Factors affecting the fatigue life of bearing steel

The comprehensive sign of the intrinsic quality of bearing steel is fatigue life, some scholars put forward the view that: reducing the oxygen content still has 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? After the reduction in the amount of oxide inclusions, the excess sulfide and become a negative factor affecting the fatigue life of steel. 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.

So what factors affect the bearing steel fatigue life? Share the following.

1, the impact of nitride on fatigue life

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

Test results show that: the oxygen content of steel to below 20ppm, 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 the 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 increase the hardness of steel, strength, especially contact fatigue life improvement effect is objective.

2, the effect 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 oxygen. As the oxygen content decreases, oxide inclusions will be reduced; nitrogen content and oxygen content, the same and nitrides exist as a function of the relationship, but because the oxide in the steel distribution is more dispersed, playing the same role as the carbide fulcrum 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 origin of metal fatigue. Most of the stress concentration between the oxide, point inclusions and matrix, when the stress is large enough to produce cracks, and rapid expansion and damage. The lower the plasticity of inclusions, the shape of the more sharp edges, the greater the stress concentration.

3, the impact of sulfide on fatigue life

Steel sulfur content is almost all in the form of sulfides. Higher sulfur content in steel, the steel sulfide correspondingly higher, but because the sulfide can be well surrounded by oxide, reducing the impact of the oxide on fatigue life, and the nature of inclusions, size and distribution. The more inclusions there are, the lower the fatigue life must be, taking into account other influencing factors. In the bearing steel sulfide is fine diffuse distribution, and mixed into the oxide inclusions, even if the metallographic method is difficult to identify. The test confirmed: in the original process based on the amount of Al to reduce the oxide ﹑ sulfide play a positive role. This is because Ca has a fairly strong desulfurization ability. Inclusions have little effect on the strength, but 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 that: steel inclusions is a brittle phase, the higher the volume fraction, the lower the toughness; the larger the size of inclusions, the faster the decline in toughness. For the toughness of the deconstruction fracture, the smaller the size of the inclusions, the smaller the spacing of the inclusions, the toughness not only does not fall, but to improve, if the crystal brittle phase arrangement is denser, 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 different inclusions.