Microstructural Evolution during Creep of Alloy 800HT in the Temperature Range 600 deg C to 1000 deg C

摘要

The microstructure of SANICRO 31HT (alloy 800HT) creep tested to a maximum of 85,388 hours in the temperature range of 600 deg C to 1000 deg C was investigated. Coarse Ti(C, N) precipitates were found to form in the melt and remained stable after solution annealing and aging at all temperatures investigated. M_(23)C_6 precipitating in the range of 600 deg C to 700 deg C was found in two different distributions: as intergranular precipitates and as small intragranular particles. The gamma'precipitation sequence was followed at 600 deg C, 650 deg C, and 700 deg C, and the volume fraction and precipitate diameter was assessed using energy-filtered transmission electron microscopy (EFTEM). The gamma'precipitates grew and coarsened slowly at 600 deg C (10 to 30 nm) but somewhat faster at 650 deg C (25 to 50 nm). The volume fraction was largest at 600 deg C. The gamma' precipitates formed at 700 deg C were not homogeneously distributed in the matrix. Instead, gamma' was observed in the immediate vicinity of M_(23)C_6 and Ti(C, N) precipitates. Atom probe field ion microscope analysis of gamma'-precipitate showed that it contained slightly more Ti than Al and that its Fe content was a few atomic percent. Nitrogen uptake was very pronounced in creep-deformed material aged at 1000 deg C, and A1N formed as grain boundary needles and in the interior of grains.