Factors affecting the life of bearing steel
The comprehensive sign of the internal quality of bearing steel is the fatigue life. Some scholars put forward the view that reducing the oxygen content has not yet played a role in greatly 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 reducing oxygen content improve the fatigue life of bearing steel? China bearing network (abbreviated as China axis network) sharing reason: after the reduction of oxide inclusion, 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 we fully tap the material potential and greatly improve the fatigue life of bearing steel.
What factors affect the fatigue life of bearing steel?
1. Effect of nitrides 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 reduction of the average size of inclusions in steel. Limited by technology, a considerable number of inclusion particles less than 0.2in are not included. The existence of these tiny nitride particles has a direct impact on the fatigue life of bearing steel. Ti is one of the strongest elements to form nitrides. It has small specific gravity and is easy to float up. Some 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 should be controlled.
The test results show that the oxygen content in the steel decreases below 20ppm, the nitrogen content increases, the size, type and distribution of non-metallic inclusions are improved, and the stable inclusions are significantly reduced. Although the nitride particles in the steel increase, their particles are very small and dispersed in the grain boundary or crystal, which becomes a favorable factor, so that the strength and toughness of the bearing steel are well matched, greatly increasing the hardness and strength of the steel, especially the improvement effect of contact fatigue life.
2. Effect 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. During solidification of liquid steel, oxygen dissolved by elements such as aluminum, calcium and silicon forms oxides. The content of oxide inclusions is a function of oxygen. With the decrease of oxygen content, oxide inclusions will decrease; Nitrogen content, like oxygen content, also has a functional relationship with nitride. However, due to the relatively dispersed distribution of oxide in steel, it plays the same fulcrum role as carbide, so it does not damage the fatigue life of steel.
Steel destroys the continuity of metal matrix due to the existence of oxide, 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 oxides, point inclusions and matrix. When the stress reaches large enough, cracks will occur, expand rapidly and destroy. The lower the plasticity and the sharper the shape of the inclusion, the greater the stress concentration.
3. Effect of sulfide on fatigue life
Almost all the sulfur content in steel 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 around the oxide, the influence of oxide on fatigue life is reduced. Therefore, the influence of the number of inclusions on fatigue life is not absolute, which is related to the nature, size and distribution of inclusions. The more a certain inclusion, the lower the fatigue life. Other influencing factors must be comprehensively considered. In bearing steel, sulfide is dispersed in fine shape and mixed into oxide inclusions, which is difficult to identify even by metallographic method. The test shows 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 quite strong desulfurization capacity. Inclusions have little effect on 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. Famous experts point 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 improve. If the arrangement of brittle phases in the crystal is dense, the dislocation stacking distance can be shortened and cleavage fracture is not easy to occur, so as to improve the cleavage fracture strength. Someone has made a special test: A and B batches of steel belong to the same steel, but the inclusions contained in them are 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 area shrinkage, steel B is slightly lower than steel a, which 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 a short-life material with low fatigue limit. When the cyclic stress of steel sample is slightly higher than the fatigue limit of steel a, the service life of steel B is only 1 / 10 of that of steel a. A. B inclusions in steel are 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 have the same size and uniform distribution; B steel contains some large particle inclusions with uneven distribution. This fully shows that the expert’s view is correct.
Additional information
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