In order to make the above-mentioned material factors affecting the life of the bearing in an optimal 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 ° C) austenitizing and cooling to 630 ° C isothermal normalizing to obtain pseudo The pearlite structure is eutectoidally analyzed or cooled to 420 ° C to obtain a bainite structure. It can also be rapidly annealed by wrought residual heat to obtain fine-grained pearlite structure to ensure fine and uniform distribution of carbides in the steel. When the original structure in this state is austenitized by quenching, the undissolved carbides aggregate into fine particles in addition to the carbides dissolved in the austenite.
When the original microstructure in the steel is constant, the carbon content of the quenched martensite (that is, the austenite carbon content after quenching heating), the amount of retained austenite and the amount of undissolved carbide mainly depend on the quenching heating temperature and the holding time. As the quenching heating temperature increases (time is constant), the amount of undissolved carbides in the steel decreases (the carbon content of the quenched martensite increases), the amount of retained austenite increases, and the hardness first increases as the quenching temperature increases. After reaching the peak, it decreases as the temperature increases. When the quenching heating temperature is constant, as the austenitizing time is prolonged, the amount of undissolved carbides decreases, the amount of retained austenite increases, and the hardness increases. When the time is long, the tendency is slowed down. When the carbides in the original structure are fine, since the carbides are easily dissolved into the austenite, the hardness peak after quenching is shifted to a lower temperature and appears in a shorter austenitizing time.
In summary, after quenching of GCrl5 steel, the amount of undissolved carbide is 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-described level of microstructure is reliably controlled, it is advantageous to obtain high comprehensive mechanical properties, thereby having a high service life. It should be noted that the original structure of the finely dispersed carbides, when quenched and heated, the undissolved fine carbides will aggregate and grow to make them coarse. Therefore, for the original tissue bearing parts with such quenching heating time should not be too long, using rapid heating austenitizing quenching process, will obtain higher comprehensive mechanical properties.
In order to make the bearing parts have a large compressive stress on the surface after quenching and tempering, a carburizing or nitriding atmosphere may be introduced during quenching heating to perform surface carburizing or nitriding for a short time. Since the actual austenite carbon content is not high when the steel is quenched and heated, it is much lower than the equilibrium concentration shown on the phase diagram, so carbon (or nitrogen) can be absorbed. When austenite contains higher carbon or nitrogen, its Ms decreases, and the martensite transformation occurs in the surface layer after quenching than in the inner layer and the core, resulting in a large residual compressive stress. After the GCrl5 steel was heated and quenched by carburizing atmosphere and non-carburizing atmosphere (both low temperature tempering), the contact fatigue test showed that the surface carburizing life was 1.5 times higher than that of non-carburizing. The reason is that the surface of the carburized part has a large residual compressive stress.
