Factors affecting fatigue life of bearing steel

The comprehensive mark of the internal quality of bearing steel is fatigue life. Some scholars have put forward the view that reducing oxygen content still has not played a significant role in improving the fatigue life of bearing steel. In fact, only by reducing the content of oxides and sulphides can we fully tap the material potential and greatly improve the fatigue life of bearing steel.

What factors affect the fatigue life of bearing steel? As follows:
The effect of nitride on fatigue life
Some scholars pointed out that the volume fraction of nitride decreased while nitrogen increased in steel, which was due to the decrease of average size of inclusions in steel, and a considerable number of inclusions less than 0.2in were not counted due to the limitation of technology. It is precisely the existence of these small nitride particles, which has a direct impact on the fatigue life of bearing steel. Ti is one of the strongest elements to form nitrides, with small proportion and easy to float up. Some ti remains in the steel to form multi angular inclusions. This inclusion is easy to cause local stress concentration and fatigue crack, so it is necessary to control the formation of such inclusions.
The results show that the oxygen content in steel is lower than 20ppm, nitrogen content is improved, the size, type and distribution of nonmetallic inclusions are improved, and stable inclusions have been significantly reduced. Although the number of nitride particles in steel increases, the particles are very small and distributed in the grain boundary or crystal, which is a favorable factor. The strength and toughness of bearing steel are well matched, and the hardness and strength of steel are greatly increased, especially the effect of improving contact fatigue life is objective.
The effect of oxide on fatigue life
The oxygen content in steel is an important factor affecting the material. The lower the oxygen content, the higher the purity, the longer the corresponding rated life. The oxygen content in steel is closely related to oxide. During solidification of molten steel, oxygen dissolved by aluminum, calcium, silicon and other elements forms oxide. The content of oxide inclusions is a function of oxygen. With the decrease of oxygen content, oxide inclusions will decrease; nitrogen content and oxygen content are also functional relations with nitrides. However, due to the dispersion of oxides in steel, they play the same fulcrum as carbides, so it does not play a destructive role in fatigue life of steel.
Because of the existence of oxides, steel destroys the continuity of metal matrix, and because the expansion coefficient of oxide is smaller than that of bearing steel, when it bears alternating stress, it is easy to produce stress concentration and become the origin of metal fatigue. Most of the stress concentration occurs between oxide, point inclusions and matrix. When the stress reaches enough, cracks will occur and will expand rapidly and destroy. The lower the plasticity of inclusions, the more sharp the shape, the greater the stress concentration.
The effect of sulfide on fatigue life
The sulfur content in steel is almost all in sulfide form. The sulfur content in steel increases, so the sulfide in steel increases correspondingly. However, because sulfide can be well surrounded by oxides, the influence of oxides on fatigue life is reduced. Therefore, the influence of the number of inclusions on fatigue life is not absolute, and is related to the nature, size and distribution of inclusions. The more inclusions, the lower the fatigue life, and other factors must be considered comprehensively. Sulfide in bearing steel is distributed in fine shape and mixed with oxide inclusions, which is difficult to identify even by metallographic method. The experiment shows that on the basis of the original technology, increasing Al content plays a positive role in reducing oxides and sulphides. This is because CA has a strong desulfurization capability. Inclusions have little influence on strength, but it is harmful to steel toughness, and its harm degree depends on the strength of steel.
According to fracture analysis, the fracture process of GCr15 steel is mainly cleavage and quasi cleavage fracture mechanism. Xiaojimei, a famous expert, pointed out that inclusions in steel are brittle phases, the higher the volume fraction, the lower the toughness, the larger the size of inclusions, the faster the toughness decreases. For the toughness of cleavage fracture, the smaller the size of inclusions and the smaller the spacing between inclusions, the toughness will not decrease, but increase. If the brittle phases in the crystal are arranged closely, the dislocation stacking distance can be shortened and cleavage fracture is not easy to occur, thus improving the cleavage fracture strength. Some people have done the tests specially: the two batches of steel A and B belong to the same steel class, but the inclusion conditions of each group are different. After heat treatment, the two batches of steel A and B have the same tensile strength of 95 kg/mm', and the yield strength of a and B steel is the same. In terms of elongation and surface shrinkage, steel B is still qualified if it is slightly lower than that of steel a. After fatigue test (rotating bending), it is found that steel a is long-life material with high fatigue limit; steel B is short life material, and fatigue limit is low. When the cyclic stress of steel sample is slightly higher than the fatigue limit of a steel, the life of steel B is only 1/10 of that of steel a. A. B inclusions in steel are oxides. From the total inclusion, the purity of steel a is worse than that of B steel, but the oxide particles of steel a are the same and distributed uniformly; steel B contains some inclusions with large particles, and the distribution is not even. This fully shows that Mr. Xiao Jimei's view is correct.
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