Search

International audience; The industrial importance of carbonitriding is owed to the exceptional wear and fatigue resistance it imparts to treated steel parts. This resistance is related to microstructural changes occurring during the enrichment treatment and upon cooling. Here, the effects of interstitial contents, in particular nitrogen, and transformation temperature were investigated in 23MnCrMo5 steel. Samples were homogeneously enriched in the austenitic phase and the isothermal transformation of the enriched austenite between 750 °C and 600 °C was studied. CrN was found to precipitate during the enrichment treatment. During subsequent isothermal holding, CrN precipitate as fine platelets in nitrogen containing samples. At equal carbon content, ferrite formed faster and in finer grains in presence of nitrogen. Preexisting CrN facilitate ferrite nucleation resulting in more numerous ferrite grains. The intense nitride precipitation is the main origin for enhanced hardness in nitrogen-enriched alloys. The exact mechanism leading to the observed microstructures could not be determined and remains under investigation. In particular, the high nitrogen supersaturation of ferrite required to produce the observed fraction of CrN has to be explained.

Journal of alloys and compounds 658, 832 - 838(2016). doi:10.1016/j.jallcom.2015.11.007, 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., Published by Elsevier, Lausanne

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.

PSA., leader in the fabrication of Diesel engines, aims at increasing its productivity for the manufacture of forged steel crankshafts and controlling perfectly their quality. All industrial partners of PSA are contributing to the achievement of this goal. Detailed metallurgical studies and numerical simulation have been used internally to describe the microstructural transformations involved. It has been shown that numerical simulation allows to identify trends for the effect of the modification of various parameters without the need for expensive and time-consuming prototype studies.