STUDIES OF THE WELDABILITY OF STEEL SHEETS BY LASER WELDING PROCESS

Abstract

Advanced high strength steels, such as dual-phase and martensitic steels, are increasingly being used by automotive industry to decrease the thickness of steel sheet used in parts without sacrificing part strength. When welded, the martensite within the dual-phase steel tempers, reducing the heat-affected zone (HAZ) hardness compared to the base material, locally reducing the strength. This process is known as Heat Affected Zone softening (HAZ softening). In this dissertation report the HAZ softening of laser welded dual phase steel is studied. The dual phase steel is produced by the inter-critical heat treatment followed by the rapid water quenching. The heat treatment of low carbon steel SAE 1012 is done at two intercritical temperatures i.e. 760 °C and 820 °C so that the martensitic content of the dual phase steel can be varied. For welding of the produced dual phase steel, the two modes i.e. continuous mode and Sine mode of laser beam were used. It was observed that the more softening occurred in the heat affected zone of the welds welded in sine mode due to more heat input. It was also confirmed that the softening depends on the martensitic content of the dual phase. More softening in the heat affected zone was found in the dual phase steel produced at the intercritical temperature 820 °C. The HAZ softening reduces the mechanical properties of the weld. The properties like tensile strength, hardness, formability and fatigue life are mainly affected due to HAZ softening compromising the strength of the weld. To reduce the softening of laser welded dual phase steel, in-process cooling is applied. Inprocess cooled weld showed some reduction in the HAZ softening. The reason behind this is that the heat affected zone remains exposed to a temperature range of 250 °C to 600 °C for less period of time. As it is known that tempering of martensite is diffusion process and diffusion needs time for its occurrence so less exposure of the weld at a particular temperature prevents less diffusion of carbon in to the dislocation sites and it in turn reduces the softening.