Effect of carbide precipitation behavior at high temperatures on microstructure and mechanical properties of M50 steel

By means of microstructure observation, phase analysis and mechanical-property tests, the effect of carbide precipitation behavior at high temperatures on microstructure and mechanical properties of M50 steel was studied. Results show that during the quenching process, the 5-min heat holding at 790 °C promotes the precipitation of carbides along grain boundaries. During the process, the carbon and alloying elements such as Cr, Mo and V are depleted near grain boundaries, where the solution strengthening effect and corrosion resistance are reduced, so after the corrosion treatment, the grain boundary characteristics are more obvious. After isothermal quenching, compared to the steel without short-term heat holding, the retained austenite in the steel with the short-term heat holding at 790 °C is more stable and the content is more. However, after tempering, the precipitation of grain boundary carbides reduces the stability of retained austenite, and the content of retained austenite in the steel with the short-term heat holding is lower than that without the holding. The short-term heat holding at the high temperature results in the formation of a carbide shell in grain boundaries, and when the steel is subjected to impact, tensile, and rotational bending loads, cracks are easy to initiate at grain boundaries, which significantly reduces the impact toughness, plasticity and rotational bending fatigue properties of the steel, and the hardness and the strength of the steel are also little reduced.