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Advantages of cryogenic treatment and recent developments. By Newmaker, cold treatment generally refers to treatments at temperatures between 0–100°C as ordinary cold treatment, while treatments at temperatures below -130°C are considered cryogenic treatment; it is one of the latest strengthening treatment processes. According to the process, cryogenic treatment can be divided into the rapid heating after cryogenic treatment method and the cold-heat cycling method. The thermal cycling stabilization treatment involves first cooling the parts to temperatures of –40°C to –90°C or even lower, holding them at those temperatures for a certain period of time. After that, the parts are heated to a temperature at which their mechanical properties are not compromised (usually between 80°C and 190°C), and they are held at that temperature for another period of time. This cycle is repeated multiple times. “The \"cold treatment followed by rapid heating\" method was proposed by Japanese researcher Koji Wado. In this method, after quenching the workpiece or mold, cold treatment is not carried out immediately; instead, it is first placed in a water bath and then in a treatment tank at –80°C or –180°C. That is, –80°C represents normal cold treatment ； —180°C is used for cryogenic treatment, with the holding time calculated at 1 hour per inch of volume. After insulating, take it out and place it in hot water to heat it up quickly. In countries such as the United States, the former Soviet Union, and Japan, cryogenic technology is not only used for high-speed steel, bearing steel, and die steel in order to improve the wear resistance and strength of these materials and thus extend the overall service life of the components, but it is also employed to modify aluminum alloys, copper alloys, cemented carbides, plastics, glass, and other materials through cryogenic treatment. Improve uniformity, stabilize dimensions, reduce deformation, and extend service life. ⑵ Mechanism of cryogenic treatment: The quenching process of steel is one that results in the formation of martensite; however, quenching does not enable all of the austenite in steel to be converted into quenched structure. After heat treatment, some austenite remains in various types of steel, and the amount of this remaining austenite varies depending on the type of steel and the heating temperature. Additionally, there is also a certain amount of residual stress present. They have more or less an impact on the performance of the workpiece. Cryogenic treatment enables further transformation of austenite in steel into martensite, and it can improve and eliminate the distribution of residual stresses in the steel as well as promote the formation of more fine carbides, thereby achieving dispersion strengthening. In materials without phase transformation, it causes distortion of the grain boundaries, thus enhancing the properties of the matrix. ⑶ Advantages of cryogenic treatment: The greatest advantage of SSZ treatment is the increase in the hardness of the workpiece due to the martensitization of γR, which in turn improves the workpiece’s resistance to wear and abrasion. At the same time, to prevent aging deformation, Dr. Pahlen’s research shows that workpieces subjected to cryogenic treatment have the following advantages: ① γR is essentially almost completely transformed into M ； ② The wear resistance is improved compared to workpieces that have not been treated with SSZ or those that have only undergone conventional cold treatment ； ③ The process of tissue refinement and the precipitation of fine carbides took place ； ④ The hardness is almost the same as that of the CSZ-treated workpieces. ⑷ Precautions for cryogenic treatment: ① Do not place workpieces that have not been cooled to room temperature after quenching directly into the cryogenic device to avoid cracking. ② Workpieces that have cooled to room temperature should be placed in the cryogenic device as soon as possible to prevent the stabilization of austenite, which could affect the treatment results. ③ Generally, steel should not be tempered before cryogenic treatment, while high-speed steel can undergo cryogenic treatment after being tempered once. Recent Advances in Cryogenic Treatment Research: Over the past decade or so, and especially in the last two years, research institutions in China, led by universities such as Gansu University of Technology, Hebei University of Technology, and Central South University of Technology, have been working on the processes and mechanisms related to cryogenic treatment of tool steel, die steel, cutting tool steel, measuring tool steel, as well as non-ferrous metals. Significant results have been achieved; some of these results have already been applied in production, while others are still in the pilot stage. For example, Gansu University of Technology found that after deep cooling of the high-speed steel W18Cr4V, not only were the strength, toughness, and wear resistance of the alloy improved, but the service life of the tools and dies manufactured from it could also be increased by 2 to 5 times. In explaining this phenomenon, it did not rely solely on traditional mechanisms such as the transformation of residual austenite into martensite, grain refinement, and the precipitation of dispersed carbides; instead, it proposed its own entirely new and more detailed explanations. ①The toughness of die materials under repeated impact conditions is related to the morphology and distribution of residual austenite, while the strength and red hardness of cutting tool materials are associated with martensite desolvation and fine carbon phases. ②TEM observations showed that cryogenic treatment resulted in the distribution of dispersed carbides along the twinning bands of martensite, with diameters ranging from 3 to 10 nm; the crystal structure of these carbides was of the M6C type. ③Crystal structure analysis by X-ray diffraction revealed that the axis ratio of the martensite lattice decreased after cryogenic treatment, which also confirms that carbide desorption and decomposition occurred in the martensite. ④Using a self-designed computer processing system for low-temperature dynamic microstructure, it was observed that during cryogenic treatment, residual austenite in metal materials undergoes in-situ dynamic transformation into martensite, and there is a incubation time for this transformation; the transformation occurs first near the edges of the specimen before spreading inward. There is a distinct isothermal martensitic transformation at -196°C. It was also found that during the rapid heating phase after cryogenic treatment, a small amount of martensitic transformation occurred, but the transformation rate was slow and the amount of transformation was limited. ⑤Positron annihilation experiments revealed changes in the density of point defects after cryogenic treatment. The point defect density of ferrous metals increases after cryogenic treatment, while that of non-ferrous metals decreases. Changes in the density of point defects have a significant impact on the properties of metal materials. For example, in non-ferrous copper alloys, a decrease in the vacancy density as a result of cryogenic treatment leads to a reduction in the material’s resistivity, an increase in its strength, and an increase in its density. (end)