Control of Material Factors Affecting Bearing Life
The company mainly produces three core series of products: cylindrical roller bearings, self-aligning roller bearings, and thrust self-aligning roller bearings. The product can be adapted to multiple industrial fields such as metallurgical equipment, mining machinery, heavy machinery, engineering equipment, etc., and can meet the operational and load-bearing requirements of equipment under different working conditions.
SZ Bearings
To optimize the material factors affecting bearing life, it is first necessary to control the initial microstructure of the steel prior to quenching. Technical measures that can be adopted include: austenitizing at a high temperature (1050°C) and rapidly cooling to 630°C for isothermal normalizing to obtain a pseudo-eutectoid fine-grained pearlitic microstructure, or cooling to 420°C for isothermal treatment to obtain a bainitic microstructure. Alternatively, rapid annealing using residual heat from forging or rolling can be employed to obtain a fine-grained pearlitic structure, ensuring that carbides in the steel are fine and uniformly distributed. In this initial microstructure, during austenitization by quenching heating, aside from the carbides dissolved into the austenite, the undissolved carbides will aggregate into fine grains.
When the initial microstructure of the steel is fixed, the carbon content of the quenched martensite (i.e., the carbon content of the austenite after quenching), the amount of retained austenite, and the amount of undissolved carbides primarily depend on the quenching temperature and holding time. As the quenching temperature increases (with holding time constant), the amount of undissolved carbides in the steel decreases (the carbon content of the quenched martensite increases), while the amount of retained austenite increases. Hardness initially increases with rising quenching temperature, reaches a peak, and then decreases as the temperature continues to rise. When the quenching temperature is constant, as the austenitizing time increases, the amount of undissolved carbides decreases, the amount of retained austenite increases, and hardness increases; however, this trend slows down as the time becomes longer. When the carbides in the original microstructure are fine, they dissolve easily into the austenite, causing the hardness peak after quenching to shift to a lower temperature and appear at a shorter austenitizing time.
In summary, the optimal microstructural composition for GCr15 steel after quenching is approximately 7% undissolved carbides and 9% retained austenite (with an average carbon content of approximately 0.55% in the cryptocrystalline martensite). Furthermore, when the carbides in the as-cast structure are fine and uniformly distributed, reliably controlling the microstructure composition at the aforementioned levels facilitates the attainment of high comprehensive mechanical properties, thereby ensuring a long service life. It should be noted that in a primary microstructure with fine, uniformly distributed carbides, the fine, undissolved carbides tend to agglomerate and grow during the quenching and holding phase, causing coarsening. Therefore, for bearing components with such a primary microstructure, the quenching heating time should not be too long; adopting a rapid heating austenitizing and quenching process can yield higher comprehensive mechanical properties.
To ensure that bearing components retain significant compressive residual stress on the surface after quenching and tempering, a carburizing or nitriding atmosphere can be introduced during the quenching heating process to perform short-term surface carburizing or nitriding. Since the actual carbon content of the austenite in this steel is not high during quenching heating—far below the equilibrium concentration shown on the phase diagram—it can absorb carbon (or nitrogen). When the austenite contains higher levels of carbon or nitrogen, its Ms temperature decreases. During quenching, the surface layer undergoes martensitic transformation later than the inner layers and core, resulting in significant residual compressive stress.
Contact fatigue tests on GCr15 steel treated with quenching in both carburizing and non-carburizing atmospheres (both followed by low-temperature tempering) show that the service life of surface-carburized parts is 1.5 times longer than that of non-carburized parts. The reason for this is that the surface of carburized parts possesses higher residual compressive stress.
The primary material factors affecting the service life of high-carbon chromium steel rolling bearing components and their control measures are as follows:
(1) The carbides in the steel’s original microstructure prior to quenching must be fine and uniformly distributed. This can be achieved through high-temperature austenitization at 630°C or 420°C, or by utilizing a rapid annealing process using residual heat from forging or rolling.
(2) For GCr15 steel after quenching, the microstructure should consist of cryptodmartensite with an average carbon content of approximately 0.55%, about 9% argon, and approximately 7% uniformly distributed, spherical undissolved carbides. This microstructure can be controlled by adjusting the quenching temperature and duration.
(3) After quenching and low-temperature tempering, the part’s surface must retain significant compressive stress, which helps improve fatigue resistance. This can be achieved by performing a short-term surface carburizing or nitriding treatment during the quenching heating process to ensure the surface retains significant compressive stress.
(4) Steel used for manufacturing bearing components must have high purity, primarily by reducing the content of O₂, N₂, P, oxides, and phosphides. Technical measures such as electroslag remelting and vacuum metallurgy can be employed to ensure the oxygen content of the material is preferably <15 ppm.