A review of the literature pertaining to the mechanical properties, solidification and segregation effects in nodular cast iron has been made. A series of investigations concerning the influence of microsegregation on mechanical properties of :pearlitic, ferritic and austenitic nodular cast iron have then been reported. The influence of section size on the tensile and impact properties of cornmercial purity and refined ferritic nodular cast iron has been studied. It has been shown. that an increase in section caused a decrease in impact transition temperature of the commercial purity material without greatly affecting the impact transition temperature of the purer material. This effect has been related to increased amounts of segregation effects such as cell boundary carbides in heavier sections of the commercial purity material. Microsegregation studies on the materials used in this thesis have been carried out using an electron probe microanalyser. This technique has shown that concentrations of chromium and manganese and depletions of nickel and silicon occurred at eutectic cell boundaries in nodular cast iron and were often associated with brittle carbides in these areas. These effects have been shown to be more prevalent in heavier sections. The nature of segregation during the solidification of nodular cast iron has been studied by quenching samples of nodular iron during the solidification process. Micro-analysis of such samples has shown that segregation of manganese and chromium occurs by a gradual build-up of these elements at the solid/liquid interface. The microstructures of the quenched specimens revealed carbide filaments connecting graphite nodules and areas of quenched liquid. These filaments have been used as evidence for a revised hypothesis for the solidification of nodular cast iron by a liquid diffusion mechanism. A similar series of experiments has been carried out on two high nickel austenitic irons containing 0.5 per cent manganese and 4 per cent manganese respectively. In both these materials a decrease in elongation was experienced with increasing section. This effect was more drastic in the 4 per cent manganese material which also contained much greater amounts of cell boundary carbide in heavy sections. Micro-analysis of samples of the 4 per cent manganese material quenched during solidification revealed that manganese concentrated in the liquid and that nickel concentrated in the solid during solidification. No segregation of silicon occurred in this material. Carbide filaments appeared in the microstructures of these specimens. A discussion of all the above effects in terms of current concepts is included.

Through thickness microstructure and mechanical properties of defect-free electron beam welded 20 mm thick AISI 316L austenitic stainless steel has been investigated as a function of beam power. The weld microstructure is characterised by a columnar and equiaxed dendritic ferrite in an austenite matrix. The dendritic structure was finer at the bottom of the weld zone. A microstructural boundary called “Parting” was seen along the weld centreline. Tensile tests, using a digital image correlation technique, demonstrated that the highest strain was concentrated in the fusion zone. The bottom section of the weld metal exhibited a yield strength of about 14 – 52 MPa higher than the top section. The ultimate tensile strength in the bottom of the weld was also about 4% higher than the top. The final fracture was detected in the parting region. It was observed from the EBSD scan that the grains in the weld zone contained a weak orientation and showed a high Schmid factor intensity with interception between some strong grains and soft grains at the weld centreline boundary. This explains the high weld ductility and the failure to happen in the parting region. Dissimilar welding of 20 mm thick AISI 316L stainless steel to TiAl6V4 using electron beam welding process was carried out. A successful joint was possible through using of copper sheet with 1.5 mm thick as a transition layer between the two metals. Preheating the weld samples was performed to lower the heat input and reduce the residual stresses. A double pass welding technique was applied to achieve full weld penetration. The weld microstructure was studied by SEM, EDS and XRD. The sensitivity of the microstructure to cracking was evaluated by a microhardness test of the weld cross-section. The weld region near the stainless steel contained Fe and Cu in solid solution. While the weld area near the titanium alloy characterised by the copper solid solution with Cu-Ti and Cu-Fe- Ti intermetallic phases. Ti-Fe intermetallic compounds was suppressed and replaced by relatively soft Cu-Ti intermetallics, which significantly improved the joint toughness. However, the formation of Ti-Cu at the Ti/Cu interface makes this region still susceptible to cracking.

This thesis investigates the combustion of fuel compositions representative of those produced by the integrated steelmaking process. As organisations strive for improvements in utilisation efficiency with increasingly complex technologies, more detailed understanding is required to accurately simulate combustion of the potentially weak and dilute fuels, and thereby aid design processes. Dynamic fuel properties have been characterised through experimentation, in addition to a comparison of numerically simulated results obtained from chemical kinetics. The parameters identified to investigate fuel behaviour were laminar burning velocity and Markstein Length, and characterised with regard to operational instability in practical combustion systems. The design and construction of a suitable experimental rig is detailed, as required to facilitate the accurate determination of burning velocities by quantifying the outward propagation of spherical flames. A regressive analytical technique was developed based on previous studies, nonlinearly relating propagation to change in stretch rate. The developed solution was benchmarked against analogous studies in literature, and ensured experimental performance was accurate and repeatable for the well documented combustion of methane. Steelworks gases were tested to attain representative burning velocities, with significant attention paid to the change resulting from fluctuation in blast furnace gas H2 fraction. The study characterised the observed sensitivity to change in flame speed and discussed the implications with regard to practical combustion systems. Several methods of reducing the measured fluctuation are subsequently quantified, including change in ambient condition, and relative humidity. Non-monotonic behaviour was observed for the latter effect, with a suggested trade-off between a chemically catalytic influence on intermediate species, and lowering of flame temperature. Consequently this suggested water addition could be an effective mechanism for the reduction of H2 induced flame speed variation for blast furnace gas, and influence other synthesised fuels comprising large quantities of CO, including BOS gas. Additional steelworks gases were blended in different ratios to assess dynamic combustive properties relating to fuel flexibility, and the effectiveness of minimising fluctuation in combustion behaviour.

Les émissions élevées de CO2 de la filière industrielle de fabrication des tuyaux en fonte résultent pour l'essentiel de l'emploi massif de carbone fossile, charbon et coke, comme combustible et agent réducteur dans les procédés. Substituer du carbone issu de biomasse au carbone fossile en vue de réduire ces émissions de CO2, avec application au cas d'une usine lorraine, a été l'idée de départ de ce travail. Différents types de biomasse ont été envisagés. Le bois et la paille seraient localement disponibles en quantité suffisante pour autoriser une substitution partielle de 20 % du coke. Cette biomasse doit cependant être traitée thermiquement (séchage et pyrolyse) avant de satisfaire les spécifications techniques imposées par les procédés métallurgiques (agglomération, cokerie, haut fourneau). Six scénarios d'usage de la biomasse au haut fourneau ou à l'agglomération ont été sélectionnés. Pour les évaluer sur la base de leur mérite environnemental, nous avons réalisé une analyse de cycle de vie comparative en suivant une approche du type berceau à la porte. Point original de notre étude, l'inventaire de cycle de vie est issu de modèles systémiques des principaux procédés, spécifiquement développés et intégrés à un logiciel de diagrammes de flux. Grâce à ces modèles et notamment celui du haut fourneau, on a pu déterminer les taux de remplacement du coke par trois types de biomasse (charbon de bois en blocs, charbon de bois pulvérisé, bois torréfié pulvérisé) et prédire les modifications de fonctionnement qu'entrainerait l'injection de biomasse. Les résultats de l'analyse de cycle de vie montrent que l'injection de bois, carbonisé ou torréfié, sous forme pulvérisée aux tuyères du haut fourneau serait une solution intéressante d'un point de vue environnemental : baisse des émissions de gaz à effet de serre (-15 %) et des impacts sur la santé humaine et sur les écosystèmes. A l'heure actuelle cependant, le coût économique d'une telle solution demeure excessif / High CO2 emissions from the industrial production of pig iron pipes are mainly due to the massive use of fossil carbon, coal and coke, as a fuel and/or a reducing agent in the processes. The use of biogenic carbon as a substitute for fossil carbon in order to reduce the CO2 emissions, in the case of an existing plant in Lorraine, was the starting point of our work. Different types of biomass were investigated. Local resources for wood and straw would be sufficient for a partial substitution for 20% of the coke. However, this kind of biomass should first undergo a thermal pretreatment step (drying and pyrolysis) to fit the technical requirements for the metallurgical processes (sinter plant, coke oven and blast furnace). Four scenarios of using biomass in the blast furnace or at the sinter plant were selected. To evaluate their environmental performances, we produced a comparative life cycle assessment, using a cradle-to-gate approach. An original feature of our study lies in the use of systems modeling for the determination of the life cycle inventories of the main processes, which were modeled using a flowsheeting software. From these models, particularly the model of the blast furnace, we could evaluate the replacement ratio of three biomass types (charcoal lumps, charcoal fines and torrefied wood fines) for the coke and predict the influence of biomass injection in the blast furnace on its operation. The results of the life cycle assessment show that the injection of pulverized, torrefied or carbonized, wood through the tuyeres of the blast furnace would be an attracting solution from an environmental point of view. It would reduce greenhouse gas emissions by 15%, as well as mitigate the impacts on human health and ecosystems. Yet, the costs associated to such a solution are currently too high

L’amélioration de la solidification de lingots d’acier industriels de plusieurs tonnes demeure un défi scientifique. Lors de cette étape cruciale, des hétérogénéités chimiques – à l’échelle du mètre – peuvent se développer. Le mouvement des grains solides qui se forment et se déplacent dans le bain liquide est un des leviers d’action sur la macroségrégation. L’inoculation permet d’agir sur le nombre, la taille et la morphologie de ces grains via des ajouts dans le métal liquide. Dans ce travail, les nuances 42CrMo4 et 34Cr4 ont été inoculées pour plusieurs formats de lingots. Des techniques de caractérisation (2D et 3D) ont été développées pour la comparaison des structures de solidification dendritiques avant et après inoculation. Une première série de lingotins (50 g) a montré l’effet affinant de poudres à bonne cohérence cristallographique avec la ferrite et l’effet grossissant d’un ferroalliage de cérium. Après une étude à plus grande échelle (8 kg) pour les meilleurs candidats (poudres de TiN, CeO2, Si3N4 et ferroalliage de cérium), un ajout de cérium a été fait dans un lingot industriel de 6,2 t. Le cérium, responsable de la croissance des grains équiaxes pour l’ensemble des lingots caractérisés. Il agit probablement sur les tensions interfaciales solide/liquide et moule/liquide tout en remplaçant les sites de germination nativement présents dans le métal liquide par des inclusions au cérium qui ne servent pas de sites de germination / Improving the solidification of large industrial steel ingots remains a scientific challenge. During casting chemical heterogeneities (macrosegregation), sometimes in the scale of meters, can arise. Solid grain motion is one phenomena responsible for macrosegregation. Inoculation allows the number, size and morphology of these grains to be modified through additions to the liquid metal. In this work 42CrMo4 and 34Cr4 grade steel ingots of several sizes were modified with potential inoculants. Characterization techniques (2D and 3D) were developed to compare dendritic solidification structures before and after inoculation. The first series of small ingots (50 g) showed grain refinement for powder additions with a low lattice misfit between the inoculant and solidifying melt, and a coarsening effect when ferrocerium was added. The best inoculant candidates (TiN, CeO2, Si3N4 powders and cerium ferroalloy) were tested in medium sized ingots, after which the cerium addition was tested with a 6,2 t industrial ingot. Cerium is responsible for coarsening the grain size in all sizes of cast ingots investigated. This is likely due to a modification of the solid/liquid and liquid/mold interfacial energy along with the replacement of existing sites with cerium inclusions which are not active during nucleation