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Évolutions microstructurales de dépôts épais d'acier inoxydable austénitique 316L obtenus par projection dynamique par gaz froid ("cold spray") sous l'effet d'un traitement thermique ; conséquences sur les propriétés d'usage

Abstract : In the nuclear and marine industries, some components are subjected to degradation, due to severe in-service conditions. In-site reparation appear to be a suitable solution, especially for complex components, difficult to replace. With this aim in mind, the use of cold gas dynamic spraying, known as “cold spray”, is particularly promising as it is a solid-state process and thus avoids heat-affected zone formation. Yet,successive particle impacts at supersonic velocities on the substrate during projection cause a strong deformation of the particles and severe work hardening of the impacted areas. This generates unstablemicrostructures in the as-sprayed state. In addition, if a large coating thickness is targeted, residual stresses accumulation can lead to crack initiation and propagation at particle-particle interfaces and/or at the substrate-coating interface. This thesis work is based on 316L cold sprayed coating heat treatment to adapt its microstructure as well asits in-use properties. To do so, it is necessary to better understand the mechanisms involved during these treatments, particularly at the particle-particle interfaces, as they are the main place of coating adhesion and the zones of strain concentration. The thesis is structured in three parts: i) microstructural analysis of as-sprayed coatings; ii) the development of an optimal heat treatment, in regard to microstructure and industrial application; iii) the study of its effect on the coating in-use properties. Heat treatment development involved a study of recovery and recrystallization phenomena, mainly by highlighting the three-stage recrystallization path, starting in the particles areas which are most affected by impacts during spraying, then propagating to the particles core zones and finally reaching the whole coating.This approach has led to a better understanding of the competition between the different thermally activated mechanisms and has led to the selection of a short-cycle treatment. This treatment, due to its high heating rate and short holding time, short-circuits some effects of slower treatments (as cavitation or extensive evolution of undesirable phases), resulting in a fine-grained recrystallized microstructure. Subsequently, the mechanical and corrosion properties of those heat-treated coatings were studied. It was shown that this short-cycle treatment allowed a considerable ductility restoration, while preserving corrosion resistance. A reproducible, with sufficient in-use properties, repair solution for 316L components with cold spray process has been identified.
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Submitted on : Thursday, July 28, 2022 - 10:43:57 AM
Last modification on : Wednesday, August 3, 2022 - 3:06:49 AM

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  • HAL Id : tel-03739758, version 1

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Laury-Hann Brassart. Évolutions microstructurales de dépôts épais d'acier inoxydable austénitique 316L obtenus par projection dynamique par gaz froid ("cold spray") sous l'effet d'un traitement thermique ; conséquences sur les propriétés d'usage. Matériaux. Université Paris sciences et lettres, 2022. Français. ⟨NNT : 2022UPSLM008⟩. ⟨tel-03739758⟩

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