4. The transformation of the surface layer structure controls the carbon content of the carburized bearing steel to gradually decrease from the surface layer to the center. After heat treatment, the structure of the surface layer at different depths is different. After quenching, the matrix structure of the carburized bearing steel from the surface to the core is: needle-like Martensite → acicular martensite + lath martensite → lath martensite. Quenching heating temperature is an important factor in martensitic quenching. On the one hand, higher quenching heating temperature is conducive to the uniform diffusion of carbon and other alloying elements in austenite; on the other hand, at higher quenching heating temperature Under the temperature, more carbides dissolve, and the effect of pinning grain boundaries is weakened, which will promote the growth of austenite grains. The morphology and size of the quenched martensite determine the hardness, strength and toughness of the steel, and grain refinement is the only method that can improve both the strength and toughness of the material. Therefore, a reasonable quenching temperature and holding time are selected. Very important. High-speed railway bearings need to withstand greater impact loads, and bearing steels are not only required to have high wear resistance, high fatigue resistance, but also good impact toughness. For carburized bearing steel, selecting a reasonable secondary quenching temperature can obtain excellent surface structure and properties. The G20CrNi2Mo steel commonly used in high-speed railway bearings is carburized at 945 ℃, then pre-cooled and quenched at 860 ℃, and then subjected to secondary quenching at 830 ℃. The martensite becomes finer, and fine and uniform carbide particles are dispersed and precipitated on the matrix, and the wear resistance is significantly improved. Fig.6 Microstructure comparison of G20CrNi2Mo steel before and after secondary quenching Each upgrade mainly depends on the increase of the thrust-to-weight ratio of the engine. Now the dn value of the engine main shaft has reached 2.4×106mm·r·min-1, the working temperature of the engine main shaft bearing is close to 300 ℃, and it has been subjected to huge impact loads and cyclic fatigue stress for a long time. In order to meet the higher performance requirements of aviation bearings, my country has developed the second generation of high temperature carburized bearing steel G13Cr4Mo4Ni4V (ASTM M50NiL). According to YB/T 4106-2000 'High temperature carburized bearing steel for aircraft engines', the surface hardness after carburizing, quenching and tempering can reach 62~63 HRC, with high wear resistance and good Fatigue resistance, core lath martensite provides good impact toughness. Figure 7 shows the common carburizing heat treatment process for G13Cr4Mo4Ni4V steel. The temperature during vacuum carburization is generally 890 ℃, then quenched at 1 100 ℃, and then tempered at 545 ℃ for 3 times, 2 h each time. Fig.7 Diagram of carburizing heat treatment process for G13Cr4Mo4Ni4V steel The matrix structure of the layer is mainly cryptocrystalline martensite, and the structure of the transition zone contains both flaky martensite and low-carbon lath martensite, in which the size of the martensitic lath is smaller than that of the core, which It is due to the low carbon content in the core that the austenite grains are coarser after solution treatment at high temperatures, resulting in the growth of martensite laths more easily. Fig. 8 Microstructure of M50NiL steel after carburizing heat treatment Fig. 8 Microstructure of M50NiL steel after carburizing heat treatment Literature [65] studied the effect of bainite isothermal quenching on the microstructure and properties of the carburized layer of G23Cr2Ni2Si1 Mo steel: at 200 ℃ After 8 h of lower isothermal temperature, the carburized layer obtained a considerable amount of acicular lower bainite (Fig. 9a); it can be clearly seen in Fig. 9b that there is a thin film between the nano-scale bainitic ferrite laths retained austenite. The fine bainitic ferrite lath greatly increases the phase boundary area, which is beneficial to hinder dislocation slip and resist plastic deformation, and the carbon-rich film-like retained austenite can effectively hinder the growth of microcracks, so it is beneficial to Improve toughness and wear resistance.