I. Material factors affecting bearing life
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) When the original microstructure of martensite high carbon chromium steel in hardened steel is granular pearlite, the carbon content of quenched martensite in 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 martensite is not uniform in the micro-region, and the carbon concentration around the carbide is higher than the far-away ferrite portion, so that the temperature at which martensite transformation begins to occur 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 matrix with the best failure resistance.
(2) Retained austenitic high carbon chromium steel in hardened 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 effects of Ar content on the hardness and contact fatigue life of GCr15 steel after quenching and low temperature tempering were 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, Ar undergoes a small amount of deformation, 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 cause stress concentration and rupture, thereby reducing the life. It should be noted that the beneficial effect of Ar must be under the steady state of Ar. If it is spontaneously transformed into martensite, the toughness of steel will be drastically reduced and embrittled.
(3) The amount, morphology, size and distribution of undissolved carbides in hardened steel in hardened steel are affected by both the chemical composition of the steel and the original structure before quenching, and the austenitizing conditions. The impact of the study on the effect of undissolved carbides on bearing life is less. 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, quenching of undissolved carbides also affects 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 carbides 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 Â°C, Since martensite has the same carbon content and high hardness, a small increase in undissolved carbides 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 amount), 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.
(4) Residual stress after quenching and tempering After the 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 remaining, it will offset the tensile stress of the same value, and the actual tensile stress value 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).
(5) Impurity content of steel Impurities in steel include non-metallic inclusions and harmful elements (acid-soluble). Their damage to steel properties is 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.
Second, the control of material factors affecting bearing life
In order to make the above-mentioned material factors affecting the life of the bearing in an optimal state, it is first necessary to control the original structure of the steel before quenching. The technical measures that can be taken are: high temperature (1050 Â° C) austenitizing and cooling to 630 Â° C isothermal normalizing to obtain pseudo The pearlite structure is eutectoidally analyzed or cooled to 420 Â° C to obtain a bainite structure. It can also be rapidly annealed by wrought residual heat to obtain a fine-grained pearlite structure to ensure fine and uniform distribution of carbides in the steel. When the original structure in this state is austenitized by quenching, the undissolved carbides aggregate into fine particles in addition to the carbides dissolved in the austenite.
When the original microstructure in the steel is constant, the carbon content of the quenched martensite (that is, the austenite carbon content after quenching heating), the amount of retained austenite and the amount of undissolved carbide mainly depend on the quenching heating temperature and the holding time. As the quenching heating temperature increases (time is constant), the amount of undissolved carbide in the steel decreases (the carbon content of the quenched martensite increases), the amount of retained austenite increases, and the hardness first increases as the quenching temperature increases. After reaching the peak, it decreases as the temperature increases. When the quenching heating temperature is constant, as the austenitizing time is prolonged, the amount of undissolved carbides decreases, the amount of retained austenite increases, and the hardness increases. When the time is long, the tendency is slowed down. When the carbides in the original structure are fine, since the carbides are easily dissolved into the austenite, the hardness peak after quenching is shifted to a lower temperature and appears in a shorter austenitizing time.
In summary, after quenching of GCrl5 steel, the amount of undissolved carbide is about 7%, and the retained austenite is 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 above-described level of microstructure is reliably controlled, it is advantageous to obtain high comprehensive mechanical properties, thereby having a high service life. It should be noted that the original structure of the finely dispersed carbides, when quenched and heated, the undissolved fine carbides will aggregate and grow to make them coarse. Therefore, for the original tissue bearing parts with such quenching heating time should not be too long, using the rapid heating austenitizing quenching process, will obtain higher comprehensive mechanical properties.
In order to make the bearing parts have a large compressive stress on the surface after quenching and tempering, a carburizing or nitriding atmosphere may be introduced during quenching heating to perform surface carburizing or nitriding for a short time. Since the austenite actually has a low carbon content when the steel is quenched and heated, it is much lower than the equilibrium concentration shown on the phase diagram, so that carbon (or nitrogen) can be absorbed. When austenite contains higher carbon or nitrogen, its Ms decreases, and the martensite transformation occurs in the surface layer after quenching than in the inner layer and the core, resulting in a large residual compressive stress. After the GCrl5 steel was heated and quenched by carburizing atmosphere and non-carburizing atmosphere (both low temperature tempering), the contact fatigue test showed that the surface carburizing life was 1.5 times higher than that of non-carburizing. The reason is that the surface of the carburized part has a large residual compressive stress.
Third, the conclusion <br> <br> factors and the degree of control the main material of high carbon chromium steel bearing parts of life:
(1) The carbides in the original structure of the steel before quenching are required to be fine and diffuse. High temperature austenitizing at 630 Â° C, or 420 Â° C high temperature can also be achieved by using a forging hot flash rapid annealing process.
(2) After quenching the GCr15 steel, it is required to obtain a microstructure of cryptocrystalline martensite having an average carbon content of about 0.55%, about 9% of Ar, and about 7% of undissolved carbide in a round state. The quenching heating temperature and time can be utilized to control the microstructure.
(3) After quenching and quenching of the parts, it is required to have a large compressive stress on the surface, which contributes to the improvement of fatigue resistance. A treatment process of surface carburizing or nitriding for a short time during quenching heating may be employed, so that a large compressive stress remains on the surface.
(4) Steel for the manufacture of bearing parts is required to have a high degree of purity, mainly to reduce the content of O2, N2, P, oxides and phosphides. Electroslag remelting, vacuum smelting and other technical measures can be used to make the material oxygen content â‰¤ 15PPM.
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