Crystallographic Analysis of Proeutectoid Ferrite/Austenite Interface and Interphase Precipitation of Vanadium Carbide in Medium-Carbon Steel

To clarify the mechanism of interphase precipitation of vanadium carbide (VC) in a medium-carbon steel, orientation relationships (ORs) and plane orientations of ferrite/austenite interfaces were investigated. It was found that a large part of grain boundary ferrite holds near-K-S OR with at least one side of austenite adjacent to grain boundary regardless of V addition. By the V addition, a fraction of grain boundary ferrite holding near the K-S OR with both sides of austenite is decreased remarkably. Furthermore, only non-K-S ferrite/austenite interfaces migrate dominantly in the V-added alloy in contrast to the V-free alloy. Ferrite/austenite interface orientations are not fixed crystallographically but are randomly distributed in terms of ferrite and austenite orientations. Those results do not agree with the ledge mechanism originally proposed by Honeycombe. Thus, it is proposed that the ledge mechanism is extended to the non-K-S interface, which partially consists of coherent and less-mobile interfaces.