Heat treatment

The process of heat treatment

 

Heat treatment process generally includes heating, insulation, cooling three processes, sometimes only heating and cooling two processes.

 

Heating is one of the most important processes in heat treatment. There are many heating methods for metal heat treatment. The first uses charcoal and coal as heat source, and then uses liquid and gas fuel. 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, and the oxidation and decarbonization often occur (that is, the carbon content of the surface of the steel parts is reduced), which has a bad effect on the surface properties of the parts after heat treatment. Therefore, metals should usually be heated in controlled or protective atmosphere, molten salt and vacuum, or by coating or packaging.

 

The heating temperature is one of the important technological parameters in the heat treatment process. Selecting and controlling the heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies with the object of heat treatment and the metal material being treated, but it is generally heated above the phase change temperature to obtain high temperature microstructure. 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. When high energy density heating and surface heat treatment are used, the heating speed is very fast, and there is no heat preservation time in general, but the heat preservation time of chemical heat treatment is often longer.

 

Cooling is also an indispensable step in the process of heat treatment. Cooling methods vary with the process, 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.