This research has an aim to modify and improve the microstructures and hardness property of cast nickel-based superalloy, IN-738 by modified chemical composition of various Al additions as well as different reheat treatment conditions. IN-738 was modified by adding aluminium in 1, 2 and 3% by weight and casting with vacuum arc melting process. Then all casted alloys were performed with 5 different solutioning temperatures of 1125°C, 1145°C, 1165°C, 1185°C and 1205°C for 6 hours and all following with the same precipitation aging temperature of 845°C for 24 hours. The various obtained microstructures of the casting and reheat treatments were observed and investigated by SEM to evaluate the average area fraction and size of γ’ precipitated particles. The mechanical property was evaluated by Vickers hardness. From the obtained results, it was found that fine microstructure, which would be the most appropriate for using at elevated temperature, was obtained by the Al addition with 1%wt. following with solutioning temperature range between 1125°C – 1145°C/6 hours and precipitation aging at 845°C/24 hours. The microstructures received consist of high area fraction of γ’ particles in cubic shape with the proper size. However, the maximum hardness value of 683 HV was obtained from the alloy with 3%wt. Al addition following with solutioning temperature of 1205°C.
In this research study, the effect of long-term aging after various solutioning temperatures on final microstructure was investigated. The cast nickel base superalloy, GTD-111, usually has standard reheat treatment as follows: solutioning treatment at 1448 - 1478 K for 7.2 ‑ 21.6 ks and aging at 1118 K for 86.4 ks. However, from previous research works, the density of γ՛ phase was not reached the optimum value. Therefore, extension of aging time was performed in the study from 90 to 180, 270, 360, 720, 1080 and 1440 ks in order to increase density or volume fraction of precipitating γ՛ particles. From the results, it was found that longer aging time provided higher values of both area fraction and size of γ՛ particles. However, increase in aging time resulted in the hardness decrease.
The objective of this research work is to investigate the effect of Aluminum addition in cast nickel base superalloy grade Inconel-738 by vacuum arc melting process on microstructural modification and oxidation behavior at elevated temperatures of 900°C and 1000°C. The Al element, basically, could be added in cast nickel base superalloys in proper amount to form precipitated intermetallic phase with nickel atoms as gamma prime phase (γ’, Ni3Al) to increase mechanical properties by blocking dislocation movements at elevated temperatures. Furthermore, Al can assist nickel base superalloy to form protective oxide film, Al2O3 for better oxidation resistance at very high temperatures (over 980°C). In this research, all casted samples of Inconel-738 with various Al additions for 1, 2 and 3 percent by weight were standard heat treated consisting of solution treating at 1125°C for 6 hours and following with precipitate aging at 845°C for 24 hours. The oxidation tests were carried out at temperatures of 900°C and 1000°C up to 110 hours. From all obtained results, it was found that the sample that has the most microstructural stability after long-term heating as simulated working conditions is Inconel-738 sample with 2%wt. Al addition. Furthermore, more Al addition had resulted in higher oxidation resistances for both testing temperatures.
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