The crystallography of cementite precipitation in the bainite transformation

Abstract

The orientation relationships between cementite and ferrite in both upper and lower bainite have been determined using transmission electron microscopy and diffraction from thin foils of isothermally transformed Cr-Mo-B steels. In lower bainite the tempering relationship: (001) Fe 3C ∥(211) α, [100] Fe 3C ∥[01̄1] ga [010] Fe 3C ∥[11̄1̄] ga was obeyed in every case except one, while in upper bainite just under 2 3 of the observed diffraction patterns were consistent with this relationship. The remaining upper bainite patterns could only be analysed by relating both the cementite and the ferrite to the parent austenite. A stereographic method of carrying out this analysis is described. The orientation relationship results suggest a transformation mechanism in which eementite always precipitates within the ferrite in lower bainite, while in upper bainite the eementite and ferrite can form separately in the parent austenite and need not be related, except via their common parent phase. These conclusions are supported by the results of habit plane and direction determinations for eementite in both upper and lower bainite.