(1) Application in Iron and Steel Industry
Metal cryogenic treatment is a branch of metal heat treatment. It was not until 1965 that the United States applied metal cryogenic treatment to industrial production for the first time. In China, about 2000, metal cryogenic treatment was gradually applied to industrial production. Now it has entered many fields, such as automotive parts manufacturing, tooling manufacturing, roll cold treatment, precision mechanical parts manufacturing and tool manufacturing. According to GB/T 25743-2010 steel parts cryogenic treatment, metal cryogenic treatment includes tool steel, spring steel, bearing steel, stainless steel, heat resistant steel, carburizing steel and carbonitriding steel. According to the existing research and application results, the role of metal cryogenic treatment is summarized as follows:
1. Retained austenite transforms martensite. At low temperature (below Mf point), the retained austenite decomposes and transforms into martensite, which improves the hardness and strength of the workpiece. Some scholars believe that cryogenic treatment can completely eliminate retained austenite; others believe that cryogenic treatment can only reduce the amount of retained austenite, but can not completely eliminate it; others believe that cryogenic treatment can change the shape, distribution and sub-structure of retained austenite, thus improving the strength and toughness of steel. From our application analysis, cryogenic can eliminate more than 80% of the Paralympic Games.
2. Ultrafine carbides are precipitated from martensite. The lattice constant of Fe tends to decrease after martensite is cooled and volume shrinkage, which strengthens the driving force of carbon atom precipitation. However, due to the difficulty of diffusion at low temperature and the shorter diffusion distance, a large number of dispersed ultrafine carbides are precipitated on martensite matrix to achieve the effect of dispersion strengthening, thus significantly improving the wear resistance.
3. Residual compressive stress is produced on the surface. The cooling process may cause the plastic flow of defects (micro-holes, internal stress concentration areas). Residual compressive stress is produced on the surface of the vacancy during the rewarming process, which can reduce the damage of the defect to the local strength of the material and ultimately improve the abrasive wear resistance.
4. Tissue refinement. Tissue refinement improves both strength and toughness of the workpiece, which mainly refers to the fragmentation of the original coarse lath martensite and the homogeneity, compactness and fineness of the structure.
5. Stable size. Due to the transformation of the paralympic, the refinement of the material structure and the reduction of the material defects, the size of the cryogenic workpiece is more stable in the subsequent use, which is of great significance to the precision workpiece.
(2) Application of Aluminum Alloy Industry
Aluminum alloy produces internal stress in the process of production and processing. The traditional method is to reduce internal stress by aging heating treatment. However, due to the low melting point of the aluminum alloy itself and the increase of aging temperature, the strength of the material will inevitably be reduced. Therefore, the aging treatment after quenching of aluminium alloys is usually carried out at a lower temperature (less than 200-250 C), which affects the effect of stress relief, generally only about 10-35%.
Using cryogenic treatment can effectively reduce the residual stress of 20-90%, and its greatest advantage is that it can improve the strength, hardness, wear resistance and structural stability of materials. There is no limit on the shape and size of the workpiece to be processed. The cryogenic treatment before cutting can also significantly improve the tendency of serious deformation and improve the structural stability of materials. For some precise aluminium alloy parts, it has great application value.
(3) Application of Copper Alloy Industry
A few scholars in China have begun to study the cold treatment of copper alloys. From the existing research situation, it is generally believed that at low temperature, copper alloys undergo phase transformation from retained austenite to martensite similar to steel materials and refine grains, thus improving the strength and hardness of copper alloys. However, for cryogenic treatment, the microstructure of copper alloys changes. In order to improve the physical mechanism of copper alloy properties, there is still a lack of sufficient experimental and theoretical basis to elaborate.
As an indispensable application material in manufacturing industry, the research on cryogenic treatment of copper alloys is of great significance. At present, there is no definite conclusion about the transformation mechanism of cryogenic treatment of copper alloys, but its principle and function have been gradually recognized, which lays a foundation for further research. With the development of cryogenic treatment technology for copper alloys, the research on its transformation mechanism and technological measures has become more systematic, which will inevitably promote the application of copper alloys.
(4) Other metal industries
In the past, due to the limitation of conditions, the research on metal cryogenic field in our country is too little. The research on metal cryogenic field in America, Japan, Germany and other countries is ahead of our country, not only in the research results, but also in the degree of technicians'understanding of deep cryogenic. Such as Custom 465 475 developed by Carpenter Technologies Inc. in the United States, the material design considers that the full phase transformation can be carried out at - 80 C, and then the aging strengthening treatment is carried out. The tensile and yield strength can reach more than 1800 Mpa, and the fracture toughness is good. It is the steel with the best combination of strength and toughness in PH stainless steel, which is used in the United States Joint Strike Aircraft. Tie rod, flaps track, slotted wing track, transmission device, engine anti-vibration pad and landing gear and other components, but there is no similar brand in China at present.
Compared with heating, cryogenic technology is still a new field. Kuwaijani is an early enterprise engaged in the development of liquid nitrogen cryogenic equipment in China. It has rich R&D and practical experience, and has many mature products. It is willing to cooperate with users to promote the popularization and use of metal cryogenic technology.