Phase distributions in the compound layer of Ferritic Nitrocarburized (FNC) 4140 steel using alloy specific Kunze diagrams as a function of KN and KC in the heat treat gas atmosphere

As part of a project to develop simulation software for the FNC process, alloy specific Lehrer and Kunze diagrams were developed using ThermoCalc for 4140 steel to determine the phase composition and distribution during FNC heat treating. The Kunze diagrams were developed to determine the phases in the compound layer on the steel in equilibrium with the nitriding and carburizing atmospheres (KN and KC) as a function of temperature. The phases in the compound layer were determine be g ‘Fe₄N and e Fe_(2-3)N. The volume fraction of each phase is a function of KN and KC. The simulation results are compared with experimental results. The effects of e Fe_(2-3)N stoichiometry are discussed in terms of porosity development and phase distribution. The microstructural profile in the compound layer and the hardened case are interpreted using diffusion paths on the Kunze diagrams and isopleths of the quasi-ternary diagrams.