Crystallography and Morphology of Niobium Carbide in As-Cast HP-Niobium Reformer Tubes
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HEAT-RESISTANT Fe-Cr-Ni-C austenitic stainless steels have become the dominant alloys for use in steam reforming and pyrolysis applications by the petrochemical industry. For example, depending on the process requirements, centrifugally cast alloy reformer tubes typically operate at temperatures between 1123 K and 1323 K (850 C and 1050 C) and internal pressures between 2 and 5 MPa for up to 100,000 hours. The combination of high temperatures and moderate stresses causes creep to be the dominant failure mechanism experienced in service. Initially, the HK (25 pct Cr, 20 pct Ni) alloy series replaced HT (18 pct Cr, 37 pct Ni) superalloys, providing tubes with comparable creep performance while reducing cost (due to the decrease in Ni). Subsequently, the HK series was superseded by the HP series (25 pct Cr, 35 pct Ni), which has shown higher creep strength and oxidation resistance than its predecessors. Extensive work has been carried out to optimize the microstructure of all of these alloys to maximize their high-temperature strength. In the as-cast condition, the standard 25Cr-35Ni HP microstructure consists of a dendritic network of primary Cr7C3 (commonly known in the literature as M7C3) in an austenite matrix, which is relatively free of precipitation. On exposure to service temperatures, the primary Cr7C3 transforms to Cr23C6 and extensive secondary precipitation of Cr23C6 occurs within the matrix. The grain boundary carbide network is believed to inhibit grain boundary sliding, while the secondary matrix carbides KARL G. BUCHANAN, Doctoral Candidate, and MILO V. KRAL, Professor, are with the Department of Mechanical Engineering, University of Canterbury, Private Bag 4800, Christchurch, New Zealand. Contact e-mail: [email protected] Manuscript submitted December 1, 2010. Article published online January 19, 2012 1760—VOLUME 43A, JUNE 2012
obstruct dislocation motion.[1] However, due to the rapid diffusion of chromium through the austenite matrix (in comparison to vanadium, niobium, and titanium), the high coarsening rate of Cr23C6 causes it to possibly be the least effective of the available carbides for strengthening high-temperature stainless steels.[2] Addition of niobium, a strong carbide forming element, was reported to cause partial replacement of Cr7C3 with NbC (MC), refining and fragmenting the primary carbide network. The amount of replacement, refinement, and fragmentation of the as-cast carbide network is a function of the niobium content.[3] Accelerated creep testing showed that niobium-modified HP alloys have increased rupture life and lower minimum creep rates in comparison to the standard HP composition (i.e., without niobium).[3] However, due to the decomposition of niobium carbide to G phase (Ni16Nb6Si7) during exposure to elevated temperatures, the long-term strengthening effect of niobium is relatively unknown. In the present work, the microstructures of two ascast niobium-modified HP alloys were characterized with specific attention being paid to the primary NbC precipitates. Past studies
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