Corrosion and Mechanical Performance of Grade 92 Ferritic-Martensitic Steel After Exposure to Supercritical Carbon Dioxi
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THE supercritical carbon dioxide (s-CO2) Brayton cycle is currently being considered for next-generation power systems for uses across nuclear, solar, and fossil power sources. Compared to the current industry standard, the Rankine cycle, the Brayton cycle has been found to produce higher efficiencies at comparable operating temperatures.[1,2] Implementing an s-CO2 Brayton cycle, however, will not be possible without a thorough understanding of the effects of the environment on candidate materials.[3] Grade 92 ferritic-martensitic steel (P92 or NF616)— along with other 9-12 pct Cr ferritic steels—is being considered for medium temperature applications due to its good strength and creep properties up to its highest suggested service temperature of 620 C.[4–10] These high-temperature properties are a result of precipitation of M23C6 carbides and MX carbonitrides along grain boundaries. A homogeneous distribution of these ANDREW BRITTAN is with the University of WisconsinMadison, 1500 Engineering Drive, Madison, WI, 53706 and also with the Oregon State University, 1891 SW Campus Way, Corvallis, OR, 97331. Contact e-mail: [email protected] JACOB MAHAFFEY is with the Sandia National Laboratory, Albuquerque, NM, 87123. MARK ANDERSON is with the University of Wisconsin-Madison. Manuscript submitted October 10, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
precipitates nucleates and remains relatively small during initial heat treatment of P92, due to the stabilizing effect of W.[11,12] The stabilization of precipitates causes no detrimental coarsening after further aging at 600 C. However, after 1000 hours of aging at 600 C Laves-phase precipitates and coarsens to above 0.5 lm, resulting in embrittlement, though the effect of this on creep strength is not confirmed.[7,13] The mechanical properties of 9-12 pct Cr ferritic steels are highly dependent on the size, density, distribution, and morphology of both strengthening precipitates.[5] The P92 microstructure consists of 30–50 lm prior-austenite grains which are transformed during the heat treatment process to a dual phase structure of ferrite and tempered martensite, resulting in a grain size of 1-10 lm.[14] This fine grain structure allows for high strength from the near-homogeneous precipitation of strengthening precipitates along grain boundaries. Carbonitrides are not expected to significantly coarsen with additional carbon deposition, as they are likely limited by the availability of their forming metals, trace alloying elements V and Nb. Conversely, the forming metals of M23C6 carbides are primary alloying constituents (Fe, Cr, W, and Mo). The abundance of these constituents leads to enhanced coarsening when carbon is added to the matrix, in this case by environmental exposure to CO2.[15–18] Additionally, the refined microstructure of P92 creates a dense network of diffusion pathways for carbon, potentially enhancing the depth-dependent
effects of carburization when exposed to the CO2 environment. Oxidation of P92 and similar 9-12 pct Cr steels in CO2 has bee
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