In situ XRD experiments on the growth of expanded austenite using different process gases

Austenitic stainless steel has many usages, because of its low cost, good workability and reasonable corrosion resistance. Its low hardness and wear resistance is however an undesirable property. Low temperature N or C treatment results in the formation of the expanded austenite. This “phase” contains colossal amount of interstitially solved N/C which increase the hardness, but no nitride/carbide formation takes place, thus the corrosion resistance is not compromised.

In this work we show our result of in-situ nitrocarburising 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 totak thickness of the expanded austenite can be determined. Applying both N and C resulted in an almost 10 times faster growth of the expanded austenite than with N only. The growth is thermally activated. The activation energy 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.