Heat treatment refers to a metal thermal processing process in which the material is heated, held, and cooled in the solid state to obtain the desired structure and properties. In the process from the Stone Age to the Bronze Age and the Iron Age, the role of heat treatment has gradually become known.
As early as 770 BC to 222 BC, Chinese people have discovered in production practice that the performance of steel will change due to the influence of temperature and pressure deformation. The softening treatment of white cast iron is an important process for manufacturing agricultural tools.
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 connected to each other and cannot be interrupted. Heating is one of the important processes of heat treatment. There are many heating methods for metal heat treatment. The first use of charcoal and coal as a heat source, and the application of liquid and gaseous fuels. Electrical applications make heating easy to control and environmentally friendly. These heat sources can be used for direct heating, or by indirect heating of molten salt or gold, or floating particles. When the metal is heated, the workpiece is exposed to the air, and oxidation and decarburization (ie, the carbon content on the surface of the steel part) often occur, which has a detrimental effect on the surface properties of the parts after the heat treatment. Thus, the metal should generally be heated in a controlled or protective atmosphere, in a molten salt and in a vacuum, or protected by a coating or packaging process. Heating temperature is one of the important process parameters of the heat treatment process. Selecting and controlling the heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies depending on the metal material to be treated and the purpose of the heat treatment, but is generally heated above the phase transition temperature to obtain a high temperature structure. In addition, the transformation takes a certain time, so when the surface of the metal workpiece reaches the required heating temperature, it must be kept at this temperature for a certain period of time, so that the internal and external temperatures are uniform, and the microstructure is completely transformed. This period of time is called the holding time. When high-energy density heating and surface heat treatment are used, the heating rate is extremely fast, and generally there is no holding time, and the heat treatment time of the chemical heat treatment tends to be long.