Bearing Appearance and Outer Surface Quality
Source: China Bearing Network | Time: 2014-02-14
Rolling bearings can fail due to several mechanisms, including external contact fatigue, abrasive wear, adhesive wear, and corrosion. These failures often occur on the outer and inner surfaces of the bearing during operation. Therefore, the quality of the outer surface is crucial for ensuring the reliability and longevity of the bearing. The surface quality of a rolling bearing involves several aspects, such as surface appearance, material properties, microstructure, residual stress, wear, and corrosion resistance. The outer surface of the bearing may undergo changes in its microstructure and mechanical properties due to various processing conditions, such as cold or hot working, lubrication, and grinding. The outer surface layer that experiences these changes is known as the "surface heat-affected layer." If this layer is caused by grinding, it's specifically called the "grinding heat-affected layer." Analyzing this layer is a key step in assessing the quality of the bearing's outer surface and is an essential part of failure analysis. During the grinding process, two main factors influence the formation of the heat-affected layer: grinding heat and grinding force. 1. **Grinding Heat**: The high temperatures generated during grinding can reach up to 1000–1500°C within milliseconds. This extreme heat can cause several effects on the surface: - **Oxide Layer Formation**: A thin layer of iron oxide (about 20–30 nm thick) forms due to oxidation from oxygen in the air and the steel itself. The thickness of this layer correlates with the depth of the heat-affected zone, making it a reliable indicator of grinding quality. - **Amorphous Layer**: High temperature can melt the surface layer, which then solidifies rapidly, forming a very thin amorphous structure (around 10 nm). This layer has high hardness but is easily removed during fine grinding. - **Tempering Layer**: If the temperature is high enough to exceed the tempering temperature of the workpiece, a tempering layer forms beneath the surface, reducing hardness. - **Secondary Quenching**: When the surface reaches the austenitizing temperature, rapid cooling can lead to martensitic transformation, creating a hard quenching layer. However, this can also result in cracks if the temperature is too high. - **Cracks**: The stress induced by secondary quenching can cause microcracks, especially at grain boundaries, potentially leading to larger cracks over time. 2. **Grinding Force Effects**: In addition to heat, the forces applied during grinding—such as cutting force, friction, and clamping force—can cause plastic deformation and work hardening on the surface. These include: - **Cold Plastic Deformation**: As the grinding wheel wears, the abrasive grains may plow into the surface, creating a plastic deformation layer. The depth of this layer increases with more aggressive grinding. - **Thermoplastic Deformation**: High temperatures reduce the material’s elastic limit, allowing plastic deformation under pressure. This effect becomes more pronounced as the surface temperature rises. - **Work Hardening**: Mechanical deformation during grinding can increase the hardness of the surface layer, detectable through microhardness testing and metallography. Other surface layers, such as decarburization layers formed during casting or heat treatment, can also affect bearing performance. If not fully removed during subsequent machining, they may lead to softening and early failure. In conclusion, the quality of the outer surface of a bearing plays a critical role in its performance and lifespan. Understanding and controlling the effects of grinding and other processes is essential for maintaining high-quality bearings.
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