Creep Strength Of Austenitic Steels Research Articles

Progress in Creep-Resistant Steels for High Efficiency Coal-Fired Power Plants

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.

Effect of nickel content on creep strength of Fe-15Cr-Ni-2.5Mo-0.3Ti austenitic steels for fast breeder reactor fuel cladding tube

The fuel cladding tubes of fast breeder reactors (FBR) are subjected to internal pressure caused by fission product gases at temperatures above 923 K under fast neutron irradiation. Therefore, resistance to swelling and creep strength are the principle requisites for cladding materials. Austenitic steels such as type 316 steel have been selected for cladding material. The resistance to swelling of austenitic steels is known to be suppressed by cold working [1] and increasing nickel content [2]. However, the effect of nickel content on the creep strength of austenitic steels is not well understood. This letter describes the effect of nickel content ranging from 15 to 35 wt % on the creep strength of cold-worked Fe-15Cr-Ni-2.5Mo -0.3Ti austenitic steels strengthened by the precipitation of MC type (TIC) carbides. The chemical compositions of the test materials are given in Table I. Only the nickel content was varied, ranging from 15 to 35 wt %, and the other compositions, including molybdenum, titanium, phosphorus and boron (elements that have been found to be effective in improving the creep strength), were held constant. The test materials were melted in a vacuum-induction furnace and cast into 10 kg ingots. Th%ingots were hot forged and hot rolled to sheets of 6 mm thickness. After solution treating at 1373K and cold rolling to 2.5ram thickness, they were final solution treated at 1373 K or 1423 K. The sheets were cold rolled for 20% before creep-rupture testing. Creep-rupture tests were performed at 973 K using plate specimens 2 mm thick and with gauge section 6 mm wide and 25 mm long. The microstructures of the specimens were examined by transmission electron microscopy (TEM).