The comprehensive sign of bearing steel internal quality is fatigue life. Some scholars put forward the view that reducing oxygen content has not yet 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 material be fully exploited and the fatigue life of bearing steel be greatly improved.

Why can't reducing oxygen content improve fatigue life of bearing steel? China bearing network (abbreviated as: China bearing network) share the reason: after the reduction of oxide inclusions, the excess sulfide has become an adverse 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.


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


1. Effect of nitride on fatigue life


Some scholars point out that the volume fraction of nitride decreases with the increase of nitrogen in steel, which is due to the decrease of the average size of inclusions in steel. Due to the limitation of technology, a considerable number of inclusions less than 0.2 in are not included. 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 small specific gravity and is easy to float up. Some ti remains in the steel to form multi angular inclusions. This kind of inclusion is easy to cause local stress concentration and produce fatigue crack, so the generation of this kind of inclusion should be controlled.


The results show that the oxygen content in the steel is reduced to less 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 there are more nitride particles in the steel, the particles are very small and disperse in the grain boundary or in the grain, which becomes a favorable factor. The strength and toughness of the bearing steel are well matched, and the hardness and strength of the steel are greatly increased, especially 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, and the longer the corresponding rated life is. Oxygen content in steel is closely related to oxide. During solidification, oxygen dissolved by Al, Ca and Si forms oxide. The content of oxide inclusion is a function of oxygen. With the decrease of oxygen content, oxide inclusions will decrease; Nitrogen content is the same as oxygen content, and also has a functional relationship with nitride. However, due to the dispersed distribution of oxides in the steel, it plays the same role as carbide as fulcrum, so it has no destructive effect on the fatigue life of steel.


Because of the existence of oxide, the continuity of metal matrix is destroyed. And because the expansion coefficient of oxide is less than that of bearing steel matrix, when bearing alternating stress, it is easy to produce stress concentration and become the birthplace of metal fatigue. Most of the stress concentration occurs between the oxide, the point inclusion and the matrix. When the stress is large enough, the crack will appear, and it will expand rapidly and destroy. The lower the plasticity and the sharper the shape of inclusions, the greater the stress concentration.


3. Effect of sulfide on fatigue life


The sulfur content in steel is almost all in the form of sulfide. With the increase of sulfur content in steel, the sulfide content in steel increases correspondingly. However, because sulfide can be well surrounded by oxides, the effect of oxides on fatigue life is reduced. Therefore, the effect of the number of inclusions on fatigue life is not absolute, which is related to the nature, size and distribution of inclusions. The more inclusions there are, the lower the fatigue life will be. Other factors must be considered comprehensively. The sulfide in bearing steel is fine dispersed and mixed with oxide inclusion, which is difficult to identify even by metallographic method. The results show that on the basis of the original process, increasing Al content plays a positive role in reducing oxides and sulfides. This is because CA has very strong desulfurization ability. Inclusions have little effect on the strength, but do great harm to the toughness of steel, and the degree of harm depends on the strength of steel.


According to the fracture analysis, the fracture process of GCr15 steel is mainly cleavage and quasi cleavage fracture mechanism. Xiao Jimei, a famous expert, pointed out that inclusion in steel is a kind of brittle phase. The higher the volume fraction is, the lower the toughness is; The larger the size of inclusions, the faster the toughness decreases. As for the toughness of cleavage fracture, the smaller the size of inclusions and the smaller the distance between inclusions, the toughness will not decrease, but increase. If the brittle phases are densely arranged in the crystal, the distance between dislocations 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: two batches of steel A and B belong to the same steel grade, but their inclusions are different.


After heat treatment, the two batches of steel A and B have 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, steel B is still qualified if it is slightly lower than steel a. After fatigue test (rotary bending), it is found that steel a has long life and high fatigue limit; Steel B has short life and low fatigue limit. When the cyclic stress of steel sample is slightly higher than the fatigue limit of steel a, the life of steel B is only 1 / 10 of that of steel a. A. The inclusions in steel B are all oxides. From the total amount of inclusions, the purity of steel a is worse than that of steel B, but the oxide particles of steel a are uniform in size and distribution; B steel contains some large particles of inclusions, and the distribution is not uniform. This fully shows that Mr. Xiao Jimei's view is correct.


[warm tips] [Zhongyi bearing network] part of the information comes from the Internet, striving to be safe, timely and accurate. The purpose is to transmit more information, which does not mean that our network agrees with its views or is responsible for its authenticity. If the information reprinted in this website involves copyright and other issues, please contact this website in time.