Materials at High Temperature Vol 16, Issue 4, 1999
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Fossil Energy Materials
Conference
Introduction:
This issue of Materials at High Temperatures comprises a selection of papers that were presented at the Fossil Energy Materials Conference of the US Department of Energy’s (DOE’s) Advanced Research and Technology Development (AR&TD) program, held in Knoxville, Tennessee in May 1999. The conference provides an annual forum for review of projects in progress, and the results reported are documented each year in a conference proceedings. Given the diverse nature of the materials issues addressed and the different rates of progress in each project, information appropriate for wider public dissemination becomes available from given projects at different times, hence the range of topics addressed by the papers assembled in this issue.
The overall AR&TD Fossil Energy Materials program is structured to address materials-related issues identified as potential barriers to the successful operation of fossil fuel-fired power plants. Topics addressed in the program are identified through analysis of materials issues associated with processes needed to meet the goals of the processes of interest in the DOE’s fossil power generation initiatives, to determine where current materials are deficient, or where knowledge is lacking. Approaches are developed to overcome these potential barriers, and efforts are prioritized to ensure that information required by designers and operators is generated in a timely fashion. Where and when appropriate, programs are transitioned from laboratory research functions to provide direct support to prototype or field testing activities. The main topic areas currently addressed in the program are: x Coatings and Protection of Materials, aimed at areas in the fire-side and steam-side circuits of combustion and gasification processes where unusually severe conditions result in unacceptably rapid loss of conventional materials of construction.
x New Alloys, intended to meet the requirements of processes intended to achieve increasingly high levels of efficiency for materials capable of operation at higher temperatures, pressures, and in increasingly difficult environments. This effort includes incremental improvements to existing materials, the development of routes for extending the capabilities of metallic materials to higher temperatures, as well as ceramic materials for use in advanced steam cycles, gasification-based combined cycles, related processes such as pyrolysis reactors, and recuperators for gas turbine and reciprocating engines.
x Functional Materials, which are required in such processes as those involved in gas separation, fuel cells, and hot gas clean-up systems.
x Ultrahigh Performance Materials, which tend to involve higher risks and are somewhat longer range, but which have potential for significant advances in the temperature capability of metallic or ceramic materials. These materials would have application in, for instance, components required for very high-temperature duty in the hot gas path of hydrogen combustion systems, or for very hightemperature heat transfer surfaces required in indirectly-fired cycles.
If you would like to have further information on the projects highlighted by these papers, or about the AR&TD Fossil Energy Materials Program in general, please contact the Program Office at Oak Ridge National Laboratory, Oak Ridge, Tennessee at (865) 574-4572, or visit our Web site at: http://www.ornl.gov/fossil/, and the DOE Fossil Energy Web site at http://www.fe.doe.gov/.
Roddie R. Judkins
David P. Stinton
Robert W. Swindeman
Ian G. Wright Oak Ridge, December 1999
Corrosion behavior of weldable Fe–Al alloys in oxidizing–sulfidizing environments
S.W. Banovic, J.N. DuPont and A.R. Marder
Energy Research Center, Lehigh University, Bethlehem, PA 18015, USA
The objective of the present study was to investigate the corrosion behavior of weldable Fe–Al alloys in environments representative of low NOx gas compositions, i.e., high partial pressures of sulfur [p(S2)] and low partial pressures of oxygen [p(O2)]. Using thermogravimetric techniques, binary alloys with 0–12.5 wt% Al were exposed in oxidizing–sulfidizing environments [p(S2) = 10–4 atm and p(O2) = 10–25 atm] at 500–700°C for various times up to 100 h. Post-exposure characterization consisted of surface and cross-sectional microscopy in combination with energy dispersive spectroscopy and/or electron probe microanalysis. It was found that the Fe–Al alloys exhibited three different stages of corrosion behavior: inhibition, breakdown, and steady-state. Observance and/or duration of these stages was directly related to the aluminum content of the alloy. The inhibition stage was characterized by growth of a thin, gamma alumina scale that suppressed rapid degradation of the underlying substrate for alloys with greater than 7.5 wt% Al. During the breakdown stage, mechanical failure of the initially formed alumina scale, and the inability to re-establish itself, resulted in the growth of nodular sulfide products due to short circuit diffusion of sulfur and iron through the passive layer. This typically occurred on alloys with 7.5 wt% Al. The final stage (steady-state) found the diffusional growth of thick sulfide scales on alloys with less than 7.5 wt% Al that led to relatively high weight gains. Overall, the results from this study indicate that weldable Fe-Al compositions, approaching 10 wt% Al, have excellent corrosion resistance in aggressive low NOx gas compositions at service temperatures below 600°C. With the potential promise for applications requiring a combination of weldability and corrosion resistance in moderately reducing environments, these alloys are viable candidates for further evaluation for use as sulfidation resistant weld overlay coatings.
Keywords: weldable Fe–Al alloys, oxidizing–sulfidizing environments
Development of a scaled-up chemical vapor infiltration system for tubular geometries
K. J. Probst1,T. M. Besmann2, J. C. McLaughlin2,T. J. Anderson1 and T. L. Starr3
1Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA
2Metals and Ceramics Division, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831-6063, USA
3Department of Chemical Engineering, University of Louisville, Louisville, KY 40292, USA
Silicon carbide-based heat exchanger tubes are of interest to energy production and conversion systems due to their excellent high temperature properties. Fiber-reinforced SiC is of particular importance for these applications since it is substantially tougher than monolithic SiC, and therefore more damage and thermal shock tolerant. This paper reviews a program to develop a scaled-up system for the chemical vapor infiltration of tubular shapes of fiber-reinforced SiC. The efforts include producing a unique furnace design, extensive process and system modeling, and experimental efforts to demonstrate tube fabrication.
Keywords: Chemical vapor infiltration, fiber-reinforced SiC, composites
Intermetallic reinforced Cr alloys for high-temperature use
M.P. Brady, J.H. Zhu, C.T. Liu, P.F.Tortorelli, L.R.Walker, C.G. McKamey,
J.L.Wright, C.A. Carmichael, D.J. Larson, M.K. Miller and W.D. Porter
Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6115, USA
A new family of Cr(Ta)-Cr2Ta intermetallic alloys based on Cr-(6-10)Ta (at%) is currently under development for structural use in oxidizing environments in the 1,000–1,300°C temperature range. These alloys show excellent strength and creep resistance and good oxidation resistance at high temperatures in air. Oxidation resistance comparable to commercial reactive element doped chromia-forming alloys and creep resistance comparable to single-crystal superalloys have been demonstrated. To date, only modest room-temperature fracture toughness (in the 11–12 MPa mg range) has been achieved. Preliminary results of a promising approach to improve room-temperature fracture toughness via ductilization of Cr with MgO additions are discussed.
Keywords: Intermetallic reinforced Cr alloys, Cr, oxidation, Laves phase, mechanical properties, MgO
Development of nondestructive evaluation methods for hot gas filters
W.A. Ellingson, E.R. Koehl, J.G. Sun, C. Deemer, H. Lee and T. Spohnholtz
Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439, USA
Rigid ceramic hot gas candle filters are currently under development for high-temperature hot gas particulate cleanup in advanced coal-based power systems. The ceramic materials for these filters include monolithics (usually non-oxides), oxide and non-oxide fiber-reinforced composites, and recrystallized silicon carbide. A concern of end users in using these types of filters, where over 3000 may be used in a single installation, is the lack of a data base on which to base decisions for reusing, replacing or predicting remaining life during plant shutdowns. One method to improve confidence of usage is to develop nondestructive evaluation (NDE) technology to provide surveillance methods for determination of the extent of damage or of life-limiting characteristics such as thermal fatigue, oxidation, damage from ash bridging such as localized cracking, damage from local burning, and elongation at elevated temperatures. Although in situ NDE methods would be desirable in order to avoid disassembly of the candle filter vessels, the possible presence of filter cakes and/or ash bridging, and the state of current NDE technology prevent this. Thus, off-line NDE methods, if demonstrated to be reliable, fast and cost effective, could be a significant step forward in developing confidence in utilization of rigid ceramic hot gas filters. Recently, NDE methods have been developed which show promise of providing information to build this confidence. Acousto-ultrasound, a totally nondestructive method, together with advanced digital signal processing, has been demonstrated to provide excellent correlation with remaining strength on new, as-produced filters, and for detecting damage in some monolithic filters when removed from service. Thermal imaging, with digital signal processing for determining through-wall thermal diffusivity, has also been demonstrated to correlate with remaining strength in both new (asreceived) and in-service filters. Impact acoustic resonance using a scanning laser vibrometer has been demonstrated to allow detection of changes in frequency which may be correlated to remaining strength. These methods have been shown to be applicable to clay-bonded SiC filters, recrystallized SiC filters, CVI–SiC composite filters and oxide composite filters. Other NDE methods under development include: (a) fast, high spatial-resolution X-ray imaging for detecting density variations and dimensional changes; (b) air-coupled ultrasonic methods for determining through-thickness compositional variations; and (c) acoustic emission technology with mechanical loading for detecting localized bulk damage.
Keywords: nondestructive evaluation, hot gas filters
Reduction in defect content of oxide dispersion strengthened alloys
A.R. Jones and J. Ritherdon
University of Liverpool, UK
Evidence from the literature has been considered for factors controlling the occlusion and retention of gas in powders, particularly those produced by mechanical alloying (MA). A series of experiments has been performed on oxide dispersion strengthened Fe3Al and PM 2000 powders to determine recovery, recrystallisation and vacuum degassing behaviour. As-MA ODS Fe3Al powders begin to recover and recrystallise at lower temperatures than PM 2000 (550°C c.f. 750°C, respectively) and recrystallise to a finer grain size (~10_m maximum mean linear intercept c.f. ~50_m in PM 2000). Both powders outgassed hydrogen, as well as smaller quantities of nitrogen and water vapour. However, PM 2000 released more hydrogen than Fe3Al. Both powders contained small pores which are likely to have entrained molecular hydrogen. Defects contained in consolidated secondary recrystallised Fe3Al included porosity, residual stringers of fine grains and intrusions. The latter were larger, more numerous and slightly elongated within the fine grained regions suggesting an association.
Keywords: defect control, oxide dispersion strengthened alloys
Tensile deformation behaviour of two aluminium alloys at elevated temperatures
D. Ravi Kumar* and K. Swaminathan
Department of Metallurgical Engineering, Indian Institute of Technology, Madras 600 036, India
Present addresss: Department of Mechanical Engineering, Indian Institute of Technology, New Delhi 110 016, India
The tensile deformation behaviour of two recently developed aluminium alloys in the temperature range 200–550°C is characterized in this paper. The aluminium alloys studied here are an automotive stamping grade Al–Mg–Mn alloy and an Al–Li–Cu alloy. Tensile properties at elevated temperatures were determined under different temperature-strain rate combinations. An analysis of deformation and fracture behaviour at elevated temperatures is also presented. The Al–Mg–Mn alloy and the Al–Li–Cu alloy exhibited extended ductility or mild superplasticity at elevated temperatures. Metallographic and fractographic studies revealed appreciable grain growth and cavitation at elevated temperatures. The fracture elongation of Al–Mg–Mn alloy decreased beyond 430°C. Pronounced apparent strain hardening was observed in the case of the Al–Li–Cu alloy in the temperature range 525–550°C at a very low strain rate. This could be due to dynamic grain growth and/or dislocation structure evolution.
Keywords: tensile deformation, strength, ductility, metallography, fractography, grain growth, cavitation
Improved creep-resistance of austenitic stainless steel for compact gas turbine recuperators
P.J. Maziasz1, R.W. Swindeman1, J.P. Montague2, M. Fitzpatrick2, P.F. Browning2, J.F. Grubb3, and R.C. Klug3
1Oak Ridge National Laboratory, Oak Ridge, TN 37831-6115, USA
2Solar Turbines, Inc., San Diego, CA, USA
3Allegheny-Ludlum, An Allegheny-Teledyne Co., Brackenridge, PA, Wallingford, CT, or Leechburg, PA, USA
Primary surface recuperators (PSRs) are compact heat-exchangers made from thin-foil type 347 austenitic stainless steel, which boost the efficiency of land-based gas turbine engines. Compact recuperators are also an essential technology for some new microturbines. Solar turbines uses foil folded into a unique corrugated pattern to maximize the primary surface area for efficient heat transfer between hot exhaust gas on one side, and the compressor discharge air on the other side of the foil. Allegheny-Ludlum produces 0.003–0.004 inches thick foil for a range of current turbine engines using PSRs that operate up to 660°C. One goal of this team-effort project is to modify the processing to enable improved creep resistance of such 347 stainless steel foils at 650–700°C. Laboratory-scale processing modification experiments recently have demonstrated that dramatic improvements can be achieved in the creep resistance of such typical 347 stainless steel foils. The modified processing enables fine NbC carbide precipitates to develop during creep at 650–700°C, which provides strength even with a fine grain size. Such improved creep-resistance allows greater flexibility in optimizing the cost-performance relationship as increased demands are placed on the PSR at higher operating temperatures. The next challenges are to better understand the nature of the improved creep resistance in these 347 stainless steel foil, and to achieve similar improvements with scale-up to commercial foil production.
Keywords: Austenitic stainless steel, compact gas turbine recuperators
Characterizing coal-gasifier slag–refractory interactions
J.Rawers, J.Kwong, and J.Bennett
US Department of Energy, Albany Research Center, Albany Oregon, USA
To characterize refractory degradation and loss in commercial coal-gasifier combined cycle power systems, cup-type tests were conducted on high chromium-alumina, sinter-bonded refractories under laboratory conditions designed to simulate commercial operations of temperature, atmosphere, and slag interactions. These tests provided qualitative results from which the slag–refractory interactions can be characterized. These high chromium refractories were generally inert with respect to the coal slag components. However, in this study preliminary findings did show (1) iron ( oxide) in the slag reacted with chrome sesquioxide to produce a Cr–Fe spinel at the slag–refractory interface, and (2) chrome was soluble in the molten slag. Comparison of cup-type test results with data from operating commercial plants suggests that the principal loss of refractory material in a coal-gasifier combustion chamber is chrome dissolution into the slag. Tests are currently underway to determine if minor modifications to the combustion process might increase refractory life.
Keywords: coal-gasifier, slag–refractory interactions
Development of slurry-based Ca0.5Sr0.5Zr4(PO4)6 (CS-50) coatings for SiC in fossil energy applications
Virginie M.Vaubert, Mark A. Janney, and David P. Stinton
Oak Ridge National Laboratory, PO Box 2008, Bldg. 4515, MS-6063, Oak Ridge, TN 37831-6063, USA
Coatings of Ca0.5Sr0.5Zr4(PO4)6 (CS-50), an ultra-low thermal-expansion material, have been successfully developed to protect SiC-based materials from sulfate salt corrosion. The coatings were applied to fiber-reinforced, SiC-matrix composite heat exchanger tubes and clay-bonded SiC. Aqueous slurries were prepared and applied to the SiC substrates. One coating process for heat exchanger material substrates, consisted of mechanically dipping the samples in a slip. A spin-coating process has also been developed that produced thin, non-obstructing coatings for hot-gas filter substrates. The thermal shock and corrosion resistance of coated parts were evaluated, and preliminary results are encouraging for the use of CS-50 as a protective coating.
Keywords: CS-50 coatings, sulfate salt corrosion