International audienceThe complex microstructures developed during post-welding heat-treatment in the vicinity of the fusion line between a ferritic and austenitic steel were examined in the case of submerged arc welded 18MND5/309L dissimilar joints. Quantitative measurements of the carbon distribution in the as-welded and post-weld heat-treated conditions were performed by both wavelength dispersive spectrometry and secondary ion mass spectrometry. The extent of carbon diffusion was confirmed by hardness profiles performed by nanoindentation. On the low-alloy ferritic side, decarburization resulted in cementite dissolution allowing the evolution of the bainitic structure toward a large-grained ferritic region. In the weld metal, the carbon content reached unusually high levels and an intense precipitation of chromium-rich carbides was observed in both the interfacial martensitic layer and the austenitic weld metal. The evolution of the precipitation as a function of the distance from the interface was analyzed in terms of crystallography, chemistry, volume fractions, and size distributions. Automated crystal orientation mapping in a transmission electron microscope allowed identification of the precipitates extracted on carbon replicas from both the martensitic and austenitic matrices. A 3D reconstruction of the carbides population in the martensitic layer was performed by serial cutting with a focused ion beam: M7C3 and M23C6 were found to coexist in the two carburized regions, but displayed different sizes, compositions, and morphologies, depending on their location with respect to the fusion line. This evolution in terms of precipitation was analyzed taking into account the local microstructure and composition

Dissimilar welds close to the fusion boundary exhibit a variety of solidification microstructures that strongly impact their service behavior. Investigations were therefore undertaken to clarify the origins of the morphological and microstructural evolutions encountered in a 18MND5/309L dissimilar joint produced by submerged arc welding, using a combination of microstructural characterizations, thermodynamic computations, and solidification modelling. An unexpected evolution was observed in the solidification mode, from primary austenite towards primary ferrite with increasing growth rate. Solidification of austenite at the fusion boundary was assigned to its epitaxial growth on the metastable austenitic structure of the base metal resulting from an incipient melting mechanism. The evolution of the solidification mode toward primary ferrite was explained based on computations of the solute built up between austenite cells followed using the so-called "interface response function model". Analyzing macro-and microstructural characteristic lengths with the published solidification model and data enabled evaluation of local values of the solidification rate, thermal gradient, and cooling rate close to the fusion boundary, thus providing useful data for numerical modelling of the submerged arc-welding process.

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A low copper reactor pressure vessel steel was characterised by atom probe tomography after neutron irradiation at different fluences. The specimens were irradiated within the frame of the Surveillance Program of a production reactor. Roughly spherical clusters enriched in nickel, manganese, silicon and, in a lesser extent, phosphorus and copper were observed at all fluences. The chemical composition of these clusters shows no evolution with fluence, as well as their diameter, close to 3 nm. Their number density increases linearly with the neutron fluence. A continuous segregation of the elements found in the clusters is also observed along dislocation lines, with similar enrichments.

Many components used in the pressure vessel of French pressurized water reactors are made of austenitic stainless steels : 316L for screws, 304L for core barrel and baffle assembly, etc. Among these components, the most irradiated ones undergo an average damage rate of about 1 dpa/an at temperatures which may reach about 400°C. Such irradiations may have very detrimental effects on the in-service behaviour of austenitic steels. In order to forecast these effects, Electricité de France has defined important programmes of studies. In the framework of these programmes, we have carried out a review concerning the effects of irradiations at relatively low temperatures (i. e. 500°C) on cold-worked 316 stainless steel. We were particularly interested in the evolution of microstructure as well as tensile (with or without notch), creep, fatigue (with or without creep) and fracture toughness properties. In some cases, we have also analysed the influence of cold-work level, irradiation temperature and test temperature. For some mechanical properties, the data was sufficient to qualitatively predict the properties of cold worked 316 stainless steels after long — term irradiations under PWR conditions.