The rolling bearing's working life and reliability largely depend on the steel's smelting quality. Because of the characteristics of bearing steel, the requirement for steel's smelting quality is stricter than general industrial steel, such as the chemical component, clarity, uniformity.
1. Requirement for precise chemical component
General bearing steel is mainly high-carbon chromium bearing steel (hypereutectoid steel) that has 1% carbon, 1.5% chromium and a little manganese and silicon. Chromium can improve the steel's heat treatment performance, hardenability, structure uniformity, temper resistance, rust prevention and grinding performance. When the chromium content exceeds 1.65%, after quenching, it increases the retained austenite of steel and carbide's instability, reduce the hardness, dimensional stability and steel's impact toughness and fatigue strength. So, we usually control the chromium content under 1.65% for getting the satisfying characteristics.
2. Requirement for high precise dimension
Because of most bearing parts is through pressure forming, there are high requirement for the precision of bearing dimension. For saving materials and improving labor productivity, most bearing ring is under forging forming. The steel ball is under cold forging or hot forming. Small size roller is under cold forging, too. If the precision of steel's dimension isn't high, we can't calculate the material's weight and size. And then we can't make sure the quality of bearing parts and can do damage to equipment and mould.
3. Requirement for high clarity
The clarity of steel means the number of nonmetallic inclusion of steel. The higher clarity, the less nonmetallic inclusion of steel. The hazardous inclusion such as oxide, silicate is the main reason that cause the bearing's early fatigue peeling and reduce the bearing's lifetime signally. Especially the brittle inclusion, it affects the quality of precision-machined surface badly when it peeling off from the metallic matrix. So we must reduce the inclusion content of bearing steel for improving bearing's lifetime and reliability.
4. Strict requirement for surface defect and internal defect
For bearing steel, the surface defect includes crack, slag inclusion, rag, scab, scale cinder and so on, the internal defect includes shrinkage cavity, bubble, flakes and so on. These defects have bad effect on bearing's processing, performance and lifetime. So in the standard of bearing materials, these defects can't be allowed.
5. Strict requirement for carbide nonuniformity
If there is huge carbide nonuniformity in bearing steel, the structure and hardness would be uneven under the heat treatment process. The structure nonuniformity of steel has big effect on contact fatigue strength. What's more, huge carbide nonuniformity is easy to reduce the bearing's lifetime and cause that the bearing parts have crack when quenching. So in the standard of bearing materials, there are explicit requirements for different specification of steel.
6. Strict requirement for decarburization layer of surface
If the decarburization layer of surface exceeds the defined limit and not be cleaned out before heat treatment, the hardening crack would come into being and then cause the parts' being scrapped. So in the standard of bearing materials, there are strict rules for decarburization layer of steel surface.
7. Other requirements
In the standard of bearing materials, there are strict rules for bearing steel's smelting methods, oxygen content, annealed hardness, fracture, residual elements, spark test, condition of delivery, symbol and so on.