Thickness of small sample for creep property evaluation of 9Cr steel base metal of piping on site

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.