Investigation into mechanical properties of heat-affected zones in high strength S690 steel welded sections

Over the past two decades, many experimental investigations reported about significant reductions in the mechanical properties of high strength S690 steel after welding. In general, under a heating and cooling cycle induced by welding, the high strength steels are heated up rapidly, and then cooled down gradually. Depending on heat input energy during welding and plate thicknesses, phase transformation, recrystallization, and grain growth may take place within the heat affected zones of the welded sections. It is essential to quantify any reduction to the mechanical properties of the heat affected zones in the welded sections in order to predict their structural behaviour. A series of experimental and numerical investigations into the effects of multi-pass welding onto the mechanical properties of typical welded sections of the high strength S690 steel is presented in this paper. Through a physical welding simulation for a wide range of heat treatment onto a total of 48 steel coupons, the effects of phase transformations onto the mechanical properties of the steel were quantified in terms of modified stress-strain curves within specific ranges of critical temperatures Tcr, when compared with those of the base metal, obtained from tensile tests. Heat transfer analyses of welding were then performed to identify specific regions, i.e. critically transformed regions CTR, within the heat affected zones which had experienced Tcr, and hence, had undergone phase transformation, leading to reduced mechanical properties. Structural models with these CTRs and their corresponding mechanical properties are found to be able to predict the deformation characteristics of the welded sections with a high level of accuracy, when compared with test data.