Abstract
A class of low-alloy steels often referred to as SA508 represent key materials for the manufacture of nuclear reactor pressure vessels. The alloys have good properties, but the scatter in properties is of prime interest in safe design. Such scatter can arise from microstructural variations but most studies conclude that large components made from such steels are, following heat treatment, fully bainitic. In the present work, we demonstrate with the help of a variety of experimental techniques that the microstructures of three SA508 Gr.3 alloys are far from homogeneous when considered in the context of the cooling rates encountered in practice. In particular, allotriomorphic ferrite that is expected to lead to a deterioration in toughness, is found in the microstructure for realistic combinations of austenite grain size and the cooling rate combination. Parameters are established to identify the domains in which SA508 Gr.3 steels transform only into the fine bainitic microstructures.
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Notes
‘M’ refers to metal atoms.
We henceforth use the subscript r to identify transformation temperatures for bainite (B Sr) and martensite (M Sr) as well, recorded during continuous cooling. These will differ from the fundamental B S and M S temperatures either because undercooling occurs due to kinetic reasons or because the chemical composition of austenite is altered by prior transformation into other phases.
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Acknowledgments
The authors are grateful to Rolls-Royce plc for sponsoring this work and in particular to Dan Cogswell for his support. Discussions with staff at Sheffield Forgemasters, particularly with Jesus Talamantes-Silva, are also appreciated.
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Manuscript submitted December 19, 2013.
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Pous-Romero, H., Bhadeshia, H.K.D.H. Continuous Cooling Transformations in Nuclear Pressure Vessel Steels. Metall Mater Trans A 45, 4897–4906 (2014). https://doi.org/10.1007/s11661-014-2433-8
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DOI: https://doi.org/10.1007/s11661-014-2433-8