Coupled-solute drag effects on ferrite formation in Fe-C-X systems
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13/3/04
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Coupled-Solute Drag Effects on Ferrite Formation in Fe-C-X Systems H.I. AARONSON, W.T. REYNOLDS, and G.R. PURDY The influence of X upon proeutectoid ferrite/microstructurally defined bainite formation in Fe-C-X alloys, where X is Co, Cr, Cu, Mn, Mo, Ni, Ni, Si, or V, is examined in terms of the competing influences of the coupled-solute drag effect (C-SDE) and the shifting in the paraequilibrium Ae3 curve. The relative strength of the C-SDE was estimated primarily in terms of the influence of X upon the Wagner interaction parameter for C-X interaction in austenite, g12. Changes in the Ws (Widmanstätten-start temperature) with X additions at a constant pct C are ascribed almost entirely to shifts in the paraequilibrium Ae3. Influence of X upon the steady-state nucleation rate of ferrite allotriomorphs at austenite grain faces is largely explicable upon the same basis, though additional effects appear to be exerted by Mn and Ni upon relative values of the interfacial energies involved in the nucleation process. Development of a bay in the TTT curve for initiation of ferrite formation occurs more readily with increasingly negative g12 and increasing carbon concentration. When g12 1, considerably larger X and C concentrations are required to form what may sometimes be described as a “virtual bay,” only the lower portion of which is experimentally detectable. As g12 becomes increasingly negative (but not as much when g12 is increasingly positive), the influence of paraequilibrium Ae3 shifts is much diminished. Widmanstätten sideplate formation is also increasingly suppressed and replaced by grain boundary and twin boundary allotriomorphs at temperatures between the upper nose and the bay of the ferrite-start TTT curve. Changes in carbide precipitation patterns and replacement of (Fe, X)3C by alloy carbides directly follow from these alterations in ferrite morphology. When g12 is sufficiently negative, incomplete transformation occurs below the bay temperature. Much higher proportions of X and C are required to produce this effect when g12 1. Still more negative values of g12 may be required to develop the degenerate ferrite microstructures below the bay that result from increasing C-SDE restrictions on growth and the consequent frequently repeated sympathetic nucleation.
I. INTRODUCTION
THE Workshop of which this article is a part, as well as other sessions at the Fall 2002 TMS/ASM Meeting, attest to a remarkable revival of basic research interest in the effects of substitutional alloying elements, X, upon ferrite formation in Fe-C-X alloys—the model system for studying alloying element effects upon the industrially most important phase transformation in solid alloys. This circumstance has encouraged the authors to attempt an assessment of the comparative effects of the commonly studied alloying elements upon various aspects of ferrite formation in (more or less) high-purity Fe-C-X alloys. Although much recent work in this area has focused upon Fe-C-Mo alloys, there is sufficie
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