29th HATFIELD MEMORIAL LECTURE: Ferrite

The morphology of ferrite grains formed from austenite is first considered, then the refinement of ferrite grain size by controlled rolling as exemplified by HSLA steels. The influence of alloying elements is briefly examined, followed by a detailed discussion of the precipitation of carbides during and after the formation of ferrite. It is shown that the γ/α interfaces are preferred sites for the precipitation of alloy carbides (interphase precipitation), which occurs on both planar and curved boundaries. In the former, the precipitation is associated with the movement of small ledges across the γ/α interface, whereas the curved bands of precipitate are formed on high-energy boundaries which move by bowing around the particles. A change in the nature of the γ/α interface often leads to fibrous carbide growth, while more rapid rates of cooling lead to the formation of supersaturated ferrite and subsequent precipitation of carbides on dislocations. The dislocations in the ferrite are shown to play an important role in the subsequent coarsening of the carbide dispersion, whether they form at the interfaces or on the dislocations. Finally, the observations on the formation and growth of carbide dispersions are related to the structure and properties of microalloyed steels.