Local microstructure evolution of a V-containing Fe–Cr–Ni–Mo weld metal subjected to post-weld heat treatment

Abstract

The high-strength steels are widely used in modern shipbuilding and marine facilities. However, welding of >900 MPa grade steels is particularly challenging owing to the lack of understanding of fine microstructure for the weld metal (WM). The present investigation is mainly focused on a V-containing Fe–Cr–Ni–Mo WM fabricated by gas metal arc welding (GMAW) in aspects of localized microstructure evolution during post-weld heat treatment (PWHT). Microanalysis of the localized microstructure of inter-dendritic regions (IDRs) and dendrite core region (DCRs) for WMs was conducted by transmission electron microscopy (TEM), transmission kikuchi diffraction (TKD) and atom probe tomography (APT), complimented by nanoindentation hardness testing. It is found that the microstructure of WM in as-welded (AW) and PWHT condition was characterized by elongated martensitic laths with high-density dislocations in IDRs, while that in DCRs was coarse bainitic microstructure with relatively fewer dislocations. Most of retained austenite (RA) formed during cooling after welding was decorated martensite lath boundaries within IDRs owing to the segregation of austenite stabilizing elements after solidification. For PWHT condition, more numerous and smaller nano-scale MC carbides were observed in IDRs compared to DCRs. Furthermore, V atoms were mainly responsible for the nucleation of nano-scale MC carbides, and the growth and coarsening process of them were mainly controlled by diffusion of Mo atoms. Besides, the RA could also decompose into nano-scale MC carbides during PWHT. The IDRs of the PWHT specimens showed the average nanohardness value of 7.4 ± 2.3 GPa, which was higher than that of the DCRs (4.0 ± 0.2 GPa).