Unveiling M-A chain connectivity: key to optimizing low-temperature toughness in narrow-gap laser wire-feed welding offshore steel

• Research has identified the chain-like distribution of martensite-austenite constituents within the critical heat-affected zone as the primary cause of low-temperature toughness deterioration in thick-walled offshore steel laser welds. • Quantitative analysis confirms that the connectivity index and average chain length of these brittle phases, rather than grain size, govern crack propagation pathways and fracture mechanisms. • A novel martensite-austenite matrix chain-guided fracture mechanism is proposed, elucidating how elastoplastic mismatch between matrix chains and the ferritic matrix accelerates void nucleation and aggregation. Narrow-gap laser wire‒feed welding shows promise for welding thick offshore steel plates, but the origins of heat-affected zone (HAZ) toughness remain controversial. This study examined 20 mm thick EH36 steel joints made under different laser power conditions. Combined characterization techniques were used to explore the relationships between power and toughness and their underlying mechanisms. The results show that the impact toughness changed nonmonotonically with increasing laser power, reaching a peak at approximately 5.5 kW, whereas a shift from dimple to quasicleavage fracture occurred at 6.5 kW, causing the toughness to decrease. The main reason for the toughness loss was identified as the chain-like martensite‒austenite constituents in the intercritical HAZ (ICHAZ). Clarifying the chaining coefficient and conducting quantitative analysis on the average chain length, combined with spatial correlation to high-KAM bands, confirmed that failure occurred mainly along interfaces: the elastoplastic mismatch between the martensite–austenite constituent (M-A) and ferrite increased interfacial stress triaxiality, leading to the formation of voids preferentially at the ends of lath-like M-A and chain intersections. These then propagated quickly along the M-A chains and coalesced. Therefore, the enhanced crack connectivity through the M-A chains in the ICHAZ, rather than the grain size in the coarse-grained HAZ (CGHAZ), mostly explained the decrease in toughness at high power.