Three stages of heat treatment process

According to different process methods, heat treatment plants of iron and steel can be divided into three categories:

One piece heat treatment (including annealing, normalizing, quenching, quenching + tempering, tempering and others).

Surface heat treatment (including surface quenching and tempering, etc.)

Three chemical heat treatment (including carburizing, nitriding, carbonitriding, nitrocarburizing, nitriding other non-metal, metallizing, multi-element nitriding, solution nitriding), all heat treatment is composed of three stages of heating, heat preservation and cooling.

The hot part of steel consists of three stages: heating, heat preservation and cooling, sometimes only heating and cooling. These processes are interlinked and uninterrupted. Heating is one of the important processes of heat treatment. There are many heating methods for metal heat treatment, the first one is to use charcoal and coal as heat source, and then use liquid and gas fuel.

1. When the metal is heated, the workpiece is exposed to the air, which often occurs oxidation and decarburization (that is, the carbon content on the surface of steel parts is reduced), which has a very adverse effect on the surface performance of parts after heat treatment. Therefore, the metal should be heated in controllable or protective atmosphere, molten salt and vacuum, and can also be protected by coating or packaging.

2. Heating temperature is one of the important process parameters of heat treatment process. The selection and control of heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies with the metal material being treated and the purpose of heat treatment, but it is generally heated above the phase change temperature to obtain high temperature structure.

In addition, the transformation needs a certain time, so when the surface of the metal workpiece reaches the required heating temperature, it must be kept at this temperature for a certain time, so that the internal and external temperatures are the same, and the microstructure is completely transformed, which is called the holding time. When high energy density heating and surface heat treatment are used, the heating speed is extremely fast, and there is generally no heat preservation time, while the heat preservation time of chemical heat treatment is often longer.

3. Cooling is also an indispensable step in the process of heat treatment. The cooling method varies with the process, mainly controlling the cooling speed. Generally, the cooling speed of annealing is the slowest, that of normalizing is faster, and that of quenching is faster. But there are different requirements due to different kinds of steel. For example, air hardening steel can be quenched at the same cooling rate as normalizing.

According to the process classification, the metal heat treatment process can be generally divided into three categories: overall heat treatment, surface heat treatment and chemical heat treatment.

According to the 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 to obtain different structures and different properties. Steel is the most widely used metal in industry, and the microstructure of steel is also the most complex, so there are many kinds of heat treatment process.

The whole heat treatment is a metal heat treatment process that heats the whole workpiece and cools it at a proper speed to obtain the required metallographic structure and change its whole mechanical properties. There are four basic heat treatment processes of iron and steel: annealing, normalizing, quenching and tempering.