Machinery's Handbook, 31st Edition
Heat Treatment of Steel 479 this paste and heat slowly until dry, then proceed to harden. Still another method is to heat the work to a blue color, or about 600°F (316°C), and then dip it in a strong solution of salt water prior to heating in the lead bath. The lead is sometimes removed from parts having fine projections or teeth, by using a stiff brush just before immersing in the cooling bath. Removal of lead is necessary to prevent the formation of soft spots. Tempering in Sand.— The sand bath is used for tempering certain classes of work. One method is to deposit the sand on an iron plate or in a shallow box that has burners beneath it. With this method of tempering, tools such as boiler punches, etc., can be given a varying temper by placing them endwise in the sand. As the temperature of the sand bath is higher toward the bottom, a tool can be so placed that the color of the lower end will become a deep dark blue when the middle portion is a very dark straw, and the working end or top a light straw color, the hardness gradually increasing from the bottom up. Double Tempering.— In tempering high-speed steel tools, it is common practice to re- peat the tempering operation or “double temper” the steel. Double tempering is done by heating the steel to tempering temperature, say 1050°F (566°C), and holding it at that temperature for 2 hours. It is then cooled to room temperature, reheated to the same tem- perature for another two-hour period, and again cooled to room temperature. After the first tempering operation, some untempered martensite remains in the steel. This mar - tensite is not only tempered by a second tempering operation but is relieved of internal stresses, thus improving the steel for service conditions. The hardening temperature for the higher-alloy steels may affect the hardness after tempering. For example, molybde- num high-speed steel heated to 2100°F (1149°C) had a 61 RC (Rockwell C scale) after tempering, whereas a temperature of 2250°F (1232°C) resulted in hardness of 64.5 RC after tempering. Annealing, Spheroidizing, and Normalizing Annealing of steel is a heat-treating process in which the steel is heated to some elevated temperature, usually in or near the critical range, is held at this temperature for some pe- riod of time, and is then cooled, usually at a slow rate. Spheroidizing and normalizing may be considered as special cases of annealing. The full annealing of carbon steel consists in heating it slightly above the upper critical point for hypoeutectoid steels (steels of less than 0.85 percent carbon content) and slightly above the lower critical point for hypereutectoid steels (steels of more than 0.85 percent carbon content), holding it at this temperature until it is uniformly heated and then slowly cooling it to 1000°F (538°C) or below. The resulting structure is layerlike, or lamellar, in character due to the pearlite that is formed during the slow cooling. Anealing is employed 1) to soften steel for machining, cutting, stamping, etc., or for some particular service; 2) to alter ductility, toughness, electrical or magnetic characteristics or other physical properties; 3) to refine the crystal structure; 4) to produce grain re orientation; and 5) to relieve stresses and hardness resulting from cold-working. The spheroidizing of steel, according to the American Society of Metals, is “any process of heating and cooling that produces a rounded or globular form of carbide.” High-carbon steels are spheroidized to improve their machinability especially in continuous cutting operations, such as are performed by lathes and screw machines. In low-carbon steels, spheroidizing may be employed to meet certain strength requirements before subsequent heat treatment. Spheroidizing also tends to increase resistance to abrasion. The normalizing of steel consists in heating it to some temperature above that used for annealing, usually about 100°F (56°C) above the upper critical range, and then cooling it in still air at room temperature. Normalizing is intended to put the steel into a uniform, unstressed condition of proper grain size and refinement so that it will properly respond to further heat treatments. It is particularly important in the case of forgings that are to be later heat treated. Normalizing may or may not (depending on the composition) leave steel in a sufficiently soft state for machining with available tools. Annealing for machinability
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