The Use and Performance of Oxidation and Creep-Resistant Stainless Steels in an Exhaust Gas Primary Surface Recuperator Application

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. Oxidation of the thin metal foil recuperator walls is a major concern, necessitating the use of heat-resistant alloys. Water vapor, present in the exhaust gas as a by-product of combustion, has been shown to be detrimental to the elevated temperature oxidation resistance of some ferrous alloys currently used for recuperators, e.g., Type 347 stainless steel. The walls of the primary surface recuperator are also subjected to a complex state of stress. Creep deformation can cause the compressor discharge air passages to expand, thus restricting exhaust gas flow and increasing the turbine backpressure. The material of construction must, therefore, be resistant to both oxidation and creep deformation. Long-term oxidation, stress-rupture, and creep test results and analysis will be presented for both commercially available and developmental austenitic stainless steel foil materials. A 20Cr-25Ni austenitic stainless steel containing a small addition of Nb was found to exhibit good creep strength when compared to current alloys of construction. This alloy also possesses excellent resistance to attack in environments containing high levels of water vapor. Oxide volatility and breakaway oxidation were not observed after 10,000 hours of exposure at temperatures as high as 760°C (1400°F).

Effect of gas flow rate on oxidation behaviour of alloy 625 in wet air in the temperature range 900–1000 °C

“…The alloy has a large variety of applications for instance in aerospace industries, chemical processing, pollution control equipment, waste‐to‐energy boilers and recuperators . Recently, alloy 625 was considered as a potential construction material for gas turbine recuperators and its oxidation behaviour in water vapour containing exhaust gases was studied during long‐term exposures in a wide temperature range .…”

“…Refs. ) and its significance for the overall oxidation performance is well established when considering materials behaviour at temperatures around and above 900 °C . Especially from extensive studies carried out at Chalmers University of Technology (Göteborg, S) it was shown that the formation of volatile chromium species may also substantially affect materials performance at much lower temperatures around 600 °C.…”

“…For NiCr‐base alloys, this sudden change in oxidation kinetics does generally not occur due to the relatively low growth rates of the less protective oxides NiO and NiCr 2 O 4 . The change in oxidation kinetics upon occurrence of a critical Cr depletion occurs more gradually and therefore the expression ‘critical Cr‐depletion’ may be more appropriate than ‘breakaway oxidation’ .…”

The Use and Performance of Wrought 625 Alloy in Primary Surface Recuperators for Gas Turbine Engines

“…The results vary from study to study, but water vapor tends to accelerate the rate of oxidation, leads to the formation of layered scales, and increases the amount of chromium required to form a protective oxide film. Work focused specifically on Fe-Cr-Ni alloys suggests that higher chromium and nickel contents (or conversely, a low iron content) increase oxidation resistance in environments containing water vapor [24][25][26][27][28][29][30][31][32][33]. Studies involving austenitic stainless steels noted that a thin chromium oxide layer is established on the sample surface during initial exposure to air containing water vapor.…”