Remaining lifetime assessment of 141,000-hour service-aged forge 91-pipe 91 steel header welds with novel in-situ digital image correlation

Condition assessment of welded steam components is a crucial activity for safe power plant operations. In this work, remaining lifetime of a 141,000-hour service-aged large bore girth weld between an SA-182 forged 91 (F91) reducer and an SA-335 pipe 91 (P91) header was assessed with a specially designed in-situ high temperature digital image correlation (DIC) system. Nonuniform creep deformation/damages in this service-aged multi-pass F91-P91 welds were quantified and characterized. The in-situ DIC strain measurements reveal the nominal creep strain averaged over the gauge length was consistently below 3%, and the localized creep strain value at failure in the HAZ was ∼10%. Advanced metallurgical characterization was executed in the as-received and posttest conditions to determine the contributions of preexisting creep damage, the so-called “soft zone” in the HAZ, and microstructural variations in the HAZ to the observed Type IV failure. Creep failure occurred in the most predamaged region and this location is adjacent to, but not inside, the documented soft zone before the test. The higher fraction of inclusions in F91 steel contributed significantly to the lower creep resistance observed in both base metal and HAZ.