9Cr鋼配管母材における微小サンプル採取位置の評価

抄録

In the evaluation of the creep property of piping base metals, an important consideration is from which region in the thickness direction small specimens are taken. We conducted microstructural observation, composition analysis, hardness measurement and the creep test to evaluate creep properties in the thickness direction of Gr.91 boiler piping base metals used for a long time in three ultra-supercritical (USC) power plants. In the microstructure near the outer surface, no affected layer was observed. Regarding the composition near the outer surface, carbon concentration fluctuations were observed in the region of about 1 mm at maximum from the outer surface, but a region beyond that showed no fluctuations in carbon and chromium concentrations. Regarding hardness, a change in hardness was observed in the region of about 0.1 mm to 0.2 mm from the outer surface, but in all other regions, the hardness in the thickness direction was almost constant. Regarding the creep test, except for some pipings, no significant difference in creep rupture time was observed in the thickness direction. Therefore, the creep property of Gr.91 boiler piping base metals is considered to be constant in the thickness direction, except for the region about 1 mm from the outer surface.