Vanadium micro-alloying effect on heat affected zone microstructure in welded joints for structural applications

The thermally heat affected zone in the inter-critical temperature range (ICRCGHAZ) appears to be one of the most fragile sections in the welding of high-strength micro-alloy steels (HSLA). Following multiple heating cycles in the temperature range between Ac1 and Ac3, the heat affected zone with coarsened grain undergoes a toughness and fatigue resistance loss, mainly caused by the formation of residual austenite (RA). The presence of micro-alloying elements in HSLA steels promotes variations in the microstructure and in the distribution of some microstructural constituents, more or less beneficial, which allow to improve the mechanical performance of a welded joint. To this aim, to highlight the influence of vanadium on the microstructural variations of a C-Mn steel in the ICRCGHAZ area, the micro-alloyed S355 grade with a V content of 0.1 wt.% was considered. Five double-pass welding thermal cycles were simulated, using a dilatometer, with the maximum temperature of the secondary peak in the inter-critical area, in the range between 720 degrees C and 790 degrees C. The work reports results related to microstructural evolution with main attention to the formation of RA and consequent influence on the Vickers hardness values.