Abstract  
                The purpose of this study was to examine the effect of re-heat treatment conditions on the microstructural refurbishment and development in long-term serviced gas turbine blades, which were made of superalloy grade IN-738. The microstructures of cast polycrystalline nickel base superalloy, IN-738, operated by Electricity Generating Authority of Thailand (EGAT) for long-term services (70,000 hrs) were investigated by SEM in secondary electron mode after different re-heat treatment conditions. It is well-recognized that mechanical properties such as tensile and both low and high cycle fatigue at elevated temperatures as well as creep strengths are all strongly dependent on the morphology of the tested or serviced microstructures. This is concerning to grain structure, strengthened intermetallic precipitating ?’ phase (in size, shape, area distribution and volume fraction), carbide type and its morphology as well as grain (dendrite, in case of casting superalloy) boundary morphology. Therefore, re-heat treatment was linked to the manner, in which rejuvenates microstructures approached in order to re-exploit the high temperature strength of the alloy. It was found that SEM micrograph of the exposed specimen after long-term service could be nearly recovered 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 higher solution annealing temperature resulted in less volume fraction of coarse gamma prime precipitates and less homogeneous in microstructure than the lower one.