Creep behavior of carbon free Fe-20Cr-30Ni-2Nb (at.%) steels strengthened by Fe2Nb Laves phase at 1073 K has been examined in air and water vapor atmosphere, in order to identify the effect of water vapor on creep properties. The steel studied has the compsition with 0.03 at.% B. Solution treated (1473 K/2 h) specimen and pre-aged (1073 K/240 h) specimen were used to intentionally change the area fraction of the Laves phase on grain boundaries (ρ) of 0% and 80%, respectively. Creep test was carried out at 1073 K/70 MPa in air and water vapor. Under air atmosphere, the specimen with ρ=0% ruptured after 293 h, whereas the specimen with ρ= 80% exhibits the rupture time of 833 h. In addition, the latter specimen shows smaller creep rate than the former in entire creep stage. This pronounced extension of rupture life is caused by the "grain boundary precipitation strengthening" (GBPS) effect by Laves phase, as proposed by authors. Under water vapor atmosphere, the specimen with ρ= 0% exhibits the minimum creep rate smaller than that of 8.0x10-4 /h of the specimen tested in air, and the rupture life is extended to 376 h. The pre-aged specimen with ρ= 80% shows no difference in creep properties regardless of the atmoshere. These results suggest that the grain boundaries not covered by the Laves phase are somehow sensitive to the enviroment, and the grain boundary Laves phase is effective in reducing the enviromental effect. Thus, the novel steels strengthened by the GBPS are promising for use of further advanced ultra-super critical (FA-USC) power plant to be operated above 700 oC. Detailed oxide layer formed with and without water vapor, and effect of oxide layer on the formation and morphology of the grain boundary Laves phase will be presented. Part of this study was carried under the research activities of “Advanced Low Carbon Technology Research and Development Program” (ALCA) in JST (Japan Science and Technology Agency).
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