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.

1. Effect of nitride on fatigue life

Some scholars have pointed out that the volume fraction of nitrous compounds decreases with the increase of nitrogen in steel, which is due to the decrease of the average size of inclusions in steel. Due to technical limitations, a considerable number of inclusion particles smaller than 0.2 in are not included in the calculation. It is precisely the existence of these tiny nitride particles that has a direct impact on the fatigue life of bearing steel. Ti is one of the strongest elements to form nitride. It has a small specific gravity and is easy to float up. Some Ti will remain in the steel to form multi-angular inclusions. This kind of inclusion is easy to cause local stress concentration and fatigue crack, so it is necessary to control the generation of this kind of inclusion.

The experimental results show that the size, type and distribution of non-metallic inclusions are improved and the stable inclusions are obviously reduced when the oxygen content in steel is lower than 20ppm and the nitrogen content is increased. Although the nitrogen particles in the steel increase, but the particles are very small, and in the grain boundary or the distribution of the dispersion state, become a favorable factor, so that the strength and toughness of bearing steel has been well matched, greatly increase the hardness and strength of steel, especially the improvement effect of contact fatigue life is objective existence.

2. Influence of oxide on fatigue life

The oxygen content of steel is an important factor affecting the material quality. The lower the oxygen content, the higher the purity and the longer the rated life of steel. There is a close relationship between oxygen content and oxide in steel. In the solidification process of molten steel, aluminum, calcium, silicon and other elements dissolve oxygen to form oxides. The oxide inclusion content is a function of oxygen. With the decrease of oxygen content, the oxide inclusions will decrease. Nitrogen content is the same as oxygen content, and also has a functional relationship with nitride, but because the oxide in the steel distribution is more dispersed, and plays the role of the fulcrum with the same role of carbide, so the fatigue life of steel does not play a destructive role.

Because of the existence of oxides, steel destroys the continuity of metal matrix, and because the expansion coefficient of oxides is less than that of bearing steel matrix, when subjected to alternating stress, it is easy to produce stress concentration, and becomes the birthplace of metal fatigue. Most of the stress concentration occurs between oxide, point inclusions and matrix. When the stress is large enough, cracks occur and rapidly expand to destroy. The lower the plasticity of inclusions is, the sharper the shape is, and the greater the stress concentration is.

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