The early failure modes of rolling bearings mainly include: cracking, plastic deformation, wear, corrosion and fatigue. Under normal conditions, the main intrinsic factors of contact fatigue are hardness, strength, toughness, wear resistance, corrosion resistance and internal stress state. Outside the service conditions)
1. Martensite in hardened steel
High carbon chromium steel original structure: granular pearlite
Quenching + low temperature tempering: carbon content in quenched martensite M, which obviously affects the mechanical properties of steel
The carbon content of GCr15 steel quenching M is 0.5% to 0.56%, and the comprehensive mechanical properties with the strongest failure resistance can be obtained.
M: cryptocrystalline martensite, the measured carbon content is the average carbon content.
2. Retained austenite in hardened steel
High carbon chromium steel may contain 8% to 20% Ar (residual austenite) after normal quenching.
The Ar in the bearing parts has advantages and disadvantages, and the Ar content should be appropriate.
The hardness and contact fatigue life of Ar%↑ increase with the peak value, and then the beneficial effect of reducing Ar must be under the stable state of Ar. If it is spontaneously transformed into martensite, the toughness of steel will be drastically reduced. And embrittlement.
When the load of the bearing is small: Ar is slightly deformed, which reduces the stress peak, and strengthens the deformed Ar processing and stress-strain induced martensite transformation. The influence of the increased amount of Ar on the contact fatigue life is reduced. When the load is large: the large plastic deformation of Ar and the local body will locally cause stress concentration and break, thus reducing the life.
3. Undissolved carbides in hardened steel
The amount, morphology, size and distribution of undissolved carbides in hardened steel are affected by the austenitizing conditions due to the chemical composition of the steel and the original microstructure before quenching.
When the load (especially the carbide is non-spherical) and the matrix cause stress concentration and cracks, the toughness and fatigue resistance are lowered.
Quenching of undissolved carbides affects the properties of steel
It affects the carbon content and Ar content and distribution of quenched martensite, which has an additional effect on the properties of steel.
Excessive quenching of undissolved carbides is detrimental to the overall mechanical properties and failure resistance of the steel.
A small amount of undissolved carbide after bearing steel quenching is necessary for wear resistance. Fine grained cryptocrystalline martensite is required. Less undissolved carbides are required (less quantity), smaller (small size), and uniform (the sizes are very small, And evenly distributed), round (each carbide is spherical).
Properly reducing the carbon content of bearing steel is one of the ways to improve the service life of parts.
4. Residual stress after quenching and tempering
After quenching and low temperature tempering, the bearing parts still have large internal stress.
When the surface residual compressive stress increases, the fatigue strength increases (excessive residual stress may cause deformation of the part). When the residual internal stress is tensile stress, the fatigue strength is lowered.
5. Impurity content of steel
Impurities: non-metallic inclusions harmful elements (acid soluble), such as the higher the oxygen content, the more oxide inclusions, the influence of impurities in steel on the mechanical properties and the failure resistance of the parts and the type, nature, quantity and size of the impurities and Shape-related effects generally reduce toughness, ductility, and fatigue life. For bearing parts that work under high stress, the oxygen content of the steel must be reduced. The MnS inclusions in the steel can be wrapped in an ellipsoid shape. Large oxide inclusions have little or even beneficial effect on fatigue life reduction.
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