Abstract
We have analyzed the evolution of the ferrite fraction and average ferrite grain size during partial cyclic austenite-to-ferrite and ferrite-to-austenite phase transformations in an Fe-0.25C-2.1Mn (wt pct) steel using three-dimensional neutron depolarization (3DND). In the 3DND experiments, the ferrite fraction is derived from the rotation angle of the neutron polarization vector, and the average grain size is determined from the shortening of the polarization vector. From these, the number density of ferrite grains is derived, which indicates that grain nucleation is negligible during partial cycling in the intercritical regime and that all transformation kinetics can be attributed to growth processes only. In the multiple successive cyclic partial transformations, the interfacial migration rate was found to be sluggish due to Mn partitioning. The transformation kinetics determined with 3DND was compared to the predicted behaviors for diffusion-controlled simulations under local equilibrium and para-equilibrium interfacial conditions. It was found that the simulation predictions under local equilibrium only qualitatively capture the transformation kinetic with a difference of one order of magnitude in the variation in the ferrite fraction during cycling. The cyclic behavior of this Fe-0.25C-2.1Mn (wt pct) steel shows that the austenite-ferrite interface indeed migrates back and forth during cycling, while at the same time, there is a gradual increase in both the ferrite fraction and the average ferrite grain size over subsequent cycles. The intrinsic cyclic behavior is only visible after subtracting the effect of the progressive interfacial migration into austenite. The present study demonstrates the advantage of 3DND in studying partial cyclic phase transformations over conventional experimental approaches.
Highlights
THE kinetics of the austenite-to-ferrite (c-a) and the ferrite-to-austenite (a-c) transformations in low-alloyed steels have attracted extensive attention due to their practical importance and scientific challenges.[1,2,3,4,5] During the austenite-to-ferrite transformation, the ferriteManuscript submitted February 14, 2018
The evolution of the ferrite fraction and the ferrite grain size during slow partial cyclic austenite-ferrite phase transformations in Fe-0.25C-2.1Mn steel has been studied in detail with 3DND experiments
The number density of the ferrite grains was estimated, and the results demonstrate that during cycling additional nucleation is proven to be negligible or even absent
Summary
THE kinetics of the austenite-to-ferrite (c-a) and the ferrite-to-austenite (a-c) transformations in low-alloyed steels have attracted extensive attention due to their practical importance and scientific challenges.[1,2,3,4,5] During the austenite-to-ferrite transformation, the ferriteManuscript submitted February 14, 2018. As observed with synchrotron X-ray diffraction, ferrite nucleation occurs in a certain temperature (or time) range, where new nuclei continuously form until a maximum density is reached.[6] Once nucleated, the growth of a ferritic grain, i.e., the interfacial migration, is controlled by interfacial mobility and diffusion of solute elements in the vicinity of the moving interface. To explore the effect of the alloying elements M (=Mn, Ni, Co, etc.) on interfacial migration in Fe-C-M steels, extensive studies have been performed using conventional isothermal or continuous heating and cooling experiments.[7,8,9,10] in such experiments where nucleation and interfacial migration take place simultaneously, the impossibility to determine the nucleation rate during the entire transformation process unavoidably leads to nonnegligible uncertainties in the derivation of the interfacial mobility and investigating the effect of the alloying elements. To avoid the effect of nucleation on the transformation kinetics, the concept
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