The creep rupture strengths of welded joints of Modified 9Cr-1 Mo steel and 1 2Cr steel tend to decrease as comparedwith those of base metals. Therefore, the softening behavior at the heat affected zone (HAZ) was examined by means of mainly welding thermal cycle simulation, and the improvement of the creep rupture strength of GTAW joint was investigated from the standpoint of precipitation strengthening and/or solid solution strengthening, Consequently, the maximum HAZ softening was found at

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... peak temperature around 900'C, which was closely associated with the lack of fine Nb and V carbonitrides coherent with the matrix in addition to the fine grained microstructure formed by recovery and polygonalization of the tem pered martensite. When conducting creep rupture test for their welded joints, the fracture location sh ifted from the base metal or the weld metal into the softened HAZ with increasing the Larson Miller parameter. As far as the improvement of the creep rupture strength of GTAW joints is concerned, the control of morphology of Nband V carbonitrides in the base metal prior to welding by using the thermo-mechanical control process (TMCP) was not effective. On the contrary, the creep rupture properties tended to be improved by increasing W and Cu contents simi[ar to the case of base metal. In conclusion, since the effect of precipitation strengthening is reduced at softened HAZ, the decrease in the creep rupture strength of weldmentto some extent must be taken into account, and it is of importance to improve the strength of weldment by increasing that of base metal resulting from mainly solid solution strengthening. KEY WORDS: Modified 9Cr-1 Mo steel; 1 2Cr steel; creep rupture strength; welded joint; HAZ softening; Nband V carbonitrides; Larson Miller parameter. l.