Stress rupture factors and weld strength reduction factors for Grade 91 weldments in the codes and literatures have been reviewed. Stress rupture factors for weld metals proposed for Code Case N-47 in the mid 1980’s was defined as the average rupture strength of the deposited filler metal to the average rupture strength of the base metal. Remarkable drop in creep rupture strength of weldments is significant issue of Grade 91, especially in the low-stress and long-term regime. A premature failure of Grade 91 weldments in the long-term, however, is caused by Type IV failure which takes place in the fine grained heat affected zone (FG-HAZ), rather than fracture in the deposited weld metal. The stress rupture factor of the Grade 91 steel, therefore, was based on the creep rupture strength of cross weld test specimens. Time and temperature dependent stress rupture factors for Grade 91 have been estimated based on the average creep rupture strength of cross weld test specimen to the average creep rupture strength of base metal.

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