Heat treatment process generally includes heating, heat preservation and cooling, sometimes only heating and cooling. These processes are interconnected and uninterrupted.
When the metal is heated, the workpiece is exposed to the air and often oxidizes and decarburizes (that is, the carbon content on the surface of steel parts is reduced), which has a very adverse effect on the surface properties of the parts after heat treatment. Therefore, the metal should be heated in a controlled or protective atmosphere, molten salt and vacuum, and can also be protected by coating or packaging.
Heating temperature is one of the important process parameters of the heat treatment process. Selecting and controlling the heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies with the metal material being treated and the purpose of heat treatment, but it is generally heated above the phase transformation temperature to obtain high-temperature structure. In addition, the transformation takes a certain time, so when the surface of the metal workpiece reaches the required heating temperature, it is also necessary to maintain this temperature for a certain time, so that the internal and external temperatures are consistent, and the microstructure transformation is complete. This period of time is called the holding time.
When high energy density heating and surface heat treatment are used, the heating speed is extremely fast, and generally there is no heat preservation time, while the heat preservation time of chemical heat treatment is often longer.
Cooling is also an indispensable step in the process of heat treatment. The cooling method varies with the process, mainly controlling the cooling speed.