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: Control of Material Factors Affecting 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.
With the increase of quenching heating temperature (time is certain), the amount of undissolved carbide in the steel decreases (the carbon content of quenched martensite increases) and the amount of residual austenite increases.
The hardness first increases with the increase of quenching temperature, reaches the peak value, and then decreases with the increase of temperature. When the quenching heating temperature is constant, the amount of residual austenite increases and the hardness increases.
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.
Control of Material Factors Affecting Bearing Life
To sum up, GCrl5 steel has a good microstructure composed of insoluble carbide of about 7% and retained austenite of 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 microstructure composition of the above level is reliably controlled, it is conducive to obtain high comprehensive mechanical properties and high service life.Control of Material Factors Affecting Bearing Life
It should be pointed out that for the original structure with fine dispersed carbides, when quenching, heating and holding, the insoluble fine carbides will aggregate and grow up and coarsen them.
Therefore, the quenching and heating time of bearing parts with this original structure should not be too long. Using rapid heating austenitizing quenching process will obtain higher comprehensive mechanical properties.
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). Control of Material Factors Affecting Bearing Life
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.
After heating and quenching in carburizing atmosphere and non carburizing atmosphere (both tempered at low temperature), the contact fatigue test shows that the service life of surface carburized steel is 1.5 times higher than that of non carburized steel. The reason is that the surface of carburized parts has large residual compressive stress.