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

Corrosion of 316L in Liquid Tellurium at 551DC

International audienceGrade 316L stainless steel was exposed to liquid tellurium at 551oC. Corrosion was rapid, leading to more than 100 et956;m loss of steel section in 30 min. The corrosion product was a mixed telluride scale, which thickened according to parabolic kinetics whilst simultaneously dissolving at its outer surface into the liquid tellurium. A mathematical model based on diffusion-controlled scale growth coupled with dissolution at a rate controlled by liquid phase diffusion is shown to describe scaling and dissolution kinetics successfull

Corrosion in liquid metals from database acquisition to life time prediction

International audienceIn nuclear power plant, materials can be corroded by liquid metals that are either flowing as coolant in first and secondary circuit (lead alloys Pb-Bi) or stagnant as fission product in cladding (tellurium). Steel corrosion process depends (i) on the concentration of dissolved corrosive element as O or C that can lead to oxidation or carburization, similarly to corrosion processes in gas phase; (ii) on the potential existence of intermetallic containing elements originating from both liquid metal and steel (FeTe$_2$, Cr$_3$Te$_4$); (iii) on capability of steel dissolution in the liquid metal. For all these corrosion processes, thermodynamic databases have to be developed in order to model the corrosion mechanisms. The obtained corrosion model is supposed to be used for life time prediction. This paper deals with both thermodynamic data acquisition (Ni solubility limit in liquid lead bismuth alloy) with thermodynamic database development (Calphad method) and corrosion experiments with mechanism modelling. Liquid metal environment considered are liquid Te and Pb-Bi alloy. Corroded steels are austenitic steel

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

International audienceTo 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

Corrosion behavior of a AISI-316 stanless grade type in nitrogen environment for a sodium-gas heat exchanger

International audienc

Comparison of 15Cr-15Ni austenitic steel cladding tubes obtained by HPTR cold pilgering or by cold drawing

International audienceDue to their high void swelling resistance, work-hardened titanium stabilized austenitic steels have been chosen as cladding material for sodium cooled fast reactor. In this study, HPTR cold pilgering process is compared to cold drawing at the last shaping step of the tube processing. The effects of the cold work accumulation and heat treatments are studied in connection with the microstructure (grain size), the hardness and the texture. The following results were found:-As larger amount of cold work can be applied by HPTR cold pilgering, a lower number of intermediate heat treatments are required. In addition, the bending of the tube is significantly reduced after each pass for this process allowing for a limitation of the straightening operations.-For both processes, optical micrographs show micrometric titanium carbide precipitates and the presence of deformation twins on a few grains on the final tube. A significant grain size refinement from $\approx$45 \mu^m to $\approx$17 \mu^m can be obtained by reducing annealing temperature from 1403K down to 1353K. For the latter, the precipitated mass fraction measured by selective dissolution of the alloy matrix is the largest, revealing a possible negative impact on swelling under irradiation.-Through-wall Vickers hardness profiles show an increase of the hardness at the outer diameter for HPTR cold pilgering whereas the hardness profile remains continuous for cold drawing. It is found that the Q-factor has an influence on these wall-thickness hardness profiles.-For each process, neutron diffraction measurements on finished tubes reveal two main fiber texture components and parallel to the tube axis with differences in their relative intensities

Comportement en traction et en oxydation de fibres SiC

Dans le contexte du développement d’un matériau composite SiC/SiC pour le gainage du combustible des réacteurs rapides à caloporteur gaz, on a examiné les propriétés de fibres micrométriques à base de carbure de silicium à faible teneur en oxygène dans des conditions représentatives du fonctionnement du système. Des essais de traction lente sur fibre individuelle ont été mis en œuvre dans une machine dédiée sous vide secondaire de la température ambiante à 1700 °C. On observe que, dans une large gamme de température, les fibres présentent un comportement fragile avec une baisse des propriétés quand la température s’élève. D’autre part, une étude par thermogravimétrie a été réalisée à 1100 °C sous hélium avec 20 Pa d’oxygène et indique qu’après environ 60 h de traitement, les fibres s’oxydent suivant une loi d’allure globalement parabolique. La surface se couvre d’une fine couche d’oxyde de silicium identifiée en microscopie électronique. Les prochaines étapes du travail s’attacheront à caractériser l’influence des traitements d’oxydation sur le comportement en traction des fibres

Corrosion Behaviour Of A Aisi-316 stainless steel Grade Type In Nitrogen Under Pressure (180 Bar) At 515°c.

International audienceIn the framework of research on Generation IV nuclear power plant, French research is focused on Sodium Fast Reactor. In the proposed design, heat exchange between the reactor core and the secondary circuit is performed via the Energy Conversion System (ECS). This specific ECS design involves that sodium (from the core) heat is transferred to gaseous nitrogen (from the secondary circuit) through 2 mm thickness wall channels. The considered material for this application is the 316L stainless steel. However this stainless steel grade has not been studied in nitrogen environment at working conditions (180 bar and 515DC) and few data are available in literature to assess nitriding risk