The early failure modes of rolling bearings mainly include cracking, plastic deformation, wear, corrosion and fatigue. Under normal conditions, they are mainly contact fatigue. In addition to the service conditions, the failure of bearing parts is mainly limited by the hardness, strength, toughness, wear resistance, corrosion resistance and internal stress state of the steel. The main intrinsic factors that affect these performance and status are as follows.
1.1, martensite in hardened steel
When the original structure of high carbon chromium steel is granular pearlite, the carbon content of quenching martensite under quenching and low temperature tempering obviously affects the mechanical properties of steel. The strength and toughness are about 0.5%, the contact fatigue life is about 0.55%, and the crush resistance is about 0.42%. When the carbon content of the quenched martensite of GCr15 steel is 0.5% to 0.56%, the anti-failure ability is the strongest. Comprehensive mechanical properties.
It should be noted 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 the martensite is not uniform in the micro-region, and the carbon concentration around the carbide is higher than that away from the original ferrite portion of the carbide, so that the temperature at which they begin to undergo martensite transformation is different. Thereby, the growth of the martensite grains and the display of the microscopic morphology are suppressed to become cryptocrystalline martensite. It can avoid the microcracks that are prone to occur in the quenching of high carbon steel, and its substructure is dislocation-type lath martensite with high strength and toughness. Therefore, only when the medium carbon cryptocrystalline martensite is obtained when the high carbon steel is quenched, the bearing parts can obtain the substrate with the best failure resistance.
1.2. Retained austenite in hardened steel
High carbon chromium steel may contain 8% to 20% Ar (residual austenite) after normal quenching. The Ar in the bearing parts has its advantages and disadvantages. In order to eliminate the disadvantages, the Ar content should be appropriate. Since the amount of Ar is mainly related to the austenitizing condition of quenching heating, how much it affects the carbon content of quenched martensite and the amount of undissolved carbide, it is difficult to correctly reflect the influence of Ar amount on mechanical properties. For this reason, the austenitic conditions were fixed and the austenite thermal stabilization treatment process was used to obtain different amounts of Ar. The influence 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 with the peak value, but the peak Ar content is different, the hardness peak appears at about 17% Ar, and the contact fatigue life The peak appears around 9%. When the test load is reduced, the influence of the increase in the amount of Ar on the contact fatigue life is reduced. This is because when the amount of Ar is small, the effect on the strength reduction is small, and the effect of toughening is more obvious. The reason is that when the load is small, a small amount of deformation of Ar occurs, which reduces the stress peak and strengthens the deformed Ar processing strengthening and the stress strain induced martensite transformation. However, if the load is large, the large plastic deformation of Ar and the base will locally generate stress concentration and rupture, thereby reducing the life. It should be noted that the beneficial effect of Ar must be under the stable state of Ar. If it is spontaneously transformed into martensite, the toughness of the steel will be drastically reduced and embrittled.
1.3. Undissolved carbides in hardened steel
The quantity, morphology, size and distribution of undissolved carbides in hardened steel are affected by the chemical composition of steel and the original structure before quenching, and by the austenitizing conditions. The undissolved carbides are related to bearing life. There are fewer impact studies. Carbide is a hard and brittle phase. In addition to its good wear resistance, the load will cause stress (especially that the carbide is non-spherical) and the matrix will cause stress concentration, which will reduce the 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 the quenched martensite, which has an additional effect on the properties of the steel. In order to reveal the effect of undissolved carbide on the properties, steels with different carbon contents are used. After quenching, the martensite has the same carbon content and Ar content and the undissolved carbide content is different. After tempering at 150 °C, Since martensite has the same carbon content and high hardness, a small increase in undissolved carbide has little increase in hardness, and a crush load reflecting strength and toughness is reduced, and contact fatigue life sensitive to stress concentration is Obvious reduction. Therefore, excessive quenching of undissolved carbide is detrimental to the overall mechanical properties and failure resistance of the steel. Properly reducing the carbon content of bearing steel is one of the ways to improve the service life of parts.
In addition to the influence of the amount of quenched undissolved carbide on the material properties, the size, morphology and distribution also have an effect on the material properties. In order to avoid the harm of undissolved carbides in the bearing steel, it is required that the amount of undissolved carbides is small (small quantity), small (small size), uniform (small difference between the sizes, and evenly distributed), round (each carbide is present) spherical). It should be noted that it is necessary to have a small amount of undissolved carbide after quenching of the bearing steel, which not only maintains sufficient wear resistance, but also is a necessary condition for obtaining fine-grained cryptocrystalline martensite.
1.4, residual stress after quenching and tempering
After quenching and low temperature tempering, the bearing parts still have large internal stress. The residual internal stress in the part has two advantages and disadvantages. After the heat treatment of the steel, the fatigue strength of the steel increases with the increase of the surface residual compressive stress. On the contrary, when the residual internal stress is the tensile stress, the fatigue strength of the steel is lowered. This is because the fatigue failure of the part occurs when subjected to excessive tensile stress. When the surface has large compressive stress, it will offset the tensile stress of the same value, and the actual tensile stress of the steel will be reduced to make the fatigue strength. When the limit value is increased, when the surface has a large tensile stress, it will be superimposed with the tensile stress load, so that the actual tensile stress of the steel is significantly increased, even if the fatigue strength limit value is lowered. Therefore, the residual compressive stress on the surface of the bearing parts after quenching and tempering is also one of the measures to improve the service life (of course, excessive residual stress may cause deformation or even cracking of the parts, and sufficient attention should be paid).
1.5, the impurity content of steel
Impurities in steel include non-metallic inclusions and harmful elements (acid-soluble). Their hazards to steel properties are often mutually reinforcing. For example, the higher the oxygen content, the more oxide inclusions. The effect of impurities in steel on mechanical properties and failure resistance of the part is related to the type, nature, quantity, size and shape of the impurities, but generally has the effect of reducing toughness, plasticity and fatigue life.
As the size of the inclusions increases, the fatigue strength decreases, and the higher the tensile strength of the steel, the greater the tendency to decrease. The oxygen content in the steel is increased (increased oxide inclusions), and the bending fatigue and contact fatigue life are also reduced under high stress. Therefore, it is necessary to reduce the oxygen content of the steel for manufacturing for bearing parts that work under high stress. Some studies have shown that MnS inclusions in steel are ellipsoidal in shape and can enclose oxide inclusions that are more harmful, so they have little or even beneficial effect on fatigue life reduction, so they can be controlled from a wide range.
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