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

Abstract

A metastable phase diagram for the dynamic transformation of austenite at temperatures above the Ae 3 A method is proposed for calculation of the pseudobinary phase diagram associated with the dynamic transformation of austenite to ferrite. Here the driving force is taken as the difference between the austenite flow stress at the moment of initiation and the yield stress of the fresh Widmanstätten ferrite that takes its place. The energy opposing the transformation consists of the Gibbs energy difference between austenite and ferrite at temperatures above the Ae 3 and the work of accommodating the shear displacements and dilatation associated with the phase change. A metastable phase diagram is calculated for a 0.30 wt.% Mn-0.01 wt.% Si steel by balancing the driving force against the three obstacles. The results show that, under dynamic conditions, the ferrite phase field extends all the way from room temperature to that for the formation of delta ferrite.Keywords: Dynamic transformation; Austenite; Ferrite; Fe-C phase diagrams; Thermomechanical processing IntroductionIn the early work on dynamic transformation (DT), it was shown that DT takes place at temperatures up to 166 8C above the Ae 3 [1, 2]. More recently, this temperature range has been extended to 1 350 8C, i. e. to 480 8C above the paraequilibrium Ae 3 [3,4]. The observation that Widmanstätten ferrite [4 -7] can be formed throughout the austenite phase field indicates that the conventional Fe-C phase diagram does not apply when austenite is being deformed. It is the aim of this paper to propose an approach for the derivation of phase diagrams that applies to 'dynamic' conditions and to provide an example of such a diagram for a particular steel.In order to deal with the effect of deformation on the phases present, three quantities must be evaluated that play particularly important roles, the first of which is totally absent under conventional 'static' conditions. In the latter case, the driving force is the free energy difference between the original and the replacement phase, the latter being of lower energy. In the dynamic case, however, the free energy difference is of opposite sign, so that it becomes a barrier to the transformation instead. Under these conditions, it is the flow stress difference between the work hardened austenite and the much softer ferrite that replaces it that provides the driving force [8,9].The other quantities are the work of dilatation and of shear accommodation [10]. Unlike the situation at ambient temperatures, at which these energies are stored elastically, the high temperature versions are not stored reversibly, but involve dissipation by means of plastic work. It is shown below how these three quantities can be evaluated over the experimental temperature range and expressed algebraically. These expressions can then be introduced into the Fe-C database of the FactSage thermodynamic software [11]. A metastable phase diagram is in turn generated employing the modified database and valid over the temperature range an...

Determination of the Critical Stress Associated with Dynamic Phase Transformation in Steels by Means of Free Energy Method

“…The present analysis is focused on a steel with 0.06 wt % C, 0.3 wt % Mn and 0.01 wt % Si with a paraequilibrium Ae 3 temperature of 870 • C. Note that this is similar to the material used in previous investigations by the present authors [4,6,12,[15][16][17]. Hot compression tests were performed on a 100 kN MTS machine (MTS, Eden Prairie, MN, USA) equipped with a radiation furnace and a temperature controller.…”

Dynamic Transformation during the Simulated Plate Rolling of a 0.09% Nb Steel

“…Such ferrite forms displacively when the driving force for DT overcomes the total obstacle, which consists of the free energy difference between the phases as well as the lattice dilatation work and shear accommodation work. 7,8) The amounts of ferrite formed and retained were reduced by about 0.3, 1.2, 2.9, 4.2, 4.8 and 5.5% per pass on the second, third, fourth, fifth, sixth and seventh, respectively, when the interpass time was increased by 20 s. This is because longer interpass times favor the retransformation of ferrite back into austenite. Static retransformation takes place because the Gibbs energy of ferrite is higher than that of the austenite in this temperature range.…”

“…Concurrently, as the Ae 3 temperature is approached, there is a reduction in the height of the obstacle to the transformation. 7,8) The effect of increasing the length of the interpass interval is in turn associated with reducing the retained strain through static recovery.…”

Retransformation Behavior of Dynamically Transformed Ferrite during the Simulated Plate Rolling of a Low C and an X70 Nb Steel

“…[1][2][3][4][5][6][7] This has been shown to occur throughout the austenite phase field. 8,9) In earlier publications, the present authors have reported that the amounts of ferrite formed per pass increase with decreasing temperature. 9,10) Subsequently, the metastable ferrite tends to retransform to austenite to an extent that increases with the length of the interpass interval.…”

“…1) Although some studies regarding the retransformation of ferrite into austenite during holding have already been published, 1,8,10,12) these have been limited to isothermal and single pass conditions. Here this phenomenon is explored under continuous cooling and multiple pass conditions, which are closer to those applicable to industry.…”