Complex-inclusion induced fracture mechanism of ultraclean 100Cr6 bearing steel under high cycle fatigue test

Fatigue property of an ultraclean 100Cr6 bearing steel was investigated by the high-cycle rotating bending fatigue tests. It was found that the cyclic loading below 1197 MPa led to the frequent cracking initiated from the complex inclusions with the two distinctly different features: the breaking and the debonding of complex inclusions, all of them have a core of oxide agglomerate containing many cavities and a nearly 2 mu m-thick sulfide shell. Finite element simulation results reveal that this distinction depends on whether a sufficiently high residual stress (RS), which should be developed near the interface of inclusion and matrix during quenching and accumulated during the cyclic loading, can exceed the yield strength (YS) of steel matrix. Higher interfacial RS than the YS leads to the debonding of inclusion whilst lower one to the breaking of inclusion. A more uniform and thicker sulfide shell could relieve the RS to the greater extent, favoring the breaking of inclusion with longer fatigue life. Finally, we propose that the critical sizes for no fatigue crack initiated at the pure oxide, the complex inclusions with non-uniform and uniform sulfide shell are 11.2 mu m, 12.8 mu m and 17.0 mu m respectively, and the optimum size ratio of oxide core to the entire complex inclusion is 0.56-0.76. Both are of great importance on tailoring the inclusions of ultraclean bearing steel for improving the fatigue life of bearing steel.