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Solidification and phase transformations in a dissimilar steel weld 18MND5/309L/308L : evolution of microstructure and mechanical properties

Abstract : Dissimilar welds between low-alloy steel and stainless steel are numerous within the French nuclear power plants where they enable connecting the main components to the primary circuit pipes. The internal cladding (in stainless steel) of the pressure vessel (in bainitic steel) made by submerged arc welding is another case of dissimilar weld whose goal is the protection against corrosion. This PhD work aims at understanding the complex microstructures which form at the interface between both steels during welding together with their evolution during the post-weld heat-treatment at 610°C and their consequences on the mechanical behavior of the welded assembly. Starting from the base metal, one meets successively a thin layer of martensite, a fully austenitic zone and the two-phase δ/γ microstructure of the stainless steel. Microscopy techniques (SEM, EDS, EBSD) combined with thermo-kinetics calculations (Scheil-Gulliver model, dendrite tip undercooling) have allowed explaining the graded microstructure and the reasons for the observed phase transitions. During the post-weld heat-treatment, the large gradient of carbon chemical potential across the fusion line leads to cementite dissolution and grain growth on the low-alloyed side. Carbon diffusion through the interface and Cr-rich carbides precipitation in both the martensitic layer and the austenitic weld have also been observed. An in-depth characterization has been performed at different scales (from the millimeter to the atomic level) to quantify the extent of carbon diffusion and carbides precipitation during the phase transformations. A mesoscopic thermodynamic and kinetic model based on Calphad databases has been developed to fully couple long-range diffusion in a multi-component system with precipitates nucleation and growth (Numerical Kampmann-Wagner approach). It allowed a prediction of the carbon content, volume fraction and size distribution of the precipitates at any distance from the fusion line. The consequences of the high variability of microstructures on the local mechanical behavior have been examined in the last part of this work, in particular the localization of deformation and the ductile failure. Elasto-plastic constitutive laws were determined for each region of the dissimilar weld in the heat-treated state. Ductile failure mechanisms in the weak zones of the weld, namely the decarburized base metal and the stainless steel cladding layers, were investigated through in-situ observations and damage modeling.
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Fanny Mas. Solidification and phase transformations in a dissimilar steel weld 18MND5/309L/308L : evolution of microstructure and mechanical properties. Materials. Université de Grenoble, 2014. English. ⟨NNT : 2014GRENI076⟩. ⟨tel-01153912⟩

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