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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 intrinsic factors influencing contact fatigue include hardness, strength, toughness, wear resistance, corrosion resistance, and the internal stress state of the material. These properties are closely related to the microstructure and processing of the steel used in bearing manufacturing.
1. **Martensite in Hardened Steel**
High carbon chromium steel usually has a granular pearlite structure as its original microstructure. After quenching followed by low-temperature tempering, the resulting martensite contains carbon that significantly affects the mechanical properties of the steel. For GCr15 steel, the carbon content in the quenched martensite is generally between 0.5% and 0.56%, which provides optimal failure resistance. The martensite formed is often cryptocrystalline, and the measured carbon content represents an average value.
2. **Retained Austenite in Hardened Steel**
After normal quenching, high carbon chromium steel may contain 8% to 20% retained austenite (Ar). While Ar can have both beneficial and detrimental effects depending on its amount, it should be kept within an appropriate range. As the percentage of Ar increases, so does the hardness and contact fatigue life, up to a peak point. Beyond that, reducing Ar becomes necessary, but only if the remaining Ar remains stable. If it spontaneously transforms into martensite, the steel's toughness can drastically decrease, leading to embrittlement.
Under low load conditions, small amounts of Ar can deform slightly, reducing stress peaks and promoting strain-induced martensite transformation, which helps improve fatigue life. However, under high loads, large plastic deformation of Ar can cause local stress concentration and cracking, ultimately shortening the bearing's lifespan.
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. Undissolved carbides, especially non-spherical ones, can cause stress concentration at the interface with the matrix, thereby reducing toughness and fatigue resistance.
Quenching can also affect the carbon content and distribution of martensite, further influencing the overall properties of the steel. Excessive undissolved carbides are harmful to the mechanical performance and failure resistance of the steel. However, a small amount of undissolved carbide after quenching is necessary for wear resistance. Ideally, these carbides should be fine, evenly distributed, and spherical in shape to minimize their negative impact.
Reducing the carbon content of bearing steel can be an effective way to enhance the service life of the components.
4. **Residual Stress After Quenching and Tempering**
After quenching and low-temperature tempering, bearing parts still retain significant internal stresses. Surface residual compressive stress improves fatigue strength, but excessive stress can lead to part deformation. Conversely, tensile residual stress reduces fatigue strength.
5. **Impurity Content in Steel**
Impurities such as non-metallic inclusions and harmful elements like oxygen can negatively affect the mechanical properties and failure resistance of bearing components. Higher oxygen content leads to more oxide inclusions, which reduce toughness, ductility, and fatigue life. For bearings operating under high-stress conditions, minimizing oxygen content is crucial.
MnS inclusions in the steel can be elongated, while larger oxide inclusions may have little or even beneficial effects on fatigue life. Overall, controlling impurity levels is essential for ensuring long-lasting and reliable bearing performance.
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