Comparison of Ni-Based 625 Alloy and ATI 20-25+Nb™ Stainless Steel Foils After Long-Term Exposure to Gas Turbine Engine Exhaust

Abstract

A field test program to validate the high temperature oxidation resistance of the ATI 20-25+Nb™ alloy in a Solar Turbines Incorporated Mercury™ 50 gas turbine engine has exceeded 66,000 operating hours. The primary goal of this program is to assess the effect of the actual recuperator operating environment on the high temperature degradation of primary surface recuperator (PSR) materials. As PSRs are generally fabricated from thin foil materials, excessive degradation can cause perforation or collapse, leading to decreased performance or failure. To avoid such issues, PSRs are generally fabricated from highly-alloyed austenitic stainless steels or nickel-base superalloys. This field test program evaluates and compares the long-term performance of the iron-based austenitic ATI 20-25+Nb™ stainless steel (UNS S35140) with the more highly alloyed nickel-based 625 alloy (UNS N06625) in an in-situ turbine exhaust environment. Sub-size air cell samples of alloy 625 and ATI 20-25+Nb™ alloy, exposed for 66,000 hours (running time) in turbine exhaust, were removed and tested for materials characterization. Analysis showed that both alloys exhibit excellent long-term resistance to environmental degradation, even after service exposures equivalent to over 7.5 years.