1) Cooling stage
The purpose of slow cooling is to completely eliminate the residual stress. In the process of quenching and tempering, the residual stress will be produced in the metal matrix. During the transformation of retained austenite to martensite, the volume expansion will also increase the residual stress. Only slow cooling can offset the increase of residual stress and eliminate the residual stress completely. In general, we don’t pay attention to the residual stress in the matrix, but it is the residual stress in the matrix that makes the knife shear products produce cracks and other defects. Rapid cooling will increase the residual stress.
2) Insulation stage
The purpose of heat preservation is to make the retained austenite in the matrix transform into martensite as much as possible, and produce carbide particles as much as possible, because the transformation process from retained austenite to martensite is a slow process, and the length of holding time will affect the amount of transformation of retained austenite. At the same time, the life after deep cooling is mainly determined by the length of holding time However, if it is a high-quality product, it needs to use more than 24 hours of heat preservation time. The increase of service life is directly related to the length of holding time.
3) Heating stage
The main purpose of slow heating process is to prevent the generation of residual stress.
Due to the limitation of equipment, the domestic cryogenic treatment research generally adopts industrial refrigerator, that is, the workpiece is directly put into the industrial refrigerator, the holding time is relatively short, and the holding time is consistent with the diameter (mm). This method will greatly increase the residual stress, although the performance has been improved, environmental protection, high efficiency, no pollution.
In the heating stage, it can be raised to room temperature. If the special purpose of the parts is considered, such as higher working temperature, it can be increased to + 160 ℃
From the mechanism of cryogenic treatment, it can be seen that the above process curve has little relationship with the material and size of the material, but the effect after treatment is different due to material factors. Almost all tools, moulds, knives, shears, cutting tools, etc. adopt this process for cryogenic treatment.
