Littérature scientifique sur le sujet « Austeno-Ferritic steel »

A new method for determining the parameters characterising elastoplastic deformation of two-phase material is proposed. The method is based on the results of neutron diffraction, which are analysed using the self-consistent rate-independent model of elastoplastic deformation. The neutron diffraction method (time-of-flight technique) was applied and the self-consistent model was used to predict the second order stresses in austeno-ferritic duplex steel. Calculations based on the model were successfully compared with experimental results for both phases of the duplex steel.

Duplex Stainless Steels (DSS) are very attractive steels and their application is presently of increasing interest, especially as structural materials in aggressive environments. DSS are austeno-ferritic biphasic steels, having an austenite-to-ferrite phase ratio of about one, giving the best combination of mechanical and corrosion-resistance properties. However, these steels must be handled with extreme care, especially if thermal cycles are involved, owing to the formation of dangerous secondary compounds that highly worsen their excellent properties.The production of big pipes requires manufacturing welding operations on steel plates or sheets and the final products must satisfy specific requirements in terms of material properties. DSS approximately contain equal volume fraction of the phases that, in practice, cover a slightly wider range within 40/60 and 60/40. Therefore, since DSS properties depend on phase ratio, ferrite quantification on an industrial scale represents a topic of great interest, which must be as reliable as possible and, at the same time, quickly executable.The present paper gives a comparison of different methods currently employed for ferrite determination in DSS weldments, in order to understand the limits derived from the specific employed technique.

Les aciers austéno-ferritiques utilisés dans la conception de diverses pièces moulées (valves, corps de pompe, etc.) du circuit primaire principal des centrales nucléaires de 2ème génération présentent une évolution de leurs propriétés mécaniques aux températures de service comprises entre 285 °C et 325 °C. Ces alliages biphasés, qui combinent la ferrite et l’austénite, vont présenter un durcissement microstructural qui se produit au sein de la ferrite. Ce durcissement est lié à deux transformations de phases comprenant, d’une part, la décomposition spinodale en une phase α riche en Fe et une phase α’ riche en Cr, et d’autre part, la formation de phase G riche en éléments d’alliage Ni, Si, Mn, Mo. Alors que les aciers austéno ferritiques contenant du Mo présentent un durcissement plus important que les aciers sans Mo, l’observation d’un acier purement ferritique contenant du Mo présentait, au contraire, un durcissement moins important en raison de l'absence de précipitation de la phase G. Cette étude avait pour objectif de comprendre l’influence de l’austénite sur le vieillissement de la ferrite, qui était l’hypothèse principale avancée, pour expliquer la raison du durcissement moins important de l’acier purement ferritique. Pour parvenir à confronter cette hypothèse, l’utilisation d’une méthode électrochimique a permis la dissolution sélective de l’austénite pour obtenir une ferrite sans austénite, de même composition, morphologie et histoire thermo-mécanique que la ferrite avec austénite. L’étude par sonde atomique tomographique de l’évolution microstructurale de ces deux ferrites, ainsi que celle d’un acier purement ferritique pour une condition de vieillissement de 1 000 h à 400 °C, a permis l’étude et la compréhension des points suivant :- Les résultats ont montré que la présence d'austénite induit des contraintes résiduelles de compression sur la ferrite, qui ne sont pas la cause du vieillissement plus rapide des aciers austéno-ferritiques, puisqu’aucune différence significative de microstructure n’était observable entre les ferrites avec et sans austénite.- La caractérisation de l’effet du gradient de composition Cr/Ni à proximité des interfaces α/γ sur l’évolution microstructurale de la ferrite a été réalisée. L’évolution des concentrations en Cr et Ni vers les interfaces α/γ n’impacte pas la décomposition spinodale mais affecte la formation de la phase G, avec comme principal effet une forte diminution de la densité de nanoparticules vers les interfaces α/γ.- Les contraintes résiduelles étant hors de cause, l’hypothèse d’un effet de la composition chimique a été envisagé. Ainsi, les résultats ont montré que la densité numérique des particules de phase G est fortement corrélée à la concentration en impuretés de la ferrite. L’absence d’impuretés dans l’alliage ferritique semble expliquer l’absence de nanoparticules de phase G aux interdomaines α/α’ et donc le vieillissement moindre de l'alliage ferritique
Austeno-ferritic steels used in the design of various cast components (valves, pump bodies, etc.) in the primary circuit of second-generation nuclear power plants exhibit changes in their mechanical properties at service temperatures between 285°C and 325°C. These two-phase alloys, which combine ferrite and austenite, exhibit a microstructural hardening that occurs within the ferrite. This hardening is associated with two phase transformations, including, firstly, spinodal decomposition into an α phase rich in Fe and an α' phase rich in Cr, and secondly, the formation of a G phase rich in alloying elements Ni, Si, Mn, Mo. While austeno-ferritic steels containing Mo present a greater hardening than steels without Mo, the observation of a purely ferritic steel containing Mo, on the contrary, showed less hardening due to the absence of G phase precipitation. The aim of this study was to understand the influence of austenite on the aging of ferrite, which was the main hypothesis put forward to explain the reason for the lesser hardening of purely ferritic steel. To confront this hypothesis, the use of an electrochemical method allowed for the selective dissolution of austenite to obtain austenite-free ferrite, with the same composition, morphology, and thermo-mechanical history as the ferrite with austenite. The study by atom probe tomography of the microstructural evolution of these two ferrites, as well as that of a purely ferritic steel under aging conditions of 1,000 hours at 400°C, allowed for the investigation and understanding of the following points:- The results showed that the presence of austenite induces residual compressive stresses on the ferrite, which are not the cause of the enhanced aging of austeno-ferritic steels, as no significant difference in microstructure was observable between ferrites with and without austenite.- The characterization of the effect of the Cr/Ni composition gradient near the α/γ interfaces on the microstructural evolution of ferrite was conducted. The evolution of Cr and Ni concentrations towards the α/γ interfaces does not impact the spinodal decomposition but affects the formation of the G phase, with the main effect being a significant decrease in nanoparticle density towards the α/γ interfaces.- As residual stresses are not the cause of the difference in aging between purely ferritic and austeno-ferritic steels, the hypothesis of a chemical composition effect was considered. The results showed that the number density of G phase particles is strongly correlated with the impurity concentration in the ferrite. The absence of impurities in the ferritic alloy appears to explain the absence of G phase nanoparticles at the α/α’ interdomains and thus the lesser aging of the ferritic alloy

Abstract. The excellent performance of duplex and super duplex stainless steels due to their high corrosion resistance and high mechanical strength is directly related to their austeno-ferritic microstructure. However, these steels may suffer the formation of secondary brittle phases when they reach temperatures between 600°C and 950°C causing catastrophic service failure of components. In order to understand the influence of the mechanical history of the steel, the equal channel angular pressing was applied on UNS S32750 bar samples. ECAP is processed by using a hydraulic press (DE-80). Microstructural characterization was carried out on the severe plastic deformed samples by means of OM, SEM and EBSD. Ultimate tensile strength has improved with a low decreasing of elongation after the first pass. The ECAP process resulted to produce a faster precipitation of secondary phases if compared with the non-ECAP samples.