Heat treatment process generally includes heating, heat preservation, cooling three processes, sometimes only heating and cooling two processes.These processes are interconnected and uninterruptible.

Heating is one of the important steps of heat treatment.There are many heating methods for metal heat treatment, early is the use of charcoal and coal as a source of heat, and then the application of liquid and gas fuel.The application of electricity makes heating easy to control and free from environmental pollution.These sources of heat can be heated directly or indirectly through molten salts or metals, or even floating particles.

When the metal is heated, the workpiece is exposed to the air, which often occurs oxidation and decarburization (that is, the carbon content on the surface of steel parts decreases), which has a very adverse effect on the surface performance of the parts after heat treatment.As a result, the metal should normally be heated in a controlled atmosphere or protective atmosphere, molten salt and vacuum, and can also be protected by coating or packaging methods.

The heating temperature is one of the important technological parameters in the heat treatment process. The selection and control of the heating temperature is the main problem to ensure the quality of the heat treatment.The heating temperature varies with the metal material being treated and the purpose of the heat treatment, but is generally heated above the phase transition temperature to obtain the desired tissue.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 changes completely. This period of time is called the heat preservation time.When high-energy density heating and surface heat treatment are adopted, the heating speed is very fast, and generally there is no or very short heat preservation time, while the heat preservation time of chemical heat treatment is often longer.

Cooling is also an indispensable step in the process of heat treatment. The cooling method varies from process to process, mainly controlling the cooling rate.Generally, the cooling rate of annealing is * slow, while the cooling rate of normalizing is faster, and the cooling rate of quenching is faster.But there are also different requirements depending on the type of steel. For example, air-hardened steel can be hardened at the same cooling rate as normalizing.

Metal heat treatment process can be divided into overall heat treatment, surface heat treatment, local heat treatment and chemical heat treatment.According to the different heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes.Different microstructure can be obtained by different heat treatment process for the same metal, so it has different properties.Iron and steel is widely used in industry, and the microstructure of iron and steel is also complex, so the heat treatment technology of iron and steel is various.