Anisotropy effects on gaseous nitriding of austenitic stainless steel single crystals

Low temperature nitrided austenitic stainless steel exhibits strong orientation dependent case growth kinetics. Anisotropy effects during low temperature gaseous nitriding of single crystals of austenitic stainless steel with surface-normal orientations 〈100〉, 〈111〉 and 〈10 7 2〉 were investigated with thermogravimetry, X-ray diffractometry (XRD), glow discharge optical emission spectroscopy (GD-OES), nano-indentation and electron back-scatter diffraction (EBSD). Case growth kinetics were fastest for 〈100〉 and slowest for 〈111〉. Depth-profiling over the case depth was accomplished by successive layer removal combined with X-ray diffraction and nano-indentation. The results indicate that anisotropic case-growth kinetics is related to early nucleation of long-range ordered (LRO) over-stoichiometric γ’_N−Me₄N_{1+ x} (Me=metal atoms) in 〈100〉, which is inseparable from short range ordered (SRO) expanded austenite, γ_N. Depth ranges of elasto-plastic and purely plastic accommodation of the nitrogen induced lattice expansion were distinguished. In the elasto-plastic region stress-gradient enhanced diffusion of N is observed.