Creep property of abnormal structure formed in long-term used Mod.9Cr-1Mo steel weldments and its effect on creep strength of welded joint are presented. The abnormal structure is defined as a structure in which the hardness is clearly lower than that in the normal region, and the grain is coarsened. Creep strength of the abnormal structure is significantly lower than that of normal weld metal and base metal. The creep remaining life of aged materials in the actual service condition is estimated using standard specimen. The predicted creep lives with abnormal structure are shorter than those without abnormal structure and the data are correlated below 99% confidence lower limit of welded joint of Gr. 91 steel. Full thickness welded joint specimen consisting of more than half abnormal structure in the weld metal ruptured at the weld metal before the HAZ. Time to creep rupture decreased with increasing the abnormal structure ratio in thickness direction. Finally, a simplified prediction method is developed to predict creep life of welded joint with abnormal structure formed in weld metal.
