Stability of expanded austenite, generated by ion carburizing and ion nitriding of AISI 316L SS, under high temperature and high energy pulsed ion beam irradiation

Expanded austenite can be generated on austenitic stainless steels either by ion carburizing or ion nitriding. In both cases the resulting fcc crystal structure, supersaturated with nitrogen or carbon, is strongly hardened with improved wear-resistance, while maintaining the original resistance to corrosion. In this work, we have studied the stability of expanded austenite, generated by ion nitriding and ion carburizing on AISI 316L SS with N and C, under: a high temperature (225 degrees C - 504 degrees C), and b-under irradiation with high energy (30 keV - 500 key), high fluence (similar to 10(15) cm(-2)), short duration (similar to 400 ns) light (deuterium and helium) ion beams. It was found that expanded austenite is stable below 325 degrees C. Between 325 degrees C and 504 degrees C expanded austenite lattice parameter presents gradual reduction with increasing temperature. We observed microstructural changes related only to the temperature treatment. We did not observe any microstructure change due to the duration of the heat treatment. Over 504 degrees C, the lattice parameter returns to the material's austenite original parameter. On the other hand, when irradiated with pulsed ion beams, a gradual reduction of the lattice parameter corresponding to the expanded austenite with the number of pulses was observed. This behavior can be explained through the thermal shock induced on the surface by each beam, consisting in fast heating followed by fast cooling that induces the gradual exo-diffusion of N (or C). Nevertheless, after 20 ion pulses, a final lattice parameter slightly higher than the corresponding to the original austenite was found as stable limit. This residual expansion can be attributed to partial amorphization of the first few micrometers that induces stresses on the crystals of austenite which are closer to the surface layers. (C) 2013 Elsevier B.V. All rights reserved