Influence of welding pass on microstructure and toughness in the reheated zone of multi-pass weld metal of 550Â MPa offshore engineering steel

The objective of this paper is to study the influence of thermal cycles produced by the later welding passes on the properties of the sub-regions in reheated zone of multi-pass weld metal for a 550 MPa grade offshore engineering steel. A Gleeble-3500 simulator was applied to simulate microstructural evolution in sub-regions of the reheated zone and its effect on the properties. The results indicated that the reheating process changed the prior austenitic morphology from columnar structure to equiaxed structure and similar columnar structure with quasi-polygonal ferrite (QPF) or blocky M-A (martensite/austenite) constituent distributed on the grain boundaries while the matrix microstructure (acicular ferrite) changed slightly. Charpy impact results indicated that WM region (as-deposited) had the highest impact energy. However, the actual impact sample showed lower impact energy because the machined notch contained one or more brittle reheated zones. In these brittle reheated zones, the necklace-type M-A constituent was hard phase decorating prior columnar or equiaxed austenite grain boundaries, which yielded stress concentration significantly and was mainly responsible for lower toughness of the entire weld metal. Fortunately, this deterioration in toughness could be reduced as decomposition of necklace-type M-A constituent occurred due to later welding passes.