The heat treatment operation of continuing to cool the quenched workpiece from normal temperature to a lower temperature to transform the retained austenite into martensite is called cold treatment. The purpose of cold treatment is to improve the hardness and wear resistance of steel and stabilize the size of the workpiece. It is mainly used for bearings, tools and some carburized parts. Generally, the treatment method of further cooling the steel material to a temperature below zero degrees Celsius (usually 0 to -80 °C) after ordinary heat treatment is called ordinary cold treatment or subzero treatment; Usually -130～-196℃) cold treatment is called cryogenic treatment, and cryogenic treatment is often called ultra-low temperature treatment. Cryogenic treatment is a technology that places the processed workpiece in a specific and controllable low temperature environment to change the microstructure of the material, thereby improving or improving the properties of the material. Due to the change of the microstructure of the processed material in the low temperature environment, the macroscopic performance of the material is the improvement of the wear resistance, dimensional stability, yield strength, tensile strength and other aspects of the material. Application industries include aerospace, precision instrumentation, friction couples, tooling, measuring tools, textile machinery parts, automotive industry and military science and other fields. Cryogenic treatment can greatly prolong the service life of engine parts on racing cars, motorcycles, ships, skis, small racing cars, etc. The principle of cold treatment The steel is austenitized and heated and then quenched to room temperature, and the austenite is transformed into martensite, which significantly improves the hardness and strength of the steel. When the steel is quenched and cooled, the austenite must be supercooled to a certain temperature before it begins to transform into martensite. This temperature is called the martensite transformation point (Ms), and the temperature is lowered to a lower certain temperature to complete the martensite transformation. This temperature Known as the martensitic termination point (Mf) (Figure 1). The carbon content, chemical composition, and austenitizing heating temperature of the steel all affect the transformation of austenite into martensite, especially for high-carbon chromium bearing steel, there must be some residual austenite and residual stress after quenching and heat treatment. The performance of the parts will have a certain impact (excessive retained austenite and residual stress lead to dimensional instability, prone to grinding cracks, etc.). Cold treatment can further transform the austenite in the steel into martensite, improve the distribution of residual stress in the steel, and precipitate more fine carbides, thereby playing a role in dispersion strengthening. Distortion occurs, thereby enhancing matrix properties. Figure 1 The relationship between the martensite transition temperature and carbon content in steel Cold treatment of bearing steel There are also many studies on cold treatment of bearing steel, including GCr15 steel, aviation high temperature bearing steel, carburized bearing steel, etc. In addition to the reduction in aspects (reduced impact toughness, up to 21% [1]), almost all studies have shown many advantages of cold treatment: reduced Paralympic; increased hardness; increased wear resistance; increased tensile strength; Carbide precipitation, fatigue life improvement, etc. For example: Literature [2] carried out research on vacuum low-pressure carburizing and post-carburizing heat treatment process for a martensite-based aviation bearing steel (Cr-Co-Mo series high-temperature bearing steel). The test results show that the test steel After 'carburizing + quenching + cryogenic + tempering + cryogenic + tempering', good microstructure and properties are obtained, the retained austenite is effectively controlled, the network carbide is eliminated, the hardness gradient is gentle and no ' head down' phenomenon. Through the martensitic transformation and the precipitation of carbides, the surface hardness of the test steel is increased by 20HRC compared with the initial hardness in the annealed state. The carburizing heat treatment process literature [3] studied the mechanical properties of carburized bearing steel G20Cr2Ni4A after carburizing, secondary quenching and cryogenic treatment under different conditions: with the extension of cryogenic treatment time, the content of retained austenite decreased, and the carburized layer The surface hardness of the steel can be improved; at the same time, the cryogenic treatment can improve the tensile strength of the material and reduce the impact energy of the carburized steel. Discussion on several issues 1) Retained austenite content The content of retained austenite in bearing steel should be balanced according to its service conditions (application conditions), which require high dimensional accuracy and wear resistance, and may require strict processes such as cold treatment. Control the content of austenite. The content of retained austenite after quenching and tempering specified in GB/T 34891-2017 'Rolling Bearings - High Carbon Chromium Bearing Steel Parts - Heat Treatment Specifications' is shown in the following table. Table E.1 Retained austenite content of bearing parts after conventional tempering Table E.2 Retained austenite content of bearing parts after high temperature tempering 2) Austenite stabilization here mainly refers to austenite thermal stabilization, Austenite is a phase transformation hysteresis phenomenon in which the martensite transformation temperature decreases and the amount of retained austenite increases due to the isothermal retention of austenite during the cooling process. The exact mechanism of austenite thermal stabilization is not particularly clear. For the cold treatment of bearing steel, the main problem is the process of cold treatment, whether it is treated before or after tempering. The heat treatment process route needs to be determined according to the machining properties, mechanical properties and retained austenite content.