Modélisation numérique du laminage à pas de pèlerin de tubes ODS en vu de limiter les risques d'endommagement

National audiencePour les Réacteurs à Neutrons Rapides au sodium, les matériaux de gainage de référence pour les très forts taux de combustion sont les nuances ferritiques/martensitiques ODS. Ces matériaux présentent en effet des bonnes propriétés en fluage, en résilience et en résistance à l'oxydation. Toutefois la présence des oxydes ODS font de ces nuances des matériaux très difficiles à mettre en forme. Classiquement le tube de gainage est mis en forme à froid à partir d'une ébauche tubulaire par une succession de passes de laminage à pas de pèlerin et de traitements thermiques. Dans le cadre de cette étude la modélisation numérique du procédé de laminage dans une configuration de type HPTR a été entreprise. Le modèle prend en compte toute la complexité des phénomènes physiques, mécaniques ainsi que le modèle de comportement du matériau pour simuler les déformations élastoplastiques cycliques qui apparaissent au cours de la mise forme des tubes minces. La modélisation de la cinématique du procédé a déjà été réalisée. L'utilisation de capteurs numériques pour suivre le chemin de déformation de la matière lors du procédé permet d'estimer la nature et l'amplitude des déformations cycliques subies

Finite element simulation of cold pilgering of ODS tubes

International audienceThe oxide dispersion strengthened (ODS) ferritic and martensitic steels are candidate cladding materials for the new fast-neutron sodium-cooled Generation IV reactors. Typically the cladding is cold formed by a sequence of cold pilger rolling passes with intermediate heat treatments. Cracking risk prediction in pilgering is linked to the choice of an appropriate constitutive model for modeling the process. Consequently, this work aims to assess the impact of the constitutive laws on cracking risk development in pilgering conditions

High temperature mechanical strength and microstructural stability of advanced 9-12%Cr steels and ODS steels

International audienceThe present study proposes a comparison between several martensitic and ferritic steels in terms of creep strength and cyclic softening effect. The damage mechanisms are identified using fractographic observations and the microstructural evolutions are observed by TEM. The effects of a modified chemical composition on the high temperature mechanical behaviour are studied

Macroscopic and microscopic determinations of residual stresses in thin Oxide Dispersion Strengthened steel tubes

To improve the efficiency of components operating at high temperatures, many efforts are deployed to develop new materials. Oxide Dispersion Strengthened (ODS) materials could be used for heat exchangers or cladding tubes for the new GENIV nuclear reactors. This type of materials are composed with a metallic matrix (usually iron base alloy for nuclear applications or nickel base alloy for heat exchangers) reinforced by a distribution of nano-oxides. They are obtained by powder metallurgy and mechanical alloying. The creep resistance of these materials is excellent, and they usually exhibit a high tensile strength at room temperature. Depending on the cold working and/or the heat treatments, several types of microstructure can be obtained: recrystallised, stress relieved. One of the key challenges is to transform ODS materials into thin tubes (up to 500 microns thick) within a robust fabrication route while keeping the excellent mechanical properties. To prevent cracking during the process or to obtain a final product with low residual stresses, it is important to quantify the effect of the heat treatments on the release of internal stresses. The aim of this study is to show how residual stresses can be determined on different thin tubes using two complementary approaches: (i) macroscopic stresses determination in the tube using beam theory (small cuts along the longitudinal and circumferential directions and measurements of the deflection), (ii) stress determination from X-ray diffraction analyses (surface analyses, using "sin 2 Ψ" method with different hypothesis). Depending on the material and the heat treatment, residual stresses vary dramatically and can reach 800 MPa which is not far from the yield stress; comparisons between both methods are performed and suggestions are given in order to optimize the thermo-mechanical treatment of thin ODS tubes

Neutron analyses for nuclear materials: Texture, residual stresses and small angle scattering

Neutron techniques are very useful for metallurgical investigations of nuclear materials, bringing complementary results compared to others analysis techniques like XRD, TEM, APT. Especially, statistical information representative of the bulk material are obtained. Various nuclear materials examples are given here, on: texture analyses, residual stress determination and Small Angle Neutron Scattering before and after  irradiation

Neutron analyses for nuclear materials: Texture, residual stresses and small angle scattering

Neutron techniques are very useful for metallurgical investigations of nuclear materials, bringing complementary results compared to others analysis techniques like XRD, TEM, APT. Especially, statistical information representative of the bulk material are obtained. Various nuclear materials examples are given here, on: texture analyses, residual stress determination and Small Angle Neutron Scattering before and after  irradiation

Development of Adapted Material Testing for Cold Pilgering Process of ODS Tubes

International audienceDevelopment of fast-neutron sodium-cooled Generation IV reactors is resulting in extremely severe environment conditions for cladding tubes [1]. Both temperature and irradiation level will increase compared to the nowadays conditions. Due to their characteristics in irradiated environment, the oxide dispersion strengthened (ODS) ferritic and martensitic steels are natural candidate cladding materials[2]. However, they exhibit low deformation capabilities at room temperature, leading to problematic issues for forming such as pilgering. In order to improve the fabrication route for tubes, both metallurgical and numerical approaches can be conducted [3,4,5]. To reach predictive description of damage location and evolution, an adapated Latham and Cockoft model has been developed. This model is, of course, highly depending on the stress and strain prediction of the numerical model which itself is linked to the behavior law. In this work, we will describe an adapted material test developed in order to reproduce the cyclic, non uniform loading of the material during pilgering. An advanced cyclic beahvior law is introduced in the software. The model of Chaboche using 2 isotropic and 2 kinematic variables is chosen[6]. An inverse analysis procedure is used to identify both isotropic and kinematic hardening parameters. The results obtained using the identified behavior law are compared to both experimental observation and to other models including monotonic or cyclic laws identified on traditional test

Numerical simulation of the cold pilgering process of ODS tubes

International audienceFor new fast-neutron sodium-cooled GEN IV reactors, the candidate cladding materials for the very strong rates of combustion are the ferritic and martensitic ODS grades. Classically the cladding tube is cold formed by a sequence of cold pilger rolling passes with intermediate heat treatments. Within the framework of this numerical simulation study, the HPTR cold pilgering process is investigated. The model under development takes into account the complex kinematics of the process as well as the material constitutive behavior under cyclic elastic-plastic loadings. The use of numerical sensors to follow the deformation path of a volume element during the process allows estimating the nature and the amplitude of the cyclic deformations. The identification of the deformation path is the first step toward a better understanding of the critical conditions leading to damage and cracking. The material is highly textured and consequently mechanically anisotropic. The influence of the constitutive behavior on the deformation path is analyzed using two different plasticity criteria : Von Mises (isotropic), and Hill48 (anisotropic). Integration of a proper damage indicator into the proposed numerical model leads to the numerical optimization of the process in terms of maximum acceptable deformation before heat treatment, or process parameters (tools kinetics, dies profile, etc.)