Zirconium alloys are susceptible to engineering problems associated with the uptake of hydrogen throughout their design lifetime in nuclear reactors. Understanding of hydrogen embrittlement associated with the precipitation of brittle hydride phases and a sub-critical crack growth mechanism known as Delayed Hydride Cracking (DHC) is required to provide the engineering justifications for safe reactor operation. The nature of bulk zirconium hydrides at low concentrations (< 100 wt. ppm) is subject to several contradictory descriptions in the literature associated with the stability and metastability of γ-phase zirconium hydride. Due to the differing volume expansions (12-17%) and crystallography between γ and δ hydride phases, it is suggested that the matrix yield strength may have an effect on the phase stability. The present work indicated that although yield strength can shift the phase stability, other factors such as microstructure and phase distribution can be as or more important. This suggests that small material differences are the reason for the literature discrepancies. DHC is characterised by the repeated precipitation, growth, fracture of brittle hydride phases and subsequent crack arrest in the ductile metal. DHC growth is associated primarily the ability of hydrogen to diffuse under a stress induced chemical potential towards a stress raiser. Knowledge of the factors controlling DHC are paramount in being able to appropriately describe DHC for engineering purposes. Most studies characterise DHC upon cooling to the test temperature. DHC upon heating has not been extensively studied and the mechanism by which it occurs is somewhat controversial in the literature. This work shows that previous thermo-mechanical processing of hydrided zirconium can have a significant effect on the dissolution behaviour of the bulk hydride upon heating. DHC tests with γ-quenched, furnace cooled-δ and reoriented bulk hydrides upon heating and DHC upon cooling suggest that the amount of hydrogen in solution is the primary factor controlling the occurrence of DHC and consistent with the postulation that the stress induced chemical potential is the driving force for DHC. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2012-01-24 06:14:14.152

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Stress corrosion cracking (SCC) from the external surface of a buried pipeline is a serious matter and can cause significant economic and environmental losses. Despite of many research works which have been done on the understanding of crack initiation and propagation mechanisms, these mechanisms are still being debated. This research studied the crack growth behaviour of different pipeline steels including two types of X65, one X52 and one X80 pipeline steels in near-neutral pH environments. Crack growth behaviour of all steels has been found to be consistent with that of true corrosion fatigue. Crack growth rates were correlated with (K)2Kmax/f0.1. It was revealed that cracking behaviour of pipeline steels in near neutral pH environments is material dependent. Highest crack growth rate was seen in the steel which highest amount of hydrogen atoms could be generated and stored in its microstructure to contribute in cracking procedure due to hydrogen embrittlement effect. / Materials Engineering

The influence of steel metallurgy on rock bolt SCC was studied using a series of commercial carbon and low-alloyed steels. The chemical composition, their mechanical properties and the microstructures of these steels varied considerably in order to gather information for the discussion of the metallurgical influences under Hydrogen Embrittlement (HE) and Stress Corrosion Cracking (SCC) conditions. In order to understand the metallurgical influences on Rock Bolt SCC, an evaluation was carried out to fifteen commercial steels. The experiments reproduced the Stress Corrosion Cracking condition at which commercial rock bolts had failed in Australians mines. Due to the selected materials, stress and electrolyte condition it is expected that Hydrogen Embrittlement (HE) will affect the steel failure. The approach was to use the Linearly Increasing Stress Test (LIST) and exposing the sample to a dilute pH 2.1-sulphate solution, in accordance with prior studies. Stress Corrosion Cracking was evaluated by analysing the decrease in tensile strength, loss of ductility and fractography observed using Scanning Electron Microscopy (SEM). The initial series of test to the fifteen steels were performed at the free corrosion potential (f.c.p.) vs. Ag/AgCl. From this initial test only five steels (AISI 1008, AISI 4140, AISI 4145H, pipeline X-65 and X-70) did not show Stress Corrosion Cracking features. These five steel were tested in accordance with the Linearly Increased Stress Test (LIST) in the dilute pH 2.1 sulphate solution at different electronegative applied potential to minimum value of -1500mV. The experimental procedure tried to reproduce the Stress Corrosion Cracking condition to identify the most aggressive condition the steel is able to support before failing due to Stress Corrosion Cracking to then compare the theory of SCC and HE in low carbon and low alloy steel with the obtained experimental results. The investigation compared the well-known theory of SCC and HE in low carbon and low alloy steel with the obtained experimental results. Surprisingly, the experimental result did not always agree with the theory.

Ce travail de thèse a porté sur l’impact de traces d’oxygène sur la corrosion de fer pur ou d’acier en milieu H 2 S, ainsi que sur le chargement en hydrogène. Trois axes de recherche ont été suivis.Une première partie est consacrée à la chimie de la réaction H 2 S+O 2 . Nous avons d’abord identifié dans la littérature les chemins réactionnels possibles de la réaction H2S+O2 , et les méthodes d’analyse des espèces S-O en solution. Nous avons ensuite utilisé des méthodes thermodynamiques pour prédire les espèces les plus stables, qui ont été validées par des essais expérimentaux. Il ressort de cette analyse que les principaux produits solubles de la réaction H2S+O2 sont les ions sulfate, thiosulfate etsulfite, ainsi que les ions H + . La décroissance continue du pH de la solution d’essai dans un système corrosif en milieu H2S peut être un indicateur d’une pollution par l’oxygène.La seconde partie utilise des méthodes électrochimiques pour étudier les phénomènes de corrosion et de formation de dépôts en présence ou en absence d’oxygène. La vitesse de corrosion est systématiquement plus grande en présence de O2 . Un modèle d’impédance a été developpé pour l’étude de l’évolution de l’interface métal/électrolyte, et ce modèle est en accord avec la littérature et est validé par l’évaluation de la vitesse de corrosion par perte de masse.Enfin, la troisième partie est consacrée au chargement en hydrogène dans le fer pur ou dans un acier pétrolier de type X65. En absence de pollution par l’oxygène, on observe des rendements très proches de 100 % entre le courant de perméation et le courant de corrosion estimé à partir des mesures d’impédance, et ce sur plusieurs semaines. En présence d’O2 , ce rendement est systématiquement beaucoup plus faible. Par ailleurs, quelques tests menés sur l’acier pétrolier montrent également un effet important de l’oxygène sur le caractère plus ou moins protecteur du dépôt, qui n’avait pas été observé sur système modèle utilisant du fer pur / The work in this thesis presents the effect of trace concentrations of oxygen (O2 ) on corrosion and hydrogen uptake of pure iron/steel in H2S-media. Three main avenues of research were conducted.The first part is concerned with the reaction chemistry between dissolved H2S and O2. A literature review is presented, identifying a number of reaction pathways associated with the dissolved H2S and O2 reaction as well as analytical methods used to measure the resulting S-O reaction products. Subsequently, thermodynamic calculations allow for the prediction of the most stable species indissolved H2S -O2 solutions, later confirmed using spectroscopic and chromatographic methods. Such techniques reveal that the principal soluble reaction products are sulfate, thiosulfate and sulfite anions,as well as hydronium ions. From pH monitoring, a continuous rise in acidity of an H2S solution can be an indication of O2 ingress.Secondly, electrochemical methods are used to explore corrosion phenomena and deposit formation in the presence and absence of O2 pollution. Corrosion rates in the presence of O2 are reproducibly much higher if compared against those in the system lacking O2 . An electrochemical impedance spectroscopy model has been developed to study the evolution of the metal / electrolyte interface over time, which is validated using weight loss corrosion rate measurements, and represents impedance data reported in the literature on similar systems.Finally, the third part is devoted to hydrogen uptake in pure iron and petroleum industry sour-grade steel (X65). In the absence of O2 pollution, permeation efficiency yields close to 100% are observed between the permeation current and the corrosion current estimated from the impedance measurements over a period of several weeks. In the presence of O2 , however, this yield is reproducibly far lower. Moreover, some tests conducted on X65 also show a significant effect of O2 on the quite protective nature of the deposit, not observed on the model system using pure iron

A fragilização por hidrogênio é uma potencial causa de falha em diversos componentes industriais. Este tipo de fragilização ocorre principalmente em aços ao carbono e também em aços inoxidáveis martensíticos, que apresentam alta resistência mecânica. Neste trabalho, avaliou-se a influência das temperaturas de revenimento de 300 ºC, 400 ºC e 500 ºC na fragilização por hidrogênio do aço ABNT 10B22-Modificado, após o processo de carbonitretação, o qual é muito utilizado na fabricação de parafusos e fixadores. Foram confeccionados corpos de prova convencionais segundo a Norma NACE TM 177-90 e também foram usados parafusos M4 X50 fabricados conforme a Norma DIN 13 para realização dos ensaios. As amostras foram submetidas a três níveis de introdução forçada de hidrogênio (10mA/cm2, 20mA/cm2 e 30mA/cm2) por controle galvanostático, para as diferentes temperaturas de revenimento. Para efeito comparativo, para cada temperatura de revenido foram produzidas amostras isentas de hidrogênio, caracterizando a condição inicial do material. Todas as amostras foram submetidas ao ensaio de tração com baixa taxa de deformação. Os corpos de prova convencionais foram também ensaiados em tração com carregamento simultâneo de hidrogênio, enquanto os parafusos foram submetidos ao ensaio de torque de fragilização com carregamento prévio. Foram caracterizados os micromecanismos de fratura em todas as condições das amostras confeccionadas. Os resultados obtidos evidenciaram que para menores temperaturas de revenimento as amostras carregadas com hidrogênio apresentaram uma maior perda das propriedades mecânicas como resistência à tração, tenacidade e alongamento quando comparadas a condição inicial, sendo que a geometria do parafuso foi mais propensa aos efeitos deletérios do hidrogênio que os corpos de prova convencionais. O micromecanismo de fratura na camada carbonitretada foi predominantemente intergranular nas amostras carregadas com hidrogênio e o núcleo apresentou coalescimento de microcavidades na maioria dos casos, com microcavidades rasas nas amostras carregadas com hidrogênio. / Hydrogen embrittlement is a potential cause of failure in various industrial components. This type of embrittlement occurs mainly in carbon steels and in martensitic stainless steels, which have high mechanical strength. In this study, it was evaluated the influence of the tempering temperatures of 300 ºC, 400 ºC and 500 ºC on the hydrogen embrittlement effect, for the ABNT 10B22-modified steel, after the process of carbonitriding, which is widely used in the manufacture of screws and fasteners. The specimens were machined according to the NACE TM 177-90 standard and were also employed as specimens in the tests, screws M4 X50 manufactured according to the DIN 13 standard. The samples were subjected to three levels of hydrogen charge (10mA/cm2, 20mA/cm2 and 30mA/cm2) by galvanostatic control for the different tempering temperatures. For comparison, each tempering temperature sample was produced free of hydrogen, characterizing the initial condition of the material. All samples were subjected to slow strain rate tensile test. The specimens were also analyzed in conventional tensile tests with simultaneous charging of hydrogen, while the screws were subjected to the embrittlement torque testing with precharged specimens. Micromechanisms of fracture were characterized in all conditions for each of the samples. The results showed that lower tempering temperatures caused greater losses of mechanical properties such as tensile strength, elongation and toughness for the hydrogen charged samples when compared to the initial condition, and the geometry of the screw was more prone to the deleterious effects of hydrogen that conventional standardized specimens. The case fracture micromechanism was predominantly intergranular in samples charged with hydrogen and the core showed coalescence of microcavities micromechanism, in most cases, with shallow microvoids in the samples charged with hydrogen.

The phenomenon of hydrogen embrittlement in steel has been known for over 150 years. Hydrogen-resistant alloys have been developed to mitigate this effect and three types of alloys with optimised structures have been enhanced over the years: nickel alloys, stainless steels, and quenched and tempered martensitic low alloy steels. Nevertheless, those alloys are limited in terms of strength and ductility. The aim of the work presented in this thesis was to design bainitic alloys with hydrogen embrittlement resistance, and with a better combination of strength and ductility than conventional alloys. In the novel alloys, two microstructural features were produced to mitigate the damaging effects of hydrogen: 1. A percolating austenite structure, in which hydrogen diffusion is orders of magnitude lower than in bainitic ferrite. This feature was introduced to impede the ingress of hydrogen through the structure. 2. Iron carbide traps, which can form at the bainite transformation temperature. This feature was introduced to trap diffusible hydrogen and prevent it from causing damage. The alloys, designed with the aid of computer models and phase transformation theory, contained a volume fraction of retained austenite above its percolation threshold, theorised as 0.1, which was proven to form an effcient barrier to hydrogen ingress. The effective diffusivity of hydrogen, measured using an electrochemical permeation technique, was shown to decrease with increasing austenite fraction up to the percolation threshold. It was seen to plateau for austenite fractions comprised between 0.1 and 0.18, and to decrease further for fractions above 0.18. The compositions of the alloys were precisely selected to allow for iron carbides to precipitate during the bainitic transformation reaction. Until the present work, only alloy carbides V4C3, TiC and NbC had been reported to strongly trap hydrogen. The literature was very inconsistent regarding the trapping ability of cementite, with reported trap binding energies ranging from 11 to 66 kJ mol−1. The carbides produced in the alloys were identified as cementite. The cementite fraction was measured to be 0.001 ± 0.0001 for one of the designed alloys, which is the lowest ever reported carbide fraction in steel measured using a simple X-ray diffraction technique. Experimental thermal desorption spectroscopy data were used to determine the binding energy of hydrogen to cementite to be 37.5 kJ mol−1, suggesting that cementite is not a strong hydrogen trap. Further tests performed after room temperature hydrogen degassing displayed insignifcant amount of trapped hydrogen, thus confrming the reversible nature of cementite traps. The comparison of two successive transients using the electrochemical permeation technique confirmed that result. The influence of the heat treatments on the microstructures and on the mechanical properties of the designed alloys was extensively studied. The novel alloys met all the set requirements, and successfully outperformed conventional alloys in terms of strength and ductility. They did not meet the NACE TM0316-2016 standard requirement for operation in hydrogen-rich environments, likely owing to the inadequate trapping ability of cementite. Future work should focus on exploring the possible use of alternative carbides for hydrogen trapping in bainitic structures.

abstract: Hydrogen embrittlement (HE) is a phenomenon that affects both the physical and chemical properties of several intrinsically ductile metals. Consequently, understanding the mechanisms behind HE has been of particular interest in both experimental and modeling research. Discrepancies between experimental observations and modeling results have led to various proposals for HE mechanisms. Therefore, to gain insights into HE mechanisms in iron, this dissertation aims to investigate several key issues involving HE such as: a) the incipient crack tip events; b) the cohesive strength of grain boundaries (GBs); c) the dislocation-GB interactions and d) the dislocation mobility. The crack tip, which presents a preferential trap site for hydrogen segregation, was examined using atomistic methods and the continuum based Rice-Thompson criterion as sufficient concentration of hydrogen can alter the crack tip deformation mechanism. Results suggest that there is a plausible co-existence of the adsorption induced dislocation emission and hydrogen enhanced decohesion mechanisms. In the case of GB-hydrogen interaction, we observed that the segregation of hydrogen along the interface leads to a reduction in cohesive strength resulting in intergranular failure. A methodology was further developed to quantify the role of the GB structure on this behavior. GBs play a fundamental role in determining the strengthening mechanisms acting as an impediment to the dislocation motion; however, the presence of an unsurmountable barrier for a dislocation can generate slip localization that could further lead to intergranular crack initiation. It was found that the presence of hydrogen increases the strain energy stored within the GB which could lead to a transition in failure mode. Finally, in the case of body centered cubic metals, understanding the complex screw dislocation motion is critical to the development of an accurate continuum description of the plastic behavior. Further, the presence of hydrogen has been shown to drastically alter the plastic deformation, but the precise role of hydrogen is still unclear. Thus, the role of hydrogen on the dislocation mobility was examined using density functional theory and atomistic simulations. Overall, this dissertation provides a novel atomic-scale understanding of the HE mechanism and development of multiscale tools for future endeavors. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2015

A fragilização por hidrogênio é uma potencial causa de falha em diversos componentes industriais. Este tipo de fragilização ocorre principalmente em aços ao carbono e também em aços inoxidáveis martensíticos, que apresentam alta resistência mecânica. Neste trabalho, avaliou-se a influência das temperaturas de revenimento de 300 ºC, 400 ºC e 500 ºC na fragilização por hidrogênio do aço ABNT 10B22-Modificado, após o processo de carbonitretação, o qual é muito utilizado na fabricação de parafusos e fixadores. Foram confeccionados corpos de prova convencionais segundo a Norma NACE TM 177-90 e também foram usados parafusos M4 X50 fabricados conforme a Norma DIN 13 para realização dos ensaios. As amostras foram submetidas a três níveis de introdução forçada de hidrogênio (10mA/cm2, 20mA/cm2 e 30mA/cm2) por controle galvanostático, para as diferentes temperaturas de revenimento. Para efeito comparativo, para cada temperatura de revenido foram produzidas amostras isentas de hidrogênio, caracterizando a condição inicial do material. Todas as amostras foram submetidas ao ensaio de tração com baixa taxa de deformação. Os corpos de prova convencionais foram também ensaiados em tração com carregamento simultâneo de hidrogênio, enquanto os parafusos foram submetidos ao ensaio de torque de fragilização com carregamento prévio. Foram caracterizados os micromecanismos de fratura em todas as condições das amostras confeccionadas. Os resultados obtidos evidenciaram que para menores temperaturas de revenimento as amostras carregadas com hidrogênio apresentaram uma maior perda das propriedades mecânicas como resistência à tração, tenacidade e alongamento quando comparadas a condição inicial, sendo que a geometria do parafuso foi mais propensa aos efeitos deletérios do hidrogênio que os corpos de prova convencionais. O micromecanismo de fratura na camada carbonitretada foi predominantemente intergranular nas amostras carregadas com hidrogênio e o núcleo apresentou coalescimento de microcavidades na maioria dos casos, com microcavidades rasas nas amostras carregadas com hidrogênio. / Hydrogen embrittlement is a potential cause of failure in various industrial components. This type of embrittlement occurs mainly in carbon steels and in martensitic stainless steels, which have high mechanical strength. In this study, it was evaluated the influence of the tempering temperatures of 300 ºC, 400 ºC and 500 ºC on the hydrogen embrittlement effect, for the ABNT 10B22-modified steel, after the process of carbonitriding, which is widely used in the manufacture of screws and fasteners. The specimens were machined according to the NACE TM 177-90 standard and were also employed as specimens in the tests, screws M4 X50 manufactured according to the DIN 13 standard. The samples were subjected to three levels of hydrogen charge (10mA/cm2, 20mA/cm2 and 30mA/cm2) by galvanostatic control for the different tempering temperatures. For comparison, each tempering temperature sample was produced free of hydrogen, characterizing the initial condition of the material. All samples were subjected to slow strain rate tensile test. The specimens were also analyzed in conventional tensile tests with simultaneous charging of hydrogen, while the screws were subjected to the embrittlement torque testing with precharged specimens. Micromechanisms of fracture were characterized in all conditions for each of the samples. The results showed that lower tempering temperatures caused greater losses of mechanical properties such as tensile strength, elongation and toughness for the hydrogen charged samples when compared to the initial condition, and the geometry of the screw was more prone to the deleterious effects of hydrogen that conventional standardized specimens. The case fracture micromechanism was predominantly intergranular in samples charged with hydrogen and the core showed coalescence of microcavities micromechanism, in most cases, with shallow microvoids in the samples charged with hydrogen.

CoordenaÃÃo de AperfeiÃoamento de NÃvel Superior / Os aÃos maraging sÃo ligas de ultra-alta resistÃncia com vasta aplicaÃÃo na engenharia, desde vasos de alta pressÃo de operaÃÃo em processos crÃticos, componentes aeronÃuticos, aplicaÃÃes militares atà equipamentos esportivos. O presente trabalho buscou avaliar os efeitos da fragilizaÃÃo por hidrogÃnio no aÃo maraging 18% Ni da classe 300, nas condiÃÃes solubilizada e envelhecida. As amostras foram solubilizadas a 1093 Â10K por 3,6 ks, seguido de um resfriamento ao ar e envelhecidas a 753 e 843 Â10K por 10,8 ks, respectivamente e resfriadas ao ar. Foi realizada uma caracterizaÃÃo microestrutural por difraÃÃo de raios-X, correntes parasitas, medidas de dureza Rockwell e microscopia eletrÃnica e Ãptica. Para avaliar os efeitos do ingresso de hidrogÃnio nas propriedades mecÃnicas do aÃo maraging 18% Ni da classe 300 foram realizados ensaios de traÃÃo com baixa taxa de deformaÃÃo (BTD). A taxa de deformaÃÃo aplicada foi 1,0 x 10-6 s-1. Os ensaios foram realizados ao ar (meio inerte) e em soluÃÃo de 3,5% NaCl sob o potencial catÃdico de -1,2 VECS. Foi observada uma reduÃÃo de 11,06 para 3,89% no alongamento e de 61,28 para 10% na reduÃÃo de Ãrea para as amostras solubilizadas. As amostras envelhecidas a 753 Â10K por 10.8 ks apresentaram maior reduÃÃo nessas propriedades. Nesta condiÃÃo a reduÃÃo observada foi de 1929,26 MPa para amostras ensaiadas ao ar para 447,64 MPa para amostras ensaiadas em soluÃÃo de 3,5% NaCl sob potencial catÃdico no limite de resistÃncia e de 7,30 para 1,62 % no alongamento. As amostras envelhecidas a 843 Â10K, as quais apresentaram de cerca de 10% de austenita sofreram fragilizaÃÃo similar as amostras envelhecidas a 753 Â10K. Trincas secundÃrias perpendiculares a carga aplicada foram observadas nas amostras solubilizadas e ensaiadas em soluÃÃo de 3,5% NaCl sob potencial catÃdico. Os resultados indicam que a presenÃa de precipitados e de austenita revertida impedem a propagaÃÃo de trincas secundÃrias na seÃÃo longitudinal nas condiÃÃes envelhecidas. A anÃlise da superfÃcie de fratura revelou caracterÃstica de uma fratura dÃctil nas amostras ensaiadas ao ar com dimples de diferentes tamanhos e profundidades, enquanto que nas amostras ensaiadas em soluÃÃo de 3,5% NaCl sob potencial catÃdico foram observadas trincas induzidas pelo hidrogÃnio e microcavidades e regiÃes de quase-clivagem para todas as condiÃÃes estudadas. / Maraging steels are ultra high strength alloys widely used in engineering applications from high pressure vessels operating in critical processes, aircraft components, military applications to sports equipment. This work assessed the effects of hydrogen embrittlement in 18% Ni maraging grade 300 steel in the solution annealed and aged conditions. Samples were solution annealed at 1093 Â10K for 3.6 ks, followed by air cooling and aging at 753 and 843 Â10K for 10.8 ks, respectively, and cooled by air. The microstructure was characterized by X-ray diffraction, eddy current, hardness measurement and optical and electron microscopy. To assess the effects of hydrogen ingress on the mechanical properties of 18% Ni maraging grade 300 steel, slow strain rate tests (SSRT) were performed. A strain rate of 1.0x10-6 s-1 was applied. The tests were carried out in air (middle inert) and the samples immersed in the electrolyte at a simultaneous potential of -1.2 VSEC. The results showed the reduction elongation from 11.06 to 3.89% and from 61,28 to 10% in reduction of area for samples in the solution annealed condition. The greatest reductions were observed in the samples aged at 753 Â10K for 10.8 ks. In this condition the reduction from 1929.26 MPa in air tests to 447.64 MPa in ultimate tensile strength and from 7.30 to 1.62% in elongation under cathodic polarization in the 3,5% NaCl solution was observed. The samples aged at 843 Â10K for 10.8 ks, where about 10% of reverted austenite was identified, showed evidence of hydrogen embrittlement as seen in the samples treated at different conditions. Secondary cracks, perpendicular to the loading direction at the longitudinal surface of the solution annealed fractured samples immersed in 3,5% NaCl solution under cathodic potential were seen. The results evidence that the precipitates and reverted austenite difficult secondary crack propagation in longitudinal section on aged samples. Scanning electron examination showed a change in fractografic features from ductile dimples to quasi-cleavage and microvoid modes when comparing samples without (air tested) and with hydrogen ingress.