The 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 -1 . The experimental parameters were varied in order to determine the effects of strain and temperature on the formation of strain-induced ferrite and cementite. The structures observed are Widmanstätten in form and appear to have nucleated displacively.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 car...

Formation of Strain-induced Ferrite in Low Carbon Steels at Temperatures Above the Ae3

“…The absence of martensite between the plates in steel 1 is consistent with the thermodynamic analysis of Ref. 4), in which it is shown that C is only rejected during plate formation when the C level exceeds about 0.1%. Transmission electron microscopy 5) and atom probe tomography 15) revealed the presence of fine carbides on dislocations and at grain boundaries; however, due to their fine scale they could not be detected using the EBSD techniques employed in the present work.…”

“…(4), that the strain is at a maximum at the surface of the specimen, while it is zero along the axis. The amount of dynamic transformation is therefore expected to be at a maximum at the outer surface and to decrease to zero along the radius.…”

“…This is because the low solubility of carbon in ferrite implies that, during plate formation, the carbon must diffuse to the plate boundaries and partition into the austenite or precipitate as fine carbides. This requirement will retard plate growth 4) and also slow the transition from plate to polygonal ferrite. This can account for the observation that the volume fraction of DT ferrite at a given strain decreases as the carbon content is increased, Figs.…”

“…[1][2][3][4] Yada and coworkers [5][6][7] were the first to report the occurrence of dynamic transformation (referred to as DT in this work ) during the 1980's. Later, in 2000, Yada et al 8) verified that the transformation is indeed dynamic by carrying out in-situ X-ray diffraction measurements on 0.096C-Fe-Ni and 0.29C-Fe-Ni alloys.…”

Dynamic Transformation of an X70 Steel under Plate Rolling Conditions

“…The first minimum identifies the start of dynamic transformation (DT), while the second specifies that of dynamic recrystallization (DRX). As shown in earlier studies using microstructural analyses, 6,7,21) the critical strain for DT is always lower than that for DRX.…”