EXAFS investigation of low temperature nitrided stainless steel Jette Oddershede Æ Thomas L. Christiansen Æ Kenny Sta ˚hl Æ Marcel A. J. Somers Received: 21 February 2008 / Accepted: 6 June 2008 / Published online: 28 June 2008 Ó Springer Science+Business Media, LLC 2008 Abstract Low temperature nitrided stainless steel AISI 316 flakes were investigated with EXAFS and X-ray dif- fraction analysis. The stainless steel flakes were transformed into a mixture of nitrogen expanded austenite and nitride phases. Two treatments were carried out yielding different overall nitrogen contents: (1) nitriding in pure NH 3 and (2) nitriding in pure NH 3 followed by reduction in H 2 . The majority of the Cr atoms in the stainless steel after treatment 1 and 2 was associated with a nitrogen–chromium bond distance comparable to that of the chemical compound CrN. The possibility of the occurrence of mixed substitutional– interstitial atom clusters or coherent nitride platelets in nitrogen-expanded austenite is discussed. Introduction Low temperature nitriding of austenitic (fcc) stainless steel gives rise to the dissolution of colossal quantities of nitrogen in the surface adjacent region under para-equilibrium con- ditions, provided that the nitriding temperature is kept below approximately 720 K. The nitrided zone is referred to as expanded austenite or S-phase, even though essentially no new crystal structure develops. Low temperature nitriding greatly improves the tribological properties of stainless steel without impairing the corrosion properties. Consequently, the nature of expanded austenite has attracted much focus, both scientifically and technologically [1–5]. Expanded austenite is a supersaturated solid solution of nitrogen (or carbon) in stainless steel with a homogeneity range corresponding to an occupancy of y N = 0.18–0.61 of the interstitial sublattice, i.e. approximately 17–38 at.% N, respectively [6]. Such high nitrogen contents are obtained due to the high affinity between nitrogen and chromium, where nitrogen is believed to reside in the immediate vicinity of the supposed randomly distributed chromium atoms, i.e. short range order of N occurs. The thermody- namically more stable CrN does not form within typical treatment times of up to 24 h, as a consequence of the low mobility of chromium at the nitriding temperature. Virtu- ally only nitrogen is able to diffuse at a significant rate; thus nitrogen contents many times higher than the equi- librium solubility are attained. A fraction of the nitrogen content in expanded austenite, corresponding to y N = 0.18, has been observed to be more tightly bound than nitrogen in the composition range y N = 0.18–0.61; there are strong indications that the tightly bound nitrogen interacts with chromium, which leads to nitrogen trapping [6]. Extended X-ray absorption fine structure analysis (EXAFS) is a proficient tool for probing the local environ- ment around each atom type in the metal lattice of nitrided or carburised steels. In particular the local structure of the chromium atoms is of interest in this respect, since it may shed a light on the actual occurrence of the anticipated short range ordering of nitrogen atoms. The information that can be obtained comprises bond length, disorder (thermal or static) and the number of nearest neighbours. These param- eters can be related to the presence/absence of interstitially dissolved elements in the immediate surroundings of the probed metal atom. J. Oddershede Á K. Sta ˚hl Department of Chemistry, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark T. L. Christiansen (&) Á M. A. J. Somers Department of Mechanical Engineering, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark e-mail: tc@ipl.dtu.dk 123 J Mater Sci (2008) 43:5358–5367 DOI 10.1007/s10853-008-2791-y