The Effect of Prior Ferrite on Bainite Transformation Kinetics

Abstract

In this work the effect of ferrite on the formation of bainite is studied. Different ferrite fraction of up to 25% were obtained through the application of three different intercritical annealing treatments to an Fe-0.2C-3Mn-2Si (wt%) steel. Two of the heat treatments consisted of an isothermal hold at temperatures between 835°C and 950°C, which was followed by slow cooling to a bainite formation temperature of 400°C. A third heat treatment included fast cooling from top temperatures between 810°C and 950°C to the same bainite formation temperature of 400°C. The heat treated material has been studied by means of dilatometry, optical microscopy, scanning electron microscopy and Vickers micro-hardness tests. All of the involved ferrite fractions had an accelerating effect on bainite formation, when compared to samples where no prior ferrite was present. The accelerating effect was larger for smaller prior ferrite fractions. It is proposed that the amount of preferential nucleation sites for bainite is increased by the formation of prior ferrite and leads to an acceleration of bainite formation. The presence of a significant amount of silicon (2 wt%) prevented the formation of cementite and resulted in carbon enrichment of austenite during ferrite formation. For the evaluated ferrite fractions, the accelerating effect due to ferrite presence is greater than the decelerating effect caused by the carbon enrichment of austenite, the latter becoming stronger with increasing ferrite fraction. Additionally, it is proposed that, due to the increase in preferential nucleation sites, a large amount of bainite sheaves start to grow simultaneously. Together with the autocatalytic nucleation of new bainite sub-units, this could account for the high bainite formation rates that were observed in the presence of ferrite. A change in bainite morphology from degenerated upper bainite to granular bainite with increasing prior ferrite fractions was also noticed. No significant difference in hardness was observed between the samples that contained prior ferrite, but the fully austenitised samples were slightly harder. It is expected that increasing ferrite fractions will result in a decrease in overall hardness, when macro-hardness tests would be applied.