Comparative Study of Solution Heat Treatment of IN738LC Superalloy in Conventional Conditions and Salt Bath

In this study, to investigate the impact of heating rate, the IN738LC superalloy was solutionized both in standard and salt bath conditions. For this purpose, the samples were solutionized in the temperature range of 1090–1200 °C for 30–120 minutes in normal conditions and 10–120 minutes in the salt bath. Afterward, samples were aged at 850 °C for 24 hours. The results of scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM) showed that as the heating rate increased, the dissolution process improved. In contrast, as the time and temperature increased, the γ' precipitates’ volume fraction decreased in both the normal condition and the salt bath by 26.01 and 50.28 %, respectively. The size of the remaining γ' precipitates after dissolution in the salt bath was about 16 nm smaller than normal ones. The size and volume fraction of the residual precipitates in the salt bath increased at high temperatures compared to the normal ones due to the attachment mechanisms’ activation. Solubility in the salt bath reduced the size of primary γ' in the normal range and increased the volume fraction of the secondary γ' precipitates after the aging. This alloy’s hardness is strongly influenced by the characteristics of the γ precipitates and the solutionizing process variables. Therefore, the results indicated that the secondary γ' precipitates are about 16 nm smaller than the usual one, which has a remarkable impact. After aging, increasing the heating rate during the solutionizing resulted in a further rise in the hardness and reduced the hardness after solutionizing. However, in both conditions, as the time of solutionizing reduced and temperature increased, the samples’ hardness after solution decreased, and the hardness after aging increased.