The response of accumulated stress and microstructural refinement on cyclic deformation and associated mechanism in high strength bainite-martenite dual-phase steel

We elucidate here the response of accumulated stress and microstructural refinement on the ultra-high cyclic deformation and associated crack initiation mechanism in high strength bainite-martenite dual-phase steel, where non-inclusion induced crack initiation is the dominated failure mode. The non-inclusion induced crack initiation involved nucleation of microvoids in the coarse bainitic lath when the applied stress accumulated. The density of the microvoids increased with the accumulation of applied stress, and the microvoids that were near the grain boundary connected to form a microcrack. Failure occurred when the microcracks combined to form a macrocrack. The premise of crack propagation is that the total stress intensity factor at the crack tip is larger than granular bright facet (GBF) crack growth threshold value. A mathematical criterion is proposed for non-inclusion induced cracking. The study suggested that the probability of crack initiation is reduced when the micro structural constituent, bainitic lath, is refined.