Nitrogen diffusivity in expanded austenite

Expanded austenite is a metastable phase formed after nitrogen insertion into austenitic stainless steel, at elevated temperatures between 350 and 400 degreesC with nitrogen contents between 5 and 30 at.%, thus leading to a lattice expansion of up to 13%. The layer thickness can vary up to a factor of ten for different alloys at identical process conditions. The main part of the nitrogen depth profiles can be described by an complementary error function. However, close to the end of the profile, a rather sharp decrease of the nitrogen content is found. Calculating the concentration dependent nitrogen diffusivity, it can be shown that this kink corresponds to steep increase of the diffusivity by nearly a factor of ten. A systematic study of different steel grades is presented, trying to correlate the diffusivities with the alloying content and to predict the 'suitability' of austenitic stainless steel for nitriding. (C) 2003 Elsevier Science B.V. All rights reserved.