Austenitic stainless steels are frequently used for engineering applications in aggressive environments. Typical sources of component failures are associated with localized attack at grain boundaries, such as intergranular corrosion and stress corrosion cracking. To prevent premature failures, structural integrity assessments are carried out, with the aim of predicting the maximum likelihood of cracking that may develop. For accurate predictions it is of great importance to know the interaction of parameters involved in life-determining processes. This PhD thesis investigates the effect of microstructure and stress on intergranular stress corrosion cracking in Type 302 / Type 304 austenitic stainless steels. High-resolution X-ray tomography has been successfully applied to examine, for the first time in 3-dimensions, in-situ, the interaction between microstructure and crack propagation. The development and subsequent failure of crack bridging ligaments has been observed and correlated with regions of ductile tearing persistent on the fracture surface. These ductile regions were consistent with the morphology of low-energy, twin-type grain boundaries, and are believed to possess the capability of shielding the crack tip. Following this observation, a new grain bridging model has been developed, in order to quantify the effect of static stress and crack bridging on the maximum likely crack length. The model was compared and evaluated with in the literature available percolation-like models. Intergranular stress corrosion tests in tetrathionate solutions have been designed and carried out to validate the new model. The assessment comprised,(i) a thorough examination of the microstructure and analysis parameters employed,(ii) the determination of the degree of sensitisation with subsequent crack path investigations,(iii) the identification of a suitable test system with associated grain boundary susceptibility criteria,(iv) the application of Grain Boundary Engineering (GBE) for microstructure control,(v) statistical crack length assessments of calibrated IGSCC test specimens. The results of these tests showed that the new model successfully predicts the magnitude of stress and the effect of grain boundary engineering on the maximum crack lengths.

Dans les études expérimentales de la micromécanique des matériaux granulaires, la mesure des forces de contact entre particules est de nos jours toujours un challenge en comparaison avec les outils et techniques bien mieux établis pour la caractérisation cinématique à l'échelle des particules. Cette thèse de doctorat s'attaque à cet ambitieux problème. L'approche proposée implique deux aspects : (i) la caractérisation expérimentale du réseau de contact et de la cinématique à l'échelle des particules, qui peut être réalisée avec des techniques d'imagerie standards ; (ii) une approche numérique capable d'exploiter ces mesures afin de déduire les forces de contact.L'une des contraintes qu'on s'était imposée était de ne s'appuyer que sur la connaissance de la géométrie des particules ainsi que du réseau de contacts pour réaliser la déduction des forces de contact. Trois techniques numériques différentes ont été proposées à cet effet : une méthode basée sur l'élasticité des contacts (CEM), une méthode basée sur la dynamique de contact (CDM) et une méthode basée sur l'équilibre élasto-plastique de l'assemblage granulaire (QSM). Chacune de ces techniques repose sur une approche de la famille des méthodes en éléments discrets ; il s'agit respectivement de le DEM de type Cundall, la dynamique des contacts non régulière, et une approche de calcul statique élastoplastique. La non-unicité de la solution est le principal problème avec les techniques choisies, et elles sont étroitement liées à l’indétermination des forces dans le système.Les trois méthodes sont d'abord présentées et validées en les appliquant à l'estimation des forces dans les systèmes granulaires 2D générés au moyen de simulations DEM explicites. Nous prenons ces simulations comme des expériences "idéales" dans le sens où elles fournissent des données similaires à celles extraites des expériences, mais dépourvues d'erreurs de mesure. Un avantage évident de cette stratégie est d’obtenir des ensembles de forces faisant office de référence faisant foi. Sur cette base, les principaux aspects affectant la détermination des forces peuvent être étudiés. En particulier, le rôle crucial de l'histoire du chargement est mis en évidence et certaines solutions pour les prendre en compte dans la détermination des forces ont été prospectées. Une évaluation de l'influence de l'erreur de mesure a également été réalisée pour prédire l'applicabilité de chaque méthode à des expériences réelles. Une brève analyse de la variabilité des solutions est également fournie.Finalement, des tentatives ont été faites pour déduire des forces issues d’expériences effectuées dans le dispositif 1gamma2epsilon. La cinématique des particules et la connectivité ont été évaluées au moyen de la technique de corrélation d'image numérique. Les avantages et inconvénients des trois méthodes ont été éclaircis. Ils nous conduisent à envisager une utilisation combinée des trois méthodes pour tirer parti de leurs atouts respectifs. À l'avenir, il conviendra de réfléchir à la prise en compte de la stabilité de la solution -- dans l'algorithme de convergence vers une solution -- avec l'espoir de limiter la variabilité des solutions. / In the experimental study of the micro-mechanics of granular materials, measuring inter-particle contact forces is still a challenging task, if compared to the well-established tools and techniques for the kinematic characterisation at particle scale. This doctoral thesis addresses this problem. The proposed approach consists of two parts: an experimental characterisation of the granular network geometry and of particle-scale kinematics, which can be carried out with common imaging techniques such as Digital Image Correlation; a numerical approach aiming to exploit these measurements for the estimation of forces.One imposed constraint was to only make use of the rigid motions of particles, together with the knowledge of the contact network, to infer contact forces. Three different numerical techniques have been proposed to this purpose, referred to as Contact Elasticity Method (CEM), Contact Dynamics-based Method (CDM) and Quasi-Static Method (QSM). Each of these techniques is based on the formulation of common approaches in the family of Discrete Element Methods, respectively the classical Cundall-like DEM, the Non Smooth Contact Dynamics and a quasi-static approach accounting for both contact elasticity and plasticity. It is shown that memory of the history of the packing is the main concern with all the chosen techniques.The three methods are first presented and validated by applying them to the estimation of forces in 2D granular systems generated by means of explicit-time DEM simulations. We refer to these simulations as "ideal" experiments since they are meant to provide the same information that can be extracted from experiments, but without any measurement error. An obvious benefit of this strategy is to get reference force sets that are taken as ground truth. Based on this, the main aspects that affect the determination of forces can be investigated. In particular, the crucial role of history is emphasised here, and some solutions to take it into account in the force inference have been investigated.An assessment of the influence of measurement error has also been carried out, to predict the applicability of each method to real experiments. A short analysis of the variability of the solutions is also provided.Finally, some attempts have been made to infer forces from experiments carried out in the 1gamma2epsilon device. Particle kinematics and connectivity have been assessed by means of the Digital Image Correlation technique.The benefits and drawbacks of the three methods have been demonstrated. They conduct us to envision a combined usage of the three methods. In the future, studying the stability of equilibrium might help reducing the variability of the solutions.

A junção por difusão em argônio é uma técnica cujo sucesso depende da otimização de determinados parâmetros. No presente trabalho, executado sobre o aço inoxidável AISI 304L, reporta-se um estudo desses parâmetros e dos mecanismos de junção por difusão. Foram escolhidos os parâmetros gerais para junção (pressão e temperatura). Os experimentos foram efetuados nos parâmetros gerais constantes com três tempos diferentes. Três diferentes acabamentos superficiais foram produzidos; a resistência mecânica foi medida pelo ensaio de tração e diversas técnicas de caracterização foram empregadas para descrever a interface. A melhor condição de processo em termos de resistência mecânica foi 1000 C/17MPa durante 40 minutos, correspondendo a s = 552 MPa. Nestas condições não houve redução da resistência mecânica na junta soldada. As peças de aço inoxidáveis 304L foram tratadas durante a operação de solda por difusão. Estes parâmetros não se tornaram sensíveis a um ataque intergranular.As exigências ao estado de superfícies foram as seguintes:- a qualidade da superfície com rugosidade da superfície Ra= 1 até 6 m, pode ser realizada com usinagem fina,- evitar acabamento superficial com processos de atrito, e- preparações superficiais devem ser realizadas pouco antes da soldagem por difusão.

<p>Laboratory tests on microscale are reported in which amorphous silica grains were compressed in a liquid environment, namely in solutions with different silica ion concentrations for up to four weeks. Such an arrangement represents an idealized representation of two sand grains. The grain surfaces and asperities were examined in Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) for fractures, silica polymer growth, and polymer strength. Single chains of silica polymers are found to have a failure pulling force of 330 - 450 nN. </p><p>A chain of observations are reported for the first time, using Pneumatic Grain Indenter and Grain Indenter-Puller apparatuses, confirming a long-existing hypothesis that a stressed contact with microcracks generates dissolved silica in the contact (asperity) vicinity, which eventually polymerizes, forming a structure between the grains on a timescale in the order of weeks. Such structure exhibits intergranular tensile force of 1 - 1.5 mN when aged in solutions containing silica ion concentrations of 200- to 500 ppm. Stress appears to accelerate the generation of silica polymers around stressed contact regions, so does mica-silica contacts. The magnitude of intergranular tensile force is 2 to 3 times greater than that of water capillary effect between grains.</p> / Dissertation

In this study, a two phase investigation of the hydraulic conductivity parameters of silty soils was performed. In the first phase, double-ring infiltrometer tests were used to measure infiltration rates in-situ at two sites in the Piedmont physiographic province of Georgia. The efficacy of predicting saturated hydraulic conductivity for Piedmont soils via published soil surveys from the National Resource Conservation Service and pedotransfer functions was then investigated. Work focused on the development of a consistent test methodology for soils (sandy, to silts and clays) in the Piedmont, and the final test method utilized being the constant head test, using a double-ring infiltrometer with Mariotte tubes to maintain the head. In the second phase of the investigation, laboratory based measurements of the saturated hydraulic conductivity of binary mixtures of fine sand and nonplastic silt were performed to investigate the effects of particle mixtures on hydraulic conductivity. The materials used were ASTM 100/200 sand and Sil-Co-Sil 40 non-plastic silt, chosen based on the ratio of the mean particle diameters. Significant effort was invested in the development and comparison of methodologies to produce uniform specimens of the binary mixtures for hydraulic conductivity testing, with the final being modified dry tubing. Two fixed densities were used to investigate the effects of particle packing on the hydraulic conductivity of binary mixtures, with critical fines contents chosen to ensure the finer particles primarily filled the pore volume of the coarse particles. Incremental fines contents, by mass, up to this theoretical fines content were tested. The measured saturated hydraulic conductivity was evaluated in terms of fines content, global and intergranular void ratio, and confining stress. Models for predicting extreme void ratios and saturated hydraulic conductivity of binary mixtures were also investigated.

Austenitic stainless steels have a major problem during solution annealing or welding in the temperature range of 500-800 &deg / C due to the formation of chromium carbide, which causes chromium depleted areas along grain boundaries. This means that the structure has become sensitized to intergranular corrosion. Susceptibility to intergranular corrosion can be determined by means of destructive acid tests or by nondestructive electrochemical potentiokinetic reactivation (EPR) tests. The EPR test, which provides quantitative measurements, can be practiced as single loop or double loop. Single loop EPR method for AISI 304 and 304L type stainless steels was standardized / however double loop EPR (DLEPR) method has not been validated yet. In this study, the degree of sensitization was examined in AISI 304L and 316L type steels by DLEPR method whose experiments have been carried out on sensitive and nonsensitive steels to examine and determine the detailed parameters / solution temperature, concentration and scan rate of the DLEPR method. In order to determine the degree of sensitization, oxalic acid, Huey and Streicher tests were carried out and revealed microstructures and measurements of weight loss by the acid tests were then correlated with DLEPR method results, as a first step towards standardization of DLEPR method for 316L steels. Best agreement was provided with test parameters which are 1M H2SO4 + 0.005M KSCN at 3 V/hr scan rate with 30 &deg / C solution temperature. It was concluded that specimens can be classified as step, dual and ditch, if the Ir:Ia ratios were obtained to be between 0 to 0.15, 0.15 to 4.0 and 4.0 to higher respectively.

Submitted by Maykon Nascimento (maykon.albani@hotmail.com) on 2015-12-15T18:33:05Z No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Avaliacao da resistencia a corrosao de revestimentos da liga UNS N06625 depositados pelo processo PTA P.pdf: 6159981 bytes, checksum: 50da99c404daae3acef07ae743845d6a (MD5) / Approved for entry into archive by Morgana Andrade (morgana.andrade@ufes.br) on 2015-12-30T15:51:09Z (GMT) No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Avaliacao da resistencia a corrosao de revestimentos da liga UNS N06625 depositados pelo processo PTA P.pdf: 6159981 bytes, checksum: 50da99c404daae3acef07ae743845d6a (MD5) / Made available in DSpace on 2015-12-30T15:51:09Z (GMT). No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Avaliacao da resistencia a corrosao de revestimentos da liga UNS N06625 depositados pelo processo PTA P.pdf: 6159981 bytes, checksum: 50da99c404daae3acef07ae743845d6a (MD5) Previous issue date: 2015 / A busca por materiais versáteis que associem elevada resistência à corrosão em ambientes severos e propriedades mecânicas, a fim de atender as mais exigentes condições de serviço tornou-se um desafio para a indústria petroquímica, termoelétrica, naval, dentre outras. Neste contexto, a soldagem de revestimento com ligas à base de níquel se torna uma interessante opção para a construção e o reparo de equipamentos para aplicações em ambiente de corrosão severa, permitindo alcançar as características desejadas, com significativa redução de custos. Em determinadas aplicações a alta diluição dos revestimentos deve ser controlada devido aos altos teores de ferro alcançados. O objetivo deste trabalho foi avaliar a corrosão intergranular e corrosão por pite de revestimentos da liga UNS N06625, com diferentes teores de diluições, depositados pela técnica Plasma com Arco Transferido Alimentado com Pó (PTA-P) sobre um substrato de aço carbono ASTM A36. Com a avaliação e caracterização do Inconel 625 buscou-se compreender o impacto da diluição na microestrutura, propriedades e no revestimento produzido. Foram realizadas análises das propriedades mecânicas dos revestimentos por meio de ensaios de microdureza. A caracterização microestrutural consistiu de análises de microscopia ótica, Microscopia Eletrônica de Varredura (MEV) e análise de composição química por EDS. Ensaios de reativação eletroquímica potenciodinâmica de duplo ciclo (DL-EPR) e polarização potenciodinâmica cíclica foram realizados para avaliar a resistência à corrosão intergranular e por pite, respectivamente. Através deste trabalho foi possível observar que a diferença de diluição influencia de forma suave na microdureza do revestimento. As microestruturas dos revestimentos apresentaram matriz γ com fases secundárias interdendríticas. Os revestimentos não apresentaram corrosão intergranular nas condições ensaiadas e a susceptibilidade da corrosão por pite somente pôde ser avaliada através da histerese apresentada pelas curvas de polarização. / The search for versatile materials that associate high corrosion resistance and high mechanical properties in severe environments, in order to meet the more demanding operational conditions has become a challenge for the petrochemical, thermoelectric, naval and other industries. In this context, weld overlay with nickel-based alloys becomes an interesting option for construction and repair of equipments for applications in severe corrosion environments, allowing to achieve the desired characteristics with significant cost reduction. In specific applications the high overlay dilution must be controlled due to high iron content encountered. The objective of this work is to evaluate the intergranular and pitting corrosion of the alloy UNS N06625, with different dilution values, deposited through Plasma Transferred Arc with Powder (PTA-P) welding technique on an ASTM A36 carbon steel substrate. With the evaluation and characterization of the alloy UNS N06625 aimed the comprehension of the dilution effect on the coating microstructure and properties. The mechanical properties tests were analyzed through microhardness tests. The microstructural characterization consisted of analyses with the optical microscope, with the Scanning Electron Microscope (SEM) and chemical composition with the EDS module. Double-Loop Electrochemical Potentiokinetic Reactivation Test (DL-EPR) and Cyclic Potentiodynamic Polarization tests were done to evaluate intergranular and pitting corrosion resistance, respectively. With this work it was possible to observe that the dilution difference influences in a smooth way in the coating microhardness. The microstructures presented aγ matrix with secondary interdendritic phases. The coatings did not present intergranular corrosion in the test conditions and the pitting corrosion susceptibility was able to be evaluated only through the hysteresis presented in the polarization curves.

Made available in DSpace on 2014-10-09T12:37:40Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:21Z (GMT). No. of bitstreams: 1 05333.pdf: 6187094 bytes, checksum: f2e70bb302111d70efab851a434597e9 (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2016-12-08T17:19:38Z (GMT). No. of bitstreams: 1 Vilmar L Rossi.pdf: 1361290 bytes, checksum: 3d9a672479f6e21f35489add0106e893 (MD5) Previous issue date: 2004-07-16 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / A ocorrência de corrosão severa em rotores de motobombas motivou a realização de um estudo que envolveu a avaliação da influência da qualidade das matérias primas de fusão e de controle processo sobre a suscetibilidade á corrosão na liga utilizada para obtenção destas peças por processo de fundição, no caso a liga com denominação comercial 356 (Al7Si03Mg). O principal parâmetro de avaliação foi à composição química, com especial ênfase nos teores dos elementos cobre e zinco presentes em percentuais que caracterizaram estes elementos como impurezas e não como elementos de liga. Foram avaliadas as influências individuais e combinadas de teores crescentes dos elementos Cu e Zn sobre a resistência a corrosão da liga A356, uma variante da liga 356 o níveis de impurezas mais baixos, medida através de ensaios de corrosão intergranular realizados de acordo com as especificações contidas nas normas ASTM G110-92 e ASTM G01-90. Com os resultados obtidos nestes ensaios foi possível obter medidas de perda em massa e profundidade de corrosão por metalografia ótica quantitativa. Os resultados obtidos indicam que pequenos incrementos nos percentuais de Cu e Zn ocasionam aumentos significativos na tendência á corrosão da liga em questão, sendo que em alguns casos estes teores estão abaixo dos valores normalmente presentes na grande maioria das ligas comerciais utilizadas nas indústrias de fundição de alumínio.

Made available in DSpace on 2014-10-09T12:37:40Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:21Z (GMT). No. of bitstreams: 1 05333.pdf: 6187094 bytes, checksum: f2e70bb302111d70efab851a434597e9 (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP