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Theses

Étude de la précipitation de la phase σ dans un acier super austénitique

Abstract : Improving the corrosion resistance of industrial stainless steels ingots remains a scientific challenge. During ingot casting, chemical heterogeneities at the micrometer scale can arise and lead to the formation of chromium and molybdenum rich intermetallic precipitates that lower the corrosion resistance by removing these elements from the matrix. As-cast ingots exhibit σ-phase in the interdendritic region. The aim of this thesis work was to study the formation mechanism of σ phase with real solidification conditions and to study the impact of the microsegregation on the phase transformation during solution heat treatment. The characterization of samples elaborated with a Quenching Directional Solidification (QDS) set-up helped us to determine establish the solidification path, the phase transformations areas for the solidification conditions carried out and the σ-phase transformation mechanism of the steel. The complementarity of microstructural and chemical characterizations led to the conclusion that the (σ + γ2) precipitates present in the as-cast ingot are inherited from the microsegegation of molybdenum in the liquid during solidification and come from the eutectoid decomposition of the ferrite formed in the solidification interval. We also identified the complex dissolution sequence of the (σ + γ2) precipitates and the quantities that govern the slow dissolution kinetics. Thus, the interdendritic spacings, precipitates size σ + γ2) and molybdenum microsegregation in primary austenite inherited from solidification appear to be the main quantities.
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Submitted on : Tuesday, February 2, 2021 - 12:31:29 PM
Last modification on : Thursday, February 25, 2021 - 11:05:04 AM

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  • HAL Id : tel-03128605, version 1

Citation

Raphaël Marin. Étude de la précipitation de la phase σ dans un acier super austénitique. Science des matériaux [cond-mat.mtrl-sci]. Université de Lorraine, 2020. Français. ⟨NNT : 2020LORR0132⟩. ⟨tel-03128605⟩

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