heat parts that are uneven in cross section, even though the heating rate is slow. However, such parts are less apt to be cracked or excessively warped when the heating rate is slow. Soaking The object of heat treating is to bring about changes in the properties of metal. To accomplish this, the metal must be heated to the temperature at which structural changes take place within the metal. These changes occur when the constituents of the metal go into the solution. Once the metal is heated to the proper temperature, it must be held at that temperature until the metal is heated throughout and the changes have time to take place. This holding of the metal at the proper temperature is called SOAKING. The length of time at that temperature is called the SOAKING PERIOD. The soaking period depends on the chemical analysis of the metal and the mass of the part. When steel parts are uneven in cross section, the soaking period is determined by the heaviest section. In heating steels, the metal is seldom raised from room temperature to the final temperature in one operation. Instead, the steel is slowly heated to a temperature below the point at which the solid solution begins, and it is then held at that temperature until heat is absorbed throughout the metal. This process is called PREHEATING. Following the preheating, the steel is quickly heated to the final temperature. Preheating aids in obtaining uniform temperature throughout the part being heated, and, in this way, reduces distortion and cracking. When apart is of intricate design, it may have to be preheated at more than one temperature to prevent cracking and excessive warping. As an example, assume that an intricate part is to be heated to 1,500°F (815°C) for hardening. This part might be slowly heated to 600°F (315°C), be soaked at this temperature, then be heated slowly to 1,200°F (649°C), and then be soaked at that temperature. Following the second preheat, the part would be heated quickly to the hardening temperature. Nonferrous metals are seldom preheated because they usually do not require it. Furthermore, preheating tends to increase the grain size in these metals. Cooling After being heated to the proper temperature, the metal must be returned to room temperature to complete the heat-treating process. The metal is cooled by placing it in direct contact with a gas, liquid, or solid, or some combination of these. The solid, liquid, or gas used to cool the metal is called a “cooling medium.” The rate at which the metal should be cooled depends on both the metal and the properties desired. The rate of cooling also depends on the medium; therefore, the choice of a cooling medium has an important influence on the properties obtained. Cooling metals rapidly is called “quenching,” and the oil, water, brine, or other mediums used for rapid cooling is called a “quenching medium.” Since most metals must be cooled rapidly during the hardening process, quenching is generally associated with hardening. However, quenching does not always result in an increase in hardness. For example, copper is usually quenched in water during annealing. Other metals, air-hardened steels for example, may be cooled at a relatively slow rate for hardening. Some metals are easily cracked or warped during quenching. Other metals may be cooled at a rapid rate with no ill effects. Therefore, the quenching medium must be chosen to fit the metal. Brine and water cool metals quickly, and should be used only for metals that require a rapid rate of cooling. Oil cools at a slower rate and is more suitable for metals that are easily damaged by rapid cooling. Generally, carbon steels are considered water hardened and alloy steels oil hardened. Nonferrous metals are usually quenched in water. FORMS OF HEAT TREATMENT 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 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. 15-38