Effect of quenching, tempering, and cold rolling on creep deformation behavior of a tempered martensitic 9Cr-1W steel
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AN increasing steam temperature for the boiler and turbine components of a power plant, which enhances thermal efficiency, requires the development of advanced ferritic heat-resistant steels with sufficient creep-rupture strength at high temperatures. Tempered martensitic 9 to 12 Cr steels strengthened by tungsten have become of much interest for application to high-temperature plants, such as coal-fired ultrasupercritical (USC) power plants at temperatures higher than 873 K[1,2,3] and the first wall and blanket of a fusion reactor,[4] because of their sufficient creep-rupture strength. Enhancement of the thermal efficiency results in lower fuel consumption and lower carbon dioxide emissions. The improvement of creep strength has been performed for 9 to 12 Cr steels mainly by optimizing alloying elements, such as tungsten, molybdenum, and boron, to obtain desirable precipitation and solid-solution strengthening.[5] There are many studies on the effect of alloying elements on creepdeformation behavior and creep-rupture strength.[6] At present, however, the understanding of the effect of pretreatments such as tempering and cold rolling on the creepdeformation behavior is rather insufficient. The 9 to 12Cr steels are usually used after normalizing followed by temFUJIO ABE, Director, is with the Heat Resistant Design Group, Steel Research Center, National Institute for Materials Science, Tsukuba 305– 0047, Japan. Contact e-mail: [email protected] Manuscript submitted July 8, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
pering and exhibit a tempered martensitic microstructure. Tempering conditions, such as temperature and time, affect the resultant dislocation density and dislocation substructure as well as carbide distributions after tempering. On the other hand, high-temperature components, such as the boiler tubes and main steam pipes of a power plant, are usually subjected to cold working in the final stage of fabrication and plant construction. Cold working produces a deformation microstructure. A different initial microstructure in terms of the dislocation substructure and carbide distribution can cause a different microstructure evolution during creep, which can affect the creep-deformation behavior. There are very important differences in the creepdeformation behavior of various classes of materials, such as solid-solution alloys and particle-hardened alloys.[7,8] In tempered martensitic 9Cr steels containing tungsten, the present author and co-workers have revealed that the agglomeration of M23C6 carbides, the recovery of excess dislocations produced by the martensitic transformation, and the coarsening of laths proceed during creep with the aid of stress or strain and that the precipitation of Fe2W Laves phase takes place from a supersaturated solid solution of 9Cr-W steels containing 2 wt pct W or more during creep at around 873 K.[9,10,11] The effect of Fe2W Laves-phase precipitation and subsequent coarsening on the creep rate is analyzed for 9Cr steels in detail using creep rate vs time curves an
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