Effect of multiple recurring weld repairs on the properties of a 690 MPa Quenched and Tempered steel

Australia’s Collins class submarines are made from a high strength quenched and tempered steel, BIS812 EMA. This steel has a minimum 690 MPa yield strength and it consists of a combination of macro and micro-alloying elements which contribute to both its high strength and high low-temperature toughness. After long exposure to seawater environments, localised corrosion damage occurs and this can lead to a reduction of steel plate thickness which can adversely affect the structural integrity of the hull. Corrosion damage can be repaired by removing the damaged region and rebuilding the area with weld deposits. Over the life of a vessel, corrosion damage at the same location can occur and recurring repair welding is preferable to plate replacement, as it is a more cost effective and timely procedure. Additionally, recurring butt welds on the hull may be required for the long-term sustainment of the submarines. All these recurring multiple repair welds may have a detrimental effect on the mechanical properties of the base plate, that is, in the heat affected zone (HAZ). There has been limited research published on the effects of multiple repair welds on the properties of steel. From the limited research that has been conducted, the nature of the weld repair, welding type and steel type having been identified as important factors affecting the mechanical properties of the HAZ. Besides, very few of these studies were specifically conducted on quenched and tempered steels. BIS812 EMA steel is a unique steel type; it is a quenched and tempered steel with macro-alloying elements (Cr, Mo, Ni) and microalloying additions (Nb, Ti, V and B). High strength submarine steels are generally macroalloyed and do not contain microalloying additions. As such, the limited research that has been conducted is not readily transferable to the recurring welding of BIS812 EMA steel. This research is focussed on analysing the effect of multiple repair welds on the steel and understanding the underlying mechanisms for the variation of mechanical properties and microstructure.