The durability of bearing steel is usually measured by fatigue life. The research shows that the fatigue life of bearing steel can not be significantly improved by reducing oxygen content alone. In fact, reducing both oxide and sulfide content is the key to improving fatigue life.

Why is reducing oxygen content not effective in improving fatigue life? When the oxide impurities are reduced, the sulfide becomes the new fatigue life limiting factor. Only by simultaneously reducing these two impurities can we give full play to the potential of the material and significantly improve the fatigue life of the bearing steel.

The following are the main factors affecting the fatigue life of bearing steel:

Some scholars have pointed out that although the nitrogen content in steel increases, the volume proportion of nitrides decreases because the average size of inclusions decreases. The small nitride particles have a direct effect on the fatigue life of bearing steel. Titanium (Ti) is one of the powerful elements that form nitrides, and it forms polygonal inclusions that can cause local stress concentration, which can trigger fatigue cracks. Therefore, it is necessary to control the formation of such inclusions. The experimental results show that the size, type and distribution of non-metallic inclusions are improved when the oxygen content in steel is reduced below 20ppm and the nitrogen content is increased, and the stable inclusions are significantly reduced. Although the number of nitride particles increases, they are very small and evenly distributed in the grain boundaries or within the grains, which has a positive impact on enhancing the hardness, strength and contact fatigue life of the bearing steel.

The oxygen content in the steel is an important factor to determine the purity of the material, the lower the oxygen content, the higher the purity of the material, the longer the corresponding rated life. The oxide inclusion formed in the solidification process is closely related to the oxygen content. With the decrease of oxygen content, the oxide inclusion also decreases. Although the oxide distribution in the steel is more dispersed, similar to the role of carbide, but under alternating stress, the presence of oxide will destroy the continuity of the metal matrix, resulting in stress concentration, becoming the origin of fatigue cracks.

The sulfur in steel is mainly in the form of sulfide. An increase in sulfur content will lead to an increase in sulfide, but the sulfide can surround the oxide and reduce the negative impact of the oxide on fatigue life. The influence of the number of inclusions on the fatigue life is not absolute, but also depends on the nature, size and distribution of inclusions. Experiments show that increasing the content of aluminum (Al) on the basis of the existing process helps to reduce the content of oxides and sulfides. This is because calcium (Ca) has a strong desulfurization ability. Inclusions do great harm to the toughness of steel, and their degree depends on the strength of steel.

By comparing two steels (A and B) with different inclusions content, we found that although the purity of steel A is slightly lower than that of steel B, the oxide particles in steel A are the same size and evenly distributed, while steel B contains larger particles of inclusions and is unevenly distributed. This results in A steel with a higher fatigue limit and a longer service life, confirming the importance of the characteristics, size and distribution of inclusions on their fatigue properties.