THE FINE-STRUCTURE AND FORMATION MECHANISM OF LOWER BAINITE

Isothermal transformation of austenite to lower bainite was studied by optical microscopy and transmission electron microscopy (TEM), in two high-purity Fe-C-4 wt pct Mn-2 wt pct Si alloys containing 0.4 and 0. 6 wt pct carbon, in order to elucidate the fine structure and formatio n mechanism of lower bainite. The present results support a mechanism for lower bainite formation presented previously in which lower bainit e sheaves result from the formation of an aggregate of fine ferrite cr ystals with thin austenite ''gaps'' between them; carbide precipitatio n occurs within these austenite gaps. This mechanism accounts for the carbides oriented at an angle to the sheaf axis repeatedly observed in lower bainite but is inconsistent with models based on the precipitat ion of a high-volume fraction of carbides within highly supersaturated ferrite formed by high-velocity shear. Observations by a number of ot her researchers are reviewed and shown to include morphological featur es consistent with the present mechanism. Finally, the orientation rel ationships typically observed among fenite, austenite, and carbides in lower bainite are reviewed and shown in most (but not all) cases to b e consistent with the view that carbides precipitate in austenite at f errite-austenite boundaries, also in agreement with the present model.