Machinery's Handbook, 31st Edition
THERMAL TREATMENTS OF STEEL 405 brine, salt or air to form a hard, usually brittle, metastable structure known as untempered or white martensite; tempering the steel by reheating it to a temperature below the critical range in order to obtain the required combination of hardness, strength, ductility, tough ness, and structural stability (tempered martensite). Two well-known modifications of conventional quenching and tempering are “austem pering” and “martempering.” They involve interrupted quenching techniques (two or more quenching media) that can be utilized for some steels to obtain desired structures and properties while minimizing distortion and cracking problems that may occur in conventional hardening. Normalizing: The steel is heated to a temperature above the critical range, after which it is cooled in still air to produce a generally fine pearlite structure. The purpose is to promote uniformity of structure and properties after a hot-working operation such as forging or extrusion. Steels may be placed in service in the normalized condition, or they may be subjected to additional thermal treatment after subsequent machining or other operations. Annealing: The steel is heated to a temperature above or within the critical range, then cooled at a predetermined slow rate (usually in a furnace) to produce a coarse pearlite structure. This treatment is used to soften the steel for improved machinability; to improve or restore ductility for subsequent forming operations; or to eliminate the residual stresses and microstructural effects of cold-working. Spheroidize Annealing: This is a special form of annealing that requires prolonged heat ing at an appropriate temperature followed by slow cooling in order to produce globular carbides, a structure desirable for machining, cold forming, or cold drawing, or for the effect it will have on subsequent heat treatment. Stress Relieving: This process reduces internal stresses caused by machining, cold- working, or welding. The steel is heated to a temperature below the critical range and held there long enough to equalize the temperature throughout the piece. See the sections HARDENING, TEMPERING, AND ANNEALING on page 458 and Heat Treating High-Speed Steels on page 493 for more information about the heat treatment of steels. Applications.— Many factors enter into the selection of a steel for a particular applica- tion. These factors include the mechanical and physical properties needed to satisfy the design requirements and service environment; the cost and availability of the material; the cost of processing (machining, heat treatment, welding, etc.); and the suitability of available processing equipment or the cost of any new equipment required. These steel selection considerations require input from designers, metallurgists, manu facturing engineers, service engineers, and procurement specialists, and can be considered proper or optimum when the part is made from the lowest cost material consistent with sat isfying engineering and service requirements. The factors in selection can vary widely among different organizations, so that several different steels may be used successfully for similar applications. The best choice of a steel for any application most often results from a balance or trade-offs among the various selection considerations. The AISI/SAE designated “standard steels” provide a convenient way for engineers and metallurgists to state briefly but clearly the chemical composition and, in some instances, desired properties. These steels are widely recognized and used in the United States and in many other countries. There are, however, numerous nonstandard carbon, alloy, and stainless steel grades that are widely used for special applications. The following sections and tables illustrate the general characteristics and typical appli cations of most of the standard carbon, alloy, and stainless steel grades. General Application of SAE Steels: These applications are intended as a general guide only since the selection may depend on the exact character of the service, cost of material, machinability when machining is required, or other factors. When more than one steel is recommended for a given application, information on the characteristics of each steel listed will be found in the section Carbon Steels starting on page 407.
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