A precise understanding of compound layer growth on nitrocarburized steels requires fundamental investigations on nitrocarburizing of iron-based binary and ternary alloys. Such studies were extensively carried out for the case of nitriding. To this end, this study is devoted to understanding the role of V on the structure and morphology of iron-carbonitrides developed during salt bath nitrocarburizing of pure Fe and Fe-4 wt.% V alloy. Salt bath nitrocarburizing is carried out in a cyanate-based bath at 500oC and 560oC for nitriding times of 4h and 10h. Treated specimens were characterized using light and scanning electron microscopes, X-ray and Electron Back Scattered diffraction techniques, and microhardness measurements.
An unusual plate-type morphology of γ’ and ε phases growing into ferrite matrix was evidenced in the case of Fe-V alloy and usual layer type growth of compound layer occurred in a pure iron specimen. Interestingly, the ε nitride phase was developed first (i.e., present at larger depths from the specimen surface) followed by the γ’ which has been attributed to the role of C in stabilizing the ε phase and C is known to enrich at subsurface regions during nitrocarburizing. Developed nitrides and carbonitrides maintained a specific crystallographic orientation relationship with the ferrite matrix. XRD and hardness measurements indicated the development of VN precipitates in the ferrite matrix before the plate-type growth of iron nitrides and iron-carbonitrides. Such plate-type morphology of iron-nitrides developed in nitride Fe-Al, Fe-Si, and Fe-Mo was attributed to the difficulty in alloying element partitioning required for the development of nitrides. In the current work where iron-carbonitrides were growing into a ferrite matrix in which all V has already precipitated as nitride suggests the role of developed VN and associated interfacial and misfit strain fields could be responsible for the unusual plate-type morphology. Thermo-Calc software and steels thermodynamic property database (TCFE9) were utilized to predict the phase constitution expected and were utilized to explain the observed phase formation sequence.
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