Cette étude traite de la prise en compte du gradient de contrainte et du frottement pour calculer l'amorçage de fissures en fatigue. On s'intéresse notamment au dimensionnement d'une manille en Inconel 718 munie de bagues en Inconel 718montées avec interférence. Pour cela, on réalise des essais de fretting et de fatigue pour étudier les conditions d'amorçage de fissures en présence de gradient de contrainte et de frottement. Les résultats expérimentaux mettent en évidence l'effet du gradient des contraintes sur l'amorçage de fissures et justifient la prise en compte du gradient pour le dimensionnement en fatigue des structures. A travers ces essais balayant une gamme de gradient de contrainte étendue, on confirme que la présence d'un gradient de contrainte retarde l'amorçage de fissures. Les avantages et les inconvénients de plusieurs approches de fatigue multiaxiale intégrant le gradient de contrainte sont étudiés. De cette analyse, une proposition est faite permettant de décrire au mieux les résultats expérimentaux de fatigue et de fretting. Cette approche s'appuie sur l'utilisation d'une fonction de poids dépendant du gradient de la pression hydrostatique. Cette approche est similaire à des approches mises en oeuvre sur d'autres problématiques de fatigue ce qui permet de dégager quelques points de convergence pour traiter le problème du gradient de contrainte en fatigue. Un essai de fatigue sur une manille munie d'une bague montée avec interférence est réalisé. L'effet du gradient de contrainte sur l'amorçage de fissures dans ce système est trop faible pour que l'utilisation d'une approche intégrant l'effet de gradient soit pertinente. Cette observation est cohérente avec les résultats expérimentaux précédents obtenus dans cette étude. L'inconel 718 apparaît donc comme un matériau peu sensible à l'effet de gradient de contrainte. Une approche de fatigue multiaxiale de type Crossland permet une description raisonnable de la durée de vie du système. Une analyse numérique montre que la tenue en fatigue du système est peu sensible au coefficient de frottement alors que l'interférence a un impact beaucoup plus important.

Cette étude financée par SNECMA et le CNES a pour objectif de mieux comprendre et caractériser le résultats de la solidification de l'inconel 718. Cet objectif est motivé par le besoin grandissant de fiabilité sur les pièces brutes de fonderies utilisées dans l'assemblage des moteurs de fusée. En effet, lors de la solidification, la ségrégation chimique provoque la formation de phase fragile. C'est notamment le cas des phases de Laves. Ainsi, un four de fonderie sous vide et un moule instrumenté ont été mise au point afin de couler au laboratoire des plaques de géométrie variable. Le but étant d'obtenir plusieurs échantillons obtenu sous différente conditions de solidifications. Une méthode d'analyse quantitative au MEB a par la suite été mise au point pour analyser les 4 coulées exploitables. Elle a permis de quantifier la quantité d'intermétalliques en fonction de la vitesse de refroidissement. Mais aussi leur répartition vis à vis des joints de grains d'une part et l'impact de la désorientation entre chaque grain d'autre part. La ségrégation chimique a aussi pu être quantifiée selon ces mêmes paramètres.De travaux numériques ont été entrepris en parallèles avec tout d'abord la simulation macroscopique des coulées avec le logiciel Thercast. Ces simulations ont permis de connaître les conditions de solidification de l'ensemble des échantillons coulés et ont fourni des données d'entré pour les autres aspects numériques. Par exemple, avec un modèle élément fini et automate cellulaire, la formation de la structure granulaire a pu être modélisée. Enfin, la simulation de la microségrégation a permis de valider notre compréhension des phénomènes prédominant lors de la solidification de l'inconel 718. Ces simulations numériques apportent à la fois un éclairage sur les résultats expérimentaux mais aussi démontrent leurs possibilités dans l'optique de mettre en place un modèle global de solidification recouvrant ces trois échelles pour des pièces industrielles

A pipe consisting of Inconel 600 welded to grade 106-B Carbon-Steel using Inconel 182 weld filler is used to transport heavy water in nuclear reactors. A confidential report concluded that cracking is one of the problems these pipes are currently facing. Before cracking can be fully understood the mechanical properties of the weld must be determined. This thesis analyzed the pipe at various length-scales using optical microscopy, micro-hardness testing, small and large scale tensile testing and digital image correlation (DIC). This thesis successfully achieved it goals of determining the mechanical properties and creating a model of the Inconel dissimilar metal weld. It partially met the goal of observing fracture mechanisms as it was able to observe fracture in tensile samples but was not able to successfully track crack growth.

The objective of this work is to study crack path deflection under proportional and non-proportional mixed-mode fatigue and predict crack branching direction based on linear elastic fracture mechanics (LEFM) driving forces. Under proportional in-phase mixed Mode I / Mode II loading conditions, crack growth direction has previously been observed in some materials to shift from tensile-dominated Mode I to shear-dominated Mode II or mixed-mode crack growth at higher proportions of initial Mode II loading, but non-proportional loads are not well-characterized. An LEFM approach is desired in order to implement the model in crack growth software such as the boundary element-based fracture analysis package FRANC3D. A novel specimen configuration has been designed and analyzed for generation of wide ranges of mixed-mode loading conditions in a single test. This specimen and a more conventional thin-walled tubular specimen have been used to test polycrystalline nickel-base superalloy Inconel 718 under proportional in-phase and 3 kinds of non-proportional fatigue loading. Stress intensity factors for the various configurations have been analyzed with FRANC3D. Modal transition from Mode I (tensile) to Mode II (shear) crack branching has been observed in several load cases. Qualitative microscopy of fracture surfaces was used to characterize the difference between crack branch modes. An LEFM approach based on an effective stress intensity factor range, which incorporates the maximum value and range of each appropriate stress intensity (Mode I or Mode II), has been used to successfully predict the crack deflection angles, and in some cases to quantify modal transition, within each load case considered. Variability between load cases and specimen configurations points to the limitations of LEFM in providing a general predictor of crack path behavior across all types of non-proportional mixed mode loading.

As ligas a base de Níquel são frequentemente utilizadas na indústria do petróleo devido à elevada resistência mecânica e excelente resistência à corrosão. Neste cenário, a utilização da soldagem como etapa na fabricação de dutos rígidos de grande comprimento é inevitável. Assim, chapas da liga Inconel 625® foram unidas através do processo Friction Stir Welding (FSW). A soldagem foi executada com uma ferramenta de nitreto cúbico de boro policristalino e o inerente desta ferramenta foi analisado. Em um primeiro momento, na seleção de parâmetros de processo, diversas condições de processo foram analisadas para escolha da melhor solda. Após esta etapa, novas juntas soldadas foram produzidas com o parâmetro de processo adequado (velocidade de rotação da ferramenta de 200 rpm e velocidade de avanço de 1 mm/s). A caracterização da superfície de topo da solda foi realizada através da microscopia ótica, microscopia eletrônica de varredura, microdureza e ensaios de dobramento. Defeitos superficiais foram inspecionados com líquidos penetrantes. Além disso, os estados de tensões residuais foram obtidos com a difração de raios-X. Ainda, a resistência à corrosão localizada foi estudada com o ensaio de imersão em cloreto férrico e a determinação da temperatura crítica de pite (conhecida como CPT) Por fim, a susceptibilidade à corrosão intergranular foi avaliada com o ensaio Streicher Test. O processo de soldagem promoveu o refino de grão e o aumento de dureza na solda. As micrografias do metal de base (MB) mostraram carbonetos do tipo MC, M6C, M23C6. Por outro lado, a melhor junta soldada apresentou uma microestrutura livre de defeitos, homogênea, onde não foram percebidos carbonetos do tipo M6C, M23C6 nos contornos de grão, foi aprovada nos ensaios de dobramento, líquidos penetrantes e imersão em FeCl3. Entretanto, ocorreu o um desgaste da ferramenta, mas reduzido e aceitável para esta solda. Tensões residuais longitudinais trativas da ordem de 50 MPa foram observadas na zona de mistura. Considerando o valor médio, o MB alcançou uma CPT de 77 °C, enquanto no centro da solda a CPT foi de 86°C. A taxa de corrosão média alcançada após o ensaio de corrosão intergranular foi de 0,4406 mm/ano, valor que sugere uma boa qualidade da solda. / Nickel based alloys are often used in the oil and gas industry due to their great mechanical strenght as well as excelent corrosion resistance. In this scenario, welding as manufacturing step for long lenght rigid tubes is inevitable. Therefore, Inconel 625® sheets were joined by Friction Stir Welding (FSW). The welding was performed with a Polycrystalline Cubic Boron Nitride (pcBN) tool and the unavoidable tool wear was verified. At first, in the process parameter development step, different process conditions were analyzed for choosing the best welded joint. Afterwards, new welded joints were produced with the suitable process parameter (tool rotational speed of 200 rpm and welding speed of 1 mm/s). Top surface characterization of friction-stir-welded Inconel 625 was carried out by optical microscopy, scanning electron microscopy, microhardness and bending tests. Surface defects were verified by dye penetrant inspection (DPI). In addition, residual stress states were obtained by X-ray diffraction. Furthermore, localized corrosion resistance was studied by the immersion test in FeCl3 and the critical pitting temperature (CPT) determination. Finally, the susceptibility to intergranular corrosion was evaluated by Streicher Test. FSW process promoted grain refining as well as increased microhardness in the joint Base material micrographs showed different carbides as MC, M6C, M23C6. On the other hand, the best joint presented a sounds weld without defetcs, homogeneous microstructure, grain boundaries free of M6C, M23C6 carbides, approved in bending tests, DPI and the immersion test. However, a tool wear was also noted, but reduced and acceptable for this weld. Tensile longitudinal residual stresses of 50 MPa were observed in the stir zone. As an avarage, the base material CPT was 77 °C, while the CPT was 86 °C at the weld center. The mean corrosion rate after intergranular corrosion test was 0.4406 mm/year, a value which suggests a good weld quality.

Additive manufacturing (AM) is a growing process interesting many companies in many industries. Thereare multiple processes within the familty of AM, but this study focuses on laser blown powder (LBP). LBP involves a laser beam focused on the substrate with powder being blown into the laser beam. The laser beam melts both the powder and the surface of the substrate and as the laser beam moves and the melt pool solidies it leaves a bead of solid material behind. These beads are placed next to each other creating a layer which are then stacked, building the wanted geometry. As the method develops new materials are tested and this study analyses Haynes 282 powder onto Inconel 718 substrate. Multiple process parameters are involved in the LBP method and this study focuses on the impact of laser effect and scanning speed. Each value on the process parameters was inspired by previous reports with similar equipment and process. The laser effect ranges from 1600 W to 700 W, scanning speed ranges between 900 mm/min to 300 mm/min and the powder feeding rate was also varied from 4 g/min to 3 g/min. Each sample was built as a single bead and a multilayer specimen, which is ve layers and 16 beadswide at the bottom and 12 beads wide at the top. When analyzing the samples images from microscopes were mostly used for obtaining results. An image software called ImageJ allowed measurements in an image to obtain penetration depth or primary dendrite arm spacing. ImageJ also allowed measurements of porosity by turning the image binary and calculate the fraction of white and black. The results consists of numerical values and visual analysis of the bead geometry, minimum and maximum penetration, microstructure, porosity, hardness and cracks. The results show an increased bead width around 2 mm to 4 mm and decreased bead height around 0,2 mm to 0,7 mm of single beads with increased laser effect. Increased maximum penetration depth around, 200 μm to 500 μm, withincreased laser effect. More remelt between each deposited layer causing longer dendrites with increasinglaser effect. Porosity is decreased with an increased laser power, going from 0,04 % to 0,15 %. No distinct difference in hardness is observed between the samples, ranging between 255 HV to 310 HV. It is believed that aging causes the increased hardness right above the fusion zone. Cracks were found between dendrites and is believed to be caused by Laves-phases. Most results are comparable to previous similar studies, both as trends and numerical values. The statistics of the study is limited, meaning that all results should not be taken as granted but as a general guide line for more studies. The purpose and goals of the study has been met and completed.

Este trabalho tem por objetivo avaliar a capacidade de detecção e dimensionamento de descontinuidades internas em juntas soldadas dissimilares e cladeadas com a utilização da inspeção por ultrassom através da técnica de Phased Array. Este estudo se justifica pela possibilidade da aplicação desses materiais nas atividades de exploração de petróleo no campo offshore com a utilização de dutos rígidos de aço carbono com revestimento interno de Inconel. As juntas circunferenciais foram soldadas com material austenítico e foram realizados ensaios com juntas sem a realização do amanteigamento do bisel e com corpos de provas que receberam camadas de amanteigamento que tem a finalidade de aumentar a resistência à corrosão-fadiga da junta circunferencial. A tubulação soldada tinha como material de base um aço ferrítico API 5L X-52 com revestimento interno de Inconel 625. As descontinuidades foram inseridas nos passes de raiz fabricadas com o processo de soldagem TIG, nos passes de enchimento através do processo por eletrodos revestidos e na região da raiz através do processo de eletroerosão com a finalidade de simular uma falta de fusão na raiz. Para as inspeções foram adotados as varreduras setorial e linear com duas posições do cabeçote em relação à linha de centro da solda. Estas foram realizadas com o intuito de avaliar a capacidade de detecção e dimensionamento das descontinuidades através da utilização de ondas transversais com freqüência de 5 MHz e assim analisar os resultados encontrados para poder viabilizar a aplicação desta técnica como ferramenta de inspeção nestes equipamentos. Análises metalográficas serviram para realizar o dimensionamento real das descontinuidades e caracterizar a micrografia do metal de base, zona afetada pelo calor e metal de solda para poder correlacioná-los com os resultados encontrados nas inspeções por Phased Array. Assim com os resultados encontrados foi possível demonstrar a possibilidade de aplicação da técnica de Phased Array na inspeção das juntas dissimilares e cladeadas. / This work aims to evaluate the detection capability and sizing of internal discontinuities in dissimilar welded and claded joints with the use of Phased Array ultrasonic inspection technique. This study was performed due to the possibility of the application of those materials in the activities of exploration of petroleum in the offshore field with the use of rigid risers of carbon steel with internal covering of Inconel. The girths joints were welded with material austenitic and testing were accomplished with joints without the realization of the buttering of the bevel and with specimens that received layers of buttering that has the purpose of increasing the resistance to the corrosion-fatigue of the girth joint. The welded piping had as base material a ferritic steel API 5L X-52 with internal overlay of Inconel 625. The discontinuities were inserted in the root pass manufactured with the welding process TIG, in the filler pass through the process by covered electrodes and in the area of the root through the electric discharge machining process with the purpose of simulating a lack of fusion in the root. For the inspections the sectorial and linear sweepings were adopted with two positions of the transducer in relation to the line of center of the weld. This was performed with the intention of evaluating the detection capacity and sizing of discontinuities through the use of shear waves frequently of 5 MHz and like this to analyze the results found to make possible the application of this technique as inspection tool in these equipments. Metallographic analyses were to accomplish the real sizing of the discontinuities and to characterize the micrograph of the base metal, affected area for the heat and weld metal to correlate them with the results found in the inspections by Phased Array. Like this with the found results it was possible to demonstrate the possibility of application of Phased Array technique in the inspection of the dissimilar and claded joints.

As superligas de níquel apresentam boa combinação de resistência à corrosão e à oxidação, resistência mecânica, resistência à fadiga e resistência à fluência. Além disso, apresentam bom desempenho operacional em temperaturas próximas à temperatura de fusão da liga. Estudos apontam à possibilidade de se obter aço carbono revestido com a superliga de níquel Inconel 625 pelo processo de cladeamento e pelo processo de aspersão térmica. A presença de molibdênio na composição química do Inconel625 garante excelente resistência à corrosão por pites e por fresta, enquanto o nióbio confere alta soldabilidade e resistência à corrosão intergranular a essas ligas. O processo de soldagem é amplamente usado em processos industriais e pode ser aplicado em uma junta Inconel-Inconel, podendo modificar alguma propriedade dessa liga. O objetivo do presente trabalho é estudar uma junta soldada de Inconel 625, obtida pelo processo de soldagem por fusão a arco elétrico com o uso de eletrodo revestido ENiCrMo-3, quanto à morfologia estrutural, à dureza e à resistência à corrosão. A morfologia foi avaliada por microscopia óptica, por microscopia eletrônica de varredura e por espectroscopia de energia dispersiva. A propriedade mecânica de dureza foi avaliada a partir do perfil de microdureza Vickers ao longo da junta soldada Para avaliar a resistência à corrosão da junta soldada foram realizados os ensaios de polarização potenciodinâmica, reativação potenciodinâmica de varredura cíclica (norma ASTM G108-94), polarização potenciodinâmica cíclica e ensaio de imersão (norma ASTM G48-03). Também avaliou-se a susceptibilidade à corrosão por pites pela norma ASTM G61-86 a temperatura ambiente e a 90 ºC. Para a polarização potenciodinâmica, três diferentes eletrólitos foram empregados: 3,5% NaCl, 3,5% NaCl + 0,01 mol/L Na2S2O3 e 3,5% NaCl + 1M H2SO4. Os resultados obtidos mostraram que o metal de solda apresenta uma microestrutura dendrítica com precipitados de Nb e de Mo nas regiões interdendríticas. O desempenho quanto à resistência à corrosão foi satisfatório nos meios estudados, considerando as características dessas ligas. No entanto, a junta soldada apresentou maior suscetibilidade à corrosão por pites e à corrosão localizada comparativamente ao metal base. O filme passivo formado no metal base é melhor constituído do que o filme passivo formado no metal de solda. / Nickel superalloys show a good combination of corrosion, oxidation and creep resistance as well as mechanical and fatigue strength. These alloys can operate at temperatures close to their melting temperature. The Inconel 625 has desired characteristics for application in environments that require high corrosion resistance, such as in oil and gas exploitation. Some studies show the possibility of obtaining a coating of Inconel 625 onto carbon steel by the cladding process and by the thermal spray process. The molybdenum content in Inconel 625 ensures excellent resistance to pitting and crevice, while the niobium content gives high weldability to these alloys. The welding process is practically present in all industrial plants, so an Inconel-Inconel joint is unavoidable. However, the welding can lead to changes in the properties of these superalloys. Therefore, the present work aims to study the structural morphology, hardness and corrosion resistance of a welded joint of Inconel 625 obtained by the electric arc fusion welding process with ENICrMo-3 covered electrode. The morphology was evaluated by optical microscopy, scanning electron microscopy and dispersive energy spectroscopy. Hardness was studied by Vickers microhardness profile along the welded joint. In order to evaluate the corrosion resistance of the welded joint, the Potentiodynamic polarization, potentiodynamic cyclic sweep reactivation (ASTM G108-94), cyclic potentiodynamic polarization and immersion test (ASTM G48-03) were performed Also susceptibility to pitting corrosion was assessed by ASTM G61-86 at room temperature and at 90 ° C. Potentiodynamic polarization curves were performed in three different solutions to evaluate the corrosion resistance: 3.5% NaCl, 3.5% NaCl + 0.01 mol / L Na2S2O3 and 3.5% NaCl + 1M H2SO4 The results show that the weld metal has a dendritic microstructure with precipitates of Nb and Mo in the interdendritic regions. The corrosion performance in environments close to the environments of possible applications was satisfactory considering the resistance of these alloys, but the specific welded joint was more susceptibility to corrosion by pits and localized corrosion. The passive film formed in the base metal was better formed than the passive film formed in the welded metal.

Este trabalho tem por objetivo avaliar a capacidade de detecção e dimensionamento de descontinuidades internas em juntas soldadas dissimilares e cladeadas com a utilização da inspeção por ultrassom através da técnica de Phased Array. Este estudo se justifica pela possibilidade da aplicação desses materiais nas atividades de exploração de petróleo no campo offshore com a utilização de dutos rígidos de aço carbono com revestimento interno de Inconel. As juntas circunferenciais foram soldadas com material austenítico e foram realizados ensaios com juntas sem a realização do amanteigamento do bisel e com corpos de provas que receberam camadas de amanteigamento que tem a finalidade de aumentar a resistência à corrosão-fadiga da junta circunferencial. A tubulação soldada tinha como material de base um aço ferrítico API 5L X-52 com revestimento interno de Inconel 625. As descontinuidades foram inseridas nos passes de raiz fabricadas com o processo de soldagem TIG, nos passes de enchimento através do processo por eletrodos revestidos e na região da raiz através do processo de eletroerosão com a finalidade de simular uma falta de fusão na raiz. Para as inspeções foram adotados as varreduras setorial e linear com duas posições do cabeçote em relação à linha de centro da solda. Estas foram realizadas com o intuito de avaliar a capacidade de detecção e dimensionamento das descontinuidades através da utilização de ondas transversais com freqüência de 5 MHz e assim analisar os resultados encontrados para poder viabilizar a aplicação desta técnica como ferramenta de inspeção nestes equipamentos. Análises metalográficas serviram para realizar o dimensionamento real das descontinuidades e caracterizar a micrografia do metal de base, zona afetada pelo calor e metal de solda para poder correlacioná-los com os resultados encontrados nas inspeções por Phased Array. Assim com os resultados encontrados foi possível demonstrar a possibilidade de aplicação da técnica de Phased Array na inspeção das juntas dissimilares e cladeadas. / This work aims to evaluate the detection capability and sizing of internal discontinuities in dissimilar welded and claded joints with the use of Phased Array ultrasonic inspection technique. This study was performed due to the possibility of the application of those materials in the activities of exploration of petroleum in the offshore field with the use of rigid risers of carbon steel with internal covering of Inconel. The girths joints were welded with material austenitic and testing were accomplished with joints without the realization of the buttering of the bevel and with specimens that received layers of buttering that has the purpose of increasing the resistance to the corrosion-fatigue of the girth joint. The welded piping had as base material a ferritic steel API 5L X-52 with internal overlay of Inconel 625. The discontinuities were inserted in the root pass manufactured with the welding process TIG, in the filler pass through the process by covered electrodes and in the area of the root through the electric discharge machining process with the purpose of simulating a lack of fusion in the root. For the inspections the sectorial and linear sweepings were adopted with two positions of the transducer in relation to the line of center of the weld. This was performed with the intention of evaluating the detection capacity and sizing of discontinuities through the use of shear waves frequently of 5 MHz and like this to analyze the results found to make possible the application of this technique as inspection tool in these equipments. Metallographic analyses were to accomplish the real sizing of the discontinuities and to characterize the micrograph of the base metal, affected area for the heat and weld metal to correlate them with the results found in the inspections by Phased Array. Like this with the found results it was possible to demonstrate the possibility of application of Phased Array technique in the inspection of the dissimilar and claded joints.

abstract: The microstructure development of Inconel alloy 718 (IN718) during conventional processing has been extensively studied and much has been discovered as to the mechanisms behind the exceptional creep resistance that the alloy exhibits. More recently with the development of large scale 3D printing of alloys such as IN718 a new dimension of complexity has emerged in the understanding of alloy microstructure development, hence, potential alloy development opportunity for IN718. This study is a broad stroke at discovering possible alternate microstructures developing in Direct-Metal-Laser-Sintering (DMLS) processed IN718 compared to those in conventional wrought IN718. The main inspiration for this study came from creep test results from several DMLS IN718 samples at Honeywell that showed a significant improvement in creep capabilities for DMLS718 compared to cast and wrought IN718 (Honeywell). From this data the steady-state creep rates were evaluated and fitted to current creep models in order to identify active creep mechanisms in conventional and DMLS IN718 and illuminate the potential factors responsible for the improved creep behavior in DMSL processed IN718. Because rapid heating and cooling can introduce high internal stress and impact microstructural development, such as gamma double prime formations (Oblak et al.), leading to differences in material behavior, DMLS and conventional IN718 materials are studied using SEM and TEM characterization to investigate sub-micron and/or nano-scale microstructural differences developed in the DMLS samples as a result of their complex thermal history and internal stress. The preliminary analysis presented in this body of work is an attempt to better understand the effect of DMLS processing in quest for development of optimization techniques for DMLS as a whole. A historical sketch of nickel alloys and the development of IN718 is given. A literature review detailing the microstructure of IN718 is presented. Creep data analysis and identification of active creep mechanisms are evaluated. High-resolution microstructural characterization of DMLS and wrought IN718 are discussed in detail throughout various chapters of this thesis. Finally, an initial effort in developing a processing model that would allow for parameter optimization is presented. / Dissertation/Thesis / Masters Thesis Engineering 2017