The effect of initial microstructures of an 0.05 wt.% C low carbon steel, acicular ferrite (AF), Bainite (B), polygonal ferrite (PF), fine polygonal ferrite (FPF), and a microstructure produced by direct strip casting (DSC) (termed SC), on the deformation and recrystallization behaviour of cold rolled low carbon (LC) steel, was investigated. The initially prepared samples with the initial microstructures were cold rolled to 50, 70 and 90% reductions, then annealed isothermally in the temperature range 580 ??? 680 oC. The microstructures and textures produced by deformation and annealing were studied by optical microscopy, XRD, TEM, SEM and EBSD. The initial microstructures were characterized mainly by optical microscopy and EBSD. Using EBSD, the ferrite grain size of the AF, B and SC samples was considerably larger than that found by optical microscopy with a large fraction of low angle grain boundaries (LAGBs) observed within prior austenite grains. All samples exhibited a very weak texture close to random. After cold rolling, the microstructures of AF and SC contained shear bands with PF and FPF generating deformation bands. For AF and SC, the pearlite phase was more extensively elongated in rolling direction compared with PF and FPF. After 90% cold rolling reduction, PF, FPF and SC consist mainly of the texture component and AF and B . It was found that FPF recrystallized most rapidly followed by B, PF and AF with SC recrystallizing orders of magnitude more slowly due to the solution drag caused by its uniformly distributed higher Mn content. Very strong (???-fibre) texture was generated in cold rolled PF followed by FPF, with AF, SC and B generating very weak textures. The texture evolution during annealing 90% reduction PF was examined in further detail. The behaviour of nucleation and grain growth provides strong evidence of orientated nucleation as the dominant factor for CRA texture development in this material.

Despite of their costs, titanium alloys are often used for structural applications due to their high performance to density ratio that allows the manufacturers to reach the aimed mechanical properties. Users are more and more inclined to turn towards the ƓÒ-metastable alloys since they provide a wider range of processing conditions, very attractive corrosion resistance and higher strength levels in comparison to the ƓÑƓyƓÒ alloys. Nevertheless, these alloys present a high sensitivity to the variation in the applied heat- or thermomechanical treatment influencing the final mechanical properties. That is why the understanding of the relationships existing between these heat- or thermomechanical treatments and the corresponding modifications of the microstructures, as well as the influence of several characteristics of the microstructure on the mechanical properties under static and cycling loading conditions is of primary importance. This research allows to characterise and to better understand the precipitation and growth sequences of the ƓÑ phase in the Ti-LCB alloy during classical heat- and thermomechanical treatments in order to be able to predict the microstructure resulting from a defined treatment. Furthermore, to improve the understanding of the relationships existing between the manufacturing process, the corresponding microstructures modifications, and the mechanical properties, tensile tests are performed on the different microstructures and microstructural parameters playing an important role on the static properties are identified. Microstructural features governing the static fracture process in two different microstructures are determined by using a micromechanical model based on a physical understanding of the mechanisms of damage. Finally, the crack initiation and the first stage of crack propagation under high cycle fatigue conditions are investigated at a local scale on two different microstructures.

Despite of their costs, titanium alloys are often used for structural applications due to their high performance to density ratio that allows the manufacturers to reach the aimed mechanical properties. Users are more and more inclined to turn towards the ƓÒ-metastable alloys since they provide a wider range of processing conditions, very attractive corrosion resistance and higher strength levels in comparison to the ƓÑƓyƓÒ alloys. Nevertheless, these alloys present a high sensitivity to the variation in the applied heat- or thermomechanical treatment influencing the final mechanical properties. That is why the understanding of the relationships existing between these heat- or thermomechanical treatments and the corresponding modifications of the microstructures, as well as the influence of several characteristics of the microstructure on the mechanical properties under static and cycling loading conditions is of primary importance. This research allows to characterise and to better understand the precipitation and growth sequences of the ƓÑ phase in the Ti-LCB alloy during classical heat- and thermomechanical treatments in order to be able to predict the microstructure resulting from a defined treatment. Furthermore, to improve the understanding of the relationships existing between the manufacturing process, the corresponding microstructures modifications, and the mechanical properties, tensile tests are performed on the different microstructures and microstructural parameters playing an important role on the static properties are identified. Microstructural features governing the static fracture process in two different microstructures are determined by using a micromechanical model based on a physical understanding of the mechanisms of damage. Finally, the crack initiation and the first stage of crack propagation under high cycle fatigue conditions are investigated at a local scale on two different microstructures.

The effect of initial microstructures of an 0.05 wt.% C low carbon steel, acicular ferrite (AF), Bainite (B), polygonal ferrite (PF), fine polygonal ferrite (FPF), and a microstructure produced by direct strip casting (DSC) (termed SC), on the deformation and recrystallization behaviour of cold rolled low carbon (LC) steel, was investigated. The initially prepared samples with the initial microstructures were cold rolled to 50, 70 and 90% reductions, then annealed isothermally in the temperature range 580 ??? 680 oC. The microstructures and textures produced by deformation and annealing were studied by optical microscopy, XRD, TEM, SEM and EBSD. The initial microstructures were characterized mainly by optical microscopy and EBSD. Using EBSD, the ferrite grain size of the AF, B and SC samples was considerably larger than that found by optical microscopy with a large fraction of low angle grain boundaries (LAGBs) observed within prior austenite grains. All samples exhibited a very weak texture close to random. After cold rolling, the microstructures of AF and SC contained shear bands with PF and FPF generating deformation bands. For AF and SC, the pearlite phase was more extensively elongated in rolling direction compared with PF and FPF. After 90% cold rolling reduction, PF, FPF and SC consist mainly of the texture component and AF and B . It was found that FPF recrystallized most rapidly followed by B, PF and AF with SC recrystallizing orders of magnitude more slowly due to the solution drag caused by its uniformly distributed higher Mn content. Very strong (???-fibre) texture was generated in cold rolled PF followed by FPF, with AF, SC and B generating very weak textures. The texture evolution during annealing 90% reduction PF was examined in further detail. The behaviour of nucleation and grain growth provides strong evidence of orientated nucleation as the dominant factor for CRA texture development in this material.

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No. of bitstreams: 1 DissRBS.pdf: 2973698 bytes, checksum: 37eb711fcbda392563dc3155916b5d95 (MD5) Previous issue date: 2017-03-31 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Magnesium is considered a promising candidate in applications of storage hydrogen in the solid state, due, among other factors, to its low relative cost and high gravimetric capacity (7,6 wt%.). For magnesium, the use of transition metals as additives may show great improvements in the kinetics of hydrogen absorption/desorption. Different additives can have different functionalities, for example, reducing the energy barrier for H2 adsorption and acting as a heterogeneous nucleating agent in the kinetics of phase transformation. In this research, the main effects of the use of zirconium (Zr) as an additive of the magnesium for the purposes of hydrogen storage in bulks produced by rapid solidification in melt-spinning furnace and cold rolling were studied in detail. Basically, structural characteristics were related to the behavior of absorption / desorption. The alloy produced by cold rolling showed behavior of activation and absorption similar to the pure Mg which was produced by the same route. For the route processed by melt spinning, the use of zirconium as additive has presented a deleterious effect on the activation. However, after activation, the kinetics of absorption exhibit behavior similar to pure Mg. The activation had significantly improved after mechanical or thermal processing, however, the structural characteristics that permeate this effect still have a lack of studies. For both process route, the desorption kinetics presented excellent results, reducing the complete reaction in a few minutes. With the results obtained it is possible to conclude that the use of zirconium as an additive in magnesium alloys has no beneficial effect on absorption. In the other hand, the desorption is extremely attractive. After exposure to air, it was verified that both compositions presented formation of contaminants on the surface and this made the activation a difficult way. However, this effect was more latent in pure magnesium. / O magnésio é considerado um candidato promissor em aplicações de armazenagem de hidrogênio no estado sólido, devido, entre outros fatores, ao seu baixo custo relativo e elevada capacidade gravimétrica, 7,6% em peso. Para o magnésio, o uso de metais de transição como aditivos pode apresentar melhoras significativas na cinética de absorção/dessorção de hidrogênio. Diferentes aditivos podem ter diferentes funcionalidades, por exemplo, reduzir a barreira energética para a adsorção de H2 e atuar como agente inoculante de nucleação heterogênea na cinética de transformação de fase. Neste trabalho, foram estudados em detalhes os principais efeitos provenientes da utilização de zircônio como aditivo ao magnésio para fins de armazenagem de hidrogênio em massas metálicas volumosas (do inglês, bulks) produzidas por solidificação rápida em forno melt-spinning (da sigla em inglês, MS) e laminação a frio (do inglês, CR, cold rolling). Basicamente, relacionaram-se características estruturais ao comportamento de absorção/dessorção. A liga produzida por laminação a frio apresentou comportamento de ativação e absorção semelhante ao Mg puro produzido pela mesma rota. Para o material processado por melt-spinning, o uso do zircônio como aditivo apresentou efeito deletério na ativação. Contudo, após a ativação, a cinética de absorção exibe comportamento semelhante ao Mg puro. Posteriores processamentos mecânico ou térmico melhoraram significativamente a ativação, contudo, as características estruturais que permeiam tal efeito ainda carecem de estudos. Para ambas as rotas de processamento, a cinética de dessorção apresentou ótimos resultados, reduzindo em poucos minutos a reação completa. Com os resultados obtidos se conclui que o uso do zircônio como aditivo em ligas de magnésio não apresenta efeitos benéficos na absorção. Todavia, a dessorção da liga com Zr é extremamente atrativa. Após exposição ao ar, verificou-se que ambas as composições apresentaram formação de contaminantes sobre a superfície e, isso dificultou a ativação, porém, este efeito foi mais evidente no magnésio puro.

CoordenaÃÃo de AperfeiÃoamento de Pessoal de NÃvel Superior / As propriedades magnÃticas dos materiais ferromagnÃticos sÃo bastante influenciadas pelo estado microestrutural e o estado de tensÃes. Dessa forma, o monitoramento do comportamento magnÃtico dos materiais estruturais pode ser utilizado para a caracterizaÃÃo de seu estado interno. Este trabalho desenvolve um sistema de mediÃÃo experimental para a realizaÃÃo de ensaios nÃo-destrutivos baseados na avaliaÃÃo das caracterÃsticas magnÃticas do material e sua correlaÃÃo com o estado de tensÃes e microestrutura. O sistema foi aplicado na avaliaÃÃo da degradaÃÃo microestrutural causada por deformaÃÃo plÃstica a frio do aÃo A516 grau 60, provocada pelo processo de laminaÃÃo. Foi projetado e construÃdo um sensor para realizar simultaneamente a magnetizaÃÃo e o monitoramento do comportamento magnÃtico do material inspecionado. ParÃmetros operacionais do sensor como frequÃncia, corrente e espessura da camada de ar entre o sensor e a superfÃcie do material foram estudados atravÃs da simulaÃÃo com o mÃtodo dos elementos finitos. Entre os ensaios realizados pelo sistema està o ensaio de anisotropia magnÃtica, que obtÃm o valor da densidade de fluxo em vÃrios Ãngulos em relaÃÃo à direÃÃo de laminaÃÃo. O ensaio de histerese realiza o levantamento do ciclo de histerese do material, de onde à possÃvel extrair os parÃmetros campo coercitivo e perdas de histerese, que apresentam correlaÃÃo direta com o grau de deformaÃÃo plÃstica. O ensaio de energia magnetocristalina fornece uma avaliaÃÃo direta da anisotropia entre os eixos paralelo e perpendicular à direÃÃo de laminaÃÃo. O ensaio de anÃlise dos ciclos menores de histerese utiliza excitaÃÃo triangular, o que viabiliza uma avaliaÃÃo direta da permeabilidade magnÃtica diferencial, que à bastante sensÃvel ao estado interno do material e fornece uma visualizaÃÃo de como o processo de magnetizaÃÃo à influenciado pela deformaÃÃo plÃstica e pelas tensÃes residuais. Os ensaios apresentaram resultados coerentes com o estado final provocado pelo processo de laminaÃÃo a frio e com a literatura, caracterizando satisfatoriamente o estado do material estudado. / The magnetic properties of ferromagnetic materials are greatly influenced by the microstructure and stresses. Thus, the evaluation of the magnetic behavior of structural materials can be used for characterization of its internal state. This paper develops a experimental measurement system for non-destructive testing based on the evaluation of the magnetic characteristics of the material and its correlation with the state of stress and its microstructure. The system was used to evaluate the microstructural degradation caused by plastic deformation of A516 grade 60 steel, through cold rolling process. A sensor was designed and built to perform both the magnetization and the monitoring of magnetic behavior of the material tested. Sensor operating parameters such as frequency, current and thickness of the air gap between the sensor and the surface of the material were studied by simulation with finite element method. Among the tests performed by the system is the testing of magnetic anisotropy, which gets the value of the flux density at various angles relative to the rolling direction. The hysteresis test obtains the hysteresis loop of the material from which it is possible to extract the magnetic parameters coercive field and hysteresis losses, which have a direct correlation with the degree of plastic deformation. The magnetocrystalline energy testing provides a direct assessment of anisotropy between the axes parallel and perpendicular to the rolling direction. The testing of analysis of minor hysteresis loops uses triangular excitation, which enables a direct assessment of the differential magnetic permeability, which is very sensitive to the internal state of the material and provides a visualization of how the process of magnetization is influenced by plastic deformation and residual stresses . The tests showed results consistent with the final state caused by the process of cold rolling and similar research on the literature, providing an satisfactory characterization of the state of the studied material.

Global warming and air pollution are the major problems facing the world today. Therefore strict environmental legislation on the emission of harmful gases from motor vehicles has forced the automobile industry to search for alternative materials or new materials for exhaust systems. In order to produce cleaner exhaust gases, the exhaust temperature needs to be increased to approximately 900oC. Therefore, exhaust manifolds are exposed repeatedly to hot gases as they are nearest to the engine requiring good oxidation resistance, thermal fatigue properties, cold workability and weldability. One such material to meet the above characteristics is AISI 441 ferritic stainless steel, a dual stabilised Ti and Nb ferritic stainless steel. Ti and Nb are added to stainless steel to stabilise C and N due to their high tendency to form carbonitrides (Ti,Nb)(C,N) and laves phase (Fe2Nb) and Fe3Nb3C. With 18% Cr content, this steel has a good corrosion resistance at elevated temperatures. Included in many applications of this steel are those requiring deep drawing and related forming operations. However, the drawability and stretchability of ferritic stainless steels is inferior to that of the more expensive austenitic stainless steels. For instance, Columbus Stainless has experienced ridging/roping problems at times during the manufacturing process of type AISI 441 ferritic stainless steel. It is believed that this problem is related to crystallographic texture of materials which have effect on formability. The R-value in FSS can be improved through optimisation of chemical composition, which includes reducing the carbon content, and processing conditions such as reducing the slab reheating temperature, increasing annealing temperature and refining the hot band grain size. Therefore the aim of this research project was firstly to investigate effect of amount of cold reduction and annealing temperature on texture evolution and its influence on formability. The as received 4.5 mm hot band steel was cold rolled by 62, 78 and 82% reductions respectively followed by isothermal annealing of each at 900oC, 950oC and 1025oC for 3 minutes. Orientation distribution function (ODF) through X-ray diffractometer (XRD) measurement was used to characterise the crystallographic texture formed in the steel using PANanalytical X’Pert PRO diffractrometer with X’celerator detector and variable divergence. Microstructures were characterised using optical microscopy and scanning electron microscope (SEM). The results show that steels that received 78% cold reduction and annealed at 1025oC recorded the highest Rm-value and lowest ΔR-value which enhances its deep drawing capability. In addition, this steel showed the highest intensity of shifted γ-fibre, notably {554}<225> and {334}<483>. It can therefore be concluded that the γ-fibre which favours deep drawing, is optimal after 78% cold reduction and annealing at 1025oC. The second objective was to investigate the effect of (Nb+Ti) content on the crystallographic texture and the subsequent formability and ridging severity. AISI 441 ferritic stainless steel with different amount of (Nb+Ti) content was used i.e. Steel A (0.26Nb+0.2Ti), Steel B (0.44Nb+0.15Ti) and steel C (0.7Nb+0.32Ti). After a strain of 10%, steels A exhibited the least resistance against surface ridging with average roughness Ra of 1.5 μm followed by steels B with an average roughness Ra of 1.1μm. Steel C showed the highest resistance to ridging with an average roughness Ra of 0.64 μm. This was attributed to the increase in carbonitrites (NbTi)(C,N) due to increased (Nb+Ti) content which acted as nucleation sites for γ-fibre. / Dissertation (MEng)--University of Pretoria, 2013. / gm2014 / Materials Science and Metallurgical Engineering / unrestricted

Introduction 100Mo target design is key to commercially viable large scale cyclotron production of 99mTc. The target back plate supporting the 100Mo must be chemically inert to the target dissolution conditions but ideally it should also be able to dissipate the high thermal loads of irradiation, not contaminate target substrate with radionuclidic by-products, and be adequately inexpensive to allow for single use. Aluminum was selected as our target support as it satisfies these requirements. Our process entails rolling 100Mo powder into a foil of desired thickness, and then diffusion bonding [1] the foil onto an aluminum back plate. The 100Mo targets were designed to be 20×80×0.1 mm to match our TR24 cyclotron’s proton beam profile and energy. Efforts are currently underway to scale up the process to allow for simultaneous production of multiple targets at once. Material and Methods The crude enriched 100Mo foil (99.815% enrichment) was made from 100Mo powder using a horizontally mounted rolling mill and an aluminum hopper. The crude foil was rolled repeated-ly, and the space between the rollers gradually reduced until the thickness of the foil was changed from an initial thickness of 0.3 mm to a thickness of 0.1 mm. The rolled 100Mo foil was annealed under reducing atmosphere and then bonded to the aluminum target plate support under inert atmosphere in a heated press at 500 °C. Results and Conclusion By rolling 100Mo foils from powder we were able to produce uniform foils with an average density of > 98 % compared to the maximum theoretical density of 100Mo (n = 5) and thicknesses of roughly 0.1 mm. All foils produced were the desired 20 mm width (i.e. limited by the width of the opening of the hopper) and trimmed to the desired 80 mm length. The annealing process was necessary due to the brittleness of the un-annealed rolled foil and the difference in the thermal expansion coefficients of molybdenum and aluminum which caused un-annealed foils in previous experiments to crack and break off during pressing (n = 10). Surface preparation of the aluminum support plate was also found to play a critical step in the efficiency of the bond, and continuing effort to scale the above de-scribed procedure to mass produce 100Mo tar-gets is ongoing. Targets have undergone preliminary testing to 250 μA.

This research is about a costly problem in the automotive industry due to tool fracture during the splines cold rolling of steel shafts. The objective is to study the cause of this failure and propose solutions that can be implemented in the workshop.The writing starts with a brief introduction of the companies involved in shafts production and problem solving. It introduces the cold rolling process and its advantages on splines manufacturing, and it goes through relevant material and process characteristics that help to determine the cause of tool fracture.In order to understand the process failure and production flow, it has been necessary to build up an Ishikawa diagram with possible tool fracture causes. After collecting and analysing the data about the machine tool, cold rolling process and work-piece and rolling tool materials, tests and experiments have been done.It has been considered that there is a rolling tool fatigue that causes tool fracture. Beside tool fracture, two more problems with production flow instability and the right side rolling tool have been detected. Testing the material hardness of the work-piece has shown continuous hardness fluctuations from the supplier. Rolling tool misalignment has been measured by using a vernier caliper measurement device. Rolling tools material hardness analysis shows that tool is very hard and it is possible to use a tougher material which responds better to cyclic loads.Leax has tried to solve the problem by testing another lubrication and tool coatings. A modal analysis test has been performed in order to find the natural frequency of the work-piece which possibly may lead to vibration and over loading one of the rolling tools.The conclusion that has been reached is that main cause of fracture is rolling tool fatigue due to cyclic loads and it is important to use other rolling tool material. The other two detected problems, production flow instability and rigth side rolling tool fracture, should be considered as a part of the problem in order to significantly increase tools life and stabilize production flow rate.

The research describes possible reasons why the flaking tendency during cold rolling can vary between different austenitic stainless steels. The flaking phenomenon was observed after a rolling process in Granlund Tools AB’s roll reducing mill “KOR-8”. A literature review was conducted with the purpose of finding a connection between rolling process, austenitic stainless steels and flaking. The laboratory work aimed at revealing possible differences between the flaked material and materials that is known to be cold rolled with high surface finish in the particular machine. In order to come to a conclusion regarding the flaking appearance, scanning electron microscopy (SEM), Vickers hardness test and light optical microscopy (LOM) were performed. The literature review along with the laboratory results made it possible to determine the two major factors affecting the flaking behavior. Two independent analyses revealed what came to be the most important discovery in this study. LOM showed the appearance of a thick oxide scale on the surface of the flaked steel which was further confirmed by SEM-Energy-Dispersive X-ray Spectroscopy that indicated the existence of oxides on the steel’s surface. These observations along with the knowledge that oxide scales grow only during high-temperature processing led to the conclusion that the steel is not appropriate for the cold rolling process due to the earlier steel manufacturing.