Microstructural and Phase Evolution of Compound Layers Growing on α–Iron During Gaseous Nitrocarburizing

The microstructural evolution of compound layers grown on 1-mm thick α-iron substrates after nitrocarburizing at 853 K (580 °C) in NH3/H2/N2/CO gas mixtures was investigated by light optical microscopy and X-ray diffraction. The evolution of the microstructure can be divided into several stages. Starting with the formation of the carbon-rich phase cementite, which practically does not contain nitrogen, the phase constitution of the compound layer develops through successive stages of microstructural change into the direction of the nitrogen-richer and carbon-poorer phases e and γ’. These results are the consequences of (1) the kinetics of nitrogen and carbon uptake at the gas–solid interface and the considerably different solubilities of nitrogen and carbon in the α-iron substrate and (2) the occurrence of local equilibrium conditions prevailing at the solid–solid interphase boundaries in the compound layer. The change of the microstructure as a function of depth in the compound layer is shown to be compatible with so-called “diffusion paths” in the ternary Fe-N-C phase diagram.