MICROSTRUCTURE EVALUATION OF SERVICE AGED AND REJUVENATED NICKEL SUPER ALLOYS USING HOT ISOSTATIC PRESSING TREATMENT
The design life of gas turbine blades is normally 48,000 Engine Operating Hours (EOH) or 72,000 EOH based on OEM recommendation. Extending the service life of gas turbine blades is a huge advantage in the aspect of capital cost-saving in the power plant industries. The coarsening and shape transformation of γ′ precipitates is one of the main life limiting factor for gas turbine blades. The conventional heat treatment could not restore or recover the γ′ -phase precipitates to their original morphology. In this study, Hot Isostatic Pressing (HIP) treatment is used to improve the material properties of the blades to nearly zero-hour operation and prolong the life of gas turbine blades. HIP rejuvenation treatment applies high temperature and high pressure to close up the micropores, re-transform the micro phases, such as γ′ precipitates and to heal the material property. For case study, three HIP parameters were applied on Nickel–based superalloy (GTD 111). Microstructure characterization were carried out on the new, aged, and HIPed treated GTD 111 alloys. The microstructure after HIPed treatment exhibited significant improvement and recovery in γ′ precipitates morphology. The hardness property of HIPed blade samples appeared consistent at 440 HV and did not deviate significantly from new materials with an average value of 439 HV. The average size of the γ′ - precipitate was successfully reduced from 1.3 µm (degraded state) to 0.3 µm (near zero operation state). The experimental results suggested that HIP treatment is a promising procedure to restore the high temperature strength of gas turbine blades.