Variant selection of bainite on the surface of allotriomorphic ferrite in a low carbon steel

The crystallographic orientations of bainitic plates on the surface of allotriomorphic ferrite were analyzed in a low carbon Fe-0.05C-2.9Mn-1.8Si alloy which after austenitization was isothermally held successively at 725 and 340 degrees C. Nearly 80% of ferrite allotriomorphs had a Kurdjumov-Sachs orientation relationship with one of the austenite grains. In the grain that had an almost exact Kurdjumov-Sachs relationship with the ferrite allotriomorph bainitic plates nucleated with the same variant as that of ferrite. In the grain deviated slightly from the Kurdjumov-Sachs relationship bainitic plates chose the variant proximate to that of ferrite. No variant selection appeared to occur at irrational ferrite/austenite boundaries. The nucleation potency of Kurdjumov-Sachs related and irrational boundaries is discussed in terms of the ability of the variant of bainite nuclei to form a low energy boundary with the ferrite and the possible difference in the amount of carbon build-up which arises out of the difference in boundary mobility. (C) 2012 Elsevier Inc. All rights reserved.