The dynamic transformation of deformed Austenite at temperatures above the Ae3

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% H2 and the experimental temperatures were above the ortho and para-equilibrium Ae3 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 Ar3*, 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 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 Ae3. 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.Le comportement de la transformation dynamique de l'austénite lors de sa déformation a été étudié dans quatre aciers, dont les teneurs en carbone sont croissantes, déformés à des températures allant de 743 à 917°C. Ces expériences ont été réalisées par torsion sous une atmosphère contrôlée (mélange argon et 5% H2) et les températures de déformation étaient au-dessus des températures d'ortho et para-équilibre Ae3 pour chaque acier. Des taux de déformation de 0,15 à 5 ont été utilisés avec des vitesses de déformation allant de 0,4 à 4,5 s-1. Les paramètres expérimentaux ont été modifiés afin de déterminer les effets du taux de déformation et de la température sur la formation de ferrite et de cémentite induite par déformation. Les structures observées sont de type Widmanstätten et semblent avoir nucléées de façon displacive. Les départs de la transformation dynamique et de la recrystallization dynamique ont été détectés pour les quatre aciers en utilisant la méthode de double différenciation. Deux ensembles de minima sur les dérivés secondes se trouvent être associés à toutes les courbes d'écoulement. Il est montré qu'un double minimum ne peut être obtenu que lorsque l'ordre du polynôme utilisé dans la procédure d'ajustement de l'intégralité de la courbe découlement est supérieur ou égale à 7. Le premier ensemble de minima correspond à l'initialisation de la transformation dynamique. Le second ensemble est associé à la nucléation de la recrystallization dynamique. Le taux de déformation critique de la transformation dynamique est toujours inférieur à celui de la recrystallization dynamique, dans la gamme de température étudiée, et augmente légèrement avec la température. A l'inverse, le taux de déformation critique de la recrystallization dynamique diminue, de manière connue, avec la température. Les contraintes d'écoulement moyennes relatives à chaque condition expérimentale sont calculées à partir de l'intégration des courbes d'écoulement. Celles-ci sont représentées en fonction de l'inverse de la température absolue sous la forme de diagrammes de Boratto. Les températures auxquelles la chute de contrainte se produit, normalement définie comme étant la température critique supérieure Ar3*, sont déterminées à partir de ces diagrammes. Celles-ci sont d'environ 40°C au-dessus de la température critique supérieure de para-équilibre et d'environ 20 à 30°C au-dessus de celle d'ortho-équilibre. L'effet de la déformation sur l'énergie de Gibbs de l'austénite dans ces aciers est estimé en supposant que l'austénite continue de s'écrouir après l'initiation de la transformation et que sa contrainte d'écoulement et sa densité de dislocations peuvent être dérivées à partir des courbes d'écoulement expérimentales en émettant des hypothèses appropriées sur l'écoulement conjoint de deux phases. En prenant en outre en compte l'hétérogénéité de la densité de dislocation, les contributions à l'énergie de Gibbs obtenues (forces motrices) sont suffisantes pour promouvoir une transformation jusqu'à 100°C au-dessus de la température Ae3. Le temps de diffusion du carbone requis pour la formation des plaques de ferrite observées est calculé; les résultats obtenus sont cohérents avec l'apparition de diffusion interstitielle au cours de la déformation. Des calculs similaires indiquent que la diffusion substitutionnelle ne joue aucun rôle lors de la transformation dynamique. Les calculs de l'énergie de Gibbs suggèrent que la croissance de la ferrite de Widmanstätten est suivie de la diffusion du carbone dans le cas des plus faibles teneurs en carbone tandis qu'elle est accompagnée par celle-ci pour les teneurs les plus fortes

The equivalent strain in high pressure torsion

International audienceThe results of experiments carried out by means of high pressure torsion (HPT) are frequently described in terms of the Eichinger ``equivalent'' strain. When employed in conjunction with equivalent stresses derived using von Mises in conjunction with the Fields and Backofen analysis, the values of the work done/unit volume associated with the Eichinger-based flow curves are shown to be only a small fraction of the actual work done. By contrast, the values of the work done/unit volume that correspond to the shear stress/shear strain as well as the von Mises flow curves are in full agreement with the torque/twist work done/unit volume. When the principle of work conjugacy is employed, the Eichinger strain-based work done values are necessarily in agreement with the shear stress/shear strain and torque/twist results, but then the ``equivalent'' stresses are unrealistically high. It is concluded that only the von Mises equivalent stresses and strains should be employed in the description of HPT results and that the Eichinger ``equivalent'' strain should not be used for this purpose

Effects of varying twist and twist rate sensitivities on the interpretation of torsion testing data

International audienceThe Nadai and Fields and Backofen expressions are widely used to convert the torque/twist data obtained during torsion testing into shear stress/shear strain curves as well as von Mises equivalent stress/ equivalent strain curves. However, when employed in the conventional manner using average values of the twist hardening exponent N, they overestimate the critical strains for the initiation of twinning, dynamic transformation and dynamic recrystallization by comparison with the values determined using compression testing. By contrast, when the local or instantaneous values of the exponent are employed, the torsion and compression results are in good agreement. Another feature of the corrected curves is that they indicate that considerably more dynamic softening takes place during the high temperature deformation of austenite than suggested by the average N flow curves. It is shown that, despite the lack of work conjugacy between the torque-twist and stress-strain curves, the above expressions always lead to the correct constitutive behavior at the external radius. (C) 2013 Elsevier B.V. All rights reserved

Effect of cooling rate on the evolution of microstructure and mechanical properties of nonisothermally partitioned steels

In the present investigation, multiphase microstructures containing a combination of ferrite, martensite, retained austenite and carbides have been produced by altering the cooling rate in low alloy steels. The mechanical properties have been evaluated and correlated with ensuing microstructural features. The as-cast alloys were austenitized, hot rolled to about 93% reduction in thickness, followed by cooling to 200 °C on the run-out table. The cooling rates, namely 50 and 70 °C/s, were employed for this study. The steel plates were then cooled slowly to room temperature in a furnace to simulate the nonisothermal partitioning, similar to the hot-rolled coil cooling. The results show that the alloy with lower carbon and Mn content (Alloy-1) reveal ferrite formation (35.4 ± 4.1 vol%) at the cooling rate of 50 °C/s. However, at a higher cooling rate of 70 °C/s, ferrite formation was circumvented and the presence of martensite, retained austenite (6.3 ± 0.13 vol%) and carbides were observed. Although no significant difference was observed in the hardness and strength values for these two cooling rates, the presence of retained austenite at a higher cooling rate (i.e. 70 °C/s) led to better ductility and impact toughness. In the other alloy, with higher carbon and Mn addition (Alloy-2), the ferrite formation was considerably reduced even for the cooling rate of 50 °C/s. As a result, it showed higher hardness and strength (~1.5–2.0 times), with a concurrent decrease in the ductility and impact toughness, in comparison to Alloy-1

Tribological Investigation of Textured Surfaces in Starved Lubrication Conditions

The present work investigates the friction reduction capability of two types of micro-textures (grooves and dimples) created on steel surfaces using a vertical milling machine. The wear studies were conducted using a pin-on-disc tribometer, with the results indicating a better friction reduction capacity in the case of the dimple texture as compared to the grooved texture. The microscopic images of the pin surface revealed deep furrows and significant damage on the pin surfaces of the groove-textured disc. An optimization of the textured surfaces was performed using an artificial neural network (ANN) model, predicting the influence of the surface texture as a function of the load, depth of cut and distance between the micro-textures

Quench Temperature-Dependent Mechanical Properties During Nonisothermal Partitioning

The present study demonstrates the role of hot rolling and quench temperature in determining the mechanical properties of low alloy steel processed through quenching and nonisothermal partitioning (Q&P) route. The results indicate that the abrasive wear resistance does not show any significant variation with quench temperature. However, a reduction in tensile strength and an increase in charpy impact toughness and elongation is observed with increasing quench temperature. Interestingly, the retained austenite shows high thermal stability at sub-zero temperature. Furthermore, during deformation through the wear process, the retained austenite experiences the TRIP effect that leads to improvement in wear resistance. The incorporation of hot rolling prior to Q&P led to a significant improvement in strength, energy absorption capability and wear resistance due to a considerable refinement of the microstructural constituents