Microstructural characterization of the expanded austenite formed on the plasma nitrited AISI 316 L steel.

International audienceIn this work, the AISI 316 L stainless steel was plasma nitrided at 420 °C for different times ranging from 0.5 to 8 h in a 90% N2-10% H2 gas mixture. The observations by optical microscope (OM) revealed the presence of the expanded austenite γN with a mean layer thickness lying between 2.8 and 9.7 μm. The growth kinetics of γN layer was also studied. The XRD and GDOES analyses were used to experimentally characterize the γN layer. A mechanical model, based on the XRD data, was applied to determine the compressive stresses in the γN layer and strains induced by the nitrogen diffusion. The estimated values of compressive stresses inside the γN layer (in absolute values) were in the range of 2.10 to 3.70 GPa with the strains located between 6.3 and 7.94%. Based on the XRD data, it was found that the expanded austenite can dissolve up to 32 at.%N