Accelerated oxidation of type 347 stainless steel primary surface recuperators operating above 600°C

Type 347 stainless steel has traditionally been used in the manufacture of microturbine primary surface recuperators. It has been established during the past few years that the water vapor present in the microturbine exhaust gas causes accelerated oxidation of austenitic stainless steels at operating temperatures above ∼600°C (∼1110°F), which has resulted in the replacement of austenitic stainless steels with more highly alloyed Fe-based alloys and Ni-based alloys in microturbine recuperators. The effect of water vapor on type 347 stainless steel primary surface recuperators has been studied extensively by Capstone Turbine Corporation in collaboration with Oak Ridge National Laboratory. Several recuperators exposed in a Capstone C60 Micro-Turbine™ under different steady-state and cyclic operating conditions, have been microstructurally characterized. Evaluation of surface oxide scale formation and associated compositional changes has been carried out on representative sections from recuperators with operating lives ranging from ∼2,000 - 15,000 hours. Results from the microstructural and compositional analyses of the engine-tested recuperators illustrate the progression of accelerated oxidation of type 347 stainless steel at recuperator operating temperatures above 600°C.