An Investigation on the Microstructure of Used First-Stage Blades of Gas Turbine and Determination of Optimum Heat Treatment Cycle for Rejuvenation of the Blades

The Ni-base superalloy IN738LC is employed in manufacturing of the first stage blades of high power gas turbines. Its microstructure changes after 20000 hours in high temperature service. The microstructure degeneration is so that the blades loose their appropriate strength. Regarding their high price, microstructure recovery by heat treatment is a valuable job. Therefore, in this research, the effects of heat treatment parameters such as: time and temperature of homogenization, partial solution and aging temperatures, and cooling rate from homogenization and solution temperatures on the microstructure of the alloy are investigated. Microstructure of some specimens from used blades before and after applying of different heat treatment cycles was studied by SEM. Its results led to obtaining of the optimum cycle for producing optimum microstructure. Also, to verify the effect of the optimum cycle, creep test was carried out on the samples extracted from unused blades and on the samples extracted from used blades after applying the optimum heat treatment cycle. Comparison of the results confirmed the effectiveness of the optimum heat treatment cycle in rejuvenation of the blades.