Heat treatment technology is essential in metal processing. It can make metal workpiece have the required mechanical properties, physical properties and chemical properties. The processes of metal heat treatment can be roughly divided into three categories: one is overall heat treatment, the other is surface heat treatment and chemical heat treatment. In these three categories, there are also several different heat treatment processes according to the different heating medium, heating temperature and cooling method. In different treatment processes, the same metal can also obtain different structures, resulting in different properties.
1、 What are the heat treatment processes
1. Integral heat treatment
As the name suggests, integral heat treatment is a metal heat treatment process that heats the workpiece as a whole, cools it at an appropriate speed after heating, obtains the desired metallographic structure, and changes its overall mechanical properties. Among them, annealing, normalizing, quenching and tempering are the basic processing processes of the overall heat treatment of steel.
2. Chemical heat treatment
Chemical heat treatment needs to immerse metal or alloy workpieces into them for heat preservation with the help of active media at a certain temperature, so that the surface of the workpieces can be infiltrated with atoms of one or concentrated chemical elements. Through the three-stage chemical heat treatment process of heating → heat preservation → cooling, the chemical composition, structure and performance of the workpieces can be changed. At the same time, after quenching and low-temperature tempering, the surface of the workpiece can have higher hardness, wear resistance and contact fatigue, and at the same time, it can also ensure that the core has higher strength and toughness.
The main change of the workpiece after chemical heat treatment is that the technical parameters have two aspects, one is the depth of the hardened layer, the other is the surface hardness. The former needs to use Vickers hardness tester to detect the distance from the workpiece surface to the hardness reduced to 50HRC, which is the so-called hardening depth.
The surface testing of chemical heat-treated workpieces has certain similarities with the hardness testing of surface quenching heat-treated workpieces. They can be tested by Vickers hardness tester, surface Rockwell hardness tester and durometer. Except that the thickness after nitriding is relatively thin. If it is not greater than 0.7mm, Rockwell hardness tester cannot be used.
3. Surface quenching heat treatment
Surface quenching is only the quenching of the surface layer of steel parts. Its heat treatment is characterized by the rapid heating method, which quickly heats the surface of steel parts to the quenching temperature, and then quickly cools, so that the surface layer of steel parts can be hardened to a certain depth, while the heart can remain in the original state.
Surface quenching heat treatment is mainly carried out by heating or flame heating. The main technical parameters are surface hardness, local hardness and effective hardened layer depth. Vickers hardness tester or Rockwell hardness tester can be used for hardness testing. The selection of test force is directly related to the effective depth of hardened layer and the surface hardness of the workpiece.