A Comparison of Ferrite Growth Kinetics under Denitriding and Decarburizing Conditions

Abstract

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.

Interface Segregation and Nitrogen Measurement in Fe–Mn–N Steel by Atom Probe Tomography

“…It is well known that FIB milling can cause damage to the near-surface regions of a sample, resulting in atom displacements, vacancy generation, and the implantation of Ga ions (Giannuzzi & Stevie, 2005; Volkert & Minor, 2007). These effects may provide N atoms with sufficient energy to be ejected from the sample.…”

“…This concentration decays rapidly with depth, until ~15–20 nm, after which the Ga content is only present at background levels. Assuming the FIB milling to be operating in the linear cascade regime (Giannuzzi & Stevie, 2005), there is the possibility that the collision cascades caused by the incident Ga ions affect N at greater depths than those reached by the Ga ions themselves; however, in a relatively dense material such as steel, the depth of the collision cascade should not be far greater than the Ga ion penetration depth. An approximation of the collision depth in the Fe–Mn–N martensite samples is made by modeling with the stopping and range of ions in matter (SRIM) software package (Ziegler, 2013).…”

“…During ion beam bombardment, the majority of the elastic energy interactions (i.e., from the initial kinetic energy of the ion) is converted to heat (Volkert & Minor, 2007). It has been argued that in most samples during milling, specimen heating is largely confined to the collision cascade (Giannuzzi & Stevie, 2005); however, for the case of an APT sample, it is possible that some heating of the bulk needle still occurs. The shape of the APT sample has an extremely high aspect ratio at the tip apex, which yields a high amount of area exposed to the ion beam, per volume.…”

Effect of C and N and their absence on the kinetics of austenite-ferrite phase transformations in Fe-0.5Mn alloy

“…The ternary Fe-N-Mn alloys were then produced using controlled nitriding of the Fe-0.5Mn binary. The full details for these processes are given by Guo et al [14]. The exact composition of the alloys used in this study are shown in Table 1.…”

“…In another approach, the effects can be studied by conducting austenite to ferrite phase transformation experiments in series of Fe-C-Mn and Fe-N-Mn ternary steels. In this regard, a series of recent experimental studies on ferrite growth during controlled decarburizing and denitriding [14,15], have suggested interfacial segregation of Mn in presence of C, but no sign of interfacial segregation of Mn in Fe-N-Mn. These findings, based on analysis of the growth kinetic data, is consistent with the preceding accepted perception [8,16e18] that the presence of interstitial elements influences the tendency of the substitutional alloying element to segregate to the interface.…”