Chemical Potentials and Activities of Nitrogen and Carbon Imposed by Gaseous Nitriding and Carburising Atmospheres

Abstract

The nitrogen and carbon activities are the cardinal parameters for process control of nitriding, nitrocarburising, carburising, and carbonitriding. The essential elements of the thermodynamic background for the definition of the so called nitrogen and carbon activities in nitrided and carburised surface layers of iron based substrates are presented in a comparative manner. Choice of the distinct reference states is discussed in relation to the consequences for the interpretation of activities and equilibrium constants of the reactions. Practical examples are used to show how the nitrogen and carbon activities can be established; the activities

The $$\upalpha +\upvarepsilon $$ Two-Phase Equilibrium in the Fe-N-C System: Experimental Investigations and Thermodynamic Calculations

“…[7,59,60] By applying a high flow rate (500 ml min À1 ), the composition of the gas atmosphere remains constant. In an ideal case, thus, a local equilibrium at the surface of the specimen is achieved.…”

“…As discussed elsewhere, [7,12] whereas local solid-solid equilibria can occur at each depth, corresponding to the local gross (laterally averaged) composition, within the compound layer, this may not occur at the gas-solid interface, i.e., at the surface of the compound layer, where moreover at most a stationary state may be established. [10,61] Then, recognizing that in gaseous nitrocarburizing, as compared to gaseous nitriding, distinctly more reactions participate at the gas/solid interface, [59,60] it may be understood that the microstructures obtained upon nitriding are more uniform than those obtained upon nitrocarburizing (cf. Sections IV-A and IV-B).…”

Pore Formation Upon Nitriding Iron and Iron-Based Alloys: The Role of Alloying Elements and Grain Boundaries

“…It is important to realize that establishment of this equilibrium requires that thermal dissociation of NH 3 can be ignored and that the recombination of nitrogen atoms adsorbed at the surface is negligible (if the latter would not hold, a stationary state, instead of an equilibrium situation, would occur at the surface of the substrate; for full discussion see References 1,19 [20] r…”

“…The sum of the gas fluxes of NH 3 (purity 99.998 vol pct) and H 2 (purity 99.999 vol pct) were kept constant at 500 mL/ min and the NH 3 and H 2 gas fluxes were maintained such to set the required nitriding potential. [20] Nitriding experiments were carried out such that either nitrogen ferrite or nitrogen austenite was stabilized at the specimen's surface. The corresponding two sets of nitriding conditions employed in the present work are shown in Table III.…”

Normal and excess nitrogen uptake by iron-based Fe–Cr–Al alloys: the role of the Cr/Al atomic ratio

“…The application of a NH 3 /H 2 gas mixture, instead of pure N 2 gas, makes it possible to impose, at 1atm and in a temperature range of 500 -580°C, at the surface of the specimen to be nitrided, a chemical potential of nitrogen equivalent to that of thousands of atmospheres of N 2 gas. To this end the slow thermal dissociation of NH 3 under these conditions is crucial [8,9].…”