Abstract
Welding of thick low alloy steel components without elevated preheat imposes rapid cooling rates that may promote martensite formation in the coarse-grained heat affected zone (CGHAZ). Freshly formed martensite contains a distorted and supersaturated matrix with high dislocation densities. As a result, fracture and impact toughness properties of these regions are inherently low and post weld tempering procedures are typically required. Conventional postweld heat treatment (PWHT) involves extended isothermal holds at elevated temperature which allows for considerable softening of the martensitic structure but has been found to have less effect on improving impact toughness. In contrast, short-term tempering processes with rapid heating and cooling rates and short times at elevated temperature have been shown to greatly improve impact toughness through a shift in the kinetics of martensite tempering that produces high number densities of refined carbides. Temper bead welding (TBW) was developed for inservice repair of thick steel components where PWHT was impractical or not possible. With temper bead welding, the heat-affected-zone is tempered by one or more rapid re-heat cycles from overlapping weld beads or weld layers. This study compares the Charpy V-notch impact toughness and fracture morphology of the CGHAZ in the as-welded condition, with conventional PWHT, single temper bead reheat, and multiple temper bead reheats for an SA-387 Grade 22 alloy steel.
