Développement d'un procédé de nitruration pour l'aéronautique. Étude des mécanismes de durcissement sur des alliages fer-chrome nitrurés

Abstract : Nitriding of steels is a thermochemical process for improving the mechanical properties by nitrogen diffusion and associated to nitrides precipitation of the alloying elements within the treated zone. This treatment results in a significant increase in hardness and to the introduction of residual compressive stresses. The scope of the study was to develop a nitriding cycle adapted to a steel grade for heavily loaded applications such as gears for the aerospace industry. A parametric study allowed the evaluation of the parameters criticality on the behavior of the chosen steel after nitriding. Based on these results, a nitriding cycle fulfilling the defined requirements has been proposed. Hardening of the nitrided layer was studied on Fe-Cr binary alloys in order to avoid complex precipitation phenomena observed in industrial grade steel. Transmission electron microscopy analyses confirmed, through the complete interpretation of the electron diffraction pattern, that three nitrides families develop on the faces of the cubic centered structure of ferrite and obey the orientation relationships of Baker-Nutting. These semi-coherent nitrides adopt a disk-shape geometry. Image analyses allowed the identification of the microstructural parameters (size distribution of the precipitates, width, inter-particles spacing and volume fraction). The knowledge of these parameters led to the development of a hardening model, based on particles bypassing by dislocations and taking into account the disk-shape geometry of the precipitates. It has been shown that the use of results coming from diffusion - precipitation models does not permit to reliably determine the hardness gradient of the nitrided layer. This inability results principally from the impossibility of these models to describe correctly the microstructure in terms of precipitates geometry and their size distribution
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  • HAL Id : tel-02006456, version 1

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Olivier Skiba. Développement d'un procédé de nitruration pour l'aéronautique. Étude des mécanismes de durcissement sur des alliages fer-chrome nitrurés. Matériaux. Université de Lorraine, 2018. Français. ⟨NNT : 2018LORR0099⟩. ⟨tel-02006456⟩

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