8Cr4Mo4V steel is a kind of high-temperature bearing steel widely used in my country, mainly used in the manufacture of aero-engine spindle bearings. With the increasingly severe service conditions of engine main shaft bearings, the requirements for material properties are getting higher and higher, domestic and foreign scholars have carried out a lot of research work on the performance improvement of 8Cr4Mo4V steel. The author introduces the research progress of chemical composition optimization of 8Cr4Mo4V steel; focuses on analyzing the development of heat treatment technology for 8Cr4Mo4V steel, including traditional quenching and tempering, bainite isothermal quenching and dimensional stabilization and other heat treatment processes; introduces the research progress of surface strengthening technology for 8Cr4Mo4V steel And related achievements, involving surface alloying, coating deposition, shot peening and composite strengthening technologies; finally, combined with the service requirements of 8Cr4Mo4V steel and the research status of related technologies, its follow-up research directions are prospected. Bearings, as the key components of aero-engines, are mainly used to reduce the friction between the various components of the engine, transmit loads and maintain the exact position of each component. The service environment of the main shaft bearing of aero-engines is relatively harsh, and its performance directly affects the operation and use of the engine. Therefore, the selection of aero-engine main shaft bearing materials is particularly important. 8Cr4Mo4V (Cr4Mo4V, G80Cr4Mo4V, M50) steel is a typical molybdenum-based high-speed steel, and its main alloying elements include a large amount of Cr and V in addition to Mo. There are a lot of alloy carbides in 8Cr4Mo4V steel after traditional heat treatment, which ensures its high temperature hardness and wear resistance. Based on the excellent mechanical properties of 8Cr4Mo4V steel, NASA began to apply it to aerospace bearings in the mid-1980s. So far, 8Cr4Mo4V steel is still widely used in my country as a high-temperature (use temperature not greater than 316 ℃) bearing steel, mainly used in the manufacture of engine main shaft bearings. In order to further improve the properties of 8Cr4Mo4V steel, scholars at home and abroad have carried out a lot of research in recent years. The latest report in 2020 shows that in order to improve the dry friction properties of 8Cr4Mo4V steel, foreign scholars use spark plasma sintering (Spark Plasma Sintering, SPS) to directly add solid lubricant (SnS/ZnO) to the 8Cr4Mo4V steel matrix. 1. The chemical composition of 8Cr4Mo4V steel and the optimized chemical composition of traditional 8Cr4Mo4V steel are shown in Table 1, in which C with a mass fraction of 0.75%~0.85% provides the material with excellent hardenability and hardness. Cr, Mo and V are strong carbide forming elements, which can significantly improve the wear resistance of the material. Among them, Cr usually exists in the form of M23C6 in 8Cr4Mo4V steel, while Mo and V mainly exist in the form of M2C and MC. Table 1 Mass fraction of chemical composition of 8Cr4Mo4V steel % Literature [7] studied the failure mode of 8Cr4Mo4V steel, and pointed out that its fatigue spalling usually originates from coarse and irregular carbides. There are two kinds of massive carbides in 8Cr4Mo4V steel: M2C dominated by Mo and part of MC in random blocks. In order to eliminate the random massive carbides, the reference [9] redesigned the composition of 8Cr4Mo4V steel (the improved type in Table 1) through thermodynamic calculation and carried out trial production. The results show that by appropriately increasing the Cr content and reducing the V content, the The size of M2C type carbide is reduced to below 5.3μm, and the spherical carbide M23C6 is increased; the room temperature hardness of the improved 8Cr4Mo4V steel after quenching and tempering treatment is not lower than 64 HRC, and the hardness at 400 ℃ high temperature is not lower than 60 HRC, which is in line with high temperature Service requirements for bearings. Due to the lack of relevant verification at present, the actual engineering application of this composition 8Cr4Mo4V steel has not been seen. Reference [10] studied the effect of Mg on the carbides of 8Cr4Mo4V steel, and the results showed that adding a small amount of Mg could improve the crystallographic structure of the steel and optimize the network carbides in the as-cast structure of the ingot. 2. Development of heat treatment technology for 8Cr4Mo4V steel Heat treatment is a key step in determining the final properties of the material. In order to improve the properties of 8Cr4Mo4V steel, it is of great significance to study the improvement technology of the heat treatment process. 2.1 Traditional heat treatment technology and its microstructure The traditional heat treatment process of 8Cr4Mo4V steel at home and abroad is quenching and three tempering, as shown in Figure 1, the difference is that the foreign quenching and heating process is one-step preheating (red line in Figure 1a) or three-step preheating ( Figure 1a black dotted line), while domestic basically adopts one-step preheating (Figure 1b); foreign quenching and cooling process adopts graded quenching method, the main purpose is to reduce heat treatment deformation, and domestic use N2 continuous cooling. Fig. 1 Domestic and international heat treatment process of 8Cr4Mo4V steel Fig. 1 Domestic and international heat treatment process of 8Cr4Mo4V steel The microstructure and phase of 8Cr4Mo4V steel after quenching and tempering process are shown in Fig. 2: The grains of 8Cr4Mo4V steel after quenching are relatively uniform and fine , 8Cr4Mo4V steel after tempering is composed of tempered martensite, a small amount of retained austenite (volume fraction not more than 3%) and carbides; after tempering, there are irregular massive and dispersed small granular carbides, among which the block The like carbides are quenched undissolved carbides and tempered precipitated carbides. Fig.2 Microstructure of 8Cr4Mo4V steel after quenching and tempering processThe main types of carbides precipitated by tempering are M23C6 and M2C. Usually, the carbides precipitated by tempering are nanoscale in size, in the form of flakes or spheres, and can play a secondary hardening effect.