Ali Reza Eivani

In stationary components of gas turbine such as vanes, segments, and walls, domestic object damage (DOD) and foreign object damage (FOD) are the most significant degradation phenomena, then the impact behavior via the Charpy test is considered a criterion for assessing the degradation evaluation. In the present study, the effects of rejuvenation heat treatment on the microstructure and high-temperature impact behavior of a super-exposed third-stage vane in a V94.2 gas turbine engine, with approximately 105,000 EOH, made of IN738LC, were evaluated. Moreover, owing to the thin walls of the airfoil, the preparation of standard specimens is limited; therefore, another objective of this study was to determine the correlation ratio that connects the absorbed energy in the sub-size and standard specimens for the IN738LC alloy. In this study, Charpy impact specimens with varying dimensions (x×10×55 mm, x=2.5, 5, 10) were prepared then impact testing was performed at 900°C. The analysis of the absorbed energy results showed that to extrapolate the impact results from specimens sized 2.5×10×55 mm and 5×10×55 mm to the standard specimen, correction ratios of 2.3 and 1.5, respectively, should be used. Impact sub-size specimens were prepared from the thin exposed and rejuvenated airfoil section, and Charpy impact testing was performed; the absorbed energies were found to be 5.1 J and 11.3 J, respectively, by applying a correction ratio, that was lower and higher than the threshold impact energy value (8.1 J). Microstructural and fractographic examinations showed that failure in the exposed specimen occurred via intergranular cracking, whereas in the rejuvenated specimen, ductile and transgranular fracture with the formation of deep dimples led to increased absorbed energy. Finally, it was revealed that using the rejuvenation treatment, the microstructure and impact properties were essentially improved, thus enabling the refurbishment of the vane after the rejuvenation heat treatment.