The comprehensive mark of the inherent quality of bearing steel is the fatigue life. Some scholars put forward the view that the reduction of oxygen content has not 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 material potential be fully tapped and the fatigue life of bearing steel be greatly improved.
Why can't reducing oxygen content improve the fatigue life of bearing steel?After the oxide inclusion amount is reduced, the excess sulfide has become an adverse factor affecting the fatigue life of steel.Only when the content of oxide and sulfide is reduced at the same time, the material potential can be fully exploited and the fatigue life of bearing steel can be greatly improved.
What factors affect the fatigue life of bearing steel?Share below:
1, the influence of nitrogen compounds on fatigue life
Some scholars point out that with the increase of nitrogen in steel, the volume fraction of nitrogen compounds decreases, which is due to the decrease of the average size of inclusions in steel. Due to technical limitations, there are still a considerable number of particles of inclusions less than 0.2in which are not counted.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 strongest elements to form nitride, with a small specific gravity and easy to float up. 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 the generation of this kind of inclusion must be controlled.
The results show that the oxygen content in steel decreases to less than 20ppm, the nitrogen content increases, the size, type and distribution state of nonmetallic inclusions are improved, and the stable inclusions are obviously reduced.Although the number of nitride particles in the steel increases, the particles are very small and are distributed in the grain boundary or within the grain, which becomes a favorable factor. The strength and toughness of bearing steel are well matched, and the hardness and strength of steel are greatly increased, especially the improvement effect of 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, and the longer the corresponding rated life.Oxygen content in steel is closely related to oxides. In the solidification process of molten steel, aluminum, calcium, silicon and other elements dissolve oxygen to form oxides.Oxide inclusion is a function of oxygen.With the decrease of oxygen content, the oxide inclusions will decrease.The nitrogen content is the same as the oxygen content, and the nitride also has a functional relationship, but because the oxide distribution in the steel is more dispersed, and plays the same role as the fulcrum of carbide, so the fatigue life of steel did not play a damaging role.
Because of the existence of oxides in steel, the continuity of metal matrix is destroyed, and because the expansion coefficient of oxides is less than the expansion coefficient of bearing steel matrix, when subjected to alternating stress, it is easy to produce stress concentration, which becomes the birthplace of metal fatigue.Most of the stress concentration occurs between oxides, punctate inclusions and matrix. When the stress reaches enough, cracks will occur, which will rapidly expand and destroy.The lower the plasticity and the sharper the shape of the inclusion, the greater the stress concentration.
3, the influence of sulfide on fatigue life
Sulphur content in steel almost all exists in the form of sulfide.When the sulfur content in steel increases, the sulfide in steel increases accordingly. However, because the sulfide can be well surrounded by the oxides, reducing the influence of the oxides on the fatigue life, the influence of the number of inclusions on the fatigue life is not absolute, but related to the nature, size and distribution of the inclusion.The more the inclusion is, the lower the fatigue life must be. The other factors must be considered.The sulfide in bearing steel is fine and diffuse, and mixed with oxide inclusions, even using metallographic method is difficult to identify.Based on the original process, increasing the amount of Al plays a positive role in reducing oxides and sulfides.This is because CA has a fairly strong desulfurization capacity.Inclusion has little effect on the strength, but does great harm to the toughness of steel, and the degree of harm depends on the strength of steel.
The fracture process of GCr15 steel is mainly cleavage and quasi cleavage fracture mechanism according to fracture analysis.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 inclusion, the faster the toughness decreases.As for the toughness of cleavage fracture, the smaller the size of inclusions and the smaller the spacing of inclusions, the toughness is not decreased but increased. If the brittle phase in the grain is closely arranged, the dislocation stack plugging distance can be shortened, and the cleavage fracture is not easy to occur, thus improving the cleavage fracture strength.Someone has done A special test :A, B two batches of steel belong to the same steel, but their inclusion is different.
After heat treatment, the two batches of steel A and B reach 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, B steel slightly lower than A steel is still qualified.After fatigue test (rotary bending), it is found that: steel A is A long-life material, with high fatigue limit;B steel is short life material, low fatigue limit.When the cyclic stress of steel specimen 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 A and B steel are oxides.From the total amount of inclusion, the purity of A steel is worse than that of B steel, but the size of oxide particles of A steel is consistent and evenly distributed;B steel contains some large inclusions, which are also unevenly distributed.This fully shows that Mr. Xiao Jimei's view is correct.