In the last decades, significant improvements regarding the design, materials and technology of rock drills have been made. Likewise, in sheet metal forming, forming tools experience very high contact pressures when processing high strength steel sheets. In both applications components operate under extremely tough contact conditions that result in an accelerated component failure. Enhancements on mechanical properties of components material subjected to extreme contact conditions are highly required in order to withstand the application loads and prevent severe wear. The present thesis was focused on understanding of machinery component damage mechanisms under severe contact conditions. A case study of worn components used in rock drilling and sheet metal cold work was carried out. Thread joints from rock drilling and punches from sheet metal pressing were selected for the investigation. For these components, sliding contact under high contact pressure is a common load condition under the components usage. Then, to understand and quantify the influence of contact parameters, load and surface quality on material performance, laboratory simulations were performed. The results were used for a comparative analysis of the typical damage mechanisms observed in the tests and the case study of the components. The case study results showed that the threaded surfaces underwent severe plastic deformation due to the high-pressure sliding contact. The microstructure beneath the worn surface was altered and surface cracks and delamination were frequently observed at the worn surface. The dominant damage mechanism found on the investigated punches was adhesive wear. Material transfer adds friction stresses at the punch surface and ultimately, with repeated punch strokes, it leads to initiation and propagation of fatigue cracks. Wear process in thread joint and punch wear was simulated using the SOFS. The worn specimens tested experimentally showed similar wear mechanisms obtained in the case study. The thread joint wear simulation showed that the total damage at the worn surface was a result of adhesive wear, plastic deformation, and initiation and propagation of fatigue cracks. In addition, the results showed that the type of motion had a significant influence on the worn volume and crack initiation, and more severe wear was observed at reciprocal motion. The punch wear simulation showed that the friction quickly increased as work material from metal sheets transferred to the disc surface. The rate of the material transfer was strongly dependent on the combination of sheet material and tool steel. Further, the present experimental simulations were applicable to characterize and predict wear of components in the application. / Components used in rock drilling and sheet metal forming operate under harsh contact conditions that result in an early-life component failure. Wear and fatigue are considered as the most common damage mechanism for these components. Commonly, the service life of a component is designed based on its fatigue life. However, wear might have a significant effect on the components life too. Wear results in a surface damage that in turn may cause a fatigue crack initiation. Therefore, knowledge about wear of materials and components is a key factor in design and prediction of the lifetime of the components. In order to predict wear of a certain component, a thorough understanding of the component with regards to its material properties, application loads and working environment, and damage mechanisms is required. The overall aim of the present work was to define the typical wear mechanisms occurred on machinery components used in rock drilling and sheet metal forming. A comparative analysis of the case studies and results from performed laboratory tests simulated wear mechanisms in the applications highlighted wear mechanisms and factors influencing severity of wear in the applications. Obtained information is crucial for ranking and selection of the best material in the applications. / <p>The presentation will will be via zoom. PhD student will together with the supervisors will be in Karlstad while the opponent is in Luleå. </p>

In the pipe manufacturing and pipe processing industry, the demand for cost-effective pipes with high strength and good ductility is increasing. In the present study, the inductive longitudinal welding process was combined with a Quenching and Partitioning (Q&P) treatment to manufacture pipes with enhanced mechanical properties. The aim of the Q&P process is to establish a martensitic structure with increased retained austenite content. This allows for the beneficial use of both phases: the strength of martensite as well as the ductility of retained austenite. A 42SiCr steel, developed for Q&P processes, was joined at the longitudinal seam by a high-frequency induction (HFI) welding process and was subsequently heat-treated. The applied heat treatments included normalizing, austenitizing, quenching, and two Q&P strategies (Q&P-A/Q&P-B) with distinct quenching (Tq = 200/150 °C) and partitioning temperatures (Tp = 300/250 °C). Investigations of the microstructures revealed that Q&P tubes exhibit increased amounts of retained austenite in the martensitic matrix. Differences between the weld junction and the base material occurred, especially regarding the morphology of the martensite; the martensite found in the weld junction is finer and corresponds more to the lath-type morphology, compared to the base material in the circumference. In all zones of the welded tube circumference, retained austenite has been found in similar distributions. The mechanical testing of the individual tubes demonstrated that the Q&P treatments offer increased strength compared to all other states and significantly improved ductility compared to the quenched condition. Therefore, the approach of Q&P treatment of HFI-welded tubes represents a route for the mass production of high-strength tubular products with improved ductility.

No description available.

No description available.

No description available.

O desenvolvimento e utilização de novos materiais, mais leves e com propriedades mecânicas superiores aos atuais, se mostram extremamente importantes devido à redução de peso e consequentemente redução na emissão de gases poluentes que poderiam gerar. As ligas de Fe-Mn-C com elevados teores de Mn (20-30%) representam um desenvolvimento muito recente de aços austeníticos, que, através dos seus mecanismos diferenciados de deformação reúnem elevada resistência mecânica com grande ductilidade. Essa nova classe de materiais estruturais possibilita uma efetiva redução de custos na produção através do reduzido tempo de processamento (sem a necessidade de tratamentos térmicos especiais e de processamentos termomecânicos controlados). A soldagem é, atualmente, o mais importante processo de união de metais usado no setor industrial. Dentro da variada gama de processos de soldagem existentes, a soldagem por fricção e mistura mecânica (SFMM, em inglês: Friction Stir Welding - FSW) se destaca por ser um processo de união no estado sólido que apresenta uma série de vantagens sobre as tecnologias convencionais de soldagem por fusão. Do ponto de vista metalúrgico, uma das suas principais vantagens se manifesta justamente na junção de materiais dissimilares, visto que o grau de mistura de composições e as transformações de fases entre materiais incompatíveis podem ser minimizados. Outra vantagem é que há um refino de grão no cordão de solda comparado com a microestrutura fundida que se forma nos processos convencionais. Este trabalho teve como objetivo produzir em escala laboratorial os aços de alta liga ao manganês com efeito TRIP, avaliar o impacto da velocidade de rotação da ferramenta na soldagem por fricção e mistura mecânica e avaliar a microestrutura e propriedades mecânicas das juntas soldadas. A microestrutura das juntas soldadas caracterizou-se pela presença apenas da zona de mistura e do metal base, além da formação de \'anéis de cebola\' na zona de mistura, esta não mostrou sinais de transformação martensítica induzida por deformação e sofreu recristalização dinâmica para todas as velocidades de rotação investigadas com a formação de grãos refinados e com morfologia equiaxial. Os corpos de tração fraturaram todos nos metais de base, mostrando que as propriedades mecânicas da zona de mistura foram superiores à do metal base e que a variação de aporte térmico alcançada com a velocidade de rotação da ferramenta não comprometeu a qualidade das juntas soldadas. / The development and application of new light materials with superior mechanical properties is extremely important to weight reduction in vehicles and consequently reduction of greenhouse gases emission. The Fe-Mn-C steels with high Mn (20-30%) are a recent development of austenitic steels, which, due to their different mechanisms of deformation, possesses high strength and high ductility as well. In addition, this new type of structural steel allows an effective reduction of manufacturing costs due to its reduced processing time (it does not require special heat treatments and controlled thermo mechanical processing). Welding has been one of the most important processes for joining metals. Among the available welding processes, friction stir welding (FSW) is notable for being a solid state process with great advantages over the conventional welding methods. In the mettalurgical point of view, welding dissimilar materials is a significant advantage of FSW over the other process. The main reason is the reduction of mixture of material and phase transformations between the incompatible materials in the weld. Moreover, grain refinement is another advantage from the process. The present study aimed to produce laboratorial scale high Mn steels with TRIP effect, investigate the impact of tool speed ont the microstructure and mechanical properties of friction stir welded joints. The microstructure of the welded joints exhibited only the stirred zone (SZ) and the base material (BM), besides the presence of ´onion rings´ within the stirred zone. The SZ exhibited no signs of martensite suggesting that dynamic recrystallization have occurred for all the speed tested. Moreover, the grains in the SZ had equiaxial morphology and were significantly refined. The fracture of the tensile specimens occurred in the base material, bringing to light that the welding process was beneficial to the mechanical properties. Furthermore, the variation of heat input achieved with the speed did not compromise the quality of welded joints.

O desenvolvimento e utilização de novos materiais, mais leves e com propriedades mecânicas superiores aos atuais, se mostram extremamente importantes devido à redução de peso e consequentemente redução na emissão de gases poluentes que poderiam gerar. As ligas de Fe-Mn-C com elevados teores de Mn (20-30%) representam um desenvolvimento muito recente de aços austeníticos, que, através dos seus mecanismos diferenciados de deformação reúnem elevada resistência mecânica com grande ductilidade. Essa nova classe de materiais estruturais possibilita uma efetiva redução de custos na produção através do reduzido tempo de processamento (sem a necessidade de tratamentos térmicos especiais e de processamentos termomecânicos controlados). A soldagem é, atualmente, o mais importante processo de união de metais usado no setor industrial. Dentro da variada gama de processos de soldagem existentes, a soldagem por fricção e mistura mecânica (SFMM, em inglês: Friction Stir Welding - FSW) se destaca por ser um processo de união no estado sólido que apresenta uma série de vantagens sobre as tecnologias convencionais de soldagem por fusão. Do ponto de vista metalúrgico, uma das suas principais vantagens se manifesta justamente na junção de materiais dissimilares, visto que o grau de mistura de composições e as transformações de fases entre materiais incompatíveis podem ser minimizados. Outra vantagem é que há um refino de grão no cordão de solda comparado com a microestrutura fundida que se forma nos processos convencionais. Este trabalho teve como objetivo produzir em escala laboratorial os aços de alta liga ao manganês com efeito TRIP, avaliar o impacto da velocidade de rotação da ferramenta na soldagem por fricção e mistura mecânica e avaliar a microestrutura e propriedades mecânicas das juntas soldadas. A microestrutura das juntas soldadas caracterizou-se pela presença apenas da zona de mistura e do metal base, além da formação de \'anéis de cebola\' na zona de mistura, esta não mostrou sinais de transformação martensítica induzida por deformação e sofreu recristalização dinâmica para todas as velocidades de rotação investigadas com a formação de grãos refinados e com morfologia equiaxial. Os corpos de tração fraturaram todos nos metais de base, mostrando que as propriedades mecânicas da zona de mistura foram superiores à do metal base e que a variação de aporte térmico alcançada com a velocidade de rotação da ferramenta não comprometeu a qualidade das juntas soldadas. / The development and application of new light materials with superior mechanical properties is extremely important to weight reduction in vehicles and consequently reduction of greenhouse gases emission. The Fe-Mn-C steels with high Mn (20-30%) are a recent development of austenitic steels, which, due to their different mechanisms of deformation, possesses high strength and high ductility as well. In addition, this new type of structural steel allows an effective reduction of manufacturing costs due to its reduced processing time (it does not require special heat treatments and controlled thermo mechanical processing). Welding has been one of the most important processes for joining metals. Among the available welding processes, friction stir welding (FSW) is notable for being a solid state process with great advantages over the conventional welding methods. In the mettalurgical point of view, welding dissimilar materials is a significant advantage of FSW over the other process. The main reason is the reduction of mixture of material and phase transformations between the incompatible materials in the weld. Moreover, grain refinement is another advantage from the process. The present study aimed to produce laboratorial scale high Mn steels with TRIP effect, investigate the impact of tool speed ont the microstructure and mechanical properties of friction stir welded joints. The microstructure of the welded joints exhibited only the stirred zone (SZ) and the base material (BM), besides the presence of ´onion rings´ within the stirred zone. The SZ exhibited no signs of martensite suggesting that dynamic recrystallization have occurred for all the speed tested. Moreover, the grains in the SZ had equiaxial morphology and were significantly refined. The fracture of the tensile specimens occurred in the base material, bringing to light that the welding process was beneficial to the mechanical properties. Furthermore, the variation of heat input achieved with the speed did not compromise the quality of welded joints.

No description available.

The importance of fire safety design has been realised due to the ever increasing loss of properties and lives caused by structural failures during fires. In recognition of the importance of fire safety design, extensive research has been undertaken in the field of fire safety of buildings and structures especially over the last couple of decades. In the same period, the development of fire safety engineering principles has brought significant reduction to the cost of fire protection. However the past fire research on steel structures has been limited to heavier, hot-rolled structural steel members and thus the structural behaviour of light gauge cold-formed steel members under fire conditions is not well understood. Since cold-formed steel structures have been commonly used for numerous applications and their use has increased rapidly in the last decade, the fire safety of cold-formed steel structural members has become an important issue. The current design standards for steel structures have simply included a list of reduction factors for the yield strength and elasticity modulus of hot-rolled steels without any detailed design procedures. It is not known whether these reduction factors are applicable to the commonly used thin, high strength steels in Australia. Further, the local buckling effects dominate the structural behaviour of light gauge cold-formed steel members. Therefore an extensive research program was undertaken at the Queensland University of Technology to investigate the local buckling behaviour of light gauge cold-formed steel compression members under simulated fire conditions. The first phase of this research program included 189 tensile coupon tests including three steel grades and six thicknesses to obtain the accurate yield strength and elasticity modulus values at elevated temperatures because the deterioration of the mechanical properties is the major parameter in the structural design under fire conditions. The results obtained from the tensile tests were used to predict the ultimate strength of cold-formed steel members. An appropriate stress-strain model was also developed by considering the inelastic mechanical characteristics. The second phase of this research was based on a series of more than 120 laboratory experiments and corresponding numerical analyses on cold-formed steel compression members to investigate the local bucking behaviour of the unstiffened flange elements, stiffened web elements and stiffened web and flange elements at elevated temperatures up to 800°C. The conventional effective design rules were first simply modified considering the reduced mechanical properties obtained from the tensile coupon tests and their adequacy was studied using the experimental and numerical results. It was found that the simply modified effective width design rules were adequate for low strength steel members and yet was not adequate for high strength cold-formed steel members due to the severe reduction of the ultimate strength in the post buckling strength range and the severe reduction ratio of the elasticity modulus to the yield strength at elevated temperatures. Due to the inadequacy of the current design rules, the theoretical, semi-empirical and empirical effective width design rules were developed to accurately predict the ultimate strength of cold-formed steel compression members subject to local buckling effects at elevated temperatures. The accuracy of these new design methods was verified by comparing their predictions with a variety of experimental and numerical results. This thesis presents the details of extensive experimental and numerical studies undertaken in this research program and the results including comparison with simply modified effective width design rules. It also describes the advanced finite element models of cold-formed steel compression members developed in this research including the appropriate mechanical properties, initial imperfections, residual stresses and other significant factors. Finally, it presents the details of the new design methods proposed for the cold-formed steel compression members subject to local buckling effects at elevated temperatures.

Actuellement, l’industrie automobile est à la recherche d’une meilleure solution pour l’allégement de la structure de véhicule afin de diminuer la consommation de carburant et par conséquent diminuer les émissions nocives de CO2. Les aciers à très haute résistance (THR) mécanique permettent d’obtenir les tôles d’acier à section diminué avec les mêmes ou meilleurs propriétés fonctionnels. Les aciers Dual-Phase (DP), constitués majoritairement d’une phase ductile, la ferrite, et d’une phase dure, la martensite, occupent une place importante en tant que matériaux de structure destinés au challenge préoccupant l’industrie automobile. Une bonne résistance à l’endommagement est exigée pour leur utilisation en tant que des pièces de structures et de renfort pour l’automobile. Il a été bien établi que la résistance à l’endommagement des ces aciers Dual-Phase est contrôlée par leur microstructure. Ce travail de thèse s’est inscrit dans une logique de compréhension des mécanismes d’endommagement d’un acier Dual-Phase modèle, le DP 780, en fonction de différents paramètres microstructuraux. Deux mécanismes d’endommagement ont été identifiés pour l’acier DP 780 : la décohésion de l’interface ferrite/martensite et la formation de cavités autour des carbures, dans la martensite revenue. Un modèle qualitatif de mécanisme d’endommagement a été développé afin de pouvoir prédire l’endommagement de l’acier DP 780. Ce modèle qualitatif, développé pour l’acier DP 780, servira de base d’approfondissement de modèles plus élaborés et quantitatifs permettant la compréhension et la prédiction de l’endommagement des aciers Dual-Phase, de façon générale / In the automotive industry current environmental concerns require that the vehicle fuel consumption and CO2 emissions should be reduced as much as possible. It is therefore advantageous to reduce the weight of body in white components by replacing existing parts with higher strength, thinner gauge alternatives with equivalent or improved functional properties. Dual Phase (DP) steels are a class of high-strength low-alloy steels characterized by a microstructure consisting of martensite and ferrite. Dual Phase steels combine high strength levels with good ductility. Thus, DP steels are potentially very attractive for the automobile industry. In addition to the required high strength and ductility, DP steel has to be cold formed into complex shapes. It appears that DP steel damage behaviour is very complex and cannot be predicted using existing models based on standard mechanical properties. This work is concerned with the study of microstructural evolution and investigation of the relation between the microstructure and damage mechanisms in a reference DP 780 steel. Two damage mechanisms have been identified in this DP steel: ferrite/martensite interface decohesion and void formation at tempered carbides. A simple modeling for qualitative description of the observed damage formation mechanisms is proposed. This modeling permits a basic understanding of the experimentally observed trends and could be used as the starting point for a more detailed analysis in future