Machinery's Handbook, 31st Edition
476 Heat Treatment of Steel the respective steels by a conventional quench and tempering to above 400°F (204°C). Much greater toughness and ductility are obtained in an austempered piece, however, as compared with a similar piece quenched and tempered in the usual manner. Two factors are important in austempering. First, the steel must be quenched rapidly enough to the specified subtransformation temperature to avoid any formation of pearlite, and, second, it must be held at this temperature until the transformation from austenite to bainite is completed. Time and temperature transformation curves (called S-curves because of their shape) have been developed for different steels, and these curves provide important data governing the conduct of austempering as well as the other interrupted quenching methods. Austempering has been applied chiefly to steels having 0.60 percent or more carbon content with or without additional low-alloy content, and to pieces of small diameter or section, usually under 1 inch (2.5 cm), but varying with the composition of the steel. Case- hardened parts may also be austempered. Martempering: In this process the steel is first rapidly quenched from some temperature above the transformation point down to some temperature (usually about 400°F or 204°C) just above that at which martensite begins to form. It is then held at this temperature for a length of time sufficient to equalize the temperature throughout the part, after which it is removed and cooled in air. As the temperature of the steel drops below the transformation point, martensite begins to form in a matrix of austenite at a fairly uniform rate throughout the piece. The soft austenite acts as a cushion to absorb some of the stresses which develop as the martensite is formed. The difficulties presented by quench cracks, internal stresses, and dimensional changes are largely avoided; thus a structure of high hardness can be obtained. If greater toughness and ductility are required, conventional tempering may follow. In general, heavier sections can be hardened more easily by the martempering process than by the austempering process. The martempering process is especially suited to the higher-alloyed steels. Isothermal Quenching: This process resembles austempering in that the steel is first rapidly quenched from above the transformation point down to a temperature that is above that at which martensite begins to form and is held at this temperature until the austenite is completely transformed into bainite. The constant temperature to which the piece is quenched and then maintained is usually 450°F (232°C) or above. The process differs from austempering in that, after transformation to a bainite structure has been completed, the steel is immersed in another bath and is brought up to some higher temperature, depending on the characteristics desired. It is maintained at that temperature for a definite period of time, followed by cooling in air. Thus, tempering to obtain the desired toughness or ductility takes place immediately after the structure of the steel has changed to bainite and before it is cooled to atmospheric temperature. Laser and Electron-Beam Surface Hardening.— Industrial lasers and electron-beam equipment are now available for surface hardening of steels, a process referred to as laser peening . The laser and electron beams can generate intense energy fluxes and steep tem - perature profiles in the workpiece so that external quench media are not needed. This self-quenching is due to a cold interior with sufficient mass acting as a large heat sink to rapidly cool the hot surface by conducting heat to the interior of a part. The laser beam is a beam of light and does not require a vacuum for operation. The electron beam is a stream of electrons, and processing usually takes place in a vacuum chamber or envelope. Both processes may normally be applied to finished machined or ground surfaces because little distortion results. Tempering The object of tempering or drawing is to reduce the brittleness in hardened steel and to remove the internal strains caused by the sudden cooling in the quenching bath. The tempering process consists in heating the steel by various means to a certain temperature and then cooling it. When steel is in a fully hardened condition, its structure consists
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