In order to keep the above material factors affecting bearing life in a good state, it is first necessary to control the original structure of the steel before quenching. The technical measures that can be taken are: Bearing Life
austenitizing at high temperature (1050 ℃), rapid cooling to 630 ℃ isothermal normalizing to obtain pseudo eutectoid fine pearlite structure, or cooling to 420 ℃ isothermal treatment to obtain bainite structure.
It can also be rapidly annealed by forging and rolling waste heat to obtain fine-grained pearlite structure, so as to ensure the fine and uniform distribution of carbides in the steel. When the original structure in this state is austenitized by quenching and heating, in addition to the carbides dissolved in austenite, the insoluble carbides will aggregate into fine particles.
When the original structure in the steel is certain, the carbon content of quenched martensite (i.e. the carbon content of austenite after quenching heating), retained austenite and undissolved carbide mainly depend on the quenching heating temperature and holding time. Control of Material FactorsAffecting Bearing Life
When the quenching heating temperature is constant, with the extension of austenitizing time, the amount of insoluble carbide decreases, the amount of residual austenite increases and the hardness increases. Control of Material FactorsAffecting Bearing Life
When the time is long, this trend slows down. When the carbide in the original structure is fine, because the carbide is easy to dissolve into austenite, the hardness peak after quenching moves to a lower temperature and appears in a shorter austenitizing time.
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 conducive to obtain high comprehensive mechanical properties and high service life.
In order to make the surface of bearing parts remain large compressive stress after quenching and tempering, carburizing or nitriding atmosphere can be introduced during quenching and heating to carry out surface carburizing or nitriding for a short time.
Because the actual carbon content of austenite during quenching and heating of this steel is not high, which is far lower than the equilibrium concentration shown in the phase diagram, it can absorb carbon (or nitrogen).
When austenite contains high carbon or nitrogen, its MS decreases. During quenching, martensitic transformation occurs on the surface layer compared with the inner layer and the core, resulting in large residual compressive stress.
Impact on vibration and noise:
Influence on Vibration: NSK bearing test results show that cleanliness seriously affects the vibration level of rolling mill bearing, especially the vibration in high frequency band. The bearing with high cleanliness has low vibration speed, especially in high frequency band.The Cleanliness of NSK Bearing is Important to Heating
c. Effect on lubrication performance:The Cleanliness of NSK Bearing is Important to Heating
The decline of NSK bearing cleanliness will not only affect the formation of lubricating oil film, but also cause the deterioration of lubricating grease and accelerate its aging, which will affect the decline of lubricating performance of lubricating grease.The Cleanliness of NSK Bearing is Important to Heating
In the initial stage of rolling mill bearing operation, most of the lubricating grease is quickly squeezed out of the raceway (less than one minute), and the accumulated lubricating grease is discharged in a small amount one after another in the cycle of bearing migration and transformation.
According to the quality and filling amount of grease in the bearing structure of the rolling mill, this period may last for more than ten minutes or even several hours.
(1) The carbides in the original structure of steel before quenching are required to be fine and dispersed. It can be realized by high-temperature austenitizing at 630 ℃ or 420 ℃, or by rapid annealing process with forging and rolling waste heat.
(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%, AR of about 9% and insoluble carbide in uniform and circular state of about 7%.
(3) After quenching and low temperature tempering of parts, large compressive stress is required on the surface, which is helpful to improve the fatigue resistance.Control of Material FactorsAffecting Bearing Life
The surface can be carburized or nitrided for a short time during quenching and heating, so that there is a large residual compressive stress on the surface.Control of Material FactorsAffecting Bearing Life
(4) Steel for manufacturing bearing parts is required to have high purity, mainly to reduce the content of O2, N2, P, oxide and phosphide. Technical measures such as electroslag remelting and vacuum smelting can be adopted to make the oxygen content of the material ≤ 15ppm.Control of Material FactorsAffecting Bearing Life