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tool steel[′tül ¦stēl]
carbon or alloy steel for the manufacture of cutting and measuring tools, and dies for hot and cold forming, as well as parts of machines which undergo increased wear under moderate dynamic loads (ball and roller bearings, gears, and motion screws in high-precision machine tools). As a rule, tool steel contains more than 0.6–0.7 percent carbon; die steels for hot forming, which contain 0.3–0.6 percent carbon, are an exception. For improved performance characteristics, tool steels undergo thermal treatment (hardening or tempering), which increases their Rockwell C hardness (HRC) to 60–66 and the bending strength to 2.5–3.5 giganewtons per sq m (GN/m2), or 250–350 kilograms-force per sq mm (kgf/mm2). As the hardness of tool steel increases so does its wear resistance (the ability to retain fixed dimensions and shape of the operating surface under friction with high pressures). Tool steels alloyed with chromium and manganese have higher hardenability and calcinability than carbon steels. Increased red hardness of tool steel—the ability to retain high hardness and wear resistance at temperatures up to 500°-700°C—is achieved by alloying steels with tungsten, molybdenum, and vanadium. Tool steels are subdivided into three groups according to their resistance to the heat that is generated during their use (see Table 1).
Steels with low heat resistance retain great hardness up to 150°-200°C and are used for low-speed cutting of soft materials and for cold forming. Carbon steels of this group are characterized by low hardenability—upon hardening, articles with a diameter or thickness of more than 15–20 mm acquire great hardness (up to 65 HRC) only in a thin surface layer, retaining a soft and viscous core. Because of increased deformation during hardening with cooling in water, mainly tools of simple shapes, such as files, reamers, and hand taps, are made of carbon steel. Low-alloy steels, which have somewhat better hardenability, are used for tools with a small cross section requiring high and uniform hardness, such as hacksaw blades for manual cutting of metals, razor blades, and circular wood saws. Alloyed steels of this group have high hardenability (25–100 mm) and are used for measuring tools, ball races of antifriction bearings, and dies of complex shape.
|Table 1. Chemical composition of common Soviet tool steels (average, percent)|
|Steels with low heat resistance|
|Steels with high heat resistance|
|Heat-resistant steels (die steels)|
Steels with high heat resistance retain their performance characteristics at temperatures up to 250°-4O0°C. They are mainly alloyed steels with a high chromium content (up to 12 percent). They have increased wear resistance under conditions of abrasive wear, since they contain up to 20–30 percent high-hardness chromium and vanadium carbides: Me7C3 (Vickers hardness [HV], 1, 200–1,400) and MeC (2,000 HV). After thermal treatment (hardening with cooling in air, oil, or molten salts with a temperature of 150°-180°C), they acquire a hardness of up to 63 HRC. These steels are characterized by high hardenability (up to 300–400 mm) and minimal volume changes during hardening. High-chromium steels are used in manufacturing large dies that undergo increased wear, as well as surgical instruments that are stable in aggressive mediums.
Steels that are resistant to heat retain their hardness up to 560o-700°C. Tungsten and molybdenum are the basic alloying elements of these steels and provide their red hardness. Steels with an increased carbon content (0.7–1.5 percent) and high hardness (up to 64–68 HRC) are used for the manufacture of cutting tools; steels with up to 0.4 percent carbon (die steels), which have lower hardness but better viscosity, are used for hot-forming dies and pressure metal-casting molds.
REFERENCESGuliaev, A. P., K. A. Malinina, and S. M. Saverina. Instrumental’nye stali: Spravochnik. Moscow, 1961.
Geller, Iu. A. Instrumental’nye stali, 3rd ed. Moscow, 1968. (Contains bibliography.)
IU. A. GELLER