Graduation theses consist of two parts. The first one, theoretical is focused on piece of knowledge regarding nodular iron history, its mechanical properties, chemistry, production and heat treatment. Theoretical part also includes information relating to graphite and white irons that relate closely with nodular iron. Important chapter is description of isothermal refining which is the subject of these theses. The second, experimental part is focused on isothermal refining of nodular iron casting. Description of the casting introduces this section, material specification, production and application. Following chapters describe preliminary conditions for isothermal refining that is applied on every single specimen made of the casting. These specimens have been tested mechanically. Optimal procedure of isothermal refining is applied on whole the casting and characterized final mechanicals. Final mechanical properties after casting isothermal refining has been analyzed and confronted with literature.

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Made available in DSpace on 2017-07-21T20:43:47Z (GMT). No. of bitstreams: 1 Ludiere Lucas Toldo Boneti.pdf: 5901766 bytes, checksum: 4f1a93e8f2d24f82e2576b7891703690 (MD5) Previous issue date: 2014-08-29 / The present study aimed to evaluate the influence of the austempering heat treatment parameters on microstructure and mechanical properties of a ductile cast iron produced in industrial conditions, containing 3.59% carbon, 2.68% silicon, 0.46% copper (in wt%) and a carbon equivalent of 4.50%. The material was produced in a foundry located in the southwest region of Parana state, Brazil. The specimens were obtained by casting the alloy into Y-block molds. The austempering heat treatments consisted of pre-heating at 500°C, followed by austenitizing step at 870°C, 900°C and 930°C during 60 minutes. Austempering was carried out in molten metal baths at temperatures of 300°C and 370°C for 30 minutes. Microstructural characterization was carried out by light optical microscopy (LOM) with image analysis, scanning electron microscopy (SEM-FEG) and X-ray diffraction with Rietveld refinement. The mechanical properties were evaluated by tensile and Vickers hardness tests. The as-cast microstructure displayed a very heterogeneous microstructure, characterized by the presence of regions with graphite flotation and carbide containing intercellular regions. The graphite nodules showed low nodularity, of 85%, which was attributed to the fading effect of magnesium alloy. Austempered samples were characterized by the presence of bainitic ferrite, interspersed with retained austenite in films and blocks. The austempering at 300°C resulted in a finer microstructure containing smaller volume fractions of retained austenite. All heat treated samples displayed transformation gradients between graphite flotation and intercellular regions. These gradients affected the mechanical properties, as well as the fracture characteristics. The best results of mechanical properties were obtained in the specimen austenitized at 900°C followed by austempering at 300°C, allowing the ADI produced to fit into a high strength class, according to ASTM A897. The study of fracture surfaces showed a sharp transition between graphite flotation and intercellular regions. The fracture at the vicinity of graphite nodules have occurred by a ductile mechanism, characterized by dimples. It was observed a rapid transition to the cleavage mode at intercellular regions, containing solidification carbides. / O presente trabalho teve como objetivo avaliar a influência dos parâmetros de tratamento térmico de austêmpera na microestrutura e nas propriedades mecânicas de tração e dureza, em uma liga de ferro fundido nodular produzida em condições industriais. A liga em estudo possui de teor de carbono de 3,59%, silício de 2,68% e adições de 0,46% de cobre, possuindo ainda um carbono equivalente de 4,50%. O material foi produzido em condições industriais, em uma fundição localizada no sudoeste do estado do Paraná. Os corpos de prova para retirada de amostras foram obtidos pelo vazamento da liga em moldes de blocos Y. Os tratamentos térmicos de austêmpera consistiram de pré-aquecimento a 500°C, seguido da etapa de austenitização a 870°C, 900°C e 930°C, por um tempo fixo de 60 minutos. A etapa de austêmpera foi realizada em banhos de metais fundidos, em temperaturas de 300°C e 370°C, durante 30 minutos. A caracterização microestrutural de amostras foi realizada pelas técnicas de microscopia ótica, com análise de imagens, microscopia eletrônica de varredura e difração de raios X, com refinamento pelo método de Rietveld. As propriedades mecânicas foram avaliadas por meio de ensaios de tração e dureza Vickers. A liga no estado bruto de fundição apresentou microestrutura bastante heterogênea, caracterizada pela presença de regiões com flotação de grafita e regiões intercelulares contendo carbetos de solidificação. Os nódulos de grafita apresentaram baixo grau de nodularização, de 85%, resultado que foi atribuído ao efeito de fadiga térmica da liga nodularizante. A microestrutura de amostras austemperadas foi caracterizada pela presença de agrupamentos de feixes de ripas de ferrita bainítica, entremeadas por austenita retida, na forma de filmes e blocos. A austêmpera a 300°C produziu microestruturas mais refinadas e com menores frações volumétricas de austenita retida. Em todas as amostras tratadas termicamente, foi observado um gradiente de transformação entre as regiões de flotação de grafita e regiões intercelulares. Estes gradientes afetaram tanto os resultados de propriedades mecânicas, como as características de fratura dos corpos de prova. Os melhores resultados de propriedades mecânicas foram obtidos para a condição de austenitização a 900°C seguida de austêmpera, permitindo enquadrar o ADI em uma classe de alta resistência, segundo a norma ASTM A897. As análises das superfícies de fratura dos corpos de prova de tração mostraram uma mudança nas características de fratura entre as regiões de flotação de grafita e as regiões intercelulares. Nas proximidades dos nódulos de grafita houve predominância do mecanismo de fratura dúctil, caracterizada pela presença de cavidades alveolares (“dimples”), com rápida transição para o modo de fratura por clivagem nas regiões com baixos números de nódulos de grafita, contendo carbetos de solidificação.

The processes leading to the retention of small quantities of austenite following the bainite and martensite phase transformations have been examined, together with the influence of retained austenite on the properties of low alloy steels. It was found that the upper and lower bainite transformations are separate reactions, although both involve a displacive transformation mode, Growth seems to occur by the repeated nucleation of martensitic sub-units, and this leads to an apparently slow growth rate, The partitioning of carbon from bainitic ferrite into residual austenite was thermodynamically proven to occur subsequent to transformation, and was shown to be directly responsible for the 'incomplete reaction phenomenon'. The nature of sympathetic nucleation and of the limited size of bainitic sub-units was rationalised in terms of the relatively low driving force available for bainite transformations. It was shown that the retention, stability and morphology of austenite could be directly derived from the basic transformation mechanism. Under certain circumstances, the bainitic retained austenite conferred exceptional strength/toughness properties to silicon steels; these were· shown to be superior to the properties associated with tempered martensite microstructures, Using thermodynamics, a model was established which could predict the toughness behaviour of silicon steel bainites simply from a knowledge of the composition. The tempered martensite embrittlement phenomenon was not found to be directly linked to the decomposition of retained austenite films, but to the coarsening of inter- or intra-lath carbides. In dislocated martensites, it was found that the distribution and quantity of retained austenite could be rationalised in terms of the degree of accommodation between adjacent martensite variants. The incipient twins generally observed in lath martensites were shown to be accommodation defects such that the extent of twinning was the greatest when adjacent martensite units had twin-related lattices. The thermodynamics of dislocated martensites have been briefly examined, The inhomogeneous deformation behaviour of dual-phase steels has been analysed in terms of available models.

Les aciers multiphasés à effet TRIP sont étudiés depuis plusieurs années maintenant et suscitent un intérêt industriel grandissant, non seulement de la part des sidérurgistes mais également de la part des "clients" de ces derniers, à savoir les constructeurs automobiles. Il semble en effet tout à fait clair que la combinaison au sein de la même microstructure de différentes phases de l'acier (ferrite, bainite, martensite) et l'existence de l'effet TRIP (c'est-à-dire une transformation martensitique induite mécaniquement) améliore de façon importante les propriétés de résistance et de ductilité d'aciers faiblement alliés. Dans ce contexte, notre projet a eu pour objectif de trouver des alternatives aux éléments d'alliage actuellement utilisés (silicium et aluminium) pour la genèse de microstructures conférant aux aciers TRIP les propriétés de résistance et de ductilité désirées. En particulier, nous avons étudié l'un des plus utilisés en sidérurgie, à savoir le niobium. Notre travail s'est donc attaché à étudier l'effet d'additions importantes de niobium (jusque 0.12% en poids) sur les transformations de phase ayant lieu durant le traitement thermique imposé aux aciers multiphasés à effet TRIP ainsi que sur la microstructure et les propriétés mécaniques résultantes. Le niobium influence considérablement les différentes transformations de phase et en particulier la recristallisation de la matrice ferritique se produisant lors du réchauffage et du maintien intercritique. / TRIP-assisted multiphase steels have been studied for several years and arouse a growing industrial interest, not only on behalf of the steelmakers but also on behalf of the “customers” of these, namely the car manufacturers. It seems completely clear that the combination within the same microstructure of various steel phases (ferrite, bainite, martensite…) and the existence of the TRIP effect (i.e. a mechanically induced martensitic transformation) improves in a large way the properties of resistance and ductility of low-alloy steels. In this context, this project aimed to find alternatives to alloy elements currently used (silicon and aluminium) for the generation of microstructures conferring to the TRIP steels the desired properties of resistance and ductility. In particular, we studied the niobium that is one of the most used in iron and steel industry. Our work thus attempted to study the effect of important additions of niobium (until 0.12% in weight) on phase transformations taking place during the heat-treatment imposed to TRIP-aided multiphase steels and on the resulting microstructure and mechanical properties. Niobium influences considerably the various phase transformations and in particular the recrystallization of the ferrite matrix occurring during reheating and intercritical annealing.

Made available in DSpace on 2016-03-15T19:36:56Z (GMT). No. of bitstreams: 1 Ricardo Mazziero Quartarolo.pdf: 8600660 bytes, checksum: 6058bd2a64c8a23a963ef69194501521 (MD5) Previous issue date: 2015-11-30 / Increasing the energy efficiency of internal combustion engines manufactured for light vehicles is a growing demand from society due to the importance of the transport sector in relation to emissions of greenhouse gases. In a move led by the developed countries of Europe, regulatory agencies have been imposing aggressive emission reduction targets for new cars. The reduction of the mechanical losses caused by friction between engine components through the development of new technologies and materials is mandatory for the most industries involved in the manufacture of engines. The piston rings are responsible for much of the losses due to their adverse operating conditions within the combustion chamber. The interaction between the rings and the cylinder is decisive in determining beyond the mechanical friction losses, wear of the assembly, oil consumption and power. The improvement of mechanical properties of piston rings by heat treatment is an important alternative to reduce friction. The reduction of tangential force is especially important in the rings of the second channel, although they are known as scraper rings, in practice its main function is to stabilize the piston and prevent compression ring floating and consequently the flow of combustion gases into the crankcase. Results suggest that bainitic microstructure, obtained by austempering of the piston rings made from gray cast iron enables the reduction of their tangential force. This reduction has a positive relationship with the engine operating temperature exposure time as found in simulations carried out at constant temperature and logarithmic timescale. / O aumento da eficiência energética dos motores de combustão interna utilizados por automóveis leves é uma demanda crescente da sociedade, devido à representatividade do setor de transporte em relação às emissões dos gases do efeito estufa. Em um movimento liderado pelos países desenvolvidos da Europa, as agências reguladoras vêm impondo metas agressivas de redução de emissões para automóveis novos. A redução das perdas mecânicas geradas pelo atrito entre os componentes do motor através do desenvolvimento de novas tecnologias e materiais é mandatório entre a maior parte das indústrias envolvidas na fabricação de motores. Os anéis de pistão são responsáveis por grande parte dessas perdas, devido às suas condições de operação adversas dentro da câmara de combustão. A interação entre os anéis e o cilindro é decisiva para determinar além das perdas mecânicas por atrito, o desgaste do conjunto, consumo de óleo e potência. A modificação das propriedades mecânicas dos anéis de pistão através de tratamento térmico é uma alternativa importante para reduzir o atrito. Esse efeito é especialmente importante nos anéis de segunda canaleta, que apesar de serem considerados anéis raspadores, na prática tem como principal função estabilizar o conjunto e evitar a flutuação do anel de compressão e consequentemente a passagem de gases de combustão para o cárter. Os resultados sugerem que a microestrutura bainítica, obtida através da austêmpera dos anéis de pistão fabricados a partir de ferro fundido cinzento possibilita a redução da força tangencial, sendo que essa redução apresenta uma relação positiva com o tempo de exposição à temperatura de trabalho do motor conforme constatado em simulações realizadas à temperatura constante e escala logarítmica de tempo.

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.

Seit vielen Jahren wächst die Nachfrage bezüglich sparsamer Fahrzeuge; die Autohersteller konkurrieren miteinander und werben mit neuen Fahrzeugkonzepten, in denen hochmoderne Werkstoffe ihre Anwendung finden. In dieser Arbeit werden Legierungskonzepte und entsprechende Warmwalztechnologien einer ultrahochfesten bainitischen Stahlsorte mit Restaustenit vorgestellt, die der genannten Anwendung angepasst werden können. Der gewählte Werkstoff gehört zu den Stählen mit mittleren Kohlenstoffgehalten, die sich nach der - im Rahmen dieser Arbeit entwickelten - Behandlung durch hohe Zugfestigkeit bei vergleichsweise hoher Bruchdehnung auszeichnen. Es werden erweiterte Modelle zur Beschreibung der Phasenumwandlung von Stählen im Bainitgebiet vorgestellt. Die Ergebnisse aus den Experimenten wurden genutzt, um die Modelle zu ergänzen und zu evaluieren. Dabei wird nicht nur der Warmwalzprozess, sondern auch die chemische Zusammensetzung der Stähle selbst optimiert. Die hier präsentierte Arbeit erstreckt sich über die gesamte Produktionskette und zeigt geeignete Herstellungsbedingungen, die in Betriebsanlagen leicht realisierbar sind und umgesetzt wurden.:1 Einführung 10 2 Literaturauswertung 12 3 Aufgabestellung 52 4 Eigene Arbeiten 53 5 Versuchsdurchführung 55 6 Ergebnisse 67 7 Zusammenfassung und Ausblick 129 8 Literaturverzeichnis 131 9 Abbildungsverzeichnis 141 10 Tabellenverzeichnis 144 11 Angewendete Simulationsanlagen 146 12 Anhang 151

Research on bainite and martensite structures has indicated that lower bainite needles have a refining effect on the lath martensitic structure. Lower bainte needles partitions prior austenite grains and will consequently have a refining effect on the subsequent formed lath martensite. Smaller austenite grains will result in smaller lath martensitic packets and blocks and will result in enhanced mechanical properties.   In order to create a variation of lower bainte structure in a matrix of martensite, two different heat treating methods were tested. The work was focused towards the formation of lower bainite during isothermal heat treating in molten salt, above and below the MS-temperature. Both un-tempered and tempered samples were analyzed .Two different materials were tested, both were AISI 4140 but with a slightly difference in hardenability. The material provided by Ovako Steel is 326C and 326F the later had a higher hardenability. In order to better distinguish the two structures from each other when studied under a microscope, a variation of etching methods were tested.  It was possible to create a variation of lower bainite structures in a matrix of martensite.  326F shows less amount of lower bainite and provides a higher average surface hardness before tempering.

The present study focuses on developing a relationship between process variables, mechanical properties and post weld microstructure in Friction Stir Welded HSLA 65 steel. Fully consolidated welds can be produced in HSLA 65 steel by PCBN Convex-Scrolled-Shoulder-Step-Spiral (CS4) tool over a wide range of parameters. Microstructures in the nugget center (NC) are dominated by lath bainite and a few polygonal/allotriomorphic grain boundary ferrites. FSW dependent variables are related to FSW independent variables by non-linear relationship. Heat input is identified to be the best parameter index to correlate with microstructures. With increasing heat input, the volume of bainite is reduced, the shape of bainite is more curved and grain/lath size become coarser. A linear relationship was established between heat input and semi-quantitative post-weld microstructures based on the optical microstructures. Further analysis has been applied on the NC to obtain more fundamental understanding of FSW. The new approach via Orientation Imaging Microscopy (OIM) was developed to acquire quantitative microstructural data including bainite lath/packet and prior austenite grain size (PAG). A linear relationship between heat input and quantitative microstructural features in the NC have been established. Mechanical properties exhibits linear relationship with heat input. These correlations can be utilized to determine FSW weld parameter to get desired mechanical properties welds.