The steps of heat treatment

Metal heat treatment processes can be generally divided into overall heat treatment, surface heat treatment, local heat treatment and chemical heat treatment. According to the difference of heating medium, heating temperature and cooling method, each big category can be divided into several different heat treatment processes. Different heat treatment processes can be used to obtain different structures and different properties of the same metal. Steel is the most widely used metal in industry, and the microstructure of steel is the most complex. Therefore, there are many kinds of heat treatment processes. The overall heat treatment is a metal heat treatment process that heats the workpiece and then is cooled at an appropriate speed to change its overall mechanical properties. Heating heating is one of the important steps of heat treatment.
There are many heating methods for metal heat treatment. The earliest uses of charcoal and coal as heat source, and then liquid and gas fuels. The application of electricity makes heating easy to control without environmental pollution. These heat sources can be heated directly or indirectly by heating molten salts or metals and even floating particles.
When the metal is heated, the workpiece is exposed to the air, often oxidizing and decarburization (that is, the reduction of carbon content on the surface of the steel parts), which has a bad effect on the surface properties of the parts after heat treatment. Therefore, metals should normally be heated in controlled atmosphere or protective atmosphere, molten salt and vacuum, and can also be protected or heated by coating or packaging. Heating temperature is one of the important technological parameters of heat treatment process. Selecting and controlling heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies with the purpose of the treated metal and heat treatment, but generally it is heated above the phase transition temperature to obtain the desired structure. In addition, the change needs a certain time, so when the metal workpiece surface reaches the required heating temperature, it is necessary to maintain a certain time at this temperature, make the internal and external temperature consistent, make the microstructure change completely, this period is called the heat preservation time. With high energy density heating and surface heat treatment, the heating rate is very fast. In general, there is no heat preservation time or short holding time, and the heat preservation time of chemical heat treatment is often longer.
Cooling is also an indispensable step in the heat treatment process. The cooling method is different because of different processes, mainly controlling the cooling rate. The cooling rate of general annealing is the slowest, the normalizing cooling rate is faster, and quenching is faster. But there are different requirements due to different steel grades. For example, hard steel can be hardened by normalizing cooling rate.