Microstructural investigation of oxynitrocarburized components processed at different temperatures

Gas nitrocarburizing combined with a post-oxidation treatment is an interesting industrial process developed in order to improve wear and corrosion resistance of low alloyed steel components. In the present work, the microstructure resulting by a thermochemical treatment, which comprises nitriding, nitrocarburizing and post-oxidation stages, applied to an industrial 16MnCr5 component, has been studied. Both the nitriding and nitrocarburizing temperatures were systematically changed in order to study the resulting phases in the compound layer. The depth of the compound layer have been measured by optical microscopy to evaluate the effect due to the variations in the process variables during the nitrocarburizing process. Moreover, the microstructure has been investigated by means of a scanning electron microscope equipped with a electron backscatter diffraction detector in order to assess the amount and the distribution of Fe-N-C phases. A temperature increase from 510 up to 550°C during the nitriding process inside a NH3 atmosphere induces a higher fraction of ε-Fe2-3(C,N) compounds. On the contrary, nitrocarburizing at 600°C instead of 580°C under a gaseous mix of NH3 (50%), CO2 and N2 favors a greater amount of γ'-Fe4(C,N) nitrides. A greater amount of porosity in the outer part of the compound layer favors a thicker oxide film obtainable with the post-oxidation process.