Fatigue and fracture behaviour of austenitic-martensitic high carbon steel under high cycle fatigue: An experimental investigation

Dual-phase high carbon steels are widely used in mining industries for grinding applications and are thus subjected to repetitive loading for a very large number of cycles. This paper presents the high-cycle fatigue behaviour of austenitic-martensitic high carbon steel and establishes a concurrent structure-property relationship. Cantilever-type rotating bending fatigue test was carried out on hourglass specimens in the high cycle and very high cycle fatigue (VHCF) regime under ambient condition. The S-N curve exhibited a bilinear slope indicating a change in the mode of crack initiation and micromechanism of cyclic deformation. Based on the fracture surface examination, the initiation of the fatigue cracks was categorised into two groups: (a) surface induced (SI) cracks in short life region and (b) non-inclusion (NI) induced subsurface cracks with a fish-eye formation for the long life region. The stress intensity factor at the periphery of the GBF (Delta K-GBF) was observed in the range of 5.2-5.75 MPa m(1/2) which is close to the crack growth effective threshold value. The Kernel average misorientation (KAM) maps revealed that intense localised deformation occurred at the interface of the austenite-martensite phases and then gradually decreased at the interior of the austenite grains. This observation was supported by the transmission electron microscopy (TEM) studies which showed the pile-up of dislocation at the phase boundaries. The three-dimensional surface roughness parameters were found to be increasing with an increase in the number of fatigue cycles thus indicating the increase in the dislocation activity of the material in the long life regime.