Crack initiation induced by twin-martensite and inclusion in rotatory bending fatigue of a high nitrogen martensite bearing steel

Rotating bending fatigue tests, together with microstructural characterization, have been performed on the high nitrogen martensite bearing steel Cronidur30. The results show that the fatigue stress amplitude gradually decreases with the increasing number of fatigue cycles, with a fatigue strength limit of 867 MPa after 1.0 × 107fatigue cycles. Crack sources due to both inclusions and twin-martensite volumes are found from inspection of fracture surfaces, with nearly 40% of the fatigue failures identified as originating from twin-martensite volumes. Based on a detailed microstructural examination, it is concluded that twin-martensite volumes first fracture locally along the maximum shear stress direction, and these locations then act as the sources for crack propagation and finally resulting in fatigue failure. The stress intensity factors calculation of two types of crack sources initiated by the subsurface shows that the two types of crack sources play basically the same role in fatigue crack initiation. Granular bright facet (GBF) will be formed around the crack source to meet the threshold value of crack propagation. When the initial size of these two crack sources is large enough, the crack can directly propagate without forming GBF. The novel observation of twin-martensite induced crack nucleation highlights the fact that fatigue failures of the investigated bearing steel do not only originate from inclusions, and help to provide an in-depth understanding on the fatigue mechanisms of bearing steels.