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

Abstract

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 Mössbauer 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 Mössbauer spectroscopy. Three subspectra in terms of different iron sites were resolved in the Mössbauer 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.

Composition-dependent variation of magnetic properties and interstitial ordering in homogeneous expanded austenite

“…Similarly for published results on the crystal structure, available literature on Mössbauer studies and magnetic properties of expanded austenite is focused on specimens with gradients in composition and residual stresses [7,20,21,[24][25][26][27], which is likely to influence the obtained results. In the current investigation the relation between interstitial content and magnetic properties is, for the first time, explored for homogeneous samples of nitrogen expanded austenite.…”

From Micro to Nano Scales -Recent Progress in the Characterization of Nitrided Austenitic Stainless Steels

“…Mossbauer spectroscopy is a powerful tool in probing the variation in the local environment of iron atoms in ferrous alloys [84][85][86][87][88] Lei and Zhu [90] applied AES and XPS to study the expanded austenite phase formed on plasma nitride 18-8 austenitic stainless steel and found that, all the chromium and part of the iron were in the nitride state and all the nickel was in metallic state. As compared to stoichiometric nitrides, the nitrided phase contains weaker Cr-N ionic-type bonds and stronger Fe-N ionic-type bonds.…”

Nitrided Amorphous Stainless Steel Coatings Deposited by Reactive Magnetron Sputtering from an Austenitic Stainless Steel Target

“…Nitrogen is known to occupy interstitial sites in AISI 316 which results in hardening. [42] Additional strengthening mechanisms are associated with the formation of nitrides. With involvement of the alloying elements in AISI 316, the reported values can be explained by CrN/Cr 2 N nitrides (which are difficult to identify in XRD) accompanied with the formation of e-Fe 2 N-Fe 3 N nitrides.…”