Abstract
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.
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