methods of heat treatment of metals and their alloys in which the heat is produced by an electric current.
Electrothermic treatment, as opposed to flame heating, is most often used for surface hardening in an electrolyte or with high-frequency currents. A workpiece to be hardened in an electrolyte is placed in a tank filled with the electrolyte; the tank serves as an anode, and the workpiece, as a cathode. When a direct current is passed through the electrolyte, hydrogen is released. The hydrogen settles on the surface of the workpiece, whose electrical resistance is consequently increased; as a result, the workpiece is heated. The current is then turned off, and the workpiece is quenched either where it is (in the electrolyte) or in a separate quench bath. The advantages of the electrolytic process are its simplicity, the ability it provides for heating a specific area on a workpiece, and the ease with which the process may be automated. The disadvantages of the process are that temperature is difficult to regulate, productivity is low, and workpieces must be protected from corrosion.
Surface hardening with high-frequency currents makes possible the formation of a hard surface layer on a workpiece, whose core remains soft and malleable. The hardening is achieved by means of induction heating in an induction-heating unit. Depending on the size and shape of the workpiece and on the properties desired, hardening methods that use high-frequency currents may be classified as simultaneous, continuous-sequential, or sequential. The advantages of treatment with high-frequency currents are that the process is economical and has a short production time, the hardness of a surface treated with this method is greater than that of surfaces treated with other methods, and workpieces may be heated rapidly. Moreover, there is no scaling, the depth of the hardened layer may be accurately controlled, the process itself may be automated, and work conditions are improved.
N. A. SHEMELEV