AbstractThe creep properties of simulated heat-affected zone, weld metal and cross-weld specimens of tCr0.5Mo and cross-weld specimens of 2.25Cr tMo have been studied with reference to the effect of postweld heat treatment (PWHT). A reduction in the creep rupture strength of up to 20% has been observed after PWHT. Contrary to what has been previously proposed, PWHT was in the present investigation found to have only a marginal influence on the creep ductility. The creep rate can increase by up to a factor of 2-4 after postweld heat treatment, probably because of-carbide coarsening during the heat treatment. Preheating at 200°C did not affect the creep properties of 2.25Cr lMo. Overheating, on the other hand, at 650°C for 1000h reduced the rupture time of tCr0.5Mo by almost an order of magnitude. The tensile strength and yield strength of tCrO.5Mo were significantly reduced after postweld heat treatment. Reductions of up to 100-200MPa were observed both at room temperature and at 550°C in some cases.