Machinery's Handbook, 31st Edition
482 Heat Treatment of Steel Usually, an initial heating and slow cooling is followed by reheating to 1400–1450°F (760 to 788°C), quenching in oil or water, and a final tempering. More definite information is given in the following section on SAE steels. Carburizers: There are many commercial carburizers on the market in which the materi als used as the generator may be hard and soft wood charcoal, animal charcoal, coke, coal, beans and nuts, bone and leather, or various combinations of these. The energizers may be barium, cyanogen, and ammonium compounds, various salts, soda ash, or lime and oil hydrocarbons. Pack-Hardening.— When cutting tools, gages, and other parts made from high-carbon steels are heated for hardening while packed in some carbonaceous material in order to protect delicate edges, corners, or finished surfaces, the process is usually known as pack-hardening. Thus, the purpose is to protect the work, prevent scale formation, ensure uniform heating, and minimize the danger of cracking and warpage. The work is packed, as in carburizing, and in the same type of receptacle. Common hardwood charcoal often is used, especially if it has had an initial heating to eliminate shrinkage and discharge its more impure gases. The lowest temperature required for hardening should be em- ployed for pack-hardening—usually 1400 to 1450°F (760 to 788°C) for carbon steels. Pack-hardening has also been applied to high-speed steels, but modern developments in heat-treating salts have made it possible to harden high-speed steel without decarburiza - tion, injury to sharp edges, or marring the finished surfaces. See Salt Baths on page 471. Cyanide Hardening.— When low-carbon steel requires a very hard outer surface but does not need high shock-resisting qualities, the cyanide-hardening process may be em- ployed to produce what is known as superficial hardness. This superficial hardening is the result of carburizing a very thin outer skin (which may be only a few thousandths inch thick) by immersing the steel in a bath containing sodium cyanide. The tempera- tures usually vary from 1450 to 1650°F (788 to 899°C), and the percentage of sodium cyanide in the bath extends over a wide range, depending on the steel used and properties required. Nitriding Process.— Nitriding is a process for surface hardening certain alloy steels by heating the steel in an atmosphere of nitrogen (ammonia gas) at approximately 950°F (510°C). The steel is then cooled slowly. Finish machined surfaces hardened by nitriding are subject to minimum distortion. The physical properties, such as toughness, high im- pact strength, etc., can be imparted to the core by previous heat treatments and are unaf - fected by drawing temperatures up to 950°F (510°C). The “Nitralloy” steels suitable for this process may be readily machined in the heat-treated as well as in the annealed state, and they forge as easily as alloy steels of the same carbon content. Certain heat treatments must be applied prior to nitriding, the first being annealing to relieve rolling, forging, or machining strains. Parts or sections not requiring heat treating should be machined or ground to the exact dimensions required. Close tolerances must be maintained in finish machining, but allowances for growth due to adsorption of nitrogen should be made, and this usually amounts to about 0.0005 inch (0.0127 mm) for a case depth of 0.02 inch (0.508 mm). Parts requiring heat treatment for definite physical properties are forged or cut from annealed stock, heat treated for the desired physical properties, rough machined, normal- ized, and finish machined. If quenched and drawn parts are normalized afterwards, the drawing and normalizing temperatures should be alike. The normalizing temperature may be below but should never be above the drawing temperature. Ion Nitriding.— Ion nitriding, also referred to as glow discharge nitriding, is a process for case hardening of steel parts, such as tool spindles, cutting tools, extrusion equipment, forging dies, gears, and crankshafts. An electrical potential ionizes low-pressure nitrogen gas, and the ions produced are accelerated to and impinge on the workpiece, heating it to the appropriate temperature for diffusion to take place. Therefore, there is no requirement for a supplemental heat source. The inward diffusion of the nitrogen ions forms the iron
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