Effects of vanadium addition on nucleation and growth of pearlite in high carbon steel

Abstract

The influence of vanadium addition on the microstructure of high carbon steels has been investigated. A careful examination of the initial stages of austenite decomposi~ion has been made, using a range of high resolution metallographic techniques. It has been confirmed that vanadium addition results in the formation of grain boundary ferrite films, even in the eutectoid composition range. It is argued that this ferrite is the product of eutectoid transformation, and is not proeutectoid ferrite. This is because the first event is the nucleation of carbide particles along the grain boundaries. These carbides have been identified mainly as cementite. The presence of vanadium appears to change the morphology and distribution of the grain boundary cementite, so that rather than forming a grain boundary network, the cementite occurs in the form of a high density of small discrete particles along the boundaries. It is proposed that this occurs because vanadium increases the driving force for cementite nucleation. The formation of the grain boundary cementite depletes the surrounding region of carbon and encourages the formation of ferrite, but because of their discrete and fine dispersion, the cementite particles are engulfed by the more voluminous ferrite phase. In such regions, the onset of afully cooperative growth regime is delayed. Pearliteforms later at the ferrite/austenite interfaces. MST/1923