Microstructural Evolution and Creep-Rupture Behavior of A-USC Alloy Fusion Welds

Characterization of the microstructural evolution of fusion welds in alloys slated for use in advanced ultrasupercritical (A-USC) boilers during creep has been performed. Creep-rupture specimens involving INCONELA (R) 740, NIMONICA (R) 263 (INCONEL and NIMONIC are registered trademarks of Special Metals Corporation), and Haynes(A (R)) 282(A (R)) (Haynes and 282 are registered trademarks of Haynes International) have been analyzed via light optical microscopy, scanning electron microscopy, X-ray diffraction, and thermodynamic and kinetic modeling. Focus has been given to the microstructures that develop along the grain boundaries in these alloys during creep at temperatures relevant to the A-USC process cycle, and particular attention has been paid to any evidence of the formation of local gamma'-denuded or gamma'-free zones. This work has been performed in an effort to understand the microstructural changes that lead to a weld strength reduction factor (WSRF) in these alloys as compared to solution annealed and aged alloy 740 base metal. gamma' precipitate-free zones have been identified in alloy 740 base metal, solution annealed alloy 740 weld metal, and alloy 263 weld metal after creep. Their development during long-term thermal exposure is correlated with the stabilization of phases that are rich in gamma'-forming elements (e.g., eta and G) and is suppressed by precipitation of phases that do not contain the gamma' formers (e.g., M23C6 and mu). The location of failure and creep performance in terms of rupture life and WSRF for each welded joint is presented and discussed.