Effect of 12 Heat Treatment Conditions After HIP Process on Microstructural Refurbishment in Cast Nickel-Based Superalloy, GTD-111

摘要

The present work is an attempt to 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 12 heat treatment conditions for long-term serviced gas turbine blades, cast nickel based superalloy grade 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 microcrack was observed after a long time of HIP process such as for 18 ks HIP time. However, microvoids were still found even after 18 ks-HIP process at 1473 K but in a very small amount. Furthermore, during solution treatment, coarse carbides and over-exposed coarse gamma prime precipitates, which had formed previously during service, would dissolve into the matrix. 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 were performed by using scanning electron microscope and image analyzer after hot isostatic pressing and heat treatments to evaluate the rejuvenated microstructures.