Effect of Microstructural Heterogeneity on Fatigue Limit of As-quenched Low-carbon Low-alloy Martensitic Steel

Fatigue crack initiation and subsequent crack propagation behaviour in as-quenched low-carbon low-alloy steel were examined using a rotating-bending fatigue test and electron backscatter diffraction analysis to clarify the relationship between the fatigue limit and the microstructural heterogeneity of martensite. The as-quenched low-carbon low-alloy steel exhibited a low fatigue limit relative to its ultimate tensile strengths. The fatigue fracture was originated from slip deformation due to dislocation glide in the matrix. Furthermore, a tensile test revealed a low elastic limit in the steel, which can be explained by the movement of high-density mobile dislocations introduced during the transformation. These findings suggest that the low fatigue limit of as-quenched low-carbon low-alloy steel is due to its low elastic limit. Fatigue cracks initiated at prior austenite grain boundaries (PAGBs), at packet boundaries, and parallel to the block boundaries. These crack initiations were triggered by the preferential activation of slip systems parallel to the habit plane in the coarse martensite, which was nucleated at the PAGBs in the early stage of transformation and satisfied the Kurdjumov–Sachs orientation relationship (K–S OR), with not only its own parent austenite grain but also the adjacent austenite grain (i.e. the double K–S OR). Additionally, the initiated cracks were arrested at the fatigue limit. This is probably due to plasticity-induced crack closure stemming from the significant plastic deformation of the early transformed coarse martensite.