Machinery's Handbook, 31st Edition
464 Heat Treatment of Steel Tempering: As-quenched steels are in a highly stressed condition and are seldom used without tempering. Tempering imparts plasticity or toughness to the steel and is inevita bly accompanied by a loss in hardness and strength. The loss in strength, however, is only incidental to the very important increase in toughness, which is due to the relief of residual stresses induced during quenching and to precipitation, coalescence, and spheroidization of iron and alloy carbides resulting in a microstructure of greater plasticity. Alloying slows the tempering rate, so that alloy steel requires a higher tempering temperature to obtain a given hardness than carbon steel of the same carbon content. The higher tempering temperature for a given hardness permits a greater relaxation of residual stress and thereby improves the steel’s mechanical properties. Tempering is done in furnaces or in oil or salt baths at temperatures varying from 300 to 1200°F (149 to 649°C). With most grades of alloy steel, the range between 500 and 700°F (260 to 371°C) is avoided because of a phenomenon known as “blue brittleness,” which reduces impact properties. Tempering the martensitic stainless steels in the range of 800–1100°F (427 to 593°C) is not recommended because of the low and erratic impact properties and reduced corrosion resistance that result. Maximum toughness is achieved at higher temperatures. It is important to temper parts as soon as possible after quenching because any delay greatly increases the risk of cracking resulting from the high-stress condition in the as- quenched part. Surface Hardening Treatment (Case Hardening). —Many applications require high hardness or strength primarily at the surface, and complex service stresses frequently require not only a hard, wear-resistant surface, but also core strength and toughness to withstand impact stress. To achieve these different properties, two general processes are used: 1) The chemical composition of the surface is altered, prior to or after quenching and tempering; the pro cesses used include carburizing, nitriding, cyaniding, and carbonitriding; and 2) Only the surface layer is hardened by the heating and quenching process; the most common pro cesses used for surface hardening are flame hardening and induction hardening. Carburizing: Carbon is diffused into the part’s surface to a controlled depth by heating the part in a carbonaceous medium. The resulting depth of carburization, commonly referred to as case depth, depends on the carbon potential of the medium used and the time and temperature of the carburizing treatment. The steels most suitable for carburizing to enhance toughness are those with sufficiently low carbon contents, usually below 0.3 percent. Carburizing temperatures range from 1550 to 1750°F (843 to 954°C), with the temperature and time at temperature adjusted to obtain various case depths. Steel selection, hardenability, and type of quench are determined by section size, desired core hardness, and service requirements. Three types of carburizing are most often used: 1) Liquid carburizing involves heating the steel in molten barium cyanide or sodium cyanide. The case absorbs some nitrogen in addition to carbon, thus enhancing surface hardness; 2) Gas carburizing involves heating the steel in a gas of controlled carbon content. When used, the carbon level in the case can be closely controlled; and 3) Pack carburizing, which involves sealing both the steel and solid carbonaceous material in a gas-tight container, then heating this combination. With any of these methods, the part may be either quenched after the carburizing cycle without reheating or air cooled followed by reheating to the austenitizing temperature prior to quenching. The case depth may be varied to suit the conditions of loading in ser vice. However, service characteristics frequently require that only selective areas of a part have to be case hardened. Covering the areas not to be cased, with copper plating or a layer of commercial paste, allows the carbon to penetrate only the exposed areas. Another method involves carburizing the entire part, then removing the case in selected areas by machining, prior to quench hardening. Nitriding: The steel part is heated to a temperature of 900–1150°F (482 to 621°C) in an atmosphere of ammonia gas and dissociated ammonia for an extended period of time
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