Microstructure Characteristics of Rejuvenated Service Exposed Gas Turbine Blade under the Influence of Various Solution Temperature and Cooling Rates

The present research aims to investigate the effect of parameters such as full and partial solution temperature and cooling rate on the final microstructure obtained by rejuvenation heat treatment of used IN738LC Ni-based superalloy gas turbine blades. After investigating the damages caused by the high-temperature service, microstructural rejuvenation was performed using several heat treatment cycles. The final microstructure after each cycle was observed by Field Emission Gun Scanning Electron Microscopy (FEG-SEM). Experimental results indicate that the standard (conventional) heat treatment cycle could not recover service-induced deterioration of the microstructure. Full solution heat treatment at 1200 °C recovered the damages generated during service conditions, such as the continuous γ′ and carbide film at the grain boundaries, due to working at elevated temperature by dissolving and coarsening γ′. Also, the γ′ precipitate with appropriate morphology and dispersion formed during the partial solution cycle at 1120 °C. The cooling rate after complete dissolution affects the size of gamma prime after partial solutionizing. This is due to the formation of supersaturated solution as well as controlling the rate of nucleation and growth in the next stage. A higher cooling rate at this stage results in a smaller primary gamma prime. Controlling the cooling rate in the partial solutionizing stage reduces the size of the primary gamma prime to the size of the secondary gamma primes, reducing the aging time and making the rejuvenation process more manageable and better.