Thermodynamic reassessment of Fe-N and Fe-N-C systems

Kouba, R.; Rayane, Karim; Кеддам, М.

doi:10.1016/j.calphad.2020.102197

Cited by 4 publications

(8 citation statements)

References 25 publications

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“…The transformation of θ into ε and γ 0 and the respective ε/θ, ε/γ 0 , and γ 0 /θ phase boundaries observed inside the u Si,θ regions (Figure 8) are compatible with local equilibria in the well-known Fe-C-N system at 540 C, [44,45] i.e., the boundary system of the Fe-C-Si-N system at u Si ¼ 0. Likewise, the ε þ X phase mixture in the surface-adjacent, decarburized part of the compound layer inside u Si,α and u Si,Fe 23 Si 5 C 4 regions at z !…”

Section: Topology Of the Fe-c-si-n Phase Diagramsupporting

confidence: 64%

“…[12] Moreover, the transformation α ! ε þ X occurred at a nitriding temperature of 540 C, at which the growth of ε in contact with α-Fe under conditions of local equilibrium is incompatible with the Fe-C-N phase diagram [44][45][46] and available thermodynamic databases [47] containing the Fe-Si-N and Fe-C-Si-N systems. A higher Si solubility in ε than in γ 0 was hypothesized to stabilize ε with respect to γ 0 in Fe-Si steels.…”

Section: Doi: 101002/adem202100833mentioning

confidence: 99%

“…Based on the experimental data and literature information [44][45][46][47] on the respective ternary boundary systems, basic features of the topology of the Fe-rich corner of the Fe-C-Si-N can be deduced.…”

Section: Topology Of the Fe-c-si-n Phase Diagrammentioning

confidence: 99%

“…To illustrate the relative position of the phase fields in a N -a C -u Si space, the u Si section in Figure 11a is overlaid to the well-known section of the Fe-C-Si-N phase diagram at u Si,θ ¼ 0 (Fe-C-N boundary system [44][45][46][47] ) in Figure 12. Coincident diffusion paths describing the observed sequence of phases and phase transformations between the substrate and surface in u Si,α and u Si,θ regions in Figure 8 after t 2 and t 3 are indicated, together with multiphase points (Section 4.1) determining the position of the phase fields in both u Si sections with respect to each other.…”

Section: Schematic U Si Section Of the Fe-c-si-n At U Siαmentioning

confidence: 99%

“…Figure 11. a) Schematic sketch of the Fe-rich corner of the Fe-C-Si-N potential phase diagram at 540 C deduced from present experimental data and literature [44][45][46][47]. The 3D phase diagram in a N -a C -a Si space is visualized by projections along the a Si and a C axis onto the a N -a C and a N -a Si boundary planes, schematically showing the topology of the Fe-C-N[44,47] and Fe-Si-N[12,37,47] boundary systems (gray).…”

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confidence: 99%

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Nitriding of White‐Solidified Fe–C–Si Alloys: Diffusion Path Concept Applied to Inhomogeneous Microstructures

2021

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Gray-solidified cast irons are frequently used to manufacture machine and engine parts due to their low cost, good compressive strength, and high damping capacity. [1,2] Their use is, however, often limited by poor surface hardness and corrosion resistance. Both can be notably improved by a duplex surface treatment developed in recent years. [3][4][5][6] First, the surface of the cast iron is remelted, e.g., by an electron beam or plasma arc. This produces a white-solidified, ledeburitic [7,8] surface layer, which is hard and abrasion-resistant. Afterward, the material is nitrided to enhance the corrosion resistance by the formation of a dense surface layer composed of the main Fe (carbo)nitrides, γ 0 -Fe 4 (N,C) and ε-Fe 3 (N,C) 1þx (Table 1), denoted compound layer. Note that nitriding without remelting leads to less favorable microstructures because coarse graphite particles contained in gray-solidified cast irons are detrimental to the compound layer formation. [3,4,[9][10][11] Although the general advantages of the previously described duplex treatment have been demonstrated, the nitriding behavior of the white-solidified surface layers turns out to be complex and incompletely understood. [12][13][14][15][16][17][18][19] The research of the current authors [12,18,19] aims at elucidating the complex nitriding behavior of the remelted surface layers by controlled gas nitriding of white-solidified Fe-3.5 wt% C-0/1.5/3 wt% Si alloys, serving as model alloys for commercially available ferritic cast iron grades subjected to remelting and nitriding. Note that, in contrast to ferritic cast irons, pearlitic cast irons often contain non-negligible additions of Mn and Cu, which may additionally affect the nitriding behavior. [17] The microstructure of white-solidified Fe-3.5 wt% C-1.5/3 wt% Si alloys, having C and Si contents typical of cast irons, is mainly composed of coarse eutectic cementite plates, θ-Fe 3 C 1Àz (Table 1), embedded in ferrite and pearlite. [20,21] The latter result from the decomposition of primary and eutectic γ-Fe during cooling after solidification. In addition, a minor volume fraction of Fe 23 Si 5 C 4 -type silicocarbide (Table 1) is contained in the alloys. [22][23][24] The distribution of Si in the white-solidified microstructure is very heterogeneous. The Si/Fe atomic ratio in the eutectic and pearlitic θ is u Si,θ < 3 Â 10 À4 and, here, considered negligible, say u Si,θ ! 0. [12] The Si/Fe atomic ratio in Fe 23 Si 5 C 4 is u Si,Fe 23 Si 5 C 4 % 0.22; however, it might take also values slightly different from 0.22 due to a possibly mixed Fe/Si site [23,24] and extended lattice defects affecting the composition. [22] The Si content in α-Fe relates to the Si content of γ-Fe, which is affected by Si segregation during solidification. The Si content in the center of γ-Fe dendrites is typically similar to the Si content of the alloy, while the outer rim of dendrites and last-to-freeze regions are enriched in Si. [25] The Si content in eutectic γ-Fe is higher

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Section: Topology Of the Fe-c-si-n Phase Diagramsupporting

confidence: 64%

Section: Doi: 101002/adem202100833mentioning

confidence: 99%

Section: Topology Of the Fe-c-si-n Phase Diagrammentioning

confidence: 99%

Section: Schematic U Si Section Of the Fe-c-si-n At U Siαmentioning

confidence: 99%

mentioning

confidence: 99%

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Nitriding of White‐Solidified Fe–C–Si Alloys: Diffusion Path Concept Applied to Inhomogeneous Microstructures

2021

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Coupled Experimental Study and Thermodynamic Modeling of the Fe–Si–N System

You,

Shen,

Liu

et al. 2024

steel research int.

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Some key experiments are carried out to determine the solubility of N in Fe–Si liquid solutions at various compositions, temperatures, and N2 pressures using the sampling method. The Fe–Si–N system is thermodynamically modeled using the CALculation of PHAse Diagrams (CALPHAD) approach for the first time based on critical evaluation of all available experimental data. In the present thermodynamic modeling, the liquid and solid solutions of the Fe–Si–N system are described using the modified quasichemical model and compound energy formalism, respectively. In particular, the Gibbs energies of liquid, FCC_A1, and BCC_A2 solutions are carefully determined to resolve the discrepancies among existing experimental data. The solubility product of β‐Si3N4(s) in Si steels and ferrosilicon alloys, activity coefficients, and interaction parameters of N in liquid and solid Fe–Si–N solutions are determined over a wide temperature range. As an application of the database, the N solubility in Si steels and phase constitution of nitrided ferrosilicon alloys in a wide composition, temperature, and N2 pressure ranges are predicted.

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