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.).

La fabrication des composants d'une turbine hydraulique est un procédé complexe, couteux et nécessite beaucoup de métal. Une possibilité d'améliorer ce procédé est d' obtenir des formes complexes, ce qui va réduire la quantité de soudage et de post-traitement. Le nouveau procédé nécessite l'étude de nouveaux matériaux, qui possèdent une déformabilité élevée et répondent aux exigences du matériau pour une application dans les turbines hydrauliques. Les aciers supermartensitiques sont attractifs, car ils possèdent la combinaison exceptionnelle d'une haute résistance à la corrosion, une bonne soudabilité, une résistance à la traction élevée et un comportement ductile.Le but de ce travail est la détermination et la modélisation du comportement d'un acier supermartensique à hautes températures pour contrôler et optimiser le procédé de mise en forme à chaud de composants d'une roue d'une turbine hydraulique.Des paramètres différents affectent la mise en forme à chaud de l'acier. D'un côté, la connaissance de la microstructure et des transformations de phase est nécessaire pour optimiser la température du mise en forme et analyser les résultats, d'un autre côté, des essais mécaniques doivent être réalisés pour déterminer le comportement à haute température de l'acier et ses lois de comportement, ce qui permettra de modéliser le procédé de mise en forme. Ainsi, une étude détaillée de la microstructure et des transformations de l'acier supermartensique est d'abord présentée. Il a été démontré que le traitement thermique peut améliorer la microstructure du matériau et supprimer les hétérogénéités provenant de la fabrication de l'acier. Ensuite, le comportement mécanique à haute température a été étudié. Des essais de traction ont été effectués dans l'intervalle de température de 650 ° C à 1100 ° C à des vitesses de déformation entre 0,0004s-1 et 0,04s-1. Les influences de la température, de la vitesse de déformation, de la taille des grains, de la teneur et de la morphologie des phases sur le comportement mécanique ont été analysés. En outre, la caractérisation qualitative de l'endommagement a été effectuée dans le domaine austénitique. Finalement les lois de comportement ont été déterminées et la modélisation de la mise en forme à chaud d'un acier supermartensitique a pu être entreprise. Des exemples de deformation en flexion, correspondant à des formes réelles complexes, sont présentés.Les résultats de l'étude des aciers inoxydables supermartensitiques et de leurs lois de comportement à chaud permettent d'optimiser le processus de formage. / The current manufacturing of hydraulic turbine components is complex, metal-consuming and expensive process. The one way of improving this process is forming complex shapes that will decrease the amount of welding and post treatment. The new forming process requires the investigation of new materials having good deformability and satisfying the requirements to material for hydraulic turbine application. The supermartensitic steels are attractive as they have exceptional combination of high corrosion resistance, good weldability, tensile strength and ductile behavior.The aim of this work is the determination and modeling of supermartensitic stainless steel behavior at high temperature to control and optimize process of hot forming the parts of hydraulic turbine runner.Different parameters affect the hot forming of steel. On the one hand, the knowledge about steel microstructure and transformation is needed to optimize temperature of forming and analyze results, and on the other hand, the mechanical experiments need to be performed to determine high temperature behavior of steel and constitutive law which allow to model process of forming. Thus firstly detailed study of microstructure and transformation of supermartensitic steel is presented. It has been demonstrated that the heat treatment can improve microstructure of material and exclude disadvantage of steel making. Then the mechanical behavior at high temperature was investigated. Tensile tests were carried out in the interval of temperature from 650°C to 1100°C at strain rate in range from 0,0004s-1 to 0,04s-1. The influences of temperature, strain rate, grain size, content and morphology of phases on mechanical behavior have been analyzed. Additionally brief investigation of cavities at full austenite zone has been done. Finally the constitutive laws were determined and modeling of behavior is presentede Examples of bending process are reported, corresponding to real and complex shapes of turbine parts.The presented investigation of supermartensitic stainless steel and proposed constitutive laws allow optimization the process of hot forming.

Ce travail de thèse porte sur l’étude de divers mécanismes de fragilisation d’un acier inoxydable supermartensitique 16Cr-4Ni à basse teneur en carbone (Virgo™38) utilisé pour la fabrication des compresseurs centrifuges produits par GE Oil&Gas.Il a été montré que la microstructure fine, polyphasée et morphologiquement très complexe de cet alliage était sensible aux conditions de traitement thermique. Notamment, la proportion et la répartition de l’austénite de réversion, la présence de carbure de chrome et la ségrégation intergranulaire d’éléments fragilisant tel que le phosphore ont été mis en évidence suite à plusieurs traitements thermiques. La ségrégation du phosphore aux joints de grain (SPJG) est l’un des mécanismes entrainant la fragilisation du Virgo™38 sous certaines conditions de traitement thermique.La présence d’austénite de réversion n’entraine pas seulement une amélioration de la ténacité de l’alliage mais également le piégeage de l’hydrogène dans cette phase. Etant donnée la grande instabilité mécanique de l’austénite, sa transformation sous contrainte provoque la libération de cet hydrogène piégé et ainsi une surconcentration locale en hydrogène. La fragilisation par hydrogène est le deuxième mécanisme entrainant la fragilisation du Virgo™38 étudié dans cette thèse.Les mécanismes mis en jeu lors de la formation de l’austénite de réversion conduisent à la répartition hétérogène des éléments d’alliage. L’une des conséquences de cette répartition hétérogène est la corrosion sélective de l’une ou l’autre des phases de l’alliage en fonction du pH de l’environnement et de la charge appliquée. La dissolution préférentielle est le dernier mécanisme abordé. / This PhD thesis is focused on the study of embrittlement mechanisms in a 16Cr-4Ni low carbon supermartensitic stainless steel (Virgo™38) used for the construction of rotating parts of centrifugal compressors produced by GE Oil&Gas.It is shown that the morphologically complex multiphase microstructure of this alloy is very sensitive to the heat treatment conditions. In particular, the fraction and repartition of retained austenite, the presence of chromium carbides and the phosphorous grain boundary segregation (PGBS) are evidenced consecutive to several heat treatments. PGBS is one of the mechanisms leading to brittle failure of Virgo™38 after specific heat treatments.The presence of retained austenite is not only beneficial for toughness but also leads to hydrogen trapping in this phase. Due to the mechanical instability of retained austenite, its transformation under applied stress leads to the release of trapped hydrogen into newly formed martensite and thus to a local overconcentration of hydrogen. Hydrogen embrittlement is the second mechanism studied in this report.Mechanisms involved during the formation of retained austenite lead to the heterogeneous repartition of alloying elements (partitioning). One of the consequences being the selective dissolution of one or the other phase of the alloy as a function of the pH of the environment and of the load applied. Preferential dissolution is the third mechanism studied.

O desenvolvimento dos aços inoxidáveis Super-Martensíticos (SM) nasce da necessidade de implementar novas tecnologias, mais econômicas e amigáveis ao meio ambiente. Os aços inoxidáveis SM são uma derivação dos aços inoxidáveis martensíticos convencionais, diferenciando-se basicamente no menor teor de carbono, na adição de Ni e Mo. Foram desenvolvidos como uma alternativa para aços inoxidáveis duplex no uso de dutos para a extração de petróleo offshore em meados dos anos 90. Para que esses aços apresentem as propriedades mecânicas de resistência à tração e tenacidade é necessário que sejam realizados tratamentos de austenitização, seguido de têmpera, e de revenimento, onde, particularmente para este último, há várias opções de tempos e temperaturas. Como os tratamentos térmicos geram as propriedades mecânicas através de transformações de fase (precipitação) podem ocorrer alterações da resistência à corrosão. São conhecidos os efeitos benéficos da adição de Nb em aços inoxidáveis tradicionais. Por isso, o objetivo desta pesquisa foi estudar aços inoxidáveis SM contendo Nb. Foi pesquisada a influência da temperatura de revenimento sobre a resistência à corrosão de três aços inoxidáveis SM, os quais contêm 13% Cr, 5% Ni, 1% a 2% Mo, com e sem adições de Nb. No presente trabalho, foram denominados de SM2MoNb, SM2Mo e SM1MoNb, que representam aços com 2% Mo, 1% Mo e 0,11% Nb. Dado que os principais tipos de corrosão para aços inoxidáveis são a corrosão por pite (por cloreto) e a corrosão intergranular (sensitização), optou-se por determinar os Potenciais de Pite (Ep) e os Graus de Sensitização (GS) em função da temperatura de revenimento. Os aços passaram por recozimento a 1050°C por 48 horas, para eliminação de fase ferrita delta. Em seguida foram tratados a 1050 °C por 30 minutos, com resfriamento ao ar, para uniformização do tamanho de grão. A estrutura martensítica obtida recebeu tratamentos de revenimento em temperaturas de: 550 °C, 575 °C, 600 °C, 625 °C, 650 °C e 700 °C, por 2 horas. O GS foi medido através da técnica de reativação eletroquímica potenciodinâmica na versão ciclo duplo (DL-EPR), utilizando-se eletrólito de 1M H2SO4 + 0,01M KSCN. Para determinar o Ep foram realizados ensaios de polarização potenciodinâmica em 0,6M NaCl. Os resultados obtidos foram discutidos através das variações microestruturais encontradas. Foram empregadas técnicas de microscopia ótica (MO), microscopia eletrônica de varredura (MEV), simulação termodinâmica de fases através do programa Thermo-Calc e determinação de austenita revertida mediante difração de raios X (DRX) e ferritoscópio. A quantificação da austenita por DRX identificou que a partir de 600 °C há formação desta fase, apresentando máximo em 650 °C, e novamente diminuindo para zero a 700 °C. Por sua vez, o método do ferritoscópio detectou austenita nas condições em que a analise de DRX indicou valor nulo, sendo as mais críticas a do material temperado (sem revenimento) e do aço revenido a 700 °C. Propõe-se que tais diferenças entre os dois métodos se deve à morfologia fina da austenia retida, a qual deve estar localizada entre as agulhas de martensita. Os resultados foram discutidos em termos da precipitação de Cr23C6, Mo6C, NbC, fase Chi, austenita e ferrita, bem como das consequências do empobrecimento em Cr e Mo, gerados por tais microconstituintes. São propostos três mecanismos para explicar a sensitização: o primeiro é devido a precipitação de Cr23C6, o segundo a precipitação de fase Chi (rica em Cr e Mo) e o terceiro é devido a formação de ferrita durante o revenimento. O melhor desempenho quanto ao GS foi obtido para os revenimentos a 575 °C e 600°C, por 2 horas. Os resultados de Ep indicaram que o aço SM2MoNb, revenido a 575°C, tem o melhor desempenho quanto à resistência à corrosão por cloreto. Isso associado ao baixo GS coloca este aço, com este tratamento térmico, numa posição de destaque para aplicações onde a resistência à corrosão é um critério de seleção de material, uma vez que, segundo a literatura a temperatura de 575 °C está no intervalo de temperaturas de revenimento onde são obtidas as melhores propriedades mecânicas. / Supermartensitic (SM) stainless steels were developed in response to the need of new technologies that are more economical and environmentally friendly. SM steels are different from classic martensitic stainless steels due to their lower carbon content and the addition of Mo and Ni. SM steels were developed as an alternative for duplex stainless steels in oil extraction pipelines offshore in the mid-1990s. To acquire the desired mechanical properties quenching and tempering treatments can be conducted, with tempering which can be performed in different temperatures and times. Since the previous heat treatments change the mechanical properties by phase transformations (precipitation), changes in the corrosion resistance properties can be expected. The beneficial effects of Nb additions in traditional stainless steels are well known. Therefore, the aim of this investigation was to study the influence of tempering temperature on the corrosion resistance of three SM stainless steels, containing 13% Cr, 5% Ni, 1-2% Mo, with and without Nb. The steels used in this investigation were denominated SM2MoNb, SM2Mo and SM1MoNb, according to their Mo (2% or 1%) and Nb (0.11%) content. Considering that the usual types of corrosion observed in stainless steels are pitting and intergranular corrosion, the pitting potential (Ep) and the degree of sensitization (DOS) were determined in function of tempering temperature. The steels were annealed at 1050° C during 48 hours to eliminate delta ferrite phase. Afterwards, the samples were normalized for 30 minutes at 1050 ºC (air cooling) to standardize the grain size. Finally, the samples were tempered at 550 °C, 575 °C, 600 °C, 625 °C, 650 °C or 700 °C, for two hours. The DOS was measured through double loop electrochemical potentiodynamic reactivation technique (DL-EPR), using as electrolyte a solution containing 1 M H2SO4 + 0.01 M KSCN. To determine the Ep, potentiodynamic polarization tests were carried out in a 0.6 M NaCl solution. The results were discussed based on the observed microstructural changes. Optical microscopy (OM), scanning electron microscopy (SEM), phase diagram simulation using Thermo-Calc software and quantification of reversed/retained austenite were done using the following two methods: X-ray diffraction (XRD) and magnetic measurements with a ferritscope. The quantification analysis by XRD showed the formation of austenite above a temperature of 600 °C, reaching the maximum volume at 650 °C and decreasing again to zero at 700 °C. However, the ferritscope measurements detected the presence of austenite in the whole temperature range. The largest difference between the two methods used was observed for the specimens only quenched and quenched/tempered at 700 °C. It is proposed that the observed differences between XRD and ferritscope methods correspond to the presence of nanometric retained austenite, which is localized among the martensite laths. The results were discussed based on the precipitation of Cr23C6, Mo6C, NbC, Chi-phase, austenite and ferrite, and by the resulting Cr and Mo depletion caused by the precipitation of these microconstituents. Three mechanisms were proposed to explain the degree of sensitization: i) Cr23C6 precipitation, ii) Chi-phase (Cr and Mo enrichment) precipitation and, iii) ferrite formation during the tempering treatment. The best performance in terms of DOS was obtained for the tempering conditions of 575 ºC and 600 ºC. In addition, the Ep results showed that the steel SM2MoNb tempered at 575 °C offers the best pitting corrosion resistance in chloride environment. Therefore, the previous results can be used as criteria for best material and tempering temperature selection. Moreover, according to published literature, 575 °C is within the temperature range that offers the best mechanical properties.

O desenvolvimento dos aços inoxidáveis Super-Martensíticos (SM) nasce da necessidade de implementar novas tecnologias, mais econômicas e amigáveis ao meio ambiente. Os aços inoxidáveis SM são uma derivação dos aços inoxidáveis martensíticos convencionais, diferenciando-se basicamente no menor teor de carbono, na adição de Ni e Mo. Foram desenvolvidos como uma alternativa para aços inoxidáveis duplex no uso de dutos para a extração de petróleo offshore em meados dos anos 90. Para que esses aços apresentem as propriedades mecânicas de resistência à tração e tenacidade é necessário que sejam realizados tratamentos de austenitização, seguido de têmpera, e de revenimento, onde, particularmente para este último, há várias opções de tempos e temperaturas. Como os tratamentos térmicos geram as propriedades mecânicas através de transformações de fase (precipitação) podem ocorrer alterações da resistência à corrosão. São conhecidos os efeitos benéficos da adição de Nb em aços inoxidáveis tradicionais. Por isso, o objetivo desta pesquisa foi estudar aços inoxidáveis SM contendo Nb. Foi pesquisada a influência da temperatura de revenimento sobre a resistência à corrosão de três aços inoxidáveis SM, os quais contêm 13% Cr, 5% Ni, 1% a 2% Mo, com e sem adições de Nb. No presente trabalho, foram denominados de SM2MoNb, SM2Mo e SM1MoNb, que representam aços com 2% Mo, 1% Mo e 0,11% Nb. Dado que os principais tipos de corrosão para aços inoxidáveis são a corrosão por pite (por cloreto) e a corrosão intergranular (sensitização), optou-se por determinar os Potenciais de Pite (Ep) e os Graus de Sensitização (GS) em função da temperatura de revenimento. Os aços passaram por recozimento a 1050°C por 48 horas, para eliminação de fase ferrita delta. Em seguida foram tratados a 1050 °C por 30 minutos, com resfriamento ao ar, para uniformização do tamanho de grão. A estrutura martensítica obtida recebeu tratamentos de revenimento em temperaturas de: 550 °C, 575 °C, 600 °C, 625 °C, 650 °C e 700 °C, por 2 horas. O GS foi medido através da técnica de reativação eletroquímica potenciodinâmica na versão ciclo duplo (DL-EPR), utilizando-se eletrólito de 1M H2SO4 + 0,01M KSCN. Para determinar o Ep foram realizados ensaios de polarização potenciodinâmica em 0,6M NaCl. Os resultados obtidos foram discutidos através das variações microestruturais encontradas. Foram empregadas técnicas de microscopia ótica (MO), microscopia eletrônica de varredura (MEV), simulação termodinâmica de fases através do programa Thermo-Calc e determinação de austenita revertida mediante difração de raios X (DRX) e ferritoscópio. A quantificação da austenita por DRX identificou que a partir de 600 °C há formação desta fase, apresentando máximo em 650 °C, e novamente diminuindo para zero a 700 °C. Por sua vez, o método do ferritoscópio detectou austenita nas condições em que a analise de DRX indicou valor nulo, sendo as mais críticas a do material temperado (sem revenimento) e do aço revenido a 700 °C. Propõe-se que tais diferenças entre os dois métodos se deve à morfologia fina da austenia retida, a qual deve estar localizada entre as agulhas de martensita. Os resultados foram discutidos em termos da precipitação de Cr23C6, Mo6C, NbC, fase Chi, austenita e ferrita, bem como das consequências do empobrecimento em Cr e Mo, gerados por tais microconstituintes. São propostos três mecanismos para explicar a sensitização: o primeiro é devido a precipitação de Cr23C6, o segundo a precipitação de fase Chi (rica em Cr e Mo) e o terceiro é devido a formação de ferrita durante o revenimento. O melhor desempenho quanto ao GS foi obtido para os revenimentos a 575 °C e 600°C, por 2 horas. Os resultados de Ep indicaram que o aço SM2MoNb, revenido a 575°C, tem o melhor desempenho quanto à resistência à corrosão por cloreto. Isso associado ao baixo GS coloca este aço, com este tratamento térmico, numa posição de destaque para aplicações onde a resistência à corrosão é um critério de seleção de material, uma vez que, segundo a literatura a temperatura de 575 °C está no intervalo de temperaturas de revenimento onde são obtidas as melhores propriedades mecânicas. / Supermartensitic (SM) stainless steels were developed in response to the need of new technologies that are more economical and environmentally friendly. SM steels are different from classic martensitic stainless steels due to their lower carbon content and the addition of Mo and Ni. SM steels were developed as an alternative for duplex stainless steels in oil extraction pipelines offshore in the mid-1990s. To acquire the desired mechanical properties quenching and tempering treatments can be conducted, with tempering which can be performed in different temperatures and times. Since the previous heat treatments change the mechanical properties by phase transformations (precipitation), changes in the corrosion resistance properties can be expected. The beneficial effects of Nb additions in traditional stainless steels are well known. Therefore, the aim of this investigation was to study the influence of tempering temperature on the corrosion resistance of three SM stainless steels, containing 13% Cr, 5% Ni, 1-2% Mo, with and without Nb. The steels used in this investigation were denominated SM2MoNb, SM2Mo and SM1MoNb, according to their Mo (2% or 1%) and Nb (0.11%) content. Considering that the usual types of corrosion observed in stainless steels are pitting and intergranular corrosion, the pitting potential (Ep) and the degree of sensitization (DOS) were determined in function of tempering temperature. The steels were annealed at 1050° C during 48 hours to eliminate delta ferrite phase. Afterwards, the samples were normalized for 30 minutes at 1050 ºC (air cooling) to standardize the grain size. Finally, the samples were tempered at 550 °C, 575 °C, 600 °C, 625 °C, 650 °C or 700 °C, for two hours. The DOS was measured through double loop electrochemical potentiodynamic reactivation technique (DL-EPR), using as electrolyte a solution containing 1 M H2SO4 + 0.01 M KSCN. To determine the Ep, potentiodynamic polarization tests were carried out in a 0.6 M NaCl solution. The results were discussed based on the observed microstructural changes. Optical microscopy (OM), scanning electron microscopy (SEM), phase diagram simulation using Thermo-Calc software and quantification of reversed/retained austenite were done using the following two methods: X-ray diffraction (XRD) and magnetic measurements with a ferritscope. The quantification analysis by XRD showed the formation of austenite above a temperature of 600 °C, reaching the maximum volume at 650 °C and decreasing again to zero at 700 °C. However, the ferritscope measurements detected the presence of austenite in the whole temperature range. The largest difference between the two methods used was observed for the specimens only quenched and quenched/tempered at 700 °C. It is proposed that the observed differences between XRD and ferritscope methods correspond to the presence of nanometric retained austenite, which is localized among the martensite laths. The results were discussed based on the precipitation of Cr23C6, Mo6C, NbC, Chi-phase, austenite and ferrite, and by the resulting Cr and Mo depletion caused by the precipitation of these microconstituents. Three mechanisms were proposed to explain the degree of sensitization: i) Cr23C6 precipitation, ii) Chi-phase (Cr and Mo enrichment) precipitation and, iii) ferrite formation during the tempering treatment. The best performance in terms of DOS was obtained for the tempering conditions of 575 ºC and 600 ºC. In addition, the Ep results showed that the steel SM2MoNb tempered at 575 °C offers the best pitting corrosion resistance in chloride environment. Therefore, the previous results can be used as criteria for best material and tempering temperature selection. Moreover, according to published literature, 575 °C is within the temperature range that offers the best mechanical properties.

This work deals with the damage due to the simultaneous presence of hydrogen in atomic form and stress – straining.<p>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. <p>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.<p>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. <p>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. <p>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.<p>Because of previous industrial experience it was necessary to find a more appropriate qualification test method to asses the risk of HE. <p>In this work we proposed the stepwise repeated slow strain rate test (SW R – SSRT) as a qualification test method for supermartensitic stainless steels. <p>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. <p>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.).<p><p><p><p>Ce travail se rapporte à l’endommagement provoqué par la présence simultanée de l’hydrogène sous forme atomique et une contrainte (appliquée où résiduelle). <p>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. <p>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.<p>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. <p>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. <p>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. <p>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. <p>Dans ce travail on propose un essai de traction lente incrémentée (SW R – SSRT) comme méthode de qualification pour les aciers supermartensitiques. <p>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. <p>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.).<p> <p><p><p><p><p><p><p> / Doctorat en sciences appliquées / info:eu-repo/semantics/nonPublished