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).