Several thermodynamic descriptions of the Fe-N and Fe-N-C systems were proposed before now. The results of these descriptions significantly deviate from more recently obtained experimental data. The present work provides a revised thermodynamic description of these systems. The new description for the Fe-N system agrees distinctly better with the experimental data especially for the equilibrium of c 0 -Fe 4 N 1Àx and e-Fe 3 N 1+z . The new thermodynamic description for the Fe-N-C system considering the Fe-rich part of the system with less than 33 at. pct N and less than 25 at. pct C excellently agrees with the new experimental data for both the temperatures of the invariant reactions and the phase boundaries. This in particular concerns the temperature range of typical technical nitriding and nitrocarburizing treatments [723 K to 923 K, (450°C to 650°C)], within which three invariant reactions occur in the ternary system.
The present work is dedicated to investigating the occurrence of the a þ e equilibrium at temperatures typically applied for nitrocarburizing treatments. To this end, pearlitic Fe-C specimens were treated between 823 K and 863 K (550°C and 590°C) in gaseous nitriding and gaseous nitrocarburizing atmospheres, allowing control of the chemical potentials of N and C. Subsequently, the resulting compound-layer microstructures were investigated using light microscopy and X-ray diffraction. Thermodynamic calculations, adopting several models for the Fe-N-C system from the literature, were performed, showing significantly different predictions for both the sequence of the invariant reactions and their temperatures. Comparison of the experimental data and the theoretical calculations led to the conclusion that none of the models from the literature is able to realistically describe the experimentally observed constitution in the Fe-N-C system in the considered temperature range. Values/value ranges for the temperatures of the invariant reactions were obtained.
The simultaneous diffusion of N and C over the interstitial sites of the Fe-sublattice of e-iron carbonitride was studied. To this end, gas nitrocarburizing experiments of pure Fe and Fe-C alloys were performed at 853 K (580°C), leading to two different types of microstructures containing e (sub)layers. These microstructures were investigated by light microscopy, electron probe microanalysis, and X-ray diffraction in order to evaluate the components of the (N and C) diffusivity matrix. The off-diagonal components of the diffusivity matrix were shown to have significant, non-negligible values. These results provided insight into the thermodynamics of the Fe-N-C system.
Pure iron and a series of iron-based Fe-Me alloys (with Me = Al, Si, Cr, Co, Ni, and Ge) were nitrided in a NH 3 /H 2 gas mixture at 923 K (650°C). Different nitriding potentials were applied to investigate the development of pores under ferrite and austenite stabilizing conditions. In all cases, pores developed in the nitrided microstructure, i.e., also and strikingly pure ferritic iron exhibited pore development. The pore development is shown to be caused by the decomposition of (homogeneous) nitrogen-rich Fe(-Me)-N phase into nitrogen-depleted Fe(-Me)-N phase and molecular N 2 gas. The latter, gas phase can be associated with such high pressure that the surrounding iron-based matrix can yield. Thermodynamic assessments indicate that continued decomposition, i.e., beyond the state where yielding is initiated, is possible. Precipitating alloying-element nitrides, i.e., AlN, CrN, or Si 3 N 4 , in the diffusion zone below the surface, hinder the formation of pores due to the competition of alloying-element nitride (Me x N y ) precipitation and pore (N 2 ) development; alloying elements reducing the solubility of nitrogen enhance pore formation. No pore formation was observed upon nitriding a single crystalline pure iron specimen, nitrided under ferrite stabilizing conditions, thereby exhibiting the essential function of grain boundaries for nucleation of pores.
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