Effect of Cryogenic Treatment After Full Solution on Rejuvenation of Exposed IN738LC Gas Turbine Blade

In this research, the samples were exposed to full and partial solution heat treatments and quenched in the air and nitrogen each period. Also, one sample was kept in the liquid nitrogen for 2 h after cooling in the air and isothermal process. In the end, the samples were aged. At every stage, samples were investigated using scanning electron microscope (SEM) and microhardness HV test. The results showed that the rejuvenation heat treatment leads to formation of nanoprecipitates with the cubic morphology in the matrix. The percentage of nanoprecipitates after aging increased by about 87/8%. Keeping in the liquid nitrogen after full solution increased the volume fraction of the small precipitates by about 85%. After partial solution heat treatment, the precipitates became coarse and volume fraction increased to 74%. By increasing the cooling rate, microhardness increased from 620 to 786 microhardness HV after aging. After solutionizing and high rate quench, the microhardness of the blade airfoil decreased to 361 HV, which means a successful solutionizing as the main stage of rejuvenation. During rejuvenation, the size and volume fraction of γ´ precipitates decreased in the solution step and increased by aging. The use of cryogenics and the high rate of quench caused the improvement in the rejuvenation process compared to previous researches. This research can be a start for using this new method of rejuvenation.