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 the elevated temperature oxidation resistance of common stainless steels. Increasing the amount of chromium and nickel in heat-resistant alloys appears to alleviate the risk of breakaway oxidation in air containing water vapor. Alloy 625 (UNS N06625), a common wrought nickel-base superalloy, is a candidate material for recuperators. Long-term oxidation testing using 0.10 mm (0.004 inches) thick samples showed no indication of breakaway oxidation in both ambient and humidified air at 704-815°C (1300-1500°F). Significantly lower overall weight gains were observed for the samples exposed in humidified air. The experimentally observed oxidation kinetics can be explained by a model combining the simultaneous growth and evaporation Of an oxide layer. The oxidation test data and observed microstructural features agree generally well with this model.
Recuperation is a means for increasing the efficiency of a simple-cycle gas turbine. A primary surface recuperator allows for heat transfer between the exhaust and compressor discharge gas streams in a gas turbine in a highly efficient, relatively compact package. The primary surface recuperator operates at high temperatures and gas pressures and is constructed from thin metal sections. Such a primary surface recuperator design presents several unique challenges for high-temperature materials. Among these is failure due to high-temperature oxidation in the gas turbine exhaust stream environment.
Water vapor is encountered as a minor component in ambient air and in larger concentrations as a by-product of various industrial processes such as combustion. It has been known for some time that the presence of water vapor in oxidizing environments can alter the degradation process for many different metals. 1 When present in oxygen-bearing atmospheres or as the primary oxidant, water vapor appears to hasten the onset of rapid oxidation of Fe-Cr and Fe-Ni-Cr alloys at elevated temperatures. 23° The results vary from study to study, but general trends show that the presence of water vapor accelerates 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-Ni-Cr alloys suggests that higher chromium and nickel contents (or conversely, a low iron content) are beneficial. 2432
Recent studies involving 18Cr-10Ni austenitic stainless steels noted that a thin chromium oxide layer is established on the sample surface during initial exposure to air containing water vapor. The amount of chromium incorporated into this scale is not large enough to result in significant depletion of the underlying metal. Therefore, the occurrence of breakaway oxidation in this case cannot be due solely to chromium depletion of the substrate. Rapid weight gain then occurs due to the formation and growth of mixed oxide nodules. These nucleate at isolated locations after an incubation time and then spread, consuming the initially-formed protective oxide layer. This observation is supported by the fact that remnants of the initial chromium oxide layer survive on heavily degraded samples. 19'2° It is unclear if the nodules form as a direct result o