What are the factors affecting the fatigue life of rolling bearing steel

A comprehensive sign of the intrinsic quality of bearing steel is fatigue life, some scholars have put forward the view that reducing the oxygen content has still not played a significant role in improving the fatigue life of bearing steel. In fact, only at the same time to reduce the oxide and sulfide content, in order to fully exploit the material potential, significantly improve the fatigue life of bearing steel. Why reduce the oxygen content can not improve the fatigue life of bearing steel? Share the reason: after the oxide inclusions are reduced, the excess sulfide and become a detrimental factor affecting the fatigue life of steel. Only at the same time to reduce the oxide and sulphide content, in order to fully exploit the material potential, significantly improve the fatigue life of bearing steel.

What factors affect the fatigue life of bearing steel?

1, the impact of nitride on fatigue life

Some scholars point out: steel nitrogen, nitride volume fraction but down, this is due to the average size of the inclusions in steel to reduce the reason, limited by technology, there are a considerable number of less than 0.2in inclusions particles are not counted. Ti is one of the strongest elements in the formation of nitride, the specific gravity is small, easy to float, there will be a part of Ti in the steel to form multi-angle inclusions. This inclusions are likely to cause local stress concentration and fatigue cracking, so to control the generation of such inclusions.

Test results show that: the oxygen content of steel to 20ppm or less, nitrogen content has increased, the size, type and distribution of non-metallic inclusions have been improved, stable inclusions have been significantly reduced. Although the nitride particles in steel increased, but its particles are very small, and in the grain boundaries or grain is distributed in a diffuse state, become a favorable factor, so that the strength and toughness of bearing steel has been a good match, greatly increased the hardness of steel, strength, especially contact fatigue life improvement effect is objective.

2, the impact of oxide on fatigue life

Oxygen content in steel is an important factor affecting the material, the lower the oxygen content the higher its purity, the longer the corresponding rated life. Oxygen content in steel and oxide has a close relationship, the steel in the solidification process, aluminum, calcium, silicon and other elements of dissolved oxygen to form oxide. The oxide inclusion content is a function of the oxygen. With the reduction of oxygen content, oxide inclusions will be reduced; nitrogen content and oxygen content, the same and nitrides exist as a function of the relationship, but due to the distribution of oxides in steel is more dispersed, playing the same role as carbide pivot role, so the fatigue life of steel did not play a destructive role.

Steel due to the presence of oxides, the destruction of the continuity of the metal matrix, and because the expansion coefficient of the oxide is less than the bearing steel matrix expansion coefficient, when subjected to alternating stress, easy to produce stress concentrations, become the source of metal fatigue. Most of the stress concentrations are generated in the oxide, point inclusions and matrix between, when the stress is large enough to produce cracks, and rapid expansion and damage. The lower the plasticity of the inclusions, the more sharp-edged shape, the greater the stress concentration.

3, the impact of sulfide on fatigue life

Steel sulfur content almost all in the form of sulfides. Steel sulfur content increased, the steel sulfide correspondingly increased, but because the sulfide can be well surrounded by oxide, reducing the impact of the oxide on fatigue life, so the number of inclusions on the fatigue life is not a certain impact, with the nature of inclusions, size and distribution. The more inclusions there are, the lower the fatigue life must be, and other influencing factors must be taken into account. In the bearing steel sulphide is fine diffuse distribution, and mixed into the oxide inclusions, even if the use of metallographic methods are difficult to identify. Tests have confirmed that: on the basis of the original process, increase the amount of Al to reduce the oxide ﹑ sulphide play a positive role. This is because Ca has a rather strong desulphurisation capacity. Inclusions have little effect on the strength, while the toughness of the steel is more harmful, and the degree of harm depends on the strength of the steel.

The fracture process of GCr15 steel, according to fracture analysis is mainly deconstruction and quasi-deconstruction fracture mechanism. Experts Xiao Jimei pointed out: steel inclusions is a brittle phase, the higher the volume fraction, the lower the toughness; the larger the size of inclusions, the faster the toughness decline. For the toughness of the deconstruction fracture, the smaller the size of the inclusions, the smaller the spacing of inclusions, the toughness not only does not fall, but to improve, if the crystal brittle phase is arranged more dense, it can shorten the dislocation stacking distance, not easy to deconstruction fracture, thus improving the deconstruction fracture strength. Some people have done a special test: A, B two batches of steel belongs to the same steel, but each contains inclusions of different situations.