Thermodynamics of the Fe-N and Fe-N-C Systems: The Fe-N and Fe-N-C Phase Diagrams Revisited

Abstract

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.

Critical Evaluation and Optimization of the Fe-N, Mn-N and Fe-Mn-N Systems

“…In the temperature range from 450 to 550°C, the solubility of N in c 0 -Fe 4 N phase increases continuously with the increase of P 1=2 N 2 until the N content (x N ) reaches a value of 0.1997 which is very close to stoichiometric N content of c 0 -Fe 4 N. In order to reproduce these experimental data, we used both o L Fe:N;Va and 1 L Fe:N;Va interaction parameters. There are only a small difference among the previous assessment results by Du [11] and Gohring et al [12] and the present work at 500 and 550°C by Gohring et al, but the results at 450°C by Gohring et al are less consistent with experimental data by Grabke, [94] as can be seen in Fig. 5(a).…”

“…Such wrong slope of phase boundary can cause more serious error of FCC phase region with increasing temperature. Even in the later optimization by Gohring et al, [12] the phase boundary of FCC solution against HCP phase is less reliable as shown in Fig. 4(b).…”

“…The Fe-N phase diagram was calculated by Hillert and Jarl [34] Agren, [35] and Kunze [36] based on available experimental data. Then, proper thermodynamic assessment of the entire Fe-N system was performed by Frisk [8,32] using CALPHAD method and later by Du, [11] Fernandez Guillermet and Du [37] and Gohring et al [12] In the thermodynamic assessments, BWRMM was used for the liquid solution and two-sublattice model for the solid solutions. It is well known that Liquid, BCC (a-Fe and d-Fe), FCC (c-Fe), HCP (e), c 0 -Fe 4 N are stable in the binary Fe-N system.…”

“…Unfortunately, the assessments of binary Fe-N and Mn-N systems adopted in the ternary systems have several critical problems. According to the thermodynamic parameters optimized by Frisk, [8] c-FCC phase is calculated in high-N concentration region at the temperature below 470°C, and this problem was still not resolved in the more recent reassessments by Du [11] and Gohring et al [12] In addition, c 0 -Mn 4 N phase of the Mn-N system was assumed as a stoichiometric compound in Qiu, and Fernandez and Guillermet's [9] assessment, which is conflicting with experimental measurements by Hagg, [13] Brisi, [14] Juza et al, [15] Lihl et al, [16] Kudielka and Grabke, [17] Pompe [18,19] that indicate certain inhomogeneity range.…”

Crystallography of γ′-Fe4N formation in single-crystalline α-Fe whiskers

“…The growth substrates with the whiskers were nitrided in a laboratory chamber furnace in an NH 3 and H 2 gas mixture (300 l h À1 ) at 823 K and a nitriding potential of p NH 3 =p 3=2 H 2 = 0.7 atm À1/2 for 20 min (p: partial pressures in the furnace gas mixture as measured in the furnace chamber and at the exhaust). Under these conditions, 0 -Fe 4 N is the only nitride phase that should be formed in metastable equilibrium with the gas (Lehrer, 1930;Gö hring et al, 2016). After nitriding, the samples were cooled by moving them into the cold zone of the furnace under process gas.…”