The production of advanced high strength steels (AHSS) for use in the automotive and construction industries requires complex control of runout table (ROT) cooling. Advanced high strength steels require coiling at temperatures below 500 °C in order to produce a complex multi-phase microstructure. The research described here will investigate the boiling conditions that occur for moving plate experiments when steel is cooled towards low coiling temperatures. Experiments were performed on a pilot-scale ROT located at the University of British Columbia using industrially supplied steel plates. Tests were performed for four different speeds (0.3, 0.6, 1.0 and 1.3 m/s) and three different initial plate temperatures(350, 500 and 600 °C). Each plate was instrumented with thermocouples in order to record the thermal history of the plate. The results show that cooling is more effective at slower speeds within the stagnation zone for surface temperatures over 200 °C. Outside the stagnation zone regardless of speed cooling is primarily governed by air convection and radiation with minor effects from latent heat caused by splashing water. The maximum peak heat flux value increases with decreasing speed and occurs at a surface temperature of approximately 200 °C, regardless of speed. Below a surface temperature of 200 °C, speed has a negligible effect on peak heat flux. The maximum integrated heat flux seems to vary with speed according to a second order polynomial.

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Made available in DSpace on 2014-06-11T19:27:11Z (GMT). No. of bitstreams: 0 Previous issue date: 2012-03-05Bitstream added on 2014-06-13T20:16:10Z : No. of bitstreams: 1 silva_ea_me_guara.pdf: 1498613 bytes, checksum: 06fa171181e5ced1500ca8c7e2468fc9 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Este projeto é um estudo do efeito springback em quatro tipos de aços de alta resistência, sendo estes bifásico, baixo carbono, endurecível após pintura e com interstícios livres, aplicados atualmente como matéria-prima na produção de veículos. Neste contexto está inserido o desenvolvimento de novos aços avançados de alta resistência em consonância com o projeto ULSAB-AVC, que visa à produção de veículos seguros e econômicos para o século 21. A caracterização mecânica do efeito springback foi realizada por intermédio de ensaio de conformação mecânica, denominado dobramento de três pontos ao ar, como uma adaptação ao ensaio de flexão cilíndrica livre. Foram avaliadas também as propriedades mecânicas do material definidas pelo ensaio de tração, objetivando a determinação da resistência à tração, do limite de escoamento e do alongamento. Além disso, foi avaliada a caracterização microestrutural dos aços avançados, identificando e quantificando-se as fases presentes em coexistência por meio do processamento digital de imagens. Os resultados mostram que o efeito springback no aço bifásico, devido à sua alta resistência mecânica, apresenta as maiores taxas de springback e causa uma diminuição na “razão de aspecto” dos grãos que sofreram conformação mecânica e tentaram retornar às suas formas originais. Aços de baixo carbono e endurecíveis após a pintura, não sofrem efeito springback suficiente para causar alteração na forma dos grãos, sendo que a variação da “razão de aspecto” depende da combinação do alongamento e resistência mecânica destes aços. Já o aço com interstícios livres, devido a sua menor resistência mecânica, o efeito springback apresenta as menores taxas e a variação da razão de aspecto depende somente da capacidade de alongamento desse aço / This is a study of the springback effect on four kinds of high strength steel, wich are dual-phase, low carbon, bake hardening and interstitial free, currently used as feedstock in the production of vehicles. In this context is inserted the development of new advanced high steel resistance in accordance with the project ULSAB-AVC, which aims to produce safe and economical vehicle for the 21st century. The mechanical characterization of the springback effect was performed by means of a test of mechanical conformation, called three-point air bending, done by adapting to unconstrained cylindrical bending test. Were also evaluated the mechanical properties of material defined by the tensile test, in order to determine the tensile strength, yield strength and the elongation. Furthermore, was performed the microstructural characterization of advanced steels by identificating and quantificating of present phases in coexistence by means digital image processing. The results indicates that the springback effect in dual-phase steel, due to its high mechanical strength, has the highest springback rates and causes a decrease in the “aspect ratio” of the grains that suffered mechanical conformation attempting to return to its original forms. Low carbon and bake hardening steels, have not enough springback effect to cause change in the shape of the grains and the change of the “ratio aspect” depends on the combination of both elongation and mechanical strength of these steels. Yet on the interstitial free steel, due to its lower mechanical strength, the springback effect has the lowest rates and the change in “aspect ratio” depends only on elongation capacity of the steel

Atualmente tubos de aço microligados são utilizados na construção de oleodutos e gasodutos para exploração e condução de petróleo e gás natural em águas profundas. Estas aplicações demandam a utilização de ligas metálicas que apresentem elevada resistência mecânica, boa soldabilidade e excelente resistência às falhas associadas ao hidrogênio devido ao trabalho em ambientes sour. O hidrogênio atômico oriundo da corrosão do aço entra na microestrutura do material através da superfície, devido à ação do H2S do meio que inibe a reação de recombinação do hidrogênio (H0+H0=H2) e sua consequente dissipação para o meio em forma de bolhas. O hidrogênio atômico permeia através do material se movendo por difusão através do reticulado cristalino onde interage com defeitos microestruturais denominados traps, tais como inclusões, precipitados, contornos de grão e discordâncias impedindo que este continue a se movimentar. O acúmulo de hidrogênio atômico nestes traps, ao atingir a concentração crítica, leva a ocorrência de falhas seja pela redução localizada da força de coesão dos átomos do reticulado, seja pela formação de tensões internas oriundas da formação de hidrogênio molecular. Diferenças microestruturais influenciam o mecanismo de difusão e aprisionamento do hidrogênio atômico, bem como a concentração crítica de hidrogênio molecular necessária para ocorrência das falhas associadas ao hidrogênio. A deformação plástica, inerente ao processo de conformação de tubos, gera discordâncias que atuam como traps de hidrogênio causando seu aprisionamento e influenciando, portanto, a difusão através do reticulado. O objetivo do presente trabalho é avaliar o efeito da deformação plástica sobre a permeabilidade ao hidrogênio em dois tubos API 5L X65, os quais se diferenciam, principalmente, por apresentarem diferentes teores Mn, levando a diferença microestruturais significativas. Para tanto foram empregados ensaios de permeabilidade de hidrogênio, utilizando metodologia adaptada do trabalho de Devanthan e Stachursky (1962), em amostras não deformadas e deformadas até 1% e 6% de alongamento. Os exames microestruturais mostraram diferenças na fração de perlita e no tamanho médio de grão entre os dois aços. Os resultados para as amostras não deformadas mostram que a permeabilidade e a difusividade aparente do hidrogênio são menores no aço com menor teor de Mn. A deformação plástica reduziu a difusividade aparente de hidrogênio nos dois materiais, sendo esta mais acentuada para o aço com maior teor de Mn. Entretanto, a permeabilidade de hidrogênio após deformação de 1% comportou-se de maneira distinta nos dois aços estudados. Este fato evidencia a influência da composição química e, consequentemente, da microestrutura, na permeabilidade e difusividade aparente de hidrogênio nos materiais submetidos à deformação plástica. / Currently microalloyed steel pipes are used to build pipelines for oil and gas exploration and conduction in deep waters. These applications demand alloys with high mechanical strength, weldability and excellent resistance to hydrogen assisted cracking due to work in sour environment. Atomic hydrogen produced during steel corrosion reaction enters in the microstructure from the metal surface due to the presence of H2S that hinders the hydrogen recombination reaction (H0+H0=H2), hence inhibiting hydrogen dissipation to the environment as bubbles. Atomic hydrogen permeates into the material moving through the lattice by diffusion, wherein it interacts with metallurgical defects such as inclusions, precipitates, grain boundaries and dislocations hindering its transport by diffusion. The accumulation of atomic hydrogen in these traps, upon reaching a critical concentration, leads to the occurrence of failures, either by the localized reduction of the cohesive strength of the atoms in the lattice, or by the build up of internal stresses arising from the formation of molecular hydrogen. Microstructural differences influence the mechanism of atomic hydrogen diffusion and entrapment, as well as the critical molecular hydrogen concentration required for the occurrence of hydrogen assisted cracking. Plastic deformation, which is inherent of pipeline forming process, creates dislocations that act as hydrogen traps, thus affecting hydrogen diffusion through the lattice. This work aims to evaluate the effect of plastic deformation in hydrogen permeation in two API 5L X65 pipeline with differences in chemical compositions especially regarding their Mn contents, which cause significant microstructural changes. Hydrogen permeation tests were performed, using a methodology adapted from the work of Devanathan and Stachursky (1962), in non-deformed and deformed samples up to 1% and 6% elongation. The microstructural characterization shows differences between the pearlite fractions of the two pipelines and in their average grain boundary sizes. The results of the permeation tests in the non-deformed samples showed that hydrogen apparent diffusivity and permeation are lower in the steel with lower Mn content. The plastic deformation reduced the apparent diffusivity of hydrogen in the two materials, however, the diffusivity reduction was more pronounced for the steel with higher Mn content. In addition, the changes in hydrogen permeation in samples deformed up to 1% were different for the two steels. This clearly shows the influence of chemical composition and microstructure in hydrogen permeation and apparent diffusivity in the plastically deformed materials.

The production of advanced high strength steels (AHSS) for use in the automotive and construction industries requires complex control of runout table (ROT) cooling. Advanced high strength steels require coiling at temperatures below 500 °C in order to produce a complex multi-phase microstructure. The research described here will investigate the boiling conditions that occur for moving plate experiments when steel is cooled towards low coiling temperatures. Experiments were performed on a pilot-scale ROT located at the University of British Columbia using industrially supplied steel plates. Tests were performed for four different speeds (0.3, 0.6, 1.0 and 1.3 m/s) and three different initial plate temperatures(350, 500 and 600 °C). Each plate was instrumented with thermocouples in order to record the thermal history of the plate. The results show that cooling is more effective at slower speeds within the stagnation zone for surface temperatures over 200 °C. Outside the stagnation zone regardless of speed cooling is primarily governed by air convection and radiation with minor effects from latent heat caused by splashing water. The maximum peak heat flux value increases with decreasing speed and occurs at a surface temperature of approximately 200 °C, regardless of speed. Below a surface temperature of 200 °C, speed has a negligible effect on peak heat flux. The maximum integrated heat flux seems to vary with speed according to a second order polynomial. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The mitigation of blast hazards on critical reinforced concrete structures has become a major concern in regards to the safety of people and the integrity of buildings. Recent terrorist incidents and accidental explosions have demonstrated the need to study the effects of such threats on structures in order to develop effective methods of reducing the overall impact of blast loads. With the arrival of innovative materials such as steel fibre reinforced concrete (SFRC), ultra-high performance fibre reinforced concrete (UHPFRC) and high strength steel reinforcement, research is required in order to successfully adapt these new materials in blast-resistant structures. Hence, the objective of this thesis to conduct an experimental parametric study with the purpose of investigating the implementation of these innovative materials in reinforced concrete slabs and panels. As part of the study, a total of fourteen one-way slab specimens with different combinations of concrete, steel fibres and steel reinforcement are tested under simulated blast loads using the University of Ottawa Shock-Tube Facility. The test program includes three slabs constructed with normal-strength concrete, five slabs constructed with SFRC and six slabs constructed with UHPFRC. Among these specimens, four are reinforced with high-performance steel reinforcement. The specimens are subjected to repeated blast loading with gradually increasing reflected pressure and reflected impulse until failure. The performance of the slabs is studied using various criteria such as failure load and mode, maximum and residual deflections, as well as tensile cracking, spalling and secondary fragmentation control. The behaviour of all specimens is compared in different categories to determine the effects of concrete type, steel reinforcement type, steel fibre content and steel fibre type on blast performance. As part of the analytical study the response of the slab specimens is predicted using dynamic inelastic single-degree-of-freedom (SDOF) analysis. The dynamic analysis is conducted by generating load-deformation resistance functions for the slabs incorporating dynamic material properties.

Previous events have demonstrated the vulnerability of reinforced concrete infrastructure to blast loading. In buildings, ground-story columns are key structural components, and their failure can lead to extensive damages which can cause progressive collapse. To prevent such disasters, the steel reinforcement in such columns must be properly detailed to ensure sufficient strength and ductility. The use of modern concrete materials such ultra-high performance concrete (UHPC) is one potential solution to improve the blast performance of columns. UHPC shows high compressive strength, high tensile resistance and superior toughness, properties which make it ideal for use in the blast-resistant design of columns. The combined use of UHPC and high-performance steels can potentially be used to further enhance the blast resistance of columns. This thesis presents an experimental and analytical study which investigated the use of high-performance materials to increase the blast capacity and ductility of reinforced concrete columns. As part of the experimental study, a total of seventeen columns were tested under simulated blast loading using the University of Ottawa Shock-Tube. Parameters investigated included the effect of concrete type (NSC and UHPC), steel reinforcement type (normal-strength, high-strength or highly ductile), longitudinal reinforcement ratio, seismic detailing and fiber properties. The test program included two control specimens built with normal-strength concrete, five specimens built with UHPC in combination with high-strength steel, and ten columns built with highly ductile stainless steel reinforcement. Each column was subjected to a series of increasing blast pressures until failure. The performance of the columns is investigated by comparing the displacements, impulse capacity and secondary fragmentation resistance of the columns. The results show that using high-performance steels increases the blast performance of UHPC columns. The use of sufficient amounts of high-strength steel in combination with UHPC led to important increases in column blast capacity. The use of ductile stainless steel reinforcement allowed for important enhancements in column ductility, with an ability to prevent rupture of tension steel reinforcement. The study also shows that increasing the longitudinal reinforcement ratio is an effective means of increasing the blast resistance of UHPC columns The thesis also presents an extensive analytical study which aimed at predicting the response of the test columns using dynamic inelastic, single-degree-of-freedom (SDOF) analysis. A sensitivity analysis was also performed to examine the effect of various modelling parameters on the analytical predictions. Overall, it was shown that SDOF analysis could be used to predict the blast response of UHPC columns with reasonable accuracy. To further corroborate the results from the experimental study, the thesis also presents an analytical parametric study examining the blast performance of larger-scale columns. The results further demonstrate the benefits of using UHPC and high-performance steel reinforcement in columns subjected to blast loading.

No description available.

Este trabalho apresenta um estudo dos mecanismos de fadiga em mola para válvula automotiva, realizado através da análise do comportamento mecânico correlacionado ao aspecto microestrutural. As amostras foram fabricadas em aço de alta resistência Si-Cr, submetidas a tratamentos térmicos convencionais (têmpera e revenimento) e não convencionais (austêmpera), obtendo um mesmo nível de dureza (aproximadamente 610 HV). Os corpos de prova foram submetidos a ensaios de bancada com diversos níveis de carregamentos, controlados através de variação da amplitude de tensão cisalhante. Esta variação promoveu diferentes regimes de fadiga, conhecidos como alto e ultra-alto ciclo, para maiores e menores níveis de amplitudes de tensões, respectivamente. O desempenho foi medido através do cálculo da vida total em fadiga, probabilidade de sobrevivência, além da verificação do relaxamento da força elástica e da tensão residual. As amostras foram caracterizadas através de métodos metalográficos, determinação da composição química, ensaios de dureza e resistência à tração, além de técnicas difratométricas de raios-X. As superfícies de fratura foram investigadas utilizando técnicas de microscopia eletrônica de varredura Os mecanismos de início e propagação das trincas, de ambas as amostras, foram semelhantes, apesar do tratamento térmico proposto de austêmpera produzir uma microestrutura mista, com predominância de bainita inferior, contendo regiões esparsas de austenita retida. A microestrutura bainítica, resultante da austêmpera, quando comparada com as amostras martensíticas, geradas pela têmpera e revenimento, propiciou um aumento da vida total em fadiga nos maiores níveis de amplitudes das tensões, tendendo à equalização com a diminuição desta amplitude, além de apresentarem um maior relaxamento elástico, gerando uma discreta diminuição de magnitude da tensão média. Por outro lado, não houve relaxamento das tensões residuais, em ambas as amostras, após uma quantidade significativa de ciclos, portanto este fenômeno não influenciou em sua vida total. / This paper presents a study on the valve spring fatigue mechanisms, carried out through analysis of mechanical behavior correlated to microstructural aspect. The samples were made of high strength steel Si-Cr, obtained by conventional (quenching and tempering) and unconventional (austempering) heat treatments, generating a single level of hardness (approximately 610 HV). The samples were subjected to bench test with various loading levels, controlled through shear stress amplitude fluctuation. This variation has promoted distinct fatigue regimes known as high and ultra-high cycle, for greater and smaller levels of stress amplitudes, respectively. The performance was measured by calculating the fatigue total life, survival probability, verification of the elastic force and residual stress relaxation. The samples were characterized by means of metallographic methods, chemical composition analysis, hardness/tensile strength testing and X-ray diffractometry techniques. The fracture surfaces were investigated using scanning electron microscopy techniques. The cracks initiation and propagation mechanisms were similar in both samples; despite the proposed austempering heat treatment produced a mixed microstructure with predominance of lower bainite, containing sparse regions of retained austenite. The bainitic microstructure, resulting of austempering, when compared with martensitic samples, generated by quenching and tempering, led to an increase in fatigue total life under higher levels of stress amplitudes, but tending to be the same as the stress amplitude deacresases and the number of cycles increases. On the other hand, there was no residual stresses relaxation in both samples, after a significant amount of cycles, so this phenomenon did not influence the total life.

The influence of steel metallurgy on rock bolt SCC was studied using a series of commercial carbon and low-alloyed steels. The chemical composition, their mechanical properties and the microstructures of these steels varied considerably in order to gather information for the discussion of the metallurgical influences under Hydrogen Embrittlement (HE) and Stress Corrosion Cracking (SCC) conditions. In order to understand the metallurgical influences on Rock Bolt SCC, an evaluation was carried out to fifteen commercial steels. The experiments reproduced the Stress Corrosion Cracking condition at which commercial rock bolts had failed in Australians mines. Due to the selected materials, stress and electrolyte condition it is expected that Hydrogen Embrittlement (HE) will affect the steel failure. The approach was to use the Linearly Increasing Stress Test (LIST) and exposing the sample to a dilute pH 2.1-sulphate solution, in accordance with prior studies. Stress Corrosion Cracking was evaluated by analysing the decrease in tensile strength, loss of ductility and fractography observed using Scanning Electron Microscopy (SEM). The initial series of test to the fifteen steels were performed at the free corrosion potential (f.c.p.) vs. Ag/AgCl. From this initial test only five steels (AISI 1008, AISI 4140, AISI 4145H, pipeline X-65 and X-70) did not show Stress Corrosion Cracking features. These five steel were tested in accordance with the Linearly Increased Stress Test (LIST) in the dilute pH 2.1 sulphate solution at different electronegative applied potential to minimum value of -1500mV. The experimental procedure tried to reproduce the Stress Corrosion Cracking condition to identify the most aggressive condition the steel is able to support before failing due to Stress Corrosion Cracking to then compare the theory of SCC and HE in low carbon and low alloy steel with the obtained experimental results. The investigation compared the well-known theory of SCC and HE in low carbon and low alloy steel with the obtained experimental results. Surprisingly, the experimental result did not always agree with the theory.