1. Overview of cryogenic processing
1.1 Definition
In the industry, the general treatment of the material after further heat treatment to a temperature below zero degrees Celsius (usually 0 to -130 ° C) is called ordinary cold treatment; and below -130 ° C (usually -130 ° C) The cold treatment of ~-196 ° C) is called cryogenic treatment. Cryogenic treatment, often referred to as ultra-low temperature treatment, is a continuation of ordinary heat treatment, a branch of low temperature technology.
Cryogenic treatment is a new technology to improve or improve the material properties by placing the treated workpiece in a specific, controlled low temperature environment to change the microstructure of the material. The microstructure of the treated material changes due to the microstructure in the low temperature environment, and the macroscopically appears to be the improvement of the wear resistance, dimensional stability, tensile strength and residual stress of the material. the study. With the development of cryogenic technology and the improvement of testing methods, people's research on cryogenic treatment has gradually deepened. In addition to steel materials, materials have been extended to powder metallurgy, copper alloys, aluminum alloys and other non-metallic materials (such as plastics, Nylon, etc.). The application industry is spread over aerospace, precision instrumentation, friction parts, tooling, measuring tools, textile machinery parts, automotive industry and military science. A recent study by Professor Robin Rhodes of the New England Cryogenics Institute found that cryogenic treatment can extend the life of engine components on racing cars, motorcycles, ships, skis, mini-cars, etc. The emergence of cryogenic treatment technology has opened up another broad research field for the practical application and development of cryogenics in industry.
1.2 Cryogenic treatment development history
More than 100 years ago, Swiss watchmakers buried key parts of watches in the cold alpine snow mountains to increase the life of watches; and some experienced tool makers stored tools in the freezer before using the tools. A similar effect can be achieved in a month. Now it seems that they have used cold processing unconsciously.
With the development of refrigeration technology, cryogenic treatment technology appeared in the 1930s. In 1939, the Russians first proposed the concept of cryogenic treatment. However, because the low-temperature cryogenic technology was not perfect at that time, it was only theoretically explored for a long time and explored in the laboratory.Professor F. Barron of the Louisiana Polytechnic University in the United States studied five different alloy steels in the late 1960s. By comparing the samples treated with uncold treatment, low temperature -84 ° C and -190 ° C cryogenic treatment, it was found that the abrasive wear of the samples after the low temperature treatment changed significantly, but the hardness change was not obvious. The wear resistance of the sample treated at -84 °C was 2.0-6.6 times higher than that of the uncooled treatment, and the wear resistance of the sample treated at -190 °C was 2.6 times higher than that at -84 °C. The actual production process also confirmed the correctness of the results of F. Barron's research. Dayton's punches for large pot wheel engines have doubled their service life after treatment at -190 °C.
With the development of liquid nitrogen technology and thermal insulation materials, the United States first applied the cryogenic treatment in 1965, and the main application targets the aviation field. Since then, cryogenic technology has begun to attract the attention of researchers from all over the world. Immediately, scholars from various countries such as Britain, Russia, and Japan conducted extensive and in-depth research. Many studies have shown that the material has a much higher hardness and wear resistance than the ordinary cold treatment after cryogenic treatment.
China's research and development of cryogenic treatment started late. At the end of the 1980s, Chinese research scholars began to study the process and mechanism of cryogenic treatment. The materials were mainly concentrated in tool steel, die steel and high speed steel. The results show that the performance of the material after cryogenic treatment is generally improved compared with the performance after the general cold treatment. In recent years, with the development of cryogenic technology, cryogenic treatment has gradually expanded from the research of ferrous metals to non-ferrous metals and composite materials, and has made certain research progress.
2. Martensitic transformation and cryogenic treatment
The material is rapidly cooled after austenitizing, and a diffusion-free martensitic transformation occurs at a lower temperature. Martensite transformation is one of the important means of strengthening materials and is a very important solid phase transition. People have studied the martensitic transformation for nearly 100 years and formed some theories. However, some theories are still not perfect. For example, nucleation theory, shear model, etc. still have some controversy and lack of unified understanding. Domestic scholars such as Xu Zuyao, Deng Yongrui and Wang Shidao have conducted in-depth and systematic research on the thermodynamics, kinetics, crystallography, nucleation-growth and other aspects of martensitic transformation, and proposed some martensite phases. The nucleation theory and physical model have made important contributions to the development of martensitic transformation theory and the application of martensitic transformation in iron-based alloys, non-ferrous alloys, ceramic materials and other inorganic non-metallic materials.
The martensitic transformation of the material is a non-diffusion phase transition, and the lattice reorganization is completed by shearing without involving the composition change. Only the material with the isomeric transformation during the cooling process may have martensite transformation. Due to the non-diffusion of martensite transformation, phase transformation requires a large driving force and subcooling. The martensitic transformation formed by quenching and cooling, also known as heat-induced martensitic transformation, is quenched and then tempered to adjust hardness and toughness to meet the different performance requirements of various workpieces. At present, in the industrial production, the quenching process of the metal material is mainly to heat the workpiece to 30~50 ° C above the Ac3 or Ac1 of the material, and after cooling for a certain time, it is rapidly cooled in water, brine or oil. Although the quenching ability of these quenching media is very strong, since the first transformed austenite has an inhibitory effect on the transformation of untransformed austenite, only the phase change driving force is further increased, that is, the degree of subcooling is increased to make the phase transition. Continuing, so for most iron-carbon alloys, some of the retained austenite is always present after quenching. If the retained austenite content is too large, it will directly affect the quality of the tempering treatment and will not achieve the required performance of the workpiece. In addition, for some stainless steels, high alloy steels, zirconia ceramics, etc., because their Ms (martensitic transformation onset temperature) is much lower than room temperature, conventional quenching media cannot obtain all martensite after quenching the material.
Therefore, in order to improve the performance and service life of the workpiece, satisfactory quenching quality is obtained, and the retained austenite is minimized to obtain the maximum amount of martensite. At present, the cryogenic treatment method is adopted in the industry, that is, the material quenched to room temperature is continuously cooled. To a lower temperature, the retained austenite continues to transform into martensite during this process. This can further increase the hardness and wear resistance of the steel and stabilize the size of the steel.
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