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Fluage à 500°C d'un joint soudé d'un acier 9Cr-1Mo modifié. Evolution de la microstructure et comportement mécanique

Abstract : With the increase in worldwide energy demand, the nuclear industry is a way of producing electricity on a large scale and to answer to this need. For the design of a new generation of fission nuclear reactors and among six chosen fission reactor systems, France develops in particularly the Very High Temperature Reactor (VHTR) concept. This implies the use of materials that are more and more resistant to high temperature for long-term exposure. AREVA focuses on materials already used in fossil-fuel power plant, so that the mechanical behaviour of Grade 91 (Fe-9Cr-1Mo-Nb-V) has to be investigated. This ferritic-martensitic steel is considered to be a potential candidate for welded components. Such structures are combined with welded joints, which have to be studied.

Three industrial partners (AREVA, CEA, EDF) have launched a study with the Centre des Matériaux in order to investigate the creep of welded joint of Grade 91. The aim of this work is to complete the available database about the mechanical behaviour of Grade 91, base metal and welded joint, during creep tests performed at 500°C up to 4500h exposure.

Thermal aging tests, tensile tests, and creep tests were performed at 450°C and 500°C using both base metal and cross-weld samples. Several geometries of cross-weld creep specimens were tested. The microstructure has not remarkably changed after tests concerning both nature and size of precipitates, and the characteristic size of the matrix sub-structure. The creep damage is not developed in the ruptured specimens after creep tests. Only little damage by cavity nucleation and growth was found in the creep specimens. Creep fracture at 500°C takes places by viscoplastic flow, contrary to tests performed at 625°C where the creep-induced damage governs the creep rupture at least for long-term lifetime.

From creep curves of base metal and cross-weld specimens, a phenomenological model is proposed. The flow rule is a Norton power law with a stress exponent of 19 in the case of base metal and 18 in the case of welded joint. These high values of Norton exponent indicate the existence of internal stress and suggest that the mechanism which governs the creep deformation might be dislocation glide. The rupture of cross-weld creep specimens takes place in the weld metal, whereas it takes place in the base metal after tensile tests. The heat-affected zone has no obvious role in the mechanical strength of cross-weld specimens at 500°C. In this case, the creep behaviour of the weld metal can be deduced from a combination of the welded joint behaviour and the base metal behaviour. These were identified from experimental creep curves. Another method to determine the weld metal behaviour is to fit the model parameters on creep curves obtained from modified specimens used to test only the weld metal. These results are consistent with open literature data. The proposed model allows determination of a rupture time for longer-term exposure with simple equations and with a good agreement with recent results from CEA.
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Submitted on : Friday, June 26, 2009 - 11:06:13 AM
Last modification on : Thursday, September 24, 2020 - 6:30:07 PM
Long-term archiving on: : Tuesday, June 15, 2010 - 6:53:20 PM

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  • HAL Id : tel-00399352, version 1

Citation

Florian Vivier. Fluage à 500°C d'un joint soudé d'un acier 9Cr-1Mo modifié. Evolution de la microstructure et comportement mécanique. Mécanique [physics.med-ph]. École Nationale Supérieure des Mines de Paris, 2009. Français. ⟨NNT : 2009ENMP1619⟩. ⟨tel-00399352⟩

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