Machinery's Handbook, 31st Edition
Heat Treatment of Steel 465 that depends on the case depth desired. A thin, very hard case results from the formation of nitrides. Strong nitride-forming elements (chromium and molybdenum) are required to be present in the steel, and often special nonstandard grades containing aluminum (a strong nitride former) are used. The major advantage of this process is that parts can be quenched and tempered, then machined, prior to nitriding, because only a little distortion occurs during nitriding. Cyaniding: This process involves heating the part in a bath of sodium cyanide to a tem perature slightly above the transformation range, followed by quenching, to obtain a thin case of high hardness. Carbonitriding: This process is similar to cyaniding except that the absorption of carbon and nitrogen is accomplished by heating the part in a gaseous atmosphere containing hydrocarbons and ammonia. Temperatures of 1425–1625°F (774 to 885°C) are used for parts to be quenched, and lower temperatures, 1200–1450°F (649 to 788°C), may be used where a liquid quench is not required. Flame Hardening: This process involves rapid heating with a direct high-temperature gas flame, such that the surface layer of the part is heated above the transformation range, followed by cooling at a rate that causes the desired hardening. Steels for flame hardening are usually in the range of 0.30–0.60 percent carbon, with hardenability appropriate for the case depth desired and the quenchant used. The quenchant is usually sprayed on the surface a short distance behind the heating flame. Immediate tempering is required and may be done in a conventional furnace or by a flame-tempering process, depending on part size and costs. Induction Hardening: This process is similar in many respects to flame hardening except that the heating is caused by a high-frequency electric current sent through a coil or inductor surrounding the part. The depth of heating depends on the frequency, the rate of heat conduction from the surface, and the length of the heating cycle. Quenching is usually accomplished with a water spray introduced at the proper time through jets in or near the inductor block or coil. In some instances, however, parts are oil-quenched by immersing them in a bath of oil after they reach the hardening temperature. Structure of Fully Annealed Carbon Steel.— In carbon steel that has been fully an- nealed, there are normally present, apart from such impurities as phosphorus and sul- fur, two constituents: the element iron in a form metallurgically known as ferrite and the chemical compound iron carbide in the form metallurgically known as cementite . This latter constituent consists of 6.67 percent carbon and 93.33 percent iron. A certain propor- tion of these two constituents will be present as a mechanical mixture. This mechanical mixture, the amount of which depends on the carbon content of the steel, consists of alter- nate bands or layers of ferrite and cementite. Under the microscope, the matrix frequently has the appearance of mother-of-pearl and hence has been named pearlite . Pearlite con- tains about 0.85 percent carbon and 99.15 percent iron, neglecting impurities. A fully an- nealed steel containing 0.85 percent carbon would consist entirely of pearlite. Such a steel is known as eutectoid steel and has a laminated structure characteristic of a eutectic alloy. Steel that has less than 0.85 percent carbon ( hypoeutectoid steel) has an excess of ferrite above that required to mix with the cementite present to form pearlite; hence, both ferrite and pearlite are present in the fully annealed state. Steel having a carbon content greater than 0.85 percent ( hypereutectoid steel) has an excess of cementite over that required to mix with the ferrite to form pearlite; hence, both cementite and pearlite are present in the fully annealed state. The structural constitution of carbon steel in terms of ferrite, cemen- tite, pearlite and austenite for different carbon contents and at different temperatures is shown by the accompanying figure, Phase Diagram of Carbon Steel . Effect of Heating Fully Annealed Carbon Steel.— When carbon steel in the fully an- nealed state is heated above the lower critical point, which is some temperature in the range of 1335 to 1355°F (724 to 735°C) depending on the carbon content, the alternate
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