Dynamic transformation of deformed austenite at temperatures above the Ae3

Abstract

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

A metastable phase diagram for the dynamic transformation of austenite at temperatures above the Ae3

“…The second class of observation involves dynamic transformations of the type considered here. Under these conditions, an equivalent strain of one is again consistent with the experimental data, as shown below for the present steel, as well as for the other steels that have been examined to date [7]. C. Aranas Jr. et al: A metastable phase diagram for the dynamic transformation of austenite above the Ae 3 [3].…”

“…Here it can be seen that they depend on the Schmid factor applicable to the orientation of the habit plane on which transformation is taking place (as well as on the shear direction). For this type of transformation, the habit plane and shear direction were taken to be (0.506, 0.452, 0.735) and (-0.867, 0.414, 0.277), respectively [7]. A Schmid factor of 0.5 corresponds to a particularly well oriented grain, while values of m =0.4 and m = 0.3 correspond to progressively less and less suitably oriented grains.…”

“…Here m is the Schmid factor applicable to the habit plane and shear direction of the transformation, r c is the critical stress required to initiate the phase change, and c = 0.22 is the shear strain assumed here to be associated with the displacive transformation of austenite to ferrite under the present conditions [7]. The current mechanically-driven phase change is considered in this respect to resemble that of the formation of bainite or acicular ferrite at lower temperatures [7].…”

“…The current mechanically-driven phase change is considered in this respect to resemble that of the formation of bainite or acicular ferrite at lower temperatures [7]. Its dependence on temperature is illustrated in Fig.…”

Reverse-transformation austenite structure control with micro/nanometer size

“…The microstructures were determined by the reverse-transformation kinetics, which, in turn, was related to the cold deformation [17][18]. The metastable austenitic phase was transformed to the martensite after cold deformation, and the strain-induced martensite lath structure was destroyed preferentially via the large deformation to the refined structure during the cold rolling [16].…”