Metallurgical Characterization of a 114,000-Hour Service-Aged Forge 91-Pipe 91 Steel Header Weldment

In this work, multi-scale metallurgical characterizations with advanced microscopy techniques were conducted on an ex-service girth weld between a SA-182 forge 91 (F91) steel reducer to a SA335 pipe 91 (P91) steel header after 141,000 hours (16 years) service at a coal-red power plant between 1991 and 2015. Multiple metallurgical factors, including compositions, inclusions, precipitates, and hardness, were analyzed in comparison for the F91 and P91 steels. The results not only gain an in-depth understanding of creep deformation mechanisms in 9Cr steel welds, but also provide baseline information for remaining lifetime assessments of those service-aged steam components. The results show highly nonuniform creep degradation and damages were observed on the F91 and P91 sides. Base metal and heat affected zone (HAZ) on the F91 steel side experienced a higher degree of microstructure degradation with lower hardness and a higher fraction of creep cavities. Softened zones with lower hardness values were identified in both HAZs of F91 side and P91 side. However, the most creep damaged zones (CDZ) with the highest number density of cavities are not the identified softened zones. In the CDZ, creep cavities are always associated with coarse precipitates (Laves phase, Z phase) and large inclusions (Al oxides, AlN, MnS). A higher fraction of inclusions, coarser precipitates, and larger grain size in F91 steel put itself and its HAZ more vulnerable to creep damages, especially the infamous Type IV cracking.