Microstructural Characterization of the expanded austenite formed on the plasma nitrided AISI 316 L steel

In 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% N 2 -10% H 2 gas mixture. The observations by optical microscope (OM) revealed the presence of the expanded austenite γ N with a mean layer thickness lying between 2.8 and 9.7 μ m. The growth kinetics of γ N layer was also studied. The XRD and GDOES analyses were used to experimentally characterize the γ N layer. A mechanical model, based on the XRD data, was applied to determine the compressive stresses in the γ N layer and strains induced by the nitrogen diffusion. The estimated values of compressive stresses inside the γ N 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.