Vanadium alloying in S355 structural steel: effect in residual austenite formation in welded joints heat affected zone

The inter-critical heat affected zone (ICHAZ) appears to be one of the most brittle sections in the welding of high-strength micro- alloy steels (HSLA). Following multiple heating cycles in the temperature range between Ac1 and Ac3, the ICHAZ undergoes a strong loss of toughness and fatigue resistance, mainly caused by the formation of martensite-austenite constituent (MA). The presence of micro-alloying elements in HSLA steels induces variations in the formation of some microstructural constituents, more or less beneficial, which allow to improve the mechanical performance of a welded joint. The behavior in the inter-critical region of a S355 grade steel with 0.1wt% V addition is reported in this paper. 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 °C and 790 °C. The residual austenite dependence on inter-critical temperature is analyzed and related to the hardness behavior.