What factors affect bearing steel fatigue life?

What factors affect bearing steel fatigue life?

Here share some information for our customer reference,

1, the impact of nitride on fatigue life
Some scholars pointed out that: steel increase nitrogen, nitride volume fraction is down, this is due to the average size of inclusions in the steel to reduce the cause, by the technical limitations, there are still a considerable number of less than 0.2in inclusions particles are not calculated. Precisely the existence of these tiny nitride particles state, the fatigue life of bearing steel has a direct impact. ti is one of the strongest elements in the formation of nitride, small specific gravity, easy to float, there will be a part of Ti in the steel to form angular inclusions. This kind of inclusions can easily cause local stress concentration and produce fatigue cracks, so it is necessary to control the generation of such inclusions.

The test results show that: the oxygen content in the steel is reduced to less than 20ppm, the nitrogen content is improved, the size, type and distribution of non-metallic inclusions have been improved, and the stable inclusions have been significantly reduced. Nitride particles in steel, although more, but its particles are very small, and grain boundaries or intracrystalline diffuse state distribution, become a favourable factor, so that the strength and toughness of bearing steel to get a good match, greatly increase the hardness and strength of steel, especially the contact fatigue life improvement effect is objective.

2, the effect of oxides 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 oxides have a close relationship, liquid steel in the solidification process, aluminium, calcium, silicon and other elements of dissolved oxygen to form oxides. The oxide inclusion content is a function of 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, and carbide plays the same role as the role of the fulcrum role, so the fatigue life of steel does not play a destructive role.

Steel due to the presence of oxides, destroying the continuity of the metal matrix, but also due to the expansion coefficient of the oxide is less than the expansion coefficient of the bearing steel matrix, when subjected to alternating stress, easy to produce stress concentration, become the origin of metal fatigue. Most of the stress concentration is generated in the oxide, point-like inclusions and between the matrix, when the stress reaches a large enough, cracks, and rapid expansion and destruction. The lower the plasticity of the inclusions, the more pointed shape, the greater the stress concentration.

3, the effect of sulfide on fatigue life

Almost all of the sulfur content in steel in the form of sulfide. Increased sulfur content in steel, the corresponding increase in steel sulfide, but because sulfide can be well surrounded by oxides, reducing the impact of oxides on fatigue life, so the number of inclusions on the impact of fatigue life is not absolute, 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 considered. Sulfide in bearing steel is a small diffuse distribution, and mixed into the oxide inclusions, even if the metallographic method is difficult to identify. Tests have confirmed: on the basis of the original process, increase the amount of Al to reduce the oxide ﹑ sulfide play a positive role. This is because Ca has a strong desulphurisation capacity. 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 the fracture analysis is mainly deconvoluted and quasi-deconvoluted fracture mechanism. Famous experts Xiao Jimei pointed out that: inclusions in steel is a brittle phase, the higher the volume fraction, the lower the toughness; inclusions the larger the size, the faster the toughness decline. For the toughness of the deconstructive fracture, the smaller the size of the inclusions, the smaller the spacing of the inclusions, the toughness not only does not decline, but to improve, if the brittle phase in the crystal arrangement is denser, it can shorten the dislocation stacking distance, is not easy to deconstructive fracture, which improves the deconstructive fracture strength. Some people have done a special test: A, B two batches of steel belong to the same steel, but each contains inclusions in different situations.

After heat treatment, A, B two batches of steel to achieve the same tensile strength of 95 kg / mm ', A, B steel yield strength is the same. In terms of elongation and surface shrinkage, B steel is slightly lower than A steel is still qualified. After the fatigue test (rotary bending) found that: A steel is a long-life material, fatigue limit is high; B steel is a short-life material, fatigue limit is low. When the cyclic stress on the steel specimen is slightly higher than the fatigue limit of steel A, the life of steel B is only 1/10 of steel A. The inclusions in steel A and B are oxides. From the total number of inclusions, the purity of steel A is a little worse than the purity of steel B. However, the size of the oxide particles in steel A is consistent and uniformly distributed; steel B contains some large particles of inclusions, and the distribution is not uniform. This fully demonstrates that Mr Xiao Jimi's point of view is correct.

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