長期使用改良9Cr-1Mo鋼溶接部のクリープ強度特性評価

抄録

It is an important subject to characterize creep strength property of long-term operated Mod.9Cr-1Mo welded joints which are widely used for pipes in USC power plants for accurate creep damage assessment. Miniature creep specimens with 1mm diameter were machined from base metal, weld metal and heat affected zone(HAZ) of an about 80,000 hours operated longitudinal welded pipe with X shape weld groove. Creep tests of these miniature specimens were performed in inert gas atmosphere by the specially developed creep testing machine. Creep strengths of the base metal and the HAZ in the long-term used welded pipe were similar to those in an undamaged welded joint. On the other hand, creep strain rate of the weld metal in used welded pipe is much faster than that in the undamaged welded joint indicating degradation of microstructure during operation. Standard size welded joint specimens were taken from near surface and X-shape regions of the welded pipe to understand creep rupture strength. Creep rupture time of the standard welded joint specimens taken from the used welded pipe was much shorter than that of the undamaged welded joint. The standard welded joint specimens taken from the X shape region ruptured at HAZ and those taken from the near surface region ruptured at weld metal. It was suggested by finite element creep analyses of the standard welded joint specimens that the highest axial stress occurred at HAZ in the specimen taken from X-region causing Type IV failure and that occurred at weld metal in the specimen taken from near surface region causing failure at the weld metal.