Microstructural evolution of a low-alloy steel / nickel superalloy dissimilar metal weld during post-weld heat treatment

The microstructural evolution of a dissimilar metal weld (DMW) obtained by narrow-gap gas tungsten arc welding (NG-GTAW) was investigated after it was subjected to a post-weld heat treatment (PWHT). The case studied here is a joint between low-alloy steel pipes and a stainless steel steam generator using a nickel based alloy as filler material. The fusion boundary that was the focus of this work was that between the low-alloy steel (2.25Cr-1Mo) and the nickel alloy (alloy 82). The difference in matrix phase and chemical composition between the two alloys leads to a large difference in chemical potential for carbon, which is mobile at the PWHT temperature. A number of advanced characterization techniques were used to assess the gradient of composition, hardness and microstructures across the fusion line, both as welded and after PWHT. This complete analysis permits to highlight and understand the main microstructural changes occurring during the PWHT

An extension of the Kocks-Mecking model of work hardening to include kinematic hardening and its application to solutes in ferrite

International audienceIt is well known that the addition of a solute can change both work-hardening characteristics and yield stress; however, there are few available models which describe the role of a solute in relation to both the isotropic and kinematic aspects of work hardening. The current work extends the well-established approach of Kocks and Mecking to include the occurrence of cross slip and its dependence on solute content. The proposed model is compared with experimental data for the system Fe-Al by reference both to the observed work hardening in monotonic loading and the Bauschinger effect measured in reverse shear tests. The agreement between the model and the experimental data is satisfactory and suggests a new description of work hardening which includes a prediction of the ratio of isotropic and kinematic hardening for a given solute content

Characterization and modeling of ductile damage accumulation in martensitic stainless steels

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Evolution of a low-alloy steel / nickel superalloy dissimilar metal weld during post-weld heat treatment

International audienceThe design of the dissimilar metal weld investigated here is aimed at applications in the steam generator of a sodium-cooled nuclear reactor, with a multi-decade lifespan in demanding operational conditions. It consists in a narrow-gap joint between 2.25Cr-1Mo low-alloy steel and an austenitic alloy using a nickel-based alloy 82 as filler material. This study focuses on understanding the microstructural and micromechanical evolution in the near fusion boundary region between the low-alloy steel and the nickel alloy filler metal during post-weld heat treatment, using notably electron probe microanalysis and nano-indentation. The difference in matrix phase and chemical composition between the two alloys leads to a large difference in chemical potential for carbon, which is mobile at the post-weld heat treatment temperature. A number of fine-scale characterization techniques were used to assess the gradient of composition, hardness and microstructures across the fusion boundary, both as welded and after post-weld heat treatment. This complete analysis permits to highlight and understand the main microstructural and micromechanical changes occurring during post-weld heat treatment and opens the way to their long term study in service conditions

Selection and Characterization of Geological Sites able to Host a Pilot-Scale CO2 Storage in the Paris Basin (GéoCarbone-PICOREF) Choix et caractérisation de sites géologiques propices à l’installation d’un pilote pour le stockage de CO2 dans le bassin de Paris (GéoCarbone-PICOREF)

The objective of the GéoCarbone-PICOREF project was to select and characterize geological sites where CO2 storage in permeable reservoir could be tested at the pilot scale. Both options of storage in deep saline aquifer and in depleted hydrocarbon field were considered. The typical size envisioned for the pilot was 100 kt CO2 per year. GéoCarbone-PICOREF initially focused on a “Regional Domain”, ca. 200 × 150 km, in the Paris Basin. It was attractive for the following reasons: detailed geological data is available, due to 50 years of petroleum exploration; basin-scale deep saline aquifers are present, with a preliminary estimate of storage capacity which is at the Gt CO2 level, namely the carbonate Oolithe Blanche Formation, of Middle Jurassic age, generally located between 1500 and 1800 m depths in the studied area, and several sandstone formations of Triassic age, located between 2000 and 3000 m; several depleted oil fields exist: although offering storage capacities at a much lower level, they do represent very well constrained geological environments, with proven sealing properties; several sources of pure CO2 were identified in the area, at a flow rate compatible with the pilot size, that would avoid capture costs. 750 km of seismic lines were reprocessed and organized in six sections fitted on well logs. This first dataset provided improved representations of: the gross features of the considered aquifers in the Regional Domain; the structural scheme; lateral continuity of the sealing cap rocks. An inventory of the environmental characteristics was also made, including human occupancy, protected areas, water resource, natural hazards, potential conflicts of use with other resources of the subsurface, etc. From all these criteria, a more restricted geographical domain named the “Sector”, ca. 70 × 70 km, was chosen, the most appropriate for further selection of storage site(s). The geological characterization of the Sector has been as exhaustive as possible, with the reprocessing of additional 450 km of seismic lines, and the collection of a complete well-data base (146 petroleum wells). At this scale a relatively detailed characterization of the sedimentary layers could be done, in particular the formations potentially rich in aquifer units. For the Middle Jurassic carbonates observations were made on analogue sediments outcropping 150 km to the east of the Sector. A geological and numerical 3-D representation of the whole sedimentary pile of the Sector area was built. It forms a basis for constructing grids used by codes able to simulate various processes induced by CO2 injection (displacement of the fluids, pressure build-up and release, mechanical deformation, mineral interactions, control of the parameters used to check the local sealing efficiency, etc.). In parallel with that work on aquifers, GéoCarbone-PICOREF has access to all the petroleum data, including production data and reservoir modelling, of the Saint-Martin de Bossenay oil field, localized in the eastern part of the Sector. This was an opportunity to apply a comparable methodology and to test the capabilities of modelling codes to the specific case of a depleted hydrocarbon field, and to show some of the advantages of such a context with respect to a pilot-scale CO2 injection. <br> Le projet GéoCarbone-PICOREF avait pour objectif de caractériser des sites propices à la réalisation d’un pilote national de stockage du CO2 en réservoir géologique perméable. Deux types de réservoir ont été examinés : des aquifères profonds, et des gisements d’hydrocarbures en voie d’épuisement. Les sites devaient être choisis de manière que le pilote puisse tester des problématiques qui concernent les futurs stockages de grande taille. GéoCarbone-PICOREF a d’abord sélectionné une “Zone régionale” d’environ 200 × 150 km dans le bassin de Paris, qui présente les avantages suivants : l’information géologique y est largement disponible, grâce aux travaux d’exploration pétrolière depuis 50 ans ; de grands aquifères salins y sont présents, dans les carbonates du Jurassique moyen situés en général entre 1500 et 1800 m de profondeur, et dans les formations gréseuses du Trias, entre 2000 et 3000 m; il existe plusieurs gisements d’hydrocarbures en voie d’épuisement : offrant des capacités de stockage moindres, leur intérêt est d’être bien connus sur le plan géologique et d’être dotés de bonnes qualités en termes de piégeage géologique. Après avoir retraité 750 km de lignes sismiques, et avoir assemblé celles-ci selon six coupes calées sur des données de puits, on a précisé sur la Zone régionale : les grandes caractéristiques des aquifères concernés ; la localisation des failles ; la continuité des couches très peu perméables situées au-dessus des réservoirs. Ces études ont permis de choisir un “Secteur”, d’environ 70 × 70 km, au sein duquel on a ensuite affiné l’investigation géologique : 450 km supplémentaires de lignes sismiques, collecte exhaustive des données de puits, caractérisation fine des propriétés réservoir. Des observations de terrain ont été faites sur des roches équivalentes portées à l’affleurement. Un modèle géologique et informatique complet du Secteur a été construit à partir de ces données. Il permet de générer des maillages pour la simulation de divers comportements attendus suite à l’injection de CO2 (déplacement et dissipation du gaz dans les couches réservoir, modification des pressions et des contraintes, déformation mécanique des terrains, interaction entre l’eau acidifiée et les minéraux, etc.). Parallèlement, le projet a pu avoir accès à toutes les données pétrolières du gisement de Saint-Martin de Bossenay, situé dans la partie Est du Secteur. Grâce à cette opportunité, on a montré quel parti pouvait être tiré, pour un pilote, d’un gisement d’hydrocarbures déjà largement exploité, doté d’un piège géologique qui a retenu des hydrocarbures pendant des millions d’années, et sur lequel un opérateur industriel dispose d’une infrastructure et d’un savoir-faire