The master’s thesis deals with corrosion resistance of cast duplex steels. In the first part the corrosion mechanisms and their individual types are described, the second part characterizes the types of stainless steels in terms of their properties, use, structural components, and their impact on mechanical and corrosion properties of steels. Individual chapter is dedicated to duplex steels. In the experimental part the resistance of 1.4517 steel against pitting corrosion depending on the alloy contents and the PREN value was investigated. Four specimens graded according to PREN were cast. Supplemented with sample stirrer blade which was affected by corrosion during service they were subjected to corrosion test according to ASTM G48-11 standard in 6% FeCl3 solution. Cast specimens were subjected to tensile test and impact test.

<p> A phosphotriester synthesis for short deoxy oligomers was developed based on the RNA synthesis developed in Neilson's laboratory. This synthesis parallels that of Catlin and Cramer (1973) but differs significantly in a number of procedures and reagents used and represents an overall improvement in every aspect of the previous method. This synthetic procedure allows the synthesis of sufficient quantities of DNA for 1H-NMR analysis. The following sequences were successfully synthesized using this procedure: d(GCA) d(AGCT) d(ACGT) d(ACGTp) d (ACGTACGTp).</p> <p> It was found by variable temperature 1H-NMR that the trimer and the tetramer duplexes had a very low Tm (0-10°C) as compared to their RNA counter-parts which had Tms between 29-34°C. This demonstrates quantitatively that the short DNA duplex is significantly less stable than the short RNA duplex. Consequently, sequences of at least five or six bases in length will be required for model studies of DNA duplex stabilities using variable temperature NMR methods.</p> <p> A CD study of d(ACGTACGTp) in conditions of low salt (1M NaCl) and high salt (5M NaCl) demonstrated that a high salt B to Z-helix transition did not occur. Instead, the duplex remained in the right handed B form in both low and high salt.</p> / Thesis / Master of Science (MSc)

Cognitive radio had been proposed as a methodology for overcoming the inefficiency of the conventional static allocation of the available spectrum in wireless communication networks. The majority of opportunistic spectrum access schemes in cognitive radio networks (CRNs) rely on the Listen-Before-Talk (LBT) model due to the half-duplex nature of conventional wireless radios. However, LBT su ers from the problem of high collision rates and low secondary user throughput if time is misaligned among the secondary users (SUs) and the primary users (PUs). This problem can be mitigated by leveraging full-duplex (FD) communications that facilitate concurrent sensing and transmission. This thesis considers the problem of optimal opportunistic multi-channel spectrum sensing and access using FD radios in the presence of uncertain primary user (PU) activity statistics. A joint learningand spectrum access scheme is proposed. To optimize its throughput, the SU sensing period has to be carefully tuned. However, in absence of exact knowledge of the PU activity statistics, the PU's performance may be adversely a ected. To address this problem, a robust optimization problem is formulated. Analysis shows that under some non-restrictive simplifying assumptions, the robust optimization problem is convex. The impact of sensing periods on the PU collision probability and the SU throughput are analyzed, and the optimal sensing period is found via convex optimization. An "\epsilon-greedy algorithm is proposed for use by the SU to learn the PUs' activity statistics in multichannel networks. It is shown that sublinear regrets can be attained by the proposed estimation and robust optimization strategy. Simulation studies demonstrate that the resulting robust solution achieves a good trade-o between optimizing the SU's throughput and protecting the PU. / Thesis / Master of Applied Science (MASc)

Super Duplex Stainless Steels (SDSS) are widely used in offshore oil and gas industrial components. They are dual phase materials consisting of ferrite and austenite in similar ratios with high contents of chromium and presence of molybdenum. This combination of microstructure and chemical composition results in enhanced mechanical strength and corrosion resistance. However, this material has poor machinability, exhibiting the following characteristics: (i) tendency to strain-harden; (ii) extreme adhesive behaviour; and (iii) high cutting temperatures. These circumstances not only result in high tool wear rates, but also lead to poor surface integrity due to the work hardening effect, high roughness and tensile residual stress. To minimize these detrimental effects, PVD coating technologies have been widely applied to cutting tools due to their tribological properties exhibited during cutting, which reduce friction and diminish heat. In this work, three different PVD coatings were tested during the turning of super duplex stainless steel of grade UNS S32750. In addition to the tool performance, surface integrity was assessed by surface texture analysis, residual stresses and hardness profile. The electrochemical behaviour of the machined surface was evaluated by potentiodynamic anodic polarization measurements. Stress cracking corrosion (SCC) tests were also performed. Results indicate a relationship between the tool performance and surface electrochemical behaviour, where the tool with best cutting performance, AlTiN, also presented the best electrochemical behaviour. Stress cracking corrosion was found to be associated with residual stresses on the workpiece, among the three tested PVD coated tools the AlCrN/TiSiN showed lowest tensile residual stresses and lowest SCC susceptibility. The surface generated by AlTiN coated tool presented the highest levels of tensile residual stresses, resulting in a higher SCC susceptibility. / Thesis / Master of Applied Science (MASc)

O Aço Inoxidável Super Duplex UNS S32760 é um aço de alta liga com estrutura composta por uma matriz ferrítica com ilhas austeníticas, em proporções aproximadamente iguais. Esta estrutura lhe confere boas propriedades como: resistência à corrosão, em particular à corrosão por pite e por fresta, excelentes propriedades mecânicas, boa soldabilidade, alta condutividade térmica, baixo coeficiente de expansão térmica e excelente tenacidade. Suas principais aplicações geralmente ocorrem em ambientes com alta concentração de cloreto, como por exemplo, em tubos para transporte de óleo e gás, equipamentos para transporte ou em contato com materiais rico sem cloretos, em tanques e vasos para a indústria química e petroquímica. Estas aplicações tornam importante a determinação da temperatura crítica de pite (TCP). A ASTM normatiza essa determinação com as normas G-150 e G-48, que abrangem aços inoxidáveis austeníticos comuns e ligas à base de níquel, não existindo uma condição especifica para o teste em aços super duplex. O objetivo deste trabalho é a caracterização eletroquímica e o desenvolvimento de uma metodologia para a determinação da temperatura crítica de pite de aços super duplex UNS S32760. Devido à falta de normatização para estes materiais uma combinação das normas ASTM G-48 e G-150 foi empregada neste trabalho. Utilizou-se como meio uma solução contendo FeCl3 6% e HCl 1%, em massa. Foram empregadas como técnicas medidas de potencial de circuito aberto em função do tempo de imersão, curvas de polarização potenciostática anódica, cronoamperometria, espectroscopia de impedância eletroquímica (EIE), microscopia ótica (MO), microscopia eletrônica de varredura (MEV) e espectroscopia por dispersão de energia (EDS). Foram realizados tratamentos dos dados experimentais, abordando-os de maneiras diferentes para que a melhor resposta fosse obtida na determinação da TCP das ligas. Duas amostras diferentes deste material foram estudadas, uma amostra comercial e uma amostra desenvolvida pelo laboratório de pesquisa e desenvolvimento da Villares Metals, com diferentes teores de cromo e molibdênio. Apesar da variação na composição química das amostras, nas análises por MEV e EDS, as mesmas se mostraram semelhantes, quando apenas polidas, apresentando os mesmos tipos de precipitado. A microscopia ótica possibilitou visualizar a diferença na proporção de fases entre as duas ligas e as características morfológicas das mesmas. Analisando mais detalhada e minuciosamente os dados eletroquímicos, foi determinado que o melhor valor de densidade de corrente a ser utilizada como parâmetro na determinação da TCP , é 25±5 &#181;Acm-2. Utilizando este parâmetro foi mostrado que a TCP determinada é menor, quando comparada com a obtida pelas normas convencionais. A combinação das diferentes técnicas permite afirmar que a metodologia empregada determina de maneira bem mais precisa a TCP. / UNS S32760 super duplex stainless steel is a high alloy steel, containing a ferrítica matrix and austenitic islands as structure at equal proportions approximately. This structure results in good properties as high resistance to corrosion, particularly to pitting and crevice corrosion, excellent mechanical properties, good weldability, high thermic conductivity, low thermic expansion coefficient, and excellent tenacity. Their principal applications are in high chloride concentration media, for example in tubes for oil and gas, in transport equipment when in contact with high chloride concentration materials, tanks and vases for chemical and petrochemical industries. Due to these applications the evaluation of the pitting critical temperature is mandatory for these materials. G-150 and G-48 ASTM Standards are recommended for conventional stainless steels and nickel alloys, but there is not any specific standard for super duplex stainless steels. The main of this work is the characterization and the development of a methodology to evaluate the critical pitting temperature for USS S32760 super duplex stainless steels. Due to the absence of standard analyses for this materials, a combination of G-48 and G-150 standards were employed. The medium was 6% FeCl3 by mass and 1% HCl by mass solution. Open circuit potential measurements, anodic potentiostatic polarization curves, chronoamperometry, electrochemical impedance spectroscopy (EIS), optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS) were used as techniques. Different treatments for the experimental results were used in order to obtain the best value for TCP of these alloys. Two different samples of this material were studied: a commercial sample and a new composition developed at P&D Villares Metals industry, with different contents in chromium and molybdenum. The materials studied presented the same kinds of inclusions, according to SEM and EDS analyses. OM analysis showed the proportionality between the ferritic and austenitic phases and their morphological characteristics. The electrochemical studies suggest the 25 ± 5 &#181;Acm-2 value for current density to be used in the CPT e valuation. The CPT obtained is lower when compared to those one using conventional standards. The combination of different techniques permits to conclude that the proposed methodology evaluates the CPT w ith higher precision.

Les aciers inoxydables lean duplex, à faible teneur en nickel et sans molybdène, représentent une alternative économique pour les usines de dessalement et l’industrie pétrolière. Celles-ci ont la particularité d’être exposées à des milieux très agressifs dont la présence de chlorure, de sulfure d’hydrogène, de hautes températures ou encore d’acides peut provoquer des phénomènes de fissuration sous contrainte.Cette étude est divisée en deux parties, la première se consacre aux milieux purement chlorurés et la seconde aux milieux contenant du sulfure d’hydrogène. Dans les deux parties, la compréhension des phénomènes liés à la fissuration sous contrainte des lean duplex est effectuée sur la nuance S32304, puis est comparée aux comportements des nuances S32202 et S32101. L’influence des milieux bouillants sur la résistance à la fissuration des aciers inoxydables, les mécanismes de dépassivation et de dissolution sélective des lean duplex, les différents modes de fissuration en présence de sulfure d’hydrogène ou encore l’amorçage de la corrosion sous contrainte assistée par le sulfure d’hydrogène sont abordés dans ce rapport. / Lean duplex stainless steels, with low content of nickel and without molybdenum, represent an economic alternative for desalting plants and petroleum industry. For these uses, steel have the particularity to be exposed to very aggressive environments inducing phenomenon of Stress Corrosion Cracking as chlorides, hydrogen sulphur, high temperatures or acids.This study is divided in two parts dedicated to two different environments. The first part is devoted to chloride middles and the second to hydrogen sulphur middles. In both parts, the understanding of phenomenon linked to the stress corrosion cracking of lean duplex is studied on the grade steel S32304, then is compared to the behaviour of grade steels S32202 and S32101. The topics treated in this report are notably the influence of boiling environments on the cracking resistance of stainless steels, the mechanism of unpassivation and selective dissolution of lean duplex, the different modes of cracking in presence of hydrogen sulphur, and the initiation of stress corrosion cracking assisted by hydrogen sulphur

Os aços inoxidáveis duplex são largamente utilizados na fabricação de equipamentos para a indústria de óleo e gás, utilizados tanto no ambiente onshore quanto offshore. Sua grande limitação é que, com o aumento de temperatura, ocorre precipitação de fases indesejáveis, que reduzem drasticamente a resistência à corrosão e as propriedades mecânicas desses materiais. Considerando o efeito deletério da soldagem a fusão nos aços inoxidáveis duplex, a soldagem por atrito com pino não-consumível (FSW) é amplamente considerada como alternativa aos processos convencionais. Como no FSW a união dos materiais ocorre no estado sólido, muitos dos problemas de soldabilidade associados às técnicas tradicionais de soldagem por fusão são evitados. Neste trabalho, amostras retiradas da zona misturada (ZM), das zonas afetadas pelo calor (ZTA e ZTMA) e do metal de base (MB) de chapas de aço inoxidável lean duplex LDX 2404&reg (UNS S82441) soldadas por atrito com pino não-consumível foram caracterizadas microestruturalmente e tiveram sua resistência à corrosão avaliada por meio de ensaios eletroquímicos. Os resultados obtidos nos ensaios eletroquímicos indicaram que as zonas afetadas pelo calor e a ZM se mantiveram tão resistentes à corrosão localizada quanto o MB. Permitiram concluir também que a excelente resistência à corrosão da liga está associada ao teor elevado de N. / Duplex stainless steels have been widely used in the manufacture of equipments and piping for oil & gas industry, in onshore and offshore service. However, whenever they are exposed to high temperatures the precipitation of deleterious phases might occur. The precipitates drastically reduce the corrosion resistance and mechanical properties of these materials. The friction stir welding (FSW) process has been considered as an alternative to replace the conventional fusion welding processes in duplex stainless steels. FSW is a solid-state welding process avoiding many of the problems associated to conventional fusion techniques. In this study, specimens of the various zones affected and nearby the FSW, specifically the nugget or stir zone (SZ), the heat affected zones (HAZ/TMAZ) and the base metal (BM) of welded lean duplex UNS S82441 were characterized microstructurally and had their corrosion resistance evaluated by electrochemical tests. The results indicated that the HAZ/TMAZ and SZ regions presented similar localized corrosion resistance to that of the BM showing that it is a potential technique for replacing the conventional processes with advantageous corrosion resistance properties. They also concluded that the excellent corrosion resistance of the alloy is associated with the high N content.

A fase alfa linha provoca uma diminuição brusca da resistência à corrosão e perda das propriedades mecânicas dos aços inoxidáveis dúplex. Neste trabalho realizaram-se ensaios mecânicos e eletroquímicos no aço inoxidável dúplex UR 52N+ envelhecido a 475ºC por vários tempos para avaliar a sensibilidade dos ensaios à presença de alfa linha. Os ensaios de dureza mostraram aumento gradual nos valores da mesma, já os ensaios de impacto revelaram que com 12h de envelhecimento o material já perde cerca de 80% da energia absorvida inicialmente pela amostra solubilizada. No entanto, os ensaios de polarização cíclica mostraram que mudanças significativas só são percebidas para tempos de envelhecimento superiores a 96h. / Alpha prime phase leads to decreased corrosion resistance and mechanical properties losses of duplex stainless steels. In this work mechanical and electrochemical tests were performed in duplex stainless steel UR 52N+ aged at 475ºC for various periods in order to determine the sensibility of these tests to alpha prime presence. Hardness tests showed a gradual increase in its values; on the other hand, impact tests revealed that the material aged for 12h losses about 80% of energy absorption capacity of the solution annealed sample. Notwithstanding cyclic polarization tests showed that significant changes are only noted for aging times greater than 96h.

Made available in DSpace on 2017-07-21T20:42:44Z (GMT). No. of bitstreams: 1 Marcos Pontes.pdf: 4006153 bytes, checksum: 42b95748e61f5bc3de1b41ca1fbd649b (MD5) Previous issue date: 2012-02-29 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The processing by equal channel angular extrusion of a duplex stainless steel UNS32205/S31803, with ferritic-austenitic structures, was studied, processing until four passes each sample. The reason of this processing was to promote a grain refinement in the material, to achieve ultrafined grain material by severe plastic deformation. Different number of passes were used in this processing to evaluate how the number of extrusions in the same sample influences the deformation imposed to the material. After processing, some different heat treatments were done in the samples to evaluate the recrystallization process in the processed material. In this study, it was possible to verify the increasing level of deformation in the material after every new processing in the sample, this was verified by the changes in grain orientation and by microstructure evaluation. It was possible to reduce the stress level to values that can avoid the breaking of punctures during processing, this allowed to process the material with a greater number of passes in a material with a high resistance like the duplex stainless steels. Strain bands were found in the structure of the processed material, this indicates high levels of deformation. / Foi realizado o estudo do processamento de um aço inoxidável dúplex SAF2205,com microestrutura ferrítica-austenítica, por extrusão em canal angular por até quatro passes. O objetivo deste processamento foi a obtenção de um refino de grãos no material, atingindo uma estrutura de grãos ultrafinos, a partir da aplicação de deformações plásticas severas. Foram realizados diferentes modos de processamento, variando o número de passes pela matriz e depois foi avaliado como este parâmetro influencia na deformação imposta ao material. Após o processamento por ECA (Extrusão por Canal Angular), foram realizados diferentes tratamentos térmicos para avaliar a cinética de recristalização do material. Foi possível verificar o nível crescente de deformação do material após cada passe, com o acompanhamento da orientação dos grãos e avaliações microestruturais. Foi possível reduzir os níveis de tensões o suficiente para evitar a quebra de punções durante o processamento, permitindo um número maior de passes para um material com a resistência do aço inoxidável dúplex. Foram verificadas bandas de deformação no material processado, indicando níveis elevados de deformação.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Os aços inoxidáveis duplex vêm sendo cada vez mais utilizados em aplicações que demandam maior resistência à corrosão que os aços inoxidáveis tradicionais. Suas aplicações em estruturas ao ar livre, ou submersas, motivam estudos sobre seu desempenho em variadas situações, como no caso de exposição ao desgaste erosivo.O presente trabalho estuda os efeitos da erosão por impacto de partículas sólidas em superfícies de amostras de um aço inoxidável duplex UNS S32205, tanto do ponto de vista morfológico quanto do topográfico. Superfícies de amostras polidas e posteriormente submetidas a ataques erosivos com partículas de alumina em fluxo de ar, foram examinadas por microscopia eletrônica de varredura, para verificação das formas de desgaste obtidas, e submetidas a ensaios de rugosidade, para determinação do perfil e avaliação da evolução do processo erosivo. Foi feito um exame mais detalhado de formatos de crateras, para comparação com mecanismos propostos em modelos descritos na literatura e com resultados de simulações pelo método dos elementos finitos. Nesse aspecto, o método utilizado mostrou-se tão eficaz quanto aos métodos tradicionais de estudos balísticos com uma só partícula, além de mais abrangente e econômico. As medidas de rugosidade superficial e as informações estatísticas derivadas sinalizaram a possibilidade de utilização dessa ferramenta para avaliar as condições de efetiva perda de massa, que caracterizam a erosão. / Duplex stainless steels are being increasingly used in applications that require a greater corrosion resistance than traditional stainless steels. Its applications in both, outdoor or submerged structures, motivate the studies on its performance in various situations, as in the case of exposure to erosive wear. This work studies the effects of erosion by impact of solid particles on surfaces of samples of duplex stainless steel UNS S32205, from the point of view of both aspects, morphological and topographycal. Polished surfaces of the samples were subjected to erosive attacks by alumina particles in air flux, with different exposure times. These were examined by scanning electron microscopy, looking at the forms of wear obtained and the surfaces roughnesses were measured, aiming to follow the evolution of the erosive process. It was made a more detailed examination of craters, for comparison with the proposed mechanisms in models described in the literature and with the results of simulations by the finite elements method. In this aspect the method proved to be as effective as the traditional methods of ballistic studies using single particles, as well as to be more comprehensive and economic. The measures of surface roughness and the statistics from the derived data signaled the possibility of use of this tool to evaluate the beginning of the effective loss of mass, which characterizes the erosion.