Growth kinetics of the compound layers: Effect of the nitriding potential

The aim of this work is to study the effect of the nitriding potential on development of the compound layers during the gas nitriding of Armco Fe and XC38 carbon steel. The gas nitriding experiments were realized in an atmosphere of partially dissociated gas ammonia (NH3) at 520∘C under a variable nitriding potential (from 0.25 to 3.5atm−0.5) for 2 h. Using XRD analysis and SEM observations of the cross-sections of the treated samples, it was shown that the compound layer was composed of γ′ iron nitride after exceeding a critical value of the nitriding potential that depends on the substrate’s nature. A linear semi-logarithmic relationship relating the nitriding potential to the experimentally measured layer thickness for the γ′ phase was used to evaluate the critical nitriding potential giving rise to its formation on the material substrate. It was found that the required value of the critical nitriding potential for XC steel is greater than that of Armco iron.