Showing papers by "Fujio Abe published in 2016"
TL;DR: In this article, the authors comprehensively reviewed the recent progress in creep-resistant bainitic, martensitic, and austenitic steels for high efficiency coal-fired power plants with emphasis on long-term creep strength and microstructure stability at grain boundaries (GBs).
Abstract: Recent progress in creep-resistant bainitic, martensitic, and austenitic steels for high efficiency coal-fired power plants is comprehensively reviewed with emphasis on long-term creep strength and microstructure stability at grain boundaries (GBs). The creep strength enhanced ferritic (CSEF) steels, such as Grade 91 (9Cr–1Mo–0.2V–0.05Nb), Grade 92 (9Cr–0.5Mo–1.8W–VNb), and Grade 122 (11Cr–0.4Mo–2W–1CuVNb), can offer the highest potential to meet the required flexibility for ultra-supercritical (USC) power plants operating at around 600 °C, because of their smaller thermal expansion and larger thermal conductivity than austenitic steels and Ni base alloys. Further improvement of creep strength of martensitic 9 to 12Cr steels has been achieved by substituting a part or all of Mo with W and also by the addition of Co, V, Nb, and boron. A martensitic 9Cr–3W–3Co–VNb steel strengthened by boron and MX nitrides, designated MARBN, exhibits not only much higher creep strength of base metal than Grade 91, Grade 92, and Grade 122 but also substantially no degradation in creep strength due to type IV fracture in welded joints at 650 °C. High-strength bainitic 2.25 to 3Cr steels have been developed by enhancing solid solution hardening due to W and precipitation hardening due to (V,Nb)C carbides in bainitic microstructure. The improvement of creep strength of austenitic steels has been achieved by solid solution hardening due to the addition of Mo, W, and nitrogen and by precipitation hardening due to the formation of fine MX (M = Ti, Nb, X = C, N), NbCrN, M23C6, Cu phase, and Fe2(Mo,W) Laves phase. The boundary and sub-boundary hardening are shown to be the most important strengthening mechanism in creep of creep-resistant steels and is enhanced by fine dispersions of precipitates along boundaries.
76 citations
TL;DR: In this article, the authors investigated the effect of impurities on the creep strength of 300 series stainless steels for boiler and heat exchanger seamless tubes, namely, 18Cr-8Ni (JIS SUS 304HTB), 18cr-12Ni-Mo, 12Ni-Ni-Ti, and 14Ni-Nb, at 873 K to 1023 K (600 −C to 750 −C) and the maximum time to rupture was 222,705.3
Abstract: Metallurgical factors causing the heat-to-heat variation in time to rupture have been investigated for 300 series stainless steels for boiler and heat exchanger seamless tubes, 18Cr-8Ni (JIS SUS 304HTB), 18Cr-12Ni-Mo (JIS SUS 316HTB), 18Cr-10Ni-Ti (JIS SUS321 HTB), and 18Cr-12Ni-Nb (JIS SUS 347HTB), at 873 K to 1023 K (600 °C to 750 °C) using creep rupture data for nine heats of the respective steels in the NIMS Creep Data Sheets. The maximum time to rupture was 222,705.3 hours. The heat-to-heat variation in time to rupture of the 304HTB and 316HTB becomes more significant with longer test durations at times above ~10,000 hours at 973 K (700 °C) and reaches to about an order of magnitude difference between the strongest and weakest heats at 100,000 hours, whereas that of the 321HTB and 347HTB is very large of about an order of magnitude difference from a short time of ~100 hours to long times exceeding 100,000 hours at 873 K to 973 K (600 °C to 700 °C). The heat-to-heat variation in time to rupture is mainly explained by the effect of impurities: Al and Ti for the 304HTB and 316HTB, which reduces the concentration of dissolved nitrogen available for the creep strength by the formation of AlN and TiN during creep, and boron for the 347HTB, which enhances fine distributions of M23C6 carbides along grain boundaries. The heat-to-heat variation in time to rupture of the 321HTB is caused by the heat-to-heat variation in grain size, which is inversely proportional to the concentration of Ti. The fundamental creep rupture strength not influenced by impurities is estimated for the steels. The 100,000 hours-fundamental creep rupture strength of the 347HTB steel is lower than that of 304HTB and 316HTB at 873 K and 923 K (600 °C and 650 °C) because the slope of stress vs time to rupture curves is steeper in the 347HTB than in the 304HTB and 316HTB. The 100,000 hours-fundamental creep rupture strength of the 321HTB exhibits large variation depending on grain size.
7 citations
TL;DR: In this article, heat-to-heat variation in creep life has been investigated for the 9 heats of JIS SUS 304HTB (18Cr-8Ni steel) and also for the nine heats of Nb into account.
Abstract: Heat-to-heat variation in creep life has been investigated for the 9 heats of JIS SUS 304HTB (18Cr–8Ni steel) and also for the 9 heats of JIS SUS 347HTB (18Cr–12Ni–Nb steel) in the NIMS Creep Data Sheets, mainly taking the effect of Nb into account. The heat-to-heat variation in creep life of 304HTB is mainly caused by the variation in precipitation hardening due to fine NbC carbides at short times, while it is mainly caused by the variation in available nitrogen concentration, defined as the concentration of nitrogen free from AlN and TiN, at long times. The heat-to-heat variation in creep life of 347HTB is mainly explained by the variation of boron concentration, 3–27 ppm, but not by the variation of solution temperature, Nb/C atomic ratio and phosphorus concentration. Boron reduces the coarsening rate of fine M23C6 carbides along grain boundaries, which enhances the grain boundary precipitation hardening.
2 citations