Effect of V on carbide precipitation behavior and mechanical properties of brake disc steel for high-speed trains

With the increasing speed of high-speed trains, the brake disc heat load has also been increasing, particularly during emergency braking. Therefore, to address such issues, strict requirements for brake disc materials are suggested. Thus, the addition of V was implemented to improve the mechanical properties and thermal fatigue performance of brake disc steel. The effect of V on the microstructure and mechanical properties of Cr-Mo-V steel for brake discs at different quenching temperatures was investigated. The precipitation behavior of carbides at different V levels was also investigated through thermodynamics calculation using the Thermo-Calc software, carbon replica, transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS). The results indicate that the amount of V(C, N), which precipitates at high temperatures, increases; therefore, the austenite grains and martensite packets are refined. The precipitates in the tested steels after being quenched and tempered are mainly V(C, N),(Mo,V) C, M₇C₃, and M₂₃C₆. With an increase in V content, the precipitation of large size carbides, such as M₂₃C₆ and M₇C₃, is suppressed; therefore, its negative effect on toughness is reduced. With the increase in the amount of small-size (Mo,V) C, the precipitation strengthening effect is enhanced. When the quenching temperature is in the range of 880-900℃, the increments of vanadium content could refine martensite and reduce the content of large size carbides, which negatively affect toughness. Therefore, the impact energy changes little. When the quenching temperature is in the range of 920-940℃, increasing the vanadium content results in a significant increase in the (Mo, V) C content; therefore, the impact energy drops rapidly. Thus, it is concluded that the quenching temperature of tested steel should not exceed 900℃.