Optimisation en fluage et fatigue-fluage d'aciers martensitiques à 9%Cr par traitement thermo-mécanique

National audienceL'utilisation des aciers martensitiques à 9-12%Cr est envisagée dans les futurs réacteurs nucléaires de génération IV. En service, ils seront soumis à du fluage à haute température ainsi qu'à des sollicitations de fatigue-fluage. Or de récents travaux sur l'acier commercial P91 ont montré que des sollicitations cycliques de fatigue combinées au fluage conduisent à un adoucissement rapide du matériau, lié à un grossissement de sa microstructure. Des traitements thermomécaniques ont donc été réalisés dans le but d'affiner et de stabiliser la microstructure de cet acier. Le traitement thermomécanique présenté ici conduit à une martensite plus fine, émaillée de nombreux et fins précipités de type MX. Le P91 optimisé montre un net gain en termes de propriétés mécaniques par rapport à l'acier P91 à réception : sa dureté est plus élevée de 100 Hv, sa limite d'élasticité conventionnelle est supérieure de 430 MPa à 20°C et de 220 MPa à 550°C, sa durée de vie en fluage à 650°C est 20 fois supérieure et son adoucissement à 650°C est légèrement moins rapide

Relationship between microstructure and mechanical behaviour of thermomechanically optimised 9-12% Cr steels

International audienceThe development of Generation IV fission nuclear reactors and fusion nuclear reactorsrequires materials able to resist to high temperature (650°C) creep, but also to creep-fatigue.Martensitic 9-12%Cr steels are candidate materials for these applications.Recent studies on commercial P91 steel showed that cyclic loadings coupled to high temperature creep loadings lead to a strong softening effect, which affects the steelmechanical strength. This effect is due to the decrease of the dislocations density and thecoarsening of martensitic microstructure.Thermomechanical treatments, including warm-rolling in austenitic phase and tempering,have been applied to P91 in order to refine its microstructure and to improve its precipitationstate. The temperature of rolling was set at 600°C, and those of annealing at 650°C and700°C, thanks to MatCalc calculations.Microstructural observations proved that the warm-rolling and the following tempering lead toa finer martensite pinned with numerous small precipitates. In terms of mechanical propertiesimprovement, the hardness of thermomechanically treated P91 is higher than that of asreceivedP91. The yield strengths are higher than that of P91 (around 400 MPa at 20°C; andmore than 200 MPa at 550°C). Preliminary creep resu lts show that these treatments improvethe creep lifetime by at least a factor 8

Comportement mécanique à haute température des aciers martensitiques revenus pour application aux réacteurs à neutrons rapides

National audienceLa conception du futur réacteur à neutrons rapides à caloporteur sodium (SFR) a motivé de nombreuses études sur le comportement en fatigue, fluage et fatigue-fluage à haute température d'aciers martensitiques revenus (450-600°C). Depuis les années 80, l'instab ilité de leur microstructure dans certaines conditions de chargement (fatigue, fluage) a été mise en évidence dans la littérature. Cette instabilité conduit à un adoucissement qu'il convient de comprendre et de prédire. Nous avons particulièrement étudié l'effet des chargements caractéristiques des conditions en service auxquelles seront soumis les composants du réacteur SFR (faibles amplitudes de déformation cycliques, longues durées de vie en fluage), pour lesquelles les données expérimentales sont rares. Des évolutions microstructurales et un adoucissement notable ont été mis en évidence dans ces conditions. Des modèles micromécaniques basés sur les densités de dislocations continus permettent de prédire qualitativement un grand nombre des phénomènes observés

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

Influence of strain rate on P92 microstructural stability during fatigue tests at high temperature

International audience9-12%Cr creep-resistant ferritic-martensitic steels are candidates for structural components of Generation IV nuclear power plants. However, they are sensitive to softening during fatigue and creep-fatigue loading. To better understand softening mechanisms in ASTM Grade 92, fatigue tests were carried out at 823 K at various strain amplitudes. Two different values of the strain rate (2 10−3 s−1 and 10−5 s−1) were used for one strain amplitude. The softening behavior is mainly due to microstructural evolution. Examination of fractured specimens (hardness tests, TEM) shows an influence of strain rate on both increase in subgrain size and decrease in free dislocation density during cycling. Study of the evolution of isotropic, kinematic and viscous contributions to stress during fatigue tests shows a decrease in the kinematic contribution during cycling. A simplified mean field polycrystalline model based on subgrain growth is proposed in order to account for this strain rate effect. Potential impact on further creep resistance behavior is discussed

Analysis of the hysteresis loops of a martensitic steel, part I., study of the influence of strain amplitude and temperature under pure fatigue loadings using an enhanced stress partitioning method

International audienceIn order to identify the microstructural mechanisms leading to the softening effect usually presented by martensitic steels under cyclic loadings (with or without hold times), a study of the cyclic stress partition is presented. As the usual stress partitioning methods were found to be inadequate in the present case, a new method based both on Cottrell's method and on the Statistical Process Control principles, is proposed. This new method is used to distinguish between the kinematic, the isotropic and the viscous parts of the cyclic stress. The evolutions of these different stresses are evaluated for several strain amplitudes and temperatures under pure fatigue loading in this first part. It is shown that the softening effect is mainly due to a decrease of the backstress: the higher the strain amplitude, the stronger and the faster the softening effect. The isotropic stress is found to be independent of the strain amplitude, but increases when the temperature decreases. Whereas the viscous stress represents a large part of the total stress at 823 K, it becomes almost negligible below 673 K. These results are finally linked to the microstructural coarsening previously observed and modelled. Therefore, the decrease of the kinematic stress can be related to grain size effect

Effect of pre-strain on creep of three AISI 316 austenitic stainless steels in relation to reheat cracking of weld-affected zones

International audienceMicrostructural modifications induced by welding of 316 stainless steels and their effect on creep properties and relaxation crack propagation were examined. Cumulative strain due to multi-pass welding hardens the materials by increasing the dislocation density. Creep tests were conducted on three plates from different grades of 316 steel at 600 °C, with various carbon and nitrogen contents. These plates were tested both in the annealed condition and after warm rolling, which introduced pre-strain. It was found that the creep strain rate and ductility after warm rolling was reduced compared with the annealed condition. Moreover, all steels exhibited intergranular crack propagation during relaxation tests on Compact Tension specimens in the pre-strained state, but not in the annealed state. These results confirmed that the reheat cracking risk increases with both residual stress triaxiality and pre-strain. On the contrary, high solute content and strain-induced carbide precipitation, which are thought to increase reheat cracking risk of stabilised austenitic stainless steels did not appear as key parameters in reheat cracking of 316 stainless steels

Analysis of the hysteresis loops of a martensitic steel. Part I and Part II

International audienceThe second part of this work is devoted to the study of holding time effects on the cyclic plastic behaviour of a martensitic steel tested at 823 K. Both relaxation and creep holding times of various durations were applied. The enhanced stress partitioning method presented in the first part [B. Fournier, M. Sauzay, C. Cae's, M. Mottot, Mater. Sci. Eng., submitted for publication] is used to evaluate the kinematic, isotropic and viscous parts of the cyclic stress. The bulk Young's modulus is found to vary significantly during cycling for creep-fatigue tests, which might be correlated to specific environmental interaction. The viscous stress measured at the end of the holding period tends to vanish as the holding time increases. The introduction of creep holding times enabled higher viscoplastic strains per cycle to be reached and allowed a larger range of strain rates to be studied. In all the cases tested, the observed softening effect is mainly due to the kinematic stress decrease. Nevertheless, even though the kinematic stress is always found to decrease with increasing accumulated viscoplastic strain, the initial magnitude of the creep-fatigue kinematic stress (measured at the end of the first holding period) can be either higher or lower than that of the corresponding pure-fatigue test. These effects of the holding period on the kinematic stress value can be related to the viscoplastic strain rate (and to the nature of the holding time: creep or relaxation). This dependency presents a maximum at intermediate strain rate, suggesting that two competing microstructural mechanisms control the magnitude of the kinematic stress. The enhanced stress partitioning method also enables the activation volume of both the creep and fatigue deformation mechanisms to be evaluated. The observed values are compared to those found in the literatur

High temperature creep–fatigue–oxidation interactions in 9–12%Cr martensitic steels

International audienceDetailed observations of fractured specimens of 9–12%Cr martensitic steels subjected to creep–fatigue loadings at 823 K were carried out. Observations revealed that oxidation phenomena strongly influence the creep–fatigue lifetime whereas no creep damage (cavities) can be observed in the present loading conditions. Two main interaction mechanisms between creep, fatigue and oxidation damage were highlighted. These two damage mechanisms correspond to two distinct domains of loading (expressed in terms of total strain range and holding period duration). Based on the identified mechanisms a creep–fatigue lifetime prediction model is proposed. The crack initiation is approached by the Tanaka and Mura model, whereas the crack propagation phase is accounted by the Tomkins formulation

Characterization of a boron alloyed 9Cr3W3CoVNbBN steel and further improvement of its high-temperature mechanical properties by thermomechanical treatments

International audienceIn the framework of the development of Generation IV nuclear reactors and fusion nuclear reactors, materials with an improved high temperature (≅650 °C) mechanical strength are required for specific components. The 9-12% Cr martensitic steels are candidate for these applications. Previous works showed that the application of a thermomechanical treatment, including warm-rolling in metastable austenitic phase, to the commercial Grade 91 martensitic steel, allowed refining its microstructure, improving its precipitation state and its mechanical properties (hardness, tensile and creep properties). In the present paper, experimental steel called NPM, designed for good high-temperature creep resistance, is evaluated in terms of microstructure and mechanical properties, and compared to the G91 steel. Then the developed thermomechanical treatment is applied to this steel. Its microstructure is refined and its hardness and tensile properties are much better than the as-received NPM and therefore than the G91 steel. The cyclic softening effect still occurs for the optimized NPM, but this material once softened by cyclic loadings, still presents better creep properties than the as-received NPM steel, and even more than the commercial G91 steel