The heat treatment process of metal generally includes three processes: heating, insulation, and cooling. Sometimes there are only two processes: heating and cooling. These processes are interconnected and cannot be interrupted.
1.Heating
Heating is one of the important processes of heat treatment. There are many heating methods for metal heat treatment. The first was to use charcoal and coal as the heat source, and then to use liquid and gaseous fuels. 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 the air, and oxidation and decarburization often occur (that is, the carbon content on the surface of the steel part is reduced), which has a very negative impact on the surface properties of the parts after heat treatment. Therefore, metals should usually be heated in a controlled atmosphere or protective atmosphere, in molten salt, and in a vacuum. Protective heating can also be performed by coating or packaging methods.
Heating temperature is one of the important process parameters of the heat treatment process. Selecting and controlling the heating temperature is the main issue to ensure the quality of heat treatment. The heating temperature varies depending on the metal material being processed and the purpose of the heat treatment, but it is generally heated to above a certain characteristic transformation temperature to obtain a high-temperature structure. In addition, the transformation requires a certain amount of time. Therefore, 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 to make the internal and external temperatures consistent and the microstructure transformation to be complete. This period of time is called the 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 for chemical heat treatment is often longer.
2.Cooling
Cooling is also an indispensable step in the heat treatment process. The cooling methods vary depending on the process, mainly controlling the cooling rate. Generally, annealing has the slowest cooling rate, normalizing has a faster cooling rate, and quenching has a faster cooling rate. However, there are different requirements due to different steel types. For example, air-hardened steel can be hardened at the same cooling rate as normalizing.
Post time: Mar-31-2024