No description available.

No / M3/2 sintered high speed steel and composite materials processed by initial admixing of 5 wt.% TiC (to decrease wear) and 5 wt.% MnS (to minimise friction) powders, singly and in combination, were assessed in pin-on-disc tribometers specially constructed to simulate use in the automotive valve train. Pins were of the sintered materials and the mating tribological material discs of T1 high speed steel. For comparison with existing conventional materials, identical tests were performed with discs and pins of two types of spheroidal cast iron. Testing at 110 °C, employing a few drops of fresh Shell Helix Standard SAE: 15W-40, API: SJ/CF oil, in daily segments of 5000 m of sliding distance was carried out until the break of the boundary film and appearance of the early signs of seizure. Accordingly the conditions were initially elastohydrodynamic, then mixed lubrication, then boundary, and finally decaying boundary. In comparison with the baseline cast iron system, the friction, wear and lifetime performance of all the high speed steel systems was markedly superior. MnS further lowered the coefficient of friction and TiC increased the load carrying capacity of M3/2 steel. The lifetime, test distance until seizure, was the most discriminating parameter between the high speed steel systems, being 1.5¿3 times longer for the unmodified M3/2 than the composites and 10 times longer than that of the cast irons system.

Foram produzidas camadas de carbonetos por meio do tratamento termorreativo de difusão (TRD) em quatro ferros fundidos nodulares: um sem elemento de liga e outros três ligados com Cu, Cu-Ni e Cu-Ni-Mo, respectivamente; e um ferro fundido vermicular. O banho era composto por borato de sódio, alumínio, ferro-nióbio e/ou ferro-vanádio, a 1000 °C com tempo de permanência de tratamento de 2 horas. Realizou-se austêmpera, a partir dessa temperatura, em banho de sal fundido a 300 °C. As amostras foram caracterizadas por meio de microscopias óptica e MEV, EDS, DRX, ensaios de durezas Brinell e Vickers, Daimler-Benz Rockwell-C, ensaios de desgaste microadesivo, do tipo esfera fixa, e de corrosão por meio de ensaios de polarização potenciodinâmica. Os tratamentos TRD foram altamente eficientes para a produção de camadas de carbonetos uniformes e com boas aderências aos substratos, em todos os ferros fundidos em estudo. O ferro fundido nodular não ligado apresentou os menores valores de espessuras para as camadas produzidas de carbonetos simples de nióbio e de vanádio, tendo ocorrido um aumento dessas espessuras com a adição dos elementos de liga Cu e Cu-Ni, com exceção do Mo. As camadas apresentaram durezas acima de 2300, 2500 e 3050 HV para os casos dos carbonetos de Nb, V e Nb-V, respectivamente. As resistências ao desgaste das amostras foram aumentadas de 5 a 35x em relação aos substratos austemperados. As camadas apresentaram resistências a corrosão muito superiores as dos substratos, indicando a grande eficácia do tratamento TRD para todos os materiais estudados. Os compostos produzidos, constituídos por um substrato otimizado e de camadas de elevados desempenhos tribológicos, tornam-no altamente indicado para as condições severas de uso. / Carbide layers were produced by thermo-diffusion treatment (TRD) in four nodular cast irons: one without alloying element and the other three bonded with Cu, Cu-Ni and Cu-Ni-Mo, respectively; and a vermicular cast iron. The bath was composed of sodium borate, aluminum, iron-niobium and/or iron-vanadium, at 1000 °C with treatment time of 2 hours. The austempering treatment was carried out at 300 °C for 4 hours in a molten salt bath. The samples were characterized by optical microscopy and SEM, EDX, XRD, Brinell and Vickers hardness tests, Daimler-Benz Rockwell-C, microadhesive wear test (fixed ball type) and corrosion tests using potentiodynamic polarization. The TRD treatments were highly efficient for the production of uniform carbide layers with good adhesion to the substrates in all cast irons under study. Non-alloyed nodular cast iron presented the lowest thickness values for the layers produced from simple niobium and vanadium carbides, with an increase in these thicknesses with the addition of Cu and Cu-Ni alloys, with the exception of Mo. The layers achieved hardness above 2,300, 2,500 and 3,050 HV for the carbides of Nb, V and Nb-V, respectively. The wear resistance of the samples was increased from 5 to 35x in relation to the austempered substrates. The layers presented corrosion resistance far superior if compared to those of the substrates, indicating the great effectiveness of the TRD treatment for all the studied materials. The compounds, consisting of an optimized substrate and layers of high tribological performances, makes it highly suitable for severe conditions of use.

Amostras de ferros fundidos nodulares ligados com Cu, Cu-Ni ou Cu-Ni-Mo foram austemperadas, boretadas e boroaustemperadas e caracterizadas quanto à dureza e o comportamento ao desgaste microadesivo, tendo sido também estudada a cinética de formação da camada. O método de boretação utilizado foi via líquida em banho de bórax fundido, com tempos de permanência de 2 e 4 horas nas temperaturas de 850, 900 e 950ºC. Procedeu-se o tratamento direto de austêmpera, a partir dessa temperatura, em banhos de sal fundidos nas temperaturas de 240, 300 e 360ºC com tempos de permanência de 4 horas (boroaustêmpera). Realizou-se também, para fins de comparação, tratamento convencional de austêmpera. Microscopias óptica e eletrônica de varredura, EDS por raios-X, testes de dureza Brinell (substrato) e Vickers (revestimento) foram realizados, bem como ensaios de desgaste microadesivo do tipo esfera presa. A boretação resultou na formação de camadas de elevadas durezas, na faixa de 1300 a 1700 HV, e elevadas resistências ao desgaste. As resistências ao desgaste das amostras boretadas ou boroaustemperadas foram aumentadas em até 40x em relação às amostras brutas de fundição ou apenas austemperadas, indicando a grande eficácia do tratamento neste tipo de propriedade. / Samples of ductile cast iron alloyed with Cu, Cu-Ni or Cu-Ni-Mo were austempered, borided and boroaustempered and afterwards characterized for hardness and micro-adhesive wear behavior. The kinetics of layer formation were also studied. The boriding method used was liquid molten borax bath, in periods of 2 and 4 hours at temperatures of 850, 900 and 950°C. The direct austempering treatment was performed from the borax bath temperature using molten salt baths at temperatures of 240, 300 and 360°C for 4 hours (boroaustempered). For comparative purposes, the conventional austempering treatment was also conducted. Optical microscopy, scanning electron microscopy, EDX, Brinell hardness measurements (substrate) and Vickers (coating) were performed, as were the tests for micro-adhesive wear. The boriding treatment resulted in the formation of layers with high hardness, in the range of 1300 to 1700 HV and high wear resistance. The wear resistance of borided or boroaustempered samples were increased by 40 times when compared to cast irons or austempered samples, indicating the high efficiency of this type of treatment in increasing the wear resistance of this material.

Amostras de ferros fundidos nodulares ligados com Cu, Cu-Ni ou Cu-Ni-Mo foram austemperadas, boretadas e boroaustemperadas e caracterizadas quanto à dureza e o comportamento ao desgaste microadesivo, tendo sido também estudada a cinética de formação da camada. O método de boretação utilizado foi via líquida em banho de bórax fundido, com tempos de permanência de 2 e 4 horas nas temperaturas de 850, 900 e 950ºC. Procedeu-se o tratamento direto de austêmpera, a partir dessa temperatura, em banhos de sal fundidos nas temperaturas de 240, 300 e 360ºC com tempos de permanência de 4 horas (boroaustêmpera). Realizou-se também, para fins de comparação, tratamento convencional de austêmpera. Microscopias óptica e eletrônica de varredura, EDS por raios-X, testes de dureza Brinell (substrato) e Vickers (revestimento) foram realizados, bem como ensaios de desgaste microadesivo do tipo esfera presa. A boretação resultou na formação de camadas de elevadas durezas, na faixa de 1300 a 1700 HV, e elevadas resistências ao desgaste. As resistências ao desgaste das amostras boretadas ou boroaustemperadas foram aumentadas em até 40x em relação às amostras brutas de fundição ou apenas austemperadas, indicando a grande eficácia do tratamento neste tipo de propriedade. / Samples of ductile cast iron alloyed with Cu, Cu-Ni or Cu-Ni-Mo were austempered, borided and boroaustempered and afterwards characterized for hardness and micro-adhesive wear behavior. The kinetics of layer formation were also studied. The boriding method used was liquid molten borax bath, in periods of 2 and 4 hours at temperatures of 850, 900 and 950°C. The direct austempering treatment was performed from the borax bath temperature using molten salt baths at temperatures of 240, 300 and 360°C for 4 hours (boroaustempered). For comparative purposes, the conventional austempering treatment was also conducted. Optical microscopy, scanning electron microscopy, EDX, Brinell hardness measurements (substrate) and Vickers (coating) were performed, as were the tests for micro-adhesive wear. The boriding treatment resulted in the formation of layers with high hardness, in the range of 1300 to 1700 HV and high wear resistance. The wear resistance of borided or boroaustempered samples were increased by 40 times when compared to cast irons or austempered samples, indicating the high efficiency of this type of treatment in increasing the wear resistance of this material.

Les fontes à graphite sphéroïdal sont aujourd’hui très largement utilisées en raison de leurs bonnes propriétés mécaniques. La forme sphéroïdale du graphite est obtenue le plus souvent par l’ajout de magnésium ou de cérium lors de l’élaboration des fontes. Le graphite sphéroïdal peut être obtenu par graphitisation à l'état solide des fontes totalement ou partiellement solidifiées dans le système métastable. L’objectif de ce travail est d’étudier l’effet du traitement de graphitisation à l’état solide sur la croissance du graphite nodulaire d’une fonte à paroi mince qui présente une structure truitée à l'état brut de coulée. Cette fonte a été étudiée par microscopie optique, microscopies électronique à balayage et en transmission, spectroscopie Raman et spectroscopie de perte d'énergie des électrons. Des traitements thermiques assurant une graphitisation totale et partielle pour décomposer la cémentite formée à la solidification en graphite et en austénite ont été réalisés. Les nodules deviennent plus nombreux et leur taille augmente en fonction du temps de graphitisation. La microstructure après traitement thermique est composée de nodules de graphite et de ferrite. La spectroscopie Raman a été utilisée pour caractériser les nodules de graphite d’échantillons ayant été entièrement graphitisés à différentes températures dans le domaine austénitique. L’analyse par spectroscopie Raman ne montre aucune différence significative entre les spectres Raman enregistrés sur le graphite formé lors de la solidification et à l’état solide. Les caractérisations microstructurales par microscopie électronique en transmission montrent que le graphite à l’état brut de coulée présente une structure caractérisée par une zone interne où le graphite est désorienté. Une déformation mécanique due à la contraction lors de la solidification métastable induit la formation de cette zone. Cette zone disparaît par recristallisation après traitement de graphitisation totale pour former à la fin des secteurs coniques rayonnant à partir du germe et se développant vers la périphérie. Les résultats de ces travaux ont permis une meilleure compréhension de la structure de graphite nodulaire à l’état solide et montre aussi que le mécanisme de croissance du graphite nodulaire est le même lors de la solidification et de la transformation à l'état solide. / Spheroidal graphite iron castings are today widely used because of their good mechanical properties. The spheroidal shape of graphite is most often obtained by the addition of magnesium or cerium during the casting process. Spheroidal graphite can be formed at the solid-state by graphitization of cast irons which solidified partly or totally in the metastable system. The purpose of this work is to study the effect of solid-state graphitization treatment on the growth of nodular graphite of a thin wall casting which has a mottled structure at the as-cast state. This cast iron was studied using optical microscopy, scanning and transmission electron microscopy, Raman spectroscopy and electron energy loss spectroscopy. Heat treatments ensuring a total and partial graphitization to decompose the cementite formed at the solidification in graphite and austenite were realized. The nodules become more numerous and their size increases according to the time of graphitization. The microstructure after heat treatment is composed of graphite nodules and ferrite. Raman spectroscopy has been used to characterize graphite nodules in as-cast state and in samples having been fully graphitized at various temperatures in the austenite field. The results show no significant difference between Raman spectra recorded on these various samples, suggesting graphite grows with the same mechanism during either solidification or hightemperature (so-called first stage) graphitization. Transmission electron microscopy characterizations show that nodules in the as-cast material presents a multi-fold structure characterized by an inner zone where graphite is misoriented and an outer zone where it is well crystallized. In heat-treated samples, graphite nodules consist of well crystallized sectors radiating from the nucleus. These observations suggest that the misoriented zone appears because of mechanical deformation when the liquid contracts during its solidification. During heat-treatment, this zone disappears by recrystallization. The results of the present work lead to a better understanding of the nodular graphite structure in the solid state and also show that nodular graphite growth mechanism is the same during solidification and solid-state transformation.

Os Ferros Fundidos de Alto Cromo (FFAC), por apresentarem excelentes propriedades tribológicas, têm sido amplamente utilizados em aplicações específicas envolvendo elevadas perdas de material por abrasão, especialmente no setor da mineração. Entretanto, a demanda por materiais com maior resistência ao desgaste aumenta continuamente, sendo necessárias novas pesquisas nesta área. Portanto, o presente trabalho objetiva avaliar a utilização do nióbio para aumentar, ainda mais, a resistência à abrasão dos FFAC\'s. Por outro lado, quando o FFAC é utilizado na fabricação de peças com geometrias irregulares (por exemplo, rotores de bombas), o componente pode apresentar diferentes níveis de refinamento da microestrutura, entre as regiões finas e espessas, devido às variações na taxa de resfriamento. No presente trabalho foi avaliado, o efeito do grau de refinamento da microestrutura, e a interação do refinamento com a adição de nióbio, na resistência ao desgaste abrasivo dos FFAC\'s. Para tanto, foram desenvolvidos quatro estudos principais: no primeiro estudo foram fabricados blocos de FFAC variando o grau de refinamento da microestrutura e foi mostrado que: grandes incrementos no grau de refinamento resultam em maiores perdas de massa por abrasão. Nas microestruturas menos refinadas, os carbonetos de cromo M7C3, de maior tamanho, são menos susceptíveis ao micro trincamento e podem, ocasionalmente, atuar como barreiras ante os eventos abrasivos. Em uma segunda série de experimentos, foi avaliada a interação do efeito do grau de refinamento da microestrutura com a adição de nióbio em teores baixos (1 %); mostrando que, para microestruturas com alto grau de refinamento, adições de nióbio reduzem as perdas de massa por abrasão em até 50 %. Em uma terceira série de experimentos foi avaliada a interação dos efeitos da adição de nióbio e de molibdênio. Quando comparado com a liga isenta de molibdênio, adições simultâneas de nióbio e molibdênio resultaram em microestruturas mais refinadas, com maior microdureza da matriz, e com carbonetos de nióbio (NbC) de maior dureza. Para condições de desgaste abrasivo por baixos esforços, onde o desgaste foi mais acentuado na matriz, adições simultâneas de nióbio e molibdênio resultaram em aumentos da resistência á abrasão dos FFAC estudados. Na última etapa do trabalho foi adicionado 3 % de nióbio em um liga de FFAC com composição química inicial hipereutética (25%Cr/3%C), a qual apresentaria carbonetos primários de cromo M7C3 de grande tamanho que induziriam comportamento frágil do material quando exposto ao desgaste. Porém, a adição de nióbio resultou em um FFAC com microestrutura mais refinada (eutética), contendo NbC\'s compactos e por conseguinte, mais resistente ao desgaste abrasivo. / High Chromium Cast Irons (HCCI\'s), because of their excellent tribological properties, have been widely used for specific applications involving high wear rates by abrasion, especially in the mining sector. However, the demand for materials with higher wear resistance is continuously growing and thus further research is needed in this area. For that reason, the current work purposes to assess the use of niobium to further increase the wear resistance of HCCI\'s. On the other hand, when HCCI is used for manufacturing components with irregular geometries (e.g. pump impellers), the components thin and thick regions can contain different levels of structure refinement due to variation in their cooling rates. In this work, the effect of structure refinement and the interaction between structure refinement and niobium addition on the abrasion resistance of HCCI\'s were evaluated. For that purpose, four systematic main studies were developed: in the first study, blocks of HCCI were manufactured varying the structure refinement and it was shown that large increases in the degree of structure refinement result in higher wear mass losses by abrasion. In less refined microstructures, the larger M7C3 chromium carbides are less susceptible to microcracking and can occasionally act as a barrier to abrasive particles. In the second series of experiments, the interaction between structure refinement and niobium addition in low concentrations (1 %) was evaluated; showing that for more refined microstructures, niobium additions reduce the mass losses by abrasion up to 50 %. In the third series of experiments, the interaction between niobium and molybdenum additions was evaluated. Compared to molybdenum-free alloy, simultaneous additions of niobium and molybdenum resulted in a more refined microstructure, higher hardness of the matrix and harder niobium carbides (NbC). For Low Stress Sliding Abrasion (LSSA) wear configuration, where wear was more pronounced in the matrix, simultaneous addition of niobium and molybdenum resulted in increase of abrasion resistance in the studied HCCI. In the last stage of this work, 3 % of niobium were added in an HCCI alloy with hypereutectic initial chemical composition (25%Cr/3%C), which presents primary large sized chromium carbides that induce a brittle behavior of the HCCI when subjected to wear. However, the niobium addition resulted in a more refined microstructure (eutectic) HCCI containing compact-shaped NbC carbides, and consequently in more resistance to abrasive wear.

Les fontes de fer sont des alliages eutectiques de fonderie dont la structure consiste en des précipités de graphite dans une matrice riche en fer. Les propriétés mécaniques de ces alliages peuvent être grandement modifiées en agissant sur la matrice et sur la forme des précipités de graphite. Industriellement, les formes de graphite les plus courantes sont lamellaire et sphéroïdal, et l'on passe de l'une à l'autre par un traitement de sphéroïdisation (ajout de 0,02-0,05% massique de cérium ou de magnésium). D'autres morphologies peuvent apparaître en fonction des conditions de refroidissement et de la composition chimique de la fonte liquide. De fait, de très nombreux éléments d'alliage ou à l'état de traces peuvent diminuer l'efficacité du traitement de sphéroïdisation et modifier la forme du graphite. L'objectif de cette étude a été double, d'une part caractériser la structure fine de différentes formes de graphite, d'autre part réaliser des expériences de laboratoire destinées à étudier l'effet de quelques éléments, O, Ce et Sb, sur la croissance du graphite primaire. Dans cette étude, différents types de graphite (sphéroïdal, lamellaire et morcelé) issus de fontes commerciales ont été étudiés par microscopie optique, microscopies électroniques à balayage et en transmission, et spectrométrie de masse des ions secondaires. Leur caractérisation a permis de mettre en évidence certaines similitudes à l'échelle nanométrique malgré les différences de morphologies à l'échelle micrométrique. Des expériences consistant à fondre du fer pur dans des creusets de graphite, avec ou sans ajout délibéré d’antimoine ou de cérium, ont été réalisées. Les échantillons ont été élaborés par chauffage soit sous air soit sous vide primaire, puis refroidis lentement et maintenus au dessus de la température eutectique afin de faire croître de gros précipités de graphite primaire, et enfin rapidement pour figer en une structure fine le liquide résiduel. Les différentes formes de graphite obtenues lors de ces expériences ont fait l'objet des mêmes analyses métallographiques que les fontes industrielles. L’antimoine a favorisé la formation de lamelles de graphite incurvées. Les courbures de ces lamelles ont lieu par des changements dans leur direction de croissance. Le cérium a produit du graphite sous forme de plaquettes courtes et épaisses dans les échantillons élaborés sous air et du graphite sphéroïdal ou explosé dans les échantillons élaborés sous vide. Ainsi, l’un des rôles de cet élément est de désoxyder la fonte, ce qui a été confirmé par l’observation de différents oxydes de cérium dans les échantillons. L’observation de morphologies de graphite particulières dans ces échantillons suggère que le rôle du cérium ne se limite pas à la désoxydation de la fonte et des mécanismes d’action de ces éléments ont été discutés. Les caractérisations microstructurales, et en particulier la microscopie électronique en transmission, montrent que le graphite peut s’adapter à différentes conditions de croissance et adopter diverses morphologies dans les fontes. Les résultats de ces travaux ont permis une meilleure compréhension de l’effet des éléments d’addition sur la croissance du graphite et un modèle de croissance permettant de décrire les observations a été proposé. / Cast irons are eutectic foundry alloys with a structure consisting of graphite precipitates within an iron-rich matrix. The mechanical properties of these alloys can be modified by changing the nature of the matrix and the shape of the graphite precipitates. The most usual graphite shapes are lamellar and spheroidal, this modification being achieved through a spheroidisation treatment (adding 0.02-0.05 wt% of cerium or magnesium). Other morphologies can occur depending on the chemical composition of the melt and its cooling conditions. Numerous alloying elements present as traces can reduce the effectiveness of the spheroidisation treatment and modify the shape of graphite. The objectives of this study were to characterize the structure of different graphite types at a fine scale and to carry out laboratory experiments to study the effect of certain elements (oxygen, cerium and antimony) on primary graphite growth. In the present work, various types of graphite found in commercial cast irons were studied using optical microscopy, secondary ion mass spectrometry, scanning and transmission electron microscopy. Characterization of spheroidal, lamellar and chunky graphite was performed in order to reveal their structural features. Even though these precipitates seem very different at lower magnification the results of this investigation emphasize on similarities found between them. Experiments consisting in melting pure iron in graphite crucibles, with or without antimony or cerium additions were carried out. The samples were heated in air or primary vacuum, then slowly cooled and held above the eutectic temperature so as to produce large primary graphite crystals, and finally quenched to produce a fine structure from the remaining liquid. The different graphite morphologies obtained in these experiments underwent the same metallographic analyses as the commercial irons. Antimony favoured the development of curved graphite flakes by promoting frequent changes in the flakes’ growth direction. Cerium produced thick graphite platelets in the samples prepared in air and exploded graphite in the samples prepared in vacuum. One of the main roles of this element is thus to deoxidize the melt, which was confirmed by the presence of several cerium oxides in the samples. Peculiar graphite morphologies were observed in these samples, suggesting that the role of cerium is not limited to deoxidizing the iron and the mechanisms by which this element affects graphite growth were discussed. The microstructural characterizations and mainly transmission electron microscopy show that graphite can adapt to different growth conditions and adopt diverse morpholgies in cast irons. The results of the present work lead to a better understanding of the effect of elements on graphite growth and a model was proposed to describe the experimental observations.

Os Ferros Fundidos de Alto Cromo (FFAC), por apresentarem excelentes propriedades tribológicas, têm sido amplamente utilizados em aplicações específicas envolvendo elevadas perdas de material por abrasão, especialmente no setor da mineração. Entretanto, a demanda por materiais com maior resistência ao desgaste aumenta continuamente, sendo necessárias novas pesquisas nesta área. Portanto, o presente trabalho objetiva avaliar a utilização do nióbio para aumentar, ainda mais, a resistência à abrasão dos FFAC\'s. Por outro lado, quando o FFAC é utilizado na fabricação de peças com geometrias irregulares (por exemplo, rotores de bombas), o componente pode apresentar diferentes níveis de refinamento da microestrutura, entre as regiões finas e espessas, devido às variações na taxa de resfriamento. No presente trabalho foi avaliado, o efeito do grau de refinamento da microestrutura, e a interação do refinamento com a adição de nióbio, na resistência ao desgaste abrasivo dos FFAC\'s. Para tanto, foram desenvolvidos quatro estudos principais: no primeiro estudo foram fabricados blocos de FFAC variando o grau de refinamento da microestrutura e foi mostrado que: grandes incrementos no grau de refinamento resultam em maiores perdas de massa por abrasão. Nas microestruturas menos refinadas, os carbonetos de cromo M7C3, de maior tamanho, são menos susceptíveis ao micro trincamento e podem, ocasionalmente, atuar como barreiras ante os eventos abrasivos. Em uma segunda série de experimentos, foi avaliada a interação do efeito do grau de refinamento da microestrutura com a adição de nióbio em teores baixos (1 %); mostrando que, para microestruturas com alto grau de refinamento, adições de nióbio reduzem as perdas de massa por abrasão em até 50 %. Em uma terceira série de experimentos foi avaliada a interação dos efeitos da adição de nióbio e de molibdênio. Quando comparado com a liga isenta de molibdênio, adições simultâneas de nióbio e molibdênio resultaram em microestruturas mais refinadas, com maior microdureza da matriz, e com carbonetos de nióbio (NbC) de maior dureza. Para condições de desgaste abrasivo por baixos esforços, onde o desgaste foi mais acentuado na matriz, adições simultâneas de nióbio e molibdênio resultaram em aumentos da resistência á abrasão dos FFAC estudados. Na última etapa do trabalho foi adicionado 3 % de nióbio em um liga de FFAC com composição química inicial hipereutética (25%Cr/3%C), a qual apresentaria carbonetos primários de cromo M7C3 de grande tamanho que induziriam comportamento frágil do material quando exposto ao desgaste. Porém, a adição de nióbio resultou em um FFAC com microestrutura mais refinada (eutética), contendo NbC\'s compactos e por conseguinte, mais resistente ao desgaste abrasivo. / High Chromium Cast Irons (HCCI\'s), because of their excellent tribological properties, have been widely used for specific applications involving high wear rates by abrasion, especially in the mining sector. However, the demand for materials with higher wear resistance is continuously growing and thus further research is needed in this area. For that reason, the current work purposes to assess the use of niobium to further increase the wear resistance of HCCI\'s. On the other hand, when HCCI is used for manufacturing components with irregular geometries (e.g. pump impellers), the components thin and thick regions can contain different levels of structure refinement due to variation in their cooling rates. In this work, the effect of structure refinement and the interaction between structure refinement and niobium addition on the abrasion resistance of HCCI\'s were evaluated. For that purpose, four systematic main studies were developed: in the first study, blocks of HCCI were manufactured varying the structure refinement and it was shown that large increases in the degree of structure refinement result in higher wear mass losses by abrasion. In less refined microstructures, the larger M7C3 chromium carbides are less susceptible to microcracking and can occasionally act as a barrier to abrasive particles. In the second series of experiments, the interaction between structure refinement and niobium addition in low concentrations (1 %) was evaluated; showing that for more refined microstructures, niobium additions reduce the mass losses by abrasion up to 50 %. In the third series of experiments, the interaction between niobium and molybdenum additions was evaluated. Compared to molybdenum-free alloy, simultaneous additions of niobium and molybdenum resulted in a more refined microstructure, higher hardness of the matrix and harder niobium carbides (NbC). For Low Stress Sliding Abrasion (LSSA) wear configuration, where wear was more pronounced in the matrix, simultaneous addition of niobium and molybdenum resulted in increase of abrasion resistance in the studied HCCI. In the last stage of this work, 3 % of niobium were added in an HCCI alloy with hypereutectic initial chemical composition (25%Cr/3%C), which presents primary large sized chromium carbides that induce a brittle behavior of the HCCI when subjected to wear. However, the niobium addition resulted in a more refined microstructure (eutectic) HCCI containing compact-shaped NbC carbides, and consequently in more resistance to abrasive wear.

Emission regulations for heavy-duty diesel engines are becoming increasingly restrictive to limit the environmental impacts of exhaust gases and particles. Increasing the specific power output of diesel engines would improve fuel efficiency and greatly reduce emissions, but these changes could lead to increased exhaust gas temperature, increasing demands on the exhaust manifold material. This is currently the ferritic ductile cast iron alloy SiMo51, containing about 4 wt% Si and ~1 wt% Mo, which operates close to its fatigue and oxidation resistance limits at peak temperature (750C). To ensure high durability at higher temperatures, three different approaches to improving the life of exhaust manifolds were developed in this thesis. The first approach was to modify SiMo51 by adding different combinations of Cr and Ni to improve its high-temperature strength and oxidation resistance, or by applying a thermal barrier coating (TBC) to reduce the material temperature and thereby improve fatigue life. In the second approach, new materials for engine components, e.g. austenitic ductile iron and cast stainless steel, were investigated for their high-temperature fatigue and oxidation properties. In order to identify the most suitable alloys for this application, in the third the environmental effects of the corrosive diesel exhaust gas on the fatigue life of SiMo51 were investigated. The high-temperature oxidation resistance of SiMo51 at 700 and 800C in air was found to be improved by adding Cr, whereas Ni showed adverse effects. The effects of solid-solution hardening from Ni and precipitation hardening from Cr were low at 700C, with improvements only at lower temperatures. Applying a TBC system, providing thermal protection from a ceramic topcoat and oxidation protection from a metallic bond coat, resulted in only small reductions in material temperature, but according to finite element calculations still effectively improved the fatigue life of a turbo manifold. Possible alternative materials to SiMo51 identified were austenitic cast ductile iron Ni-resistant D5S and austenitic cast stainless steel HK30, which provided high durability of exhaust manifolds up to 800 and 900C, respectively. Corrosion fatigue testing of SiMo51 at 700C in diesel exhaust gas demonstrated that the corrosive gas reduced fatigue life by 30-50% compared with air and by 60-75% compared with an inert environment. The reduced fatigue life was associated with a mechanism whereby the crack tip oxidized, followed by crack growth. Thus another potential benefit of TBC systems is that the bond coat may reduce oxidation interactions and further improve fatigue life. These results can be used for selecting materials for exhaust applications. They also reveal many new research questions for future studies. Combining the different approaches of alloy modification, new material testing and improving the performance using coatings widened the scope of how component life in exhaust manifolds can be improved. Moreover, the findings on environmental interactions on SiMo51 fatigue provide a completely new understanding of these processes in ductile irons, important knowledge when designing components exposed to corrosive environments. The novel facility developed for high-temperature corrosion fatigue testing can be useful to other researchers working in this field. / <p>QC 20150507</p>