Dissertations / Theses on the topic 'Pearling'

A cryptic haplosporidian parasite was detected infecting rock oysters from the Montebello Islands in north-western Australia using a PCR targeting the parasite’s small ribosomal subunit gene. The PCR products were cloned and sequenced along with the remaining sections of the parasite’s SSU rRNA gene. Phylogenetic analysis of the sequence generated indicated a Minchinia species (Haplosporidia). The SSU sequence generated was used to develop two in situ hybridisation assays to visualise the parasite in H/E sections as well as a PCR assay to detect the parasite. The molecular assays were assessed for specificity and sensitivity and were then used to compare the parasite to previous haplosporidian parasite infections of pearl oysters. Both assays produced positive results from the infected pearl oysters but not from other closely related haplosporidian species. An SEM and TEM electron microscopy analysis was performed on spores from both parasite species. The spores of the pearl oyster parasite had two spore wall filaments wound around the spore originating for a posterior thickening while the spores of the rock oyster parasite were covered in microtubule-like structures. These data suggests pearl oysters where co-infected with both the Haplosporidium sp. and the Minchinia sp. detected in rock oysters. No evidence of a posterior thickening could be found on the spores of the rock oyster parasite. Attempts to detect the parasite at the previous geographic sites of its detection in pearl oysters resulted in detection of the Minchinia species in tropical oysters in the Kimberley region of Western Australia by in-situ hybridisation.

This thesis concerns the development of mathematical models to describe the interactions that occur between spray droplets and leaves. Models are presented that not only provide a contribution to mathematical knowledge in the field of fluid dynamics, but are also of utility within the agrichemical industry. The thesis is presented in two parts. First, thin film models are implemented with efficient numerical schemes in order to simulate droplets on virtual leaf surfaces. Then the interception event is considered, whereby energy balance techniques are employed to instantaneously predict whether an impacting droplet will bounce, splash, or adhere to a leaf.

Ayant une structure particulière avec une tête hydrophile et une queue hydrophobe, des molécules amphiphile ont de nombreuses applications importantes, comme par exemple, la fabrication des détergents, la protection et la fonctionnalisation de surfaces, etc. Des agrégats de diverses formes, micelles, véhicules, membranes etc., peuvent se former à partir des amphiphiles. La complexité de ces agrégats moléculaires rend l’étude théorique de ce type de systèmes extrêmement difficile. Jusqu’à présent, notre connaissance sur l’interaction entre des nanoparticules et des agrégats des amphiphiles reste encore incomplète. A l’aide de certaines méthodes de simulations moléculaire et une approche théorique, nous avons entrepris une série d’études pour mieux comprendre les questions fondamentales suivantes :1. Comment la présence de nanoparticules, notamment la courbure de ses surfaces, affecte l’agrégation de molécules amphiphile ?2. Comment une bicouche de lipide, une forme d’agrégat particulier des amphiphile, peut induire l’assemblage auto-organisé de nanoparticules hydrophobes ?3. Est-ce que la présence des nanoparticules peut provoquer des transitions morphologiques d’un nanotube membranaire ?
Amphiphile molecules, endowed with a particular structure containing a hydrophilic head and a hydrophobic tail, have many important applications, e.g., fabrication of detergents, surface coating or surface functionalization, etc. Molecular aggregates of various forms, micelles, vehicle, membranes, etc. can be formed from amphiphile molecules. The complexity of these molecular aggregates involving a large number of atoms make the theoretical study of these system very challenging. Up to now, our understanding of the interaction between nanoparticles and aggregates of amphiphiles remains quite incomplete. Using a variety of molecular simulation methods and some theoretical approaches (Helfrich theory and perturbation theory), we have studied the following issues in the present thesis: 1. How the presence of nanoparticles, especially due to their highly curved surfaces, affects the aggregation of the amphiphiles? 2. How a lipid bilayer, a particular amphiphile aggregate, induces the self-assembly of hydrophobic nanoparticles.3. How the morphology transition of a membrane nanotube can be induced by nanoparticles?

En s’appuyant sur des expériences canoniques revisitées, cette thèse caractérise la dynamique de gouttes, de bulles, et de films minces sur des surfaces biomimétiques texturées et imprégnées d’huile. En contact avec des liquides aqueux, ces surfaces présentent quatre phases distinctes (les textures solides, l’huile, le liquide déposé et l’air) donnant naissance à une multitude d’interfaces dont le rôle est prédominant dans les dynamiques observées. La friction visqueuse s’opposant au dévalement d’une goutte est caractérisée en fonction du rapport des viscosités de la goutte et de l’huile. Les résultats obtenus mettent en lumière le rôle essentiel du ménisque d’huile entourant le pied de la goutte. Par la suite, deux expériences mettant en jeu une ligne de contact en reculée sont étudiées. La première correspond au démouillage d’un film mince, la deuxième à l’instabilité de perlage. Dans ces deux cas, le comportement qualitatif correspond à celui reporté dans la littérature sur des surfaces solides. Cependant, une étude plus approfondie révèle que la présence de l’huile affecte significativement la dynamique. Les écoulements dans l’eau et dans l’huile doivent alors être pris en compte simultanément. Ces travaux mettent ainsi en lumière l’originalité de ces surfaces, partiellement solides et partiellement liquides
This thesis aims at characterizing drops, bubbles and thin films dynamics on biomimetic textured surfaces, impregnated with oil (known as LIS in the literature). When an aqueous liquid is deposited on such surfaces, the four phases at stake (oil, air, textures and deposited liquid) generate multiple interfaces, playing a crucial role in the various dynamics observed. The viscous friction opposing the motion of a drop on an inclined LIS at low capillary numbers is studied as a function of the oil/drop viscosity ratio. The results revealed the crucial role of the microscopic oil meniscus surrounding the foot of the drop. Then, two experiments focusing on the dynamics of a receding contact lines are studied: the dewetting of a thin aqueous film and the pearling instability. In both cases, the qualitative behavior is similar to the one reported in the literature on conventional solid surfaces. However, a deeper study reveals that the presence of oil changes quantitatively the dynamics. The flow in both the aqueous and the oil phases must then be taken into account simultaneously. The results obtained in this work highlight the originality of those surfaces, and shed new light on the very peculiar role of the oil meniscus surrounding the contact lines

The pearlite-to-austenite reversion transformation kinetics under isothermal and continuous heating conditions in a eutectoid plain-carbon steel have been measured, using a dilatometric technique on a Gleeble 1500 Thermomechanical Simulator. The isothermal data was characterized in terms of the transformation start time at temperature for the onset of the P→ γ transformation, and in terms of the Avrami parameters n and b. Under the assumption that the P→ γ transformation was additive, the Scheil equation was applied to the measured isothermal transformation start data to predict the onset of the transformation on continuous heating, and the isothermal phase transformation kinetics were used to predict the continuous heating kinetics. It was found that the kinetic model significantly underpredicted the transformation start time during continuous heating. This was attributed to the large experimental error inherent in the estimation of the isothermal transformation start time, t[formula omitted]. The model's continuous heating kinetic predictions were excellent at low heating rates, but it tended to overpredict the kinetics at higher heating rates. The problem was traced to an observed difference between the measured temperature and the programmed temperature during the high heating rate tests. When the model was modified to incorporate the actual temperature profile, its prediction of the kinetics was considerably improved. Thus the austenite reversion transformation was concluded to be experimentally additive. An average Avrami n value of 2.2 suggested that austenite was nucleating on pearlite colony corners and edges. This conclusion was verified with optical and scanning electron microscopy. Previously published data, which indicated that the pearlite-to-austenite transformation is isokinetic, was found to be based on questionable assumptions. Metallographic information suggests, however, that the nucleation sites are saturated early in the reaction. Furthermore, the isothermal austenite formation data generated in this work was found to meet the effective site saturation criterion for additivity, implying that the austenitization process would be expected to be additive. The effect of starting microstructure was evaluated by performing isothermal and continuous heating tests on two different pearlitic microstructures. It was found that, in agreement with published results, the transformation rate varied in inverse proportion with the pearlite spacing and colony size.
Applied Science, Faculty of
Materials Engineering, Department of
Graduate

Abrasion is a form of wear prominent particularly in the agricultural, mining, mineral and transportation industries. The cost of abrasive wear to the national economy is estimated to be about 1% of the gross national product, and it can compromise the safety and reliability of engineering components. The mechanism of wear is complex and dependent on all the materials involved in the process, environmental conditions and many subtle factors such as the shape of the abrading particles. Many abrasion-resistant steels are based on a quenched and tempered martensitic microstructure, because the hardness of the steel should intuitively matter in determining the wear rate. Nevertheless, the relationship between the rate of material loss and steel hardness is unlikely to be monotonic. The purpose of the work presented in this thesis was primarily to study the abrasive wear behavior of a nanostructured bainitic steel that has been successful in structural applications, is capable of mass production, and can achieve hardness levels comparable to martensitic steels without compromising ductility, toughness and fatigue resistance. A variety of wear mechanisms have been studied, in each case with a detailed characterisation of the damage, the structural evolution and a panoply of theoretical approaches. In the case of three-body abrasion, it is found that huge variations in hardness, achieved by changing the structure from pearlite, nanostructured bainite to martensite by heat treatment, do not lead to significant differences in the wear rate. This is because the wear mechanisms change, for example from severe sub-surface deformation leading to sticking in the case of pearlite, to brittle detachment of material in the martensitic state. The nanostructured bainite, on the other hand, undergoes reaustenitisation at the surface that leads to the formation of a fine martensitic layer with consequent surface hardening, in contrast to the pearlite and martensite, both of which soften at the contact surfaces. It is the presence of stable austenite in the nanostructured bainite that causes this difference, because austenitisation becomes easier to achieve. This hypothesis has been further tested by eliminating the austenite from the nanostructured bainite. The experiments confirm that a reaustenitised layer no longer forms during three-body abrasion. The softening observed on martensitic samples also disappears when similar tests are done on tempered martensite, indicating the effect of the localised heat generated during dry abrasion on untempered martensite. In contrast to three-body abrasion using silica where the weight loss is insensitive to hardness, the nanostructured bainite outperforms most commercial alloys of equivalent hardness, and sometime even harder materials, during dry rolling/sliding wear. The mechanisms involved have been rationalised in terms of structural damage mechanisms, the development of beneficial residual stresses, and detailed changes in crystallite size and dislocation character as a function of rolling. It has, in general, been possible to rationalise the observed variations in different types of wear tests and micro- or nanostructures, and it is believed that the work will be of use in designing commercially important products.

The combination of static strain aging and plastic strain reversal is important to understand for both the forming of components and also analysis of in service performance, for example, in the case of fabrication of pipeline, motor shafts or structural components in buildings and ships. Static strain aging phenomenon has been experimentally studied for the cases of forward and reverse re-straining after aging on a low carbon steel (0.16 wt% C) with a ferrite-pearlite microstructure. Torsion tests on hollow tubular samples were used for the mechanical tests. The shear strain on the surface of the sample was measured with the digital image correlation. The influence of the amount of pre-strain, aging time and temperature, and the strain path reversals on the stress-strain response after aging has been measured experimentally. A maximum increase of 46 MPa was obtained in the yield stress of the samples re-strained after full aging in the same direction as the initial straining. This maximum increase in yield stress as well as the rate of increment in yield strength during aging was almost independent of the amount of pre-strain and the increase in the flow stress occurred without a significant variation in the work hardening behavior. Further, it was shown that a yield point phenomenon was absent if the direction of re-straining after aging was reversed and the increase in the flow stress level after aging was proportional to the amount of pre-strain and increased with extended aging time. In this case, the absence of a sharp yield point after prolonged aging time led to the speculation that the activation of dislocations sources, rather than unpinning of locked dislocations in re-straining after aging was the controlling mechanisms although proof of this requires further investigation. Although it is difficult to unambiguously identify all of the underlying physical mechanisms, nevertheless, a comprehensive set of experimental results has been measured which can be used by the design engineer when considering cases where static strain ageing and strain path reversals are relevant for a ferrite-pearlite steel with 0.16 wt% carbon.
Applied Science, Faculty of
Materials Engineering, Department of
Graduate

The aim of this study is to qualitatively assess the insulation system's condition of medium voltage induction motors. In essence this aim of the research was to analyse and classify the data. The initial step was to understand the data. The literature review gives the background of the insulation system and the different tests done and its interpretations. The research methodology used has been explained along with description of the data. The aging of the insulation is a wide and complex topic, thus in this research the electrical aspects of the insulation were looked into and explained in detail. The data and the limitations of this study are also discussed. Data mining processes were used to gain insight into the data and the condition of the insulation system. The different stages of data mining are explained. The different stages are: identifying the problem, &tC1 understanding, data preparation and data analysis. An analysis was done using self-organizing maps, which is an unsupervised neural network technique. Hierarchical and K-mean clustering techniques were used to classify the data. The results of the different techniques were compared to an expert's assessment. The study is was an attempt to understand the condition of the insulation system and to classify the data according to its condition. A comparison was done between the different techniques used. The data was divided into four groups based on the voltage rating and class of insulation used in the motors. Good classification was obtained for three out of the four groups of data. In conclusion, the patterns in the different features of the data due to ageing were observed. The data was qualitatively assessed and classified into groups according to the deterioration of the insulation system using the classification techniques. Finally the results correlated well with the expert's assessment. In essence, the goals set for the research were achieved.
Thesis (M.Ing. (Electrical and Electronic Engineering))--North-West University, Potchefstroom Campus, 2007.

The rolling-sliding dry-wear behaviour of a series of bainitic steels and a standard pearlitic rail steel have been compared over a range of contact stress and creepage conditions applicable to the British Rail network. A rolling-sliding wear machine has been constructed - LEROS - which allows very high contact stresses to be combined with high creepages under well controlled conditions. Materials were tested on LEROS and on an Amsler machine. Limited vibration analyses were carried out on both machines and compared with the frequencies of disc surface periodic undulations. No direct linkage was determined. Despite better standard mechanical properties, the wear resistance of lower carbon bainitic steels was inferior to that of the pearlitic steel. A bainitic steel with the same carbon content as the pearlitic steel wore a little less, but at considerable expense to the pearlitic wheel steel counter-material in the wear couple. The wear resistance of bainitic steels depends upon the volume fraction of hard phase, such as carbide and martensite-austenite phase, for rolling-sliding as well as other types of dry wear loading. Pearlitic steel performs exceptionally well under certain rolling-sliding conditions, such as the majority seen in these tests, since the lamellar microstructure is modified so as to present a greater area fraction of carbide hard phase at the wear surface, a fraction in excess of bulk volume fraction. Recommendations are made for the dry wear applicability of the steels.

O objetivo do presente trabalho, consistiu na obtenção de ferro fundidos nodulares, a serem utilizados na fabricação de eixos, com as seguintes características mecânicas após os tratamentos térmicos adequados: limites de resistência à tração e escoamento mínimos de 700 N/mm2 e 450 N/mm2 respectivamente, alongamento mínimo de 5% e dureza entre 235 à 285 HB. A microestrutura mais adequada à obtenção das características mecânicas desejadas deveria consistir de cementita esferoidizada numa matriz ferrítica, a ser obtida com o tratamento de recozimento ou revenimento de microestruturas previamente perlíticas ou martensíticas, respectivamente. No caso do tratamento de recozimento, as estruturas iniciais consistiram de perlita grossa mais ferrita, presentes no material no estado bruto de fusão, ou, de perlita fina mais ferrita, do material normalizado. Todos os tratamentos térmicos utilizados, dependendo do tempo de tratamento, permitiram a obtenção de cementita esferoidizada. O material normalizado e recozido durante 2 horas a 700ºC, possibilitou a obtenção das propriedades mecânicas desejadas: limites de resistência à tração e de escoamento de 827 N/mm2 e 547 N/mm2 respectivamente, alongamento de 7% e dureza de 277 HB, valores estes, dentro dos limites desejados no presente trabalho. No caso do material temperado e revenido a 700ºC durante 0,5 hora, obteve-se limites de escoamento de 542 N/mm2, alongamento de 6% e dureza de 246 HB. O tratamento de recozimento realizado durante 48 horas, produziu microestrutura quase totalmente ferrítica, com pequenas áreas de agregados de carbonetos. O restante do carbono migrou para os nódulos de grafita secundária ao redor dos mesmos.
The purpose of the present work, consisted in obtaining ductile casting iron, that will be used in the shaft production, with de following mechanical properties after the appropriated heat treatments: ultimate tensile strength and yield strength values of 700 N/mm2 and 450 N/mm2 minimum respectively, elongation of 5% minimum and hardness value from 235 to 285 HB. The microstructure more adequated to obtaining the desired mechanical characteristics should consist of spheroidized cementite in a ferritic matrix, to be obtained with the annealing or tempering treatments of microstructures previously pearlitic or martensitic respectively. In the case of the annealing treatment, the initial structures consisted of thick pearlite plus ferrite, wich were in the material in the ascast state, or fine pearlite plus ferrite, of the normalized material. All the used heat treatment, depending on the time of treatment, allowed the obtainment of spheroidized cementite. The material normalized and annealed for 2 hours at 700°C made possible the obtaining of the desired mechanical properties: ultimate tensile strength and yield strength value of 827 N/mm2 and 547 N/mm2 respectively, elongation of 7% and hardness values of 277 HB, values these, in acordance with the aims initially proposed in the present work. In the case of the material quenched and tempered at 700°C for 0,5 hour, it was obtained yield strength values of 542 N/mm2, elongation of 6% and hardness value of 246 HB. The annealing treatment made during 48 hours, produced a microstructure almost totally ferritic, with small areas of aggregated carbides. The remaining of the carbon migrated to the graphite nodules producing secondary graphite around them.

This thesis deals with heat treatment of pearlitic steel beams with using of induction heating. The aim of this heat treatment is to improve mechanical properties of steel beams. In theoretical part are described pearlitic steels and theory of induction heating. In experimental part of this thesis analysis of current state of technology with its results was made. Numerical model describing current state of technology was assembled. Then this model was used to recommend adjustments in current technology of heat treatment, which leads to enlargement of heat treated depth. Experiments with adjusted parameters were executed on heat treating machine. Analysis of microstructure using LM and EM, measurement of hardness and fractographic analysis were done. The result of adjustments is enlargement of heat treated depth about half compared with current technology.

Cold drawn eutectoid steel wires have been widely used for a variety of applications, such as suspension bridges, steel cords for automobile tires, and springs. Much research has been done to increase their mechanical strength. With advances in modern production technology, both the drawing speed and the quality of drawn steels have been enhanced. After a careful literature survey, it is obvious that some issues are still controversial. As Y.S. Yang, J.G. Bae and C.G. Park mentioned, the lamellar spacing, thickness and volume fraction of cementites have all reached the nanometer regime, and the conventional theory is not enough to explain it. Besides, the cementite dissolution is a huge problem to the performance, according to Y.S. Yang and C.G. Park. To address the above issues, a systematic study has been taken on the wire drawing process under the conditions of the industrial production. Through the morphology, microscopic, mechanical and comprehensive analysis, a clear understanding of the microstructures and associated professing conditions of the high-strength carbon steel wires has been obtained. This project aims to clarify why the mechanical properties improve with the increasing strain. The project will be carried out in four stages: 1) characterisation of the microstructure of the cold drawn steel wires; 2) measurements of the modulus, hardness and toughness of steel wires; 3) modelling the deformation behaviour of the cold drawn steels. The techniques involved in the project include X-ray diffraction (XRD), focus ion beam (FIB), scan electron spectrum (SEM), Nanoindentation. A deep understanding of the relationship between composition, structure and performance will be achieved in this project. The results may provide the basis for improving cold-drawn steel wire designs.

Les aciers Dual Phase (DP) ferrito-martensitiques sont largement utilisés sous la forme de tôles minces dans la construction automobile en raison de leur excellent compromis résistance/ductilité et donc pour leur potentiel d’allègement. Ils sont élaborés par coulée continue, laminage à chaud et à froid suivis d’un recuit continu. Durant l’étape de chauffage et de maintien de ce recuit, la microstructure ferrito-perlitique déformée issue des étapes de laminage se transforme en microstructure ferrito-austénitique recristallisée. L’expérience montre que les cinétiques de recristallisation et de transformation ainsi que la distribution spatiale et morphologique des microstructures résultantes sont très sensibles aux vitesses de chauffage. Ce travail de thèse s’intéresse aux différents mécanismes expliquant cette sensibilité comme la maturation des carbures, la restauration, la recristallisation de la ferrite et la transformation austénitique et toutes leurs interactions. Ces mécanismes métallurgiques ont été caractérisés à différentes échelles et par des approches in situ sur un acier industriel puis modéliser par des approches à base physique pour guider une possible production. Après un premier chapitre dédié aux techniques expérimentales et de modélisations utilisées, le second chapitre de ce travail s’intéresse principalement à la caractérisation de la morphogénèse des microstructures ferrito-austénitique en microscopie électronique à balayage (MEB). Le troisième chapitre est une étude détaillée en Microscopie Electronique à Transmission (MET) et par modélisation thermocinétique (ThermoCalc, DICTRA) de la composition des carbures tout au long du processus, du laminage à chaud au recuit. Restauration et recristallisation sont étudiées au chapitre 4 principalement par des expériences in situ en Diffraction des Rayons X à Haute Energie (DRXHE) sur ligne de lumière synchrotron et modélisées par une approche originale à champs moyen. Enfin, le chapitre 5 propose une étude sous DICTRA pour comprendre les cinétiques de transformation austénitique en fonction des vitesses de chauffe. Cette approche est novatrice car elle prend en compte les carbures intergranulaires de la ferrite, a été conduite en conditions anisothermes et propose une analyse fine des modes de croissance de l’austénite associées au manganèse, élément clef de la composition de ces alliages
Ferrite/Martensite Dual-Phase steels are largely used in the form of thin sheets in the automotive industry for their excellent balance between resistance and strength and thus for their lightening potential. They are elaborated by continuous casting, hot- and cold- rolling, followed by a continuous annealing. During the heating and the soaking stages of this latter process, the deformed ferrite/pearlite microstructure obtained after rolling evolves is transformed into a recrystallized ferrite-austenite microstructure. The experiments show that recrystallization and austenite transformation kinetics as well as the resulting spatial and morphological distribution of the phases are highly sensitive to the heating rate. This PhD thesis aims at understanding the different metallurgical mechanisms explaining this particular sensitivity as carbides ripening, recovery, recrystallization and austenite transformation and all their possible interactions. The mechanisms were characterized at different scales and by in situ technics on an industrial steel and model by physical based approaches in order to drive future production lines. After a first chapter dedicated to the experimental and modeling methods, the second chapter deals with the characterization of the morphogenesis of ferrite-austenite microstructures by Scanning Electron Microscopy (SEM). Chapter 3 is a study by Transmission Electron Microscopy (TEM) and by thermokinetic modeling (ThermoCalc, DICTRA) of the chemical composition of carbides along with manufacturing, from hot-rolling to annealing. Recovery and recrystallization are studied in chapter 4 by the means of in situ High Energy X-Ray Diffraction (HEXRD) experiments conducted on a synchrotron beamline and modeled by an original mean-field approach. Finally, chapter 5 proposes an analysis with DICTRA to understand austenite transformation kinetics as function of heating rates. The proposed approach is innovative as it accounts for intergranular carbides in the ferrite matrix, is conducted in non-isothermal conditions and propose a fine analysis of growth modes of austenite associated to manganese, a key alloying element of the studied steels

In this thesis, the behavior of pearlitic steel was characterized under controlled wear conditions in the laboratory and service conditions in two ore mining stages, comminution and transportation. The thesis consists in three experimental chapters, divided according to the tribosystems analyzed. On all the chapters Electro Microscopy techniques for the microstructural analysis were employed. Scanning Electron Microscopy (SEM), Focused Ion Beam (FIB-SEM), Electron Backscatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM) were used. The first experimental chapter shows the analysis of the pearlite under abrasive wear with loose abrasive particles in multi-events conditions. The sample was taken from Semi-Autogenous Grinding mills (SAG) and experimental simulation was carried out in laboratory using the Dry Sand Rubber Wheel Abrasion Test (DSRW). The results show a polycrystalline layer formation in both cases, characterized by ultra-fine grains of ferrite in the layer closer to the surface. It was also concluded that the DSRW can simulate the wear produced on field (superficial and microstructural features) in conditions of higher normal load than recommended by the ASTM Standard G65. The second experimental chapter explores the characterization of the microstructure after the indenter pass in scratch test using two conditions of normal load applied and five sequences of scratch. The microstructural analysis shows the formation of two subsuperficial layers identified by the level of the microstructural alterations. In the subsuperficial layer (close to the surface), the formation of new ultra-fine grains of ferrite was observed. A second layer was observed deeper in the sample and denominated as layer of the microstructure transition, characterized by the combination of deformed (reduction of the interlamellar spacing) and pearlite colonies not affected plastically by the mechanical loading. On this layer, the crystallographic texture in RD // in samples tested at 4 N (normal load) and one-pass scratch was determined. Later, on this chapter, the microstructure in a ground rail (industrial procedure characterized as a multi-event scratch test) was analyzed. Two grinding conditions were used for the analysis with variation of the grinding linear speed and load on the grinding stones (discs). The combination of low grinding speed and high load promotes a higher deformed layer formation beneath the patch zone and low randomized orientation of the pearlite colonies. Finally, in the third experimental chapter, the pearlitic characterization was concluded with the study of samples of railway wheel and rail under wear in service and Rolling Contact Fatigue (RCF) in laboratory. The laboratorial simulation was carried out using the twin-disc rolling contact tribometer with a variation of number of cycles. The characterization of railway wheel shows that the WEL is characterized by levels of breaking and aligned cementite and zones with dissolution of the carbon atom in the ferrite to form the supersaturated carbon ferrite. The polycrystalline ferrite formation (ultra-fine grains) in the sub-superficial layer and it was identified a preferential orientation of RD // in the layer of microstructural transition. The results of the laboratory test show surface crack nucleation and propagation at low angle in the more severe deformed layer. The microstructure of the layer consists in polycrystalline ferrite and the cementite dissolution.
Nesta tese foi caracterizado o comportamento do aço perlítico em condições controladas de desgaste em laboratório e em serviço em dois estágios do processo de mineração de minério, cominução e transporte ferroviário. A tese consiste em três capítulos experimentais divididos segundo o tribosistema analisado. Em todos os capítulos do trabalho foi utilizada a técnica de microscopia eletrônica para análise microestrutural. Foi utilizado Microscopia eletrônica de varredura (MEV), Focused Ion Beam (FIB-SEM), Electron Backscatter Diffraction (EBSD) e Microscopia eletrônica de transmissão (MET). O primeiro capítulo experimental mostra a análise da perlita in condições de desgaste abrasivo com partículas soltas em eventos múltiplos. As amostras foram tiradas de um moinho semi-autógeno (SAG) e realizada uma simulação experimental do desgaste em condições controladas usando o tribômetro de roda de borracha (RWAT). Os resultados mostraram a formação de camada branca em ambas as condições de análise, consistindo em uma camada poli cristalina caracterizada pela formação de grãos ultrafinos na camada mais próxima da superfície de desgaste. Também foi concluído que a roda de borracha pode simular o desgaste produzido nos moinhos SAG tanto nas características superficiais quanto microestruturais em condições de maior severidade as comumente utilizadas na norma ASTM G65 (procedimento B). O Segundo capítulo experimental explora a caracterização da microestrutura depois da passagem do endentador no ensaio de riscamento (scratch test) utilizando duas condições de carga normal aplicada e 5 sequências de riscamento. A análise microestrutural mostrou a formação de duas camadas subsuperficiais identificadas pelo nível de alteração microestrutural. Na camada mais próxima da superfície de desgaste foi observada a formação de grãos ultrafinos de ferrita. A segunda camada identificada mais profundamente na amostra, denominada como camada de transição, é caracterizada pela combinação de colônias deformadas (redução do espaçamento interlamelar) e camadas não afetadas pelos esforços produzidos no contato. Nesta camada foi determinada a texturização em direção RD // nas amostras testadas a 4 N (carga normal aplicada) e uma passada. Posteriormente à análise de riscamento foi caracterizada a microestrutura de uma amostra tirada de um trilho esmerilhado (processo industrial que pode ser considerado como aplicação do ensaio de riscamento). Foram consideradas duas condições de esmerilhamento com variação de velocidade de esmerilhamento (deslocamento linear do veículo esmerilhador) e potência dos motores dos rebolos usada no procedimento. A combinação de baixa velocidade de esmerilhamento e alta potência nos motores controladores dos rebolos promoveu uma grande deformação nas camadas subsuperficiais na região de contato e uma baixa aleatoriedade das orientações cristalográficas das colônias de perlita. Finalmente, no capítulo três, a caracterização da microestrutura perlitica foi finalizada com o estudo de amostras de roda e trilho em condições de desgaste em campo e de Rolling Contact Fatigue (RCF) em ensaios de laboratório. A simulação experimental foi realizada utilizando o tribômetro twin-disc rolling (configuração disco-disco) com variação do número de ciclos. A caracterização da roda ferroviária mostrou a formação da camada branca caracterizada por níveis de cementita fraturada e alinhada em direção do movimento de rolamento/deslizamento com áreas de dissolução do átomo de carbono na ferrita formando uma ferrita supersaturada. Foi identificado a formação de policristais de ferrita (grãos ultrafinos) na camada mais superficial e uma orientação preferencial RD // na camada de transição. Os resultados dos ensaios de laboratório mostraram a nucleação de trincas superficiais se propagando a baixo ângulo na camada branca. A transformação microestrutural dessa camada após ensaios de laboratório consiste em policristais de ferrita e dissolução da cementita.

A pesquisa relativa ao transporte ferroviário no Brasil tem se intensificado nos últimos anos visando principalmente a melhoria no desempenho do par rodatrilho. Isso se deve à demanda de se transportar cada vez mais carga por eixo com a finalidade de reduzir o custo de transporte. Através de mudanças na microestrutura do material ou de mudanças na composição das ligas utilizadas na fabricação pode se aumentar a resistência do par roda-trilho. Neste trabalho foi analisada a influência de diferentes microestruturas (puramente perlíticas e perlíticas-bainíticas), obtidas por meio de tratamento térmico por dilatometria, nas propriedades de um aço alto carbono similar ao utilizado em rodas convencionais classe C AAR destinadas ao transporte de carga pesada. As análises empregaram técnicas metalográficas (identificação de fases presentes e distância interlamelar da perlita), ensaios magnéticos (saturação magnética e ruído magnético de Barkhausen), ensaio mecânico de dureza e ensaio tribológico de resistência ao desgaste por deslizamento. Foram caracterizadas rodas ferroviárias classe C AAR em quatro condições distintas: usadas e novas, fundidas e forjadas.
Research about rail transport in Brazil has been intensified recently due the need to improve the wheel-rail performance, considering the demand of increasing axle load aiming for transportation cost reduction. Changes in the material microstructure or in the alloys composition can improve the wheel-rail resistance. A eutectoid steel similar to that used in conventional class C AAR wheels (designed to heavy haul transportation - high loads and low speeds) was submitted to isothermal heat treatments applying dilatometry techniques at ten different cycles. Different microstructures (totally pearlitic and pearlitic-bainitic) were obtained and their influence on the mechanical and magnetic properties was analyzed. The analysis included metallographic techniques (identification of phases and pearlite interlamellar spacing), magnetic testing (magnetic saturation and magnetic Barkhausen noise) and mechanical testing (hardness and pin-ondisc tests). Railway wheels Class C ARR were characterized in four different conditions: new and used, cast and forged.

Eutectoid steel strips are designed for the production of parts for intensive use such as clutches, seat slides, and springs as they exhibit

excellent strength levels and wear resistance. These properties arise from the unique morphology of lamellar pearlite which can be considered

as a self-laminated nanoscale composite. However, a spheroidization annealing step is nowadays necessary to improve the cold forming properties before further cold rolling steps.

This thesis is aimed at improving the tensile ductility of the hot rolled products of eutectoid composition in order to eliminate the intermediate

annealing step. Two strategies are proposed.

The first is to transpose the concept of controlled rolling developed for HSLA to

eutectoid steels. Through a strict adjustment of the austenite processing and of the cooling strategy, it is possible to improve the ductility

of the final lamellar microstructure. The way the processing parameters influence the hot deformation of austenite, the eutectoid transformation and of the subsequent spheroidization annealing is deeply

investigated. It is found that refinement and pancaking of austenite

is beneficial as it reduces the pearlite block size improving the total

tensile elongation. Accelerated cooling is of paramount importance to

achieve fine Interlamellar spacing (ILS), which lead to high strength

levels and accelerate spheroidization during subsequent annealing.

The second approach involves intercritical or warm deformation. Warm processing of eutectoid steels is first explored by torsion testing

and then up-scaled to a pilot rolling-line. The interactions between thermomechanical parameters, rolling forces generated and microstructural

evolution are carefully scrutinized. During concurrent hot deformation, spheroidization of cementite takes place almost instantaneously

in both torsion and rolling. The restoration processes occurring in the ferrite matrix depends on the strain path and the strain rates. Low strain rates (0,1 s−1) and simple shear promotes the formation of a recrystallized-like HABs network of about 3μm in size.

Plane strain compression and high strain rates (10 s−1) leads to the formation of a typical recovered dislocation substructure (LABs) of 1μm in size. During annealing, no recrystallization occurs and the LABs substructure remains stable. This substructure influences drammatically the mechanical properties: the strength is very high and the work-hardening behavior is poor due to high recovery rate in the region close to the LABs. However, due to the presence of spheroidized

cementite particles the ductility of warm rolled eutectoid steels is higher than that of ultra fine grained low carbon steels.
Doctorat en Sciences de l'ingénieur
info:eu-repo/semantics/nonPublished

Abstract In this thesis, methods to process ultrafine ferritic (UFF) structures in steels, i.e. grain sizes below about 3 μm have been investigated. It is shown here, in accordance with the results in the literature, that a steel with a UFF grain size can be obtained by two methods, more or less convenient to mass production: deformation-induced ferrite transformation from fine-grained austenite (the DIF route) and the static recrystallization of various heavily cold-worked initial microstructures (the SRF/SRM route). In the present work, the influencing factors in the processing of UFF structure in the DIF route have been systematically studied in four low-carbon steels: one C-Mn steel and Nb, Nb-Ti and Nb-high Ti microalloyed steels. A high strain, a low deformation temperature close to Ar3 and a fine prior austenite grain size are beneficial to promote the formation of UFF grains. Especially by using complex pretreatments to refine the prior austenite grain size, cold rolling, repeating the low-temperature reheating cycle or using martensitic initial microstructure, a UFF grain size can be obtained in these steels at the strain of 1.2 (70% reduction) at 780 °C. By controlling the cooling rate, the type of the second phase can be adjusted. When using the static recrystallization route, it was found that UFF is difficult to obtain from a single-phase ferrite, but it is relatively readily obtained from deformed pearlite, bainite or martensite, especially in high-carbon steels with 0.3–0.8%C. In deformed pearlite, the cementite lamellae fragmented and spheroidised in the course of heavy deformation can provide numerous nucleation sites by the particle stimulated nucleation mechanism and retard the subgrain and recrystallized grain growth. Nucleation and retardation of grain growth are effective also in deformed bainite, martensite or high-carbon tempered martensite, as discussed in detail in the work. The thermal stability of UFF grained steels was tested and found to be generally excellent, but it varies depending on the processing method. The UFF structure obtained by the SRM route has a thermal stability somewhat weaker than that of the DIF route. For a given steel, UFF grains may show different grain growth modes, related to the dispersion of second phase particles. In the DIF structure, abnormal grain growth occurs at 700 °C after about 2.5 h, while in the SRM structure, normal grain growth takes place slowly at 600 °C. Carbides on the grain boundaries seem to play an important role in inhibiting grain coarsening. No coarse-grained zone was formed at the HAZ of electron beam or laser welded seams, as performed at low heat inputs (up to 1.5 kJ/cm) on thin strips. The hardness even increased from the base metal towards the HAZ and the weld metal in all seams as an indication that they were hardened during the rapid cooling.

Företaget Svensk Kärnbränslehantering AB (SKB) håller på att utveckla en slutförvaringskapsel som kommer innehålla avfall från den svenska kärnkraften. Det är dock fortfarande en process under utveckling och därför undersöks olika typer av metoder och kapselmaterial för att kunna tillverka en hållbar och säker kapsel. Kapseln består av ett hölje av kopparrör med svetsad botten och lock och en insats med stållock. Insatsen är en cylindrisk konstruktion  av segjärn och innehåller en svetsad stålkassett för att skapa utrymmen till det använda kärnbränslet. Insatsen innehåller bland annat stålrör som under tillverkning får utstå en gjutprocess med segjärn och erhåller efter det icke homogena egenskaper. Målet med undersökningen är hur stor påverkan gjutningen har på stålets kemiska sammansättning samt mikrostrukturer. Det som orsakar de inhomogena egenskaperna är främst värmebehandlingen som driver diffusionen av kol från gjutjärnet till stålet, som då ger ett hårdare men sprödare material. Med hjälp av experiment och simuleringar upptäcks hur mycket kol som diffunderar in i stålet samt ändringar i den kemiska sammansättningen i de påverkade zonerna. Identifiering av fasomvandlingar, diffusion och ändringar i mikrostrukturer är stora faktorer som i sin tur ändrar de mekaniska egenskaperna i stålet.
The Swedish Nuclear Fuel and Waste Management Company (SKB) have developed a final storage canister that will contain waste from the Swedish nuclear power plants. However, it is still in a development phase and therefore different types of methods and canister materials are investigated to produce the most durable and safe canister. The canister is made of a copper tube with a welded bottom and lid with an insert. The insert is a cylindrical construction of nodular cast iron that contains a welded steel cassette, to make space for the spent fuel, and a steel lid. The steel tubes showed inhomogeneous properties after being exposed to a casting around them. The aim of this investigation is to clarify the impact of casting on the chemical composition of the steel as well as the microstructure. The cause to the inhomogeneous properties were the diffusion of carbon from the cast iron to the steel, which then produced a harder and more brittle material. Experiments and simulations were used to see the carbon diffusion into the steel as well as what happens with the chemical composition in the affected zones. Identification of phase changes, diffusion and microstructures contributed to changes of mechanical properties in the steel.

The constantly tightening emission regulations demand the engines to be moreefficient, to get more power out of smaller engines. Higher engine loads andcomponent temperatures are causing more stresses to engine components. Therefore,a company that produces engines wanted to study if it would be possible to increasethe capabilities of the components by optimising the used material. In this final project work, a cylinder head will be studied. The cylinder heads for theengines are made of ductile cast iron. The limits of that material are near safety limits,and therefore a better material is needed. In this work are some previous studiesanalysed and tried to find how to optimise the used material. The optimised materialshould have better thermal conductivity properties combined with sufficient strengthproperties. Previous studies were analysed to gather knowledge of the elements that affect thematerial parameters. Those studies showed that copper, silicon, pearlite fraction, andthe use of chills are the elements to be optimised. Silicon and pearlite fraction waschosen as optimisation parameters because of their effect on the thermal conductivityand strength properties. Copper was chosen as an optimisation variable due to its effecton the pearlite formation. Chills were used to affect the cooling rate and thereby thepearlite formation. The work was made using MAGMASOFT™ simulation software to simulate cylinderhead casting. The simulated cylinder head was divided into 4 parts for the simulations.For those sections were then set targets for pearlite fraction according to previousstudies. The silicon content was kept constant in the simulation, based on the studiespresented in this work. Copper content was simulated with variations from 0 to 0.7weight-%, and chill heights were simulated from 20 to 60 mm and without chills. After simulating the different variables, the results were analysed. Then the selectedcasting simulation result was mapped to finite element simulation mesh to include thelocal material parameters to finite element simulation. With the finite elementsimulation, the estimated lifetime of the component was simulated. By analysing the casting simulation results, an optimal combination was found. Theoptimal material parameters for a cylinder head casting would be copper 0.5weight-%, silicon 1.9 weight-% and chills thicker than 40 mm on the flame plate. Theoptimised material gives more possibilities to develop engines even further when thecomponent demands are growing.

Die Arbeit widmet sich dem Fließverhalten von Stählen bei zyklisch inkrementeller Torsion. Dazu werden vergleichend Reineisen und der ferritisch-perlitische Stahl 42CrMo4N bei unterschiedlichen Verformungspfaden betrachtet. Vor allem in zyklischen Torsionsversuchen mit bleibendem Verformungsinkrement je Zyklus werden Fließkurvenverläufe und Verformungsverfestigung analysiert. Die Ergebnisse belegen den Einfluss des lamellaren Zementit auf das Fließverhalten des Stahls 42CrMo4N, während die Eigenschaften des Reineisens von der entstehenden Versetzungszellstruktur bestimmt werden. Die Richtungsabhängigkeit der Fließspannung und die Verläufe der Fließkurven unterscheiden sich für die betrachteten Werkstoffe deutlich. Fließortkurven dienen der quantitativen Beschreibung der Verfestigung. Die Vorgehensweise zu ihrer Ermittlung und ihre Abhängigkeit von den Versuchsbedingungen und den Verformungszuständen werden gezeigt. Bei Reineisen dominieren isotrope, bei dem Stahl 42CrMo4N kinematische Verfestigungsanteile das Fließverhalten
Ferritic-pearlitic steel 42CrMo4N and pure iron under different strain paths are compared regarding their flow behaviour. Mainly in cyclic torsion tests with resulting strain increment per cycle shear stresses and strain hardening are analysed. The results show, that the cementite lamellae determine the flow behaviour of the steel 42CrMo4N, whereas the properties of pure iron are governed by the evolving dislocation cell structure. The dependency of flow stress on the strain direction is different for the two materials. Yield surfaces describe strain hardening quantitatively. The procedure for yield point detection and the dependency of subsequent yield surfaces on experimental conditions and strain states is shown. For pure iron isotropic hardening, for steel 42CrMo4N kinematic strain hardening dominates the flow behaviour

Etude experimentale sur un acier micro-allie dans deux etats microstructuraux bainitique et ferrito-perlitique, soumis a des essais d'endurance, de fatigue oligocyclique et de fissuration. Comparaison avec les resultats obtenus sur un acier conventionnel dans un etat trempe-revenu. Influence de l'environnement (air, vide, azote et traces de vapeur d'eau) et de la frequence d'essai. Discussion du role fragilisant de la vapeur d'eau. Proposition d'un modele de propagation sous environnement actif

L’industrie automobile cherche constamment à augmenter la part des pièces fabriquées à partir de tôles minces en aciers plus résistants et à plus bas coût. Le parallèle entre les diagrammes de phases Fe-N et Fe-C montre qu’il est possible de développer des aciers similaires dans le système Fe-N. Les objectifs de cette étude étaient l’élaboration des aciers binaires Fe-N et le développement des structures équivalentes à celles dans les aciers au carbone. Les approches envisagées s’articulent autour de : Elaboration : la nitruration gazeuse en phase austénitique suivie de traitements d’homogénéisation ont permis de charger des tôles minces de fer pur en concentrations importantes d’azote. Une simulation de la diffusion de l’azote a été proposée. Genèse des microstructures : Le refroidissement lent de l’austénite Fe-N conduit à des structures perlitiques lamellaires et globulaires, constituées de ferrite et du nitrure non-stoechiométrique Fe4N. Une structure aciculaire particulière a été identifiée. Il s’agit de la ferrite se développant dans le nitrure Fe4N. Le refroidissement lent de la ferrite Fe-N sursaturée conduit à la précipitation des nitrures stables Fe4N et métastable Fe16N2. Des microstructures multiphasées [alpha+alpha'+gamma] ont été obtenues par des maintiens dans le domaine intercritique suivis de trempe. Le domaine intercritique a été réexaminé en utilisant le modèle des sous-réseaux. Des essais in-situ en MET ont relevé l’évolution des précipités de Fe16N2 dans la ferrite au cours du vieillissement à 85 °C. Des proportions importantes de l’austénite résiduelle ont été relevées, ce qui serait à la base du développement des aciers TRIP à l’azote
Car designers are seeking ways to increase the proportion of parts made of sheet steels of higher strength and lower cost. The parallel between the Fe-N and Fe-C phase diagrams shows that it is possible to develop similar steels in the Fe-N system. The objective of this study was to prepare binary Fe-N steels and to develop structures equivalent to those in carbon steels. Approaches to meet the objectives are considered: Preparations of Fe-N steels: gas nitriding in austenite domain followed by homogenization treatments allowed to introduce high amount of nitrogen in pure iron sheet. A simulation of the nitrogen diffusion was proposed to describe the weight increase during nitriding. Genesis of microstructures: The slow cooling of the Fe-N austenite led to lamellar and globular pearlitic structures composed of ferrite and nitrideFe4N. An acicular microstructure, which is the consequence of the precipitation of the ferrite in the nitride Fe4N, was also identified. The slow cooling of the supersaturated Fe-N ferrite led to the precipitation of the stable nitride Fe4N and the metastable nitride Fe16N2, which witnessed a rapid diffusion of nitrogen in ferrite at low temperature, comparable to that of carbon. The multiphase microstructures [alpha+alpha'+gamma] were obtained by intercritical treatments followed by quenching. The intercritical domain was reviewed using the sub-lattice model. In-situ TEM investigations have identified the precipitation of Fe16N2 nitride in the ferrite during the aging at 85 °C. High amount of the residual austenite have been identified, which would be the basis for development of TRIP nitrogen steels

This thesis aims to provide adequate analytical information on the spade shaped iron bars of Norrland and central Sweden. While their significance has been thoroughly debated for decades, analytical research on them has been confined to cases of single artefacts or theoretical interpretations of their value, meaning and origin. In this study a comprehensive approach is taken into consideration. Based on X-Ray fluorescence (XRF), scanning electron microscopy (SEM) and metallographical analysis this thesis seeks to facilitate new interpretations on quality, production centres and usage based on analytical results. Aiming to settle some of the long lasting questions regarding the artefacts while producing results which can further the discussion by raising new questions, previously unasked.

Limited information exists regarding the effects of light pearling on the properties of physical grain characteristics, composition, and technological and sensory properties of selected varieties of Western Canadian barley especially hulless barley genotypes with modified starch characteristics. Nine barley genotypes with different hull (hulled and hulless) and starch characteristics (normal, waxy, and high amylose (HA)) were pearled to three differing levels. Scanning electron micrographs showed that the pericarp, testa, aleurone, and subaleurone layers were completely removed in heavily pearled barley whereas only a few outer layers were removed in minimally pearled barley. Waxy starch genotype Fibar and HA starch genotypes, SH99250 & SB94893 contained high levels of soluble β-glucan (9-11%). Waxy starch genotypes exhibited higher β-glucan solubility when cooked compared to normal and HA starch genotypes. However, HA starch genotypes had lower in vitro starch digestibility which may provide a lower glycemic response in humans.

This study examines the experiences of dementia caregivers (DCs) and caregivers of older adults with non-dementia related mental illness (CMIs) through the application of Pearlin's stress process framework, which conceptualizes the caregiving experience as an interplay between various contextual factors, stressors, moderating resources, and outcomes variables. A total of seven caregivers (4 DCs, 3 CMIs) participated in this study. Participation involved a one-on-one semi-structured interview focused on tapping into the components of Pearlin's framework, following which participants completed three measures (CES-D, STAI-Trait form, SF-36) aimed at assessing levels of depression, anxiety, and physical and mental health, respectively. Qualitative data were analyzed using content analysis and quantitative data were examined in relation to qualitative results and to normative data. Qualitative analysis revealed that CMIs are exposed to particularly severe forms of problematic behaviours and that they utilize far fewer formal services and resources. In relation to quantitative findings, it was found that, in general, the CMIs experienced higher levels of emotional distress and worse physical health and functioning. Overall, results suggest a need for research focused on how formal services could be modified to best capture the needs of the diverse population of CMIs.

The temperature at which a new phase forms is an important parameter in the genesis of final microstructure. For diffusional transformation processes, prediction of this temperature on cooling, until now, relied upon empirical equations which are based on the cooling rate or degree of undercooling. To develop a method that is capable of predicting the onset of transformation in the steel, applicable to industrial processing conditions, a theoretical study has been conducted to critically examine the consequences and implications of the Scheil additivity rule in relation to the incubation of the austenite decomposition. An "ideal" isothermal transformation curve for the start of transformation from austenite to pearlite has been developed based on the assumption of the consumed fractional incubation time being additive. The curve is characteristic of the chemistry and the austenite grain size of the steel. A mathematical relationship between the experimental time to the start of transformation and the ideal incubation time was quantified, and two methods for deriving an "ideal" T77' curve from the controlled cooling experiments were established in this study. The derived "ideal" T77' curve was used for predicting the start of transformation from austenite to pearlite for three different cooling conditions; namely, continuous, semi-continuous and non-continuous cooling conditions. The predictions were consistent in all cases and compared favourably against other methods which have been employed frequently frequently to estimate the transformation start temperature for non-isothermal conditions.

碩士
國立臺灣科技大學
機械工程系
99
We have studied the phase transformations of a Mn-Al steel during low temperature aging processes. The heat treatment of the steel was solution treated at 1100℃ and followed by heating isothermally at temperatures from 850 to 500℃. The composition of the steel is Fe-14.5 Mn-6.4 Al-0.78 C (wt%). The constituent phases of the Mn-Al steel is single austenite at temperatures between 1100 and 825℃. At temperatures below 800℃, ferrite and ??carbide precipitate at the grain boundaries of the austenite matrix. Furthermore, at aging temperatures below 675℃, we found lamellar grains of ferrite and ??carbide forming ??pearlite appear in the austenitic grains as pearlitic colonies. It is a eutectoid reaction which supersaturated austenite decomposed into ferrite and ??carbide. The ??pearlite nucleated at the grain boundary areas, grow into the austenitic matrix, and form pearlitic colonies in the austenitic matrix. The ??pearlite is very similar to the pearlite in steels, except that the carbide is in different crystal structure. When the aging temperatures are lower than 625℃, in addition to ferrite and ??carbide grains in ??pearlite, we found M23C6 carbide precipitates in the lamellar ferritic grains. The appearance of the M23C6 carbide is a result of the decomposition of supersaturated ferrite into more stable ferrite and M23C6 carbide. Thus, the constituent phases of the Mn-Al steel are austenite, ferrite, ??carbide and M23C6 carbide. The M23C6 carbide precipitates inside the ferritic grains of the ??pearlite. The trends for the compositions of the constituent phases are as follows. The Mn content in various phases in the decreasing trend is M23C6, ??carbide, austenite, and ferrite. The Al content decreasing trend of the phases is ??carbide, ferrite, austenite, and M23C6 carbide. We found a K-S orientation relationship exists between k-carbide and ferrite: [01 ]κ//[ 11]α, (111)κ//(110)α. The same situation exist between M23C6 and ferrite: [01 ]C//[ 11]α, (111)C//(110)α

碩士
國立成功大學
材料科學及工程學系碩博士班
96
This study is intended to evaluate how feasible it is to suppress the intergranular embrittlement fracture behavior of spheroidal graphite (SG) cast iron with the formation of pearlite in the cast iron. Intermediate-temperature embrittlement and cyclic heating induced embrittlement are investigated in this study. In the latter case, the primary objective is to examine the effect of cyclic heating at the maximum temperature and the resultant microstructural change due to the instability of pearlite at high temperatures. Two types of SG cast iron have been chosen. One is as-cast SG cast iron that has been added with pearlite promoting elements (Cu, Mn and Cr) and has a bull’s eye structure. The other is SG cast iron which has undergone a ferritization heat treatment to achieve a matrix with lower pearlite content and is used as a contrast material. 　Experimental results indicate that the location of crack nucleation remains the same despite the existence of pearlite. The hindrance of pearlite to the propagation of cracks that emerge from eutectic cell boundary regions is also unapparent. Consistence is found in the intermediate-temperature embrittlement fracture behaviors between SG cast irons with different pearlite contents, namely that in-tergranular fractures only occur in the ferritic region. The RT~550℃ tensile strength and crack cross sections of SG cast irons with different pearlite contents reveal the differences in grain strength and grain boundary strength between pearlite and ferrite. This also leads to the higher critical fracture stress of pearlite compared to ferrite and the higher equicohesive temperature(ECT) in pearlite compared to ferrite. These are verified by the result that intergranular fractures only occur in the ferritic region of SG cast iron at specific tem-peratures. 　On the other hands, when the cyclic heating temperature reaches 800℃, significant martensitic phase transformation occurs and results in anmarked change in the tensile property. When cyclic heating temperature reaches 750℃, the decomposition of pearlite is mainly responsible for the change in the tensile property at the room temperature. More notably, such decomposition is less significant in the SG cast with lower pearlite percentage. Finally, an intermediate temperature tensile test following cyclic heating confirms that pearlite has a higher critical fracture stress than ferrite.

ABSTRACT This dissertation examines the enduring leadership of community activists Dorothy Bolden, Ella Mae Wade Brayboy, and Pearlie Dove from 1964 until 2015. Brayboy was one of the first African-American Deputy Voter Registrars in the state of Georgia, Bolden founded the National Domestic Workers Union and Dove was the first woman to head the department of education at Clark College. This dissertation inserts Dorothy Bolden, Ella Mae Wade Brayboy, and Pearlie Dove into the classic Civil Rights Movement narrative by framing their community advocacy as equal to the efforts of Atlanta’s more well-known African-American leaders. This dissertation presents Bolden, Brayboy, and Dove as career-oriented professional women who were also politically savvy community activists. These three women acquired a power base that allowed them to found organizations, create programming, and develop projects dedicated to empowering Atlanta’s black community. These women achieved a level of influence typically associated with the wealthy or the political prominent. Because the three women were grassroots organizers, this study contends that the implications of their activism have been obscured because of gender, race, and class. This study seeks to foreground Bolden, Brayboy, and Dove’s efforts in Atlanta’s Movement narrative. In this dissertation, assessments of Bolden, Brayboy, and Dove’s professional contributions as acts of protest on behalf of the black community are used to undergird a critical intervention; first, their work refutes previous ideology centering the efficacy of Movement leadership (as a social movement) as grounded in mass mobilization. Secondly, their leadership was oppositional to the standard portraiture of Movement leadership as male, ministerial, and middle-class. Finally, the women’s professional and activist emphases on economic uplift, education, and enfranchisement illustrate evidence of how sustained acts of protest, led by local leadership, impacted the community. Because there is considerably less literature focused on the historical significance of black women acquiring political power outside of elected office, this study seeks to establish the women as politically significant local leadership.

Die Arbeit widmet sich dem Fließverhalten von Stählen bei zyklisch inkrementeller Torsion. Dazu werden vergleichend Reineisen und der ferritisch-perlitische Stahl 42CrMo4N bei unterschiedlichen Verformungspfaden betrachtet. Vor allem in zyklischen Torsionsversuchen mit bleibendem Verformungsinkrement je Zyklus werden Fließkurvenverläufe und Verformungsverfestigung analysiert. Die Ergebnisse belegen den Einfluss des lamellaren Zementit auf das Fließverhalten des Stahls 42CrMo4N, während die Eigenschaften des Reineisens von der entstehenden Versetzungszellstruktur bestimmt werden. Die Richtungsabhängigkeit der Fließspannung und die Verläufe der Fließkurven unterscheiden sich für die betrachteten Werkstoffe deutlich. Fließortkurven dienen der quantitativen Beschreibung der Verfestigung. Die Vorgehensweise zu ihrer Ermittlung und ihre Abhängigkeit von den Versuchsbedingungen und den Verformungszuständen werden gezeigt. Bei Reineisen dominieren isotrope, bei dem Stahl 42CrMo4N kinematische Verfestigungsanteile das Fließverhalten.
Ferritic-pearlitic steel 42CrMo4N and pure iron under different strain paths are compared regarding their flow behaviour. Mainly in cyclic torsion tests with resulting strain increment per cycle shear stresses and strain hardening are analysed. The results show, that the cementite lamellae determine the flow behaviour of the steel 42CrMo4N, whereas the properties of pure iron are governed by the evolving dislocation cell structure. The dependency of flow stress on the strain direction is different for the two materials. Yield surfaces describe strain hardening quantitatively. The procedure for yield point detection and the dependency of subsequent yield surfaces on experimental conditions and strain states is shown. For pure iron isotropic hardening, for steel 42CrMo4N kinematic strain hardening dominates the flow behaviour.