The nitriding kinetics of iron-chromium alloys; the role of excess nitrogen: Experiments and modelling
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I. INTRODUCTION
NITRIDING of iron-based alloys is a process that can lead to pronounced improvement of the fatigue, corrosion, and wear properties of workpieces in countless practical applications.[1] To optimize the properties obtained, the growth kinetics of the surface adjacent nitrided zone, i.e., the thickness increase as dependent on the applied nitriding parameters (time, temperature, nitriding potential, and specimen composition) has to be known. If pure iron is nitrided, three different Fe-N phases can occur successively:[2] -Fe [N] (ferrite) with a bcc structure, where the nitrogen is statistically distributed over the octahedral interstitial sites of the bcc Fe–sublattice; the -Fe4N1x phase based on an fcc Fe sublattice, where the nitrogen atoms are distributed in an ordered way over the octahedral interstitial sites; and the -Fe2N1x phase based on an hcp Fe sublattice, where the nitrogen atoms are distributed over the octahedral interstitial sites in a more or less ordered way. The amount of nitrogen dissolved in iron depends on the nitriding temperature and the chemical potential of nitrogen in the nitriding atmosphere, which in the case of gaseous nitriding can be adjusted by varying the composition of the mixture of the ammonia and the hydrogen in the gas phase.[3,4,5] If the nitriding conditions allow the formation of , it will be formed at the surface. As a second layer, will be formed below the layer. The surface layer containing the and the phases (sublayers) is called the “compound layer.” The last “layer,” underneath , the so-called “diffusion zone,” consists of -Fe (the original substrate) where the nitrogen atoms are dissolved interstitially (previously, discussed).[5–9]
R.E. SCHACHERL, Scientist, is with the Institute for Physical Metallurgy, University of Stuttgart, D-70569 Stuttgart. Contact e-mail: r.schacherl@ mf.mpg.de P.C.J. GRAAT, Scientist, formerly with the Max Planck Institute for Metals Research, D-70569 Stuttgart, Germany, is Researcher, Philips Center for Industrial Technology (CFT), 5656 AA Eindhoven, The Netherlands. E.J. MITTEMEIJER, Director, Max Planck Institute for Metals Research, is Professor, Institute of Physical Metallurgy, University of Stuttgart. Manuscript submitted December 30, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
If the iron matrix (substrate) contains alloying elements with a relatively high affinity for N, nitrides of these elements can be formed during nitriding and these thus occur in the diffusion zone. Chromium is often used as an alloying element in nitriding steels, because of the relatively strong Cr-N interaction,[10,11,12] which allows the formation of chromium nitride.[13,14] Experimentally determined and theoretically calculated sections at constant temperature of the ternary phase diagram Fe-Cr-N are given in References 15 and 16. Such results pertaining to the relatively low temperatures and high nitriding potentials (corresponding to about 3100 atm) used in this work are not available. During the nitriding proces
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