Microstructural evolution and mechanical properties of laser beam welded joints between pure V and 17-4PH stainless steel

The microstructures and mechanical properties of joints between 17-4PH stainless steel and pure V were investigated. During laser welding of these joints, the laser beam was first placed at the middle of the joint interface (LW-0) and then shifted by 0.1 mm (LW-0.1) or 0.2 mm (LW-0.2) towards the 17-4PH side. The effects of post-weld heat treatment on the mechanical properties of the welded joints were examined. The microstructure of the LW-0 joint, which was mainly composed of (FeV)(ss) due to uniform mixing of Fe and V, changed to an (Fe)(ss)-rich phase at the 17-4PH//FZ side and (FeV)(ss) at the FZ//V side upon shifting the laser beam towards the 17-4PH side. A small fraction of sigma-phase was formed, which induced crack initiation and propagation, when the laser beam was not shifted. The grain morphologies in the fusion zones were highly dependent on the Fe-V composition. At a V content above 28 at.%, (FeV)(ss) readily forms and a coarse grain structure was observed; below this concentration Fe-rich phases formed with a columnar grain structure upon solidification. The hardness and tensile properties of the joints were dependent on the V concentration in the melt zone. The hardness increased in the order of LW-0.2 < LW-0.1 < LW-0, whereas the tensile strength decreased in the order of LW-0.2 > LW-0.1 > LW-0. The hardness significantly increased after annealing due to the precipitation of the sigma-phase, which was dispersed throughout the fusion zones of LW-0 and LW-0.1, or at the FZ//V interface for LW-0.2.