The comprehensive indicator of the inherent quality of bearing steel is fatigue life. Some scholars have put forward a point of view: reducing the oxygen content has not played a significant role in improving the fatigue life of bearing steel. In fact, only by reducing the content of oxides and sulfides at the same time can the material potential be fully tapped and the fatigue life of bearing steel can be greatly improved.
Why can’t reducing the oxygen content improve the fatigue life of bearing steel? Reason for sharing: After the oxide inclusions are reduced, the excess sulfide becomes an unfavorable factor affecting the fatigue life of the steel. Only by reducing the oxide and sulfide content at the same time can the material potential be fully tapped and the fatigue life of the bearing steel can be greatly improved.
So what factors affect the fatigue life of bearing steel? Share as follows:
1. The influence of nitride on fatigue life
Some scholars have pointed out that the volume fraction of nitrides in steel increases, but the volume fraction of nitrides decreases. This is due to the reduction of the average size of inclusions in steel. Due to technical limitations, there are still a considerable number of inclusions smaller than 0.2in. Counted. It is the existence of these tiny nitride particles that have a direct impact on the fatigue life of the bearing steel. Ti is one of the strongest elements that form nitrides. It has a small specific gravity and is easy to float. A part of Ti will remain in the steel to form angular inclusions. This kind of inclusion is easy to cause local stress concentration and fatigue cracks, so it is necessary to control the generation of this kind of inclusion.
The test results show that the oxygen content in the steel is reduced to less than 20 ppm, the nitrogen content is increased, the size, type and distribution of non-metallic inclusions are improved, and the stable inclusions are significantly reduced. Although the number of nitride particles in the steel increases, their particles are very small and distributed in a dispersed state at the grain boundary or within the grain, which becomes a favorable factor, which makes the strength and toughness of the bearing steel a good match, and greatly increases the hardness and strength of the 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. Oxygen content in steel is closely related to oxides. During the solidification of molten steel, oxygen dissolved in aluminum, calcium, silicon and other elements forms oxides. The oxide inclusion content is a function of oxygen. As the oxygen content decreases, the oxide inclusions will decrease; the nitrogen content is the same as the oxygen content, and also has a functional relationship with the nitrides, but because the oxides are more dispersed in the steel, they play the same role as the fulcrum of the carbides. , So it has no destructive effect on the fatigue life of steel.
Due to the presence of oxides, steel destroys the continuity of the metal matrix, and because the expansion coefficient of the oxide is smaller than that of the bearing steel matrix, it is prone to stress concentration when subjected to alternating stress, which becomes the birthplace of metal fatigue. Most of the stress concentration occurs between oxides, point-like inclusions and the matrix. When the stress is large enough, cracks will occur, which will rapidly expand and be destroyed. The lower the plasticity of the inclusion and the sharper the shape, the greater the stress concentration.
3. The influence of sulfide on fatigue life
Almost all of the sulfur content in steel exists in the form of sulfides. The higher the sulfur content in steel, the higher the sulfide in the steel, but because the sulfide can be well surrounded by oxides, reducing the impact of oxides on fatigue life, the number of inclusions does not affect fatigue life. Absolutely, 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 considered comprehensively. In the bearing steel, the sulfide is distributed in a fine shape, and is mixed with oxide inclusions, and it is difficult to identify even if the metallographic method is used. Tests have proved that on the basis of the original process, increasing the amount of Al has a positive effect on reducing oxides and sulfides. This is because Ca has a fairly strong desulfurization ability. Inclusions have little effect on the strength, but have greater harm to the toughness of steel, and the degree of harm depends on the strength of the steel.
According to the fracture analysis, the fracture process of GCr15 steel is mainly cleavage and quasi-cleavage fracture mechanisms. The well-known expert Xiao Jimei pointed out: Inclusions in steel are a kind of brittle phase, the higher the volume fraction, the lower the toughness; the larger the size of the inclusions, the faster the toughness decreases. For the toughness of cleavage fracture, the smaller the size of the inclusions and the smaller the spacing of the inclusions, the toughness will not decrease, but will increase. If the brittle phases in the crystal are arranged densely, the dislocation packing distance can be shortened. Cleavage fracture is less likely to occur, thereby increasing the cleavage fracture strength. Someone has done a special experiment: the two batches of steel, A and B, belong to the same steel grade, but they contain different inclusions.
After heat treatment, the two batches of steel materials A and B reach the same tensile strength of 95 kg/mm’, and the yield strength of steel materials A and B is the same. In terms of elongation and area reduction, B steel is slightly lower than A steel and still qualified. After fatigue test (rotational bending), it is found that: A steel is a long-life material with a high fatigue limit; B steel is a short-life material with a low fatigue limit. When the cyclic stress of the steel sample is slightly higher than the fatigue limit of the A steel, the life of the B steel is only 1/10 of that of the A steel. The inclusions in steel A and B are oxides. From the perspective of the total amount of inclusions, the purity of A steel is worse than that of B steel, but the oxide particles of A steel have the same size and are evenly distributed; B steel contains some large-particle inclusions and the distribution is not even. . This fully shows that Mr. Xiao Jimei’s point of view is correct.
Post time: Jul-28-2021