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.

Heat treatment refers to a metal heat processing process in which materials are heated, insulated, and cooled in a solid state to obtain the desired structure and properties. In the process of advancing from the Stone Age to the Bronze Age and the Iron Age, the role of heat treatment has gradually been recognized.