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).

Primary surface recuperators (PSR’s) for land-based industrial gas turbines are typically constructed from heat-resistant alloys such as austenitic stainless steels or nickel-base superalloys. The water vapor present in gas turbine exhaust has been shown to increase the rate of chromium oxide volatility, which in turn can cause rapid oxidation of the underlying metal. As PSR’s are generally fabricated from thin foil materials, excessive degradation can cause perforation, leading to failure of components. The results of an extensive laboratory test program to characterize the performance of heat-resistant alloys will be summarized, outlining the different modes of attack and means for their mitigation. These results will be compared to an investigation carried out using sub-size recuperator components which were exposed to a full-flow exhaust stream during gas turbine operation for times ranging from a few weeks to over one year.

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. High temperature oxidation and creep are major concerns, necessitating the use of heat-resistant alloys for the recuperator panels. Most current recuperator designs specify austenitic stainless steel foil as the material of construction. Water vapor, present in the exhaust gas as a by-product of combustion, has been shown to be detrimental to elevated temperature oxidation resistance, particularly for ferrous alloys. The protective chromium oxide scale breaks down rapidly in the presence of water vapor due to the formation of fast-growing iron oxide nodules. Increasing the amount of chromium and nickel appears to alleviate the risk of breakaway oxidation. Alloy 625, a common wrought Ni-Cr-Mo superalloy, is a candidate material for recuperator air cells. Long-term oxidation testing of 100 micron thick samples performed at 704-815ºC (1300-1500ºF) did not result in breakaway oxidation but indicated a tendency towards weight loss. The observed oxidation kinetics can be explained by a model combining the simultaneous growth and evaporation of an oxide layer. Evaporation of chromia appears to be accelerated in the presence of water vapor by the formation of volatile species such as chromium oxyhydroxide. Comparing these results to those of other nickel-base superalloys and high-alloy content stainless steels revealed that minor element chemistry can be modified to mitigate evaporation by the formation of an external spinel oxide layer. IntroductionRecuperation is a means for increasing the efficiency of a simple-cycle gas turbine. A primary surface recuperator allows for heat transfer between the turbine exhaust and compressor discharge gas streams in a gas turbine in a highly efficient, relatively compact package. This serves to increase the temperature of the air entering the combustor, effectively decreasing the amount of fuel required to reach the desired turbine inlet temperature. Figure 1 illustrates a recuperated cycle in schematic form.

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Primary surface recuperators (PSR’s) for land-based industrial gas turbines are typically constructed from heat-resistant alloys such as nickel-base superalloys or highly-alloyed austenitic stainless steels. The water vapor present in gas turbine exhaust has been shown to increase the rate of chromium oxide volatility, which in turn can cause rapid oxidation of the underlying metal. As PSR’s are generally fabricated from thin foil materials, excessive degradation can cause perforation, leading to failure of components. The results of a field test program will be summarized, based on analysis of recuperator stand-in components which were exposed to a full-flow exhaust stream during gas turbine operation for times ranging from 500 hours to 21,500 hours. A significant effect of service on the materials of construction was observed, with near-surface chromium depletion and microstructural instability evident after the longest exposures. These results will be combined with those from an extensive laboratory test program to evaluate the performance of heat-resistant alloys during recuperator service, outlining the different modes of degradation and means for their mitigation.