Rejuvenation Heat Treatment of Single Crystal Gas Turbine Blades

Thermal degradation of precipitation-hardened nickel based superalloys has been demonstrated to be reversible through full solution rejuvenation heat treatment processing. The specific concern with full solution rejuvenation heat treatment of single crystal alloys is the formation of recrystallized grains on surfaces with residual stress. The threshold temperature for recrystallization and the effect of heat treatment temperature and time on recrystallization depth were evaluated on service run industrial gas turbine blades comprised of nickel based single crystal alloys René N5 and RR2000. Recrystallization of rejuvenated blades was observed on the root surfaces of blades which had been shot peened at original manufacture and/or during a prior repair. Blades which did not receive peening at manufacture were free of recrystallization in critical areas following full solution rejuvenation heat treatment. Given that gas turbine blade roots operate at relatively low temperatures compared to the airfoil, creep is not considered a life limiting damage mechanism for this region of the blade. Rather, high cycle fatigue is considered the primary damage mechanism of concern. As such, fatigue testing of shot peened and heat treated (recrystallized) René N5 specimens was carried out at 650°C at various stress levels in comparison with baseline (non-recrystallized) specimens to determine the extent to which recrystallization would limit fatigue endurance at blade root operating conditions. It was found that recrystallization did not reduce the fatigue endurance relative to baseline samples at the tested conditions. The findings indicate that repair including full solution rejuvenation heat treatment of previously peened blades comprised of René N5 alloy is feasible provided that recrystallization be limited to root surfaces.