INTRODUCTION
Recuperators for developmental high-efficiency small gas turbine engines will require low-cost materials with both creep and corrosion resistance at 700°C or higher. Because of the accelerated corrosion attack caused by water vapor in the exhaust gas and pressurized inlet air, alloys used in current recuperators, such as type 347 stainless steel, do not have sufficient oxidation resistance to meet lifetime goals at these higher temperatures. More highly alloyed steels and Ni-base alloys can meet the requirements but may be prohibitively expensive. Model Fe-Ni-Cr and Fe-Cr alloys are being tested in air plus 10% water vapor in order to determine compositions with appropriate environmental resistance for this application and to determine the effect of minor alloy additions. Results indicate that Ni and Cr contents in the 20wt.% range prevent accelerated attack observed in leaner alloys and that Si is the most critical minor (<0.5 wt.%) alloy addition for long-term oxidation resistance in these environments.
Small (30-200kW) gas turbine engines or microturbines are one solution to the issues of improving power quality and overloaded transmission lines. 1-2 Sited at or near the user's facility, a microturbine could provide base load or peak load power with much lower emissions than similar-sized diesel or natural gas fired reciprocating engines. The waste heat from the microturbine also could be used for climate control or heating water at the facility. However, compared to large gas turbine engines, microturbines have relatively low operating efficiencies and relatively high initial and installation costs (per kilowatt). As part of the Department of Energy's Distributed Energy Resources program, the Advanced Microturbine Systems program is seeking to improve the efficiency of microturbines and to lower their costs by developing partnerships among engine manufacturers, materials suppliers and materials scientists. 3
One well-known way to improve the efficiency of microturbines from 15-20% to 25-30% is by incorporating a recuperator or heat exchanger to use the exhaust gas to heat the compressed air entering the combustor. 4 Most recuperators have used stainless steels, such as type 347, because of its excellent combination of creep and corrosion resistance. In order to further increase efficiency, higher turbine inlet temperatures are needed which also may require recuperators to operate at higher temperatures. A temperature increase is a particularly critical problem for most recuperators because they tend to employ thin-walled components to maximize the heat transfer in a minimal volume and weight. With a 75- 125#m thick type 347 foil used in both primary surface and plate-and-fin type recuperators, 5 there is only a limited reservoir of Cr available to achieve desired operating lifetimes of 20,000-50,000h. Also with the limited Cr reservoir, any accelerated corrosion attack will result in a greatly reduced component lifetime. With recuperator operating temperatures moving towards 700°C and higher, Fe-base chromia- forming alloys like type 347 stainless steel can be susceptible to accelerated attack (i.e. high rates of metal wastage) due to water vapor. 6-16 The common observation is the breakdown of the normally protective Cr-rich surface oxide and the formation of Fe-rich oxides. Water vapor is present in the exhaust gas as a combustion product but also can have a significant concentration in the compressed inlet air because of the higher total pressure.
The effect of water vapor on the oxidation behavior of ferritic and austenitic alloys is being widely studied and, while the exact mechanism may not be understood, it is clear that increasing the alloy Cr co
