Laves phase is not detrimental but promising as strengthening species, unlike common sense. For sustainable and reliable energy supply sources, advanced thermal power plants (A-USC) is receiving more attention since shutdown of nuclear power plants by an unprecedented devastating earthquake in Japan. For A-USC in reality, new materials with superior long-term creep rupture strength longer than 105 hours under 100 MPa at 973 K are required. Besides some Ni base alloys, none of the conventional austenitic heat resistant steels such as SUS 347 HTB meet this requirement. Why? This is due to microstructure instability of major strengthening spices/metallic carbides during high temperature exposure. Then, we recently proposed a new type of austenitic heat resistant steels, a carbon free Fe-20Cr-30Ni-2Nb (at%), strengthened by two types of equilibrium intermetallics of GCP (geometrically close-packed) and TCP (topologically close-packed) phases, based on our phase diagram study on Fe-Ni-M (M: transition metals) ternary systems at elevated temperatures. The model steel exhibits excellent creep properties to meet the requirement for A-USC. The superior creep rupture strength is caused by Fe2Nb (TCP) phase precipitated at the grain boundaries (GBs). The higher the fraction (ρ) of GBs covered by the Laves phase, the lower the creep rate along with the following equation: (creep rate)/(creep rate)0=(1-ρ), where the (creep rate)0 is the creep rate with ρ=0. We call the novel strengthening mechanism as “Grain-boundary Precipitation Strengthening (GBPS)”. This result suggests an important message that creep deformation occurs along grain boundaries. Note that the steel with ρ=90 % exhibits excellent creep elongation of 77 %, tensile ductility of 33 %, and Charpy absorption energy of 30 J/cm2 at room temperature, indicating no embrittlement effect by the GB Laves phase. It should also noted that Fe2Nb Laves phase shows tremendous solid solution softening by Ni in solution and plastically deformed at room temperature. Thus, Laves phase is no more detrimental but promising. In this talk, we cover the nature of Fe2Nb Laves phase and design concept of the novel austenitic heat resistant steels strengthened by the Laves phase. 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), and “Fundamental Studies on Technologies for Steel Materials with Enhanced Strength and Functions” by Consortium of JRCM (The Japan Research and Development Center for Metals) .
各種検索
サポート
ヘルプ