Effect of Treatment Time on the Microstructure of Austenitic Stainless Steel During Low-Temperature Liquid Nitrocarburizing

The effect of treatment time on the microstructure of AISI 304 austenitic stainless steel during liquid nitrocarburizing (LNC) at 703 K (430 °C) was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Experimental results revealed that the modified layer was covered with the alloy surface and the modified layer depth increased extensively from 2 to 33.4 μ m with increasing treatment time. SEM and XRD showed that when the 304 stainless steel sample was subjected to LNC at 703 K (430 °C) for less than 4 hours, the main phase of the modified layer was expanded austenite. When the treatment time was prolonged to 8 hours, the abundant expanded austenite was formed and it partially transformed into CrN and ferrite subsequently. With the increased treatment time, more and more CrN precipitate transformed in the overwhelming majority zone in the form of a typical dendritic structure in the nearby outer part treated for 40 hours. Still there was a single-phase layer of the expanded austenite between the CrN part and the inner substrate. TEM showed the expanded austenite decomposition into the CrN and ferrite after longtime treatment even at low temperature.