A MECHANISM FOR THE FORMATION OF LOWER BAINITE

A diffusional mechanism for the formation of lower bainite is proposed based primarily on transmission electron microscopy (TEM) observations of isothermally reacted specimens of Fe-C-2 pct Mn alloys. The mechanism involves the initial precipitation of a nearly carbide-free ferrite"spine," followed by sympathetic nucleation of"secondary (ferrite) plates" which lie at an angle to the initial"spine." Carbide precipitation subsequently occurs in austenite at ferrite: austenite boundaries located in small gaps between the"secondary plates." An"annealing" process then occurs in which the gaps are filled in by further growth of ferrite and additional carbide precipitation; the annealing out of ferrite: ferrite boundaries between impinged"secondary plates" completes this process. This annealing stage contributes to the final appearance of lower bainite sheaves as monolithic plates containing embedded carbides. The present mechanism accounts for the single variant of carbides oriented at an angle to the sheaf axis repeatedly reported in lower bainite; it is also consistent with the previous observation of one"rough" side and one"smooth" side of lower bainite"plates." © 1990 The Metallurgical of Society of AIME.