How different steel grades react to a salt bath nitrocarburizing and post-oxidation process: Influence of alloying elements

摘要

Two different grades of steel, DC 04 (non alloyed steel) and 15 CrMoV 6 (alloyed steel) were studied; both of them were salt bath nitrided with QPQ process. They were subjected to different durations of nitriding sequences, 2h, 3h and 8h while the duration for the last sequence, i.e. oxidation remained the same. Microhardness measurements showed that the alloying elements had a strong influence on the profile and on the surface hardness value. For each grade, the maximum surface hardness value was already obtained after a 2h treatment. The surface constitution was observed by XRD; the presence of Fe3O4was noticed above the compound layers made up of Ε and γ' nitrides. Cross sectional observations by SEM revealed that the growth kinetics of compound layers follows a parabolic law. The layers' thickness and constitution have also been confirmed by EDS analysis. On increasing nitriding duration, γ' nitride content decreases while ε nitride content increases. For longer nitriding durations, mostly ε nitride and Fe3O4 are observed on the surface of both grades. It seems that for long nitriding sequences, additional elements are not essential for compound layers' nature.