Austenite Grain Growth in a 2.25Cr-1Mo Vanadium-Free Steel Accounting for Zener Pinning and Solute Drag: Experimental St
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INTRODUCTION
Light water reactors are representing 80 pct of the world nuclear power plants. On an international level, the renewal of second-generation light water reactors is to be provided by light water reactors until at least the middle of the century. Thus, safety and technological improvement of these reactors represent key issues in electrical power generation. In this framework, the 2.25Cr-1Mo steel family, already widely used in the petrochemical industry, is being considered as a potential candidate pressure vessel material for future light water reactors. The two main reasons are its good mechanical properties in the quenched and tempered conditions[1] and its very good resistance to radiation-induced embrittlement compared to usual pressure vessel steels due to its low Ni content and low residuals.[2] Quenched and tempered 2.25Cr-1Mo steels are fully bainitic, which leads to optimum tensile and toughness properties, as well as creep resistance, compared to a proeutectoid ferritic bearing materials.[3,4] However,
S. 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, PSL Research University-Centre des Mate´riaux, UMR CNRS 7633, 9 BP 87, 91003 Evry cedex, France. B. MARINI and C. TOFFOLON-MASCLET are with the DEN-Service de Recherches Me´tallurgiques Applique´es, CEA, Universite´ Paris-Saclay, Contact e-mail: [email protected] F. ROCH is with AREVA, 1 place Jean Millier 92084 Paris La De´fense Cedex. A.-F. GOURGUES-LORENZON is with MINES ParisTech, PSL Research University-Centre des Mate´riaux, UMR CNRS 7633, BP 87, 91003 Evry Cedex, France. Manuscript submitted October 17, 2016. Article published online February 13, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
during water quench of thick-walled components such as pressure vessels, a gradient in cooling rate takes place across the wall thickness and may lead to the formation of ferrite. While the occurrence of 2 pct proeutectoid ferrite does not change the mechanical properties,[5] it is generally advised to minimize the proeutectoid ferrite fraction.[6] A convenient way to control transformations upon quenching is by increasing the austenite grain size prior to the quench, as coarser austenite grains delay the ferritic transformation toward slower cooling rates.[7,8] However, and while the austenite grain size has no direct impact on the subsequent carbide precipitation during tempering[9,10] and on the tensile properties of 2.25Cr-1Mo steels,[8–11] too coarse grains can decrease the toughness,[8,11] increase the susceptibility to temper embrittlement,[12] increase the bainitic packet size[9,11] and may lead to formation of martensite[7,9] after quenching. Additionally, the austenite grain size has to be constant across the wall thickness of the considered component in order to ensure homogeneous microstructure and mechanical properties after quenching. Thus, austenite grain growth must be understood in order to control the a
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