A soldagem altera as propriedades mecânicas dos aços ligados ao níquel, em especial seu desempenho em aplicações criogênicas. Assim, este trabalho apresenta um estudo sobre a soldabilidade metalúrgica do aço com 9% níquel e tem como objetivos: identificar se a fragilização em temperatura abaixo da temperatura Ac3 ocorre em ZACs com dois e três ciclos térmicos simulados fisicamente; analisar as características da junta soldada pelo processo de soldagem por atrito linear com mistura e os efeitos da soldagem multipasse; e analisar a influência dos passes de enchimento e acabamento sobre o comportamento mecânico da ZAC da raiz de uma junta soldada pelo processo de soldagem MIG/MAG. Destacam-se entre os resultados da simulação física da ZAC: as amostras que tiveram a temperatura máxima abaixo da temperatura Ac1, no terceiro ciclo térmico, não apresentaram o efeito de redução da energia absorvida no ensaio Charpy V; a fração volumétrica de austenita retida não aumenta a quantidade de energia absorvida no ensaio Charpy V para as amostras que tiveram a temperatura máxima do segundo ciclo térmico abaixo da temperatura Ac3 (723°C); e a correlação linear múltipla sugere um modelo empírico, baseado nos dados deste trabalho, onde os fatores de fração volumétrica do microconstituinte martensita-austenita, fração volumétrica de austenita retida e tamanho de grão são mais relevantes para a quantidade de energia absorvida no ensaio Charpy V. Destacam-se entre os resultados da soldagem por atrito linear: a energia absorvida no ensaio Charpy V da zona misturada do primeiro cordão (CP1) é menor que o metal de base; o segundo cordão gera duas regiões na zona misturada do primeiro cordão, que tendem a aumentar a energia absorvida no ensaio Charpy V; os valores de energia absorvida no ensaio Charpy V apresentam correlações lineares simples com a microdureza, a fração volumétrica do microconstituinte martensita-austenita e com o tamanho de grão. Destacam-se entre os resultados da soldagem com MIG/MAG: A soldagem do aço com 9% de níquel com a liga Inconel 625 gera uma zona não misturada entre o metal de solda e a ZAC; a amostra com todos os passes de solda (CP3) apresenta a menor energia absorvida no ensaio Charpy V entre todos os experimentos; e a trinca, no ensaio Charpy V, propaga na zona não misturada no CP1 e no CP2, que tiveram as maiores energias absorvidas no ensaio Charpy. O CP3 apresenta propagação de trinca na linha de fusão e possui a menor energia absorvida no ensaio Charpy V. / Welding modify the mechanical properties of nickel steels, especially their performance in cryogenic applications. Thus, this work presents a study on the metallurgical weldability of 9% nickel steel and its objectives are: identify if the embrittlement in temperature below the Ac3 temperature occurs in HAZs with two and three thermal cycles simulated physically; analyze the characteristics of the joint welded by friction stir welding process and the effects of multipass welding; and analyze the influence of the filling and finishing passes on the mechanical behavior of HAZ from the root of a joint welded by the GMAW welding process. The results of the physical simulation of the HAZ were: the samples that had the maximum temperature below the temperature Ac1, in the third thermal cycle, did not present the effect of reduction of the energy absorbed in the Charpy V test; the retained austenite volumetric fraction does not increase the amount of energy absorbed in the Charpy V test for the samples having the maximum temperature of the second thermal cycle below the Ac3 temperature (723 °C); and the multiple linear correlation suggests an empirical model, based on the data of this work, where the volumetric fraction factors of the martensite-austenite microconstituent, retained austenite volumetric fraction and grain size are more relevant for the amount of energy absorbed in the Charpy V test. The FSW welding highlights results: the energy absorbed in the Charpy V test of the mixed zone of the first pass (CP1) is smaller than the base metal; the second pass generates two regions in the mixed zone of the first pass, which tend to increase the energy absorbed in the Charpy V test; the values of energy absorbed in the Charpy V test show simple linear correlations with the microhardness, the volumetric fraction of the martensite-austenite microconstituent and with the grain size. The GMAW welding highlights results: welding of the steel with 9% nickel with the Inconel 625 alloy makes an unmixed zone between the weld metal and the HAZ; the sample with all weld passes (CP3) shows the lowest energy absorbed in the Charpy V test among all the experiments; and the crack, in the Charpy V test, propagates in the unmixed zone in CP1 and CP2, which had the highest energies absorbed in the Charpy test. The CP3 shows crack propagation in the melting line and has the lowest energy absorbed in the Charpy V test.

La projection à froid, aussi appelée cold spray, est considérée comme un nouveau membre de la famille de laprojection thermique depuis une trentaine d'années maintenant. Cette thèse propose d'étudier le comportement endéformation des particules et du substrat et de mettre en avant les liaisons formées dans le revêtement par deuxapproches complémentaires, expérimentale et de simulation.Une méthode innovante pour observer directement la surface fracturée des particules déposées après décollementdu substrat a été testée avec succès. Par ce moyen, la surface de contact entre particule et substrat sousdifférentes conditions a été analysée.Concernant les résultats expérimentaux, une nouvelle théorie a été proposée pour expliquer le mécanisme deliaison interfaciale d'un revêtement dur de Ni sur substrat mou d'Al reposant sur l'effet de martelage répété desparticules, sur l'effet de pression du gaz principal et sur l'effet de préchauffage du substrat. La transformation dumécanisme de liaison revêtement/substrat au cours de la construction du dépôt a été mise en évidence en passantdu verrouillage mécanique à une combinaison d'une liaison mécanique et d'une liaison métallurgique, puis à laformation d'instabilités sous forme d'un mélange tourbillonnaire à l'interface. Plus de zones de liaisonsmétallurgiques sont générées sous forte pression, une plus grande déformation plastique apparaît grâce latempérature de préchauffage, et une adhérence plus forte au sein des dépôts est capable de se produire en dépitde la présence d'un film d'oxyde épais sur la surface du substrat. / Cold spraying, also called cold gas dynamic spraying, is a new coating technology which has been developed duringthe past three decade. In this study, a comprehensive investigation on particle deformation behavior and bondingbehavior between particle and substrate was conducted by experiment and numerical method.This thesis aims at presenting an innovative method to directly observe the fractured contact surface between thecold sprayed particle and substrate. By this means, the particle/substrate fractured contact surfaces were analyzedfor different conditions.Based on the experimental results, a new theory was proposed to explain the interfacial bonding mechanism of hardNi coating onto soft Al substrate. It is assumed that the particle peening effect is essential for the formation ofdiscontinuous metallurgical bonding. The dominant coating/substrate bonding mechanism is responsible of thetransformation during the coating build-up process of the initial mechanical interlocking to a combination ofmechanical interlocking and metallurgical bonding therefore of the formation of interfacial instabilities. The highcontact pressure is the relevant factor determining the particle/substrate metallurgical bonding. More metallurgicalbonding areas were generated due to strengthen peening effect of the subsequently deposited particles with higherpropelling gas pressure. Finally, stronger adhesion is able to occur despite the presence of a thick oxide film on thesubstrate surface by the preheating of the substrate. Higher temperatures help the materials to undergoes astronger plastic deformation that disrupts the oxide films. That leads to initiate an intimate contact between particleand substrate.

Le Silicium est le deuxième élément le plus abondant dans la croûte terrestre après l’oxygène. Il est produit par voie métallurgique dans un four à arc électrique, le quartz est réduit en présence de réducteurs (charbon de bois, houille et coke de pétrole). Le silicium métallurgique est principalement utilisé dans la métallurgie comme élément d’alliage, dans la chimie et l’industrie solaire. Le prix du Silicium est fonction de sa pureté. Les travaux de cette thèse se divisent en deux parties l’utilisation du Silicium Métallurgique (99% Si) pour le stockage de l’hydrogène, et la photoluminescence du ferrosilicium (disiliciure de fer) de qualité métallurgique. Des substrats de silicium métallurgique ont été soumis à une anodisation électrochimique dans une solution à base d’acide fluorhydrique. Le silicium poreux nanostructuré obtenu est légèrement différent du silicium poreux issu de substrat de silicium de qualité électronique de même résistivité. L’influence des principaux paramètres sur la génération de l’hydrogène : la porosité, la concentration, le volume et la température ont fait l’objet d’une étude détaillée. Le silicium poreux produit à partir de silicium métallurgique est un matériau de stockage d’hydrogène. Des substrats de disiliciure de fer de qualité métallurgique ont été soumis à une anodisation électrochimique. Le composé obtenu est du disiliciure de fer nanostructuré avec du silicium résiduel, ce produit est recouvert de fluorosilicate de fer hexahydraté qui a la particularité d’être luminescent. Il s’agit à ce jour de la première anodisation du disiliciure de fer, un mécanisme de gravure a été proposé et l’influence des principaux paramètres d’anodisation sur les propriétés de photoluminescence a été évaluée. / Silicon is the second most abundant element in the Earth crust after oxygen. Its use in metallurgy, building and electronic industry requires a huge fabrication level. Depending on the contamination level allowed, the price of this material varies in the orders of magnitude. This thesis focuses on the use of dirtiest metallurgical grade silicon and iron disilicide substrates for hydrogen storage and photoluminescence applications. The initial substrates were subjected to electrochemical etching in hydrofluoric acid-containing solutions. Anodization of metallurgical grade silicon substrate produces nanostructured porous silicon with somewhat shifted parameters (comparing with electronic grade porous silicon with the same resistivity), as it was studied in this thesis in details. It was shown, that metallurgical grade porous silicon can be applied as hydrogen storage material. Hydrogen generation is studied here based on the influences of some technically critical parameters: porosity, alkali concentration, volume and temperature. Electrochemical treatment of metallurgical grade iron disilicide substrates produces luminescent iron fluorosilicate hexahydrate, covering the residual nanostructured iron disilicide/silicon. Here, the influence of anodization parameters on photoluminescent properties is studied. Also, etching mechanism is proposed as for the new material never anodized.

In order to obtain a good performance and predict the properties of alloys, it is necessary to control the contents of alloying elements and to evaluate a largest inclusion in the product. Thus, improved techniques for both control of alloy elements and evaluation of the large inclusion in products will enable us to provide better qualities of the final products. In the case of one Ni alloy, (NW2201, >99 mass%Ni), the precise control technique of Mg content is important to obtain a good hot-workability. Hereby, the slag/metal reaction experiments in a laboratory have been carried out at 1873 K, so that the equilibrium Mg content and kinetic behavior can be understood. More addition of Al in the melt as well as higher CaO/Al2O3 value of slag resulted in higher amount of Mg content in Ni. For the same conditions of Al content and slag composition, the mass transfer coefficient of Mg in molten Ni was determined as 0.0175 cm/s. By applying several countermeasures regarding the equilibrium and kinetic process to the plant trials, the value of the standard deviation for the Mg content in an alloy was decreased till 0.003 from 0.007 mass%. The size measurements of largest inclusions in the various alloys (an Fe-10mass%Ni alloy, 17CrMo4 of low-C steel and 304 stainless steel) were carried out by using statistics of extreme values (SEV). In order to improve the prediction accuracy of this method, three dimensional (3D) observations were applied after electrolytic extraction. In addition, the relationship of extreme value distribution (EVD) in the different stages of the production processes was studied. This was done to predict the largest inclusion in the products at an early stage of the process. A comparison of EVDs for single Al2O3 inclusion particles obtained by 2D and 3D observations has clarified that 3D observations result in more accurate EVD because of the absence of pores. Also, it was found that EVD of clusters were larger than that of single particles. In addition, when applying SEV to sulfide inclusions with various morphologies, especially for elongated sulfides, the real maximum sizes of them were able to be measured by 3D observations. Geometrical considerations of these particles clarified the possibility of an appearance of the real maximum inclusion sizes on a cross section to be low. The EVDs of deoxidation products in 304 stainless steel showed good agreement between the molten steel and slab samples of the same heat. Furthermore, the EVD of fractured inclusion lengths in the rolled steel were estimated from the initial sizes of undeformed inclusions which were equivalent with fragmented inclusions. On the other hand, from the viewpoint of inclusion width, EVD obtained from perpendicular cross section of strips was found to be useful to predict the largest inclusion in the final product with less time consumption compared to a slab sample. In summary, it can be concluded that the improvement of the techniques by this study has enabled to precisely control of alloy compositions as well as to evaluate the largest inclusion size in them more accurately and at an earlier stage of the production process. / QC 20101222

The initial stage of liquid phase sintering, involving liquid penetration, agglomerate separation, particle spreading and growth has been investigated in experiments using tungsten heavy alloys. The particle composites used were produced by hot isostatic pressing (HIP) of pure powder mixtures of W-Ni-Fe-(Co). By using different HIP temperatures, volume fractions of tungsten, alloying elements like Cobalt and Sulphur or excluding Iron from the matrix, liquid penetration, agglomerate separation and particle growth conditions were affected. The investigations were performed mainly under microgravity (sounding rockets or parabolic trajectories by airplanes) but at high tungsten particle fractions, short sintering times or at infiltration of solid pure tungsten, they were performed at normal gravity. The liquid penetration of the tungsten agglomerates is explained by initial wetting under non-equilibrium conditions, due to the reaction between the liquid matrix and the particles, and a decrease of interfacial energy. The dissolving of tungsten gives a pressure drop in the penetrating liquid and a driving force for the liquid movement by a suggested parabolic penetration model. For cold worked tungsten, a penetration theory was proposed, where an internal stress release in the penetrated tungsten grains creates space for the advancing liquid. The spreading of the tungsten agglomerates is explained by an interagglomerate melt swelling due to a Kirkendall effect. The liquid matrix undergoes a volume increase since the diffusion rates of Ni-Fe are higher than for W and initial concentration gradients of W and Ni, Fe exists. The suggested model by Kirkendall are also used for an analysis of the interaction behaviour between solid particles and a solidification front and inclusion behaviour in iron base alloys during teeming and deoxidation. The average tungsten particles size decrease initially since part of the tungsten particles is dissolved when the non-equilibrium matrix phase is melting. When equilibrium is reached, the tungsten particles grow in accordance with the Ostwald ripening process by an approximately 1/3 power law. Larger particle fraction of particles showed a higher growth rate, due to shorter diffusion distances between the particles. Cobalt, Sulphur and absence of iron in the matrix were found to increase the growth rate of the tungsten particles due to a higher surface tension between the solid tungsten particles and the matrix melt. / QC 20100528

Two industrial studies and one set of lab scale trials have been done. In addition, a theoretical study has been done. The main focus has been on non metallic inclusion composition during the ladle refining operation in industrial steel production. Sampling has been done together with careful inclusion determination. The inclusion composition is related to different variables. In the industrial trials samples have been taken at different steps during the ladle refining period. Steel and slag composition as well as temperature and oxygen activity have been determined. The thesis is based on five supplements with different major objectives, all related to the inclusion composition. The equilibrium top slag-steel bulk and inclusions-steel bulk were investigated by comparison between calculated and measured oxygen activity values. The oxygen activity and relation to temperature has also been discussed as well as oxygen activity and temperature gradients. The effect of vacuum pressure on inclusion composition has been evaluated in a theoretical study as well as lab scale trials. The inclusion composition has been studied during the industrial ladle treatment process. The inclusion composition was related to top slag composition and other parameters during ladle treatment. The major findings in the thesis are the lack of equilibrium conditions with respect to top-slag and steel bulk before vacuum treatment. The inclusions have been found to be closer to equilibrium with the steel bulk. Al/Al2O3 equilibrium has been found to control the oxygen activity after Al-deoxidation. Evaluation of inclusion composition during the ladle refining has revealed that the majority of the inclusions showed a continuous composition change throughout the ladle refining process, from high Al2O3, via MgO-spinel to finally complex types rich in CaO and Al2O3. The final inclusion composition after vacuum treatment was found to be close to the top slag composition. Vacuum pressure has been found to have a theoretical effect on inclusion composition at very low pressures. / QC 20100712

The present thesis was to study non-metallic inclusions during vacuum degassing in ladle treatment. Emphasize was mostly given to liquid CaO-Al2O3 inclusions. A series of industrial experiments were carried out at Uddeholm Tooling AB, Hagfors, Sweden. To gain an insight into the industrial findings, laboratory investigations were also performed. Large number of steel samples were collected and examined. Liquid calcium aluminate inclusions with low SiO2 and high SiO2 were often found with spinel inclusions before vacuum degassing. Laboratory experiments showed that spinel would react with the dissolved Ca in the liquid steel forming calcium aluminate inclusions. This laboratory results agreed with the industrial observation that spinel phase was quite often found in the center of the calcium aluminate phase. After vacuum degassing, most of the inclusions were calcium aluminate liquid inclusions. When dissolved Al level was low, 2 types of liquid calcium aluminate inclusions with considerably different SiO2 contents were found to coexist even at the end of the process. In view of the lack of the thermodynamic data for SiO2 activities in the low silica region, thermodynamic measurements were conducted in the CaO-Al2O3-SiO2-MgO system. The experimental results could reasonably explain the coexistence of the two types of the liquid oxide inclusions. While the total number of inclusions decreased during vacuum degassing, the number of bigger inclusions (>11.3 μm) increased generally in used ladles. This finding was in accordance with the previous studies, wherein, ladle glaze was found to be responsible for the supply of bigger inclusions. The behaviors of several types of inclusions in liquid steel were examined using a laser scanning confocal microscope (LSCM). While alumina inclusions tended to impact on each other, agglomerate and grow very quickly, none of the other types of inclusions, such as spinel and calcium aluminate was observed to agglomerate. The results of LSCM study agreed well with the industrial observation. Examination on a huge number of inclusions did not show any indication of impact and physical growth of the inclusions, except the alumina inclusions. The removal of inclusions around open-eye in a gas-stirred ladle was experimentally studied by a cold model with oil and water. Most of the simulated inclusions were brought up to the oil phase by gas-water plume. Inclusion removal into oil layer took place when the inclusions passed through the sphere-bed of the oil layer around the open-eye. A calculation showed that the contribution of metal-gas plume in inclusion removal was much larger than that of buoyancy mechanism. The results of the industrial experiments revealed that the inclusions distribution strongly depended on stirring condition. When a ladle was stirred by both gas and induction, inclusion removal near slag layer was significant. / QC 20100803

The aim of this thesis work is to increase the knowledge of inclusion behavior at the steel-slag interface by mathematical modeling and in-situ Confocal Scanning Laser Microscope experiments. Mathematical models based on the equation of motion predicting liquid and solid inclusion behavior was first investigated. Four main forces, the buoyancy force, the added mass force, the rebound force and the drag force, act on the inclusion as it crosses the interface. There are three types of behavior an inclusion at the steel-slag interface can adopt. These are a) pass, which means that the inclusion is separated to the slag, b) remain, where the inclusion stays at the interface without being fully transferred to the slag or c) oscillate, and the inclusion rises and descends at the interface until the motion is dampened out by the interfacial forces. The studies showed the importance of accurate experimental physical property data. Application of the models to industrial conditions illustrated that useful plots could be made showing the industry how to optimize their interfacial properties in the ladle and tundish to obtain maximum inclusion separation. In-situ Confocal Scanning Laser Microscope (CSLM) experiments were carried out in order to study agglomeration of liquid and semi liquid inclusions at the steel-gas and steel-slag interfaces and in the slag. Liquid-liquid inclusion agglomeration at steel-gas and steel-slag interfaces was seen to not occur without using force. However, when already transferred to the slag the inclusions agglomerated freely due to a higher free energy force. Comparison of experimental and theoretical agglomeration force showed good agreement between experiments and theory. The main conclusion of this work is that inclusion separation is a complex field of study and there exist no model that takes everything into account. Here the tendency for inclusion transfer and how to manipulate the physical properties for inclusion separation together with agglomeration experiments have been studied. For the future maybe coupling of models for computational fluid dynamics, agglomeration, inclusion separation, dissolution and slag entrainment in addition with experimental physical property data can provide a better overview and understanding. / QC 20100823

The objective of this research is to increase the understanding of the solidification behaviour of some industrially important wrought aluminium alloys. The investigation methods range from direct investigations of as-cast ingots to laboratory-scale techniques in which ingot casting is simulated. The methods span from directional solidification at different cooling rates to more fundamental and controlled techniques such as DTA and DSC. The microstructure characteristics of the castings have been investigated by optical and Scanning Electron microscopy. Hardness tests were used to evaluate the mechanical properties. The effects of adding alloying elements to 3XXX and 6XXX aluminium alloys have been studied with special focus on the effects of Zn, Cu, Si and Ti. These elements influence the strength and corrosion properties, which are important for the performance of final components of these alloys. Solidification studies of 0-5wt% Zn additions to 3003 alloys showed that the most important effect on the microstructure was noticed at 2.5 wt% Zn, where the structure was fine, and the hardness had a maximum. Si addition to a level of about 2% gave a finer structure, having a relatively large fraction of eutectic structure, however, it also gave a long solidification interval. The addition of small amounts of Cu, 0.35 and 1.0 wt%, showed a beneficial effect on the hardness. Differences have been observed in the ingot surface microstructures of 6xxx billets with different Mg and Si ratios. Excess Si compositions showed a coarser grain structure and more precipitations with possible negative implications for surface defect formation during DC casting. The comparison of alloys of different Ti content showed that the addition of titanium to a level of about 0.15 wt% gave a coarser grain structure than alloys with a normal Ti content for grain refinement, i.e. < 0.02 wt%, although a better corrosion resistance can be obtained at higher Ti contents. The larger grain size results in crack sensitivity during DC casting. A macroscopic etching technique was developed, based on a NaOH solution, and used in inclusion assessment along DC cast billets. Good quantitative data with respect to the size and spatial distribution of inclusions were obtained. The results from studied billets reveal a decreasing number of inclusions going from bottom to top, and the presence of a ring-shaped distribution of a large number of small defects in the beginning of the casting. The present study shows how composition modifications, i.e. additions of certain amounts of alloying elements to the 3xxx and 6xxx Al alloys, significantly change the microstructures of the materials, its castability, and consequently its mechanical properties / QC 20100901

The present work was initiated to investigate the stability, structural and thermodynamic properties of transition metal carbides, nitrides and carbo-nitrides by atomistic simulations and experimentations. The interatomic pair potentials of Cr-Cr, Mn-Mn, Fe-Fe, C-C, Cr-C, Mn-C, Fe-C, Cr-Fe, Cr-N and Mn-N were inverted by the lattice inversion method and ab initio cohesive energies, and then employed to investigate the properties of Cr-, Mn- and Fe-carbides by atomistic simulations in this work. For the binary M7C3 carbide, the structural properties of M7C3 (M = Cr, Mn, Fe) were investigated by atomistic simulations. The results show that the stable structure for these compounds is hexagonal structure with P63mc space group. The cohesive energy of M7C3 calculated in this work indicates that the stability of carbides decreases with the increasing in metal atomic number. Further, the vibrational entropy of Cr7C3 was calculated at different temperatures and compared with the entropy obtained by experimentations. The comparison demonstrates that the main contribution to the entropy is made by the vibrational entropy. For the binary τ-carbides, the structural properties of Cr23C6 and Mn23C6, as well as the vibrational entropy of Cr23C6 were computed. Further, the site preference of ternary element Fe among 4a, 8c, 32f and 48h symmetry sites in Cr23-xFexC6 was studied. It has been seen that Fe atoms would firstly occupy 4a sites and then 8c sites. The lattice constant and stability of Cr23-xFexC6 were also computed with different Fe content. In order to understand the relative stability of the transition metal carbides and nitrides, the standard formation Gibbs energies of carbides and nitrides for Cr, Mn and Fe were compared. The order of carbon and nitrogen affinities for Cr, Mn and Fe was further clarified by the comparison of the interatomic pair potentials among Cr-C, Mn-C, Fe-C, Cr-N and Mn-N. It was found that Cr-N interaction was very strong in comparison with other binary interactions above and consequently, nitrogen addition would lead to a strong decrease in the thermodynamic activity of chromium in Cr-containing alloys. This was confirmed by the investigations of thermodynamic activities of Cr in the Fe-Cr-N and Fe-Cr-C-N alloys. The activities were measured in the temperature range 973-1173 K by solid-state galvanic cell method involving CaF2 solid electrolyte under the purified N2 gas. In addition, the analysis of nitrogen content and phase relationships in the Fe-Cr-N and Fe-Cr-C-N alloys equilibrated at 1173 K were carried out by inert-gas fusion thermal conductivity method, X-ray diffraction and scanning electron microscopy technique. The experimental results show that the solubility of nitrogen in the alloys decreases with the decreasing chromium content, as well as the increasing temperature. The addition of nitrogen to the alloys was found to have a strong negative impact on the Cr activity in Fe-Cr-N and Fe-Cr-C-N systems. / QC 20100929