Evolution of Minor Phases in a 9PctCr Steel: Effect of Tempering Temperature and Relation with Hydrogen Trapping
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THE current trend in the design of modern conventional thermal power plants is the increase of the steam temperature and pressure, which has lead to the development of supercritical and ultra-supercritical power plants. In these plants, the steam temperature at the entry of the high-pressure turbine is typically 813 K to 833 K (540 C to 560 C). These exigent conditions led to the development of steels resistant to creep as well as to high temperature oxidation.[1] In the last decades, the search for materials that fulfill these conditions led to the development of the family of the 9pctCr-1pctMo steels. These steels are usually employed in service in the normalized and tempered condition. Through this sequence of treatments, a particular microstructure is obtained, which is adequate to confer the required mechanical properties.[2] Particularly, the good creep resistance has been ascribed to a distribution of fine precipitates of Nb and/or V of the MX type. These particles favor the persistence of a dislocation network during prolonged periods at the service temperature.[3] Generally, the size of these particles is lower than 0.1 lm; they can be classified into three types according to their morphology and chemical composition.[4] Type I MX corresponds to spherical, Nb-rich precipitates. Type II MX designs VCAROLINA HURTADO-NOREN˜A, CLAUDIO ARIEL DANO´N, MARI´A INE´S LUPPO, and PABLO BRUZZONI, Researchers, are with the Comisio´n Nacional de Energı´ a Ato´mica (Argentina), Av. General Paz 1499, B1650KNA, San Martı´ n, Buenos Aires, Argentina. Contact e-mails: [email protected]; carohuno@ gmail.com Manuscript submitted August 13, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
rich precipitates, with cuboid- or rod-like morphology. Type III precipitates display a specific morphology, known as ‘‘wing’’ in which V-rich rods grow on a Nbrich spherical core. Thus, the characterization of precipitated phases in 9pctCr-1pctMo steels turns out to be of great interest, given the close relationship between the morphology, spatial distribution and stability of the precipitated particles and the in-service behavior of the material. The microstructure and the in-service behavior of the 9pctCr-1pctMo steels have been intensively investigated. The precipitation state has been recognized to be a function of the thermomechanical treatment of the steels, particularly of the austenitizing and tempering temperatures. The literature reports studies that employ diverse techniques with the purpose of obtaining complete and specific results according to the field of interest.[5–7] The techniques used in the present work are X-ray diffraction (XRD) with synchrotron light, field emission gun scanning electron microscopy (FEG-SEM), and transmission electron microscopy (TEM) with energydispersive analysis of X-rays (EDS). The XRD technique, in comparison with TEM, presents the advantage that a relative large surface area of the material is sampled, thus yielding average values of the relative abundance of the detected structures. Additional
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