The corrosion fatigue crack propagation behaviour of a high strength low alloy steel, N-A-XTRA 70, in a synthetic sea water solution was tested using S. E. N. specimens subjected to a loading frequency of 0.1 Hz and a load ratio of 0.6. In order to simulate the conditions encountered by a thumbnail type crack several specimens from each of the microstructural types tested, namely parent plate, heat affected zone and heat treated material, had their crack sides covered by transparent plastic covers. Severe overprotection and slight underprotection conditions were produced using cathodic protection potentials of -1400, -1300, -1200 and -700 mV (S. C. E. ). The Paris relationship da/dN = CLKm was found to be a useful tool in describing the crack propagation rate data. Results obtained, presented in the form of plots of log da/dN against log AK, show that for parent plate, H. A. Z. and heat treated material, covering the crack sides of specimens produces enhanced corrosion fatigue crack propagation rates, at cathodic protection potentials of -1400 and -1300 mV (S. C. E. ), when compared to non covered specimens. This trend was also true for H. A. Z. specimens at a potential of -700 mV (S. C. E. ). For parent plate specimens, however, covering the crack sides at a potential of -700 mV (S. C. E. ) produced reduced crack propagation rates over non covered specimens. It is believed restriced oxygen access may account for these results. Plots of the Paris exponent m and constant C for the three microstructures tested produced three lines of the form m= alnC +b where a and b were found to be dependent upon material parameters. Comparison of results with BS 4360: 50D revealed that N-A-XTRA 70 exhibited superior fatigue performance when tested in air but behaved worse under conditions of free corrosion.

No description available.

L’objectif de ce travail consiste à améliorer les propriétés mécaniques d’un acier faiblement allié à bas carbone (0,2%) sans recours à des procédés de métallurgie secondaire onéreux. Pour cela, un processus d’inoculation, déjà utilisé lors de l’élaboration des fontes, est transposé à notre nuance d’acier. Il consiste à ajouter des éléments en très faible quantité dans l’acier liquide pour en modifier le processus de solidification donc la microstructure et par conséquent les propriétés de l’acier, dont la résilience. L’ajout d’éléments en très faible quantité ne modifie pas la nuance d’acier. Ils doivent être ajoutés en fin de fusion du métal sous forme de poudre dans le jet de coulée. Les effets de ces éléments se verront sur la microstructure notamment au niveau des inclusions et de la taille des grains. Le « bon candidat » est un élément qui conduira à une répartition homogène d’inclusions de petites tailles et de forme sphérique. Il doit aussi permettre de réduire la taille des grains. Ces modifications de structure sont supposées améliorer les propriétés mécaniques de l’acier et notamment la résistance aux chocs. Une pièce de référence est réalisée pour pouvoir en étudier la microstructure. La forme retenue est un lingot parallélépipédique dimensionné à l’aide d’un logiciel de simulation de coulée afin de prévenir des principaux défauts de fonderie. La taille est adaptée à l’échelle laboratoire (capacité four 120 kg). Les charges de fusion correspondant à la nuance étudiée sont fournies par l’entreprise partenaire de la thèse, SAFE Metal. La première étape est d’obtenir un bain convenablement désoxydé ; c’est-à-dire ajouter de l’aluminium afin de piéger l’oxygène dissous pour l’évacuer. Pour mettre en évidence d’éventuels effets significatifs des différents inoculants testés, il faut partir d’un échantillon de référence contenant un nombre d’inclusions relativement élevé. Ceci est obtenu en ajoutant du soufre dans le bain liquide. Cet élément agit directement sur le nombre d’inclusions présentes dans l’acier en formant des sulfures. Pour passer à l’étape d’inoculation il a fallu créer un outillage spécifique pouvant s’adapter à l’échelle du laboratoire. Des essais sont ensuite réalisés avec différents produits à des concentrations variables. Les échantillons obtenus sont analysés par différentes techniques : analyse chimique de l’acier (spectrométrie étincelle et ICP), analyse de microstructure et de la taille de grains (par micrographie optique) et le comptage inclusionnaire. En ce qui concerne ce dernier point qui consiste à détecter les inclusions, à les compter et à les classer par leur nature, leur forme et leur taille, deux possibilités existent. Soit à l’aide d’un microscope optique mais les risques d’erreur sont importants et le processus est long et fastidieux, soit à l’aide d’un logiciel spécifique (AZtec) couplé à un microscope électronique à balayage (MEB). C’est ce choix qui a été fait, car outre le fait qu’il permette un gain de temps considérable grâce à une automatisation du processus, il est possible de connaître via une sonde EDS, la composition chimique de chaque particule. Pour tous les produits testés, il a été montré que l’inoculation n’avait que peu d’effet sur la taille des inclusions et qu’elles deviennent plus complexes en contenant plusieurs éléments chimiques. Pour certains produits, on voit apparaître des amas d’inclusions. Ces amas sont susceptibles de favoriser la fragilité de l’acier en formant des amorces de fissuration. Pour d’autres, les inclusions diminuent nettement, ont une forme globulaire et la taille des grains est affinée. Ces effets ont tendance à améliorer les propriétés mécaniques de ces aciers. La teneur d’introduction de l’inoculant est aussi déterminée pour un maximum d’efficacité. L’inoculant le plus important pourra être utilisé pour une possible industrialisation. / The objective of this work is to improve the mechanical properties of a low-carbon steel (0.2%) without the use of expensive secondary metallurgy processes. For this, a method of inoculation, already used during the development of the cast iron, is transposed to our steel grade. It consists in adding very small quantities in liquid steel in order to modify the solidification process, thus the microstructure and consequently the properties of the steel, especially resilience. The addition of elements in very small quantities does not modify the grade of steel. They must be added at the end of melting of the metal in the form of powder in the casting stream. The effects of these elements will be seen on the microstructure, particularly in terms of inclusions and grain size. The "good candidate" is an element that will lead to a homogeneous distribution of inclusions of small size and spherical shape. It must also make it possible to reduce the size of the grains. These structural modifications are supposed to improve the mechanical properties of the steel and in particular the impact resistance. A reference piece is made to study the microstructure. The retained shape is a parallelepiped ingot sized using a casting simulation software to prevent major foundry defects. The size is adapted to the laboratory scale (furnace capacity 120 kg). The load corresponding to the grade studied are provided by the SAFE Metal, the partner company. The first step is to obtain a suitably deoxidized bath; that means adding aluminum in order to trap the dissolved oxygen and to evacuate it. To demonstrate any significant effects of the various inoculants tested, it is necessary to start from a reference sample containing a relatively high number of inclusions. This is achieved by adding sulfur to the liquid bath. This element acts directly on the number of inclusions present in the steel by forming sulphides. For the inoculation stage, it was necessary to create specific tools that could be adapted to the laboratory scale. Trials are then carried out with different products at varying concentrations. The samples obtained are analyzed by various techniques: chemical analysis of steel (spark spectrometry and ICP), microstructure and grain size analysis (optical micrograph) and inclusion counting. In order to detect inclusions, count and classify them by their nature, shape and size, two possibilities exist. Either using an optical microscope but the risks of error are significant and the process is long and tedious, either using a specific software (AZtec) coupled to a scanning electron microscope (SEM). It is this choice that was made, because besides the fact that it allows a considerable saving of time thanks to an automation of the process, it is possible to know via an EDS probe, the chemical composition of each particle. For all the products tested, it was shown that inoculation had little effect on the size of the inclusions and that they became more complex by containing several chemical elements. For some products, clusters of inclusions appear. These clusters are likely to promote the fragility of steel by forming cracking primers. For others, the inclusions decrease sharply, have a globular shape and the grain size is refined. These effects tend to improve the mechanical properties of these steels. The introductory content of the inoculant is also determined for maximum effectiveness. The most efficient inoculant can be used for a possible industrialization.

A seven-element Modified Embedded Atom Method (MEAM) potential comprising Fe, Mn, Si, C, Al, Zn, and O is developed by employing a hierarchical multiscale modeling paradigm to simulate low-alloy steels, inhibition layer, and galvanized coatings. Experimental information alongside first-principles calculations based on Density Functional Theory served as calibration data to upscale and develop the MEAM potential. For calibrating the single element potentials, the cohesive energy, lattice parameters, elastic constants, and vacancy and interstitial formation energies are used as target data. The heat of formation and elastic constants of binary compounds along with substitutional and interstitial formation energies serve as binary potential calibration data, while substitutional and interstitial pair binding energies aid in developing the ternary potential. Molecular dynamics simulations employing the developed potentials predict the thermal expansion coefficient, heat capacity, self-diffusion coefficients, thermomechanical stress-strain behavior, and solid-solution strengthening mechanisms for steel alloys comparable to those reported in the literature. Interfacial energies between the steel substrate, inhibition layer, and surface oxides shed light on the interfacial nanostructures observed in the galvanizing process.

O desempenho de serras de fita bimetálica depende principalmente das características da junção de suas extremidades, executada pelo processo de soldagem topo-a-topo por centelhamento. Este é um processo de soldagem por resistência, constituído por um aquecimento elétrico, seguido por um rápido forjamento dos materiais. O presente trabalho tem como objetivo, estudar a influência da variação da corrente elétrica, força de recalque e tratamento térmico de revenimento nas microestruturas e propriedades mecânicas das juntas soldadas topo-a-topo por centelhamento. Neste estudo, foram utilizadas amostras de serras de fita bimetálica de 27 mm de largura e 0,9 mm de espessura. O material é um composto bimetálico constituído por aço rápido M42 para os dentes, soldado a um corpo de aço carbono de baixa liga e alta resistência D-6a. As microestruturas e as propriedades mecânicas das amostras soldadas foram investigadas. Foi observado que os parâmetros de soldagem e o revenimento afetam as propriedades mecânicas. A dureza na seção transversal à linha central da solda apresentou valores levemente superiores à respectiva dureza na seção longitudinal. Amostras não revenidas resultaram em maior resistência à tração, associada ao parâmetro de menor temperatura de revenimento. Ficou evidenciado que o revenimento reduziu a dureza e a resistência à tração de todas as amostras. / The performance of bimetal band saw blades depends mainly on the features of joining their ends, carried out by flash butt welding process. This is a resistance welding method, which consists of electrical heating followed by a fast material forging. The present work has the objective of studying the influence of changing the electrical current, upset force and tempering heat treatment on the microstructures and mechanical properties of the flash butt welded joints. In this study, samples of bimetal band saw blades having width of 27 mm and thickness of 0.9 mm were employed. The material consists of a composite M42 high speed steel for the teeth welded together with a D-6a ultra-high strength carbon low alloy steel for the backer. The microstructure and mechanical properties have been investigated. It is pointed out that welding parameters and tempering affected the mechanical properties. The transversal hardness to the welding center line has shown values slightly higher than the respective longitudinal hardness. Non-tempered samples gave the highest tensile strength with the parameter of lowest tempering temperature. It is also found that the tempering reduces the hardness and the tensile strength of all samples.

O desempenho de serras de fita bimetálica depende principalmente das características da junção de suas extremidades, executada pelo processo de soldagem topo-a-topo por centelhamento. Este é um processo de soldagem por resistência, constituído por um aquecimento elétrico, seguido por um rápido forjamento dos materiais. O presente trabalho tem como objetivo, estudar a influência da variação da corrente elétrica, força de recalque e tratamento térmico de revenimento nas microestruturas e propriedades mecânicas das juntas soldadas topo-a-topo por centelhamento. Neste estudo, foram utilizadas amostras de serras de fita bimetálica de 27 mm de largura e 0,9 mm de espessura. O material é um composto bimetálico constituído por aço rápido M42 para os dentes, soldado a um corpo de aço carbono de baixa liga e alta resistência D-6a. As microestruturas e as propriedades mecânicas das amostras soldadas foram investigadas. Foi observado que os parâmetros de soldagem e o revenimento afetam as propriedades mecânicas. A dureza na seção transversal à linha central da solda apresentou valores levemente superiores à respectiva dureza na seção longitudinal. Amostras não revenidas resultaram em maior resistência à tração, associada ao parâmetro de menor temperatura de revenimento. Ficou evidenciado que o revenimento reduziu a dureza e a resistência à tração de todas as amostras. / The performance of bimetal band saw blades depends mainly on the features of joining their ends, carried out by flash butt welding process. This is a resistance welding method, which consists of electrical heating followed by a fast material forging. The present work has the objective of studying the influence of changing the electrical current, upset force and tempering heat treatment on the microstructures and mechanical properties of the flash butt welded joints. In this study, samples of bimetal band saw blades having width of 27 mm and thickness of 0.9 mm were employed. The material consists of a composite M42 high speed steel for the teeth welded together with a D-6a ultra-high strength carbon low alloy steel for the backer. The microstructure and mechanical properties have been investigated. It is pointed out that welding parameters and tempering affected the mechanical properties. The transversal hardness to the welding center line has shown values slightly higher than the respective longitudinal hardness. Non-tempered samples gave the highest tensile strength with the parameter of lowest tempering temperature. It is also found that the tempering reduces the hardness and the tensile strength of all samples.

No description available.

As cargas transportadas pelas ferrovias têm aumentado consideravelmente nos últimos anos, tanto pela capacidade individual do vagão como pela quantidade numa mesma composição. Após simulações e análise de dados obtidos em vagões instrumentados, uma das maiores operadoras logísticas do país observou que seus Sistemas de Choque e Tração estão sendo submetidos a esforços longitudinais considerados extremos. Desta forma, os trens formados por vagões do tipo GDU (capacidade de 118 t) estão limitados a uma quantidade de apenas 122 vagões, perdendo produtividade em relação aos trens formados por vagões do tipo GDT (capacidade de 110 t), os quais vêm circulando com 134 vagões. Em função das cargas extremas e consequente redução da produtividade nos trens, o Cliente solicita um trabalho conjunto para o desenvolvimento de um Sistema de Choque e Tração que torne possível a formação dos trens de minério com 134 vagões do tipo GDU. Desta forma, foi desenvolvido, como alternativa ao aço convencional, um aço baixa liga com desempenho superior, protótipos dos componentes citados para ensaios de campo e respectivos corpos de prova, validados por ensaios laboratoriais, bem como os parâmetros tecnológicos do produto e do processo. Os resultados obtidos são um aumento do Limite de Resistência, Limite de Escoamento, Tenacidade e Vida em Fadiga da ordem de 18% (em função da Temperatura de Revenido adotada), com a manutenção das características de Dutilidade (Alongamento e Redução de Área). / Loads transported by rail have increased considerably in recent years, both for individual freight car capacity and total train tonnage. Following data analysis and simulations obtained in instrumented freight cars, one of the largest logistics operators in the country noted that its Draft Gear Systems are being subjected to longitudinal efforts considered extreme. Thus, trains consisting of cars type GDU (118 ton capacity) have been limited to a quantity of only 122 cars, losing productivity gains regarding trains formed by cars type GDT (110 ton capacity), which circulate with 134 cars. Because of the extreme loads and consequential reduction in productivity of the trains, the Client requests a joint work for the development of a Draft Gear System that enables the formation of ore trains with 134 cars of type GDU. Thus, an alternative to the conventional steel, a low-alloy steel with superior performance was developed from which prototypes of the applicable components for field trials and respective test coupons were produced. This has been validated by laboratory tests, as well as the technological parameters of product and process. The results have indicated an increase in Tensile Strength, Yield Strength, Toughness and Fatigue Life of approximately 18% (depending on Tempering temperature adopted) while maintaining the characteristics of Ductility (Elongation and Area Reduction).

No presente trabalho estudou-se a recristalização do aço de alta resistência baixa liga (ARBL) microligado ao nióbio (160 ppm em peso) da Companhia Siderúrgica Nacional 2336 utilizado na fabricação de reforços estruturais para a indústria automobilística. A grande utilização deste tipo de aço pelas indústrias tem motivado o desenvolvimento de novas ligas capazes de atender a requisitos críticos de estampagem combinado com alta resistência mecânica. Amostras deste aço no estado laminado a quente foram deformadas posteriormente a frio em escala piloto com reduções de 50 e 70%, reduções típicas na indústria, e recozidas em banho de sal em temperaturas variando entre 600 e 700ºC em diferentes tempos. A evolução microestrutural das amostras no estado encruado e na condição recozida foi acompanhada utilizando-se as técnicas de microscopia ótica e eletrônica de varredura. As texturas de deformação e de recristalização foram determinadas via difração de raios X. A fração volumétrica recristalizada (Xv) e o tamanho de grão foram determinados com o auxílio de metalografia quantitativa nas amostras recozidas. Medidas de microdureza Vickers foram utilizadas para construir as curvas de encruamento e da cinética de amolecimento em função da temperatura de recozimento e comparadas com curvas construídas pelo primeiro método. Os resultados mostram que a recristalização ocorre em temperaturas acima de 650°C. Com auxílio de um gráfico de Xv contra A (fração amolecida), podemos observar a ocorrência de um desvio da idealidade a 650°C e 700°C. A explicação mais provável é a ocorrência de precipitação, especialmente para o aço menos deformado. Os valores do expoente de Avrami obtidos a partir das duas abordagens (medidas de microdureza e metalografia quantitativa) são apresentados e discutidos. / The recrystallization of a high strength low alloy steel (HSLA) containing niobium (160 wt-ppm) was investigated. The HSLA steel, named CSN 2336, is used in the manufacture of structural parts for automotive industry. The increasing use of this application type of steels in automotive industries has motivated the development of new alloys able to gather the critical requirements for deep drawing combined with high strengths. Samples of this steel in hot-rolled condition were cold-rolled with reductions of 50% and 70%, commonly used in most of applications, and isothermally annealed in salt bath at temperatures between 600 and 700ºC. The microstructure of the samples in both coldworked and annealed states was imaged using light optical (LOM) and scanning electronic microscopies (SEM). Deformation and recrystallization textures were determined by X-ray diffraction. The recrystallized volume fraction (Xv) and the mean grain size were determined by quantitative metallography in the annealed samples. Vickers microhardness testing was used to determine the work hardening and the softening kinetics behaviors. The results show that recrystallization occurs at temperatures above 650°C for the most deformed steel. By plotting Xv against A (softening fraction), we observe the occurrence of a deviation from ideality at 650°C and 700°C, likely caused by solid-state precipitation, especially for the less deformed steel. The values of n (Avrami exponent) obtained from both approaches (softening curves and quantitative metallography) are presented and discussed.

No presente trabalho estudou-se a recristalização do aço de alta resistência baixa liga (ARBL) microligado ao nióbio (160 ppm em peso) da Companhia Siderúrgica Nacional 2336 utilizado na fabricação de reforços estruturais para a indústria automobilística. A grande utilização deste tipo de aço pelas indústrias tem motivado o desenvolvimento de novas ligas capazes de atender a requisitos críticos de estampagem combinado com alta resistência mecânica. Amostras deste aço no estado laminado a quente foram deformadas posteriormente a frio em escala piloto com reduções de 50 e 70%, reduções típicas na indústria, e recozidas em banho de sal em temperaturas variando entre 600 e 700ºC em diferentes tempos. A evolução microestrutural das amostras no estado encruado e na condição recozida foi acompanhada utilizando-se as técnicas de microscopia ótica e eletrônica de varredura. As texturas de deformação e de recristalização foram determinadas via difração de raios X. A fração volumétrica recristalizada (Xv) e o tamanho de grão foram determinados com o auxílio de metalografia quantitativa nas amostras recozidas. Medidas de microdureza Vickers foram utilizadas para construir as curvas de encruamento e da cinética de amolecimento em função da temperatura de recozimento e comparadas com curvas construídas pelo primeiro método. Os resultados mostram que a recristalização ocorre em temperaturas acima de 650°C. Com auxílio de um gráfico de Xv contra A (fração amolecida), podemos observar a ocorrência de um desvio da idealidade a 650°C e 700°C. A explicação mais provável é a ocorrência de precipitação, especialmente para o aço menos deformado. Os valores do expoente de Avrami obtidos a partir das duas abordagens (medidas de microdureza e metalografia quantitativa) são apresentados e discutidos. / The recrystallization of a high strength low alloy steel (HSLA) containing niobium (160 wt-ppm) was investigated. The HSLA steel, named CSN 2336, is used in the manufacture of structural parts for automotive industry. The increasing use of this application type of steels in automotive industries has motivated the development of new alloys able to gather the critical requirements for deep drawing combined with high strengths. Samples of this steel in hot-rolled condition were cold-rolled with reductions of 50% and 70%, commonly used in most of applications, and isothermally annealed in salt bath at temperatures between 600 and 700ºC. The microstructure of the samples in both coldworked and annealed states was imaged using light optical (LOM) and scanning electronic microscopies (SEM). Deformation and recrystallization textures were determined by X-ray diffraction. The recrystallized volume fraction (Xv) and the mean grain size were determined by quantitative metallography in the annealed samples. Vickers microhardness testing was used to determine the work hardening and the softening kinetics behaviors. The results show that recrystallization occurs at temperatures above 650°C for the most deformed steel. By plotting Xv against A (softening fraction), we observe the occurrence of a deviation from ideality at 650°C and 700°C, likely caused by solid-state precipitation, especially for the less deformed steel. The values of n (Avrami exponent) obtained from both approaches (softening curves and quantitative metallography) are presented and discussed.