Primary Surface Recuperator Alloy Oxidation: A Comparison of Accelerated Engine Testing to Field Operation

The Capstone C65 Microturbine primary surface recuperator (PSR) core has been manufactured from Haynes alloy HR-120 since 2005 (Microturbine is a registered trademark of Capstone Turbine Corporation; Haynes and HR-120 are trademarks of Haynes International, Inc.). When exposed to the harsh operating environment of the microturbine PSR, HR-120 forms a protective oxide scale that is resistant to the effects of the water vapor present in the exhaust gas. Long-term accelerated microturbine testing with samples in a modified PSR with a removable aft dome is ongoing at an elevated turbine exit temperature (TET) $∼100°F$ higher than normal operation. The elevated TET test engine is operated at steady-state conditions, and the engine is shut down at predetermined intervals for sample removal. Material characterization of the elevated TET samples has been carried out by Capstone Turbine Corporation in collaboration with Oak Ridge National Laboratory. The surface oxide scale formation and associated alloy compositional changes have been evaluated for elevated TET samples with operating lives ranging from $∼1800h$ to $∼26,500h$⁠. In addition, field-operated HR-120 recuperators have been sectioned and samples have been evaluated for operating lives ranging from $∼5500h$ to $∼18,000h$⁠. Results from the microstructural and compositional analyses of both the long-term steady-state elevated TET HR-120 samples and the field-operated HR-120 recuperator samples will be presented and compared.