長期使用改良9Cr-1Mo鋼ボイラ溶接部のミニチュア試験片によるクリープ強度評価

抄録

Mod.9Cr-1Mo steels are being used for boiler pipes in ultra-super critical power plants. It is known that the creep strength of the welded joint of the steel is significantly lower than that of the base metal. In this study, miniature test specimens with 1mm diameter and 5mm parallel length were machined from a base metal, weld metal and heat affected zone (HAZ) of a long term used welded joint. Using these test specimens, the creep tests were performed by the special designed creep testing machine. Creep strengths of the base metal and HAZ were similar to unused materials, but remarkable reduction of the creep strength was observed in the weld metal in comparison with the unused materials. Material constants of the Norton's law were determined from creep data obtained by the miniature creep test specimens. Creep tests were also conducted with the miniature and standard size welded joint specimens. The rupture time of the miniature test specimens was shorter than that of the standard specimens. From the results of the finite element elastic creep analysis, the reason why the rupture time of the miniature specimen is shorter than that of the standard specimen, is that triaxiality in the HAZ of the standard specimen is higher than that of the miniature specimen.