Microstructure and mechanical properties of laser welded dissimilar joints between QP and boron alloyed martensitic steels

The newly developed quenching and partitioning (QP) steel QP980 and boron alloyed martensitic steel B1500HS were butt-welded using fiber laser at a constant laser power 4.5 kW and various welding speeds of 4∼10 m/min. Retained austenite was measured in the super-critical HAZ of B1500HS (0.9%), inter-critical HAZ of B1500HS (3.4%) and inter-critical HAZ of QP980 (0.6%), while the sub-critical HAZ of QP980 witnessed decomposition of retained austenite from 8.4% to 4.4%. The joints welded at 4 m/min achieved the highest tensile strength, while the significantly softened HAZ of the B1500HS was the weakest region. The dissimilar joint welded at 6∼8 m/min fractured at the base metal of QP980, which were preferred as more crash energy can be absorbed. However, the Erichsen values of these welded joints (∼4.06 mm) significantly decreased compared with the base metal of B1500HS (7.46 mm) and QP980 steel (8.67 mm), as the crack propagated along the softened HAZ of B1500HS.