Won-Beom Lee

Thermodynamic and kinetic analysis of formation of compound layer during gas nitriding of AISI1018 carbon steel

In this study, the formation of compound layers during isothermal (520 °C) gas nitriding of AISI1018 steel has been investigated using experimental measurements as well as thermodynamic and kinetic analyses. The nitriding potential (Kn) was obtained from the measurement of the partial pressure of hydrogen gas generated during ammonia dissociation. The microstructural evolution of the compound layer was studied by using optical microscopy (OM) and scanning electron microscopy (SEM). The measured surface hardness of the nitride layer was 450HV with a nitrogen-penetration depth of about 0.1 mm after nitriding for 1440 min. The compound layer was composed of γ’ (Fe4N) and ε (Fe2-3N) phases, and the measured thickness of the compound layer increased to ∼21 mm after 1440 min. The phase stability and kinetics of the compound layer growth were discussed using a modified Lehrer diagram for AISI1018 steel, which was obtained from thermodynamic calculations and diffusion simulation using the finite difference method.