So what factors affect the fatigue life of bearing steel? Share as follows:

1. The influence of nitrides on fatigue life

Some scholars pointed out that when nitrogen is added to steel, the volume fraction of nitrides decreases. This is because the average size of inclusions in steel decreases. Due to technical limitations, a considerable number of inclusion particles smaller than 0.2in are not counted. 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 strong elements that form nitrides. It has a small specific gravity and is easy to float. 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 cracks, so the generation of this kind of inclusion should be controlled.

The test results show that the oxygen content in steel has dropped below 20ppm, the nitrogen content has increased, the size, type and distribution of non-metallic inclusions have been improved, and the stable inclusions have been significantly reduced. Although the number of nitride particles in steel increases, the particles are very small and are dispersed at the grain boundary or in the grain, which becomes a favorable factor, making the strength and toughness of bearing steel well matched, greatly increasing the hardness and strength of steel, especially the improvement effect of contact fatigue life is objective.

2. The influence of oxides on fatigue life

The oxygen content in steel is an important factor affecting the material. The lower the oxygen content, the higher the purity, and the longer the corresponding rated life. The oxygen content in steel is closely related to oxides. During the solidification process of molten steel, the dissolved oxygen of aluminum, calcium, silicon and other elements forms oxides. The oxide inclusion content is a function of oxygen. As the oxygen content decreases, the oxide inclusion will decrease; the nitrogen content, like the oxygen content, also has a functional relationship with nitrides, but because the oxides are more dispersed in the steel, they play the same fulcrum role as carbides, so they do not have a destructive effect on the fatigue life of steel.

The presence of oxides destroys the continuity of the metal matrix. Since the expansion coefficient of oxides is smaller than that of the bearing steel matrix, when subjected to alternating stress, stress concentration is easily generated, becoming the source of metal fatigue. Most stress concentration occurs between oxides, point inclusions and the matrix. When the stress reaches a large enough level, cracks are generated and rapidly expand and break. The lower the plasticity of the inclusions and the sharper the shape, the greater the stress concentration.

3. Effect of sulfides on fatigue life

Almost all sulfur content in steel exists in the form of sulfides. As the sulfur content in steel increases, the sulfides in steel increase accordingly. However, because sulfides can surround oxides well, reducing the effect of oxides on fatigue life, the effect of the number of inclusions on fatigue life is not certain, and is related to the nature, size and distribution of the inclusions. The more inclusions there are, the lower the fatigue life will be. Other influencing factors must be considered comprehensively. In bearing steel, sulfides are dispersed in a fine shape and mixed into oxide inclusions, making them difficult to identify even with metallographic methods. The test proves that: on the basis of the original process, increasing the amount of Al plays a positive role in reducing oxides and sulfides. This is because Ca has a very strong desulfurization ability. Inclusions have little effect on strength, but have a greater impact on the toughness of steel, and the degree of damage depends on the strength of the steel.