This paper comprehensively discusses the research progress of bearing steel at home and abroad from the aspects of the classification of bearing steel, the main influencing factors of performance and the development of heat treatment process, and puts forward the process optimization and inclusions that need to be solved in the development of domestic high-quality bearing steel. Methods related to problems such as carbide control, lack of testing equipment and technical evaluation indicators, in order to achieve the quantitative production and application of domestic high-quality bearing steel.
Keywords: bearing steel; quality control; inclusions; carbides; heat treatment; development trend
With the rapid development of current society, bearing steel has been widely used in various fields such as industrial machinery, equipment manufacturing, transportation, and aerospace. After decades of development, the domestic bearing and bearing steel manufacturing industry has gradually formed a relatively complete industrial system. The R&D and production of domestic bearings and bearing steel, as well as the steady improvement in the quality of bearing steel products, have gradually made China a major bearing manufacturing country in the world and are steadily moving towards the goal of becoming a manufacturing power. The annual sales of the domestic bearing industry have now reached more than 200 billion yuan, and are growing rapidly at an annual growth rate of 12% to 15%. Although the domestic bearing industry has made great progress in recent years, the existing problems still cannot be ignored. Domestic production of bearing steel is mainly concentrated in mid- to low-end bearing products, and there is even overcapacity in the low-end bearing field. Aerospace, high-speed railway and other fields require bearings with good strength, toughness, quality stability and longer service life. However, it is currently difficult for China to make significant breakthroughs in the field of high-end bearing research and development, making domestic high-quality bearings heavily dependent on imports. This also It has become one of the main factors restricting the development of China’s high-end equipment manufacturing and strategic emerging industries. The current research and development level of China’s high-end bearing steel products is significantly different from that of Sweden, Germany, Japan and other bearing manufacturing powers. Factors such as low cleanliness of bearing steel, oxide inclusions, unstable carbides and uneven distribution have greatly reduced the cost of domestic bearing steel. Its lifespan and reliability have seriously affected the development of domestic high-end equipment manufacturing and strategic emerging industries. Therefore, the development of high-quality bearing steel is currently one of the research priorities of various scientific research institutes, universities and bearing steel manufacturing industries in China. This article mainly discusses the development status of bearing steel in recent years from the aspects of bearing steel classification, performance influencing factors, heat treatment process and development direction.
Classification of bearing steel
The main function of bearings in industrial equipment is to support the rotating body, reduce its friction factor during the operation of the equipment, and ensure the operation accuracy of the mechanical equipment. Therefore, the working state of the bearings is generally line contact or point contact. Due to the small contact area, the bearing needs to withstand a compressive stress of 1500 to 5000MPa during operation. The working environment of bearings puts forward higher requirements on the strength, toughness, wear resistance and service life of bearing steel, which makes bearing steel one of the most stringent steel types among all alloy steel production requirements. In 1976, the International Organization for Standardization ISO classified bearing steel into four categories: surface-hardened bearing steel, fully-hardened bearing steel, high-temperature bearing steel and stainless bearing steel. China’s bearing steel classification is basically the same as the ISO classification, which are high carbon chromium bearing steel, carburized bearing steel, high temperature bearing steel and stainless bearing steel. The corresponding standard numbers are GB/T18254, GB/T3203, GB/T1205, GB/T3086 and YB/T688 have a total of 15 grades.
High carbon chromium bearing steel
GCr15 high carbon chromium bearing steel is the world’s first generation bearing steel. After more than a hundred years of development, its chemical composition has not changed significantly. According to statistics, the output of this steel type still accounts for more than 80% of the total annual output of bearing steel. However, the increase in bearing design size and the increasing load required require bearing steel to have higher strength, better dimensional stability and longer fatigue life.
Chromium-manganese-silicon bearing steel adds a certain amount of Si and Mn elements, which improves the hardenability of the steel, increases the strength and hardness of the bearing steel, and effectively improves the wear resistance of the steel; Mo and Cr, When the Mn element coexists, it can improve the temper brittleness of steel and improve the stability of carbides, which is crucial to improving the performance of bearing steel. This has gradually formed bearing steels based on chromium silicon molybdenum and chromium manganese molybdenum. Represents high carbon chromium bearing steel. At present, most of the high-carbon chromium bearing steels widely used in various fields are obtained by further optimizing the content of alloy elements on the basis of GCr15. The grades of high-carbon chromium bearing steels and their main chemical compositions are shown in Table 1.
| Table 1Grades and main chemical components of high carbon chromium bearing steel | % | ||||
| Trademark | C | Mn | Si | Cr | Mo |
| GCr15 | 0.95～1.05 | 0.25～0.45 | 0.15～0.35 | 1.40～1.65 | ≤0.10 |
| G8Cr15 | 0.75～0.85 | 0.20～0.40 | 0.15～0.35 | 1.30～1.65 | ≤0.10 |
| GCr15SiMn | 0.95～1.05 | 0.95～1.25 | 0.45～0.75 | 1.40～1.65 | ≤0.10 |
| GCr15SiMo | 0.95～1.05 | 0.20～0.40 | 0.65～0.85 | 1.40～1.70 | 0.30～0.40 |
| GCr15Mo | 0.95～1.05 | 0.25～0.40 | 0.20～0.40 | 1.65～1.95 | 0.15～0.25 |
Carburized bearing steel
High carbon chromium bearing steel has a high carbon content, which reduces the impact toughness of the steel. Since bearing steel has higher requirements for impact resistance and toughness in the working environment, carburized bearing steel was developed based on low alloy structural steel. Reduce the carbon content in bearing steel, carburize the steel surface, and finally develop carburized bearing steel with higher hardness and good wear resistance on the surface and good toughness in the steel core. Representative steel types of carburized bearing steel include G20CrMo, G20CrNiMo, G20CrNi2Mo, G20Cr2Ni4, G20Cr2Mn2Mo, etc.
The results of Zhao Defense et al.’s study on the impact of heat treatment process on the microstructure and properties of carburized bearing steel show that the gradient distribution of carbon content from the surface to the core of the steel directly affects the gradient distribution of steel hardness. Due to the different specific volumes of austenite and martensite , resulting in compressive stress on the surface of the carburized layer after carburizing treatment, which increases the fatigue life of the steel. Figure 1 shows the morphology of the carburized layer of bearing steel after carburizing heat treatment.
The research team developed a carburized nanobainite bearing steel. Its surface structure is shown in Figure 2. The surface structure of the steel is high-carbon nanobainite and the core structure is low-carbon martensite. Tests have shown that compared with traditional martensite carburized steel, the contact fatigue life of this nanobainite carburized steel has more than doubled, and it has good application prospects in the field of carburized bearing steel.
Stainless Bearing Steel
Typical stainless steel bearing steels include martensitic stainless steel (such as 9Cr18, 9Cr18Mo, Cr15Mo4), austenitic stainless steel (such as 1Cr18Ni9Ti), etc. The grades and chemical compositions of the main high-carbon chromium stainless steel bearing steels at home and abroad are shown in Table 2. 9Cr18 , 9Cr18Mo and other high-carbon, high-chromium martensitic stainless steels have high hardness and wear resistance. At the same time, they have excellent corrosion resistance in marine environments, certain acid solutions, and salt solutions. They are often used in corrosion resistance and wear resistance. It should be used in usage conditions with higher performance requirements. Austenitic stainless steel bearing steel is often used in bearing parts that require higher corrosion resistance but lower impact load. This steel type shows good corrosion resistance in most working environments, but due to the austenitic structure Due to its characteristics, the bearing steel cannot be used in working environments with high requirements for strength load and wear resistance.
| Table 2 Brand and chemical composition of high carbon chromium stainless bearing steel | % | ||||||||
| project | Trademark | C | Cr | Mn | Si | Mo | Ni | P | S |
| ISO | 21 | 0.95～1.20 | 16.00～18.00 | ≤1.00 | ≤1.00 | 0.35～0.75 | ≤0.50 | ≤0.040 | ≤0.030 |
| China | 9Cr18 | 0.90～1.00 | 17.00～19.00 | ≤0.80 | ≤0.80 | ≤0.035 | ≤0.030 | ||
| 9Cr18Mo | 0.95～1.10 | 16.00～18.00 | ≤0.80 | ≤0.80 | 0.40～0.70 | ≤0.035 | ≤0.030 | ||
| Japan | SUS440C | 0.95～1.20 | 16.00～18.00 | ≤1.00 | ≤1.00 | ≤0.75 | ≤0.60 | ≤0.040 | ≤0.030 |
| Germany | X102CrMo17 | 0.95～1.10 | 16.00～18.00 | ≤1.00 | ≤1.00 | 0.35～0.75 | ≤0.50 | ≤0.040 | ≤0.040 |
| USA | 440C | 0.95～1.10 | 16.00～18.00 | ≤1.00 | ≤1.00 | 0.40～0.65 | ≤0.75 | ≤0.025 | ≤0.025 |
In recent years, researchers have developed a stainless steel bearing steel G30Cr15MoN with high corrosion resistance and compared the microstructure and properties of martensitic stainless steel 440C and stainless bearing steel G30Cr15MoN. The SEM microstructure of G30Cr15MoN and 440C stainless bearing steel is shown in Figure 2. The research shows that there is composite strengthening between the finely dispersed carbonitrides Cr2(C,N) and M23C6 carbides in G30Cr15MoN, which makes G30Cr15MoN steel have Excellent properties of high hardness and high corrosion resistance.
High temperature bearing steel
The working temperature of high-temperature bearing steel is generally above 150°C. With the continuous increase in the power of large bearing equipment such as aero engines and gas turbines, the working temperature of bearings can reach above 350°C. This requires high-temperature bearing steel to have high strength and High hardness, oxidation resistance, dimensional stability and corrosion resistance.
In recent years, in order to meet the performance requirements of bearing steel in high-temperature environments, high-speed steel has gradually been used in the research and development and production of bearing steel. For example, 8Cr4Mo4V is a high-speed steel with a certain amount of Mo and V elements added. Mo element can increase its hardness at high temperatures and improve the wear resistance and corrosion resistance of steel; V is a strong carbide-forming element, and the VC carbide formed with C in steel can effectively improve the wear resistance of steel; at the same time, W can also improve the hardenability and wear resistance of steel. Based on the isothermal quenching process of 8Cr4Mo4V bainite in high-temperature bearing steel, Zhao Kaili et al. conducted solution treatment at different temperatures on 8Cr4Mo4V steel, followed by 260°C salt bath treatment. They studied and analyzed the effect of different solution temperatures on 8Cr4Mo4V steel bainite. The influence of structural transformation. The bainite structure of 8Cr4Mo4V high-temperature bearing steel after treatment and tempering at different solution temperatures is shown in Figure 3. The results show that at the solution temperatures of 1050 and 1065°C, bainite in 8Cr4Mo4V steel The structure is finer and more uniform, and its impact toughness is also better.
Factors affecting the performance of bearing steel
Cleanliness
The mass fractions of N, H, and O in steel are important indicators for measuring the cleanliness of bearing steel. Nitrogen mainly exists in the form of TiN and AlN in steel, and oxygen generally exists in the form of oxide inclusions. Hydrogen in steel can easily cause hydrogen embrittlement and internal cracks. In order to facilitate the analysis of the influence of inclusions in steel on steel, inclusions are usually divided into four categories: A-sulfide, B-Al2O3, C-silicate, and D-point-like (spherical) non-deformable inclusions. The existence of inclusions destroys the continuity and uniformity of the steel matrix structure. Under the influence of alternating loads, stress concentration is likely to occur in the areas where inclusions exist, leading to fatigue spalling of the bearings. When the stress concentration becomes more severe, microcracks will develop, and the continuous expansion of cracks will lead to bearing failure. Oxide inclusions are the most harmful defects among non-metallic inclusions to the fatigue life of bearings. The increase in the oxygen mass fraction in steel leads to a significant increase in oxide inclusions. The number and size of oxide inclusions are important factors affecting the fatigue life of bearings. Famous bearing manufacturing companies such as Japan’s Sanyo Special Steel and Sweden’s SKF Company have continuously improved their smelting processes in recent years. The mass fraction of oxygen in steel can now be stably controlled at 0.0004% to 0.0006%, and the quality stability and service life of the products have been improved. A huge improvement. In recent years, although domestic control of the mass fraction of oxygen in steel has gradually moved closer to European and American standards, due to large fluctuations in the mass fraction of oxygen in steel, product quality stability is insufficient. In addition, since Ti is a strong carbon and nitride-forming element, if not properly controlled, Ti can easily form titanium nitride and titanium carbonitride inclusions with C, N, O and other elements in the steel, seriously reducing the fatigue life of the bearing steel. Bearing steel manufacturers at home and abroad have strict requirements on the control of Ti mass fraction in steel. SKF production standards stipulate that the residual Ti mass fraction in bearing steel should not be higher than 0.0012%. Shanyang Special Steel requires that the Ti mass fraction should be controlled between 0.0014 and 0.0012%. 0.0015%.
Carbide and tissue defects
The improvement of the cleanliness of bearing steel makes the size and uniformity of carbides have a more significant impact on the fatigue life of bearings. Research shows that the larger the size and lower the uniformity of carbides in steel, the greater the probability of bearing failure; the lower the content of carbides, the longer the fatigue life of the bearing. Bearing steel contains a large number of alloy carbide-forming elements, such as Mn, Cr, Mo and other elements, which can easily cause segregation of carbides, resulting in uneven distribution of carbides, thereby reducing the fatigue life of the bearing. The main forms of carbides in steel are carbide liquid precipitation, network carbides and ribbon carbides. With the improvement of smelting quality, carbide liquid precipitation is eliminated; network carbides need to be eliminated through a certain hot rolling process (such as controlled rolling and controlled cooling); for medium carbon bearing steel or carburized bearing steel, due to Its carbon content is low, and it mainly controls band-shaped carbides. It requires spheroidization to refine the carbides and improve the uniformity of the carbides in the steel. In addition to the fine control of carbides in the steel, high-end bearing steel also has extremely strict requirements for low-magnification structures. Structural defects such as central porosity, shrinkage holes, and segregation need to be controlled at an extremely low level to achieve high-quality bearing steel. product quality standards. At present, there is still a big gap between domestic bearing steel manufacturers’ control of structural defects, dimensional accuracy, etc. and those of bearing manufacturing powers such as Sweden and Japan. While controlling the size and uniformity of carbides and inclusions, they also need to strictly control the size and uniformity of carbides and inclusions. Control the level of looseness to avoid white spots, shrinkage holes and other structural defects in the tissue to improve the stability of product quality.
Smelting process
The current domestic smelting process has gradually been integrated with international advanced special steel production enterprises, and the smelting process of scrap steel + blast furnace hot metal → ultra-high power electric arc furnace smelting → LF refining → VD/RH vacuum degassing → IC/CC is generally adopted.
Based on the current smelting process and advanced smelting equipment, the oxygen mass fraction of domestic bearing steel can basically be controlled to 0.0006% to 0.0008%. Some advanced steel companies can even control the titanium mass fraction to 0.0015%. The control level of inclusions in steel There has been a significant improvement. However, regarding the refined control during the smelting process, such as the insufficient control of components and process parameters, and the lack of detection technology, the quality stability of domestic bearing steel products cannot be guaranteed. The performance of steel produced in different batches varies greatly, especially The product quality of high-end bearing steel cannot be effectively guaranteed. There is still much room for improvement compared with international manufacturing levels in terms of carbide distribution and low-magnification structural defect control in bearing steel. In view of this, China can introduce advanced foreign smelting processes such as vacuum induction + vacuum self-consumption (VIM + VAR) to improve the cleanliness of bearing steel and improve the size and distribution uniformity of inclusions and carbides in the steel.
Heat treatment process
Improving the heat treatment process can not only refine the grains, but also refine the carbides and improve the uniformity of carbide distribution. The high hardness martensite in traditional high carbon chromium bearing steel makes this steel have good wear resistance and excellent contact fatigue resistance. However, due to poor toughness, residual tensile stress is prone to exist on the surface of the workpiece during quenching, and it is hydrogen embrittled. The high sensitivity greatly reduces the service life of high carbon chromium bearing steel in working environments with high performance requirements. To this end, based on the traditional martensite quenching and tempering process, new heat treatment processes such as bainite isothermal quenching and martensite-bainite complex phase quenching have been developed.
Taking GCr15 high carbon chromium bearing steel as an example, Figure 4 shows three common heat treatment processes for this steel type. Process 1 is a conventional martensite quenching and tempering process; Process 2 is a bainite isothermal quenching heat treatment process; Process 3 is a martensite-bainite composite quenching heat treatment process. Figure 5 shows the microstructure morphology of GCr15 bearing steel after heat treatment under martensite quenching and tempering, bainite isothermal quenching, martensite-bainite complex phase quenching and bainite temperature change quenching processes.
Spheroidizing annealing
The conventional heat treatment of bearings includes spheroidizing annealing and quenching + low temperature tempering. On the one hand, spheroidizing annealing makes the spherical carbide particles evenly distributed on the ferrite matrix, thereby obtaining a granular pearlite structure; on the other hand, spheroidizing annealing can reduce the hardness of the material and enable the steel to obtain good cutting performance, which is the final result. Process (quenching + low temperature tempering) to prepare the tissue. Yang Hongbo et al. treated GCr15 bearing steel with different spheroidizing annealing times and performed the martensite-bainite complex phase quenching process. They found that as the spheroidizing annealing time continued to increase, the bainitic ferrite strips gradually became wider. trend, the SEM microstructure of GCr15 bearing steel after isothermal quenching at different spheroidizing annealing times is shown in Figure 6. When the annealing time is 150 minutes, the bainite ferrite strips become narrower. Analysis shows that when the spheroidizing annealing time is 150 minutes, during the lower bainite transformation process, a large amount of undissolved carbides hinder the growth of bainitic ferrite strips, causing the distortion energy to increase and promoting the formation of new bainite. Ferrite flakes are produced, so the spheroidizing annealing time directly affects the distribution of carbides in the steel. Precise control of the spheroidizing annealing time can refine the size of bainitic ferrite flakes, thereby effectively improving the comprehensive quality of bearing steel. performance.
The cooling rate of conventional spheroidizing annealing is generally 10~25℃/h. If the cooling rate is too fast, it will be detrimental to the cutting process; if the cooling rate is too slow, the size of the carbide will increase. Therefore, an isothermal spheroidizing annealing process was developed, and the processing time of this spheroidizing annealing process can generally be shortened to 10 to 18 hours; James M et al. proposed another periodic spheroidizing annealing process that can shorten the processing time, but this process The heat treatment control steps are relatively complicated and have certain limitations in actual production applications.
Martensite quenching and tempering process
The pre-treated (spheroidized annealing) and quenched steel is subjected to low-temperature tempering to improve the strength, hardness, wear resistance and fatigue life of the bearing steel. The heating temperature for final treatment (quenching + low temperature tempering) is generally Ac1 ~ Accm. Taking GCr15 bearing steel as an example, the quenching temperature is generally 820~860°C. If the temperature is too high or too low, it will affect the overall performance of the steel. The low-temperature tempering temperature is generally 150~170°C. In order to obtain a more stable tempered structure and eliminate the internal stress in the structure as much as possible, a longer tempering time should be ensured. The microstructure of GCr15 bearing steel martensite after quenching and tempering is shown in Figure 7. The GCr15 bearing steel was quenched at 850°C for 30 minutes and low-temperature at 160°C for 3 hours. Finally, the tempering Tempered structure composed of martensite, a small amount of retained austenite and carbides.
Bainite austempering process
As the performance requirements for bearing steel continue to increase, the quenching-tempering process of bearing steel is also constantly improving. High-carbon chromium bearing steel is subjected to lower bainite isothermal quenching to obtain a final structure composed of lower bainite, martensite, retained austenite and carbides. Studies by Zhang Zengqi and others have shown that the impact toughness of GCr15 bearing steel after bainite isothermal quenching is more than double that of steel after conventional martensite quenching and tempering.
ZHAOJ et al. conducted isothermal quenching treatments on high carbon chromium bearing steel at 200°C for 2, 6, 12, and 72 hours respectively, and obtained acicular bainite structures with volume fractions of 2.1%, 40.8%, 60.2%, and 86.6%. , Figure 8 shows the transmission electron microscope images of the two samples after isothermal for 6h and 12h respectively at 200°C. It can be observed that it consists of lath-like bainitic ferrite, film-like retained austenite and ε-carbide. The final structure was measured by the average line intercept method and the data was corrected. It was concluded that the structure was nanobainite laths with an average thickness of approximately 50nm±15nm.
The study found that compared with the conventional heat treatment process, the heat treatment process of nanobainite bearing steel has a lower isothermal temperature, which increases the driving force of phase transformation and increases the nucleation point of bainite. However, the disadvantage of this process is that it prolongs the isothermal time during the bainite transformation process and increases the production and manufacturing costs. Therefore, for the nanobainite heat treatment process, its composition design should be further optimized and the heat treatment process should be improved to shorten the bainite heat treatment process. The formation time during the body transformation process reduces the manufacturing cost of production.
Development direction of bearing steel
With the continuous development of domestic industrial manufacturing, China has made a national strategic deployment of “Made in China 2025” based on the international industrial transformation situation. As one of the indispensable key components in the equipment manufacturing field, bearings are related to the future development direction of high-end manufacturing in the domestic industrial field and determine China’s progress towards becoming a world manufacturing power. Overall, China’s bearing industry has made significant improvements after decades of development. However, in the field of high-end bearings, due to insufficient precision in the control of the size and distribution of inclusions, carbides, and low-magnification structural defects in steel , so that the quality stability of high-quality bearing products cannot be guaranteed. In view of this, in the future, in order to meet the long-life and high-reliability requirements of high-end bearing products, the domestic bearing industry can consider the following aspects in its research direction. (1) Process improvement. Based on the conventional martensite quenching and tempering treatment of bearing steel, new heat treatment processes such as bainite isothermal quenching, martensite-bainite isothermal quenching, and bainite temperature change quenching have been developed. Regarding the bainite bearing steels that are currently being studied more, first of all, attention should be paid to the applicability of the bainite austempering process, and the selection of the heat treatment process should be determined based on the working environment of the bearing and the actual performance requirements; secondly, for bainite In the improvement of isothermal quenching media, in the future, we should try to avoid excessive use of toxic nitrate salts and develop more environmentally friendly quenching media; thirdly, due to the low isothermal temperature of bainite, the processing time of the entire heat treatment process is too long, which undoubtedly increases Enterprise manufacturing costs, therefore reducing the bainite transformation time should be one of the focuses of future research. In addition, although the smelting process of domestic bearing steel is basically in line with international standards, domestic scrap steel smelting accounts for a large proportion, the vacuum degree during the refining process is not up to standard, and the oxygen content in the steel fluctuates greatly, resulting in the control of non-metallic inclusions and carbides. Difficulty meeting rating criteria. Finally, we should also learn from foreign advanced smelting processes such as vacuum degassing and inclusion homogenization to achieve localization of ultra-clean and ultra-long life bearing steel. (2) Internal quality control. First, there should be more refined detection and control standards for the control of oxygen mass fraction and the distribution uniformity of inclusions in steel. In the future, the mass fraction of oxygen in steel should be stabilized below 0.0006%, and the mass fraction of titanium should be Less than 0.0015%, reducing or eliminating fatigue spalling and fracture caused by hard and brittle inclusions in steel, and minimizing the impact of inclusions on steel quality; secondly, in view of the prominent problem of carbide instability and even exceeding the standard in domestic bearing steel , advanced processes such as controlled rolling and controlled cooling, periodic spheroidizing annealing, and cyclic induction spheroidizing annealing should be used to eliminate carbide segregation in steel as much as possible, improve the uniformity of carbide distribution, and achieve microstructure refinement and uniformity; second Third, optimize the continuous casting process during the smelting process, reduce low-magnification structural defects in the steel, reduce the looseness and shrinkage holes in the center of the cast slab, strictly control component segregation, and improve the quality of the continuous cast slab. (3) Surface modification. In view of the increasingly complex working environment, we should try our best to optimize the surface properties of the material and extend the surface treatment process based on the original surface treatment processes such as surface carburizing and carbonitriding, combined with new surface modification technologies such as surface coating and cladding. In order to improve the fatigue life of bearings, we have developed bearing steel products that adapt to different working environments and realized the development of bearing steel from single nature to diversification. (4) Testing equipment and technical evaluation standards. First of all, the production concentration of the domestic bearing industry is low, and the product quality of various bearing manufacturing companies is uneven. Since most small and medium-sized enterprises lack high-precision testing equipment, it is difficult to detect microscopic inclusions, network carbides, surface defects, etc. , eventually leading to substandard products flowing into the market; secondly, the domestic bearing industry standards do not have rating requirements for the content of some harmful elements, inclusions and the size and distribution of carbides, and the control of decarburized layers and dimensional accuracy is not strict enough; in addition, currently, Foreign countries have relevant control standards for the detection and assessment of residual austenite and residual stress in steel. However, China’s detection and analysis of residual stress in steel has not yet been included in the control indicators. In the future, a unified standard should be formulated based on the actual development of the bearing industry at home and abroad. The company’s technical evaluation system and complete testing and rating standards strictly control product quality and improve the quality stability of domestic bearing products.
