Microstructure and toughness of weld heat-affected zone in 785MN m −2 HSLA steel

The effect of microstructure on the toughness of the heat-affected zone (HAZ) in a 785 MN m⁻² HSLA steel has been investigated with narrow-gap welds. Various microstructure regions, i.e. a coarse-grained austenite region, afine-grained region, a partially austenitized region, and a tempered region, were formed periodically adjacent to the fusion line. This structural change was caused by reheating owing to successive weld-metal deposition. Therefore, specimens produced by simulating the multipass weld thermal cycles were also examined to assess the local microstructure and toughness in the HAZ. The transformation behaviour of simulated HAZs subject to reheating thermal cycles was related to the toughness. The partially austenitized structure, within which carbon enrichment occurred during reheating, was decomposed into high-carbon martensitic islands with poor toughness during subsequent cooling. The ductile–brittle transition properties of the HAZ are controlled by the fraction of the partially transformed structure in the HAZ.