1. Material factors affecting bearing life

The early failure forms of rolling bearings mainly include fracture, plastic deformation, wear, corrosion, and fatigue, and under normal conditions, contact fatigue is the main form. The failure of bearing components is mainly constrained by the hardness, strength, toughness, wear resistance, corrosion resistance, and internal stress state of steel, in addition to service conditions. The main internal factors that affect these performance and states are as follows.

01. When the original structure of the martensite high carbon chromium steel in the quenched steel is granular pearlite, the carbon content of the quenched martensite in the quenching and low temperature tempering state obviously affects the mechanical properties of the steel. The strength and toughness are about 0.5%, the contact fatigue life is about 0.55%, and the crushing capacity is about 0.42%. When the carbon content of quenched martensite of GCr15 steel is 0.5%~0.56%, the comprehensive mechanical properties with the strongest resistance to failure can be obtained. It should be pointed out that the martensite obtained in this case is cryptocrystalline martensite, and the measured carbon content is the average carbon content. In fact, the carbon content in martensite is uneven in the micro region, and the carbon concentration near the carbide is higher than that far from the original ferrite of carbide, so they begin to undergo martensite transformation at different temperatures, thus inhibiting the growth of martensite grains and the display of microscopic morphology, and becoming cryptocrystalline martensite. It can avoid the micro cracks that are easy to appear when high carbon steel is quenched, and its substructure is dislocation lath martensite with high strength and toughness. Therefore, only when medium carbon cryptocrystalline martensite is obtained during quenching of high carbon steel can the bearing parts obtain the matrix with the best anti failure ability.

02. After normal quenching, residual austenite in quenched high carbon chromium steel can contain 8% to 20% Ar (residual austenite). The Ar content in bearing parts has both advantages and disadvantages. In order to promote the advantages and eliminate the disadvantages, the Ar content should be appropriate. Since the amount of Ar is mainly related to the austenitizing conditions of quenching heating, and its amount will affect the carbon content of quenched martensite and the amount of undissolved carbide, it is difficult to correctly reflect the effect of the amount of Ar on mechanical properties. To achieve this, the austenite transformation heat stabilization treatment process was used to obtain different amounts of Ar under fixed austenite conditions. The effect of Ar content on the hardness and contact fatigue life of GCr15 steel after quenching and low-temperature tempering was studied here. With the increase of austenite content, both hardness and contact fatigue life increase, and then decrease after reaching the peak. However, the peak Ar content is different, with the hardness peak appearing around 17% Ar and the contact fatigue life peak appearing around 9%. When the test load decreases, the impact of increasing Ar on contact fatigue life decreases. This is because when the amount of Ar is not high, the effect on strength reduction is not significant, while the toughening effect is more obvious. The reason is that when the load is small, a small amount of deformation occurs in Ar, which not only reduces the stress peak, but also strengthens the deformed Ar by processing and martensite transformation induced by stress and strain. But when the load is large, the plastic deformation with larger Ar and the matrix will locally generate stress concentration and fracture, thereby reducing the lifespan. It should be pointed out that the beneficial effect of Ar must be in the stable state of Ar. If it spontaneously transforms into martensite, the toughness of steel will be sharply reduced and embrittlement will occur.

03. The quantity, morphology, size, and distribution of undissolved carbides in quenched steel are influenced not only by the chemical composition of the steel and the original structure before quenching, but also by the austenitizing conditions. There is relatively little research on the impact of undissolved carbides on bearing life. Carbide is a hard brittle phase, which not only benefits wear resistance, but also causes stress concentration and cracks when loaded (especially when the carbide is non spherical) with the matrix, thereby reducing toughness and fatigue resistance. In addition to its own influence on the properties of steel, quenched undissolved carbides also affect the carbon content and Ar content and distribution of quenched martensite, thus exerting additional influence on the properties of steel. In order to reveal the influence of undissolved carbide on properties, steels with different carbon content are used. After quenching, the martensite carbon content and Ar content are the same, but the undissolved carbide content is different. After tempering at 150 ℃, because the martensite carbon content is the same, and the hardness is high, a small increase in undissolved carbide has little effect on the hardness increase, and the crushing load reflecting strength and toughness is reduced, The contact fatigue life sensitive to stress concentration is significantly reduced. Therefore, excessive quenching of undissolved carbides is harmful to the comprehensive mechanical properties and failure resistance of steel. Reducing the carbon content of bearing steel appropriately is one of the ways to improve the service life of components. Quenched undissolved carbides not only have an impact on material properties in terms of quantity, but also have an impact on material properties in terms of size, morphology, and distribution. In order to avoid the harm of undissolved carbides in bearing steel, it is required to have a small amount of undissolved carbides (small in quantity), a small size (small in size), a uniform distribution (with small differences in size), and a circular shape (each carbide particle is spherical). It should be pointed out that it is necessary for bearing steel to have a small amount of undissolved carbide after quenching, which not only can maintain sufficient wear resistance, but also is a necessary condition for obtaining fine grain cryptocrystalline martensite.

04. The residual stress of bearing parts after quenching and tempering still exhibits significant internal stress after quenching and low-temperature tempering. There are two states of residual internal stress in the parts: favorable and unfavorable. After heat treatment of steel parts, as the residual compressive stress on the surface increases, the fatigue strength of the steel increases. Conversely, when the residual internal stress on the surface is tensile stress, the fatigue strength of the steel decreases. This is because the fatigue failure of the component occurs when subjected to excessive tensile stress, and when there is residual compressive stress on the surface, it will offset the same amount? The actual tensile stress value of the steel is reduced, resulting in an increase in the fatigue strength limit value. When there is a large residual tensile stress on the surface, it will be superimposed with the tensile stress load, resulting in the actual fatigue strength of the steel?? Is the tensile stress significant? Large, even if the fatigue strength limit value decreases. Therefore, increasing the residual compressive stress on the surface of bearing parts after quenching and tempering also improves their service life? One of the measures? Of course it's too big? Residual stress may cause deformation or even cracking of parts, and sufficient attention should be given.

05. Impurity content in steel. Impurities in steel include non-metallic inclusions and harmful element (acid soluble) content, and their harm to steel performance is often mutually reinforcing. For example, the higher the oxygen content, the more oxide inclusions there are. The influence of impurities in steel on mechanical properties and failure resistance of parts is related to the type, nature, quantity, size, and shape of impurities, but they usually have the effect of reducing toughness, plasticity, and fatigue life.

As the size of inclusions increases, the fatigue strength decreases, and the higher the tensile strength of steel, the greater the decreasing trend. The increase in oxygen content (oxide inclusions) in steel leads to a decrease in bending fatigue and contact fatigue life under high stress. Therefore, it is necessary to reduce the oxygen content of manufacturing steel for bearing parts that work under high stress. Some studies have shown that MnS inclusions in steel, due to their elliptical shape and the ability to encapsulate harmful oxide inclusions, have a relatively small impact on reducing fatigue life and may even be beneficial. Therefore, they can be controlled more broadly.

2. Control of material factors affecting bearing life

In order to make the above material factors affecting bearing life in the best state, it is first necessary to control the original structure of the steel before quenching. Technical measures can be taken: high (1050 ℃) austenitizing speed cooling to 630 ℃ isothermal normalizing to obtain pseudo eutectoid fine pearlite structure, or cooling to 420 ℃ isothermal treatment to obtain bainite structure. Rapid annealing with residual heat from forging and rolling can also be used to obtain fine-grained pearlite structure, ensuring the fine and uniform distribution of carbides in the steel. When the original structure in this state undergoes quenching and austenitizing, in addition to the carbides dissolved in the austenite, the undissolved carbides will aggregate into fine particles. When the original structure in the steel is constant, the carbon content of quenched martensite (that is, the carbon content of austenite after quenching and heating), the amount of retained austenite and the amount of undissolved carbide mainly depend on the quenching heating temperature and holding time. With the increase of quenching heating temperature (that is, the time is constant), the amount of undissolved carbide in the steel decreases (the carbon content of quenched martensite increases), and the amount of retained austenite increases, The hardness first increases with the increase of quenching temperature, reaches its peak, and then decreases with the increase of temperature. When the quenching heating temperature is constant, as the austenitizing time prolongs, the number of undissolved carbides decreases, the number of residual austenite increases, and the hardness increases. Over time, this trend slows down. When the carbides in the original structure are small, the hardness peak after quenching shifts to a lower temperature and appears in a shorter austenitizing time due to the easy dissolution of carbides into austenite.

To sum up, the best structure composition is that the undissolved carbide of GCr15 steel after quenching is about 7%, and the residual austenite is about 9% (the average carbon content of cryptocrystalline martensite is about 0.55%). Moreover, when the carbides in the original structure are small and evenly distributed, reliable control of the above level of microstructure composition is beneficial for obtaining high comprehensive mechanical properties and thus having a long service life. It should be pointed out that the original structure with fine dispersed carbides will aggregate and grow up during quenching, heating, and insulation, causing it to coarsen. Therefore, for bearing parts with this original structure, the quenching and heating time should not be too long. Adopting rapid heating and austenitizing quenching process will achieve higher comprehensive mechanical properties.

In order to residual large compressive stress on the surface of bearing parts after quenching and tempering, carburizing or nitriding atmosphere can be introduced during quenching and heating, and short-term surface carburizing or nitriding can be carried out. Due to the low actual carbon content of austenite during quenching and heating of this type of steel, which is much lower than the equilibrium concentration shown on the phase diagram, it can absorb carbon (or nitrogen). When the austenite contains higher carbon or nitrogen, its Ms decreases. During quenching, the martensite transformation occurs in the surface layer after the inner layer and the center, resulting in greater residual compressive stress. After heating and quenching GCr15 steel in both carburized and non carburized atmospheres (both tempered at low temperatures), contact fatigue tests showed that the life of surface carburized steel was 1.5 times longer than that of non carburized steel. The reason is that the surface of carburized parts has a large residual compressive stress.

3. Conclusion

The main material factors and control levels that affect the service life of high carbon chromium steel rolling bearing parts are:

(1) The carbides in the original structure of steel before quenching are required to be small and dispersed. High temperature austenitization at 630 ℃ or 420 ℃ high temperature can be used, or rapid annealing process with residual heat from forging and rolling can be used to achieve this.

(2) After quenching, it is required to obtain the microstructure of cryptocrystalline martensite with an average carbon content of about 0.55%, about 9% Ar and about 7% undissolved carbide in uniform and round state. This microstructure can be obtained by controlling the quenching heating temperature and time.

(3) After quenching and low-temperature tempering of parts, it is required to have a large residual compressive stress on the surface, which helps to improve fatigue resistance. Short term carburization or nitriding of the surface during quenching and heating can be used to achieve? There is significant compressive stress on the surface residue.

(4) The steel used for manufacturing bearing parts requires high purity, mainly reducing the content of O2, N2, P, oxides, and phosphides. Technical measures such as electroslag remelting and vacuum smelting can be adopted to ensure that the oxygen content of the material is ≤ 15PPM.