Carbon and nitrogen effects on microstructure and kinetics associated with bainitic transformation in a low-alloyed steel
Résumé
The effect on the bainitic transformation of carburizing, nitriding and carbonitriding 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. CrN nitrides precipitated in austenite during the enrichment in nitrogen and were quantified. Enrichment in nitrogen leads to a fine bainitic ferrite microstructure, which contains the CrN nitrides as well as some AlN, VN and MnSiN2 precipitates, with much smaller number density. The precipitates are frequently gathered into aggregates. By using HEXRD, it was possible to monitor ferrite, austenite, cementite and CrN mass fraction during the isothermal bainitic transformation, whereas HEXRD experiments in previous studies focused mostly on carbide-free bainite. Ferrite formed first, followed by cementite precipitation and, in the case of nitrided steels, increase of CrN mass fraction. Enrichment in carbon and/or nitrogen tends to slow down the bainitic transformation compared to the initial steel, as expected from their austenite-stabilizing character. However, comparison between enriched steels shows that kinetics is not simply related to the total interstitial content when both carbon and nitrogen are simultaneously involved. Possible role of CrN precipitated in austenite on ferrite formation acceleration is discussed.