The effect of vanadium micro-alloying on the microstructure of welded joints in high-strength structural steels

The balance between high strength and toughness in high-strength-low-alloy (HSLA) steels can be defined by the thermal cycles in the heat-affected zone (HAZ) of a welded joint, during a double-pass welding process with secondary heating in the inter-critical zone (IC CG HAZ). After multiple heating cycles in the temperature range between Ac1 and Ac3, the steel undergoes a strong loss of toughness and resistance to fatigue, mainly caused by the formation of residual austenite (RA). This study aims to investigate the influence of vanadium addition on the behavior of IC GC HAZ in S355-grade HSLA steel. The welding thermal cycles were simulated, considering five different inter-critical temperatures, between 720 and 790 °C. The addition of vanadium as a micro-alloy to an S355 structural steel was found to increase the mechanical strength of the IC GC HAZ zone of a welded joint without compromising toughness and fatigue resistance. This result is obtained through the generation of a bainitic microstructure with dispersion of fine regions of residual austenite and a fine and uniformly distributed precipitation.