Effect of microstructural heterogeneities on variability in low-temperature impact toughness in multi-pass weld metal of 420 MPa offshore engineering steel

Multi-pass steel weld metal is heterogeneous in nature due to spatial variations of multiple thermal cycles which leads to variations in mechanical properties. In this research, 420 MPa offshore steel was welded in the 1G, 2G, and 3G positions, and the fusion zone was divided into as-deposited weld metal (AD-WM) and reheated zone (RHZ). Subsequently, the sub-regions of the RHZ were identified, and their microstructural constituents, including martensite–austenite (MA) constituents and inclusions, were characterized and quantified. The results indicated that reheating the weld metal by the subsequent passes decides the area fraction of the RHZ in the weld metal and thus the impact toughness. The low heat input in the 2G sample led to the formation of alternate layers of AD-WM and RHZ throughout the fusion zone. The higher cooling rate led to the formation of fine acicular ferrite (AF) microstructure and thus high impact toughness (120 kJ). Increasing the heat input in the 3G and 1G samples resulted in the transformation of the AF to polygonal ferrite (PF), thereby decreasing the impact toughness. Increased number of passes in the 1G specimen led to higher area fraction of the RHZ (39.5%) than in the 3G sample (34%). Thus, the 3G sample exhibited higher low-temperature impact toughness (80 kJ) than the 1G sample (47 kJ). Thus, higher heat input and number of layers would result in multiple heating of the RHZ, which can be detrimental to the toughness.