Bainite morphologies in a 0.2 C-1.5 Mn steel

The isothermal transformation products of austenite over a wide range of temperatures and times in the bainitic range in a 0.2 wt.% C-1.5 wt.% Mn steel have been studied by transmission electron microscopy in order to characterise the bainitic microstructures in low-carbon low-alloy steels. Widmanstatten ferrite has formed with alternate layers of austenite (martensite) as a transition product at 600 and 500-degrees-C that has finally transformed on further isothermal transformation to either pearlite (at 600-degrees-C) or upper bainite (at 500-degrees-C). This type of transformation product was referred to as Bl bainite by earlier investigators, but on the basis of the present investigation it is concluded that such ferrite-austenite (martensite) structures are not bainitic as this is not the final transformation product either at 600 or 500-degrees-C. Both upper bainite and lath-type lower bainite are formed at 450-degrees-C while the transformation product has been only lath-type lower bainite at 400-degrees-C.