Adjusting and controlling heating temperature are the main point to ensure the quality of heat treatment. The heating temperature varies with the metal material and the purpose of heat treatment, but it is generally heated to more than a characteristic transition temperature, in order to obtain high temperature structure. In addition, the transformation takes time, so when the surface of the metal workpiece reaches the required heating temperature, it must keep for a certain time. Then, the internal and external temperatures are consistent, so the transformation of the microstructures is complete. This period is called temperature preservation time. If high energy density heating and surface heat treatment are used, the heating speed is very fast. Generally, there is no heat preservation time. The heat preservation time of chemical heat treatment is often longer.