Factors Affecting Bearings Life and Its Control

The early failure forms of rolling bearing mainly include fracture, plastic deformation, wear, corrosion and fatigue. Under normal conditions, it is mainly contact fatigue. In addition to the service conditions, the failure of bearing parts is mainly restricted by the hardness, strength, toughness, wear resistance, corrosion resistance and internal stress state of steel. The main internal factors affecting these performances and states are as follows.Factors affecting bearing life and its control

1. martensite in quenched steel：When the original structure of high carbon chromium steel is granular pearlite, the carbon content of quenched martensite obviously affects the mechanical properties of the steel under the condition of quenching and low temperature tempering. 
2. The strength and toughness are about 0.5%, the contact fatigue life is about 0.55%, and the compressive collapse 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 strong failure resistance can be obtained.
3. 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 away from the original ferrite part of the carbide. 
4. Therefore, they begin to undergo martensitic transformation at different temperatures, which inhibits the growth of martensitic grains and the display of microstructure and becomes cryptocrystalline martensite.
5.  It can avoid the micro cracks that are easy to appear when high carbon steel is quenched, and its substructure is dislocation plate and strip martensite with high strength and toughness. Therefore, only when medium carbon cryptocrystalline martensite is obtained when high carbon steel is quenched can the bearing parts obtain the matrix with anti failure ability.
6. retained austenite in quenched steel：After normal quenching, high carbon chromium steel can contain 8% ~ 20% AR (residual austenite). AR in bearing parts has both advantages and disadvantages. In order to promote advantages and eliminate disadvantages, the content of AR 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 insoluble carbide, it is difficult to correctly reflect the influence of AR on mechanical properties.
7. Therefore, the austenite heat stabilization process was used to obtain different ar content under the fixed austenite condition. The effect of AR content on the hardness and contact fatigue life of GCr15 steel after quenching and low temperature tempering was studied. With the increase of austenite content, the hardness and contact fatigue life increase, and then decrease after reaching the peak, but the peak ar content is different. 
8. The hardness peak appears at about 17% AR, while the contact fatigue life peak appears at about 9%. When the test load is reduced, the influence of the increase of AR on the contact fatigue life is reduced. This is because when the amount of AR is small, it has little effect on the reduction of strength, while the toughening effect is more obvious. 
9. 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 processing and stress-strain induced martensitic transformation. 
10. However, if the load is large, the larger plastic deformation of AR and the matrix will produce local stress concentration and fracture, so as to reduce the service life. It should be pointed out that the favorable effect of AR must be that under the stable state of AR, if it spontaneously transforms into martensite, the toughness of steel will be sharply reduced and embrittlement will occur.
11. insoluble carbide in quenched steel：The quantity, morphology, size and distribution of undissolved carbides in quenched steel are affected not only by the chemical composition of steel and the original structure before quenching, but also by austenitizing conditions. There are few studies on the influence of undissolved carbides on bearing life. Factors affecting bearing life and its control
12. Carbide is a hard brittle phase. In addition to being beneficial to wear resistance, it will produce cracks due to stress concentration with the matrix (especially when the carbide is non spherical), which will reduce toughness and fatigue resistance. 
13. Quenched insoluble carbides not only affect the properties of steel, but also affect the carbon content and AR content and distribution of quenched martensite, which has an additional impact on the properties of steel. 
14. In order to reveal the influence of undissolved carbides on properties, steels with different carbon content are used. After quenching, the carbon content and AR content of martensite are the same, but the undissolved carbides content is different. After tempering at 150 ℃, due to the same carbon content and high hardness of martensite, a small increase of undissolved carbides has little effect on the increase of hardness, and the crushing load reflecting strength and toughness is reduced, The contact fatigue life sensitive to stress concentration is significantly reduced. 
15. Therefore, too much quenched insoluble carbide is harmful to the comprehensive mechanical properties and failure resistance of steel. Properly reducing the carbon content of bearing steel is one of the ways to improve the service life of parts.
16. In addition to the quantity of quenched undissolved carbides, the size, morphology and distribution also affect the properties of the material. In order to avoid the harm of undissolved carbides in bearing steel, it is required to have less undissolved carbides (small quantity), small carbides (small size), uniform carbides (the size difference is very small, and the distribution is uniform), and round carbides (each carbide 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 can not only maintain sufficient wear resistance, but also be a necessary condition for obtaining fine grain cryptocrystalline martensite.
17. residual stress after quenching and tempering：After quenching and low temperature tempering, bearing parts still have large internal stress. The residual internal stress in parts can be divided into advantages and disadvantages. After heat treatment, the fatigue strength of steel increases with the increase of surface residual compressive stress. Factors affecting bearing life and its control
18. On the contrary, when the surface residual internal stress is tensile stress, the fatigue strength of steel decreases. This is because the fatigue failure of the part occurs when it bears excessive tensile stress. When there is a large residual compressive stress on the surface, it will offset the tensile stress of the same value, reduce the actual tensile stress of the steel and increase the limit value of fatigue strength. When there is a large residual tensile stress on the surface, it will be superimposed with the tensile stress load to significantly increase the actual tensile stress of the steel, even if the limit value of fatigue strength is reduced.
19. Therefore, it is also one of the measures to improve the service life to make the surface of bearing parts remain large compressive stress after quenching and tempering (of course, excessive residual stress may cause deformation and even cracking of parts, which should be paid enough attention).
20. impurity content of steel：The impurities in steel include non-metallic inclusions and harmful elements (acid soluble) content. Their harm to steel properties is often mutually reinforcing. For example, the higher the oxygen content, the more oxide inclusions. 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 reduce toughness, plasticity and fatigue life.
21. With the increase of inclusion size, the fatigue strength decreases, and the higher the tensile strength of steel, the greater the decreasing trend. When the oxygen content in steel increases (oxide inclusions increase), the bending fatigue and contact fatigue life also decrease under high stress. 
22. Therefore, it is necessary to reduce the oxygen content of manufacturing steel for bearing parts working under high stress. Some studies show that MNS inclusions in steel are ellipsoidal in shape and can wrap harmful oxide inclusions, so they have little effect on the reduction of fatigue life and may even be beneficial, so they can be controlled broadly.
23. Control of material factors affecting bearing life：In order to keep the above material factors affecting the bearing life in a state, it is first necessary to control the original structure of the steel before quenching. The technical measures that can be taken are: Factors affecting bearing life and its control
24. austenitizing at high temperature (1050 ℃), rapid cooling to 630 ℃ isothermal normalizing to obtain pseudo eutectoid fine pearlite structure, or cooling to 420 ℃ isothermal treatment to obtain bainite structure. It can also be rapidly annealed by forging and rolling waste heat to obtain fine-grained pearlite structure, so as to ensure the fine and uniform distribution of carbides in the steel. When the original structure in this state is austenitized by quenching and heating, in addition to the carbides dissolved in austenite, the insoluble carbides will aggregate into fine particles.
25. When the original structure in the steel is certain, the carbon content of quenched martensite (i.e. the carbon content of austenite after quenching heating), retained austenite and undissolved carbide mainly depend on the quenching heating temperature and holding time. 
26. With the increase of quenching heating temperature (time is certain), the number of undissolved carbide in the steel decreases (the carbon content of quenched martensite increases), the number of retained austenite increases, and the hardness first increases with the increase of quenching temperature, After reaching the peak value, it decreases with the increase of temperature.
27.  When the quenching heating temperature is constant, with the extension of austenitizing time, the amount of insoluble carbide decreases, the amount of residual austenite increases and the hardness increases. When the time is long, this trend slows down. When the carbide in the original structure is fine, because the carbide is easy to dissolve into austenite, the hardness peak after quenching moves to a lower temperature and appears in a shorter austenitizing time.
28. To sum up, the microstructure of GCrl5 steel after quenching is composed of insoluble carbide of about 7% and retained austenite of about 9% (the average carbon content of cryptocrystalline martensite is about 0.55%). Moreover, when the carbides in the original structure are fine and evenly distributed, when the microstructure composition of the above level is reliably controlled, it is conducive to obtain high comprehensive mechanical properties and high service life. 
29. It should be pointed out that for the original structure with fine dispersed carbides, when quenching, heating and holding, the insoluble fine carbides will aggregate and grow up and coarsen them. Therefore, the quenching and heating time of bearing parts with this original structure should not be too long. Using rapid heating austenitizing quenching process will obtain higher comprehensive mechanical properties.
30. In order to make the surface of bearing parts remain large compressive stress after quenching and tempering, carburizing or nitriding atmosphere can be introduced during quenching and heating to carry out surface carburizing or nitriding for a short time. Because the actual carbon content of austenite during quenching and heating of this steel is not high, which is far lower than the equilibrium concentration shown in the phase diagram, it can absorb carbon (or nitrogen). 
31. When austenite contains high carbon or nitrogen, its MS decreases. During quenching, martensitic transformation occurs on the surface layer compared with the inner layer and the core, resulting in large residual compressive stress. After heating and quenching in carburizing atmosphere and non carburizing atmosphere (both tempered at low temperature), the contact fatigue test shows that the service life of surface carburized steel is 1.5 times higher than that of non carburized steel. The reason is that the surface of carburized parts has large residual compressive stress.