How does heat treatment of bearing materials affect bearing life?

Jul 24, 2023

The early failure forms of rolling bearings mainly include cracking, plastic deformation, wear, corrosion and fatigue, and are mainly contact fatigue under normal conditions. In addition to 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 that affect these performances and states are as follows.

1.1 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 steel in the state of quenching and tempering at low temperature. 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 comprehensive mechanical properties with the strongest failure resistance can be obtained. 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-area, and the carbon concentration near the carbide is higher than that of the original ferrite part away from the carbide, so they start to undergo martensite transformation at different temperatures, thereby inhibiting the growth of martensite grains and the display of micromorphology to become cryptocrystalline martensite. It can avoid the 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, it is possible for the bearing parts to obtain the matrix with the best failure resistance.

1.2 Retained austenite in quenched steel after normal quenching, high-carbon chromium steel can contain 8% to 20% Ar (retained austenite). Ar in bearing parts has both advantages and disadvantages. In order to promote the advantages and eliminate the disadvantages, the Ar content should be appropriate. Since the amount of Ar is mainly related to the austenitization conditions of quenching and heating, its amount will affect the carbon content of quenched martensite and the amount of undissolved carbides, so it is difficult to correctly reflect the influence of Ar amount on mechanical properties. To this end, the austenitic condition was fixed and the austenitizing heat stabilization treatment process was used to obtain different Ar content. 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, both the hardness and contact fatigue life increase, and then decrease after reaching the peak value, but the peak Ar content is different. The hardness peak appears at about 17% Ar, while the contact fatigue life peak appears at about 9%. When the test load decreases, the influence of the increase of Ar content on the contact fatigue life decreases. This is due to the fact that when the amount of Ar is small, it has little effect on the reduction of strength, but the effect of toughening is more obvious. 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 through processing and stress-strain-induced martensitic transformation. However, if the load is large, the large plastic deformation of Ar and the matrix will cause local stress concentration and rupture, thereby reducing the service life. It should be pointed out that the beneficial effect of Ar must be in the stable state of Ar. If it transforms into martensite spontaneously, the toughness of the steel will be sharply reduced and it will be brittle.

1.3 Undissolved carbides in quenched steel. The quantity, shape, 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 affected by the austenitizing conditions. There are few studies on the influence of undissolved carbides on bearing life. Carbide is a hard and brittle phase. In addition to being beneficial to wear resistance, it will cause stress concentration with the matrix during loading (especially when the carbide is non-spherical) and cause cracks, which will reduce toughness and fatigue resistance. Quenching undissolved carbides not only affect the properties of the steel, but also affect the carbon content and Ar content and distribution of the quenched martensite, thereby having an additional impact on the properties of the steel. In order to reveal the influence of undissolved carbides on performance, steels with different carbon contents are used. After quenching, the martensite has the same carbon content and Ar content but the undissolved carbide content is different. After tempering at 150°C, because the 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, and the contact fatigue life that is sensitive to stress concentration is significantly reduced. Therefore, excessive quenching undissolved carbides are 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 influence of the quantity of quenched undissolved carbides on the properties of the material, the size, shape and distribution of the carbides also have an impact on 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 number), small (small size), uniform (small difference in size and uniform distribution), and round (each carbide is spherical). It should be pointed out that it is necessary for the bearing steel to have a small amount of undissolved carbides after quenching, not only to maintain sufficient wear resistance, but also to obtain fine-grained cryptocrystalline martensite.

1.4 Residual stress after quenching and tempering. Bearing parts still have large internal stress after quenching and tempering at low temperature. Residual internal stresses in a part are both favorable and detrimental states. After heat treatment of steel parts, 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 parts occurs when excessive tensile stress is applied to the surface. When a large compressive stress remains on the surface, it will offset the tensile stress of the same value, so that the actual tensile stress value of the steel is reduced, and the fatigue strength limit value is increased. Therefore, it is also one of the measures to improve the service life to make the surface of the bearing parts have a large residual compressive stress after quenching and tempering (of course, excessive residual stress may cause deformation or even cracking of the parts, which should be given enough attention).

1.5 Impurity content of steel. Impurities in steel include non-metallic inclusions and harmful elements (acid-soluble) content, and their harm to steel performance is often mutually reinforcing, such as the higher the oxygen content, the more oxide inclusions. The impact of impurities in steel on the mechanical properties and failure resistance of parts is related to the type, nature, quantity, size and shape of impurities, but usually 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 decreasing trend. The oxygen content in the steel increases (the oxide inclusions increase), and the bending fatigue and contact fatigue life also decrease under high stress. Therefore, for bearing parts that work under high stress, it is necessary to reduce the oxygen content of the steel used for manufacturing. Some studies have shown that MnS inclusions in steel are ellipsoidal in shape and can wrap more harmful oxide inclusions, so they have little effect on the reduction of fatigue life and may even be beneficial, so they can be controlled leniently.

In order to keep the above-mentioned material factors that affect 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 include: high temperature (1050°C) austenitization and rapid cooling to 630°C for isothermal normalizing to obtain pseudo-eutectoid fine pearlite structure, or cold to 420°C for isothermal treatment to obtain bainite structure. Rapid annealing with waste heat from forging and rolling can also be used to obtain a fine-grained pearlite structure to ensure that the carbides in the steel are fine and evenly 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 gather into fine grains. When the original structure in the steel is constant, the carbon content of quenched martensite (i.e. the carbon content of austenite after quenching heating), the amount of retained austenite and the amount of undissolved carbide mainly depend on the quenching heating temperature and holding time. As the quenching heating temperature increases (the time is constant), 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 quenching temperature. After reaching the peak value, it decreases with the increase of temperature. When the quenching heating temperature is constant, as the austenitization time prolongs, 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 fine, because the carbides are easy to dissolve into austenite, the hardness peak after quenching moves to a lower temperature and appears in a shorter austenitization time.

To sum up, the undissolved carbides of GCrl5 steel after quenching are 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-mentioned level of microstructure composition is reliably controlled, it is beneficial to obtain high comprehensive mechanical properties and thus have a high service life. It should be pointed out that for the original structure with fine and dispersed carbides, when quenching, heating and heat preservation, the undissolved fine carbides will gather and grow to make them coarser. Therefore, the quenching and heating time of bearing parts with such an original structure should not be too long, and the rapid heating and austenitizing quenching process will obtain higher comprehensive mechanical properties. In order to make the surface of the bearing parts retain a large compressive stress after quenching and tempering, a carburizing or nitriding atmosphere can be introduced during quenching and heating for short-term surface carburizing or nitriding. Since the actual carbon content of austenite is not high when the steel is quenched and heated, it is much lower than the equilibrium concentration shown on the phase diagram, so it can absorb carbon (or nitrogen). When the austenite contains higher carbon or nitrogen, its Ms decreases, and the surface layer undergoes martensitic transformation than the inner layer and core during quenching, resulting in a larger residual compressive stress. After GCrl5 steel is heated and quenched in carburizing atmosphere and non-carburizing atmosphere (both are tempered at low temperature), it can be seen from the contact fatigue test that the life of surface carburizing is 1.5 times higher than that of uncarburizing. The reason is that the surface of carburized parts has a large residual compressive stress.

The main material factors and control degree that affect the service life of high carbon chromium steel rolling bearing parts are:

(1) The carbides in the original structure of the steel before quenching are required to be fine and dispersed. It can be achieved by high temperature austenitization at 630°C or 420°C, or by rapid annealing process with waste heat from forging and rolling.

(2) For GCr15 steel after quenching, it is required to obtain the microstructure of cryptocrystalline martensite with an average carbon content of about 0.55%, about 9% Ar and about 7% undissolved carbides in a uniform and round state. This microstructure can be controlled by quenching heating temperature and time.

(3) After the parts are quenched and tempered at low temperature, a large compressive stress remains on the surface, which helps to improve the fatigue resistance. The treatment process of short-term carburizing or nitriding on the surface can be adopted during quenching and heating, so that a large compressive stress remains on the surface.

(4) The steel used in the manufacture of bearing parts requires high 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 oxygen content of the material ≤15PPM.