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

Abstract

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 σ-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 σ-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.