Creep degradation in welds of Mod.9Cr-1Mo steel

Creep life assessment technology has not yet been developed for Mod.9Cr-1Mo steel welded joints due to the unique degradation and failure mechanisms in the heat affected zone. Nevertheless, there is strong demand from power plant operators for the development of nondestructive damage detection and life assessment technology for weldments of this steel. In this study creep rupture testing using a large-size welded joint was conducted, and creep and damage detection tests were carried out to elucidate the degradation mechanism. Also the microstructure and hardness changes in the heat affected zones of the welds were investigated to clarify the degradation mechanisms of such welds in comparison with base metal during creep. In general, the changes in hardness distribution along the welds were difficult to correlate with the life consumption, although degradation in welds due to creep was successfully detected. Accordingly, a new approach to degradation evaluation and creep life assessment by hardness measurement method for Mod.9Cr-1Mo steel welds is proposed.