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April 6, 2016

Defense thesis of Elisabeth CHAUMIN

from Center of materials of MINES ParisTech

Defense thesis of Elisabeth CHAUMIN

Elisabeth CHAUMIN will defend her thesis called

"Stress corrosion cracking of Alloy 82 in hydrogenated steam at 400°C: influence of microstructural and mechanical parameters on initiation of SCC cracks"

in avril, the 6th, at 2pm

at Neurospin, CEA de Saclay, 91191 Gif-sur-yvette - anphithéatre de Neurospin

Abstract : In Pressurize Water Reactors (PWR), Stress Corrosion Cracking (SCC) is the mean degradation mode of components pieced together by welding. Nickel based alloys are, among others, used in dissimilar metal welding (DMW). International report showed only 3 cracking cases in Alloy 82 out of 300 cracking cases concerned on nickel based alloys DMW in primary water circuit. The aim of this study is to identify which microstructural and local mechanism parameters at microstructure scale provide the initiation of SCC cracks. Characterizations performed on specimen surface to identify those parameters are composed ofchemical composition analysis and EBSD analysis (Electron Back-Scattered Diffraction) to know the morphology and the crystallography of grains for microstructure features on one hand, and experimental strain fields measured by Digital Imaging Correlation (DIC) of gold microgrids deposed by electronic lithography on U-bend specimen surface and stress fields calculated along grains boundaries by finite element for local mechanical features on the other hand. The correlation between those characterizations and localization of initiation sites of SCC cracks, obtained on U-bend specimens tested in autoclave in hydrogen steam water at 400°C and 188 bar for 3500 hours, confirmed the susceptibility of the Alloy 82 in SCC conditions with intergranular SCC cracks. The perpendicular position to the loading direction (mode I) is the worst conditions for grains boundary in SCC. The others points concern the chemical composition (precipitation, impurities) around grain boundary and the grain boundary type which is more susceptible when it is a High Angle Grain Boundary. It is following by the mechanical characterization (stress and strain gradient) along grain boundary. This methodology can be used to other material and helped to define which microstructural and mechanical parameter can be define the initiation of SCC cracks. - See more at: http://www.mines-paristech.fr/Formation/Doctorat/Soutenances-a-venir/Detail/Elizabeth-CHAUMUN/51881#sthash.m8cmPw5U.dpuf

 

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