Suppressing self-tempering-induced M–A coarsening and carbide cluster formation to enhance fracture and fatigue resistance of 500 MPa wind power steel

This study investigates the influence of the water-cooling termination temperature during thermomechanical controlled processing (TMCP) on the microstructural evolution, low-temperature impact toughness, and fatigue crack growth behavior of 500 MPa-grade steels for wind power applications. By adjusting the cooling process, the formation of carbide clusters was suppressed and the morphology and size of M–A constituents were optimized. Two experimental steels, WC620 and WC500, were fabricated under different water-cooling termination conditions. The microstructures were characterized using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD). The results reveal that both steels consist of ferrite and bainite matrices. In WC620, coarse M–A constituents and tempering-like carbides were observed, whereas WC500 exhibited finer grains, a higher fraction of high-angle grain boundaries (HAGBs), and smaller M–A constituents. These refined microstructural characteristics significantly enhance low-temperature toughness and crack propagation resistance. The CTOD value increased from 0.119 mm in WC620 to 0.546 mm in WC500, while the fatigue crack growth threshold ΔK th also increased markedly, indicating superior fatigue performance. Fractographic observations combined with EBSD analysis further elucidate the crack initiation and propagation mechanisms. The results demonstrate that lowering the water-cooling termination temperature promotes refinement of M–A constituents, increases the fraction of HAGBs, and eliminates carbide clusters, all of which effectively inhibit crack propagation. This work provides a fundamental understanding and practical guidance for optimizing cooling parameters to enhance the fracture performance of high-strength steels used in wind power structures.