When metal is heated, the workpiece is exposed to the air, and oxidation and decarburization often occur (that is, the carbon content on the surface of the steel part is reduced), which has a very negative impact on the surface properties of the parts after heat treatment. Therefore, metals should usually be heated in a controlled atmosphere or protective atmosphere, in molten salt, and in a vacuum. Protective heating can also be carried out by coating or packaging methods.
Heating temperature is one of the important process parameters of the heat treatment process. Selection and control of the heating temperature are the main issues to ensure the quality of heat treatment. The heating temperature varies with the metal material being processed and the purpose of the heat treatment, but it is generally heated to above the phase transformation temperature to obtain a high-temperature structure. In addition, the transformation requires a certain amount of time, so when the surface of the metal workpiece reaches the required heating temperature, it must be maintained at this temperature for a certain period of time to make the internal and external temperatures consistent and the microstructure transformation to be complete. This period of time is called the holding time.
