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Lien entre microstructure et transition ductile-fragile des aciers trempés-revenus à haute résistance

Abstract : This thesis is a part of Ascometal project, which aims at guiding the development of new high strength steels used foroffshore drilling parts in oil and gas domain. The aim of the project is to predict the ductile-to-brittle transition curve in Charpyimpact tests by considering microstructural parameters. The main two objectives of this study are: improving the knowledge of the link between the microstructure and fracture properties and building a microstructurally-based predictive tool for the impacttoughness. The chosen scientific approach includes four steps described as follows: 1) building of a database of six microstructures. These microstructures were quantitatively characterized at different scales. 2) The elastic-plastic behavior was studied by varying the test temperature, the strain rate, the sampling direction and the tempering temperature. 3) The fracture behavior was studied by using an instrumented Charpy impact test in a test temperature range of -196°C to +60°C. 4) Establishment of the links between microstructure and fracture properties. Mechanical analysis by the finite element method was used in some cases in order to determine the local stress. In this work we discuss the effect of strain rate, the test temperature, the sampling direction and the tempering temperature on the elastic-plastic behavior of steels containing martensite and upper bainite. Fractography was used to characterize the fracture mechanism in the ductile region, in the transition and in the brittle domains. The effect of tempering of ductile fracture damage is also discussed. The local approach to fracture is adopted for the purpose of determining the critical cleavage stress at -196°C. A quantitative link is established between the critical fracture stress and the carbide size. The effect of the upper bainite on the cleavage fracture stress is also discussed. Finally, a microstructurally-based global model for impact toughness is proposed in this thesis. The model takes the intercarbide spacing and the fracture appearance transition temperature into account to describe the instrumented Charpy impact curve. Complementary studies have to be made in order to deeply understand the dependence of the fracture appearance temperature on the austenite grain size, the packet size and the upper bainite fraction; they will make the global model for impact toughness more robust.
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  • HAL Id : tel-02513106, version 1

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Frank Tioguem Teagho. Lien entre microstructure et transition ductile-fragile des aciers trempés-revenus à haute résistance. Matériaux. Université Paris sciences et lettres, 2019. Français. ⟨NNT : 2019PSLEM040⟩. ⟨tel-02513106⟩

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