Heat treatment refers to the material in the solid state, through heating, insulation and cooling means, in order to obtain the desired organization and performance of a metal thermal processing process.

Process characteristics

Heat treatment process generally includes heating, heat preservation, cooling three processes, sometimes only heating and cooling two processes. These processes are interconnected and uninterrupted. Heating is one of the important processes of heat treatment. There are many heating methods for metal heat treatment, the earliest is the use of charcoal and coal as a heat source, the recent application of liquid and gas fuel. The application of electricity makes the heating easy to control and no environmental pollution. These heat sources can be used for direct heating or indirect heating through molten salt or gold, or even floating particles. When the metal is heated, the workpiece is exposed to the air, and oxidation and decarburization often occur (that is, the surface carbon content of the steel part is reduced), which has a very adverse effect on the surface performance of the part after heat treatment. Therefore, the metal should usually be heated in a controlled atmosphere or a protective atmosphere, molten salt and vacuum, and can also be heated under protection by coating or packaging methods. Heating temperature is one of the important process parameters of heat treatment process, and the selection and control of heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies with the metal material being treated and the purpose of the heat treatment, but it is generally heated above the phase transition temperature to obtain a high temperature structure. In addition, the transformation takes a certain amount of time, so when the surface of the metal workpiece reaches the required heating temperature, it must be maintained at this temperature for a certain period of time, so that the internal and external temperatures are consistent, so that the microstructure is completely transformed. This time is called the holding time. When using high-energy density heating and surface heat treatment, the heating speed is extremely fast, generally there is no holding time, and the holding time of chemical heat treatment is often longer.

Cooling is also an indispensable step in the heat treatment process. The cooling method varies from process to process, mainly to control the cooling rate. Generally, the cooling rate of annealing is the slowest, the cooling rate of normalizing is faster, and the cooling rate of quenching is faster. However, there are different requirements due to different steel types. For example, empty hard steel can be hardened with the same cooling rate as normalizing.