Development of Ultra-high Purity (UHP) Fe-Based Alloys with High Creep and Oxidation Resistance for A-USC Technology
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ODUCTION
AUSTENITIC steel has been used as a structural material in several energy production applications due to its good oxidation resistance.[1–3] It also has high creep strength, which relies on its mechanical stability at high temperatures.[2,4] Excellent efficiency conversion in boiler/steam turbines, for instance, is achievable by increasing the steam temperature beyond 700 C.[5–7] The current challenge is the use of austenitic steels capable of operation in advanced ultra-supercritical water conditions (A-USC) at 700 C and 35 MPa.[8] The commercial heat-resistant Ni-Fe base alloy HR6W meets the requirement in A-USC conditions with approximately 900-hour creep-rupture strength at 700 C and a stress level of 150 MPa.[9–11] The high creep strength of HR6W is attributed mainly to precipitation strengthening by stable intermetallic Fe2W Laves, but it has a high cost because of the large amount of Ni content (45 wt pct). Additionally,
FETHI HAMDANI, NISHITH K. DAS, and TETSUO SHOJI are with the Frontier Research Initiative, New Industry Creation Hatchery Center, Tohoku University, 6-6-10, Aramaki Aza Aoba, Aoba-Ku, Sendai 980-8579, Japan. Contact e-mail: fethi.hamdani@ fri.niche.tohoku.ac.jp Manuscript submitted July 25, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
carbides susceptible to forming in this alloy that contain approximately 0.1 wt pct carbon can coarsen over the operating time. It is well known that the coarsening of carbides is a detrimental process to the creep strength. Steam oxidation resistance is also a concern. Exposure to SCW poses a serious challenge in terms of hydrogen-accelerated oxidation with water vapor species.[12,13] Based on thermodynamic calculations, a few ppm of carbon and nitrogen in Fe-Cr-Ni-Nb-X, where X corresponds to reactive elements (Mo or Zr), leads to the precipitation of carbonitrides. Additionally, the presence of trace elements of S or P plays an important role in creep cavitations along grain boundaries, which weaken the creep strength. For this reason, ultra-high purity (UHP) austenitic Fe-based alloys were investigated in the present study. An induction melting furnace with a cold copper crucible is used to produce austenitic Fe-based model alloys.[14] The addition of Mo improves the creep properties of austenitic Fe-based alloy by solid solution hardening. Stabilizing elements, such as Nb and Zr, greatly improve the creep strength of austenitic Fe-based alloys via precipitation of fine intermetallic compounds. Yamamoto et al.[15] reported that fine dispersion of Fe2Nb Laves and d-Ni3Nb within the c-Fe matrix of Fe-18Cr-31Ni-3Nb-0.26Zr alloy (wt pct) at 750 C remarkably improved the creep properties. Zr addition stabilized fine d-Ni3Nb compounds and allowed the formation of nanosized (Ni, Zr, Nb)-enriched precipitates. However, it was reported that the
precipitation of coarse Fe2Nb Laves in Fe-15Cr-20Ni-2.5Al-2.5Mo-0.9Nb (wt pct) at 750 C was related to the high Nb content.[16] The formation of micro-sized Fe2Nb Laves contributed unfavorably to the creep re
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