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 °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.