Reheating of Metal

The purpose of reheating of metal before pressure working (rolling, forging, stamping, and forge welding) is to impart to it the required plasticity; in the case of heat treatment or thermochemical treatment, the purpose is to change the metal’s mechanical, physical, or chemical properties.

Metal is reheated mainly by heat transfer from without in heating or heat-treatment furnaces or by the generation of heat directly within the metal by passing an electric current through it or by exciting an induction current in it. During reheating in furnaces, heat is supplied to the surface of the metal and then flows into its interior. When an electric current is passed through the metal, heat is released throughout its volume. This makes possible a high rate of heating. This method of contact heating is suitable for articles of moderate cross section and considerable length. In induction reheating of metals, heat is generated in a thin surface layer, from which it flows into the interior, as in the case of furnace heating.

The temperature to which metal is reheated before pressure working is selected on the basis of the properties and intended use of the metal (for example, for aluminum the temperature is 250°-500°C, and for steel it is 115O°-13O0°C), as well as on the basis of the characteristics of the processing equipment. The temperature of the thermal or thermochemical treatment depends on the purpose of the treatment and the temperatures of the structural transformations of the metal or alloy (200°-1150°C). The quality of reheating of a metal is indicated by the uniformity of temperature over the surface and throughout the volume of metal.

Reheating of metal in an atmosphere of fuel combustion products in internally fired furnaces or in air in electric furnaces takes place with oxidation and decarburization of the metal surface. Such reheating is called dark reheating and is mainly used before pressure working. For heat-treatment purposes, however, preference is given to “light” reheating in an inert gas atmosphere, which does not react with the metal surface. In a number of cases, reheating of metal is performed under vacuum. In the case of thermochemical treatment, the metal is reheated in an atmosphere that reacts with it—for example, by removing carbon from it or saturating it with carbon. Reheating in active or neutral atmospheres is performed in furnaces heated by radiation tubes or electric heating elements, as well as in muffle furnaces.