Resistance Spot Welding of QP Stainless Steels: Role of Heat Treatment and Microstructure on Joint Strength

Abstract

QP steels and other third generation AHSS possess outstanding combinations of strength and ductility, making them very attractive for the automotive sector. However, an Achilles heel of these materials is their rather limited weldability. Despite the obvious importance of this problem, very few works have been published characterising 3rd generation AHSS welded joints. For the current contribution, 3 novel QP stainless steels were investigated. Resistance spot welded joints were prepared following QP treatment of sheets. Results following paint baking revealed a high nugget hardness and a microstructure of martensite, retained austenite and delta ferrite. Cross tension strength was highest for the alloy with an optimum dispersion of retained austenite which improved weld metal toughness despite the high hardness. A further improvement in cross tension strength was realised on tempering at high temperatures, leading to an 60% improvement in strength, thanks to improved toughness of the martensite constituent.