Microstructural and mechanical investigations on the heat treatment rejuvenation of a long-term service-exposed GTD-111 Ni-based superalloy

Abstract

Increasing firing temperature has been known to increase gas turbine machines' power generation and cycle efficiency. The Ni-based superalloys are excellent candidate materials with superior mechanical properties, oxidation, and corrosion resistance for elevated temperature applications. GTD-111 is a Ni-base superalloy that is utilized at high temperatures and stresses as a hot section blade of heavy gas turbines. However, working in such harsh environments is extremely degrading and causes microstructural and mechanical deterioration during long-term operation. In order to regenerate the microstructure and mechanical properties after specific service hours, rejuvenation heat treatments are applied. Nevertheless, a few reports investigated the effect of rejuvenation heat treatments on microstructure and mechanical properties of turbine blade components after long-time exposure, which are made of cast GTD-111 Ni-base superalloys. This study investigated the effect of various rejuvenation heat treatments on the microstructural changes, and high temperature-low stress creep properties of service-exposed GTD-111 turbine blades. The different parameters, such as solution temperature, cooling rate, and aging, are considered. It was found that a proper selection of solution temperature, cooling rate, and aging heat treatment can restore the microstructure and mechanical properties of a service-exposed blade close to an un-exposed condition. Among all heat treatment regimes, full solution at 1190 °C for 2 h (cooling rate 14 °C/min) + aging at 845 °C for 24 h (air cooling) showed the optimum restoration parameters from both microstructural and mechanical aspects.