Heat treatment refers to a metal thermal processing process in which the material is in a solid state by means of heating, heat preservation and cooling to obtain the expected structure and properties. In the process from the Stone Age to the Bronze Age and Iron Age, the role of heat treatment is gradually recognized by people.

Metal heat treatment is one of the important processes in machinery manufacturing. Compared with other processing technologies, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but changes the microstructure inside the workpiece or changes the chemical composition of the workpiece surface to give or improve the performance of the workpiece. It is characterized by an improvement in the intrinsic quality of the workpiece, which is generally not visible to the naked eye. In order to make the metal workpiece have the required mechanical properties, physical properties and chemical properties, in addition to the reasonable selection of materials and various forming processes, heat treatment process is often essential. Steel is the most widely used material in the machinery industry. The microstructure of steel is complex and can be controlled by heat treatment. Therefore, heat treatment of steel is the main content of metal heat treatment. In addition, aluminum, copper, magnesium, titanium, etc. and their alloys can also be heat treated to change their mechanical, physical and chemical properties to obtain different performance.
Metal heat treatment process can be roughly divided into three categories: overall heat treatment, surface heat treatment and chemical heat treatment. According to the different heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. The same metal adopts different heat treatment processes to obtain different structures and thus have different properties. Steel is the most widely used metal in industry, and the microstructure of steel is also the most complex, so there are many types of steel heat treatment processes. The overall heat treatment is a metal heat treatment process that heats the workpiece as a whole, and then cools it at an appropriate rate to obtain the required metallographic structure to change its overall mechanical properties.