No description available.

ABSTRACT This work deals with the damage due to the simultaneous presence of hydrogen in atomic form and stress – straining. The aim of this work is twofold: to better understand the hydrogen embrittlement mechanisms and to translate the acquired knowledge into a more appropriate qualification test. The phenomena of hydrogen entry and transport inside the metals, together with the different types of damages due to the presence of hydrogen, are presented. The analysis of the most important models proposed up to now for hydrogen embrittlement (HE) indicated that the slow dynamic plastic straining is a key factor for the embritteling process. There is a synergistic effect of hydrogen – dislocations interactions: on one hand hydrogen facilitates the dislocations movement (according to the HELP mechanism) and on the other hand dislocations transport hydrogen during their movement when their velocity is lower than a critical value. This work is focused on supermartensitic stainless steels, base and welded materials. The interest on these materials is due to their broad use in offshore oil production. First, the material’s characterisation with regards to hydrogen content and localisation was performed. This was conducted in charging conditions that are representative of industrial applications. Because of previous industrial experience it was necessary to find a more appropriate qualification test method to asses the risk of HE. In this work we proposed the stepwise repeated slow strain rate test (SW R – SSRT) as a qualification test method for supermartensitic stainless steels. This test method combines hydrogen charging, test duration, plastic, dynamic and slow strains. Thus, this test method is coherent with both the model HELP proposed for hydrogen embrittlement and the observations of industrial failures. The stepwise repeated slow strain rate test (SW RSSRT) is interesting not only as a qualification test of martensitic stainless steels, but also as a qualification test of conditions for using these materials (type of straining, range of strain and stress, strain rate, hydrogen charging conditions, etc.). RESUME Ce travail se rapporte à l’endommagement provoqué par la présence simultanée de l’hydrogène sous forme atomique et une contrainte (appliquée où résiduelle). Ce travail a comme but une meilleure compréhension du mécanisme de la fragilisation par l’hydrogène (FPH) et la recherche d’un essai de qualification qui soit cohérent avec ce mécanisme. Les phénomènes liés à l’entrée et au transport de l’hydrogène au sein des métaux, ensemble avec les différents types d’endommagements dus à la présence de l’hydrogène, sont présentés. L’analyse des modèles proposés jusqu’au présent pour la fragilisation par l’hydrogène (FPH) suggère que la déformation lente plastique dynamique est le facteur clé pour le processus de la fragilisation. Il y a un effet synergétique des interactions entre l’hydrogène et les dislocations: d’un coté l’hydrogène facilite le mouvement des dislocations (d’après le modèle HELP) et d’un autre coté les dislocations transportent l’hydrogène pendant leur mouvement, pourvu que leur vitesse soit en dessous d’une valeur critique. Le travail a été conduit sur des aciers supermartensitiques, matériau de base et soudé. L’intérêt pour ces matériaux réside de leur large utilisation dans la production du pétrole en offshore. D’abord, le matériau a été caractérisé du point de vu de la teneur et de la localisation de l’hydrogène. Les essais ont été conduits dans des conditions représentatives pour les cas industriels. L’expérience industrielle d’auparavant indique qu’il est nécessaire de trouver un test de qualification plus approprié pour estimer la susceptibilité à la fragilisation par l’hydrogène. Dans ce travail on propose un essai de traction lente incrémentée (SW R – SSRT) comme méthode de qualification pour les aciers supermartensitiques. L’essai combine le chargement en hydrogène, la durée d’essai, la déformation lente, plastique et dynamique. Donc, cette méthode d’essai est cohérente avec le modèle HELP proposé pour FPH et les observations des accidents industriels. Cet essai est intéressant pas seulement comme essai de qualification pour les aciers supermartensitiques, mais aussi comme essai de qualification pour les conditions d’utilisation des ces matériaux (type de déformation, niveau de déformation et contrainte, vitesse de déformation, conditions de chargement en hydrogène, etc.).

No description available.

Incremental step load testing was used in accordance with ASTM F1940 to rank a number coating processes used in the fastener industry for their propensity to cause internal hydrogen embrittlement. The results showed that coating permeability has a first order effect, while the quantity of hydrogen introduced by the process has a second order effect. Pure zinc electroplating processes, alkaline and acid, were found to be the most embrittling, owing to the low permeability of zinc. The least embrittling processes were zinc-nickel, alkaline and acid, owing to the high permeability of Zn-Ni coatings. Non-electrolytic processes, namely phosphating, mechanical galvanising, DacrometRTM and Magni 555RTM were found to be non-embrittling. Hot dip galvanising was found to be highly embrittling, evidently due to trapped hydrogen being released by the thermal shock of up-quenching upon immersion in molten zinc. The full effect of up-quenching on the metallurgical and mechanical properties of high strength steel requires further investigation.

Pulse plated nickel and nickel based superalloys have been used extensively in the Ariane 5 space launcher engines. Large structural Ariane 5 space launcher engine components such as combustion chambers with complex microstructures have usually been manufactured using electrodeposited nickel with advanced pulse plating techniques with smaller parts made of nickel based superalloys joined or welded to the structure to fabricate Ariane 5 space launcher engines. One of the major challenges in manufacturing these space launcher components using newly developed materials is a fundamental understanding of how different materials and microstructures react with hydrogen during welding which can lead to hydrogen induced cracking. The main objective of this research has been to examine and interpret the effects of microstructure on hydrogen diffusion and hydrogen embrittlement in (i) nickel based superalloy 718, (ii) established and (iii) newly developed grades of pulse plated nickel used in the Ariane 5 space launcher engine combustion chamber. Also, the effect of microstructures on hydrogen induced hot and cold cracking and weldability of three different grades of pulse plated nickel were investigated. Multiscale modelling and experimental methods have been used throughout. The effect of microstructure on hydrogen embrittlement was explored using an original multiscale numerical model (exploiting synthetic and real microstructures) and a wide range of material characterization techniques including scanning electron microscopy, 2D and 3D electron back scattering diffraction, in-situ and ex-situ hydrogen charged slow strain rate tests, thermal spectroscopy analysis and the Varestraint weldability test. This research shows that combined multiscale modelling and experimentation is required for a fundamental understanding of microstructural effects in hydrogen embrittlement in these materials. Methods to control the susceptibility to hydrogen induced hot and cold cracking and to improve the resistance to hydrogen embrittlement in aerospace materials are also suggested. This knowledge can play an important role in the development of new hydrogen embrittlement resistant materials. A novel micro/macro-scale coupled finite element method incorporating multi-scale experimental data is presented with which it is possible to perform full scale component analyses in order to investigate hydrogen embrittlement at the design stage. Finally, some preliminary and very encouraging results of grain boundary engineering based techniques to develop alloys that are resistant to hydrogen induced failure are presented. Keywords: Hydrogen embrittlement; Aerospace materials; Ariane 5 combustion chamber; Pulse plated nickel; Nickel based super alloy 718; SSRT test; Weldability test; TDA; SEM/EBSD; Hydrogen induced hot and cold cracking; Multiscale modelling and experimental methods.

Le polyamide 11 (PA11) est soumis à des conditions particulières d’utilisation (contraintes mécaniques, température élevée en présence d’eau) au cours desquelles ses caractéristiques mécaniques et physico-chimiques peuvent être altérées. Notamment l’hydrolyse peut générer une fragilisation du matériau par coupure des chaines macromoléculaires. On peut trouver dans la littérature des rapprochements entre propriétés macroscopiques et propriétés microstructurales afin de caractériser la transition ductile-fragile du polyamide. Mais peu d’attention a été portée jusqu’à présent sur la modélisation du comportement mécanique évoluant avec le vieillissement en lien avec la diffusion d’eau dans le matériau.L’objectif de ce travail de thèse est donc de pouvoir prédire la transition ductile-fragile des polyamides, du PA11 en particulier, au cours d’un vieillissement en température et en milieu aqueux acide.La première partie du travail de thèse a porté sur la compréhension de la dégradation mécanique du PA11 en fonction d’un vieillissement hydrique et thermique (à pH4 et à 110°C) en s’appuyant sur une campagne expérimentale afin de relier l’évolution du comportement mécanique macroscopique à celle de descripteurs physico-chimiques. Tout d’abord, des essais de vieillissement en milieu acide et en température ont été effectués pour évaluer la dégradation aux temps longs du PA11. Puis, pour décorréler les effets de l’hydrolyse des effets de recuit, des essais de vieillissement thermique uniquement, en milieu neutre, ont été réalisés. Des essais de traction à la rupture, et des essais de fluage ont ensuite été menés dans le but de mesurer d’éventuelles modifications des propriétés mécaniques au cours du vieillissement. Différentes caractérisations physico-chimiques (DSC, GPC, FTIR, viscosimétrie) ont permis de relier des changements morphologiques à différentes échelles du matériau à l’évolution de ces grandeurs mécaniques macroscopiques. Dans un second temps, une modélisation de la transition ductile-fragile du PA11 au cours du vieillissement a été mise en place. L’évolution de descripteurs physico-chimiques a été intégrée dans le formalisme d’un modèle mécanique développé à IFPEN pour rendre compte du comportement biphasique des polymères semi-cristallins. Le modèle permet de décrire l’impact du vieillissement sur le comportement mécanique du PA11, en particulier l’initiation ainsi que la propagation d’une striction le cas échéant. Enfin, une modélisation de la rupture du PA11 par fragilisation est proposée. / The prediction of the long-term behaviour of polyamides (PAs) is a major challenge for the structural design of various technological equipment in the field of energy and transport. PAs are very ductile when new, but they are sensitive to hydrolysis.Structure-properties relationships of PAs during ageing in water have been somewhat studied in literature. But little attention has been paid so far to the modeling of the mechanical behavior evolving with ageing in relation to the diffusion of water in the material.This work aims to be able to predict the ductile-brittle transition of polyamides, PA11 in particular, during ageing in temperature and acidic aqueous medium.The first part of the work focused on understanding the mechanical degradation of PA11 as a function of hydric and thermal ageing (at pH4 and at 110 ° C), based on an experimental campaign in order to link the evolution of the macroscopic mechanical behavior to that of physicochemical descriptors. First, acid and temperature ageing tests were performed to evaluate the long-term degradation of PA11. Then, in order to decorrelate the effects of the hydrolysis of the annealing effects, tests of thermal ageing only, in neutral environment, were carried out. Tensile tests at break and creep tests were then performed in order to measure possible changes in the mechanical properties during ageing. Different physicochemical characterizations (DSC, GPC, FTIR, viscosimetry) made it possible to link morphological changes at different scales of the material to the evolution of these macroscopic mechanical quantities.Secondly, a modeling of the ductile-brittle transition of PA11 during ageing was put in place. The evolution of physicochemical descriptors has been integrated into the formalism of a mechanical model developed at IFPEN to account for the biphasic behavior of semi-crystalline polymers. The model makes it possible to describe the impact of ageing on the mechanical behavior of PA11, in particular the initiation as well as the propagation of a neck if necessary. Finally, a model of the rupture of PA11 by embrittlement is proposed.

No description available.

<p> The role of plastic deformation in the initiation and propagation of cracks in Cu-17Al alloy embrittled by liquid mercury has been studied. It is proposed that extensive plastic deformation and work hardening must occur at the crack-tip during propagation in order to raise the local flow stress to a critical level at which the maximum normal stress is equal to the cohesive strength. A crack initiation mechanism is proposed involving grain boundary diffusion enhanced by the stress concentration at the head of dislocation pile-ups at the grain boundary. Micro-cracks are formed at the weakened grain boundary and a period of stable crack growth made over a period of increasing applied stress may be necessary before the crack is long enough to become unstable.</p> / Thesis / Master of Science (MSc)

Inconel X-750 is a Ni-rich super-alloy with high strength and creep resistance. In CANDU reactors, it is used as tight fitting fuel channel annulus spacers (in the form of a spring). Unlike other reactor designs, the CANDU reactor has a high thermal neutron flux spectra, which, combined with the higher thermal neutron cross section of 58Ni results in an enhancement of the radiation damage and the internal production of helium and hydrogen. In recent years, it has been observed these spacers are losing ductility and strength following irradiation. The mechanical property evolution of these components is dependent on the irradiation temperature and dose. The primary degradation mechanism remains unclear, and thus provides the focus of this investigation. Inconel X-750 irradiated to these extreme conditions have never been examined prior to this research. The microstructural characterization included in this dissertation include: fractography, microstructural evolution and mechanical property evolution. The discussion of the microstructural evolution is focused on the characterization of helium bubbles. The bubbles form homogeneously in the matrix and aligned along sinks such as dislocations, grain boundaries and precipitates. Electron energy loss spectroscopy (EELS) has been used to probe individual nano-sized bubbles to provide insights into the helium density, helium-to-vacancy ratio, and pressures through the use of a hard sphere equation of state (HSEOS). In addition to understanding the influence of irradiation on helium bubbles, the evolution of secondary strengthening precipitates, gamma prime, are of interest as these precipitates play an important role on the strength and creep resistance of the unirradiated material. The stability of these precipitates with irradiation is thus an important factor to consider with respect to the microstructural degradation. The microstructure is linked to the mechanical properties via microhardness testing on adjacent material. A major contribution to this field is an approach to utilize a focused ion beam (FIB) and transmission electron microscopy (TEM) to perform high ii resolution failure analysis of an intergranular fracture surface. Although this technique is not altogether revolutionary, the application of this approach towards post irradiation examination of heavily irradiated Inconel X-750 is unique. This approach provides direct evidence of likely degradation mechanisms, and provides insights for future post irradiation failure analysis for other applicable nuclear components. Working with ex-service material creates some complications with respect to known and unknown variables making it difficult to assess all factors responsible for material degradation. To compliment the program, a controlled proton irradiation program has been performed to gain additional insights into in the effects of irradiation on the microstructure and mechanical property evolution of Inconel X-750, due to the inability to perform controlled experiments with in-service reactor components. In addition to providing a detailed analysis of a CANDU component’s degradation, this study provides comprehensive information on irradiation damage processes applicable to other reactor core components. / Thesis / Doctor of Philosophy (PhD)

The aim of this research is to identify problematic microstructural features as hydrogen traps in linepipe steel that serve to increase the hydrogen embrittlement susceptibility. A comparison is made between the hydrogen trapping capacity and associated hydrogen embrittlement susceptibility of Ca-treated X60 grade steel skelp and X70 grade steel skelp: the latter typically being more susceptible to hydrogen-induced cracking in sour environments. Through-thickness variations in the steel skelp microstructure were characterized across multi-length scales using light optical microscopy (LOM) and scanning electron microscopy (SEM) equipped with X-ray energy dispersive spectroscopy (EDS). Key features under study include the composition, shape, and distribution of non-metallic inclusions, as well as differences in features present between the quarterline (¼ and ¾ depths) and centerline (½ depth) microstructures. The type, count, and average size of inclusions present in both steel skelp grades were analyzed using an automated SEM-EDS technique called ASPEX®. Major types of inclusions detected in both grades of steel skelp include those containing Ca, Al, Mn, Mg and Ti as major elements. Overall, the area fraction of inclusions detected in the X70 steel was larger than those detected in the X60 with the exception of Ti-containing inclusions, which had a larger area fraction within the X60 steel. Comparing the number of detected inclusions shows that there was overall slightly less Ca-containing inclusions and significantly less Ti-containing inclusions detected in the X70 steel but there was generally more Al-containing, Mg-containing, and Mn-containing inclusions than those detected in the X60 steel. Thermal desorption spectroscopy (TDS) measurements were made on samples prepared from the ¼, ½, and ¾ depths of X60 and X70 steel skelps after galvanostatic cathodic charging in an As2O3-containing solution using an applied current density of −10 mA/cm2. Hydrogen release was measured using a HYDROSEEL® probe while the sample was heated from 20°C to 650°C to detect temperature values at which hydrogen gas release peaks occurred, and thus provide information on types of reversible and/or irreversible traps present. The TDS results suggests that non-metallic inclusions indeed serve as irreversible traps along with grain boundaries and dislocations, which serve as reversible traps. Hydrogen permeation measurements were also made on samples prepared from the ¼, ½, and ¾ depths after galvanostatic cathodic charging in an As2O3-containing solution using an applied current density of −10 mA/cm2. Hydrogen gas release was measured using a HYDROSEEL® probe while the sample remained at room temperature (~20°C), providing information regarding the potency of reversible hydrogen traps when subjected to a flux of hydrogen. Only reversible traps can be detected at room temperatures due to their low binding energies. Higher temperatures are required to overcome the larger binding energies associated with irreversible traps. The hydrogen permeation results indicate no significant effect of through-thickness variations in the X60 steel, but the centreline depth of the X70 steel skelp trapped a larger quantity of hydrogen than either of the two quarterline depths, indicating the presence of a distinct problematic trap. The X70 steel skelp was also observed to trap more hydrogen than the X60 steel skelp. The observed hydrogen trapping capacity was linked to the hydrogen embrittlement susceptibility by comparing the uniaxial tensile behaviour of centreline samples with and without hydrogen charging applied as a pre-treatment step. Hydrogen charging was achieved by galvanostatic cathodic polarization at an applied current density of −10 mA/cm2 for 24 h in an NH4SCN-containing solution while simultaneously loading the samples to 85% of the yield strength using a proof ring tensile test cell. An increase in hydrogen embrittlement as a result of pre-charging was confirmed through tensile plots by comparing the area of reduction and failure strain of charged samples to uncharged samples. A decrease in both values was observed in the charged samples indicating a loss in ductility as a result of hydrogen charging. Fracture surfaces were imaged using SEM and inclusions of interest were analyzed for elemental composition using EDS. Inclusions observed along the fracture surfaces include oxysulfides of Ca and Al, oxides of Mg, Al-Ca-Si oxides, and Al2O3-containing inclusions which are likely to be heterogeneous Al-Ca-O inclusions. / Thesis / Master of Applied Science (MASc)