The microstructure and fracture mode of physically simulated heat-affected zones of a weld metal used with 500 MPa offshore steel – part 2: fractographies, inclusions and microstructures

Abstract

Welded joints in 500 MPa offshore steels are often the weakest points in structures, and multiple welding passes for thick sections can affect the microstructure of the weld metal. While much research focuses on heat-affected zones (HAZ) in the base metal, this study examines the HAZ in the weld metal, comparing the original weld’s microstructure, inclusions, and fracture modes to simulated coarse-grained (CGHAZ-W) and intercritical HAZs (ICHAZ-W) in the weld metal. The original weld was produced using submerged arc welding, and HAZs with different cooling times (t_8/5 = 5, 15, 30 s) were simulated using a Gleeble 3500 thermomechanical simulator. Microstructural analysis, inclusion measurements, and fractography were performed using a field emission scanning electron microscope. The results showed that thermal cycles altered the microstructure of CGHAZ-W and ICHAZ-W compared to the original weld. The shortest cooling time (t_8/5 = 5 s) led to slight hardening in CGHAZ-W. Inclusions, mainly oxides or oxysulfides, increased significantly in CGHAZ-W, but not in ICHAZ-W. Most samples exhibited ductile fractures, with microvoid nucleation, although local brittle regions were detected, linked to grain boundary ferrite and side-plate ferrite in an acicular ferritic matrix.