The amount, morphology, size and distribution of undissolved carbides in hardened steel are affected by the chemical composition of steel and the original structure before quenching, as well as the austenitizing conditions. The influence of undissolved carbides on bearing life is seldom studied. Carbide is a hard and brittle phase, in addition to the wear resistance advantage, the load bearing (especially the carbide is not spherical) and the matrix will cause stress concentration and crack, which will reduce the toughness and fatigue resistance. Quenched undissolved carbides not only affect the properties of steel, but also affect the content and distribution of carbon and Ar in quenched martensite, which has an additional effect on the properties of steel. To reveal not dissolve carbide impact on performance, using different carbon content of steel, the martensite after quenching carbon content is the same as the Ar content without soluble carbon content of different state, after 150 ℃ tempering, due to the martensitic carbon content is the same, and the hardness is higher, and therefore did not dissolve a small amount of carbide increased to higher hardness value is not big, The crushing load, which reflects the strength and toughness, decreases, and the contact fatigue life, which is sensitive to stress concentration, decreases significantly. So too much quenched undissolved carbide 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 the parts.

In addition to the quantity of quenched undissolved carbides, the size, morphology and distribution also affect the properties of materials. In order to avoid the harm of undissolved carbides in bearing steel, undissolved carbides are required to be less (less quantity), small (small size), uniform (small size difference between each other, and uniform distribution), round (each carbide is spherical). It should be pointed out that a small amount of undissolved carbide is necessary after quenching of bearing steel, which can not only maintain sufficient wear resistance, but also is a necessary condition for obtaining fine-grained cryptocrystalline martensite.

2. Residual stress after quenching and tempering

Bearing parts still have large internal stress after quenching and tempering at low temperature. There are two states of residual internal stress in parts: good and bad. After heat treatment, the fatigue strength of steel increases with the increase of residual compressive stress on the surface; on the contrary, when the residual internal stress on the surface is tensile stress, the fatigue strength of steel decreases. This is due to the large parts of the fatigue failure appeared under tensile stress, when the surface has larger residual compressive stress, tensile stress of the same numerical, and make the actual values under tensile stress of steel is reduced, the higher fatigue strength limit, when the surface has larger residual tensile stress, and under tensile stress superposition of load and the tensile stress in the steel actual bear obvious increase, Even if the fatigue strength limit is reduced. Therefore, the larger residual compressive stress on the surface of the bearing parts after quenching and tempering is also one of the measures to improve the service life (of course, excessive residual stress may cause deformation or even cracking of the parts, so enough attention should be paid to it).

3, the content of impurities in steel

Impurities in steel include non-metallic inclusions and the content of harmful elements (acid soluble), which tend to contribute to the damage of steel performance, such as the higher the oxygen content, the more oxide inclusions. The influence of impurities in steel on mechanical properties and failure resistance of parts is related to the type, nature, quantity, size and shape of impurities, but usually has the effect of reducing toughness, plasticity and fatigue life.

The fatigue strength decreases with the increase of inclusion size, and the decreasing trend increases with the increase of tensile strength. With the increase of oxygen content in steel (the increase of oxide inclusions), the bending fatigue and contact fatigue life will decrease under high stress. Therefore, for bearing parts that work under high stress, it is necessary to reduce the oxygen content of steel used for manufacturing. Some studies have shown that MnS inclusions in steel have little or even beneficial effect on fatigue life reduction because of their ellipsometry shape and the ability to encapsulate more harmful oxide inclusions, so they can be controlled broadly.

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