Material factors affecting bearing life
The early failure modes of rolling bearings mainly include cracks, 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 restricted by the hardness, strength, toughness, wear resistance, corrosion resistance and internal stress state of the steel. The main internal factors affecting these performance and status are as follows.
1 Martensite in quenched steel
When the original structure of high-carbon chromium steel is granular pearlite, the carbon content of quenched martensite in the quenched and tempered state will obviously affect the mechanical properties of the 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 quenched martensite carbon content of GCr15 steel is 0.5% to 0.56%, the strongest failure resistance can be obtained The comprehensive mechanical properties.
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 not uniform in the micro region. The carbon concentration near the carbide is higher than the part far from the ferrite of the carbide, so they start to undergo martensite transformation at different temperatures. Thus, the growth of martensite grains and the display of microscopic morphology are suppressed to become cryptocrystalline martensite. It can avoid microcracks 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 the high-carbon steel is quenched to obtain medium-carbon cryptocrystalline martensite, the bearing parts can obtain the matrix with the best resistance to failure.
2 Retained austenite in quenched steel
After the high carbon chromium steel is normally quenched, it can contain 8%-20% Ar (retained austenite). Ar in the bearing parts has advantages and disadvantages. In order to benefit from the disadvantages, the Ar content should be appropriate. Since the amount of Ar is mainly related to the austenitizing conditions of the quenching heating, its amount will affect the carbon content of the quenched martensite and the amount of undissolved carbides, and it is difficult to accurately reflect the influence of the amount of Ar on the mechanical properties. For this reason, the austenite condition is fixed, and the austenitizing thermal stabilization process is used to obtain different Ar content. The influence of Ar content on the hardness and contact fatigue life of GCr15 steel after quenching and tempering is studied here. With the increase of the austenite content, both the hardness and the contact fatigue life increase, and then decrease after reaching the peak value. However, the peak value of Ar content is different. The hardness peak value appears at about 17% Ar, while the contact fatigue life The peak appears at 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 not much, it has little effect on the strength reduction, but 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 not only reduces the stress peak, but also strengthens the deformed Ar processing and stress-strain-induced martensitic transformation. However, if the load is large, the large plastic deformation of Ar and the local stress concentration and cracking of the matrix will cause the life to be reduced. It should be pointed out that the beneficial effect of Ar must be under the stable state of Ar. If it spontaneously transforms into martensite, the toughness of the steel will be sharply reduced and embrittlement.
3 Undissolved carbides in quenched steel
The quantity, morphology, size and distribution of undissolved carbides in quenched steel are not only affected by the chemical composition of the steel and the original structure before quenching, but also by the austenitizing conditions. The impact of undissolved carbides on bearing life Less impact research. Carbides are hard and brittle phases. In addition to being beneficial to wear resistance, cracks will occur due to stress concentration with the matrix during load (especially the carbides are non-spherical), which will reduce toughness and fatigue resistance. In addition to its own effect on the properties of steel, quenched undissolved carbides also affect the carbon content and Ar content and distribution of quenched martensite, thereby having an additional impact on the properties of steel. In order to reveal the influence of undissolved carbides on performance, steels with different carbon content were used, and after quenching, the martensite carbon content and Ar content were the same but the undissolved carbide content was different. After tempering at 150°C, Since martensite has the same carbon content and higher hardness, a small increase in undissolved carbides has little effect on the increase in hardness. The crushing load reflecting strength and toughness is reduced. The contact fatigue life sensitive to stress concentration is obviously decased. Therefore, excessive quenching of undissolved carbides is harmful to the comprehensive mechanical properties and failure resistance of steel. Appropriately reducing the carbon content of bearing steel is one of the ways to improve the service life of parts.
In addition to the quantity of quenched undissolved carbides affecting the material properties, the size, morphology, and distribution also affect the material properties. In order to avoid the hazards of undissolved carbides in bearing steel, it is required that the undissolved carbides be small (small in number), small (small size), uniform (small difference in size from each other, and evenly distributed), round (each carbide is present) spherical). It should be pointed out that a small amount of undissolved carbides in bearing steels after quenching is necessary, not only to maintain sufficient wear resistance, but also to obtain fine-grained cryptocrystalline martensite.
4 Residual stress after quenching and tempering
After being quenched and tempered at low temperature, the bearing parts still have relatively large internal stress. The residual internal stress in the part has advantages and disadvantages. After the heat treatment of the steel, as the residual compressive stress on the surface increases, the fatigue strength of the steel increases. On the contrary, 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 part occurs when it is subjected to excessive tensile stress. When a large compressive stress remains on the surface, it will offset the tensile stress of the same value, and the actual tensile stress of the steel will be reduced, so that the fatigue strength The limit value is increased. When there is a large tensile stress remaining on the surface, it will be superimposed with the tensile stress load to make the actual tensile stress of the steel increase significantly, even if the fatigue strength limit value is reduced. Therefore, it is also one of the measures to improve the service life to make the bearing parts have a large residual compressive stress on the surface after quenching and tempering.
5 Steel impurity content
Impurities in steel include non-metallic inclusions and harmful elements (acid-soluble) content, and their harm to steel performance 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 have the effect of reducing toughness, plasticity and fatigue life.
As the size of inclusions increases, the fatigue strength decreases accordingly, and the higher the tensile strength of steel, the decreasing trend increases. As the oxygen content in steel increases (increased oxide inclusions), bending fatigue and contact fatigue life are also reduced under high stress. Therefore, for bearing parts that work under high stress, it is necessary to reduce the oxygen content of steel for manufacturing. Some studies have shown that the MnS inclusions in steel, because they are ellipsoidal in shape and can pack more harmful oxide inclusions, have little effect on the reduction of fatigue life and may even be beneficial, so they can be controlled widely.
Control of material factors affecting bearing life
In order to make the above-mentioned material factors affecting bearing life in the best 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℃) austenitizing and rapid cooling to 630℃ isothermal normalizing to obtain pseudo Eutectoid fine pearlite structure, or cool to 420 ℃ isothermal treatment to obtain bainite structure. The forging and rolling waste heat can also be used for rapid annealing to obtain a fine-grained pearlite structure to ensure that the carbides in the steel are fine and uniformly distributed. When the original structure in this state is austenitized by quenching and heating, in addition to the carbides dissolved in the austenite, the undissolved carbides will aggregate into fine grains.
When the original structure in the steel is constant, the carbon content of quenched martensite (that is, the carbon content of austenite after quenching heating), the amount of retained austenite and the amount of undissolved carbides mainly depend on the quenching heating temperature and holding time As the quenching heating temperature increases (a certain time), the amount of undissolved carbides in the steel decreases (the carbon content of quenched martensite increases), the amount of retained austenite increases, and the hardness first increases with the increase of the quenching temperature. After reaching the peak, it decreases as the temperature increases. When the quenching heating temperature is constant, with the extension of the austenitizing time, the number of undissolved carbides decreases, the number of retained austenite increases, and the hardness increases. When the time is longer, this trend slows down. When the carbides in the original structure are small, the carbides are easily dissolved into austenite, so the hardness peak after quenching shifts to a lower temperature and appears in a shorter austenitizing time.
In summary, after quenching, the undissolved carbide of GCrl5 steel is about 7%, and the retained austenite is about 9% (the average carbon content of cryptocrystalline martensite is about 0.55%) is the best structure. 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 beneficial to obtain high comprehensive mechanical properties, thereby having a high service life. It should be pointed out that the original structure with fine and dispersed carbides, when quenching and heating, the undissolved fine carbides will aggregate and grow, making it coarser. Therefore, the quenching heating time for bearing parts with this original structure should not be too long, and the rapid heating austenitizing quenching process will obtain higher comprehensive mechanical properties.
In order to make the bearing parts have relatively large compressive stress on the surface after quenching and tempering, a carburizing or nitriding atmosphere can be introduced during quenching and heating to perform surface carburizing or nitriding for a short time. Since the actual carbon content of austenite during quenching and heating of this steel is not high, which is much lower than the equilibrium concentration shown on the phase diagram, it can absorb carbon (or nitrogen). When austenite contains higher carbon or nitrogen, its Ms decreases, and the surface layer undergoes a martensite transformation after the inner layer and the core during quenching, resulting in greater residual compressive stress. After GCrl5 steel is heated and quenched in a carburizing atmosphere and a non-carburizing atmosphere (both subjected to low temperature tempering), the contact fatigue test shows that the life of surface carburizing is 1.5 times longer than that of non-carburizing. The reason is that the surface of carburized parts has a large residual compressive stress.
The main material factors that affect the service life of high-carbon chromium steel rolling bearing parts and the degree of control are:
(1) The carbides in the original structure of steel before quenching are required to be fine and dispersed. High temperature austenitization can be used at 630°C or 420°C, and it can also be achieved by forging and rolling waste heat rapid annealing process.
(2) For GCr15 steel after quenching, it is required to obtain a microstructure of cryptocrystalline martensite with an average carbon content of about 0.55%, about 9% of Ar, and about 7% of undissolved carbides in a uniform and round state. This kind of microstructure can be controlled by quenching heating temperature and time.
(3) After the parts are quenched and tempered at low temperature, a relatively large compressive stress is required on the surface, which helps to improve the fatigue resistance. The surface treatment process of carburizing or nitriding for a short time during quenching and heating can be used to make the surface retain a large compressive stress.
(4) The steel used for manufacturing bearing parts requires high purity, mainly to reduce the content of O2, N2, P, oxides and phosphides. Technical measures such as electroslag remelting and vacuum smelting can be used to make the material’s oxygen content ≤15PPM.
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