Interstitial ordering of nitrogen and carbon in laser nitrided and laser carburized austenitic stainless steel

The distribution of carbon and nitrogen atoms on the octahedral interstitial sites of the face-centred-cubic austenite phase in Fe-C and Fe-N alloys, especially in austenitic stainless steel, is still causing controversy. In this work, results of Mossbauer experiments are presented in order to advance the understanding of this interstitial occupation. Therefore, laser carburized and laser nitrided austenitic stainless steel was investigated by means of x-ray diffraction and Mossbauer spectroscopy. Three subspectra in terms of different iron sites were resolved in the Mossbauer spectra for these iron-carbon and iron-nitrogen austenites. The isomer shifts, the quadrupole splittings and in particular the subspectra fractions depend on the type of the introduced atom and undergo changes when increasing the carbon or nitrogen content. This is discussed in connection with the existing ordering models for interstitial atoms. No clear evidence could be found for a perfect random occupation, nor for a perfect ordered occupation of the interstitials. Nevertheless, there seems to a tendency for a weak attractive interaction for nitrogen interstitials, and for a stronger repulsive force for the carbon interstitials in laser nitrided/carburized austenitic stainless steel.