Gamma Prime Precipitation, Dislocation Densities, and TiN in Creep-Exposed Inconel 617 Alloy
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ormerly with the Department of Engineering, University of Leicester, Leicester, LE1 7RH, U.K. is now Postdoctoral Research Associate, Dalton Cumbrian Facility, University of Manchester, Westlakes Science and Technology Park, Moor Row, Whitehaven, Cumbria CA24 3HA, U.K. HELEN V. ATKINSON, Head of Department, SARAH V. HAINSWORTH, Graduate Dean for the University of Leicester, and SIMON P. GILL, Reader, are with the Department of Engineering, University of Leicester. Contact e-mail: [email protected] Manuscript submitted December 19, 2014. Article published online October 29, 2015 178—VOLUME 47A, JANUARY 2016
INTRODUCTION
INCONEL 617 (UNS N06617) was originally introduced by Hosier and Tillack in 1972[1] and is a solid-solution strengthened austenitic Ni-based superalloy containing ~23 wt pct Cr, 12 wt pct Co, and 9 wt pct Mo with small additions of Al, Ti, and Fe. Cobalt and molybdenum are responsible for providing solid-solution strengthening, while aluminum and chromium confer the oxidation resistance at high temperatures. This alloy is one of the preferred structural materials for steam boilers and turbine, components to be used in advanced fossil-fuelled power plants to accommodate demanding operating conditions. Inconel METALLURGICAL AND MATERIALS TRANSACTIONS A
617 has been successfully used at temperatures up to 873 K (600 C) in conventional fossil-fuelled power plants and is currently being investigated for higher temperature applications in advanced steam power plants (‘ultrasupercritical’) envisioned to operate at 973 K (700 C)/37.5 MPa and above.[1–4] There have been recent studies on this particular alloy at higher temperatures [1073 K (800 C) and above] for heat exchanger applications in nuclear power reactors (e.g., Chomette et al.,[5] Kaoumi and Hrutkay[6] and Roy et al.[7]) but this study focusses on the 923 K/973 K (650 C/700 C) regime more suited to steam power plant. In addition to solid-solution strengthening, the grain structure of Inconel 617 is stabilized by precipitation of carbides, and some strengthening of the alloy is derived from inter- and intra-granular carbides and carbo-nitrides of types M23C6 and Ti (C, N).[8] Titanium and aluminum may additionally contribute to precipitation hardening through intermetallic precipitation of c¢. After exposure to high temperature 1273 K (1000 C) and creep conditions, precipitation and coarsening continually remove strengthening elements from the c-matrix and significantly degrade the high temperature creep and mechanical properties of the alloy.[9,10] Several structural characterization studies[11–20] have been conducted on thermally and creep-exposed Inconel 617 alloys. These papers have been primarily focused on the precipitation of M23C6 and Ti (C, N) in the thermal and creep-exposed alloy, and there has been less emphasis on the role of c¢ in the microstructure. In previous studies of c¢ in Inconel 617, Mankins et al.[8] performed a study on samples exposed for 215 to 10,000 hours at temperatures up to 1366 K (1093 C). The major precipitate th
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