A TEM Investigation of the Stepped Bainite Reaction in Silicon Steels

Abstract
The bainitic reaction in steels has been extensively studied, however it is still controversial whether it proceeds by a diffusional or a shear mechanism. In a previous investigation of the bainite reaction in a Fe-3.9Si-0.9C steel the transformation was considered to be the result of two competing elementary mechanisms, i.e., the shear transformation of the γ-iron lattice and the diffusion of interstitial carbon away from the transformation interface. According to this model, the transition from upper to lower bainite occurs when the rate of interstitial carbon traversing the γ/α interface becomes of the same order with the velocity of the shear front. This interpretation stipulates that the carbon content corresponding to the γ/α and T_o curves of austenite is critical. Using step quench experiments and determining the percentage of the residual austenite and its carbon content it was demonstrated that both X_γ/α and X_To contents of austenite play an important role in the initiation of its decomposition and its incomplete transformation. In this paper, the microstructural and crystallographic characteristics of the bainite products obtained through the step quenching experiments are examined, using TEM and Electron diffraction. The results are compared to those obtained by the corresponding bainitic transformation in a single step. The obtained results clearly corroborate the aforementioned model of transformation, which necessitates both a shear mechanism of the iron lattice and a redistribution of carbon between the parent phase and its transformation products.