Overview of ODS Alloy Development
- ORNL
The overall goal of this effort is to address the materials-related barriers to expediting the use of oxide dispersion-strengthened (ODS) alloys in components required in the U.S. Department of Energy's (DOE's) Office of Fossil Energy's advanced coal combustion, gasification, and utilization processes to operate at temperatures higher than are possible with conventionally-strengthened alloys. The project is focused on the needs of the broad range of ferritic ODS-alloys, especially the FeCrAl-based alloys, with the purpose of developing a detailed understanding of the behavior of ODS alloys in all phases of their use, including fabrication, service performance, life prediction, mode of failure, repair, and refurbishment. The intended output of this project is a compilation of information that facilitates the assessment of the applicability of ODS alloys to the needs of high-temperature equipment required in DOE's advanced power plants. Particular emphasis has been placed on processing modifications to increase the hoop strength of ODS tubes, on joining, and on oxidation-limited life prediction. Approaches for modifying the alloy grain structure to increase the creep strength in the transverse/hoop direction (for tubes) have involved the examination of processes that modify the secondary recrystallization behavior of the alloy, with the intention of producing larger, more equiaxed grains, or a spiral grain structure that reduces the number of transverse grain boundaries. Processing using a commercial-scale pilgering/flow-forming/cross-rolling facility indicated the potential of this route, but also revealed some practical difficulties, especially since such processing of ODS alloys must be done at temperatures above approximately 600 C. Research to provide guidance for such processing has involved measurements using well-controlled, hot torsion testing, as well as trials with a small-scale cross-rolling mill. Progress has been made with joining approaches that involve inertia welding, and diffusion bonding, and the creep strength of butt joints made by some of the process variations approaches the transverse creep strength of the parent alloy. A particular feature of the ferritic ODS-FeCrAl alloys is that they exhibit relatively low oxidation rates up to 1200 C (2192 F), due to the formation of a protective Al2O3 film. In service, the mechanical load applied to these ODS alloys will be below the stress level at which significant creep occurs, so that it is likely that the effective service lifetime will be determined by the rate of environmental degradation. Operation at the very high temperatures possible with these alloys means that, should the ability to form a protective oxide scale be lost, the ensuing oxidation damage could be very rapid. However, since during protective oxidation there is very little loss in alloy section due to the thin alumina scales formed, it is doubtful that mechanical means for monitoring the rate consumption of the alloy would provide sufficient accuracy for assessment of remaining lifetime. Hence, there is a need for some form of lifetime model to provide reliable prediction of remaining lifetime in typical service environments. Efforts to develop a lifetime model are in progress for the condition where the life-limiting process is oxidation in air (or oxygen); some data also have been generated in reducing environments (where there is potential for sulfidation attack); and in steam. Comparison of predictions from the model with results from lifetime testing in air indicate that good progress is being made.
- Research Organization:
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
- Sponsoring Organization:
- FE USDOE - Office of Fossil Energy (FE)
- DOE Contract Number:
- DE-AC05-00OR22725
- OSTI ID:
- 964685
- Resource Relation:
- Conference: 19th Annual Fossil Energy Materials Conference, Knoxville, TN, USA, 20050509, 20050509
- Country of Publication:
- United States
- Language:
- English
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