Micro-alloying effect on the inter-critical grain coarsened heat affected zone of a S355 steel welded joint (INVITED PAPER)

In this paper the effect of micro-alloying on the inter-critical heat affected zone (ICHAZ) of a welded joint is studied. The motivation of such study is in the well-known recognition of such zone as the lowest ductility one in a welded joint of high-strength low alloyed steels (HSLA). Following repeated heating cycles in in with temperature ranging Ac1 /Ac3, the ICHAZ will face with an evident toughness and fatigue behavior reduction especially due to martensite-austenite constituent (MA) formation. Microalloying in high strength steels causes the generation of some phases in the matrix able to increase the mechanical properties of the joint. In this paper we report an investigation related to up to 1000 ppm vanadium addition in the welded joint of a structural S355 steel. The inter- critical zone of ta double pass welded joint is here reproduced by dilatometer, with second peak temperature ranging 720°C-790°C. The effect of second peak temperature on hardness and microstructure is reported, especially focusing on residual austenite and precipitation, as main element affecting the fatigue behavior.