A furação é um processo caracterizado por apresentar dificuldades em relação à formação e remoção do cavaco da região de corte, e à geração de altas temperaturas devido ao cisalhamento e encruamento do material no fundo do furo, principalmente, na usinagem do aço inoxidável austenítico. Com isso a presença do fluido de corte com boas propriedades de refrigeração e lubrificação faz-se necessária para preservar a vida da broca e garantir um acabamento satisfatório do furo. Contudo, com a tendência mundial de diminuição da quantidade de fluido de corte dispendido, tem-se buscado técnicas diferentes de aplicação que utilize o lubrirrefrigerante em quantidades reduzidas. Associado a essas novas técnicas, vem sendo empregado cada vez mais o uso de partículas sólidas adicionadas ao fluido com o intuito de aumentar sua eficiência. Um tipo de partícula, que vem sendo aplicada em diversos setores produtivos devido a suas ótimas propriedades mecânicas, químicas e elétricas, é o grafeno. Sendo assim, pretende-se analisar o comportamento da furação do aço inoxidável austenítico AISI 304 utilizando flocos de multicamadas de grafeno dispersos em fluido de corte empregando o projeto de experimentos Box-Behnken A aplicação do fluido é feita externamente utilizando a técnica de quantidade reduzida com três vazões: 1,5 l/h; 2,0 l/h e 2,5 l/h. O comportamento do processo é avaliado pelos esforços de corte (força de avanço e momento torsor), pela rugosidade (média e total), e pelos desvios (dimensional e de circularidade) na entrada e na saída do furo. O desempenho do grafeno é avaliado pela comparação dos resultados da sua utilização com os de outras duas condições de aplicação do fluido de corte sem adições: em quantidade reduzida e em abundância. Teve-se como resultado que todas as variáveis respostas analisadas sofreram alguma influência da velocidade de corte e/ou da vazão do fluido aplicado em quantidades reduzidas. Os benefícios da lubrificação e refrigeração do grafeno são sentidos em alguns resultados, como nos menores valores médios de rugosidade e de desvio dimensional. Mas, na análise do processo como um todo, não se afirma que o fluido de corte com flocos de multicamadas de grafeno aplicado externamente em quantidade reduzida traz mais benefícios que o sem adições. / Drilling is a machining process characterized by difficulties in the formation and removal of the chip from the cutting region and the generation of high temperatures due to shearing and hardening of the material at the bottom of the hole, especially in the machining of austenitic stainless steel. Thus the presence of cutting fluid with good cooling and lubrication properties is necessary to preserve the drill life and ensure a satisfactory hole finish. However, with the worldwide trend of decreasing the amount of cutting fluid expended, different application systems have been sought which use the coolant in reduced amounts. Associated with these new techniques, the presence of solid particles added to the fluid has been increasingly used in order to enhance its efficiency. One of these particles, which have been applied in several productive sectors due to its excellent mechanical, chemical and electrical properties, is graphene. Therefore, it is intended to analyze the drilling behavior of AISI 304 austenitic stainless steel using multilayer graphene flakes dispersed in the cutting fluid using the Box- Behnken Design The application of the fluid is done externally by means of reduced quantity lubricant with three flows: 1.5 l/h, 2.0 l/h and 2.5 l/h. The behavior of the process is evaluated via thrust force and torque, average and total roughness, and dimensional and circularity deviations at the input and output holes. The performance of graphene is evaluated by comparing the results of its use with two other cutting fluid conditions: quantity reduced and abundance. It was found that all the analyzed responses variables had some influence of the cutting speed and/or the flow of the applied fluid in reduced quantities. The benefits of graphene lubrication and cooling are felt in some results, such as the lower values of average roughness and dimensional deviation. However, in the analysis of the process as a whole, it is not possible to state that the multilayer graphene flakes cutting fluid externally applied brings more benefits than without additions.

Os colares das sondas de prospecção petrolíferas são fabricadas de aço inoxidável austenítico devido as suas características eletromagnéticas e de corrosão. Normalmente, a temperatura de serviço é de aproximadamente 300ºC, meio aquoso e a sonda é submetida a carregamentos cíclicos. Várias foram as tentativas de desenvolvimento de um aço que atendesse as necessidades de serviço destes colares. Neste trabalho foram determinadas as propriedades mecânicas de tração e de fadiga de baixo ciclo de um aço inoxidável austenítico do sistema Fe-Mn-Cr-N, quando ensaiado mecanicamente nas temperaturas ambiente e à 300ºC. Foi observado que os valores dos limites de escoamento e de resistência e o alongamento diminuem significativamente quando a temperatura é elevada para 300ºC. Como conseqüência destas alterações, ocorreu uma diminuição da vida em fadiga. A comparação entre as curvas tensão-deformação monotônica e cíclica mostrou que este material, ensaiado à 300ºC e nas amplitudes de deformações estudadas, apresenta amolecimento cíclico. A expressão encontrada para a relação deformação-vida é dada por &#916&#949t/2=0,0054 (2Nf)-0,064 + 0,438 (2Nf)-0,595 com ponto de transição em 2Nt = 9,5 x 103. Quanto aos métodos estimativos das propriedades mecânicas de fadiga, obtidos a partir das propriedades mecânicas de tração, foi verificado que o Método da Inclinação Universal Modificado se aproxima mais da curva experimental obtida neste trabalho. A análise fratográfica mostrou a presença de estrias no estágio II de propagação de trinca. / In this work the monotonic and low cycle fatigue mechanical properties of an Cr-Mn-N austenitic stainless steel, used to produce drill collars used for deep drilling in offshore industry, were evaluated. The low cycle fatigue testing was carried out according to the ASTM E606 standard, under strain control and R = - 1, at temperature of 300°C, which is the temperature that the drill collar reaches in service. It was observed that the yield and the ultimate tensile strength and the elongation decreased sharply with increasing temperature. As a consequence, in both high and low cycle regions, the fatigue life decreased with increasing temperature. The comparison between the stress - strain monotonic and cyclic curves, showed that this material exhibited cyclic softening for the applied strain amplitudes. The correlation between strain and number of cycles to failure was can be given by &#916&#949t/2 = 0,0054 (2Nf)-0,064 + 0,438 (2Nf)-0,595 with transition in 2Nt = 9,5x103 reverses. The experimental results were compared with some models used to predict the fatigue life based on the tensile monotonic properties. The Modified Universal Shopes a better fitting with the experimental data. The fractographic analysis showed the presence of stage II striations.

Orientador: Maria Clara Filippini Ierardi / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-16T17:12:39Z (GMT). No. of bitstreams: 1 Larosa_MariaAparecida_D.pdf: 10660809 bytes, checksum: a977970425db7202c5a36a0e2a30a789 (MD5) Previous issue date: 2010 / Resumo: No presente trabalho, o aço inoxidável austenítico ASTM F 138, empregado na fabricação de implantes ortopédicos temporários e permanentes, foi submetido a tratamento de superfície por laser, com o intuito de analisar as modificações microestruturais resultantes deste tratamento e seus efeitos na resistência à corrosão e ao desgaste microabrasivo. A caracterização microestrutural foi feita por microscopia óptica, microscopia eletrônica de varredura, microdureza Vickers e difração de raios-X. Para analisar o comportamento em corrosão do aço, antes e após o tratamento, foram realizados três tipos diferentes de ensaios eletroquímicos usando solução de Ringer como eletrólito. O comportamento em desgaste foi avaliado a partir de ensaios por microabrasão do tipo esfera-sobre-placa, utilizando como abrasivos uma suspensão de partículas de carboneto de silício (SiC) em água destilada e uma suspensão de SiC em solução de Ringer. Após o tratamento por laser, o aço apresentou uma microestrutura refinada constituída por dendritas de austenita, com um aumento de aproximadamente 23% na dureza e com melhoria frente à corrosão e ao desgaste. / Abstract: In this work, ASTM F 138 austenitic stainless steel, used in temporary and permanent orthopaedic implants manufacture, was subjected to laser surface treatment. The purpose is analyze the microstructural changes resulting from this treatment and their effects on the microabrasive wear and corrosion resistance. The microstructural characterization was performed by optical microscopy, scanning electron microscopy, Vickers microhardness and X-ray diffraction. In order to analyze the corrosion behavior, three different types of electrochemical tests were carried out before and after treatment, using Ringer's solution as electrolyte. The wear behavior was evaluated using ball-cratering microabrasive wear tests, using slurry of silicon carbide particles (SiC) in distilled water and slurry of SiC in Ringer's solution. After laser treatment, the steel presented a refined microstructure consisted of austenite dendrites, with a 23% hardness increase and improved corrosion and wear resistance. / Doutorado / Materiais e Processos de Fabricação / Doutor em Engenharia Mecânica

Made available in DSpace on 2016-03-15T19:37:01Z (GMT). No. of bitstreams: 1 Eva Paganini Martins.pdf: 2852717 bytes, checksum: 4c8bb41c27d4434bc48bef3b6484ab0f (MD5) Previous issue date: 2010-03-05 / This paper deals with the effects of cold work on the nitriding kinetics and surface micro-hardness of the 3XX family of austenitic stainless steels. Nitriding is a thermo-chemical treatment based on the nitrogen diffusion and, under some specific circumstances, carbon in lower quantities, in to a ferrous matrix. The nitriding process improves particularly high cycle fatigue resistance, wear resistance, and corrosion resistance for carbon steels. To study the cold work effect on the material, samples of 304 and 316 were strained in tension tests to conventional deformation of 10%, 25% and up to rupture point prior to the nitriding process. A sample without deformation was kept for reference. It is possible to verify, based on the test performed, that the AISI 304 has smaller deformation capacity when compared to the 316 in terms of elongation on strain test presenting a higher degree of cold work deformation. Also the 304 presents a reduction on nitrided layer thickness and a smaller percentage of nitrogen in the nitrided layer after the cold work (difference of 1% point versus 0,5 for the 316) It is also possible to observe the smaller density on the 304 after the cold work which can be attributed, according to the literature, to the formation of cold work martensite during the cold work process. / O presente trabalho apresenta uma avaliação dos efeitos de encruamento na cinética do processo de nitretação das ligas de aço inoxidável austeníticos da série 3XX e no perfil de dureza das superfícies nitretadas. Nitretação é um tratamento termoquímico baseado na difusão de nitrogênio, em algumas circunstancias carbono em pequenas quantidades, na matriz ferrosa. Para estudar os efeitos do encruamento no material, amostras de 304 e 316 foram encruadas em ensaio de tração convencional, em 10%, 25% e até a ruptura e posteriormente nitretadas. Ainda manteve-se uma a mostra sem deformação para fins de comparação. Com base nos ensaios avaliados é possível verificar que o AISI 304 tem menor capacidade de deformação que o AISI 316 em termos de alongamento no ensaio de tração apresentando maior taxa de encruamento em relação ao 316. Além disso o 304 apresenta redução na camada nitretada proporcional ao grau de deformação plástica a frio e menor percentual de nitrogênio na camada após deformação (queda de aproximadamente 1 ponto percentual enquanto no 316 a diferença mantém-se em torno de 0,5 pontos). Ainda é possível observar a diminuição da densidade no 304, o que pode ser atribuído, segundo a literatura, a formação de martensita e deformação durante o processo de deformação plástica a frio.

La fissuration en relaxation (FER) peut apparaître dans les zones affectées par la chaleur de larges pièces soudées pendant leur utilisation entre 500 et 700°C. Il est admis que ce phénomène est induit par la relaxation à haute température de champs de contraintes résiduelles initialement introduits lors du soudage. L'objectif de ce travail est d'identifier, dans les aciers de type AISI 316L, les caractéristiques de ce type de matériaux, ainsi que les forces motrices, responsables du développement de la fissuration en relaxation.La méthodologie proposée consiste à reproduire les conditions de la FER dans cinq aciers de type AISI 316L de compositions chimiques différentes en utilisant des éprouvettes de type « Compact Tension » (CT) pré-comprimées. L'étude des éprouvettes à l'aide du MEB, de l'EBSD, du MET et de la tomographie X a révélé que de l'endommagement intergranulaire s'était développé dans quelques une d'entre elles. Le niveau d'endommagement mesuré dans chaque éprouvette dépend de la nuance de l'acier, de la température et de la durée d'exposition thermique, et du rayon d'entaille.Ce travail implique également la prédiction numérique des champs de déformations et de contraintes résiduelles dans les éprouvettes à l'aide d'une nouvelle loi de comportement viscoplastique à variables internes. La comparaison entre les champs de contraintes résiduelles prédits dans les éprouvettes de type CT et les distributions d'endommagement mesuré par tomographie a permis de déduire le niveau de contrainte résiduelle critique nécessaire pour l'initiation de la FER. Finalement, les distributions d'endommagement mesurées expérimentalement ont pu être correctement prédites numériquement avec une loi phénoménologique d'endommagement scalaire alimentée par les prédictions du modèle de comportement viscoplastique. / Stress relaxation cracking can potentially be found in the heat affected zone of large welded parts after service in the 500-700°C temperature range. This phenomenon, known as reheat cracking (RC), is driven by the high temperature relaxation of residual stress fields initially introduced during welding. The main objective of this doctoral thesis is to identify the material and microstructural characteristics as well as the driving forces responsible for RC damage development in AISI 316L-type austenitic stainless steels.The proposed methodology relies on the reproduction of RC conditions in five chemically different AISI 316L-type steels using pre-compressed CT-like specimens. Subsequent investigation using SEM, EBSD, TEM and X-ray tomography revealed that intergranular damage had developed in some of the specimens. The extent of damage was found to depend on the steel grade, the temperature and duration of the thermal exposure, and the notch radius.The numerical investigation of the local residual stress and strain fields in the specimens was carried out using a novel internal state variable-based viscoplastic constitutive model. A comparison between the predicted residual stress fields in the CT-like specimens and the intergranular damage distributions measured by X-ray tomography enabled the threshold level of local residual stresses associated with the initiation of stress relaxation microcracks to be inferred. Finally, the distribution of the measured local RC damage was modelled numerically by explicitly linking a suitable phenomenological scalar damage law with the above constitutive model. The corresponding results were found to be consistent with the observed damage distributions.

Les travaux de thèse ont permis de consolider et de compléter les connaissances sur les mécanismes de déformation et les évolutions microstructurales à chaud d’un acier inoxydable austénitique 316Nb. Comprendre la variation du comportement microstructural observée sur différentes pièces obtenues par forge libre permettra de pérenniser les connaissances et d’optimiser les gammes de forgeage. Du fait d’une variabilité de l’état de recristallisation, mise en évidence sur des pièces d’essai, et de son impact sur les propriétés mécaniques, des traitements thermomécaniques simplifiés sont réalisés en laboratoire afin de comprendre la genèse de ces différents états métallurgiques. L’influence de la température, du taux et de la vitesse de déformation ainsi que de la vitesse de refroidissement après l’essai (distinction entre les mécanismes dynamiques et post-dynamiques) est tout d’abord étudiée. De multiples passes de déformation, dans des conditions isothermes et anisothermes, sont ensuite appliquées afin de suivre les évolutions post-dynamiques de la microstructure entre les passes. Le rôle du traitement thermique post-déformation sur la microstructure (recristallisation statique) est étudié. Enfin, l’effet de la microstructure initiale, en termes de taille de grains et de composition chimique, notamment la teneur en niobium en solution solide, a été considéré.La recristallisation dynamique ne domine pas l’évolution de la microstructure, de par notamment une restauration dynamique avancée et une taille de grains élevée. Néanmoins, aux hautes températures et pour de faibles taux de déformation, une migration dynamique des joints de grains conduit à la formation progressive de nouveaux grains recristallisés. La recristallisation post-dynamique est très dépendante des conditions de déformation. A composition chimique donnée, la taille de grains (dans la gamme 60 – 250 µm) affecte peu la cinétique de recristallisation dynamique et post-dynamique. L’augmentation de la teneur en niobium de la solution solide entraîne, via sans doute un effet de traînage de soluté et une éventuelle modification de l’énergie de défaut d’empilement à haute température, un retard considérable de l’apparition de la recristallisation. Au cours du traitement thermique post-déformation, l’état de recristallisation final est essentiellement dépendant de la composition chimique (teneur en niobium de la solution solide et présence de ferrite δ résiduelle). Les précipités de niobium générés dans les conditions de déformation usuelles n’ont pas d’influence directe sur la cinétique de recristallisation. / Mechanical properties and microstructure of 316Nb austenitic stainless steel may show some variability in hot forging products. This work aimed at improving knowledge about hot deformation mechanisms and microstructural evolution of this steel. Obtaining a homogeneous microstructure requires deep understanding of the hot deformation behaviour and mechanisms. In thick-walled components, both work hardening, dynamic recovery and recrystallization govern hot workability. Static and post-dynamic phenomena can induce further metallurgical evolution during interpass time and cooling. The influence of deformation temperature, strain, strain rate, cooling rate on recrystallization mechanisms has been studied by using hot torsion tests. Multiple-pass tests with isothermal and non-isothermal interpass allowed understanding post-dynamic mechanisms. Static phenomena were investigated using various annealing conditions. The effects of initial microstructural features such as grain size and chemical composition, specifically niobium solute content, on the hot deformation behaviour were eventually considered.The extent of dynamic recovery, coarse initial grain size, solute drag, and pinning of grain boundaries by fine Nb(C,N) particles strongly hinder dynamic recrystallization which does not dominate the metallurgical evolution over the range studied, in contrast to results reported on 316 steel. However, bulging of grain boundaries as a prelude to dynamic recrystallization was observed at low strains and high temperature. Grain boundary serrations progressively lead to the formation of subgrain boundaries, then of new high angle boundaries. A particular dynamic recrystallization mechanism explains progressive elimination of annealing twins. Interaction with dislocations depends on locally activated slip systems and whether they are common to both twin and parent grain. At moderate strain levels, post-dynamic recrystallization occurs by rapid growth of nuclei that depends on deformation temperature, and applied strain and strain rate. For a given chemical composition, neither dynamic nor post-dynamic recrystallization is affected by the initial grain size over the range studied. Increasing the free niobium content promotes solute drag and niobium carbide precipitation, which significantly delay recrystallization. The microstructure after annealing essentially depends on the availability of solute atoms such as niobium and on residual δ-ferrite. Nb(C,N) precipitates formed during hot deformation do not significantly influence recrystallization kinetics.

Type 304 stainless steel is used in various applications where corrosion resistance is required. This material is selected for weldability and corrosion resistance, but it can suffer stress corrosion cracking (scc), corrosion fatigue, pitting and crevice corrosion in chloride environments. The aim of this project was to produce Type 304 containers with intentional stress corrosion cracks, to serve as test samples for future weld repair trials. A test rig was constructed which used thermal stress to crack Type 304 tube samples; a central Type 310 stainless steel bar contained a heating element, so that the bar serve as a heat source and a stressing element. The rig was filled or half filled with magnesium chloride solution. The elastic strain in the tube sample was directly related to the temperature difference between the central bar and the sample. The thermal stress was sufficient to cause stress corrosion cracking. The tests were terminated when the first crack extended through the wall thickness of the 304 tube sample. The distribution and depth of cracks were determined after the tests. The test procedure caused the formation of multiple cracks in the tube sample. The temperature controller caused cyclical variation in the bar temperature and hence in the thermal stress. However, the temperature variation did not have an effect on cracking; corrosion fatigue did not contribute to cracking, and the cracks had the classic branched transgranular morphology of chloride cracking. Copyright / Dissertation (MSc)--University of Pretoria, 2009. / Materials Science and Metallurgical Engineering / unrestricted

A furação é um processo caracterizado por apresentar dificuldades em relação à formação e remoção do cavaco da região de corte, e à geração de altas temperaturas devido ao cisalhamento e encruamento do material no fundo do furo, principalmente, na usinagem do aço inoxidável austenítico. Com isso a presença do fluido de corte com boas propriedades de refrigeração e lubrificação faz-se necessária para preservar a vida da broca e garantir um acabamento satisfatório do furo. Contudo, com a tendência mundial de diminuição da quantidade de fluido de corte dispendido, tem-se buscado técnicas diferentes de aplicação que utilize o lubrirrefrigerante em quantidades reduzidas. Associado a essas novas técnicas, vem sendo empregado cada vez mais o uso de partículas sólidas adicionadas ao fluido com o intuito de aumentar sua eficiência. Um tipo de partícula, que vem sendo aplicada em diversos setores produtivos devido a suas ótimas propriedades mecânicas, químicas e elétricas, é o grafeno. Sendo assim, pretende-se analisar o comportamento da furação do aço inoxidável austenítico AISI 304 utilizando flocos de multicamadas de grafeno dispersos em fluido de corte empregando o projeto de experimentos Box-Behnken A aplicação do fluido é feita externamente utilizando a técnica de quantidade reduzida com três vazões: 1,5 l/h; 2,0 l/h e 2,5 l/h. O comportamento do processo é avaliado pelos esforços de corte (força de avanço e momento torsor), pela rugosidade (média e total), e pelos desvios (dimensional e de circularidade) na entrada e na saída do furo. O desempenho do grafeno é avaliado pela comparação dos resultados da sua utilização com os de outras duas condições de aplicação do fluido de corte sem adições: em quantidade reduzida e em abundância. Teve-se como resultado que todas as variáveis respostas analisadas sofreram alguma influência da velocidade de corte e/ou da vazão do fluido aplicado em quantidades reduzidas. Os benefícios da lubrificação e refrigeração do grafeno são sentidos em alguns resultados, como nos menores valores médios de rugosidade e de desvio dimensional. Mas, na análise do processo como um todo, não se afirma que o fluido de corte com flocos de multicamadas de grafeno aplicado externamente em quantidade reduzida traz mais benefícios que o sem adições. / Drilling is a machining process characterized by difficulties in the formation and removal of the chip from the cutting region and the generation of high temperatures due to shearing and hardening of the material at the bottom of the hole, especially in the machining of austenitic stainless steel. Thus the presence of cutting fluid with good cooling and lubrication properties is necessary to preserve the drill life and ensure a satisfactory hole finish. However, with the worldwide trend of decreasing the amount of cutting fluid expended, different application systems have been sought which use the coolant in reduced amounts. Associated with these new techniques, the presence of solid particles added to the fluid has been increasingly used in order to enhance its efficiency. One of these particles, which have been applied in several productive sectors due to its excellent mechanical, chemical and electrical properties, is graphene. Therefore, it is intended to analyze the drilling behavior of AISI 304 austenitic stainless steel using multilayer graphene flakes dispersed in the cutting fluid using the Box- Behnken Design The application of the fluid is done externally by means of reduced quantity lubricant with three flows: 1.5 l/h, 2.0 l/h and 2.5 l/h. The behavior of the process is evaluated via thrust force and torque, average and total roughness, and dimensional and circularity deviations at the input and output holes. The performance of graphene is evaluated by comparing the results of its use with two other cutting fluid conditions: quantity reduced and abundance. It was found that all the analyzed responses variables had some influence of the cutting speed and/or the flow of the applied fluid in reduced quantities. The benefits of graphene lubrication and cooling are felt in some results, such as the lower values of average roughness and dimensional deviation. However, in the analysis of the process as a whole, it is not possible to state that the multilayer graphene flakes cutting fluid externally applied brings more benefits than without additions.

L'extension de la durée de vie des centrales nucléaires est considérée comme un défi énergétique mondial. C’est pourquoi il est nécessaire d’analyser les risques et d’étudier les effets de différents facteurs susceptibles de présenter un risque pour une exploitation sûre à long terme. Ces structures, souvent de grandes dimensions, sont soumises au cours de leurs vies à des chargements complexes combinant des sollicitations mécaniques variées, multiaxiales, avec des valeurs moyennes non nulles associées à des fluctuations de température dans un environnement agressif comme celui de circuit primaire de Réacteur à Eau Pressurisée (REP). Dans ce contexte, la présente étude est inscrite dans le cadre de l’identification de l’effet du milieu REP et de l’équibiaxialité du chargement sur la durée de vie en fatigue des aciers inoxydables austénitiques présents dans les tuyauteries primaires d’un Réacteur à Eau Pressurisée.Pour cela, le CEA a développé, en collaboration avec EDF et FRAMATOME, le dispositif expérimental «FABIME2e» destiné à l’étude de la fatigue équibiaxiale dans le milieu REP.Dans un premier temps, une présentation des résultats expérimentaux et numériques obtenus avec le dispositif FABIME2e est réalisée. Ces résultats montrent l’effet aggravant de l’équibiaxialité du chargement et des conditions REP de l’environnement sur la durée de vie en fatigue des aciers inoxydables austénitiques. Cependant cet effet aggravant reste couvert par la courbe de Design définie par les codes de conception de l’industrie nucléaire. Un nouveau critère de fatigue est ensuite proposé pour prédire la durée de vie en fatigue équibiaxiale tout en prenant en compte l’effet d’environnement REP.Ce travail ouvre plusieurs perspectives industrielles et scientifiques sur l'interprétation mécanique des essais de fatigue équibiaxiale dans l’environnement REP et sur le critère de fatigue proposé. / The lifetime extension of the nuclear power plants is considered as a major energy challenge. For this reason, the risk analysis and the study of various effects of different factors that could potentially represent a hazard to a safe long term operation are necessary. These structures, often of large dimensions, are subjected during their life to complex loading combining varying mechanical and multiaxial loads, with non-zero mean values associated with temperature fluctuations and Pressurized Water Reactor (PWR) environment.In this context, the present study is part of the identification of the PWR environment effect and the equibiaxiality of the loading on the fatigue life of the austenitic stainless steels present in the primary pipes of a Pressure Water Reactor.For this purpose, the CEA has developed, in partnership with EDF and FRAMATOME, the experimental device «FABIME2e» for studying the equibiaxial fatigue in the PWR environment.First, a presentation of the experimental and numerical results obtained with the FABIME2e device is realized. These results show the aggravating effect of equibiaxial loading and environmental PWR conditions on the fatigue life of austenitic stainless steel. This effect remains covered by the Design curve defined by the design codes of the nuclear industry. Second, a new fatigue criterion is proposed to predict the lifetime in equibiaxial fatigue. This criterion takes into account the effect of the PWR environment.This work opens up several industrial and scientific prospects on the mechanical inter-pretation of equibiaxial fatigue tests in PWR environment and on the proposed fatigue criterion.

This project was ordered by Sandvik Materials Technology and was performed by a group of students at Uppsala university. The purpose of the project was to study precipitation behavior and structure stability in six advanced heat resistant alloys. Each sample were subjected to a creep rupture test in 600 or 700°C depending on the alloy type. Two parts of each alloy where examined; one part which had been affected by creep and another part which was unaffected by creep. A literature study was performed first to gain knowledge of the scientific theory utilized in this project, namely creep, precipitation hardening, and about the different materials which were analyzed. Preliminary results for the phase composition of the materials were obtained from a Thermo-Calc (TC) simulation. The SEM-images showed nothing noteworthy for any sample due to the roughness of the sample surfaces. The EDS-analysis showed chromium depletion in the centers of the aged samples of HT9 and Sanicro® 75X. Other minority phases such as Cr23C6 in Sanicro®70, P-phase and a titanium nitride phase in sanicro® 60X, VB in Esshete 1250 and Sigma-phase in 4C54 were identified using EDSmapping. It was found that when using XRD to analyze the phase compositions of small samples it is impractical to have the samples cast in bakelite beforehand. The XRD-results obtained in this project showed that more than 90% of the XRD diffractogram for every sample was graphite, which made the identification of minority phases impossible. The quality of the LOM-images varied greatly between samples, for 4C54 grain sizes were measured in all images, for Esshete 1250 grain sizes were measured for the crept sample, and for Sanicro® 60X measurements could only be taken from one image. Most of the sample preparation was insufficient to achieve the test results necessary for complete microstructural analysis and phase analysis of the samples. The mistakes in the practical steps of the project were noted and improvements for these mistakes are presented in the conclusion.