Cooling is also an indispensable step in the heat treatment process. The cooling method varies depending on the process, mainly controlling the cooling speed. Generally, annealing has the slowest cooling speed, normalizing has a faster cooling speed, and quenching has a faster cooling speed. However, there are different requirements depending on the type of steel. For example, air hardened steel can be hardened at the same cooling rate as normalizing.

Metal heat treatment is one of the important processes in mechanical manufacturing. Compared to other processing processes, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but rather endows or improves the working performance of the workpiece by changing the microstructure inside the workpiece or changing the chemical composition of the workpiece surface. Its characteristic is to improve the internal quality of the workpiece, which is generally not visible to the naked eye. In order to achieve the required mechanical, physical, and chemical properties of metal workpiece, in addition to reasonable selection of materials and various forming processes, heat treatment processes are often essential.