Heat treatment process generally includes three processes of heating, heat preservation and cooling, sometimes only two processes are heated and cooled. These processes are mutually cohesive and uninterrupted.
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 source, and then use liquid and gas fuel. The application of electricity makes the heating easy to control and no environmental pollution.
These heat sources can be directly heated or indirectly heated by molten salts or metals, or even floating particles. When the metal is heated, the workpiece is exposed to the air, often occurs oxidation, decarbonization (that is, steel parts surface carbon content reduction), which has a very adverse effect on the superficial of the parts after heat treatment. Thus metals should normally be heated in a controlled atmosphere or in a protective atmosphere, in molten salts and in a vacuum, and can also be protected by coating or packaging methods.
Heating temperature is one of the important technological 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 treated metal material and the purpose of heat treatment, but is generally heated to a certain characteristic transition temperature above to obtain high temperature tissue. In addition, the transition requires a certain amount of time, so when the metal workpiece surface to meet the required heating temperature, but also at this temperature to maintain a certain time, so that the internal and external temperature consistent, so that the microstructure changes completely, this time is called holding time. High energy density heating and surface heat treatment, heating speed is very fast, generally there is no insulation time, and chemical heat treatment of heat preservation time is often longer.
Cooling is also an indispensable step in the process of heat treatment, the cooling method is different from the process, mainly to control the cooling rate. Generally, the cooling rate of annealing is the slowest, the cooling speed of normalizing is faster, and the cooling speed of quenching is quicker. But also because of different grades and different requirements, such as the hollow steel can be the same as the cooling speed of the fire hardening.
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