Tempering Procedure Effects on the Impact Toughness in Simulated Heat Affected Zone of Grade 22 Steel

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, impact toughness properties of these regions are inherently low and postweld 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 exposures to elevated temperatures 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 in-service repair of thick steel components where PWHT is impractical or not possible. With TBW, the heat-affected zone is tempered by one or more rapid reheat cycles from overlapping weld beads or weld layers. This study compares the Charpy V-notch (CVN) impact toughness and fracture morphology of the CGHAZ in the as-welded condition, with conventional PWHT, with a single TBW reheat and with multiple TBW reheats for an SA-387 Grade 22 alloy steel.