AbstractThe effect of various post-weld heat treatment (PWHT) cycles on the as welded heat affected zone (HAZ) microstructures of C–Mn steels microalloyed with niobium, or niobium plus vanadium, has been studied. Single pass welds were produced at an arc energy of 3·5 kJ mm−1; examination was carried out using optical and electron microscopy, along with hardness and crack tip opening displacement testing. As welded, the C–Mn–Nb HAZ contained a significant proportion of auto tempered martensite. After PWHT at 550°C, isolated hard regions remained, but at 600°C all hard regions had been removed, with a concomitant increase in cleavage resistance. In contrast, ferrite with aligned second phase with lower hardness was found mainly in the as welded HAZ of the C–Mn–Nb–V steel. When the PWHT temperature was raised, HAZ hardness increased to a maximum at 600°C; overaging would be required to obtain improved toughness, although this would soften the parent plate. The results indicate that the current practice of specifying a common heat treatment procedure for steels to a given specification is not satisfactory; allowance should be made for the particular composition and as welded HAZ microstructure.MST/1190