Heat treatments to improve weldability of new multiphase high strength steels

Thanks to their excellent mechanical properties, the use of multiphase High Strength Steels increases in the automotive industry. Dual Phase steels, Complex Phase steels and TRIP steels are now used to reduce car weight and improve passive safety in case of crash. However, due to their relatively high carbon content (at least higher than micro-alloyed steels commonly used), welding operations become more critical. The main welding process currently used for body-in-white assembly is resistance spot welding. This process is characterized by a high processing speed, resulting in high cooling rates. As a result, the obtained welds exhibit a martensitic structure with high hardness. Welds are then sensitive to the notch effect and present low mechanical properties. Spot welds on HSS are particularly sensitive to peeling conditions. Indeed, the interface between both plates can act as a notch, promoting cracking of the weld in a typical interfacial failure, which is rejected by the car makers. The aim of this paper is to demonstrate that a careful choice of the process parameters can significantly improve the mechanical resistance of the welds. The spot welding process allows the use of in-situ pre- and post-heat treatments. This technique has been developed for several multiphase High Strength Steels to improve mechanical strength of the welds without excessive extension of the total weld cycle.