ON THE CRYSTALLOGRAPHY OF BAINITIC TRANSFORMATION IN STEELS

Abstract

<p>Bainite is a low temperature transformation product of austenite decomposition in steels. Its unique range of microstructures offers promising combinations of strength with ductility. At low transformation temperatures the crystallography of a phase transformation often plays an important role in the overall microstructure and how it develops. Therefore in this study the structures of ferrite/cementite and ferrite/austenite interfaces in bainite were investigated from a crystallographic viewpoint. After describing these interfaces, the idea of interphase boundary nucleation of cementite on a ferrite/austenite interface was investigated.</p> <p>An O-line model (a special case of the O-Lattice) was used to explain the observed experimental results on orientation relationship, habit plane and good matching direction between ferrite and cementite. The calculated orientation relationship was used in an NCS (near coincident site) model to describe several possible edge facets of cementite precipitates. The major observed edge facet in cementite is deviated from the more favored interfaces based on the NCS model. This deviation could imply that the edge facets are non-equilibrium interfaces whose orientations and morphologies are kinetically determined.</p> <p>Focused Ion Beam sectioning, conventional transmission electron and optical microscopy were used to shed more light on the three dimensional nature of a complex cementite-free bainitic microstructure. The faceted interfaces of bainitic ferrite were characterized and it was shown that the habit plane contains edge misfit dislocations. The orientation of the bainitic ferrite lath did not match an O-line model. Transformation time was considered to play an important role on the orientation and morphology of the bainitic laths and interfacial dislocation character.</p> <p>Finally, with the aid of known crystallographic relations and interfaces between the ferrite/cementite, ferrite/austenite and austenite/cementite phases, a model for cementite nucleation was proposed. This interphase boundary nucleus is assumed to form on a coherent ferrite/austenite interface and to possess ferrite/cementite and austenite/cementite calculated habit planes as two main facets surrounding the nucleus. It was shown that cementite nucleation would be viable if interfacial energies of all surrounding facets of a nucleus are in a semi-coherent energy range.</p>