Failure behavior and mechanical properties in the resistance spot welding of quenched and partitioned (Q&P) steels

This study consisted of a comparative evaluation of the mechanical properties and failure behavior of QP980 steel with those of DP980 steel spot welds. Moreover, this paper contributes to the existing knowledge of the resistance spot welding behavior of QP steels by analyzing their microstructure, hardness profile, tensile shear strength, cross tension strength, and failure behavior. It was found that the cross tension and tensile shear test samples all failed in a partial or full button pull-out mode, with an acceptable ductility ratio and cross tension strength as per AWS. Fractography of the testing samples showed that the crack propagated along the fusion boundary, an occurrence previously reported in the literature for QP980, but with no comprehensive explanation regarding the mechanism behind the behavior. This work discusses the similarities between this behavior and welds exhibiting a possible softened region at the fusion boundary, a phenomenon known as the halo ring. The role of this softened region on the failure behavior of the welds is described.