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 re-heat treatment conditions. It was found that SEM micrograph of the exposed specimen after long-term service could be recovered in some degree by a re-solution treatment followed by two-step 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 long-term exposed microstructure. However, it was also found that the lower (1125?C) and/or higher (1175?C) solution annealing temperatures resulted in less total volume fraction of coarse gamma prime precipitates and less homogeneous distribution in microstructure than the medium (1150?C) one. Furthermore, the increase of inserted primary aging temperatures (925?C and 1055?C) resulted in an increase of gamma prime particle size, especially in coarse gamma prime particles. The re-heat treatment condition with solutioning at 1150?C for 2 hours following with primary aging at 1055?C for 1 hour and secondary aging at 845?C for 24 hours was the most proper one comparing to other re-heat treatment condition in this study. Such microstructure consists of the maximum total volume fraction of gamma prime particles (56.34%), which was very close to the expected total volume fraction of 60% in previous works. The microstructure also provided the maximum average size of both coarse and very fine gamma prime particles. This modified re-heat treatment condition even resulted in the superior microstructure comparing to the reheat treated microstructure with standard heat treatment.