The use and performance of wrought 625 alloy in primary surface recuperators for gas turbine engines

Abstract

Recuperation increases the efficiency of a gas turbine engine by extracting heat from the exhaust gas stream and using it to pre-heat the compressor discharge air. High temperature oxidation and creep are major concerns, necessitating the use of heat-resistant alloys for the recuperator panels. Most current recuperator designs specify austenitic stainless steel foil as the material of construction. Water vapor, present in the exhaust gas as a by-product of combustion, has been shown to be detrimental to elevated temperature oxidation resistance, particularly for ferrous alloys. The protective chromium oxide scale breaks down rapidly in the presence of water vapor due to the formation of fast-growing iron oxide nodules. Increasing the amount of chromium and nickel appears to alleviate the risk of breakaway oxidation. Alloy 625, a common wrought Ni-Cr-Mo superalloy, is a candidate material for recuperator air cells. Long-term oxidation testing of 100 micron thick samples performed at 704-815°C (1300-1500°F) did not result in breakaway oxidation but indicated a tendency towards weight loss. The observed oxidation kinetics can be explained by a model combining the simultaneous growth and evaporation of an oxide layer. Evaporation of chromia appears to be accelerated in the presence of water vapor by the formation of volatile species such as chromium oxyhydroxide. Comparing these results to those of other nickel-base superalloys and high-alloy content stainless steels revealed that minor element chemistry can be modified to mitigate evaporation by the formation of an external spinel oxide layer.