Transport of nitrogen and carbon in duplex stainless steel 318LN during nitrocarburizing

Active screen plasma nitrocarburizing (ASPNC) without substrate bias has been used to investigate the nitrogen and carbon transport in duplex steel between 380 °C and 480 °C for 3–7 h. Despite a continuous supply of carbon and nitrogen during the whole process, carbon is always found in front of the nitrogen. For most process parameters, the total carbon/nitrogen ratio is nearly constant. However, the formation of CrN at elevated temperatures seems to restrict the nitrogen uptake whereas at longer treatment times, where a closed carbon-rich layer can form on the surface, a reduced carbon content within the modified layer was observed. For the current experiments, the diffusion of both interstitial elements is nearly identical in both the austenite and ferrite component of the base material. In-situ XRD during sputter etching complements ex-situ XRD data showing a known formation of expanded austenite together with a progressing series of phases interpreted as expanded ferrite, hcp expanded ε′-martensite and finally a highly disordered, nearly X-ray amorphous structure in the ferritic component.