What are the factors that affect the fatigue life of rolling bearing steel?

The comprehensive sign of the inherent quality of bearing steel is the fatigue life. Some scholars have put forward the view that reducing the oxygen content still does not play 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 we fully tap the material potential and greatly improve the fatigue life of bearing steel. Why can't the fatigue life of bearing steel be improved by reducing oxygen content? Sharing reason: after the reduction of oxide inclusions, the excess sulfides become an unfavorable factor affecting the fatigue life of steel. Only by reducing the content of oxide and sulfide at the same time can the material potential be fully tapped and the fatigue life of bearing steel be greatly improved.

What factors affect the fatigue life of bearing steel?

1. Effect of nitride on fatigue life.

Some scholars have pointed out that with the increase of nitrogen in steel, the volume fraction of nitride decreases, which is due to the decrease of the average size of inclusions in steel. Due to the limitation of technology, there are a considerable number of particles smaller than 0.2in inclusions that are not calculated. It is precisely the existence state of these tiny nitride particles that has a direct impact on the fatigue life of bearing steel. Ti is one of the strong zui elements that form nitrides, with a small specific gravity and easy to float, and a part of 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 production of this kind of inclusion.

The experimental results show that the oxygen content in the steel is lower than 20ppm, the nitrogen content is increased, the size, type and distribution of non-metallic inclusions are improved, and the stable inclusions are obviously reduced. Although the nitride particles in the steel increase, the nitride particles are very small and distribute in a dispersed state at the grain boundary or in the grain, which becomes a favorable factor, which makes a good match between the strength and toughness of the bearing steel and greatly increases the hardness and strength of the steel. in particular, the improvement effect of contact fatigue life is objective.

2. Effect of oxide on fatigue life.

The oxygen content in steel is an important factor affecting the material quality. The lower the oxygen content is, the higher the purity is, the longer the rated life is. There is a close relationship between oxygen content and oxides in steel. Oxides are formed by dissolved oxygen of aluminum, calcium, silicon and other elements in molten steel during solidification. The content of oxide inclusion is a function of oxygen. With the decrease of oxygen content, the oxide inclusion will decrease, and the nitrogen content, like the oxygen content, also has a functional relationship with nitride, but because the oxide is dispersed in steel, it plays the same fulcrum role as carbide. Therefore, it does not play a destructive role in the fatigue life of steel.

Due to the existence of oxide, steel destroys the continuity of metal matrix, and because the expansion coefficient of oxide is less than that of bearing steel matrix, it is easy to produce stress concentration when bearing alternating stress, which becomes the birthplace of metal fatigue. Most of the stress concentration occurs between oxide, point inclusion and matrix. When the stress is large enough, cracks will occur and rapidly propagate and destroy. The lower the plasticity of the inclusion is and the more sharp the shape is, the greater the stress concentration is.

3. Effect of sulfides on fatigue life.

Almost all of the sulfur content in steel exists in the form of sulfides. With the increase of sulfur content in steel, the sulfide in steel increases accordingly, but because sulfide can well surround the oxide and reduce the effect of oxide on fatigue life, the effect of the number of inclusions on fatigue life is not definite. It is related to the nature, size and distribution of inclusions. The more certain inclusions, the lower the fatigue life, and other influencing factors must be comprehensively considered. Sulfides in bearing steel are finely dispersed and mixed with oxide inclusions, which is difficult to identify even by metallographic method. The experimental results show 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 strong desulphurization ability. The inclusion has little effect on the strength, but does great harm to the toughness of the steel, and its harm degree depends on the strength of the steel.

The fracture process of GCr15 steel is mainly cleavage and quasi-cleavage fracture mechanism according to fracture analysis. Expert Xiao Jimei pointed out that inclusions in steel are brittle phases, the higher the volume fraction, the lower the toughness, and the larger the size of the inclusions, the faster the toughness decreases. For the toughness of cleavage fracture, the smaller the size of inclusions and the smaller the distance between inclusions, the toughness not only does not decrease, but increases instead. If the arrangement of brittle phases in the grain is dense, the stacking distance of dislocations can be shortened and cleavage fracture is not easy to occur, so as to improve the cleavage fracture strength. Some people have made a special test: two batches of steel An and B belong to the same type of steel, but their inclusions are different.