Abstract
               The present work has an aim to compare the effect of final heat treatment conditions of rejuvenation process on microstructures of two long-term serviced cast polycrystalline nickel base superalloys. The work has also an attempt to possibly obtain the most suitable and practicable repair-condition, which could provide the desired microstructural characteristics by rejuvenation method of hot isostatic pressing (HIP) followed by various heat treatment conditions for long-term serviced gas turbine blades, casting nickel base superalloys grade IN-738 and GTD-111, operated by Electricity Generating Authority of Thailand (EGAT). The hot isostatic pressing could mostly heal any internal structural voids and cracks (by means of sintering), which were generated during service. It was found that no any microcrack was observed after longer time of HIP process such as for 5 hours HIP time. However, microvoids were still found even after 5 hours of HIP process at 1200?C but in very small amount. Therefore, both size and amount of remain microvoids should be considered comparing to both obtained original material and after long-term service one. Furthermore, during solution treatment, coarse carbides and over-exposed gamma prime precipitates, which formed previously at the grain boundaries during service by creep mechanism, would dissolve into the matrix. Then specimens will be processed through a series of precipitation aging, which re-precipitates the strengthening phase to form the proper morphology in size and shape as well as distribution that is almost similar to the new one. Metallurgical examination of the microstructure had been performed by utilizing scanning electron microscope after hot isostatic pressing and heat treatment to evaluate the micro-defects elimination.