After heat treatment, the hardness and mechanical properties of steel are greatly improved, but there are still the following problems after heat treatment:
1. Retained austenite. The proportion of austenite is about 10% – 20%. Because austenite is very unstable, it is easy to transform into martensite when subjected to external force or environmental temperature change. If the specific volume of austenite and martensite is not the same, it will cause irregular expansion of material and reduce the dimensional accuracy of workpiece.
2. The grains are coarse and the carbide is supersaturated by solid solution.
3. Residual internal stress. The residual internal stress after heat treatment will reduce the fatigue strength and other mechanical properties of the material, and easily lead to the deformation of the workpiece in the process of stress release.
Advantages of cryogenic process
Through the unremitting research of many metal material researchers at home and abroad, cryogenic and ultra cryogenic treatment process is considered to be the best method to solve the above problems
Its advantages are as follows
- 1. It transforms the retained austenite with lower hardness into hard, more stable martensite with higher wear resistance and heat resistance.
- 2. The grain boundary, edge and interior of martensite decompose and refine, and a large number of Ultra-fine Carbides are precipitated. Supersaturation of supersaturated martensite decreases in cryogenic process, and the precipitated Ultra-fine Carbides keep coherent relationship with matrix, which can make martensite lattice distortion and reduce, micro stress decrease, while fine dispersed carbides can be used in material plastic deformation At the same time, the precipitation of Ultra-fine Carbides uniformly distributed on the martensite matrix weakens the catalytic effect of grain boundary. The refinement of matrix structure not only weakens the segregation degree of impurity elements at the grain boundary, but also plays a role in strengthening the grain boundary. Therefore, the comprehensive mechanical properties of the material are improved in three aspects: the toughness of the material is improved, the impact toughness is high, the tempering stability and fatigue resistance of the matrix are improved; the wear resistance performance is improved; and the dimensional stability is improved. The purpose of strengthening matrix, improving heat treatment quality, reducing tempering times and prolonging die life are achieved.
- 3. After cryogenic treatment, the internal thermal stress and mechanical stress of the material are greatly reduced, and due to the plastic flow in the hole making or standing force concentration position during the cooling process, the compressive stress will be generated on the surface of such vacancy during the heating process. This kind of compressive stress can greatly reduce the damage of defects on the local performance of the workpiece, thus effectively reducing the possibility of the metal workpiece to have the shape crack Ability.