Material factors affecting the bearing life The early failure of rolling bearing, the main rupture, plastic deformation, wear, corrosion and fatigue, under normal conditions is of contact fatigue. Bearing parts failure in addition to service conditions, mainly by the steel hardness, strength, toughness, wear resistance, corrosion resistance and stress condition. The main internal factors affecting these properties and conditions are as follows.
1 residual austenite in quenching steel High carbon chromium steel after quenching by normal, can contain 8% ~ 20%Ar (residual austenite). Bearing parts in the Ar has its advantages and disadvantages, in order to promote, the content of Ar should be appropriate. As the amount of Ar is mainly related with the quenching of austenitizing condition, how much it will also affect the quenching martensite carbon content and the amount of undissolved carbides, difficult to correctly reflect the effect of Ar content on the mechanical properties. Therefore, the fixed austenitic condition, using austenitic heat stabilization process, in order to obtain different amount of Ar, in this study the effect of Ar content and low temperature tempering after quenching of GCr15 steel hardness and contact fatigue life. With the increase of the content of retained austenite, hardness and contact fatigue life increases, reaches a peak and then decreased correspondingly, but the content of Ar and the peak difference, hardness peak appeared at about 17%Ar, and the contact fatigue life peaked at around 9%. When the test load is reduced, because Ar increased effect on contact fatigue life decreases. This is because when the amount of Ar not much has little effect on the intensity, and the toughening effect is obvious. The reason is the load is small, Ar has small deformation, not only reduce the stress peak, and the Ar processing has been deformed reinforcement and stress and strain induced martensite transformation and strengthening. But if the load is large, Ar plastic deformation and matrix will produce local stress concentration and rupture is larger, so that the reduced life. It should be pointed out that, the beneficial effect of Ar must be under the Ar steady state, if the spontaneous transformation to martensite, will make the steel toughness decreases sharply and embrittlement.
Undissolved carbide 2 hardened steel Hardened steel of undissolved carbide quantity, morphology, size, distribution, which is affected by the chemical composition and quenched steel before the original organization, but also by the effect of austenitizing condition, the undissolved carbide research less influence on bearing life. Carbide is hard and brittle phases, except in favor of wear resistance, load will cause (especially the carbides are non spherical) stress concentration and crack cause and matrix, which will reduce the toughness and fatigue resistance. Quenching undissolved carbide in addition to affect its performance of the steel, but also the effect of the carbon content and the Ar content and distribution of quenching martensite, resulting in additional influence on the performance of steel. In order to reveal the effects of undissolved carbides on properties, using different carbon content of steel, after quenching the martensitic carbon content and the same Ar content and undissolved carbide content in different state, after 150 ℃ tempering martensite, because with the same carbon content, and the hardness is higher, so a small amount of undissolved carbides. The increase in hardness value is not, reflect the strength and toughness of the crushing load decreases, the stress concentration sensitivity of the contact fatigue life decreased. Therefore the quenching undissolved carbides and failure resistance comprehensive mechanical properties of steel is harmful. Appropriate to reduce the amount of carbon bearing steel is one of the ways to improve the service life of the product. Quenching undissolved carbide except the number of influential on the properties of the material, size, morphology, distribution and influence on material properties. In order to avoid the harm of undissolved carbide in bearing steel, require fewer undissolved carbides (number), small (small size), uniform (the size of each other is very small, and uniform distribution), round (each grain carbides are spherical). It should be pointed out that, bearing steel after quenching and a small amount of undissolved carbide is necessary, not only can maintain sufficient wear resistance, but also obtain necessary conditions for fine grain cryptocrystalline martensite.
3 of martensite in quenching steel High carbon chromium steel microstructure as granular pearlite, the tempering quenching temperature, quenching martensite carbon content, significantly affect the mechanical properties of steel. Strength and toughness, at around 0.5%, the contact fatigue life at around 0.55%, the compressive capacity in about 0.42%, when GCr15 steel quenched martensite carbon content is 0.5% ~ 0.56%, can obtain the anti failure mechanical capability is the strongest performance. It should be pointed out that, the martensite is obtained in this case is hidden crystal martensite, carbon content measured is the average carbon content. In fact, the amount of carbon in martensite in the micro region is not uniform, carbon concentration near the carbide carbide around than that far from primary ferrite part, so they began to martensitic transformation temperature is different, thus inhibiting the martensite crystal growth and morphology of display and become hidden crystal martensite. It can be easy to avoid micro cracks in high carbon steel quenching, and the sub structure for strength and toughness are higher dislocation plate martensite. Therefore, only when the high carbon steel during quenching of medium carbon martensite bearing parts can be anti failure matrix ability best.
The impurity content of 5 steel Impurities in steel including non metallic inclusions and harmful elements (acid soluble) content, their harm to the performance of the steel is often encourage each other, such as oxygen content is higher, the more oxide inclusions. The effects of impurity in steel and impurity ability against the failure on the mechanical properties and parts of the types, properties, the number, size and shape, but usually have lower toughness, ductility and fatigue life of. With the increase of the size of inclusions, the fatigue strength decreased correspondingly, and the tensile strength of the steel is high, decreasing trend of increase. Increased oxygen content in steel (oxide inclusion increased), bending fatigue and contact fatigue life under the action of high stress decreases. Therefore, in the high stress bearing parts working under reduced oxygen content, made of steel is necessary. Some research shows that, MnS inclusions in steel, because the shape of ellipsoid, and can wrap harmful oxide inclusions, so the reduction of fatigue life influence may be even less useful, so it can be lenient control. Influence factors of control material bearing life In order to make the material factors influencing the bearing life at its best, first of all need the original organization control before quenching steel, technical measures can be taken: high temperature (1050 ℃) austenitizing cooling to 630 ℃ isothermal normalizing pseudo eutectoid fine pearlite, or cool to 420 ℃ isothermal treatment, obtain bainite. Also can use forged heat annealing, obtain fine granular pearlite, in order to ensure the fine carbides in the steel and uniform distribution. The state of the original organization in quenching austenitization, in addition to dissolve in austenite carbides, undissolved carbide will gather in fine grained. When the original microstructure in steel is certain, the carbon content of the martensite (i.e. quenching after heating austenite carbon content), the amount of retained austenite and the amount of undissolved carbides depends mainly on the quenching heating temperature and holding time, along with the quenching temperature increased (time), reducing the number of undissolved carbides in steel (quenched martensite increased carbon content), the amount of residual austenite is increased, hardness with increasing quenching temperature increases, reaches a peak and then decreased with the increasing of temperature. When the quenching temperature is constant, with increasing austenitizing time, reduce the number of undissolved carbides, the amount of residual austenite increases, the hardness increases, a long time, this trend slowed down. When the original carbides are small, because carbide is easy to dissolve into austenite, so that after quenching hardness peak shifts to lower temperature and in austenitizing time shorter. In summary, GCrl5 steel after quenching undissolved carbide at around 7%, the retained austenite in about 9% (the average carbon content of martensite in about 0.55%) is the best organization. Moreover, when the original microstructure of fine carbides, distribution, composition on the microstructure and reliable control of the level, to obtain the mechanical performance, which has a high service life. It should be pointed out that, with the original fine dispersed carbide, soaking time, fine carbide undissolved will grow up together, make its coarsening. Therefore, the original structure with the bearing parts quenching heating time should not be too long, the austenitization quenching process of rapid heating, will receive comprehensive higher mechanical properties. In order to make the bearing parts after quenching and tempering surface residual compressive stress can be larger, carburizing or nitriding enters in the quenching atmosphere, for a short time, surface carburizing or nitriding. Because the steel quenching austenite actual carbon content is not high, far below the phase diagram showing the equilibrium concentration, therefore can absorb carbon (or nitrogen). When austenitic containing higher carbon or nitrogen, the Ms decreased, quenching at the surface than the inner and core of after martensite transformation, the greater compressive residual stresses. GCrl5 steel in carburizing atmosphere and non carburizing atmosphere heating quenching (both by low temperature tempering) after treatment, the contact fatigue experiments, surface carburizing carburizing life than did not increase by 1.5 times. The reason is that the part surface carburizing with compressive residual stresses larger.
Conclusion The main factors influencing the materials of high carbon chromium steel rolling bearing service life of parts and control degree: (1) carbide steel in the original tissue before quenching in small, dispersed requirements. Can be made of high temperature austenitizing at 630 ℃, or high temperature of 420 DEG C, can also be realized by forging and rolling heat fast annealing process. (2) for GCr15 steel after quenching, for hidden crystal martensite, 9% the average carbon content of about 0.55% Ar and about 7% of undissolved carbides in microstructure of uniform, round state. The quenching temperature and time to control the microstructure of the obtained. (3) parts quenching tempering requirements of surface residual compressive stress, which helps in improving fatigue resistance. The processing technology of surface carburizing or nitriding quenching in short time when heating, the surface residual compressive stress is larger. (4) made of steel bearing parts, require a higher purity, is mainly O2, N2, decrease the content of P, oxides and phosphorus. The electroslag remelting, vacuum smelting technology measures to make the material content ≤ 15PPM is appropriate.