Function: By changing the microstructure inside the workpiece or changing the chemical composition on the surface of the workpiece, the working performance of the workpiece is endowed or improved, and the internal quality of the workpiece is improved.
Extended data
The heat treatment process generally includes three processes of heating, heat preservation and cooling, and sometimes there are only two processes of heating and cooling. These processes are interrelated and uninterrupted. Heating is one of the important processes of heat treatment. There are many heating methods for metal heat treatment. Charcoal and coal were first used as heat sources, and later liquid and gas fuels were used.
The application of electricity makes heating easy to control and has no environmental pollution. These heat sources can be used for direct heating or indirect heating through molten salt or metal or even floating particles.
When the metal is heated, the workpiece is exposed to air, which often leads to oxidative decarbonization (that is, the carbon content on the surface of steel parts is reduced), which has a very adverse impact on the surface properties of parts after heat treatment. Therefore, metals should usually be heated in controlled or protective atmosphere, molten salt and vacuum, and can also be protected by coating or packaging.
Heating temperature is one of the important technological parameters in 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 to be treated and the purpose of 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 time, so when the surface of the metal workpiece reaches the required heating temperature, it must be kept at this temperature for a certain time, so that the internal and external temperatures are consistent and the microstructure is completely transformed. This time is called holding time. When using high energy density heating and surface heat treatment, the heating speed is extremely fast, and there is generally no holding time, while the holding time of chemical heat treatment is often longer.
Cooling is also an essential step in the heat treatment process, and the cooling mode varies from process to process, mainly controlling the cooling speed. Generally, the cooling rate of annealing is the slowest, that of normalizing is faster, and that of quenching is faster. However, due to different steel grades, the requirements are also different. For example, air hardened steel can be hardened at the same cooling rate as normalizing.
Baidu encyclopedia-heat treatment
Baidu encyclopedia-metal heat treatment process