Bearing Knowledge--Material factors and control affecting bearing life

Material Factors Affecting Bearing Life  

The early failure modes of rolling bearings are mainly rupture, plastic deformation, wear, corrosion and fatigue, and under normal conditions are mainly contact fatigue.  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 performance and state are the following.


1. Martensite in Quenched Steel  

When the original microstructure of high carbon chromium steel is granular pearlite, the carbon content of quenched martensite obviously affects the mechanical properties of steel under the condition of quenching and low temperature tempering.  The strength and toughness are about 0.5 %, the contact fatigue life is about 0.55 %, and the crushing resistance is about 0.42 %. When the carbon content of quenched martensite of GCr15 steel is 0.5 % – 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-regions. The carbon concentration near the carbide is higher than the part far away from the ferrite of the carbide, so the temperature at which they begin to undergo martensite transformation is different. 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 normal quenching, high carbon chromium steel can contain 8% to 20% Ar (retained austenite). Ar in the bearing parts has advantages and disadvantages. In order to benefit from the advantages and eliminate 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. Carbide is a hard and brittle phase. In addition to being beneficial to wear resistance, cracks will occur due to stress concentration with the matrix during load (especially the carbide is 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 Obvious reduction. 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 amount 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 the bearing parts are quenched and tempered at low temperature, they still have a 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 (of course, excessive residual stress may cause deformation or even cracking of the parts, and sufficient attention should be given).


5. Impurities content of steel  

The impurities in steel include non-metallic inclusions and harmful elements ( acid soluble ) content, and their harm to the properties of steel is often mutually reinforcing. For example, the higher the oxygen content is, the more oxide inclusions are.  The influence of impurities in steel on mechanical properties and failure resistance of components is related to the type, nature, quantity, size and shape of impurities, but it usually reduces toughness, plasticity and fatigue life.

With the increase of inclusion size, the fatigue strength decreases, and the higher the tensile strength of steel, the decreasing trend increases.  When the oxygen content in the steel increases ( oxide inclusions increase ), the bending fatigue and contact fatigue life decrease under high stress.  Therefore, it is necessary to reduce the oxygen content of manufacturing steel for bearing parts working under high stress. Some studies have shown that MnS inclusions in steel are ellipsoidal in shape, and can wrap harmful oxide inclusions, so it has little effect on fatigue life reduction or even may be beneficial, so it can be controlled from a wide range.


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, 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%) 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 surface of bearing parts residual large compressive stress after quenching and tempering, the atmosphere of carburizing or nitriding can be introduced during quenching and heating to carry out short-term surface carburizing or nitriding.  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 in the phase diagram, carbon ( or nitrogen ) can be absorbed.  When the austenite contains high carbon or nitrogen, the Ms decreases, and martensite transformation occurs in the surface layer after quenching compared with the inner layer and the core, resulting in large residual compressive stress.  After GCr15 steel was heated and quenched in carburizing atmosphere and non-carburizing atmosphere ( both tempered at low temperature ), the contact fatigue test showed that the life of surface carburizing was 1.5 times longer than that of non-carburizing.  The reason is that the surface of carburized parts has large residual compressive stress.


Conclusion   

The main material factors and control degree affecting the service life of high carbon chromium steel rolling bearing parts 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, or it can 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 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 in the manufacture of bearing parts requires a high degree of 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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2021-07-28