The comprehensive symbol of the internal quality of bearing steel is the fatigue life. Some scholars put forward the view that reducing oxygen content has not played a significant role in improving the fatigue life of bearing steel. In fact, only by reducing the content of oxide and sulfide at the same time can the potential of the material be fully exploited and the fatigue life of bearing steel be greatly improved.

Why does reducing oxygen content not improve the fatigue life of bearing steel? Reason: After the reduction of oxide inclusion, the excess sulfide becomes an unfavorable factor affecting the fatigue life of steel. Only by reducing the content of oxide and sulfide at the same time, can the potential of material be fully exploited and the fatigue life of bearing steel be greatly improved.

Effect of sulfide on fatigue life

Almost all sulfur content in steel exists in sulphide form. However, the effect of the number of inclusions on fatigue life is not absolute, but is related to the nature, size and distribution of inclusions, because the sulfide can surround the oxide well and reduce the influence of oxide on fatigue life. The more constant inclusions, the lower the fatigue life, and other factors must be considered comprehensively. Sulfide in bearing steel is finely dispersed and mixed with oxide inclusions, which is difficult to identify even by metallographic method. The results show that the increase of Al content has a positive effect on the reduction of oxides and sulfides. This is because Ca has a fairly strong desulfurization capacity. The inclusions have little effect on the strength, but do great harm to the toughness of steel, which depends on the strength of steel.

According to fracture analysis, the fracture process of GCr15 steel is mainly cleavage and quasi cleavage fracture mechanism. Xiao Jimei, a famous expert, pointed out that the inclusion in steel is a brittle phase, the higher the volume fraction, the lower the toughness; The larger the inclusion size is, the faster the toughness decreases. For the toughness of cleavage fracture, the smaller the inclusion size and the smaller the distance between the inclusions, the toughness will not decrease, but increase. If the brittle phases in the grain are densely arranged, the dislocation plug distance can be shortened, and cleavage fracture is not easy to occur, so as to improve the cleavage fracture strength. Someone has done A special test :A, B two batches of steel belong to the same steel, but their inclusions are different.

After heat treatment, two batches of steel A and B reach the same tensile strength of 95 kg/mm’, and the yield strength of steel A and B is the same. In terms of elongation and surface shrinkage, B steel slightly lower than A steel is still qualified. After the fatigue test (rotational bending), it is found that A steel is A long-life material with high fatigue limit; B Steel is short life material with low fatigue limit. When the cyclic stress of steel sample is slightly higher than the fatigue limit of A steel, the life of B steel is only 1/10 of A steel. Inclusions in A and B steel are oxides. In terms of the total amount of inclusions, the purity of A steel is worse than that of B steel, but the oxide particles of A steel have the same size and uniform distribution. B steel contains some large – grained inclusions with uneven distribution. This fully shows that Mr. Xiao Jimei’s point of view is correct.

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