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We had proposed a creep life assessment method for boiler piping that can consider the heat-to-heat variations of the creep properties of each welded joint, where the creep properties of the welded joint are estimated from those of each base metal. In the method, it is assumed that the creep properties of each base metal in actual pipe are approximately constant in the wall thickness direction of the pipe, and test results with a small sample cut from base metals at an outer surface of the pipe are useful for representing the creep properties of the pipe. In this work, the assumption was examined for six pipes of modified 9Cr-1Mo steel, which had been used for longer than 100,000 h in 600 °C class thermal power stations, and the thickness of the small sample taken from the piping on site was determined. The microstructure, chemical composition, hardness, creep void, uniaxial creep life and small punch creep life were investigated in detail in the wall thickness direction of the pipes. No difference was observed for these items, except in an area less than 0.4 mm from the outer surface of the pipes, which means that the assumption in the assessment method is valid except in this area. On the basis of the results, we determined the thickness of the small sample taken from the 9Cr steel piping as 2.0 mm. Then, we applied the creep life assessment method using the small sample to 600 °C class boiler piping on site, and demonstrated its usefulness for diagnoses of the piping.

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.