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Factors affecting bearing life and control methods
The early failure modes of rolling bearings typically involve cracking, plastic deformation, wear, corrosion, and fatigue. Under normal operating conditions, the primary internal factors influencing contact fatigue include hardness, strength, toughness, wear resistance, corrosion resistance, and the internal stress state. These properties are significantly affected by both the material composition and the service environment.
1. **Martensite in Hardened Steel**
In high-carbon chromium steel, the original microstructure is usually granular pearlite. After quenching followed by low-temperature tempering, the resulting martensite contains carbon that greatly influences the mechanical properties of the steel. For GCr15 steel, the carbon content in the quenched martensite typically ranges from 0.5% to 0.56%, which provides optimal mechanical properties and resistance to failure. The martensite formed is often referred to as cryptocrystalline martensite, with its carbon content representing an average value.
2. **Retained Austenite in Hardened Steel**
High-carbon chromium steel may retain between 8% and 20% austenite after standard quenching. Retained austenite has both advantages and disadvantages, and its content should be carefully controlled. As the percentage of retained austenite increases, so does the hardness and contact fatigue life, up to a peak point. Beyond that, reducing the amount of retained austenite becomes beneficial only when it remains stable. If it transforms into martensite spontaneously, the toughness of the steel can drastically decrease, leading to embrittlement.
Under low load conditions, retained austenite undergoes slight deformation, which helps reduce stress peaks and promotes the formation of strain-induced martensite. This reduces the negative impact of increased retained austenite on the contact fatigue life. However, under high loads, the large plastic deformation of retained austenite can lead to local stress concentration and cracking, thereby shortening the bearing’s life.
3. **Undissolved Carbides in Hardened Steel**
The quantity, shape, size, and distribution of undissolved carbides in hardened steel depend on the austenitizing conditions, as well as the chemical composition and original microstructure before quenching. When non-spherical carbides are present, they can cause stress concentration at the interface with the matrix, weakening the material's toughness and fatigue resistance.
Quenching can also affect the carbon content and distribution of martensite, further influencing the steel's properties. Excessive undissolved carbides can negatively impact the overall mechanical performance and failure resistance of the steel. However, a small amount of undissolved carbide is necessary for wear resistance. Ideally, the carbides should be fine, evenly distributed, and spherical to ensure good mechanical behavior.
Reducing the carbon content of bearing steel appropriately is one effective way to enhance the service life of components.
4. **Residual Stress After Quenching and Tempering**
After quenching and low-temperature tempering, bearing components still carry significant residual internal stresses. Surface compressive residual stress improves fatigue strength, while excessive stress may lead to part deformation. On the other hand, tensile residual stress decreases fatigue strength.
5. **Impurity Content in Steel**
Impurities such as non-metallic inclusions and harmful elements (e.g., oxygen) have a major impact on the mechanical properties and failure resistance of bearings. Higher oxygen content leads to more oxide inclusions, which generally reduce toughness, ductility, and fatigue life. For bearings operating under high stress, minimizing oxygen content is crucial. While some inclusions like MnS can be elongated and less detrimental, large oxide inclusions may even have a minor or neutral effect on fatigue life.