Microstructure characteristics and mechanical performances of the dissimilar welded joint of Q&P980 and Q&P1180 steels subjected to the paint-baking heat treatment

The dissimilar welded joints of Q&P980 and Q&P1180 steels with and without paint-baking heat treatment were investigated and their microstructures were characterized using confocal laser scanning microscope (CLSM), electron backscatter diffraction (EBSD), scanning electron microscope (SEM) and transmission electron microscope (TEM) in order to understand the change of mechanical performances of the welded joints. The two Q&P steels have slight differences in compositions, but the obviously varying fraction of tempered martensite and thus the difference in the tensile strength. Prior to paint-baking heat treatment, the dissimilar welds exhibit a lower martensite fraction of the intercritical heat-affected zone (ICHAZ) in the Q&P980 side compared to the Q&P1180 side, and it is not affected by the paint-baking treatment. After the treatment, the occurrence of carbides and the coarsening of martensite in the whole welded joint, as well as the decomposition of retained austenite in the ICHAZ, are observed. The paint-baking treatment also causes the decreasing microhardness values of all regions. Especially, the significantly decreasing microhardness in the fusion zone compared to other heat-affected zones results in a partial pull-out failure mode compared to a full pull-out failure mode without paint-baking treatment. The change in failure mode and the softening of the martensitic matrix are found to be responsible for the increase in the peak load and the energy absorption of the dissimilar welded joint of Q&P980 and Q&P1180 steels.