The effects of Ta on phases and mechanical properties in conventional 718 type compositions have not been fully explored. While Ta and Nb have similar atomic sizes, the solubility of Ta in nickel is much greater than that of Nb. This difference in solubility would affect the initial segregation on solidification and subsequent phase reactions. To study the role of Ta in phase reactions and on alloy stability in a homogeneous material, a comparative study of conventional P/M 718 and P/M Ta 7 18 was undertaken. The results of this study showed that the heat treatment used for conventional 7 18 did not produce an significant strengthening phases in Ta 718 and a modified heat treatment was to reci necessa~ p p itate ?/' and 1/ strengthening phases. The ?/' phase in Ta 718 is still present at 1750 F. The r to delta transition in Ta 718 is more sluggish and occurs at higher temperatures than in conventional 718 materials. Data from tensile tests at 1400'F indicates that Ta 718 has a higher temperature capability than conventional 718.

State-of-the-art superalloys are useful for high temperature applications, in large part, because they form protective alumina surface films by the selective oxidation of aluminum from the alloy. The adherence of the alumina to the alloy is crucial to maintaining oxidation resistance, particularly under thermal cycling conditions. It is now well established that small additions of reactive elements, such as yttrium hafnium, and cerium substantially improve the adherence of alumina films to alloy substrates. While the effects produced by the reactive elements are widely known the mechanisms whereby they improve adherence are not completely understood. Over the last fifty years a number of mechanisms have been proposed. However, it has recently become clear that a major effect of the reactive elements is to tie up sulfur in the alloy and prevent it from segregating to the alloy/oxide interface and weakening an otherwise strong bond. This paper describes the results of a study of the control of sulfur content in alumina-forming nickel-base superalloys and NiAl by three methods:1. Addition of Reactive Elements (Y and Hf). 2.Desulfurization in the solid state. 3.Desulfurization in the liquid state. Additionally, calculations have been performed to determine how much sulfur is available to segregate to the scale/alloy interface and how this quantity is influenced by the type and amount of reactive element in the alloy and the level to which the alloy is desulfurized. Finally, the results from experiments to desulfurize the alloys are described and cyclic oxidation measurements are used to evaluate the calculations.

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.

The Oak Ridge National Laboratory (ORNL) and ATI Allegheny-Ludlum began a collaborative program in 2004 to produce a wide range of commercial sheets and foils of the new AL20-25+Nb stainless alloy, specifically designed for advanced microturbine recuperator applications. There is a need for cost-effective sheets/foils with more performance and reliability at 650–750°C than 347 stainless steel, particularly for larger 200–250 kW microturbines. Phase I of this collaborative program produced the sheets and foils needed for manufacturing brazed plated-fin (BPF) aircells, while Phase II provided foils for primary surface (PS) aircells, and modified processing to change the microstructure of sheets and foils for improved creep-resistance. Phase I sheets and foils of AL20-25+Nb have much more creep-resistance than 347 steel at 700–750°C, and foils are slightly stronger than HR120 and HR230. Preliminary results for Phase II show nearly double the creep-rupture life of sheets at 750°C/100 MPa, with the first foils tested approaching the creep resistance of alloy 625 foils. AL20-25+Nb alloy foils are also now being tested in the ORNL Recuperator Test Facility.

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.

No abstract

Recuperation is a means for increasing the efficiency of a simple-cycle gas turbine, allowing for heat transfer between the exhaust and compressor discharge gas streams to occur 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, presenting challenges for high temperature materials selection. This paper discusses a joint Solar Turbines-ATI Allegheny Ludlum project which was undertaken to address the issue of elevated temperature attack in the presence of high levels of water vapor and its relevance to alloys intended for use in primary surface recuperators. An overview of the alloy selection process will be presented, followed by a detailed study of the two most promising alloy candidates. Breakaway oxidation was mitigated by using alloys with higher nickel and chromium content, and oxide scale evaporation was controlled with selected minor element additions.