The latest progress of cryogenic treatment research: in recent ten years, especially in the past two years, domestic scientific research institutions headed by Gansu University of technology, Hebei Institute of technology and Central South University of technology have been committed to the research of cryogenic treatment process and mechanism of tool steel, die steel, cutting tool steel, measuring tool steel and non-ferrous metal, and have achieved certain results, and some achievements have been met Some achievements are still in the pilot stage.
For example, Gansu University of technology has found that not only the strength, toughness and wear resistance of the alloy have been improved, but also the service life of the tools and dies made of it can be increased by 2-5 times. In explaining this phenomenon, it is not limited to the traditional mechanism of transformation of retained austenite to martensite, grain refinement and precipitation of dispersed carbides, instead, it puts forward its own brand-new and more advanced detailed explanation.
① The toughness of die material under multiple impact is related to the morphology and distribution of retained austenite, and the strength and red hardness of tool material are related to the micro carbon flower of martensite desolvation.
② TEM observation shows that there are dispersed carbides distributed in the twinning zone of martensite, with a diameter of 3-10nm. The crystal structure of the carbides is M6C type.
③ Through the analysis of X-ray diffraction crystal structure, it is found that the axial ratio of martensite lattice decreases after cryogenic treatment, which also proves that the carbide desolvation occurs in martensite.
④ With the aid of a computer processing system designed by ourselves, the in-situ dynamic microstructure transformation from retained austenite to martensite has been observed in the process of cryogenic treatment, and there is incubation time during the transformation from retained austenite to martensite. The transformation first occurs near the edge of the test block, and then develops to the inner depth. There is an obvious isothermal martensitic transformation at – 196 ℃. At the same time, it is also found that a small amount of martensite transformation occurs in the intense heating stage after cryogenic treatment, but the transformation speed is slow and the transformation amount is less.
⑤ Through positron annihilation test, it is found that the density of point defects changes after cryogenic treatment. The density of point defects of ferrous metals increases after cryogenic treatment, while that of nonferrous metals decreases after cryogenic treatment. The change of point defect density has a lot of influence on the properties of metal materials. For example, the vacancy density of non-ferrous copper alloy after cryogenic treatment will decrease the resistivity, increase the strength and increase the density.