Dynamic Transformation Of Austenite Research Articles

The Role of Hot Deformation Texture on Dynamic Transformation of Austenite to Ferrite in a 9%Cr Alloy Steel

The Role of Hot Deformation Texture on Dynamic Transformation of Austenite to Ferrite in a 9%Cr Alloy Steel

Effect of Number of Roughing Passes on the Dynamic Transformation of Austenite during Simulated Plate Rolling

When austenite is deformed within the austenite phase field, it partially transforms dynamically into ferrite. Here, plate rolling simulations were carried out on an X70 steel using rough rolling passes of 0.4 strain each. The influence of the number of roughing passes on the grain size and volume fraction of induced ferrite was determined. Up to three roughing passes applied at 1100 °C followed by 5 finishing passes at 900 °C were employed. The sample microstructures were analysed by means of metallographic techniques. Both the critical strain to the onset of dynamic transformation as well as the grain size decreased with pass number during the roughing simulations. For the finishing passes, the mean flow stresses (MFS`s) applicable to each schedule decreased when a higher number of roughing passes was applied. The volume fraction of dynamically formed ferrite retained after simulated rolling increased with the roughing pass number. This is ascribed to the increased amount of ferrite retransformed into austenite and the finer grain sizes produced during roughing. The forward transformation is considered to occur displacively while the retransformation into austenite during holding takes place by a diffusional mechanism. This indicates that both dynamic transformation (DT) and dynamic recrystallization were taking place during straining.

On the Role of Chromium in Dynamic Transformation of Austenite

The effect of Chromium (Cr) on the dynamic transformation (DT) of austenite to ferrite at temperatures up to 430 °C above Ae3 was studied in a medium-carbon low-alloy steel. Hot compression tests were performed using Gleeble 3800® thermomechanical simulator followed by microstructural examinations using electron microscopy (FESEM-EBSD). Driving force calculation using austenite flow stress and ferrite yield stress on an inverse absolute temperature graph indicated that Cr increases the driving force for the transformation of austenite to ferrite; however, when the influence of stress and thermodynamic analysis are taken into account, it was observed that Cr increases the barrier energy and therefore, emerges as a barrier to the transformation. An analysis, based on lattice and pipe diffusion theories is presented that quantifies the role of stress on the diffusivity of Cr and is compared with other the main alloying elements such as C, Si and Mn and its impact, positive or negative, on the DT barrier energy. Finally, a comparison is made on the differential effects of temperature and stress on the initiation of DT in medium-carbon low-alloy steels.

Influence of strain rate on dynamic transformation of austenite in an as-cast medium-carbon low-alloy steel

In this work, the effect of strain rate on the dynamic transformation (DT) of austenite to ferrite during high temperature forging of an as-cast medium-carbon low-alloy steel was investigated. Hot deformation experiments were carried out in the 1150 °C to 1200 °C temperature range (400–450 °C above Ae3) and 0.25 s−1 to 2 s−1 strain rate range using a Gleeble 3800® thermomechanical simulator. The critical strains for DT and dynamic recrystallization (DRX) were determined to be in the 0.08 to 0.18 strain range. A microstructural analysis was conducted using optical and electron microscopy. The kernel average misorientation (KAM) technique was applied to electron back-scattered diffraction (EBSD) images to quantify the internal misorientation of grains for the characterization of DT ferrite. Furthermore, it was found that an increase in strain rate decreased the amount of dynamically formed ferrite under the same applied strain and testing temperature. The obtained results were correlated with the influence of deformation parameters on carbon diffusivity and its impact on the growth of dynamically formed ferrite. It was found that an increase in the strain rate decreased the diffusion distance of carbon, which was the responsible mechanism for decreased ferrite formation at higher strain rates.

Effect of Grain Size and Residual Strain on the Dynamic Transformation of Austenite under Plate Rolling Conditions

When austenite is deformed within the austenite phase field, that is, above the equilibrium transformation temperature Ae3, it partially transforms dynamically into ferrite. Here, rough rolling simulations are conducted at 1100 °C on an X70 steel to study the influence of grain size and residual strain on this type of transformation. Three simulated roughing passes are applied with pass strains of 0.4 at a strain rate of 1.0 s−1. The resulting stress–strain curves are recorded and analyzed. The volume fractions of ferrite and martensite (prior austenite) are determined on quenched samples by means of metallographic techniques. It is observed that dynamic transformation took place during the first roughing pass and that the volume fraction of transformed ferrite increases with the cumulative strain. Both the critical strain to the onset of dynamic transformation as well as the grain size decrease with pass number.

Dynamic Transformation during Plate and Strip Rolling

Torsion simulations were carried out of both plate (long interpass times) and strip (short interpass times) rolling. Both isothermal and continuous cooling conditions were employed. The dynamic transformation of austenite to ferrite was observed under all conditions and at all temperatures within the austenite phase field. About 8 to 10 volume percent ferrite was formed in a given pass, leading to about 50 - 70 % ferrite at the end of selected simulations. During the interpass intervals, some retransformation to austenite took place, the amount of which increased with holding time and temperature and decreased with the addition of alloying elements. It is shown that the driving force for the transformation is the softening associated with the replacement of work-hardened austenite grains by the softer alpha phase. The implications with respect to rolling load (i.e. mean flow stress) are also discussed.

Effect of Mn and Si on the dynamic transformation of austenite above the Ae3 temperature

In the mechanical activation model of dynamic transformation, the stress applied to the austenite is considered to be the driving force for the transformation. Here the yield stress of the newly formed Widmanstätten ferrite that replaces the austenite and that must continue to be deformed is taken into account as an additional obstacle to the transformation. This is estimated from hardness data. In this way, the driving force is redefined to consist only of the softening that takes place during the process and therefore only of the difference between the current flow stress of the work-hardened austenite and the yield stress of the fresh Widmanstätten ferrite that takes its place. According to this model, this driving force is opposed by the chemical free energy difference between the austenite and the Widmanstätten ferrite as well as the lattice dilatation work and shear accommodation work that accompany the transformation. In order to assess the effects of Mn and Si addition on the transformation, the high temperature flow stress data obtained by Wray (1984) by means of tensile tests on four Fe–Mn–Si alloys are employed to determine how these alloying additions affect the driving and opposing forces. The present type of model provides a more detailed analysis of the phenomenon than any previously available model and in this way shows that Mn addition is expected to lead to only a small decrease in the temperature range over which displacive ferrite can form, while Si addition is predicted to have a much larger effect on this interval.

Influence of Cooling Path on the Dynamic Transformation of Austenite-to-Ferrite in Low Carbon Manganese Steels

For an effective thermomechanical process control, knowledge of the start and finish temperatures of the austenite transformation is critical. Continuous Cooling uniaxial-Tension (CCT*) is a useful way to measure these values. Therefore, the dynamic transformation of austenite to ferrite (γ → α) was investigated in C-Mn steels to understand the hot ductility behaviour of these steels after varying the cooling paths i.e. simulating “hard” and “soft” cooling rates of the strand during continuous casting. Results show that hard cooling into the dual phase (γ + α) region significantly increases the dynamic transformation temperatures due to a higher driving force owing to double transformation and precipitation of AlN during the process. A comparison of dynamic versus static transformation (Ar3 and Ar1) temperatures and equilibrium transformation temperatures gives a better understanding of the contribution of strain to transformation during casting or hot deformation.

Dynamic Transformation during the Torsion Simulation of Strip Rolling

Seven-pass strip rolling simulations were carried out on a 0.06%C and a 0.09%C-0.036%Nb steel. The rolling loads (mean flow stresses or MFS’s) did not increase as the temperature decreased during the simulation. This is ascribed to the occurrence of dynamic transformation. The simulation results are compared to the high temperature flow curves determined on eight plain C and Nb-modified steels in both compression and torsion and at a series of temperatures and strain rates. When the associated MFS’s are plotted against inverse absolute temperature in the form of Boratto diagrams, the stress drop temperatures, normally defined as the upper critical temperature applicable to rolling, Ar3*, are shown to be about 40 degrees above the paraequilibrium and about 20-30 degrees above the orthoequilibrium Ae3’s. These drops are ascribed to the dynamic transformation of austenite to ferrite, a softer phase. The characteristics of the ferrite produced dynamically are described and the transformation is shown to be displacive in nature, leading to the appearance of fine Widmanstätten plates. These plates coalesce into polygonal grains on further deformation and on holding.

Role of mechanical activation in the dynamic transformation of austenite

When austenite is deformed above the equilibrium transformation temperature Ae3, it is dynamically transformed into Widmanstätten ferrite by a displacive mechanism. On removal of the load it is slowly retransformed into austenite by diffusional processes. The forward transformation has recently been explained in terms of a thermodynamic model in which the lower free energy of austenite is raised above that of normally unstable ferrite as a result of the additional stored energy associated with the dislocations introduced by straining. This model is here shown to be unable to account for the initiation of transformation at critical strains of about 0.1, at which only low densities of dislocations are present. Of particular importance is the observation that dynamic transformation can be initiated at temperatures 100°C and more above the Ae3 and that the critical strain actually decreases with increasing temperature and increasing chemical free energy barrier. This discrepancy is removed by allowing for mechanical (stress-based) activation of the transformation. The latter provides the energy required to accommodate the shear of the parent austenite into Widmanstätten plates, as well as the volume change or dilatation accompanying ferrite formation. The work of dilatation and the shear accommodation work, omitted from the previous analysis, are introduced here as barriers to the transformation that are overcome by the applied stress. This modified approach is able to account for the very rapid forward (mechanically activated) transformation compared with the much slower reverse transformation that takes place in the absence of stress.

Dynamic Transformation of a Low Carbon Steel at Temperatures above the Ae3

The dynamic transformation of austenite was studied in a 0.06%C low carbon steel by deforming to strains of 0.25–5.0 at strain rates of 0.04 and 0.4 s–1 over the temperature range 877–917°C. All these temperatures are at or above the conventional Ae3. Two critical strains were detected, the first (about e=0.2) was for the formation of strain-induced ferrite; the second (about e=1.5) was the critical strain for the reverse transformation, which was gradual and only observed in specimens deformed at the lower strain rate. After deformation, the strain-induced ferrite was stable for about 60 s of isothermal holding. However, after this time, the reverse transformation began to take place, approaching saturation in about 240 s. The influence of strain, strain rate and temperature on the dynamic transformation is described. The results indicate that reverse transformation is unlikely to take place in the finishing stands of strip mills due to the high strain rates and short interpass times involved. It is also suggested that conventional phase diagrams do not apply to austenite undergoing deformation in rolling mills.