The dynamic transformation behavior of deformed austenite was studied in four steels of increasing carbon contents that had been deformed over the temperature range 743 -917°C. These experiments were carried out in torsion under an atmosphere of argon and 5% H 2 and the experimental temperatures were above the ortho and para-equilibrium Ae 3 temperatures of the steels.Strains of 0.15 -5 were applied at strain rates of 0.4 -4.5 s The onsets of dynamic transformation and dynamic recrystallization were detected in the four steels using the double-differentiation method. Two sets of second derivative minima were found to be associated with all the flow curves. It is shown that double minima can only be obtained when the polynomial order is at least 7. The first set of minima corresponds to the initiation of dynamic transformation (DT). The second set is associated with the nucleation of dynamic recrystallization (DRX). The critical strain for DT is always lower than for DRX in this range and increases slightly with temperature. Conversely, the critical strain for DRX decreases with temperature in the usual way.The mean flow stresses (MFS's) pertaining to each experimental condition were calculated from the flow curves by integration. These are plotted against the inverse absolute temperature in the form of Boratto diagrams. The stress drop temperatures, normally defined as the upper critical temperature A r3 *, were determined from these diagrams. These are shown to be about 40°C above the paraequilibrium and about 20 -30°C above the orthoequilibrium upper critical ii transformation temperatures. This type of behavior is ascribed to the occurrence of the dynamic transformation of austenite to ferrite during deformation.The effect of deformation on the Gibbs energy of austenite in these steels was estimated by assuming that the austenite continues to work harden after initiation of the transformation and that its flow stress and dislocation density can be derived from the experimental flow curve by making suitable assumptions about two-phase flow. By further taking into account the inhomogeneity of the dislocation density, Gibbs energy contributions (driving forces) are derived that are sufficient to promote transformation as much as 100°C above the Ae 3 . The carbon diffusion times required for formation of the observed ferrite plates and cementite particles are calculated; these are consistent with the occurrence of interstitial diffusion during deformation. Similar calculations indicate that substitutional diffusion does not play a role during dynamic transformation. The Gibbs energy calculations suggest that growth of the Widmanstätten ferrite is followed by C diffusion at the lower carbon contents, while it is accompanied by C diffusion at the higher carbon levels.iii References 138 Chapter 8 RÉSUMÉ Conclusions 139Chapter 9 . Under these conditions, the time in the deformation zone is 5 ms and the elapsed time during a strain increment of 1% is about 100 µs [3]. 76 Contributions to Original

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Compression tests were carried out on a 0.06wt%C-0.3wt%Mn-0.01wt%Si steel at temperatures high in the austenite phase field. Eight deformation temperatures were selected in the range from 1000 to 1350 ⁰C at 50 ⁰C intervals. The quenched samples were examined using optical microscopy and EBSD techniques. It was observed that dynamic transformation took place and that the volume fraction of transformed ferrite first decreased with temperature (up to 1050 ⁰C) and then increased as the delta ferrite temperature domain was approached. The EBSD results revealed the presence of Widmanstätten ferrite plates under all testing conditions, right up to 1350 ⁰C.

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Compression tests were carried out at a strain rate of 1 s -1 on a 0.06%C-0.3%Mn-0.01%Si steel over two temperature ranges: i) 920°C to 980°C, and ii) 500 to 750°C. Optical and scanning electron microscopy images indicated that significant volume fractions of Widmanstätten ferrite were formed dynamically above the Ae3 temperature. The ferrite plates coalesced into polygonal grains during straining. The double differentiation method was applied to the stress-strain curves, providing average values for the dynamic transformation (DT) and dynamic recrystallization (DRX) critical strains of 0.12 and 0.20, respectively. These results are interpreted in terms of the flow softening-based transformation model by calculating both the driving forces promoting the transformation as well as the energy barriers that oppose it. The model predicts the temperature range over which DT can occur as well as the observed critical strains.