The heat treatment process generally consists of three processes: heating, heat preservation and cooling. Sometimes only two processes are heated and cooled. These processes are interconnected and uninterruptible.
Heating is one of the important steps of heat treatment. There are many heating methods of metal heat treatment, and the first is to use charcoal and coal as heat sources to apply liquid and gas fuel. The application of electricity makes heating easy to control and free of environmental pollution. These heat sources can be heated directly, or indirectly heated by molten salt or metal, or floating particles.
Metal heating workpiece when it is exposed to the air, often occur oxidation and decarburization (that is, the surface carbon content lower) steel parts, the performance on the surface of the parts after heat treatment have a bad effect. Therefore, metals should usually be heated in a controlled atmosphere or in a protective atmosphere, in molten salt and in a vacuum, and can be protected by coating or packaging.
Heating temperature is one of the important process parameters of heat treatment process, and selecting and controlling the heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies with the purpose of the treated metal material and heat treatment, but is generally heated to the phase transition temperature to obtain the desired tissue. Another change will take time, so when the heating temperature of metal surface is achieved, still must in the temperature keeping time, make internal and external temperature, the microstructure change completely, this paragraph of time is called the heat preservation time. With high energy density heating and surface heat treatment, the heating speed is extremely fast. Generally, there is no heat preservation time or heat preservation time, while the heat preservation time of chemical heat treatment is usually longer.
Cooling is an indispensable step in the process of heat treatment. The cooling method is different due to different processes, mainly controlling the cooling rate. Generally, the cooling rate of the annealing is the slowest, the cooling speed is fast, and the quenching speed is faster. But there are different requirements for steel, such as air hard steel, which can be hardened by the same cooling speed.
The heat treatment process can be divided into general heat treatment, surface heat treatment, local heat treatment and chemical heat treatment. According to different heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. Different heat treatment processes can be used for the same metal, which can have different performance. Steel is the most widely used metal in industry, and iron and steel microstructure is the most complex, so there are many kinds of steel heat treatment technology. The overall heat treatment is to heat the workpiece as a whole and then cool it at an appropriate rate to change the metal heat treatment process of its overall mechanical properties. The overall heat treatment of steel has the basic process of annealing, normalizing, quenching and tempering.