Microstructural analysis after reheat treatments and longterm heating in cast nickel base superalloy, GTD-111

Abstract

The microstructures of cast polycrystalline nickel base superalloy, GTD-111, operated by Electricity Generating Authority of Thailand (EGAT) for long-term services were investigated by SEM after different reheat treatment conditions. It was found by SEM investigation that SEM micrograph of the exposed specimen after long-term service could be recovered in some degree by a re-solution treatment followed by primary and secondary aging treatments. During solution treatment, the coarse carbides and gamma prime precipitates were partially dissolved into the matrix. Then specimens were heat treated through series of aging resulting in uniformly dispersed precipitation of gamma prime particles, which is more uniform than those in the longterm exposed microstructure. However, it was also found that the after various reheat treatment conditions, the microstructures are much more uniform than that of the received one. Generally, solution treatment with higher temperature and/or longer duration provided much more uniform microstructural characteristics with higher density of γ particles and in more proper size and shape of the particles than those of lower temperature/shorter duration solutioning. After longterm heating, all microstructures became more degraded, which microstructures with low temperature and/or short duration solutioning provided the most degraded microstructure.