Measurement and Prediction of Phase Transformation Kinetics in a Nuclear Steel During Rapid Thermal Cycles
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To ensure the safe operation of nuclear power plants, it is critically important that the large pressure vessels used in their construction are of the highest integrity. To manufacture such components, for example reactor pressure vessels or steam generators, it is usual for a number of forged sections to be joined together by welding. This is somewhat problematic for design engineers, since although they may be confident of the mechanical properties of bulk forgings, the characteristics of the connecting welds are often less well-understood. In engineering applications, it is often
G. OBASI, E.J. PICKERING, and M. PREUSS are with the School of Materials, University of Manchester, Oxford Road, Manchester, M13 9PL, UK. Contact e-mail: [email protected] A.N. VASILEIOU, Y.L. SUN, D. RATHOD, J.A. FRANCIS, and M.C. SMITH are with the School of Mechanical, Aerospace and Civil Engineering, University of Manchester, Oxford Road, Manchester, M13 9PL, UK. Manuscript submitted July 5, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
found that welds can be a source of structural weakness and failure, and hence there is a great deal of interest in assessing their integrities in nuclear pressure vessels. Residual stresses are known to exacerbate weld failures when they occur through fast fracture,[1,2] fatigue,[3,4] creep[5,6] and stress corrosion cracking.[7] Since experimental evaluation of the stress fields present in large pressure-vessel welds is usually not feasible, advanced numerical techniques are required to predict their distribution. This is an especially formidable task for ferritic steels, because the stresses arising from welding depend not only on the differential thermal contractions experienced across the weld area during cooling, but also on the particular phase transformations that occur when austenite decomposes upon cooling.[8–10] The strain associated with the austenite-to-ferrite transition is a volume expansion. Its effect on a weld’s residual stress field will depend on the temperature at which it occurs, and can be accounted for to a reasonable extent if the phase evolution behavior with temperature is known.
However, there are several complicated effects that are influenced by the different microconstituents (e.g., bainite, martensite) that can be formed when austenite decomposes on cooling, and each of these microconstituents possesses its own characteristic transformation behavior and mechanical properties. Not only will each constituent yield at a different stress, thereby redistributing residual stresses to differing degrees, but complex phenomena can also result in strains being distributed non-uniformly across different microconstituents. For instance, the formation of a harder microstructure (martensite or bainite) in a softer phase (austenite) of a different density can lead to strain concentrations in particular regions of material (so-called Greenwood-Johnson plasticity[11,12]), and the selection of particular crystallographic variants of martensite can also influence the total str
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