WELD SOLIDIFICATION BEHAVIOR AND PERFORMANCE OF NICKEL-BASE SUPERALLOY GH750 FOR USE IN 700°C ADVANCED ULTRA-SUPERCRITICAL BOILER

A new nickel-base superalloy GH750 has been developed as boiler tube of advanced ultra-supercritical (A-USC) power plants at temperatures about and above 750 degrees C in China. This paper researched the weld solidification of GH750 filler metal, microstructure development and property of GH750 welded joint by gas tungsten arc weld. Liquid fraction and liquid composition variation under non -equilibrium state were calculated by thermo-dynamic calculation. The weld microstructure and the composition in the dendrite core and interdendritic region were analyzed by SEM(EDX) in detail. The investigated results show that there is an obvious segregation of precipitation -strengthening elements during the weld solidification. Titanium and Niobium are the major segregation elements and segregates in the interdendritic region. It was found that the changing tendency of the elements' segregation distribution during the solidification of GH750 deposit metal is agree with the thermodynamic calculation results. Till to 3,000hrs' long exposure at 750 degrees C and 800 degrees C, in comparison with the region of dendrite core of solidification microstructure, not only the coarsening and the accumulation of y' particles are remarkable in the interdendritic region, but also the small quantity of the blocky and needlelike ri phases from. The preliminary experimental results indicate that the weakening effect of creep-rupture property of the welded joint is not serious compared with GH750 itself.