Heat treatment refers to a metal thermal processing process in which the material is in a solid state by heating, holding, and cooling to obtain the desired structure and properties.

Metal heat treatment processes 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 major category can be divided into several different heat treatment processes. The same metal uses different heat treatment processes to obtain different structures and thus have different properties.

The heat treatment process generally includes three processes of heating, heat preservation and cooling, and sometimes only two processes of heating and cooling. These processes are interconnected and uninterrupted.

Holistic heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate speed to obtain the required metallographic structure to change its overall mechanical properties. The overall heat treatment of steel has four basic processes: annealing, normalizing, quenching and tempering.

Surface heat treatment is a metal heat treatment process that only heats the surface of the workpiece to change its surface mechanical properties. The main methods of surface heat treatment are flame quenching and induction heating. Commonly used heat sources are flames such as oxyacetylene or oxypropane, induced currents, lasers and electron beams.

Chemical heat treatment is a metal heat treatment process that changes the chemical composition, structure and properties of the surface of the workpiece. The difference between chemical heat treatment and surface heat treatment is that the former changes the chemical composition of the surface layer of the workpiece.

Induction heating heat treatment is to place the workpiece in the inductor, which is usually a hollow copper tube with input medium frequency or high frequency alternating current (300HZ-300KHz or higher). An alternating magnetic field generates an induction current of the same frequency in the workpiece. This induction current is unevenly distributed on the workpiece, strong on the surface and weak on the inside. Using this skin effect, the surface of the workpiece can be quickly heated. In a few seconds, the surface temperature rises to the temperature required for heat treatment.

Selection of induction heating frequency: Select the frequency according to the requirements of heat treatment and heating depth. The higher the frequency, the shallower the heating depth. High-frequency (above 10KHZ) heating depth is 0.5 ～ 2.5mm, generally used for heating small and medium-sized parts, such as small modulus gears and small and medium shaft parts. Medium frequency (1 ~ 10KHZ) heating depth is 2-10mm, generally used for heating shafts with large diameter and gears with large and medium modulus.

Induction heating surface quenching has the advantages of good surface quality, low brittleness, difficult oxidation and decarburization of the quenched surface, and small deformation. Therefore, induction heating equipment has been widely used in metal surface heat treatment.