Macrosegregation and Microstructural Evolution in a Pressure-Vessel Steel
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NTRODUCTION
MACROSEGREGATION refers to the chemical segregation which can arise over macroscopic distances during casting. In the large steel ingots used to produce pressure vessels, these distances can range from a few centimeters to a few meters. All macrosegregation originates from solidification phenomena: liquid and solid phases are either enriched or depleted in alloying elements through microsegregation, and the advection of these phases over large distances (when they move relative to one another) leads to solute being redistributed unevenly over the length scales described.[1,2] Macrosegregation during the casting of large steel ingots can give rise to the defects shown in Figure 1(a). A-segregates, the particular focus of this investigation, are channels of enriched material that form along a direction roughly antiparallel to gravity during solidification. They are formed by the motion of enriched interdendritic fluid through the mushy zone, which is driven by thermosolutal convection, Figure 1(b). This type of convection is not only controlled by the dissimilar temperatures of the interdendritic and bulk liquids, but also compositional disparities. In the case of steels, the interdendritic liquid will be less dense than the bulk due to the segregation of light elements like carbon and silicon and will hence tend to rise through the mushy zone. Persistent well-defined channels of soluterich liquid form, because as enriched liquid moves upwards and toward the center of the casting it heats up, but does not change in composition, and so remelts the solid surrounding it (or prevents its further growth).
E.J. PICKERING, Ph.D. Student, and H.K.D.H. BHADESHIA, Tata Steel Professor of Metallurgy, are with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, U.K. Contact e-mail: [email protected] Manuscript submitted October 22, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
Any chemical inhomogeneities introduced by macrosegregation should be expected to deliver different microstructures, and hence inconsistent mechanical properties. For designers of pressure vessels, such variations are highly undesirable, and yet, (to the author’s knowledge) investigations have not been made into how macrosegregation in pressure-vessel steels can affect microstructural evolution. Here, the effects of segregation on the development of microstructure in the pressure-vessel steel SA508 Grade 3 are investigated in detail. The results of dilatometry measurements, partial-transformation experiments, and scanning and transmission electron microscopy (SEM and TEM) are used to accomplish this. The origins of and severity of the segregation present are discussed, and suggestions are made for both how it might be prevented, and how its effects might be suppressed.
II.
EXPERIMENTAL
SA508 Grade 3 material was obtained from a production component, having been cast as part of a large 200-tonne ingot before being forged, quenched, and tempered. The solidification time during ingot casting was 40 hours, and forgin
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