The formation of expanded austenite in Cr-Ni austenitic stainless steels like AISI 316L is not completely understood despite its technological relevance. In this work, we present an in-situ X-ray diffraction study on the growth kinetics of the expanded austenite. We applied a lowtemperature nitrocarburizing treatment using a mixture of NH 3 , N 2 , H 2 , and C 2 H 4 gases at atmospheric pressures in a novel and custom built chamber attached to a Bruker D8 Advance diffractometer. The nitrocarburizing temperature was varied between 340 and 440 C, and the possible effects of the gas amount were also tested. The thickness of the growing layer was determined from the shrinkage of the unmodified austenite peak. The growth rate coefficient was calculated using the linear-parabolic equation. The resulting coefficients follow the Arrhenius law with the activation energy of 165 6 12 kJ/mol. This value is in good agreement with the diffusion activation energy for heavy interstitials like carbon and nitrogen. The expanded austenite peak was modelled by a multilayer approach, where each 0.5 lm sublayer has a constant lattice parameter. The lattice expansion is analyzed as a function of the Boltzmann-variable (g ¼ 0.5 Â t À1/2). The expanded austenite layer in this metric has a constant width. Furthermore by rescaling with the lattice expansion of the first sublayer, it is possible to create a scale-independent master curve. These findings indicate that thickening of the expanded austenite is purely diffusion controlled, while the extent of strain is set by the uptake rate of the gas atoms. Published by AIP Publishing.
In this work we show our result of in-situ nitrocarburizing and nitriding treatments AISI316L specimens. Part of the samples have been depassivated ex-situ and coated with a Ni layer, while other specimens received in-situ depassivation. Processing was carried out in a custom built reaction chamber attached to a Bruker D8 Advance diffractometer. We monitored the 111 peak of both the base material and expanded austenite. From the shrinkage of the base material peak the total thickness of the expanded austenite can be determined. Applying both N and C resulted in a more than 10 times faster growth of the expanded austenite than with N only. The growth is thermally activated. The activation energy for nitrocarburizing is 164 kJ/mol. This is in agreement with the activation energy of the diffusion of interstitials. Detailed analysis of the expanded austenite peak allowed the derivation of a “master curve” for the composition depth profile. This suggest that two interacting process controls the evolution. The width of the reaction zone is limited by the diffusion at low concentration side. The total concentration is determined by the reaction at the interface.
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