Rotational bending fatigue life and fatigue crack initiation mechanism of Cr4Mo4V bearing steel
GUO Jun1,2, YANG Mao-sheng2, LU De-hong3, LI Xin-yu2,3
1. Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, China;
2. Institute for Special Steel, Central Iron and Steel Research Institute, Beijing 100081, China;
3. Faculty of Material Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China
Abstract：The rotational bending fatigue life and fatigue crack initiation mechanism of Cr4Mo4V bearing steel were studied by means of rotating bending fatigue test at room temperature. The rotational bending fatigue test was carried out on a PQ1-6 rotary bending fatigue test machine. The fatigue limit and S-N curve were measured by the up-down method. The fracture of the fatigue specimen was observed by SEM, and the crack initiation source type and crack propagation behavior were analyzed. The effect of defect size on fatigue life was analyzed by the ratio σ'/σw,defect of the nominal stress amplitude at the location of the defect to the measured ultimate fatigue strength of the defect. The results show that the safety fatigue limit of Cr4Mo4V bearing steel is 1019MPa, and the S-N curve data of Cr4Mo4V bearing steel shows a downward trend and a large dispersion. Fracture observation shows that there are five types of initiation, namely surface defects leading to initiation, near-surface carbides leading to initiation, near-surface non-metallic inclusions leading to initiation, internal non-metallic inclusions leading to initiation and internal carbides initiation. Internal crack fractures have fish-eye features. When the fatigue life exceeds 107 cycles, a granular bright area (GBF) is observed in the vicinity around the internal initiation source carbides. The fatigue fracture was observed with broken carbides, and the broken carbide increased the crack growth rate. Carbides cracking can occur in the bearing steel Cr4Mo4V under the action of cyclic stresses. These fractured carbides attract the crack tip leading to faster growth rate. Influence of carbide size on the near surface initiation can be quantitatively analyzed by using the critical volume density of carbides. The values of σ'/σw,defect are higher than 1 and the larger the σ'/σw,defect, the shorter the fatigue life is.
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