Duplex stainless steels are commonly used in environments that are expected to produce hydrogen i. e. in sour environments and sea water applications, often under cathodic protection. Under these conditions there is a concern about their susceptibility to hydrogen embrittlement. The effect of hydrogen, both external and internal, on the mechanical properties and the fracture characteristics of duplex stainless steels Type 2205 and 2507 have been studied by slow strain rate techniques using smooth tensile specimens. Specimens were strained to failure in air after high pressure hydrogen thermal charging, in a hydrogen atmosphere, in a hydrogen sulphide environment under open circuit potential condition, and whilst cathodically polarized at different potentials in distilled water with 100 wppm potassium sulphate added, in 3.5% aqueous sodium chloride, or in NACE solution. All the environments produced a major reduction in ductility that increases linearly with decrease in strain rate. The severity of the embrittlement depended upon whether the supply of hydrogen was external or internal. Internal hydrogen, as in thermally charged specimens, produced a more profound loss in ductility than straining in a hydrogen atmosphere and prolonged room temperature aging of these specimens, for up to 3 years, resulted in insignificant recovery of ductility, emphasizing the role of the austenite as a hydrogen reservoir. Provision of hydrogen at very high fugacities (cathodic polarization) during straining indicated that the potential at which loss in ductility is first noted corresponds to the hydrogen evolution potential for the particular solution involved. The presence of chloride ion seems to have no significant effect on the loss in ductility- The presence of hydrogen sulphide in the environment, however, introduced the complication of extensive chemical attack during and after crack propagation. The loss in ductility increased as the pH of the solution decreased and, irrespective of pH, maximum embrittlement occurred at some particular temperature between 20 and 90'C. The latter is attributed to the two competing processes of hydrogen ABSTRACT embrittlement and corrosion. A minimum chloride ion concentration of 300 wppm seems necessary to maintain the maximum embrittlement. The ultimate tensile strength of the steel is not affected by hydrogen since cracking only occurs after it is exceeded. Cracks initiate and grow preferentially through the ferrite phase, with fracture surfaces exhibiting quasi-cleavage features; the austenite often failed in a ductile mode. The proportion and distribution of the two phases has a significant effect on the degree of embrittlement. The presence of greater amounts of austenite seems to inhibit crack propagation, but may act as a hydrogen source or reservoir for the embrittlement of the ferrite phase. Straining of the as received weldments, which had been annealed after welding, showed no evidence of hydrogen embrittlement, but an attempt was made to simulate via heattreatment the structures that could occur in the heat affected zone of the weld and these structures had inferior mechanical properties in the presence of hydrogen.

The hydrogen embrittlement of two HSLA steels was studied in conditions typical of the marine environment. Double cantilever beam specimens, heat treated to produce the microstructure in the heat affected zone of a weld, were tested in seawater containing sulphate reducing bacteria (SRB) over a range of cathodic protection (CP) potentials and the threshold stress intensities ([Threshold Stress Intensity]) were recorded. The hydrogen concentration absorbed by the steel ([Surface Hydrogen Concentration]) was measured and shown to be higher at more negative CP potentials and significantly increased when SRB were present. An inverse relationship was established between log [Threshold Stress Intensity] and [Surface Hydrogen Concentration]. It was concluded that crack propagation occurs by a single mechanism whether or not SRB are present. Three point bend specimens of both steels were machined from welded plate. Corrosion fatigue tests were carried out in seawater with and without SRB. The presence of active SRB caused increased crack growth rates. Sediment samples were collected from the River Mersey and the base of a North Sea platform. In addition, SRB were added as an inoculum to artificial seawater. SRB numbers were enumerated and their activities assessed by measuring the concentrations of sulphide generated. Hydrogen permeation tests were performed on steel held at a range of CP potentials and exposed to each environment. Measurements were also carried out in seawater containing chemically prepared sulphides. Hydrogen absorption was shown to be enhanced when SRB were present and to be related to the total sulphide (TS) concentration in the environment. High hydrogen concentrations were produced by chemically prepared sulphides and the nature and thickness of the sulphide film appeared to be important in determining the extent of hydrogen absorption. Chemically produced sulphide gave sustained levels of absorbed hydrogen, but those generated biogenically decayed rapidly unless the TS concentration was maintained in the solution.

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A detailed investigation has been carried out to study the effects of heat treatment on the susceptibility to temper embrittlement (Ductile-Brittle transition temperature and low energy fracture characteristics) of eleven experimental steels. These experimental alloys represented a range of compositions related to engineering steels and corresponded to a nominal composition of 0.34 mass%C with alloying additions of Cr, Ni and Mo in varying combinations. These alloys were doped with P and Sb to study the effect of these additions on the susceptibility to temper embrittlement. These steels were investigated in the pearlitic, bainitic and tempered martensitic conditions to establish the role of microstructure. Heat-treated alloys were characterised by fracture studies including Izod impact testing and subsequent electron microscopy. Simultaneous Auger electron spectroscopy and energy dispersive X-ray analysis coupled with with Secondary Ion Mass Spectrometry were used to study the fracture surfaces and bulk compositions of the embrittled structures. It has been established that intergranular embrittlement in a quenched and tempered martensite microstructure was associated with the presence of P, whereas the initial intergranular embrittlement in a bainitic microstructure was associated with the segregation of Sb. It is suggested that the lower C activity produced in tempered martensite structures allows P migration to the grain boundaries causing intergranular embrittlement which was attributed to the development of M[7]C[3].Alloys in isothermally transformed bainitic condition showed that the predominant carbide precipitate was M3C which increased the C activity at the prior austenite grain boundaries with a resultant decrease in P concentration and consequently an absence of intergranular failure in the early stages of embrittlement. The increased C activity continued to prevent appreciable P segregation but was not sufficient to inhibit the co-segregation of Ni and Sb after extended ageing times when the bainitic alloys began to fail by intergranular fracture. After prolonged ageing increased Ni and Sb concentrations at the grain boundaries were associated with the formation of a fine grain boundary precipitate which was low in Cr. The tendency to fail by the low energy intergranular mode of failure was always greater in the tempered martensites, even when the bainites were significantly harder.

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Dans un contexte industriel en évolution permanente, les aciers austénitiques Fe-Mn-C sont développés afin d’obtenir une résistance mécanique élevée tout en conservant une ductilité considérable. Cependant, ces aciers présentent une sensibilité à différentes formes d'endommagement par l'hydrogène, notamment la corrosion sous contrainte. L'objectif de cette étude est de caractériser l'influence de la composition chimique et de l'état microstructural de ces alliages sur leur sensibilité aux phénomènes de fragilisation par l’hydrogène (FPH), lorsqu’ils sont soumis à des phénomènes de corrosion aqueuse. Ce travail est introduit par une présentation générale de la métallurgie des aciers austénitiques Fe-Mn-C ainsi que des méthodes expérimentales utilisées. Le traçage isotopique de l’hydrogène avec du deutérium par analyse SIMS a permis d’étudier les mécanismes de diffusion de l’hydrogène dans ces alliages. L’étude du comportement en corrosion aqueuse de ces alliages, par des tests électrochimiques et des immersions au potentiel libre dans des environnements aqueux deutérés, a mis en évidence l’influence des éléments d’addition sur la prise d’hydrogène associée à la corrosion.La sensibilité à la FPH de ces aciers a été caractérisée par des essais de traction in situ. Ils montrent que les interactions hydrogène-plasticité ont un rôle essentiel sur les mécanismes régissant la FPH. Cette étude a également montré une forte influence des éléments d’addition sur la FPH. Finalement, les résultats de cette étude nous ont permis de discuter des mécanismes associés à l’influence des éléments d’addition sur la sensibilité à la FPH de ces aciers. / The automotive industry is a sector in constant evolution, in which the lightening of structures by the use of new alloys, in order to save energy, is one of the main objectives. In this context, austenitic Fe-Mn-C steels are developed in order to obtain high mechanical strength associated with considerable ductility. However, these steels are sensitive to different forms of hydrogen damage, in particular stress corrosion cracking. The objective of this study is to characterize the influence of the chemical composition and the microstructural state of these alloys on their sensitivity to hydrogen embrittlement (HE) phenomena associated with corrosion process in aqueous media. This work starts with a general presentation of the metallurgical properties of austenitic Fe-Mn-C steels and of the experimental techniques. Then, the isotopic tracing of hydrogen with deuterium by SIMS analysis allowed studying hydrogen diffusion mechanisms in these alloys. The corrosion of these steels in aqueous media have been studied by electrochemical tests and immersions at the rest potential in deuterated solution ; the influence of alloying elements on the hydrogen absorption during corrosion war characterized in detail. In situ tensile tests were used to characterize the HE susceptibility of these steels. They show that hydrogen-plasticity interactions play a predominant role in the HE mechanisms. This study also showed a strong influence of alloying elements on HE. Finally, the results of this study allowed discussing the mechanism involved in the role of alloying elements on the HE susceptibility of these steels.

Hydrogen Embrittlement (HE) is a very common failure mechanism induced crack propagation in materials that are utilized in oil and gas industry structural components and equipment. Considering the prediction of HE behavior, which is suggested in this study, is one technique of monitoring HE of equipment in service. Therefore, multi-scale constitutive models that account for the failure in polycrystalline Body Centered Cubic (BCC) materials due to hydrogen embrittlement are developed. The polycrystalline material is modeled as two-phase materials consisting of a grain interior (GI) phase and a grain boundary (GB) phase. In the first part of this work, the hydrogen concentration in the GI (Cgi) and the GB (Cgb) as well as the hydrogen distribution in each phase, were calculated and modeled by using kinetic regime-A and C, respectively. In the second part of this work, this dissertation captures the adverse effects of hydrogen concentration, in each phase, in micro/meso and macro-scale models on the mechanical behavior of steel; e.g. tensile strength and critical porosity. The models predict the damage mechanisms and the reduction in the ultimate strength profile of a notched, round bar under tension for different hydrogen concentrations as observed in the experimental data available in the literature for steels. Moreover, the study outcomes are supported by the experimental data of the Fractography and HE indices investigation. In addition to the aforementioned continuum model, this work employs the Molecular Dynamics (MD) simulations to provide information regarding bond formulation and breaking. The MD analyses are conducted for both single grain and polycrystalline BCC iron with different amounts of hydrogen and different size of nano-voids. The simulations show that the hydrogen atoms could form the transmission in materials configuration from BCC to FCC (Face Centered Cubic) and HCP (Hexagonal Close Packed). They also suggest the preferred sites of hydrogen for each case. The connections between the results for different scales (nano, micro/meso and macro-scale) were suggested in this dissertation and show good agreements between them. We finally conclude that hydrogen-induced steel fracture and the change of fracture mode are caused by the suppression of dislocation emission at crack tip and changing in the material structure due to accumulation of hydrogen, which is driven by the stress fields. This causes the brittle fracture to occur as inter-granular in the GB and trans-granular in the GI.

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