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Étude de l'endommagement en fluage d'assemblages soudés en acier martensitique à 9% Cr - Grade 91

Abstract : The research of the best thermodynamic efficiency, with the decrease of CO2 emissions, imposes more and more drastic terms of use for the components of Thermal Power Stations. This demand led to the development of the new Grade 91 martensitic steels with 9-12% chromium, associates the usual properties of martensite with solid solution and precipitation hardening. This grade presents exceptional creep properties, but also an unusual damage mode at the welded joint (internal cracks generated in the HAZ - Heat Affected Zone -). Depending on the operating conditions (temperature - stress), the creep damage mode differ, resulting in a specific fracture location. Under industrial conditions (540 - 610 °C) Grade 91 has a cavitation damage mode with a type IV fracture, located in the HAZ, close to the base metal. While ormer studies analysed the microstructural evolution induced by creep, no quantitative relationship seems to have been established between these modifications and service life. However, the links between stress evolution and microstructural modifications during the creep test are not elucidated. The aim of this work carried out with the Institut de Soudure (IS) is to identify the origin and to understand this damage mode in order to propose one or more monitoring factors for follow the residual life time of a welded joint. The objective was to follow, during creep test, the evolution of stresses, microstructure and damage in the base metal and in the weakness zone of the welded joint. Creep tests on welded joints were carried out at a temperature and stress level leading to damage similar to the operating conditions and interrupted at different stages of the service life. The samples were then characterised in situ by diffraction in synchrotron radiation during creep under the same conditions (stress-temperature), to determine the distribution of internal stress along the welded joint. The samples were then investigated using microscopy, microanalysis and diffraction techniques, including scanning electron microscopy (SEM) and transmission electron microscopy (TEM), Energy dispersive spectroscopy (EDS), transmission electron diffraction (SAED) and backscatter electron diffraction (EBSD). In addition, the welded joint mechanical properties were monitored by micro-hardness mapping.
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Submitted on : Wednesday, May 19, 2021 - 4:51:13 PM
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  • HAL Id : tel-03230284, version 1

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Solenne Collomb. Étude de l'endommagement en fluage d'assemblages soudés en acier martensitique à 9% Cr - Grade 91. Science des matériaux [cond-mat.mtrl-sci]. Université de Lorraine, 2020. Français. ⟨NNT : 2020LORR0219⟩. ⟨tel-03230284⟩

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