Improved hardenability of medium-carbon steel by tuning V distribution at grain boundaries

Vanadium (V) segregation at grain boundaries has been proven to enhance hardenability of steels, while methods of maximizing the effect of V and further improving the mechanical properties of steels still need to be explored continuously. This study introduces a two-step austenitization isothermal process to investigate its effects on enhancing the hardenability and mechanical properties of V-microalloyed steel, i.e., 40CrNiMoV steel. The results demonstrate that the two-step austenitization isothermal process (1000°C × 30 min followed by 860°C × 2 min) enhances both the hardenability and mechanical properties of the test steel compared to conventional single-step treatment (1000°C × 30 min). The specific two-step austenitization isothermal process, i.e., undergoing the first step of the austenitization isothermal process, followed by cooling to 860°C and isothermal holding for 2 min, promotes the uniform segregation of V at grain boundaries in the test steel. This leads to a significant reduction in grain boundary energy, thereby stabilizing austenite and delaying α-phase transformation, which substantially enhances the hardenability. The exceptional hardenability of the test steel promotes the formation of a high density of high-angle grain boundaries in the post-treatment microstructure. Additionally, it refines the martensite/bainite lath thickness. Such dual effects synergistically improve the mechanical properties of the test steel.