Morphology of Proeutectoid Ferrite at Austenite Grain Boundaries in Low Carbon Steels

Abstract

This paper describes morphologic features of proeutectoid ferrite (α) formed at austenite (γ) grain boundaries in three kinds of steel ; a HSLA steel with 0.04%Nb and 0.04%V, a commercial 0.2%C steel and an Fe-0.1%C alloy. Shape of the proeutectoid α at γ grain boundaries strongly depended on the austenitizing temperature. At the higher temperature, the proeutectoid (α) grains precipitated in the shape of film along planar γ grain boundaries, and a coalescence of several α grains with the same orientation made the film-shaped α in which many subgrain boundaries were observed. At the lower austenitizing temperature, on the other hand, they precipitated in the shape pf globe (allotriomorph) form along curved γ grain boundaries. Each a grain had a different orientation and led to allotriomorph. This trend was commonly observed in all the steels investigated.