The heat treatment process generally includes three processes of heating, heat preservation and cooling, and sometimes only heating and cooling processes. These processes are connected with each other and cannot be interrupted. Heating is one of the important processes of heat treatment. There are many heating methods for metal heat treatment. The earliest use of charcoal and coal as heat sources led to the use of liquid and gaseous fuels. The application of electricity makes heating easy to control and no environmental pollution. These heat sources can be used for direct heating, as well as for indirect heating via molten salts or metals as well as floating particles.
When the metal heat treatment is heated, the workpiece is exposed to air and oxidation and decarburization often occur (ie, the carbon content on the surface of the steel part is reduced), which has an adverse effect on the surface properties of the heat-treated parts. Thus metals should generally be heated in controlled atmospheres or protective atmospheres, in molten salts and in vacuum, but also protected by coatings or packaging methods.