Effect of Mn and Si on the dynamic transformation of austenite above the Ae3 temperature

In the mechanical activation model of dynamic transformation, the stress applied to the austenite is considered to be the driving force for the transformation. Here the yield stress of the newly formed Widmanstätten ferrite that replaces the austenite and that must continue to be deformed is taken into account as an additional obstacle to the transformation. This is estimated from hardness data. In this way, the driving force is redefined to consist only of the softening that takes place during the process and therefore only of the difference between the current flow stress of the work-hardened austenite and the yield stress of the fresh Widmanstätten ferrite that takes its place. According to this model, this driving force is opposed by the chemical free energy difference between the austenite and the Widmanstätten ferrite as well as the lattice dilatation work and shear accommodation work that accompany the transformation. In order to assess the effects of Mn and Si addition on the transformation, the high temperature flow stress data obtained by Wray (1984) by means of tensile tests on four Fe–Mn–Si alloys are employed to determine how these alloying additions affect the driving and opposing forces. The present type of model provides a more detailed analysis of the phenomenon than any previously available model and in this way shows that Mn addition is expected to lead to only a small decrease in the temperature range over which displacive ferrite can form, while Si addition is predicted to have a much larger effect on this interval.