Growth of nitrided layers on Fe–Cr alloys

Chromium is an important alloying element present in numerous commercial steels. A systematic study on the nitriding behavior of Fe–Cr alloys is helpful in predicting the properties of nitrided Cr-alloyed steels. Aspects such as microstructural evolution, growth kinetics, and mechanical properties should be particularly emphasized. Fe–Cr alloys containing 5, 10, and 20 wt.% Cr have been arc melted and subsequently plasma nitrided under a N2–80% H2 atmosphere. The microstructure of the resulting nitrided layers was characterized and the microhardness profiles evaluated. Thicker layers developed on low chromium alloys. Differences in hardness profiles were also observed as a function of chromium contents. Nitriding Fe–5% Cr alloys resulted in two discrete fronts, refereed to as the diffusion front and the transformation front. Transformed regions sustained a decrease in hardness from 1000 down to 600 HV, associated with the conversion of homogeneously dispersed fine precipitates into coarser needle like particles immersed in the ferritic matrix. Similar behavior was not observed for the other alloys, where both fronts developed simultaneously.