Metal heat treatment is a process in which a metal workpiece is heated to a suitable temperature in a certain medium, and after it is kept at this temperature for a certain period of time, it is cooled at different speeds. The process generally includes three processes of heating, heat preservation, and cooling, and sometimes there are only two processes of heating and cooling. These processes support each other and are closely linked.
Heating is one of the important processes of heat treatment. There are many heating methods for metal heat treatment. The first is to use charcoal and coal as heat sources, and then use liquid and gas fuels. The application of electricity makes heating easy to control, and there is no environmental pollution. These heat sources can be used for direct heating, or indirect heating by molten salt or metal, or even floating particles.
When the metal is heated, the workpiece is exposed to the air, and oxidation and decarburization (that is, the carbon content on the surface of the steel part is reduced) often occur, which has a very negative effect on the surface properties of the parts after heat treatment. Therefore, metals should usually be heated in a controlled atmosphere or protective atmosphere, molten salt and vacuum, and can also be heated by coating or packaging methods.
The heating temperature is one of the important process parameters of the heat treatment process. The selection and control of the heating temperature is the main issue to ensure the quality of the heat treatment. The heating temperature varies with the metal material being processed and the purpose of the heat treatment, but it is generally heated to a temperature above the phase transition temperature to obtain a high-temperature structure. In addition, the transformation takes a certain 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 complete the microstructure transformation. This period of time is called the holding time. When high-energy density heating and surface heat treatment are used, 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 indispensable step in the heat treatment process. The cooling method varies from process to process, and the main thing is to control the cooling rate. Generally, annealing has the slowest cooling rate, normalizing cooling rate is faster, and quenching cooling rate is faster. However, there are different requirements due to different steel grades. For example, hollow hard steel can be quenched at the same cooling rate as normalizing.
Metal heat treatment processes can be roughly divided into three categories: overall heat treatment, surface heat treatment and chemical heat treatment.
Post time: Dec-16-2021