Modeling of Gaseous Ferritic Nitrocarburizing of Steels

You, Haoxing

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Modeling of Gaseous Ferritic Nitrocarburizing of Steels

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Gaseous nitriding and ferritic nitrocarburizing (FNC) are typical thermochemical surface treatment methods to improve the wear and corrosion resistance, as well as fatigue endurance of steel parts such as shafts and gears. During the treatment, the atomic nitrogen and carbon are introduced into steel surface, a compound layer (also called white layer) composed of ε-Fe2-3(C,N) and γ’-Fe4N iron (carbon-)nitrides will form on the steel surface, and underneath the compound layer, there will be a diffusion zone with alloy (carbon-)nitrides dispersed. The compound layer can improve the wear and corrosion resistance of the steel while the diffusion zone is in favor of the fatigue endurance. The modified mechanical properties of the steel parts mainly depend on the thickness, phase constitution of the compound layer as well as the depth of the diffusion zone, which are governed by several critical process parameters including temperature, nitriding potential, carburizing potential, and process duration. The objectives of the current project are to develop a fundamental understanding of gaseous nitrocarburizing of steels and to establish a computational model for gaseous nitrocarburizing process. Two series of gaseous FNC trials have been carried out, in the first FNC trial, three different materials were selected for investigations, AISI 4140, AISI 1018 and cast iron (ASTM A536 80-55-06). The samples were nitrocarburized at 579 °C with 4 atm-1/2 nitriding potential. Three different carburizing potentials were applied and for each carburizing potential, there were four different process durations. The samples from both FNC trials were characterized with the same procedure. To obtain the total weight gain during the FNC process, the weight of each individual sample was recorded before and after the FNC treatment. The morphology of the compound layer was observed with optical microscope, SEM, and EDS mapping. The average compound layer thickness was measured from the micrographs and the compound layer growth kinetics were fitted with parabolic law. The compound layer phase constitution was analyzed with XRD and Rietveld refinement. The alloy specific potential diagrams were developed with Thermo-Calc, and the diffusion path was determined on the potential diagrams and isopleths to interpret the microstructural profiles in the compound layer. The microhardness profile was tested with line scans in the diffusion zone. The nitrogen and carbon concentration profiles were measured by OES with a layer-by-layer method. The effective nitrogen diffusion coefficient in specific steel was determined with a reversed method from the experimentally measured nitrogen concentration profiles and interpreted with the diffusion with trapping model.

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