**Basic Requirements for the Quality of Bearing Steel**
The service life and reliability of rolling bearings are heavily dependent on the quality of the steel used in their production. Compared to ordinary industrial steels, bearing steel has much stricter requirements due to its specific application characteristics, such as chemical composition, purity, microstructure, and uniformity.
1. **Strict Chemical Composition Requirements**
Bearing steel is typically high-carbon chromium steel, with a carbon content around 1% and chromium content around 1.5%. Some types may contain small amounts of manganese and silicon. Chromium improves hardenability, heat treatment performance, and tempering stability, while also enhancing rust resistance and grindability. However, if the chromium content exceeds 1.65%, it can lead to retained austenite after quenching, reducing hardness and dimensional stability, as well as causing carbide segregation and lowering impact toughness and fatigue strength. Therefore, the chromium content in high-carbon chromium bearing steel is usually kept below 1.65%.
2. **High Precision Dimensional Requirements**
Rolling bearing steel must have high dimensional accuracy, as most bearing components are formed through forging or cold heading. Poor dimensional accuracy can lead to issues during machining, affect product quality, and even damage equipment and molds.
3. **Strict Purity Standards**
Purity refers to the amount of non-metallic inclusions in the steel. The fewer the inclusions, the better the quality. Oxides and silicates are particularly harmful, as they can cause early fatigue failure and reduce bearing life. Brittle inclusions are especially damaging, as they can detach from the metal surface during processing, leading to surface defects that compromise performance.
4. **Low-Order and High-Power Microstructure Requirements**
The low-order structure refers to porosity, segregation, and other macroscopic imperfections, while the high-power structure includes annealing structure, carbide distribution, and others. A uniform microstructure is essential for consistent mechanical properties and long service life.
5. **Severe Requirements for Surface and Internal Defects**
Surface defects like cracks, slag inclusions, and scale, as well as internal defects like shrinkage cavities, bubbles, and segregation, must be strictly controlled. These defects can significantly impact the performance and lifespan of the bearing.
6. **Carbide Uniformity Requirements**
Uneven carbide distribution can lead to uneven hardness and microstructure after heat treatment, which affects fatigue strength and increases the risk of cracking during quenching.
7. **Decarburization Layer Control**
Excessive decarburization layers can lead to quenching cracks if not removed before heat treatment, resulting in defective parts.
8. **Additional Requirements**
Other important factors include oxygen content, annealing hardness, fracture analysis, residual elements, spark testing, delivery conditions, and marking standards.
By meeting these rigorous standards, bearing steel ensures optimal performance, durability, and reliability in a wide range of applications.
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