The comprehensive sign of the inner quality of bearing steel is the fatigue life. Some scholars put forward the point of view that reducing oxygen content has not significantly improved the fatigue life of bearing steel. In fact, only by reducing the oxide and sulfide content at the same time can we fully tap the material potential and greatly improve the fatigue life of bearing steel.
Why can't reducing oxygen content improve the fatigue life of bearing steel? Reason: After the oxide inclusion is reduced, the excess sulfide becomes an adverse factor affecting the fatigue life of steel. 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.
What factors affect the fatigue life of bearing steel? Share as follows:
1. Influence of nitride on fatigue life
Some scholars point out that when nitrogen is increased in steel, the volume fraction of nitrides decreases, which is due to the decrease of the average size of inclusions in steel. Due to technical limitations, a considerable number of inclusions 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 influence on the fatigue life of bearing steel. Ti is one of the strongest elements to form nitrides, with small specific gravity and easy to float, and some Ti will remain in the steel to form angular inclusions. Such inclusions tend to cause local stress concentration and fatigue cracks, so the generation of such inclusions should be controlled.
The test results show that the oxygen content in steel is reduced to less than 20ppm, the nitrogen content is increased, the size, type and distribution of nonmetallic inclusions are improved, and the stable inclusions are obviously reduced. Although the number of nitride particles in the steel is increased, the particles are very small and distributed in the grain boundary or within the grain, which becomes a favorable factor, which makes the strength and toughness of the bearing steel get a good match, greatly increases the hardness and strength of the steel, especially the improvement effect of the contact fatigue life is objective.
2. The influence of oxide on fatigue life
The oxygen content in steel is an important factor affecting the material, the lower the oxygen content, the higher the purity, the longer the corresponding rated life. There is a close relationship between oxygen content and oxide in steel. During the solidification process of liquid steel, oxygen dissolved by aluminum, calcium, silicon and other elements forms oxide. The oxide inclusion content is a function of oxygen. With the decrease of oxygen content, the oxide inclusion will decrease. Like oxygen content, nitrogen content also has a functional relationship with nitride. However, because oxide is dispersed in steel, it plays the same fulcrum role as carbide, so it has no destructive effect on the fatigue life of steel.
Due to the existence of oxides, steel damages the continuity of metal matrix, and because the expansion coefficient of oxides is smaller than that of bearing steel matrix, when subjected to alternating stress, it is easy to produce stress concentration and become the origin of metal fatigue. Most of the stress concentration occurs between the oxides, the point inclusions and the matrix. When the stress is large enough, cracks will be generated, and the cracks will expand rapidly and fail. The lower the plasticity and the sharper the shape of the inclusions, the greater the stress concentration will be.
3. Influence 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 in steel will increase correspondingly. However, because sulfide can surround the oxide well, the influence of the oxide on the fatigue life is reduced, so the influence of the number of inclusions on the fatigue life is not absolute, and is related to the nature, size and distribution of inclusions. The more inclusions there are, the lower the fatigue life will be. Other influencing factors must be considered. Sulfide in bearing steel is dispersed in fine shape and mixed with oxide inclusions, which is difficult to be identified even by metallographic method. Based on the original process, increasing the amount of Al plays a positive role in reducing the oxides and sulfides. This is because Ca has a fairly strong desulfurization capacity. Inclusions have little influence 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 fracture analysis, the fracture process of GCr15 steel is mainly cleavage and quasi-cleavage fracture mechanism. Xiao Jimei, a famous expert, pointed out that the inclusion in steel is a brittle phase, and the higher the volume fraction, the lower the toughness; The larger the size of inclusions, the faster the toughness decreases. For the toughness of cleavage fracture, the smaller the size of inclusions and the smaller the spacing of inclusions, the toughness will not decrease but increase. If the brittle phases are arranged closely in the grain, the distance between dislocations and plugs can be shortened, and the cleavage fracture is not easy to occur, so as to improve the cleavage fracture strength. A special test has been done :A, B two batches of steel belong to the same kind of steel, but each contains different inclusions.
After heat treatment, the two batches of steel A and B reached the same tensile strength of 95 kg/mm', and the yield strength of steel A and B was the same. In terms of elongation and shrinkage, B steel is slightly lower than A steel is still qualified. After the fatigue test (rotary bending), it is found that steel A is a long life material with high fatigue limit. B steel is a short life material with low fatigue limit. When the cyclic stress on the 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. The inclusions in steel A and B are oxides. From the perspective of the total amount of inclusions, the purity of steel A is worse than that of steel B, but the oxide particles of steel A have the same size and uniform distribution. B steel contains some large particles of inclusions, which are also unevenly distributed. This fully shows that Mr. Xiao Jimei's view is correct.