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Étude du comportement mécanique à chaud de l'acier P91 : vers la compréhension du rôle des mécanismes intra/intergranulaires sur la tenue en fluage. Application aux structures soudées

Abstract : The goal of this study is to develop a micromechanical approach, to identify and model the elementary mechanisms in connection with the microstructural heterogeneities accountable for the deformation and for the creep damage of P91 steels, material used for applications at high temperature in thermal power plants. This study deals with the use of kinematic full-fields measurements by digital image correlation coupled with finite element numerical simulations for the identification of constitutive equation parameters of materials presenting a strongly heterogeneous microstructure, leading to gradients of mechanical properties. The welded joint in P91 presents various microstructures along the joint: base metal, ICHAZ (intercritical heat affected zone), FGHAZ (fine grain heat affected zone), CGHAZ (coarse grain heat affected zone) and weld metal, which will be characterized by uniaxial tensile tests and tensile-relaxation tests performed at room temperature and at 625°C. These experimental tests enable to establish a constitutive law for each part of the welded joint and for a wide range of strain rate.A particular attention has then been focused on the behaviour at high temperature (creep) for which the intergranular glide plays an essential role, in particular within the ICHAZ. To characterize the grain boundary gliding mechanisms, a technique of microlithography is used. This mechanism is modelled with cohesive zones. A cohesive constitutive equation has been proposed and validated with a simplified four grain configuration. This model is able to take into account the dislocation creep – diffusion creep transition by delaying the beginning of the grain boundaries sliding when the strain rate increases.This study is granted by the EDF-GDF-SUEZ-GRT gas – Mines ParisTech – Ecole des Ponts ParisTech chair on the "durability of materials and structures for the energy ".
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Mathieu Touboul. Étude du comportement mécanique à chaud de l'acier P91 : vers la compréhension du rôle des mécanismes intra/intergranulaires sur la tenue en fluage. Application aux structures soudées. Autre. Ecole Nationale Supérieure des Mines de Paris, 2012. Français. ⟨NNT : 2012ENMP0075⟩. ⟨pastel-00819229⟩

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