Crystallography and Morphology of MC Carbides in Niobium-Titanium Modified As-Cast HP Alloys
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TRODUCTION
HIGH alloy Fe-Cr-Ni-C austenitic stainless steels have become the principal alloys for use in steammethane reforming furnaces within the petrochemical industry.[1] Each furnace contains a large array of vertically oriented centrifugally cast tubes through which a mixture of methane and steam is flowed across nickel oxide catalysts in order to obtain a mixture of hydrogen, carbon monoxide, carbon dioxide, and water commonly known as synthesis gas (or syngas). Generally, the tubes operate at temperatures between 1123 K to 1323 K (850 C to 1050 C), internal pressures between 1 and 3.5 MPa, and are expected to withstand service lives in excess of 100,000 hours. The combination of high temperatures and moderate stresses causes creep to be the dominant failure mechanism experienced by these tubes in service. The HP austenitic alloys (25 pct Cr and 35 pct Ni) are the latest in a series of heat-resisting (H-series) stainless steels developed to provide high-temperature strength, ductility, and corrosion resistance in the oxygen-, carbon-, and sulfur-rich environments typical of these furnaces. Extensive work has been carried out to optimize HP alloys’ microstructure in order to maximize creep resistance.[2–5] 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 (c) matrix that is relatively free of precipitation. On exposure to service KARL G. BUCHANAN, formerly Ph.D. Candidate with the Department of Mechanical Engineering, University of Canterbury, Private Bag 4800, Christchurch, New Zealand, is now Postdoctoral Fellow with the CEA Saclay, Gif-sur-Yvette, France. MILO V. KRAL, Professor, and CATHERINE M. BISHOP, Senior Lecturer, are with the Department of Mechanical Engineering, University of Canterbury. Contact e-mail: [email protected]. Manuscript submitted May 21, 2013. Article published online April 8, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
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 obstruct dislocation motion.[6] 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 reportedly causes it to possibly be the least effective of the available carbides for strengthening high-temperature stainless steels.[7] In recent variants of the HP alloy, creep strength increases have largely been realized through the use of Nb and/or Ti additions, which modify the primary precipitates (formed during solidification) and secondary precipitates (formed during exposure to the service temperatures). The use of Nb and/or Ti causes the partial replacement of the primary Cr7C3 with Nb-rich MC precipitates.[2–5] In HP alloys that are modified with Nb alone, the MC precipitates obtain a lamella
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