【文章內(nèi)容簡介】 cedures are clearcut in terms of technique and application． whereas in other instances， descriptions or simple explanations are insufficient because the same technique frequently may be used to obtain different objectives． For example, stress relieving and tempering are often acplished with the same equipment and by use of identical time and temperature cycles． The objectives， however， are different for the two processes． The following descriptions of the principal heat treating processes are generally arranged according to their interrelationships． Normalizing consists of heating a ferrous alloy to a suitable temperature (usually 50176。 F to 100176。F or 28℃ to 56℃ ) above its specific upper transformation temperature． This is followed by cooling in still air to at least some temperature well below its transformation temperature range． For lowcarbon steels, the resulting structure and properties are the same as those achieved by full annealing； for most ferrous alloys, normalizing and annealing are not synonymous. Normalizing usually is used as a conditioning treatment, notably for refining the grains of steels that have been subjected to high temperatures for fing or other hot working operations. The normalizing process usually is succeeded by another heat treating operation such as austenitizing for hardening, annealing, or tempering. Annealing is a generic term denoting a heat treatment that consists of heating to and holding at a suitable temperature followed by cooling at a suitable rate. It is used primarily to soften metallic materials, but also to simultaneously produce desired changes in other properties or in microstructure. The purpose of such changes may be, but is not confined to, improvement of machinability, facilitation of cold work (known as inprocess annealing), improvement of mechanical or electrical properties, or to increase dimensional stability. When applied solely to relive stresses, it monly is called stressrelief annealing, synonymous with stress relieving. When the term “ annealing” is applied to ferrous alloys without qualification, full annealing is applied. This is achieved by heating above the alloy’ s transformation temperature, then applying a cooling cycle which provides maximum softness. This cycle may vary widely, depending on position and characteristics of the specific alloy. Quenching is a rapid cooling of a steel or alloy from the austenitizing temperature by immersing the workpiece in a liquid or gaseous medium. Quenching medium monly used include water, 5% brine, 5% caustic in an aqueous solution, oil, polymer solutions, or gas (usually air or nitrogen). Selection of a quenching medium depends largely on the hardenability of material and the mass of the material being treating (principally section thickness). The cooling capabilities of the abovelisted quenching media vary greatly. In selecting a quenching medium, it is best to avoid a solution that has more cooling power than is needed to achieve the results, thus minimizing the possibility of cracking and warp of the parts being treated. Modifications of the term quenching include direct quenching, fog quenching, hot quenching, interrupted quenching, selective quenching, spray quenching, and time quenching. Tempering. In heat treating of ferrous alloys, tempering consists of reheating the austenitized and quenchhardened steel or iron to some preselected temperature that is below the lower transformation temperature (generally below 1300 ℃ or 705 ℃ ). Tempering offers a means of obtaining various binations of mechanical properties. Tempering temperatures used for hardened steels are often no higher than 300 ℃ (150 ℃ ). The term “ tempering” should not be confused with either process annealing or stress relieving. Even though time and temperature cycles for the three processes may be the same, the conditions of the materials being processed and the objectives may be different. Stress relieving. Like tempering, stress relieving is always done by heating to some temperature below the lower transformation temperature for steels and irons. For nonferrous metals, the temperature may vary from slightly above room temperature to several hundred degrees, depending on the alloy and the amount of stress relief that is desired. The primary purpose of stress relieving is to relieve stresses that have been imparted to the workpiece from such processes as forming, rolling, machining or welding. The usual procedu