Effect of Heat Treatments after HIP Process on Microstructure Refurbishment in Cast Nickel-Based Superalloy, IN-738
Keywords:Nickel base superalloy structure degradation Hot Isostatic Pressing (HIP), Rejuvenation, Re-heat treatment, Microstructural healing
The aim of this research work is to investigate the most convenient and practicable repair procedure, which could provide the appropriate microstructure characteristics in exposed turbine blades. The blades were cast from nickel-based superalloy IN-738 and were long-term exposed under conditions of land base gas turbine operation for 50,000 hours by EGAT of Thailand. The rejuvenation process includes the hot isostatic pressing (HIP) which is followed by a series of various heat treatment conditions involving either precipitation aging and or both solutioning and precipitation aging. The HIP was carried out at a pressure of 100 MPa for 5 hours at 1,200°C Then HIPed specimens were vacuum heat treated. The hot isostatic pressing could mostly heal any internal structural voids and cracks, which were generated during service. It was found that any microcrack was observed after the HIP process was exposed for 5 hours. However, microvoids were still found even after the HIP process but in very small amounts. Therefore, both the size and the amount of remained microvoids should be evaluated with respect considering both obtained virgin original material and after long-term exposed material. Furthermore, during solution treatment, coarse carbides and over-exposed gamma prime precipitates, which were formed and modified previously at the grain boundaries during service by the creep mechanism, are expected to the matrix. Then specimens were 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 an optical microscope and a scanning electron microscope after hot isostatic pressing and heat treatment to evaluate the micro-defects elimination.
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