This thesis describes the metallurgical and environmental factors that influence hydrogen embrittlement of super duplex stainless steels and presents a model to predict the rate at which embrittlement occurs. Super duplex stainless steel has an austenite and ferrite microstructure with an average fraction of each phase of approximately 50%. An investigation was carried out on the metallurgical and environmental factors that influence hydrogen embrittlement of super duplex stainless steels. Tensile specimens of super duplex stainless steel were pre-charged with hydrogen for two weeks in 3.5% NaCl solution at 50º C at a range of applied potentials to simulate the conditions that exist when subsea oilfield components are cathodically protected in seawater. The pre-charged specimens were then tested in a slow strain rate tensile test and their susceptibility to hydrogen embrittlement was assessed by the failure time, reduction in cross-sectional area and examination of the fracture surface. The ferrite and austenite in the duplex microstructures were identified by analysing their Cr, Ni, Mo and N contents in an electron microscope, as these elements partition in different concentrations in the two phases. It was shown that hydrogen embrittlement occurred in the ferrite phase, whereas the austenite failed in a ductile manner. An embrittled region existed around the circumference of each fracture surface and the depth of this embrittlement depended on the hydrogen charging time and the potential at which the charging had been carried out. The depth of embrittlement was shown to correlate with the rate of hydrogen diffusion in the alloy, which was measured electrochemically using hydrogen permeation and galvanostatic methods. A two-dimensional diffusion model was used to calculate the hydrogen distribution profiles for each experimental condition and the model could be employed to provide predictions of expected failure times in stressed engineering components.

To reduce the greenhouse effect, the use of renewable fuel has to be increased. As renewable fuel has different characteristics compared to fossil fuel regarding content of trace metals, alkali, chlorine and sulphur, the corrosion characteristics in high temperature energy processes have to be evaluated. This thesis concerns high temperature corrosion in the superheater region of a boiler and the syngas cooler area of a gasifier. For the superheater region, laboratory exposures were performed. The methods included a salt dip exposure, where samples were dipped in an equimolar solution of ZnCl2 and KCl, and two salt bed exposures with different chlorine concentrations, 10 and 20 wt%. Ranking of the materials showed that a Ni content above 10 wt% and Cr above 20 wt% reduced corrosion rates in the salt dip and in the 10% Cl salt bed exposure. For exposure in the 20% Cl bed, even higher alloying was needed. An alumina forming austenitic steel showed future potential in sulphidising-chlorinating environments. For the gasifier region, the effect of HCl in a simulated gasifier atmosphere was studied and also samples exposed in the syngas section of a biomass gasifier were investigated. Metal loss was low for all exposures and it was observed that chlorine had minor influence. For the plant exposed samples, a difference compared to that reported for coal gasifiers was the absence of FeS for the lowest alloyed steel. Instead, a deposit with pronounced content of Zn, Ca, S and O was present on the surface. Zinc was suggested to mitigate corrosion. Thermodynamic modelling was used to explain phases present and to predict the nitridation behaviour of an alumina forming austenitic steel. Equilibrium and kinetic modelling of the nitridation showed good coherence with the observed microstructures. However, the kinetic modelling resulted in larger nitridation depths than observed experimentally which was attributed to the presence of a thin oxide layer on the surface of the samples. / <p>QC 20160510</p>

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.

Made available in DSpace on 2017-07-21T20:43:48Z (GMT). No. of bitstreams: 1 Dannilo Eduardo Ferreira.pdf: 9079253 bytes, checksum: c166172d47d556878be0589dd0a79c96 (MD5) Previous issue date: 2015-08-31 / Conselho Nacional de Desenvolvimento Científico e Tecnológico / Austenitic stainless steels have a wide range of applications and are widely used in applications requiring high corrosion resistance, and CF-8M class are generally submitted to corrosive environments in liquid media. The microstructure of these steels has an austenitic matrix and a variable amount of delta ferrite resulting from the casting process, which is affected by process variables like cooling rate, and chemical composition of the melt. In this work were characterized four alloys of CF-8M class austenitic stainless steels. The chemical compositions differ only in chromium and nickel contents in order to generate different amounts of chromium equivalent (Creq) and nickel equivalent (Nieq) according to the Schoefer diagram with ratio Creq/Nieq ranging between 0.95 and 1.35. The variation in the value of the ratio Creq/Nieq aims to observe the influence of chemical composition on solidification mode and the phase transformations during processing of the alloy. The alloys were cast in the form of pins which samples were removed for characterization. The microstructural characterization of alloys was made with the techniques of optical microscopy with image analysis, Field emission microscope (SEM / FEG) with energy dispersive X-ray spectrometry and electron backscatter diffraction (EDS / EBSD) to compositional map. The ferritoscopy technique, EBSD phase map and XRD were used to determine the ratio between phases. Thermal analysis by DSC determined the solidification interval for alloys in the study and phase transformations resulting during cooling to room temperature. The results showed that alloys 1, 2 and 4 solidify with FA mode, as expected. The alloy 3 showed a corresponding microstructure of eutectic/peritectic solidification, not presented in any of the existing diagrams in the literature. The measures of δ-ferrite fraction for all leagues were valid for the values proposed by Schoefer diagram. The examination of the micrographs of alloys shows the variety of microstructures that a CF-8M steel can display only altering its chemical composition. The EDS/EBSD maps showed a higher segregation of alloying elements between the phases for alloys 1, 2 and 4, due to the solidification mode and further transformation in solid state. The alloy 3 showed a smaller difference in composition between phases and less variation within the same phase, which features a eutectic/peritectic solidification. The comparison between the different regions of the samples presented variation in the fraction and morphology of ferrite. The range of solidification of the four alloys in the study was within a maximum level of 4 ° C, expected to solidification with ferrite as the first phase to form from the liquid. It was observed for the alloys to solidify with FA mode a phase transformation in the solid state in the range 1320 °C to 1340 °C, corresponding to γ transformation. The energy of this transformation is inversely proportional to the amount of ferrite observed in the final microstructure of alloys. / Os aços inoxidáveis austeníticos têm uma vasta gama de aplicações, sendo muito utilizados em aplicações que requerem grande resistência à corrosão, sendo as ligas da classe CF-8M geralmente submetidas a ambientes corrosivos em meios líquidos. A microestrutura desses aços apresenta uma matriz austenítica e uma quantidade variável de ferrita delta resultante do processo de fundição, a qual é afetada por variáveis de processo como a taxa de resfriamento e composição química da liga. No presente trabalho foram caracterizadas quatro ligas de aços inoxidáveis austeníticos da classe CF-8M. As composições químicas variam somente os teores de cromo e níquel com o intuito de gerar valores diferentes de cromo equivalente (Creq) e níquel equivalente (Nieq) segundo o diagrama de Schoefer com relações Creq/Nieq variando entre 0,95 e 1,35. A variação no valor da relação Creq/Nieq tem por objetivo observar a influência da composição química no modo de solidificação e nas transformações de fase durante o processamento das liga. As ligas foram vazadas em forma de pinos dos quais foram retiradas as amostras para caracterização. A caracterização microestrutural das ligas foi feita com auxílio das técnicas de microscopia óptica com análise de imagens, microscópio eletrônico de varredura de efeito de campo (MEV/FEG) com microanálise química e difração de elétrons retro espalhados (EDS/EBSD) para mapeamento composicional. A técnica de ferritoscopia, mapeamento de fases por EBSD e difração de raios X foram utilizadas para a determinação da proporção entre fases. As análises térmicas por DSC determinaram o intervalo de solidificação para as ligas em estudo e as transformações de fase decorrentes ao longo do resfriamento até a temperatura ambiente. Os resultados mostraram que as ligas 1, 2 e 4 se solidificam pelo modo FA, como esperado. A liga 3 apresentou uma microestrutura correspondente à solidificação eutética/peritética, não sendo apresentada em nenhum dos diagramas existentes na literatura consultada. As medidas da fração de ferrita para todas as ligas foram válidas para os valores propostos pelo diagrama de Schoefer. A análise das micrografias das ligas mostra a variedade de microestruturas que um aço CF-8M pode apresentar apenas alterando sua composição química. Os mapeamentos por EDS/EBSD apresentaram uma maior segregação de elementos de liga entre as fases para as ligas 1, 2 e 4, devido ao modo de solidificação e posterior transformação no estado sólido. A liga 3 apresentou uma menor diferença de composição entre as fases bem como uma menor variação dentro da mesma fase, o que caracteriza uma solidificação eutética/peritética. A comparação entre as regiões distintas das amostras apresentaram variação nas fração e na morfologia da ferrita. O intervalo de solidificação das quatro ligas em estudo ficou dentro de um patamar máximo de 4 °C, esperado para a solidificação com ferrita como primeira fase a se formar a partir do líquido. Foi observada para as ligas com modo de solidificação FA uma transformação de fase no estado sólido no intervalo de 1320°C a 1340 °C, correspondendo a transformação →γ. A energia dessa transformação é inversamente proporcional à quantidade de ferrita observada na microestrutura final das ligas.

Re-processed oxide fuel product from the Thermal Oxide Reprocessing Plant (THORP) is stored in Type 316L stainless steel, using a design of several nested cans, with the outer can providing the safety case containment barrier. The research reported in this PhD thesis aims to support the safety case related to these storage cans, by identifying and characterising susceptible microstructure sites and associated material surface conditions. The overarching goal of this project is to understand the propensity of THORP storage cans towards localised corrosion and Environment Assisted Cracking (EAC) in HCl and chloride-bearing atmospheric environments. The investigation focused on two possible corrosion cases: (1) understanding the effect of surface finishing on material performance in chloride-containing atmospheric environments, and (2) characterising the effects of the HCl aqueous solutions inside the can, with potential formation of HCl vapour. Microstructure investigations were carried out on surface-treated type 316L coupon specimens. The application of aqua blasting resulted in a deformed near-surface microstructure, containing compressive residual stresses to a depth of 100-120 micrometres. Subsequent laser engraving produced a recrystallized surface layer with tensile residual stresses reaching to a depth of 200 micrometres. Changes of surface roughness topography were accompanied by the development of a thick oxide/hydroxide film after laser engraving. Atmospheric exposure revealed similar corrosion attack for all samples, with laser engraving exhibiting the lowest number of corrosion sites, but with the largest average depth of attack. In addition, laser engraving led to atmospheric-induced stress corrosion cracking (AISCC) within two weeks of exposure to 386 ug/cm2 MgCl2-laden droplet deposits, with crack growth rates similar to ground U-bend samples. Strategies to reduce the likelihood of AISCC of laser-engraved components are discussed. The influence of HCl concentration and exposure temperature on the corrosion type and rate of annealed and cold rolled type 316L stainless steel has also been investigated. Cold rolling of up to 20 % reduction was introduced, with potentio-dynamic polarization measurements conducted in 0.01 - 3 M HCl aqueous solution. Results are compared to microstructures immersed under open circuit conditions, and to HCl-laden droplet deposits at temperatures up to 80C. Corrosion type diagrams are introduced to describe the transition between uniform corrosion, mixed-mode uniform with pitting corrosion, and pitting corrosion only, as a function of temperature, HCl concentration, and cold deformation. SCC tests of type 316L stainless steel have been carried out at 110C, by exposing U-Bend samples to HCl-laden droplets and HCl vapour. The humidity of the environment was controlled using defined volume fractions of H2O in a sealed environmental chamber. HCl-laden droplets with chloride deposition densities exceeding 1.5 ug/cm2 led to SCC after 90 minutes of exposure, whereas no corrosion attack was observed for samples with exposure to 0.15 ug/cm2 HCl. Increasing HCl concentrations resulted in fewer, but longer cracks, reaching up-to several hundreds of micrometres in length. HCl vapour exposure was carried out by adding various volumes of HCl solution in a beaker to the sealed test chambers. These HCl vapour tests confirmed a change of corrosion type with HCl concentration, from pitting corrosion with SCC, to the occurrence of uniform corrosion.

Transformation Induced Plasticity (TRIP) steels undergo a phase transformation when subjected to high levels of mechanical strain. This transformation from a paramagnetic austenitic parent phase to a ferromagnetic martensitic phase is irreversible and the resultant magnetic properties may therefore be used as a measure of strain history. The transformation behaviour of TRIP steels has been recognised as a potential smart characteristic and various proposals have appeared aimed at producing a structure that performs its primary structural function as well a strain sensing function simultaneously. However the strain induced nature of the transformation implies that transformation will occur in areas of high stress concentration and therefore engineered stress concentration features will be required to provide a consistent measure of the changes in the magnetic properties of the material as a function of applied load. In order to predict the performance of smart TRIP steel sensors, an analysis method capable of quantifying the effectiveness of a component in its dual role as structure and sensor is needed. The thesis addresses the development of a methodology for correlating the changing magnetic permeability of TRIP steel sensors and structures with martensitic transformation behaviour. The prediction of the deformation behaviour including transformation is implemented by considering a mechanical analysis based on the finite element method and a constitutive model incorporating strain-induced martensitic transformation kinetics. .Extensions to the model which allow for a wide range of deformation rates and temperatures are also discussed. In order to demonstrate the application of the methodology, an analysis of a simple tensile element used in strain measurement applications is presented. The analysis also includes the effect of temperature on the performance of the sensor. An analysis of a design proposal for a smart aircraft bolt is also included to investigate the effects of geometry, particularly engineered stress concentrations, and sensor placement. / Thesis (Ph.D.)-University of Natal, Durban, 2003.

No description available.

A routine non-destructive test method based on Phased Array Ultrasonic Testing (PAUT) has been developed and applied for the inspection of the first 11 T dipole prototype magnet half shell welds, and the test results are compared with the radiography and visual inspection results of the same welds. A manual scanner and alignment system have been developed and built to facilitate the inspection of the 5.5 m long welds, and to assure reproducibility of the PAUT results. Through the comparison of distance readings and signal amplitude for different focus lengths, a focal law with focus at 25 mm sound path has been selected for the routine inspection of the 15 mm thick austenitic stainless steel 11 T dipole welds. The defocusing properties (beam spread) due to the cylindrical geometry of the half shells and the sound path distance to the area of interest were taken into account. Dedicated sensitivity calibration weld samples with artificial defects (side-drilled-holes) have been designed and produced from 11 T dipole prototype austenitic stainless steel half shell welds. These provide representative calibration for the strongly attenuating and scattering austenitic stainless steel weld material. One scan with two phased array probes aligned parallel to the weld in 2 mm distance from the weld cap edge, and one scan with the probes aligned parallel to the weld in 12 mm distance from the weld cap edge are sufficient to show if the inspected welds fulfil the requirements of weld quality level B according to ISO 5817. The standard test duration for the two scans of the two 5.5 m long horizontal welds of the 11 T dipole magnets is about one day, provided that no defects are found that need to be characterized in more detail.

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.

Resumo: Este projeto de pesquisa teve como objetivo estudar toda evolução que governa o crescimento, distribuição e classificação dos pites em aços inoxidáveis austeníticos ABNT 304 e 310S obtidos no estado como recebido e tratados, submetidos a diferentes tempos de exposição em névoa salina. A originalidade deste trabalho baseou-se no desenvolvimento de uma técnica para caracterização morfológica e da corrosão localizada associando com os aspectos de descrição de formas, tamanho, parâmetros específicos e populacionais. O aço inoxidável 304 apresentou morfologia irregulares > região de transição A> cônicos >hemisféricos > região de transição B, sendo que irregulares e região de transição A em maiores quantidades no sistema. Na liga tratada a cada tempo de exposição, a quantidade de pites é da mesma ordem, ou seja, região de transição A, irregulares, cônicos, hemisféricos, região de transição B, sendo que os pites na região de transição A e irregulares estão presentes em maior quantidades no sistema. O aço inoxidável 310S, como recebido, exibiu a seguinte morfologia: pites hemisféricos > região de transição A > região de transição B > irregulares > cônicos, sendo que os pites estão presentes em maiores quantidades em: hemisféricos e região de transição A. A quantidade de pites na liga tratada a cada tempo de exposição é da mesma ordem: região de transição B > hemisféricos > região de transição A > cônicos >irregulares. O aço inoxidável 304 apresentou maior resistência à corrosão por pite do que o aço inoxidável 310S, e também diferenças na distribuição, classificação e mediana dos tamanhos dos pites, conforme os resultados obtidos na avaliação estatística da superfície e perfis dos mesmos. / Abstract: This research project has had as objective to study all evolution that governs the growth, distribution and classification of pits in ABNT 304 and 310S austenitic stainless steels obtained in the state as receveid and treated, submitted to different times of exposition in saline mist. The originality of this work has been based on the development of a technique for morphologic characterization and of located corrosion associating with the shape description, size specific and population parameters. The 304 stainless steel has presented the following morphology: irregular > transition region A > conic > hemispherical > transition region B, in which irregular and transition region A > are in larger amounts in the system. In the treated alloy at each exposition time, the pits amount is at the same order, in other words, transition region A, irregular, conic, hemispherical, transition region B, in which the pits in the transtion region A and irregular are present in larger amounts in the system. The 310S stainless steel, as receveid, has exhibited the following morphology: hemispherical pits > transition region A > transtion region B > irregular > conic, in which the pits are present in large amounts in: hemispherical pits and transition region A. The pits amount in the treated alloy at each time of exposition has the same order: transition region B > hemispherical > transition region A > conic> irregular. The 304 stainless steel has presented pit corrosion resistance larger than the 310S stainless steel, and also differences in the distribution, classification and pits sizes medium, according to the results obtained in the statistical evaluation of their surface and profiles. / Orientador: Eduardo Norberto Codaro / Coorientador: Luis Rogério de Oliveira Hein / Banca: Roberto Zenhei Nakazato / Banca: Hamilton de Felipe / Banca: Gilberto Luis Jardim Pinto da Silva / Banca: Koshun Iha / Doutor