Mechanisms of secondary crack initiation, propagation and closure during the water quenching process in medium-carbon martensitic steel

A polycrystalline elastic-plastic phase field model is proposed to reveal the mechanisms of secondary crack initiation, propagation and closure during the water quenching process in medium-carbon martensitic steel. The formation of martensite variants during the quenching process is considered in our model. Moreover, this model can account for the influence of the elastic stress and plastic strain generated after the martensitic transformation during the quenching process on the fracture process. The simulation results show that secondary cracks initiate at the grain boundary region near the primary crack due to its induction. Additionally, they can also initiate at multiple locations in the high-angle grain boundary regions far from the primary crack. This occurs due to elastic stress concentration and plastic strain localization in these regions. Then secondary cracks mainly propagate along prior austenite grain boundary areas. The tensile stress on both sides of the crack tip is the main driving force for crack initiation and propagation. As the external loading increases, the stress at the crack tip gradually transitions into compressive stress, ultimately leading to the closure of the crack in the grain boundary regions. More importantly, these propagation paths of secondary cracks are consistent with the experimental results. Compared with intracrystalline defects, grain boundary defects are more likely to induce crack initiation and propagation. Therefore, this model can offer theoretical guidance for solving the issue of water quenching cracking in medium-carbon martensitic steel.