Cracking of Grade 91 Steel Welds in Longer-Term Service – Case Studies

High temperature steam piping systems fabricated using Grade 91 (9Cr-1Mo-V-Nb) steel components have been in service since the early 2000’s, with many systems now having accumulated over 100,000 hours of operation. There are now an increasing number of failures associated with longer-term service damage under normal operating conditions rather than due to initial fabrication and installation issues. This paper presents three recent example cases of cracking in Grade 91 steel welds in longer-term service: one trunnion attachment weld and two girth butt welds. All the cases were in large diameter hot reheat piping, with the service exposure of the welds ranging from approximately 85,000 to 150,000 hours. Cracking in all cases occurred by creep damage (cavitation and microcracking) in the partially transformed heat-affected zone (PTZ, aka Type IV zone) in the base metal adjacent to the welds. The location and morphology of the cracking are presented for each case along with operating conditions and potential contributors to the cracking, such as system loading, base metal chemical composition, and base metal microstructure.