**Basic Requirements for the Quality of Bearing Steel**
The service life and reliability of rolling bearings are closely tied to the quality of the steel used. Unlike ordinary industrial steels, bearing steel must meet far more stringent requirements in terms of chemical composition, purity, structure, 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 also include small amounts of manganese and silicon. Chromium enhances heat treatment performance, hardenability, and tempering stability, while improving corrosion resistance and grindability. However, if the chromium content exceeds 1.65%, it can lead to residual austenite after quenching, reducing hardness, dimensional stability, and fatigue strength. Therefore, the chromium content is usually kept below 1.65%.
2. **High Precision Dimensional Accuracy**
Rolling bearings require precise dimensions because most parts are formed through pressure processing. This includes cold heading or hot rolling for balls and rings. If the steel's dimensional accuracy is poor, it can lead to issues in cutting, component assembly, and even damage to equipment and molds.
3. **High Purity Requirements**
Purity refers to the number of non-metallic inclusions in the steel. These inclusions—such as oxides and silicates—are harmful and can cause early fatigue failure and reduced bearing life. Brittle inclusions are particularly damaging as they can detach during processing, affecting surface quality and performance.
4. **Strict Low- and High-Magnification Structure Requirements**
Low-magnification structures refer to looseness, intermediate looseness, and segregation, while high-magnification structures include carbide distribution, meshing, and banding. Poorly distributed carbides can reduce impact toughness and lead to cracking during quenching. These structural issues significantly affect the performance and lifespan of bearings.
5. **Severe Requirements for Surface and Internal Defects**
Surface defects like cracks, slag inclusions, and scale can weaken the material. Internal defects such as shrinkage cavities, bubbles, and segregation can severely impact the bearing’s functionality and longevity. These defects are strictly prohibited in bearing steel specifications.
6. **Strict Carbide Non-Uniformity Standards**
Uneven carbide distribution can result in inconsistent hardness and microstructure after heat treatment, leading to cracks and reduced bearing life. Specifications clearly define acceptable levels of carbide uniformity.
7. **Decarburization Layer Restrictions**
Excessive decarburization on the surface can cause quenching cracks if not removed before heat treatment. Bearing standards impose strict limits on the depth of the decarburized layer.
8. **Other Critical Requirements**
Additional factors such as oxygen content, annealing hardness, fracture analysis, residual elements, spark testing, delivery condition, and marking are also strictly regulated in bearing steel specifications.
In summary, bearing steel must meet exacting standards in every aspect to ensure the long-term performance and reliability of rolling bearings. Each requirement plays a vital role in maintaining the quality and function of the final product.
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