&Ccedil / EMTAS Steel Plant produces alloy steels used principally by the automotive industry. Demands of customers for cleaner steel with &ldquo / fewer, smaller and homogenously distributed inclusions&rdquo / are getting tighter with time. The tight demands of the customers are checked by ultrasonic testing and faulty regions are cut off and scrapped which results in production losses in steel plants. Decreasing production losses due to scrapping by producing cleaner steels is therefore a major concern. The objective of this study is to investigate factors affecting the cleanliness of the steels produced by &Ccedil / EMTAS. With this objective, the steelmaking practice used was thoroughly investigated and the possible origins of inclusions in the final product was tried to be found. Based on the findings, process parameters like temperature, time, quantities of additives to steel, intensity of stirring etc. were varied with the objective of determining the optimum production practice.

Extensive research in recent years has failed to develop any essentially new method of large scale tungsten production. A new tungsten powder production technique from calcium tungstate (CaWO4) has recently been reported. In this thesis, this technique was further explored from the aspects of electrochemical reduction mechanism and kinetics, applicability to scheelite concentrates and industrial production. Cyclic voltammetry, constant potential and constant current electrochemical reduction tests were performed to determine the reversible cell potential. Analyses of the experimental results revealed that at least 2.2 V was required to compensate the potentials for the accompanying cell reaction and the electrode polarizations. A cell reaction was proposed by associating the experimental results and the Gibbs Energy changes of the possible reactions. An experiment (mixture) design was created to optimize the process parameters of the electrochemical reduction of CaWO4 to W in molten CaCl2-NaCl eutectic mixture. Temperature, applied voltage and the length of Kanthal wire winding of the CaWO4 pellets were selected as the process parameters and allowed to vary between the predetermined minimum and maximum values. The rates of the electrochemical reductions were interpreted from the variations of current and total charge vs. time graphs under different conditions. The analysis pointed out 640oC and 2.81 V from the created mixture design for the fastest reduction and it was seen that the effect of Kanthal wire winding on the output current was less pronounced when compared to the other two parameters. Another set of experiments was performed by full factorial design to investigate the cleaning procedure needed to remove calcium containing byproducts after electrochemical reduction experiments. Three levels were determined prior to the experiments for the selected three parameters / temperature, acid concentration and exposure time. Main effect and interaction graphs for calcium percent as a function of process parameters were plotted. Calcium contents of the samples were determined by XRF measurements. A 300 g/day capacity tungsten production line was manufactured to take the process one step closer to industrialization. Problems at larger scale were addressed as incomplete reduction, oxidation of graphite and corrosion of cathode materials. After careful research, AISI 316 Ti steel was found to impart sufficient resistance to highly corrosive environment. Oxidation of graphite anode inside the cell was lowered to acceptable levels by continuous nitrogen flow. Metallic tungsten powder was obtained from rich and flotation concentrates of Uludag Etibank Volfram Plant (closed in 1989) together with mainly iron. It was seen that tungsten and iron do not make compounds at the temperatures used for reduction (600-750oC). A basic diffusion model in the electrolyte was developed to better understand the decrease in current values and incomplete reduction encountered during large scale production. The model was used to simulate the recorded current vs. time graphs of selected experiments.

The aim of this research was mainly to investigate desulfurization behaviour of ladle furnace slags of a low-sulfur steel. First, the change of the amount of unstable oxides (FetO and MnO) in slags was studied from the converter till the end of ladle furnace treatment for different steel quality groups. It was found that the change in the amount of unstable oxides could be examined best with low-C steel qualities. Then, the change of activity of iron oxide was investigated for a member of low-C steel quality group, namely 4937K, a low-S steel. The relation between the degree of desulfurization at the ladle furnace, % DeS (measured), and the change in activity of iron oxide, % Decrease in aFetO, was examined first using an empirical expression obtained from literature. Then, this relationship was studied using steel oxygen activity measurements conducted at the ladle furnace. With this method, a regression formula was proposed for 4937K slags relating activity coefficient of iron oxide, gama(FetO), with the major slag components. The results obtained from steel oxygen activities were found to be representative for 4937K steel-slag system showing similar behaviour with the ones from the empirical expression obtained from literature. Similar results were found using Temkin equation and the polymeric anion model. In relation to formation of less harmful inclusions to suppress adverse effects of sulfur, morphological studies of some steels produced in ERDEMiR plant were performed. D-type globular inclusions with severity values of 1-2 were observed.

In this study ferroboron (Fe - 14 at %B) was synthesized in crystalline form (Fe + Fe2B) via electrodeoxidation. For this purpose, Fe2O3 and H3BO3 were mixed in suitable proportions via spex mill. The powder was cold pressed and sintered at 900 &deg / C yielding a two phase structure Fe3BO6 and Fe2O3. The sintered pellets were electro-deoxidized in CaCl2 by applying 3.1 Volts at 850&deg / C for 12 hours. This yielded Fe and Fe2B in proportions slightly deviating from the target composition. The chemical pathway of reduction is inspected by the help of the available thermodynamic data and the x-ray characterization of partially reduced samples. CaO and the formation of Ca3B2O6 were found to be effective in the mid-steps of this electrodeoxidation process.

Electrochemical reductions of porous SiO2 pellets and bulk SiO2 plate were investigated in molten CaCl2 and/or CaCl2-NaCl salt mixture. The study focused on effects of temperature, particle size of the starting material, electrolyte composition and cathode design on the reduction rate. The behavior of the cathode contacting materials was also examined. Moreover, cyclic voltammetry study was conducted to investigate the mechanism of the electrochemical reaction. Mainly, XRD analysis and SEM examinations were used for characterizations. The rates of electrochemical reduction were interpreted from the variations of current and accumulative electrical charge that passed through the cell as a function of time under different conditions. The results showed that reduction rate of SiO2 increased slightly with increasing temperature or decreasing the particle size of SiO2 powder. Higher reduction rate was obtained when porous pellet was replaced by bulk SiO2 plate. Use of Kanthal wire mesh around the SiO2 cathode increased but addition of NaCl to the electrolyte decreased the reduction rate. X-ray diffraction results confirmed the reduction of SiO2 to Si in both CaCl2 salt and CaCl2-NaCl salt mixture. However, silicon produced at the cathode was contaminated by the nickel and stainless steel plates which were used as the cathode contacting materials. Microstructures and compositions of the reduced pellets were used to infer that electrochemical reduction of SiO2 in molten salts may become a method to produce solar grade silicon (SOG-Si). In addition, overall reduction potential of SiO2 pellet against the graphite anode and the potential of the cathode reaction at 750&deg / C in molten CaCl2-NaCl salt mixture were determined as 2.3 V (at 1.19 A current) and 0.47 V, respectively by cyclic voltammetry.

The purpose of this study was to synthesize calcium phosphates doped with titanium and fluoride ions in different combinations. Pure and doped calcium phosphates were synthesized by a precipitation method. The synthesized materials were sintered at 1100&ordm / C and 1300&ordm / C for 1h. The ceramics were characterized by density measurements to determine the effect of sintering temperature. Presence of phases and bonds were characterized by XRD diffraction and FTIR spectroscopy. Grain sizes of the samples were obtained by SEM. Microhardness test was applied on the samples to determine the mechanical properties of the samples. It was observed that Ti4+ addition decreased the density of samples significantly at 1100&deg / C, whereas increasing the sintering temperature to 1300&deg / C caused an increase. Increasing the F- ion amount increased the densification at 1100&deg / C when molar ratios were 1.0, 1.25, 1.50 and decreased the density at 1300&deg / C when Ca /P molar ratios were 1.0, 1.25, 1.67 and 2.0. Ti4+ and F- co-doped samples showed variety in their density behaviour after the sintering at 1100&ordm / C and 1300&ordm / C. The XRD analyses demonstrated that for Ca to P ratio 1 at 1100&deg / C, &beta / -CPP phase, when sintering temperature was raised to 1300&deg / C, as a second phase of &beta / -CPP and &alpha / -TCP observed. Increasing the molar ratio to 1.25 and 1.50 demonstrated &beta / -TCP and/or &beta / -CPP and &beta / -TCP/ HA at 1100&deg / C and &beta / -TCP and/or &beta / -CPP, &alpha / -TCP, TiO2 and HA, &alpha / -TCP, TiO2 phases at 1300&deg / C, respectively. In higher Ca/P molar ratios of 1.67 and 2.0, HA, &beta / -TCP, &alpha / -TCP, CaO, TiO2, CaTiO3 and HA, CaO, &alpha / -TCP, CaTiO3 phases were determined. Increasing the sintering temperature to 1300&deg / C resulted in transformation to &alpha / -TCP. In FTIR spectroscopy analysis, when the samples with molar ratio of 1, 1.25, 1.50, 1.67 and 2.0, sintered at 1100&deg / C, the characteristic bands of &beta / -CPP, OCP/&beta / -TCP, &beta / -TCP/HA, HA and HA were observed, respectively. With increasing the sintering temperature to 1300&deg / C, samples with molar ratio of 1.0 and 1.25 revealed additional secondary characteristic peaks of HA and &beta / -TCP. SEM images revealed that sintering temperature and ion amounts of dopants had significant effect on grain sizes of the samples. The grain sizes were generally increased when sintering temperature rose from 1100&deg / C to 1300&deg / C. The &mu / -hardness test demonstrated that Ti4+ and F- ions in large amounts had positive effect on the mechanical properties at the sintering temperatures of 1100&deg / C and 1300&deg / C

In this study the effects of different austempering times and temperatures on impact toughness, hardness and fracture toughness properties of 35NiCrMoV12.5 gun barrel steel are investigated. 300 &deg / C, 325 &deg / C and 350 &deg / C were chosen as austempering temperatures. Isothermal holding times at these temperatures were chosen as 1 minute, 10 minutes, 1 hour and 10 hours. It was found that, 350 &deg / C being an exception, austempering temperature and impact toughness has an inverse relationship and impact toughness increases as isothermal holding time increases. However this behavior is valid until some point. Prolonged transformation times causes toughness to decrease. Hardness measurements revealed that, as isothermal holding time increases, hardness decreases. In order to compare the mechanical properties obtained by austempering with that of conventional cooling and tempering, 400 &deg / C was chosen as the tempering temperature and applied to both charpy impact and fracture toughness specimens. It was found that conventional cooling and tempering produced tougher structures. Size of the fracture toughness specimens might have caused an undesired situation such as incomplete transformation to bainite. Optical and scanning electron microscopy was used in order to analyze the microstructures obtained after each treatment. It was observed that the majority of the morphologies occurred is lower bainite. On the other hand, martensitic structures were observed almost at every temperature.

In this thesis the finite element computer code QUEANA simulating the quenching of axisymetric parts and determining the residual stress state was improved by adding pre- and post-processors. The code was further verified by additional numerical experiments and comparison of the results with commercial software &ldquo / MARC&rdquo / . The possible applications of this code are optimization of industrial quenching processes by controlling the evolution of internal stresses and dimensional changes.

Due to the environmental issues related with fuel consumption and additionally passenger safety, aluminum space frame chassis is promising a big opportunity to design a lightweight structure with a high stiffness. Despite the lower stiffness and strength of aluminum in comparison to the conventional steel chassis, it can be compensated with changing thickness and design of structure by space frame geometry In this study, instead of using steel for automobile chassis, main goal is producing a space frame structure with using aluminum in an extrusion and sand casting processes and improve the stiffness. Chassis is designed according to calculations for moment of inertia, torsional and bending stiffness and in sufficient structural stiffness which can compete with steel chassis. Static finite element analysis was carried out to understand the chassis bending, torsional stiffness and fatigue behaviors. For frontal collisions, dynamic finite element analysis was also done to determine increases in the energy absorbance, specific energy absorbance and peak force for passenger safety. Aluminum profiles were produced by hot extrusion and joined with sand casting parts by TIG welding to manufacture a space frame structure. For main chassis profile, 6063 series of aluminum alloy was selected due to availability for extrusion process, weldability and having sufficient tensile strength and percent elongation and treatment response. Three point bending test was carried out to determine flexural strength. Moment of inertia calculations were done. Some parts such as side frame and shock absorber tower were produced by sand casting method. A similar composition to Silafont &ndash / 36 aluminum alloy was selected because of its high fluidity and good mechanical properties / despite it is a die cast alloy. Tensile, hardness and Charpy impact test were conducted to determine the mechanical characteristics of Silafont - 36 sand cast alloy. In addition to microstructure features and thermal analysis were also carried out to achieve sufficient alloy properties. Heat affected z one was investigated by hardness and tensile test to determine the mechanical properties change after welding process. In this space frame development study, A, B and C pillar parts were produced by Al &ndash / Si sand casting and T6 heat treatment then welded together by TIG welding and finally assembled on the bottom chassis frame produced by using 6063 extrudes welded by 4000 series electrodes. The space frame chassis was studied by also computer simulation to test and see critical points which must be modified during manufacturing. Besides the experimental and theoretical studies, space frame was also produced at the same time. According to the experimental results, the feasibility of the production of lightweight and solid chassis structure was achieved.

In the scope of this study, TiNi foams with porosities in the range of 39-64 vol% were processed from prealloyed powders by Mg space holder technique. Porous TiNi alloys displayed homogeneously distributed spherical pores with interconnections, which is suitable for bone ingrowth. Porous Ti-50.8 at%Ni alloys were processed by sintering at 1200 &deg / C for 2 h to analyze the microstructure as well as mechanical behavior. SEM, TEM and XRD studies were conducted for the characterization of microstructure and phase analyses in addition to the mechanical characterization performed by monotonic and superelasticity compression tests as well as compressive fatigue tests. It was observed that stress required to trigger martensitic transformation was decreased via increasing porosity. The monotonic compression test results also indicated that altering the porosity content of TiNi foams leads to different monotonic compression behaviors. It was observed that the foams display more bulk deformation like behavior as a composite structure composed of TiNi and macropores when the porosity content was low. As the porosity content has increased, the struts became more effective and deformation proceeds by the collapse of favorable struts. On the other hand, cyclic superelasticity tests results indicated that maximum achieved and recovered strain values at the end of fifth cycle increase while the fraction of strain recovered at the end of fifth cycle decreases with decreasing porosity content. Furthermore, the fatigue lives of the processed foams were observed to vary within a band which has a width decreasing with decreasing &sigma / max / &sigma / y yielding an endurance limit ranging in between 26-89 MPa or 0.5-0.6 &sigma / y. Fractography studies on the failed foams after fatigue testing revealed that the failure occurs by the coalescence of micro-cracks initiated from pore walls leading to macro-cracks aligned at 45o with respect to the loading axis. In addition to the mentioned characterization studies, the effects of sintering temperature and time on TiNi foams with 58 vol% porosity as well as heat treatment on the microstructure and the mechanical behavior of TiNi foams with 49 vol% porosity were analyzed with SEM and compression tests. Aging of TiNi foams with 49 vol% porosity at 450 &deg / C for 1.5 h has shown that the presence of Ti3Ni4 precipitates improve the superelastic response.