PEARLITE PHASE-TRANSFORMATION IN SI AND V STEEL

Systematic research has been undertaken on the effects of single and combined additions of vanadium and silicon on the phase transformation and microstructure of pearlitic steels. Both alloy additions were found to result in the formation of nonlamellar products in the vicinity of austenite grain boundaries in hypereutectoid compositions (0.77 to 0.95 wt pet C). The products comprise discrete initial cementite particles and grain boundary ferrite, which is embedded with interphase precipitates of vanadium carbide. As the carbon content is increased further (up to 1.05 wt pet), the amount of grain boundary ferrite gradually decreases without any dramatic change in the morphology of the initial cementite particles. No continuous embrittling grain boundary cementite network was formed. The aspect ratios of the grain boundary cementite particles were decreased from 60:1 to 25:1 by the addition of the alloy elements. A comprehensive model has been suggested to explain these effects. Other effects of these alloy elements on the microstructure of pearlitic steels have also been examined. For given austenitization conditions, an increase in carbon and vanadium content produced a decrease in austenite grain size. Silicon was found to increase the rate of interphase precipitation of vanadium carbides.