Assessment of effect of miniature sample scoop on creep life of 9Cr steel piping

Abstract

The effect of taking a miniature sample scoop on the creep life of ASME Grade 91 (9Cr steel) piping was experimentally assessed. Internal pressure tests were conducted at 650oC on tubular specimens of virgin and long-term used 9Cr steels having scoop on the outer surface, which simulate taking a sample scoop. When the ratio of scoop depth to wall thickness of the specimen was 8%, the creep life decreased to 85% of that of scoop-free specimen. On the other hand, no creep life reduction was observed in the materials with the depth ratio of 5%. Finite element analyses were conducted on the tubular specimens under the test conditions and actual-size piping under actual service conditions, where the ratio of the scoop depth was about 5% in both cases. The stress and strain concentration states near the scoop of the actual-size piping were almost the same as those of the tubular specimens, indicating that the test results were valid for the actual piping. Thus, it was concluded that taking a miniature sample to a scoop of up to 5% of the thickness of the steel has a negligible effect on the creep life of the piping.