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Plasma nitriding of austenitic stainless steel (ASS) at moderate temperature (~400°C) produces a modified near surface layer, often called S phase, which shows significantly improved tribological properties, conserved or improved (electro)chemical properties and induced ferromagnetism. Here, the study of the nature of the S phase in nitrided ASS 304L is presented. A combination of global probes (X-ray diffraction (XRD), nuclear reaction analysis, glow discharge optical emission spectroscopy) and local probes (field ion microscopy, extended x-ray absorption fine structure spectroscopy) is employed to reveal the phase structure, morphology, atomic ordering and chemical environment. While XRD shows only the presence of the presence of the S phase, the local analysis techniques consistently demonstrate the heterogeneous nature of the nitrided layer. It consists of nanometric CrN precipitates embedded in a Fe4N-like matrix. The size of the CrN precipitates is found to be larger at the surface than at the nitrided layer-steel interface. Moreover, X-ray spectroscopic investigations disclose three different intermetallic distances and different chemical environments for Fe, Cr and Ni, accompanied with a large static disorder. These findings suggest that the presence of the interstitial nitrogen can destabilize the homogeneous matrix element distribution in ASS 304L even at the rather low temperature of 400°C. Based on the heterogeneous nature of the S phase revealed in plasma nitrided ASS 304L, an alternative insight into its remarkable combination of properties is presented.

The nature of the near-surface γN phase produced by low-temperature (∼400 °C) plasma-assisted nitriding of an austenitic stainless steel 304L is studied. A combination of global probes (X-ray diffraction, nuclear reaction analysis, glow discharge optical emission spectroscopy) and local probes (field ion microscopy, conversion electron Mössbauer, X-ray absorption near edge structure and extended X-ray absorption fine structure spectroscopies) is employed to reveal the morphology, phase structure, atomic ordering and chemical state of the obtained γN phase. The results consistently reveal the heterogeneous nature of the nitrided layer consisting of nanometric CrN precipitates embedded in a Fe4N-like matrix. The size of the precipitates is found to be larger at the surface than at the nitrided layer–steel interface. The precipitates have irregular, sphere-like shapes. Moreover, X-ray spectroscopic investigation revealed three different intermetallic distances and different chemical environments for Fe, Cr and Ni, accompanied by a large static disorder. These findings suggest that the presence of the interstitial N destabilizes the homogeneous element distribution in 304L even at such low temperatures. This leads to the segregation into Cr-rich zones that are coherent with the Fe4N matrix. Possible atomistic decomposition mechanisms are discussed. Based on the heterogeneous nature of the γN phase revealed in 304L, an alternative view of its remarkable combination of properties such as large hardness, induced ferromagnetism and preserved corrosion resistance is considered.

The correlation between the grain orientation and the magnetic and elastic properties of plasma nitrided polycrystalline 316L austenitic stainless steel (ASS) is investigated.Plasma nitriding was performed at 400°C for 30 minutes under a pressure of 7.5 Pa using a 60 sccm N2 and 40 sccm H2 mixture. The grain orientation in a delimited area was obtained from electron backscatter diffraction (EBSD). The magnetic properties of the assessed grains were locally investigated by polar magneto-optical Kerr effect (MOKE) and magnetic force microscopy (MFM). Young's modulus Ehkl cartography was obtained in 3D using nanoindentation tomography. Plasma nitriding results in the nitrogen incorporation into the octahedral sites and expansion of the ASS lattice. This induces ferromagnetism at room temperature due to the formation of expanded austenite, where the diverse grains exhibit different magnetic properties, with a resulting effective magnetic easy axis along an unusual direction. Moreover, nitriding results in a complete reversal of the elastic behavior anisotropy: while the non-nitrided 316L ASSshows the typical elastic anisotropy of fcc-type metals with a maximum of Ehkl for the oriented grains, the maximum of Ehkl is observed for the oriented grains in the expanded austenite. These observations are discussed on the basis of the expansion due to the nitrogen incorporation and associated lattice rotation and residual stress effects.