**Analysis of the Causes Affecting the Grinding Metamorphic Layer of FAG Bearings**
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Source: China Bearing Network | Date: March 29, 2014
During the grinding process of FAG bearings, several factors can lead to the formation of a metamorphic layer on the surface of the workpiece. These include both **grinding heat** and **grinding force**, which significantly impact the quality and performance of the bearing.
### 1. Effects of Grinding Heat
The high temperatures generated during grinding can cause various changes in the surface layer of the bearing. The contact area between the grinding wheel and the workpiece generates a large amount of energy, leading to localized high temperatures that can reach up to **1000–1500°C** within milliseconds. This extreme heat can result in several types of surface damage:
- **Oxide Layer Formation**: High temperature oxidation leads to the formation of a thin iron oxide layer (approximately 20–30 nm thick). The thickness of this layer correlates with the grinding conditions and serves as an important indicator of grinding quality.
- **Amorphous Structure**: The intense heat can melt the surface material, creating a very thin amorphous layer (about 10 nm) that cools rapidly. Although it has high hardness, it is easily removed during fine grinding.
- **High-Temperature Tempering**: If the temperature exceeds the tempering range but not the austenitizing point, it can cause a softening effect, reducing the hardness of the surface layer.
- **Secondary Quenching**: When the surface reaches the austenitizing temperature, rapid cooling can lead to martensitic transformation, resulting in a secondary quenching layer. This layer is typically accompanied by a high-temperature tempering zone.
- **Cracking**: Secondary quenching can create stress concentrations, especially in the high-temperature tempering region. This increases the likelihood of cracks forming along grain boundaries, potentially leading to catastrophic failure if severe.
### 2. Effects of Grinding Force
In addition to heat, the mechanical forces from the grinding wheel—such as cutting force, friction, and pressure—can also induce surface deformation:
- **Cold Plastic Deformation Layer**: As abrasive grains cut or plow into the surface, they cause plastic deformation. This is more pronounced when the grinding wheel becomes dull or when the feed rate increases.
- **Thermoplastic Deformation**: At elevated temperatures, the surface becomes softer and more malleable, leading to further deformation under grinding force. This type of deformation does not fully recover during the process.
- **Work Hardening Layer**: Mechanical deformation can increase the hardness of the outer layer, which may be detected using microhardness tests or metallography.
### Additional Factors
Other factors such as **decarburization** from casting or heat treatment processes can also affect the surface properties of the bearing. If not fully removed during subsequent machining, these layers can lead to premature failure.
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