OM Study of Effect of HIP and Heat Treatments on Microstructural Restoration in Cast Nickel Based Superalloy, IN-738

The present work aims at studying and searching for the most suitable and practicable repairing condition for microstructural restoration, which could provide the desired microstructural characteristics by rejuvenation method of hot isostatic pressing (HIP), followed by 12 heat treatment conditions for long-term serviced gas turbine blades made of cast nickel base superalloy grade IN-738 operated by Electricity Generating Authority of Thailand (EGAT). During heat treatment process, coarse carbides and overexposed coarse gamma prime precipitates, which had formed previously during service, would dissolve into the matrix during solution treatment. Then specimens were processed through a series of precipitation aging, which re-precipitated the strengthening phase to form the proper morphology in size and shape as well as distribution that is almost similar to the new one. Metallography examination had been performed by using light optical microscopy after hot isostatic pressing and heat treatments to evaluate the rejuvenated microstructures.