Effect of inhomogeneity on the microstructural evolution and mechanical behaviour of a vanadium-containing Fe–Cr–Ni–Mo weld metal

Abstract

Abstract Precipitation and mechanical behavior of a novel Fe–Cr–Ni–Mo weld metal containing different V contents in as-welded (AW) and post-weld heat treatment (PWHT) conditions were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), and atom probe tomography (APT). The results showed that the dislocation density increased significantly with an increase in the V content for PWHT weld metal owing to a higher volume fraction of the discrete nano-scale MC carbides pinning the dislocations. Therefore, strengthening contributions were mainly attributed to the precipitation strengthening and dislocation strengthening for PWHT weld metals containing V. The segregation of strong carbide forming elements (V and Mo) at inter-dendritic (ID) region, which was produced by solute redistribution during solidification, exhibited a major contributor to the continuous diffusion of C element from dendrite core (DC) to ID region during PWHT, so-called uphill diffusion. Also, TEM and APT showed that an inhomogeneous distribution of carbides owing to the V and Mo elemental segregation as well as the higher density dislocation at ID segregation, exhibiting that nano-scale M2C or MC carbides were mainly precipitated in ID region whilst large M3C carbides were mainly developed in DC region for PWHT weld metal.