Validation of ATI 20-25+Nb™ Alloy at 50,000 Hours in the Recuperator of a Gas Turbine Engine

Abstract

A field test program to validate high temperature oxidation resistance of the ATI 20-25+Nb™ alloy in a Solar Turbines Incorporated Mercury™ 50 gas turbine engine has exceeded 50,000 operating hours. A primary goal of this program is to assess the effect of the actual recuperator operating environment on high temperature degradation of primary surface recuperator (PSR) materials. As PSRs are generally fabricated from thin foil materials, excessive degradation can cause perforation, leading to failure of components. To avoid such problems, PSRs are generally fabricated from nickel-base superalloys or highly-alloyed austenitic stainless steels. Previously, ATI 20-25+Nb (UNS S35140) stainless steel was developed and experimentally shown to have excellent creep resistance and good environmental resistance for a PSR application. This field test program evaluates the long-term performance of this alloy in situ in turbine exhaust and compares it with the more highly alloyed Ni-based 625 superalloy. Sub-size air cell samples of alloy 625 and ATI 20-25+Nb alloy exposed for 50,000 hours (running time) in turbine exhaust were removed and tested for materials characterization. Analysis showed that both alloy 625 and ATI 20-25+Nb alloy exhibit excellent long-term resistance to environmental degradation, even after service exposure equivalent to over 5.5 years.