Bearing is composed of inner and outer rings, rolling body (ball, roller or needle roller) and retainer four parts, in addition to the retainer, the rest are made of bearing steel. When the bearing works, the bearing inner and outer rings and the bearing rolling body bear the action of high frequency and variable stress. The working conditions of bearings are very complicated. The load is concentrated on a very small area of the rolling body. In theory, the ball bearings are acting on a point; For the roller is acting on a line, and the contact area between the rolling body and the ring is also very small (point/line contact), so the bearing parts at work, the rolling body and the ring surface of the unit area to bear a lot of pressure, generally up to 1500-5000N/mm2; Bearing rotation, but also to withstand the effect of centrifugal force, the force increases with the increase of speed; There is not only rolling but also sliding between the rolling body and the ring, so there is friction between the rolling body and the ring. Under the combined action of the above several forces, fatigue cracks first occur in the part with low fatigue strength on the surface of the ring or rolling body, and then fatigue spalling is formed, which causes the bearing damage and failure. The normal form of bearing damage is contact fatigue damage, common plastic deformation, indentation, wear, crack and so on.

Although bearing life and reliability are related to bearing design, processing and manufacturing, lubrication conditions, installation, maintenance and other factors, the high quality and reliability of bearing materials are the key. Rolling bearing parts should work at high speed and for a long time under complex stress states and high stress values such as tensile, compression, bending, shear and alternation. Therefore, rolling bearings are required to have:

1) High plastic deformation resistance,

2) High friction and wear resistance,

3) High rotation accuracy and dimensional accuracy,

4) Good dimensional stability,

5) Long life and high reliability.

For bearings working under special conditions, there are special requirements, such as high temperature resistance, low temperature resistance, corrosion resistance and diamagnetic properties, etc