Microstructure Evolution and Precipitates Stability in Inconel Alloy 740H during Creep

Abstract Inconel alloy 740H is designated for boiler sueprheater/reheater tubes and main steam/header pipes application of advanced ultra-supercritical (A-USC) power plant at operating temperatures above 750°C. Microstructure evolution and precipitates stability in the samples of alloy 740H after creep-rupture test at 750°C, 800°C and 850°C were characterized in this paper by scanning electron microscopy, transmission electron microscopy and chemical phase analysis in details. The phase compositions of alloy 740H were also calculated by thermodynamic calculation. The research results indicate that the microstructure of this alloy keeps good thermal stability during creep-rupture test at 750°C, 800°C and 850°C. The precipitates are MC, M23C6 and γ′ during creep-rupture test. The temperature of creep test has an important effect on the growth rate of γ′ phase. No harmful and brittle σ phase was found and also no γ′ to η transformation happened during creep. Thermodynamic calculations reveal almost all the major phases and their stable temperatures, fractions and compositions in the alloy. The calculated results of phase compositions are consistent with the results of chemical phase analysis. In brief, except of coarsening of γ′, Inconel alloy 740H maintains the very good structure stability at temperatures between 750°C and 850°C.