Stability of Expanded Austenite During Annealing in Vacuum

In situ X-ray diffraction has been used to investigate the stability of expanded austenite during annealing in vacuum for the austenitic stainless steel 316Ti, the super-austenitic stainless steel 904L, and the duplex steel 318LN. Expanded austenite has been formed using plasma immersion ion nitriding before. Time-of-flight secondary ion mass spectrometry before and after annealing yielded complementary information regarding nitrogen depth profiles and CrN precipitation using cluster analysis. The decay of expanded austenite during annealing was found to be thermally activated with an activation energy of 1.8 ± 0.3 eV, starting within five minutes at 550 °C and taking more than two hours below 450 °C. The decay occurs simultaneously throughout the whole nitrogen-containing zone-and not at the surface as during nitriding. Nitrogen diffusion occurring in parallel slightly complicates the data analysis. Further transmission electron microscopy investigations are necessary to understand the microstructure after annealing in vacuum. The limit for operating hard and wear-resistant expanded austenite layers at elevated temperatures of up to 350 °C is given, however, by nitrogen diffusion and not the decay into CrN.