds of tool steels have much greater resistance than the chromium hot-work steels to softening during high-temperature exposure. The improved resistance to softening is accomplished by the addition of substantial amounts of tungsten, which in addition to the chromium, produce large volume fractions of stable alloy carbides in the microstructure. The latter distributions of carbides, however, also reduce toughness.

MOLYBDENUM HOT-WORK TOOL STEELS

These kinds of tool steels The H42 steel listed in this category in Table 2-2 has softening resistance comparable to the tungsten hot-work steels and thus, offers an alternate choice, depending on availability and cost. Molybdenum, another strong alloy carbide-forming element, is added with chromium, vanadium, and tungsten to provide a large volume fraction of alloy carbides, which results in stable microstructures when exposed to high-temperature forming operations.

TUNGSTEN HIGH-SPEED TOOL STEELS

These kinds of tool steels are used for high-speed cutting tool applications. The alloy content of strong carbide-forming elements, especially tungsten in amounts of 18% or more, is so high that these steels do not soften even when exposed to temperatures where the tool appears red. The T steels, by virtue of their relatively high carbon and very high alloy contents, have high hardenability and are processed to develop microstructures with large volume fractions of high-temperature stable carbides, which create excellent wear resistance and red hardness.

MOLYBDENUM HIGH-SPEED TOOL STEELS

These kinds of tool steels similar to the T group of high-speed steels, are used for high speed cutting tool applications. Molybdenum replaces some of the tungsten of the T steels, but the performance characteristics of the T and M steels, except for the slightly higher toughness of the M steels, are essentially the same. Alloying considerations (namely, the formation of stable alloy carbides and the addition of cobalt to increase red hardness) and heat-treating processing are similar for both types of high-speed steels. However, the M steels more readily decarburize and require more care in hardening.

ULTRAHARD HIGH-SPEED TOOL STEELS

These kinds of tool steels are capable of being hardened to 70 HRC, compared to the maximum hardness of 65/66 HRC attainable in the other high-speed steels. The very high hardness represents an optimization of the heat-treating and alloying design characteristic of high-speed steels i.e., very high densities of primary alloy carbides and fine alloy carbides produced by secondary hardening at high tempering temperatures and is achieved by higher carbon contents together with high contents of cobalt and the strong carbide-forming elements. Toughness at very high hardness levels is a major consideration in the application of the ultrahard high-speed steel, and often these steels are used with hardness below the maximum attainable.