The heat treatment process generally includes three processes of heating, heat preservation and cooling.
Sometimes there are only two processes of heating and cooling.
These processes are connected to each other and cannot be interrupted.
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 sources, followed by liquid and gaseous fuels.
Electrical applications make heating easy to control and environmentally friendly.
These heat sources can be used for direct heating, or by indirect heating of molten salts or metals, or floating particles.
When the metal is heated, the workpiece is exposed to the air, and oxidation and decarburization (ie, the carbon content on the surface of the steel part) often occur, which has a detrimental effect on the surface properties of the parts after the heat treatment.
Thus, the metal should generally be heated in a controlled or protective atmosphere, in a molten salt and in a vacuum, or protected by a coating or packaging process.
Heating temperature is one of the important process parameters of the heat treatment process.
Selecting and controlling the heating temperature is a major problem in ensuring the quality of the heat treatment.
The heating temperature varies depending on the metal material to be treated and the purpose of the heat treatment, but is generally heated above the phase transition temperature to obtain a high temperature structure.
The transition takes a certain amount of time.
Therefore, when the surface of the metal workpiece reaches the required heating temperature, it must be kept at this temperature for a certain period of time, so that the internal and external temperatures are uniform, and the microstructure is completely transformed.
This period of time is called the holding time.
When high-energy density heating and surface heat treatment are used, the heating rate is extremely fast, and generally there is no holding time, and the heat treatment time of the chemical heat treatment tends to be long.
Cooling is also an indispensable step in the heat treatment process.
The cooling method varies from process to process, mainly to control the cooling rate.
Generally, the annealing rate is the slowest, the normalizing cooling rate is faster, and the quenching cooling rate is faster.
However, there are different requirements depending on the type of steel.
For example, an empty hard steel can be hardened at the same cooling rate as a normal fire.