Models were derived for monolayer and bilayer growth into a substrate in which diffusion of the solute governs the growth kinetics, as in gas-solid reactions, for example. In the models, the composition dependence of the solute diffusivity in the phases constituting the layers was accounted for by appropriate definition of an effective diffusion coefficient for a (sub)layer. This effective diffusion coefficient is the intrinsic diffusion coefficient weighted over the composition range of the (sub)layer. The models were applied for analyzing the growth kinetics of a γ′-Fe4N1-x monolayer on an α-Fe substrate and the growth kinetics of an e-Fe2N1-z/γ′-Fe4N1-x bilayer on an α-Fe substrate, as observed by gaseous nitriding in an NH3/H2-gas mixture at 843 K. The kinetics of layer development and the evolution of the microstructure were investigated by means of thermogravimetry, layer-thickness measurements, light microscopy, and electron probe X-ray microanalysis (EPMA). The effective and self-diffusion coefficients were determined for each of the nitride layers. The composition dependence of the intrinsic (and effective) diffusion coefficients was established. Re-evaluating literature data for diffusion in γ′-Fe4N1-x on the basis of the present model, it followed that the previous and present data are consistent. The activation energy for diffusion of nitrogen in γ′-Fe4N1-x was determined from the temperature dependence of the self-diffusion coefficient. The self-diffusion coefficient for nitrogen in e-Fe2N1-z was significantly larger than that for γ′-Fe4N1-x. This was explained qualitatively, considering the possible mechanisms for interstitial diffusion of nitrogen atoms in the close-packed iron lattices of the e and γ′ iron nitrides.

Layer-Growth Kinetics on Gaseous Nitriding of Pure Iron Evaluation of Diffusion Coefficients for Nitrogen in Iron Nitrides

As a prerequisite for the predictability of properties obtained by a nitriding treatment of iron based workpieces, the relation between the process parameters and the composition and structure of the surface layer produced 111ustbe known. At present, even the description of thermodynamic equilibrium of pure Fe–N phases has not been fully achieved. It is shown that taking into account the ordering of nitrogen in the e and γ'iron nitride phases leads to an improved understanding of the Fe–N phase diagran1. Although consideration of thermodynamics indicates the state the system strives f01~ the nitriding result is determined largely by kinetics. Nitriding kinetics are shown to be characterised by localnear equilibria and stationary states at surfaces and intelfaces, and the diffusion coefficient of nitrogen in the various phases, for which new data are presented. The necessary background for process control of gaseous nitriding by monitoring the partial pressure of oxygen in the furnace using a solid...

Thermodynamics, kinetics, and process control of nitriding

Thermodynamics, kinetics and process control of nitriding

Solid state phase transformations in metals, and more precisely the science of transformation interfaces, is a key point to understand the formation of nano/microstructure, and thus, as a result, many physical properties such as mechanical properties, conductivity, thermoelectric and magnetic properties of materials.

Steels are by far the most widely used metallic alloys, and a deep understanding of their microstructure is essential to tailor their service properties. The transformation of high temperature parent austenite to ferrite is one of the main issues controlling the final microstructures, and for more than a century, this has driven metallurgists to investigate in detail this solid state transformation, and, particularly, the details of austenite to ferrite interface migration.

In this paper, we review the evolution of the different concepts and experiments developed in the last century to investigate this transformation mechanism.

After a brief introduction, most of the physical models developed, which reduce the α/γ interface into a mathematical body with its own properties, are reviewed and discussed with regard to experimental data. The increased availability of highly sophisticated experimental and modelling tools in recent decades has considerably clarified the perceptions of transformation interfaces. These recent advances are presented, and their contribution to the field of migrating austenite–ferrite interfaces are highlighted in a third section. In the fourth section, the latest developments in experimental methods, which now allow the quasi atomistic direct characterization of the interface chemistry, are presented. The observed conditions at the interfaces can be compared with model predictions, which is believed to be a critical step for the refinement of the theoretical concepts guiding the understanding of the interface migration. Finally, in the concluding section, the present situation of the field is summarized, and some perspectives regarding the expected future developments are sketched.

Overview of the current issues in austenite to ferrite transformation and the role of migrating interfaces therein for low alloyed steels

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Fundamentals of nitriding and nitrocarburizing

This study is within the framework of a research program dedicated to defining the optimal conditions for the nitriding of iron and steels at atmospheric pressure by using various mixtures, NH3-N2-H2 and NH3-Ar. After studying the mechanisms of phase formation and mass transfer at the gas-solid interface, a mathematical model is developed in order to predict the nitrogen transfer rate in the solid, the nitride layer growth rate, and the nitrogen concentration profiles. In order to validate the model and to show its possibilities, it is compared with thermogravimetric experiments, analyses, and metallurgical observations (X-ray diffraction, optical microscopy, and electron microprobe anal-ysis). The results obtained allow us to demonstrate the sound correlation between the experimental results and the theoretical predictions. By applying the model to the iron-nitrogen binary system, when the e/γ/α configuration referred to the Fe-N phase diagram is formed, we have experimentally determined the effective diffusion coefficient of nitrogen in the e phase. The latter is constant for a composition of the e nitride between 8 and 9.5 wt pct nitrogen. All the results obtained show that it is possible, by means of dynamic gas flow regulation, to eliminate the incubation period and to control the thickness, composition, and structure of the compound layer at the beginning of the treatment.

Control of iron nitride layers growth kinetics in the binary Fe-N system

Application of a Mathematical Model of Iron-Nitride Layer Growth during Gas Phase Nitriding

L'etude vise a definir les conditions optimales de nitruration, a la pression atmospherique, au moyen des melanges gazeux NH 3 -N 2 -H 2 , NH 3 -Ar et NH 3 -Ar-C 3 H 8 . Apres etude des mecanismes de formation des phases et de transfert de matiere a l'interface gaz-solide, nous avons developpe un modele mathematique permettant de prevoir la vitesse de transfert de l'azote dans l'etat solide, la vitesse de croissance des couches de nitrures et de carbonitrures et les profils de concentration en azote. Pour valider ce modele et montrer ses possibilites, nous l'avons confronte a des experiences de thermogravimetrie, a des analyses et observations metallurgiques (rayons X, microscopie optique et microsonde electronique). Tous les resultats obtenus montrent qu'il est possible, par une regulation dynamique des debits de gaz a l'entree du reacteur, de controler des l'instant initial du traitement, l'epaisseur, la composition et la structure des couches de combinaison.

Ph. Bilger

Papers

Control of iron nitride layers growth kinetics in the binary Fe-N system

Application of a Mathematical Model of Iron-Nitride Layer Growth during Gas Phase Nitriding

Cinétique de croissance des couches en système polyphasé : Application aux systèmes Fe-N et Fe-N-C