Carbide Precipitation in 2.25 Cr-1 Mo Bainitic Steel: Effect of Heating and Isothermal Tempering Conditions
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LIGHT water reactors are representing 80 pct of the world nuclear power plants. On an international level, the renewal of second-generation nuclear power plants is still to be provided by light water reactors until at least the middle of the century. Safety and technological improvements of light water reactors will thus represent key issues for the next decades. In this framework, a 2.25 Cr-1 Mo steel is being considered as potential candidate pressure vessel material for future light water reactors. The two main reasons are its good mechanical properties in the quenched and tempered condition[1] and its good resistance to radiation-induced embrittlement compared to usual pressure vessel steels.[2] In such material, the precipitation of carbides in the bainitic matrix during its formation due to quenching and during the subsequent tempering is of primary SYLVAIN DE´PINOY is with the DEN-Service de Recherches Me´tallurgiques Applique´es, CEA, Universite´ Paris-Saclay, 91191, Gif sur Yvette, France, and also with the MINES ParisTech, Centre des Mate´riaux, PSL Research University, UMR CNRS 7633, BP 87, 91003, Evry Cedex, France. Contact e-mail: [email protected] CAROLINE TOFFOLON-MASCLET, STE´PHANE URVOY, JUSTINE ROUBAUD, and BERNARD MARINI are with the DEN-Service de Recherches Me´tallurgiques Applique´es, CEA, Universite´ Paris-Saclay. FRANC¸OIS ROCH is with AREVA, 1 place Jean Millier, 92084, Paris La De´fense Cedex, France. ERNST KOZESCHNIK is with the Institute of Materials Science and Technology, TU Wien, Getreidemarkt 9,Vienna 1060, Austria. ANNE-FRANC¸OISE GOURGUESLORENZON is with the MINES ParisTech, Centre des Mate´riaux, PSL Research University, UMR CNRS 7633. Manuscript submitted November 22, 2016. Article published online March 9, 2017 2164—VOLUME 48A, MAY 2017
importance. Various types of carbides with different thermal stabilities can be found depending on the tempering time and temperature, namely, M3C, M2C, M7C3, M23C6, and M6C.[3] The size, nature, and phase fraction of carbides have a direct influence on the mechanical properties of the material, for instance by improving creep strength[4] and by inducing secondary hardening.[5] Thus, tight control of carbide precipitation through manufacturing heat treatments is a key point for the targeted industrial application. Pioneering work on carbide precipitation in 2.25 Cr-1 Mo steel was published by Baker & Nutting in 1959,[3] and can be summarized as follows. Upon tempering, the cementite M3C is the first type of carbide that precipitates, followed by metastable carbides M2C and M7C3. With further tempering, these three metastable carbides are replaced by the equilibrium carbides M23C6 and M6C. This precipitation sequence has been widely confirmed since then[5–8] for martensitic and bainitic 2.25 Cr-1 Mo steels, with the exception of Vyrostokova et al.[9] and Janovec et al.[10] who reported that M23C6 carbides dissolve to the benefit of M7C3 carbides in vanadium-bearing 2.25 Cr-1 Mo steels. However, the domain of stability of carbides is highly depe
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