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Influence des éléments d'alliage sur les mécanismes de nano-précipitation et sur les mécanismes de durcissement d'alliages modèles (Fe-Cr et Fe-Cr-C) et d'aciers industriels nitrurés

Abstract : This work deals with high tech industries. It concerns the behavior of gas-nitrided (520°C) alloyed steels, in terms of nano-precipitation mechanisms, with incidence on hardening. Nano-precipitates characterization was performed by HRTEM and nano-probe EDX, complemented by thermodynamic phases calculations, with the help of Thermo-Calc software. The study carried out on nitrided binary (Fe-Cr) and ternary (Fe-Cr-C) alloys, allowed to propose a new explanation about nitrogen excess: nitrides (Fe,Cr)N precipitation. Chromium and initial microstructure effects on hardening were determined, and precipitation mechanisms explained. Investigations conducted on about 10 nitrided industrial steels, confirmed that chromium and aluminium are the most influent alloying elements on hardening. The 32CrMoV13 and 30CrAlMo6.12 industrial steels characterization resulted in identifying the role of those elements and the one of initial microstructure on precipitation mechanisms, and their impact on hardening. Nitrogen excess, systematically pointed out above the surface, is related to (Fe,Cr,Al,Mo,Mn,V)N mixed nitrides and iron nitrides (in presence of Al) precipitation. It was demonstrated that nanometric platelets MN are mainly responsible for hardening. Concerning industrial application, this study led to optimize the nitrided industrial steels composition, in order to reduce thermochemical cycles, while keeping mechanical properties
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Caroline Ginter. Influence des éléments d'alliage sur les mécanismes de nano-précipitation et sur les mécanismes de durcissement d'alliages modèles (Fe-Cr et Fe-Cr-C) et d'aciers industriels nitrurés. Autre. Institut National Polytechnique de Lorraine, 2006. Français. ⟨NNT : 2006INPL024N⟩. ⟨tel-01748883⟩

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