Ferrite and carbide morphologies in and below the “bay region” of an Fe-0.19 Pct C-2.30 Pct Mo alloy

Abstract

Optical and transmission electron microscopy were used to study ferrite and carbide morphology from temperatures just above to well below the bay temperature in an Fe-0.19 pct C-2.30 pct Mo alloy. Suppression of idmanstätten ferrite morphologies at the bay and their reappearance below, initially in highly degenerate form, previously reported by Boswellet al. in Fe-C-2 pct Mo alloys, were confirmed. These effects were again explained in terms of a solute drag-like effect. Just above the bay, the usual fibrous and interphase boundary Mo₂C structures appear. However, fibrous carbides disappear immediately below the bay whereas interphase boundary carbides continue to form until approximately 60 K below the bay. In a narrow temperature range below the bay, large, widely spaced Mo₂C laths are also observed. Many laths are associated with terraces and risers of ferrite superledges; some, however, nucleate on dislocations within ferrite. Nucleation of Mo₂C on such dislocations occurs throughout the temperature range investigated (898 to 748 K). At the lower temperatures studied, (Fe, Mo)₃C appears as plates approximately 10³ longer than those of Mo₂C also formed at these temperatures. Explanations for these observations on carbides are offered in terms of interactions between ferrite and carbide precipitation and the concepts of ledgewise diffusional growth Orientation relationships between ferrite and carbides in several types of microstructure are reported, but the equivocal nature of the information which these data supply about transformation mechanism is emphasized.