A Comparison of Ferrite Growth Kinetics under Denitriding and Decarburizing Conditions

Recent years have seen an increasing emphasis on the identification of transitions in growth modes during the diffusional decomposition of austenite to ferrite in Fe-C-X alloys. Of particular interest are transitions between the extremes of non-partitioned growth represented by the ParaEquilibrium (PE) and local equilibrium negligible partition limits. Identification of such transitions requires high-quality measurements of ferrite growth kinetics, and recent uses of the decarburization approach have allowed access to very high-precision growth kinetic measurements. However, one of the limitations of the decarburization approach is that the lower limit of its applicability is the eutectoid temperature and this has so far compromised its usefulness to probe the temperature dependence of kinetic transitions. In this contribution, analogous denitriding studies have been performed that allow access to much lower temperatures than are possible using decarburization. It is shown that the kinetics of ferrite growth in the Fe-N-Mn system become closer to the PE limit as the temperature is lowered. The growth kinetic data are interpreted quantitatively in the framework of the Zurob et al. model that includes diffusional dissipation due to Mn diffusion across the migrating interface. Furthermore, comparisons are made between decarburization and denitriding in Fe-Mn alloys of the same Mn content at the same temperature. The interface velocities are much faster under denitriding conditions, and this allows inferences to be made about the effect of the interface velocity on dissipation and contact conditions. It is also suggested that the binding energy of Mn to the migrating interface may be slightly higher in the Fe-C-Mn system than in the Fe-N-Mn system, and it is speculated that this is due to the strong segregation of C to the interface and the associated co-segregation of Mn.