Machinery's Handbook, 31st Edition
Heat Treatment of Steel 485 compared with a rate of 3 Btu/in 2 /min (5 J/mm 2 /min) for the same material at room tem- perature when placed in a furnace with a wall temperature of 2000°F (1093°C); 2) absence of tendency to produce oxidation or decarburization; 3) exact control of depth and area of hardening; 4) close regulation of degree of hardness obtained by automatic timing of heating and quenching cycles; 5) minimum warpage or distortion; and 6) possibility of substituting carbon steels for higher-cost alloy steels. To the designer, the principal advantage of induction hardening lies in its application to localized zones. Thus, specific areas in a given part can be heat treated separately to the respective hardnesses required. Parts can be designed so that the stresses at any given point in the finished piece can be relieved by local heating. Parts can be designed in which welded or brazed assemblies are built up prior to heat treating with only internal surfaces or projections requiring hardening. Types of Induction Heating Equipment.— Induction heating is secured by placing the metal part inside or close to an “applicator” coil of one or more turns through which alter nating current is passed. The coil, formed to suit the general class of work to be heated, is usually made of copper tubing through which water is passed to prevent overheating of the coil itself. The workpiece is either held in a fixed position or rotated slowly within or close to the applicator coil. Where the length of work is too great to permit heating in a fixed position, progressive heating may be employed. Thus, a rod or tube of steel may be fed through an applicator coil of one or more turns so that the heating zone travels progres sively along the entire length of the workpiece. The frequency of the alternating current used and the type of generator employed to supply this current to the applicator coil depend on the character of the work to be done. There are three types of commercial equipment used to produce high-frequency cur- rent for induction heating: 1) motor generator sets that deliver current at frequencies of approximately 1000, 2000, 3000, and 10,000 cycles; 2) spark gap oscillator units that produce frequencies ranging from 80,000 to 300,000 cycles; and 3) vacuum tube oscil - lator sets, which produce currents at frequencies ranging from 350,000 to 15,000,000 cycles or more. Depth of Heat Penetration.— Generally speaking, the higher the frequency used, the shallower the depth of heat penetration. For heating clear through, for deep hardening, and for large workpieces, low power concentrations and low frequencies are usually used. For very shallow and closely controlled depths of heating, as in surface hardening and local ized heat treating of small workpieces, currents at high frequencies are used. For example, a 1 ⁄ 2 -inch (12.7 mm) round bar of hardenable steel will be heated through its entire structure quite rapidly by an induced current of 2000 cycles. After quenching, the bar would show through hardness with a decrease in hardness from surface to center. The same piece of steel could be readily heated and surface hardened to a depth of 0.100 inch (2.5 mm) with current at 9600 cycles, and to an even shallower depth with current at 100,000 cycles. A 1 ⁄ 4 -inch (6.35 mm) bar, however, would not reach a sufficiently high temperature at 2000 cycles to permit hardening, but at 9600 cycles through hardening would be accomplished. Current at over 100,000 cycles would be needed for surface hardening such a bar. Types of Steel for Induction Hardening.— Most of the standard types of steels can be hardened by induction heating, providing the carbon content is sufficient to produce the desired degree of hardness by quenching. Thus, low-carbon steels with a carburized case, medium- and high-carbon steels (both plain and alloy), and cast iron with a portion of the carbon in combined form, may be used for this purpose. Induction heating of alloy steels should be limited primarily to the shallow-hardening type, that is, those of low alloy con tent; otherwise the severe quench usually required may result in a highly stressed surface with consequent reduced load-carrying capacity and danger of cracking.
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