EFFECTS OF VANADIUM ADDITION ON NUCLEATION AND GROWTH OF PEARLITE IN HIGH-CARBON STEEL

The influence of vanadium addition on the microstructure of high carbo n steels has been investigated. A careful examination of the initial s tages of austenite decomposition has been made, using a range of high resolution metallographic techniques. It has been confirmed that vanad ium addition results in the formation of grain boundary ferrite films, even in the eutectoid composition range. It is argued that this ferri te 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 identi fied mainly as cementite. The presence of vanadium appears to change t he morphology and distribution of the grain boundary cementite, so tha t rather than forming a grain boundary network, the cementite occurs i n the form of a high density of small discrete particles along the bou ndaries. It is proposed that this occurs because vanadium increases th e driving force for cementite nucleation. The formation of the grain b oundary cementite depletes the surrounding region of carbon and encour ages the formation of ferrite, but because of their discrete and fine dispersion, the cement particles are engulfed by the more voluminous f errite phase. In such regions, the onset of a fully cooperative growth regime is delayed. Pearlite forms later at the ferrite/austenite inte rfaces.