The various heat-treating processes are similar in that they involve the heating and cooling of metals. They differ, however, in the temperatures to which the metals are heated, the rates at which they are cooled, and, of course, in the final result. The most common forms of heat treatment for ferrous metals are annealing, normalizing, hardening, tempering, and case hardening. Most nonferrous metals can be annealed but never tempered, normalized, or case hardened. Successful heat-treating requires close control over all factors affecting the heating and cooling of metals. Such control is possible only when the proper equipment is available, and the equipment is selected to fit the particular job.

Annealing is used to reduce residual stresses, induce softness, alter ductility, or refine the grain structure. Maximum softness in metal is accomplished by heating it to a point above the critical temperature, holding at this temperature until the grain structure has been refined, followed by slow cooling.

Normalizing is a process whereby iron base alloys are heated to approximately 100F (56C) above the upper critical temperature, followed by cooling to room temperature in still air. Normalizing is used to establish materials of the same nature with respect to grain size, composition, structure, and stress.

Hardening is accomplished by heating the metal slightly in excess of the critical temperature, and then rapidly cooling by quenching in oil, water, or brine. This treatment produces a fine grain structure, extreme hardness, maximum tensile strength, and minimum ductility. Generally, material in this condition is too brittle for most practical uses, although this treatment is the first step in the production of high-strength steel.

Tempering (drawing) is a process generally applied to steel to relieve the strains induced during the hardening process. It consists of heating the hardened steel to a temperature below the critical range, holding this temperature for a sufficient period, and then cooling in water, oil, or air. In this process, the degrees of strength hardness and ductility obtained depend directly upon the temperature to which the steel is heated. High tempering temperatures improve ductility at the sacrifice of tensile, yield strength, and hardness.

The objective in casehardening is to produce a hard case over a tough core. Casehardening is ideal for parts that require a wear-resistant surface and, at the same time, must be tough enough internally to withstand the applied loads. The steels best suited to case hardening are the low-carbon and low-alloy steels. If high-carbon steel is case-hardened, the hardness penetrates the core and causes brittleness. In case hardening, the surface of the metal is changed chemically by inducing a high carbide or nitride content. The core is unaffected chemically. When heat treated, the surface responds to hardening while the core toughens. The common methods of case hardening are carburizing, nitriding, and cyaniding.