Bainite Formation in Carbon and Nitrogen enriched Low Alloyed Steels: Kinetics and Microstructures

The effect on the bainite formation of carburizing (0.6 wt.% C), nitriding (0.12 wt.% C-0.25 wt.% N) and carbonitriding (0.7 wt.% C-0.25 wt.% N) of a 23MnCrMo5 low-alloyed steel in the austenitic field was examined by in situ high-energy synchrotron X-ray diffraction (HEXRD) and transmission electron microscopy. The enrichment in nitrogen induces strong acceleration of the bainite transformation kinetics in carbonitrided steel compared to carburized steel, despite the γ-stabilizing character of nitrogen. This is attributed to the nucleation of ferrite on CrN nitrides, which precipitated during the enrichment, either at γ grain boundaries or intragranularly. AlN, VN and MnSiN2 nitrides were observed as well, with much smaller number density. They formed frequently aggregates with the CrN nitrides. The bainite microstructure is much finer than in initial or carburized steel. It shares some common features with intragranularly nucleated bainite, i. e. acicular ferrite. From HEXRD, the chronology of the phase formation (ferrite and precipitates) during bainite formation as well as cell parameter evolutions are analyzed.