Microstructural Characterization of Inconel Alloy 600 Annealed in the 700 to 1200 F Temperature Range

The possibility of grain boundary microstructural changes occurring in mill annealed Inconel Alloy 600 tubing at steam generator operating temperatures was investigated. Two aspects were considered: (1) Are microstructural changes likely to occur? (2) If they do occur, will they affect the tubing corrosion resistance? Two heats of mill annealed, thermally treated, and solution annealed Inconel Alloy 600 steam generator tubing were annealed in the 700 to 1200 F (371 to 649 C) temperature range and evaluated for microstructural changes. The data were extrapolated, and it was concluded that microstructural changes are not likely to occur at steam generator operating temperatures. This conclusion was substantiated by detailed microstructural analysis of mill annealed Inconel Alloy 600 tubing removed from operating steam generators and C-rings that had been exposed in an autoclave at 630 F (332 C) for 73,250 h. It was further concluded that if microstructural changes that are immeasurable do occur, they will not have an adverse effect on intergranular stress corrosion cracking resistance.