Heat Treatment of a Candidate Material for 700 °C A-USC Power Plants

The preliminary chemical composition optimization and heat treatment parameters of Inconel 740H were investigated systematically. Six heats of experimental alloys (designated as heat 401 to heat 406) with varied chemical compositions were subjected to standard heat treatment at four different solution temperatures. Due to its superior combination of strength, ductility, and toughness together with its minor mechanical property changes at different solution temperatures, heat 405 was chosen as an optimized one to conduct further heat treatment investigations. The evolution of grain size with solution temperature for heat 405 was studied and the optimal solution temperature was determined. After solution treatment at the optimal temperature, four different cooling rates were applied to investigate the effect of cooling rate on the size distribution modal and mean radius of the γ′ precipitates. No bi-modal size distribution of γ′ precipitates was found for all cooling rates and water cooling was recommended for Inconel 740H. In addition, the effect of pre-aging at 800 °C for 16 h on the microstructure of Inconel 740H aged for a long time was studied. It is found that pre-aging at 800 °C would result in bi-modal size distribution of γ′ precipitates after long time aging at 750 °C compared with mono-modal size distribution in samples without pre-aging, which might be caused by the difference in initial size distribution modal before Ostwald ripening of γ′ precipitates in samples with and without pre-aging at 800 °C.