Carbon Enrichment of Austenite during Ferrite-bainite Transformation in Low-alloy-steel

Abstract

The bainitic transformation kinetics and carbon enrichment of austenite during isothermal holding at 723–923 K were investigated for an Fe-0.1mass%C-0.5mass%Si-2.0mass%Mn alloy. The transformation progressed rapidly until approximately 50 s, after which transformation stasis was observed at 823 K. The carbon concentration of austenite increased as the transformation proceeded, and showed an almost constant value during stasis. It reached approximately 0.45-0.50% at 823 K, which corresponds to the carbon concentration at the T₀’ composition with an additional strain energy of 100 J/mol associated with the transformation. After stasis, a slight increase in the ferrite or bainitic ferrite fraction was observed. The carbon concentration of austenite also increased and reached approximately 0.60%, clearly exceeding the carbon concentration at the T₀ composition. These results imply that at the first stage, the bainite transformation occurs and shows the incomplete transformation, following which at the second stage, diffusional ferrite transformation proceeds. The additional strain energy associated with the transformation calculated from the carbon concentration at stasis due to the incomplete bainite transformation tends to decrease as the holding temperature increases. This indicates that strain relaxation due to the transformation occurred at higher holding temperatures.