In this thesis, the results of oxidation and decarburization experiments with pure iron-carbon alloys over the temperature range 800 - 950ºc, and in atmospheres of varying carbon monoxide - carbon dioxide ratio are reported. Most of the experimental work is concerned with a 1.065 weight percent carbon alloy at temperatures where austenite is the stable phase. Complex kinetic behaviour was observed for this alloy in the range 10 - 100 volume percent carbon dioxide. An analysis is given which in turn associates the kinetics with a pure diffusion controlled decarburization model and a pure surface controlled decarburization model. The latter best represents the interaction of the decarburization mechanism with the scaling process on Y - iron. Metallographic evidence is provided where possible in support of the results of the afore-mentioned analysis. Mathematical relations are introduced which attempt to describe qualitatively the kinetics of the carbon alloys in all the atmospheres employed. A statement is made as to the applicability of the model to carbon steels in general at temperatures where austenite is the stable phase and where the atmospheres employed are of various carbon monoxide - carbon dioxide ratio. / Thesis / Master of Science (MS)

Thesis (Ph. D.)--University of Hong Kong, 2001. / Includes bibliographical references.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

This is a thesis includes both experimental and modeling studies for high temperature slag/metal reaction system. / Bloating of metal droplets in emulsion is an important phenomenon in BOF steelmaking in controlling the kinetics of refining. This bloating controls the kinetics by mainly increasing the residence time (from ~¼th of a second to ~10-15 seconds) of the droplets in emulsion and the slag/metal surface (~5-6 times) area. The bloating behavior is determined by the decarburization kinetics. This work aims to develop fundamental understanding of the bloating phenomena through series of experiments and mathematical modeling to explore various factors affecting the kinetics of decarburization. An experimental study on varying the droplet carbon concentration, slag FeO concentration and basicity evidenced mixed controlled kinetics including transport of oxygen in the slag, interfacial (slag/metal) chemical reaction, nucleation and growth of CO bubbles. A mathematical model including these kinetic steps was developed. The model was able to demonstrate the partitioning of oxygen at the slag/metal interface into external (at the slag/metal interface) and internal (within droplet) decarburization in presence of the surface-active element sulfur. The model was developed using a single data set and validated for a wide range of experimental conditions. The model showed excellent agreement with experimental data for most of the reaction period but failed to predict a premature shutdown for droplets reacting with low conductivity slag. In order to understand this discrepancy, the slag ionic and electronic conductivity were varied which showed a premature shutdown of decarburization reaction with low conductivity slag and continuation of the reaction to the thermodynamic limit with high conductivity slag. A mechanism of generation of local electric field by accumulation of charge at the slag/metal interface was proposed to explain the premature shutdown of the reaction for low basicity slags. In all experiments with low conductivity slag sulfur was observed to delay the onset of internal decarburization. However, this effect was diminished or disappeared completely with high conductivity slag. This observation motivated additional experiments to study the competitive adsorption of oxygen and sulfur at the slag/metal interface both through experiments and modelling. It was shown that for low conductivity slag, sulfur poisoning inhibited reaction at the surface whereas for the high conductivity slags the faster transport of oxygen allowed oxygen to compete with sulfur for adsorption sites creating pathways for oxygen into the droplet. By including the possibility of competitive adsorption in the model it was possible to predict the behavior of high sulfur droplets in conductivity slags where the only modification to the model was to change the mass transfer coefficient as appropriate to the higher conductivity. Extension of this study to include silicon in the droplet showed significant effect on decarburization both in delaying bloating as well as increasing peak rate of decarburization. / Thesis / Candidate in Philosophy

No description available.

<p>Functionally graded materials offer a way of obtaining materials with superior properties. Decarburization has been used in other steels to create graded materials. These materials offer high strength and improved ductility when compared homogeneous materials of the same type. In this thesis, graded martensitic stainless steel was explored as a way to provide a very high strength material with medium ductility by partially decarburizing the materials. Different processing treatments were tried and the resulting materials characterized and mechanically tested to compare homogeneous and graded martensitic stainless steels. Mechanical testing demonstrated that decarburization has a positive effect on the tensile, rolling and Charpy impact properties. A model was also developed that attempted to capture the deformation behaviour of graded materials. Present data was not available to verify the models validity but the model was used to predict trends of a how different gradients affect fracture stresses and strains. These trends were used in an attempt to find optimum carbon distributions and maximize strength or ductility, as examples.</p> / Master of Materials Science and Engineering (MMatSE)

The metallurgical processes and the products developed from these processes have been the cornerstone on which our civilizations have developed and flourished. Many of the new materials that have been developed over centuries were often the result of serendipitous occurrences. Because of the importance of new materials to the improvement of society, it is necessary to accelerate the way in which new alloys and processes are designed, developed and implemented. Over the last two decades the computational side of materials science has thrived as a result of bigger and faster computers. However, the application of new computational methods to the development of new materials and structures is still in the early stages primarily because of the complexity of most metallurgical processes. One such process is the decarburization of steel. Because of the importance of the microstructure on the mechanical properties, changes in the near surface properties are affected by the loss of carbon in the alloy. The topics investigated in this thesis include a variety of alloys and microstructures that are considered to be important in the development of a unique structure necessary for a more efficient method of recovering natural gas and oil from underground reserves as well as structures for energy absorbing systems. Since both the material application and the structure are new, this research represents an ideal opportunity to combine processing, properties, microstructure and computations to accelerate the development of these new structures. Compared to other commercially available proppants which tend to fail in demanding environments, the thin-walled hollow metal proppants are regarded more promising due to the low density and high mechanical strength. The energy-absorbing composite material manufactured by embedding said spheres in the Mg/Al matrix material is optimized by improving sphere and matrix properties at each step in the process. Ultimately the mechanical strength, fracture toughness, and energy absorption are expected to achieve a factor of 2-5 higher than previously reported. Modeling makes it economically practical to assess the targeted materials' overall properties, behaviors and the mechanical responses in conjunction with stress environment, material properties, material dimensions among other variables, before a structure is built. Additionally, more advanced modeling can enable the quantitative descriptions of more complex metallurgical phenomena such as the effects of impurity elements and deformation under complex loading conditions.

M.Sc. (50/50) Research project submitted to School of Chemical and Metallurgical Engineering, Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, South Africa July 2017 / This study and the work done involves investigating the effects of different parameters on the decarburization process of high carbon ferrochromium melts to produce medium carbon ferrochrome, and takes into account the manipulation of the different parameters and thermodynamic models based on actual plant data. Process plant data was collected from a typical plant producing medium carbon ferrochrome alloys using AODs. The molten alloy was tapped from the EAF and charged into the AOD for decarburization using oxygen and nitrogen gas mixtures. The gases were blown into the converter through the bottom tuyeres. Metal and slag samples and temperature measurements were taken throughout the duration of each heat. The decarburization process was split into two main intervals namely first stage blow (where carbon content in the metal bath is between 2-8 wt. % C) and second stage blow (carbon mass% below 2 wt. %). The first and second blow stages were differentiated by the gas flow rates whereby the first stage was signified by gas flow ratio of 2:1 (O2:N2), whilst the stage blow had 1:1 ratio of oxygen and nitrogen respectively. The effect of Cr mass% on carbon activity and how it relates to rate of decarburization was investigated, and the results indicated that an increase in Cr 66.54 – 70.5 wt. % reduced carbon activity in the metal bath from 0.336 – 0.511 for the first blowing stage. For the second blowing stage, the increase in Cr mass % of 67.22 – 71.65 wt. % resulted in an increase in C activity from 0.336 – 0.57. The trend showed that an increase in chromium composition resulted in a decrease in carbon activity and the same increase in Cr mass% resulted in reduced carbon solubility. Based on the plant data, it was observed that the rate of decarburization was time dependent, that is, the longer the decarburization time interval, the better the carbon removal from the metal bath. An interesting observation was that the change in carbon mass percent from the initial composition to the final (Δ%C) decreased from 10.18 – 8.37 wt. % with the increase in Cr/C ratio from 8.37 – 10.18. This effect was attributed to the chromium affinity for carbon and the fact that an increase in chromium content in the bath was seen to reduce activity of carbon. It was also observed that the effect of the Cr/C ratio was more significant in the first stage of the blowing process compared to the second blowing stage. A mass and energy balance model was constructed for the process under study to predict composition of the metal bath at any time interval under specified plant conditions and parameters. The model was used to predict the outcome of the process by manipulating certain parameters to achieve a set target. By keeping the gas flow rates, blowing times, gas ratios and initial metal bath temperature unchanged, the effect of initial temperature on decarburization in the converter was investigated. The results showed that the carbon end point with these parameters fixed decreased with increasing initial temperature, and this was supported by literature. The partial pressure of oxygen was observed to increase with decrease in C mass % between the first and second blow stages. For the second stage blow the partial pressure changed from 5.52*10-12 – 2.1*10-10 and carbon mass % increased from 0.754 – 2.99 wt. %. A carbon mass % of 7.87 had an oxygen partial pressure of 4.51*10-13 whilst a lower carbon content of 1.53 wt. % had an oxygen partial pressure of 8.06*10-11. The CO partial pressure however increased with increase in carbon composition in the metal bath. When the oxygen flow rate increased, a corresponding increase in the carbon removed (Δ%C) was observed. For the first stage of the blowing process, an increase in oxygen flow rate from 388.67 – 666.5Nm3 resulted in an increase in carbon removed from 5.06 – 7.28 wt. %. The second blowing stage had lower oxygen flow rates because of the carbon levels remaining in the metal bath were around +/- 2 wt. %. In this stage oxygen flow rates increased from 125 – 286.67 Nm3 and carbon removed (Δ%C) from 0.16 – 2.093 wt. %. The slag showed that an increase in basicity resulted in an increase in Cr2O3 in the slag. As the basicity increased from 0.478 – 1.281, this resulted in an increase in Cr2O3 increase from 0.26 – 0.68. Nitrogen solubility in the metal bath was investigated and it was observed that it increased with increasing Cr mass %. The increase in nitrogen solubility with increasing Cr mass % was independent of the nitrogen partial pressures. / MT2018

A produção mundial de aço inoxidável nos últimos anos teve o maior crescimento dentre os principais metais usados na indústria. É notável a importância dos processos industriais dos aços inoxidáveis devido ao contínuo desenvolvimento desses materiais, em busca de uma melhor qualidade, com menores custos de produção e de forma sustentável. Uma das formas de obtenção do aço inoxidável é através do processo VOD, que consiste em remover o carbono do aço pela a injeção de oxigênio e reduzindo a pressão parcial do gás (CO) através da aplicação de vácuo. A primeira etapa desse processo é o sopro de oxigênio, onde é importante controlar a pressão do sistema de maneira a promover a menor oxidação do cromo sem ocorrência de transbordamentos de aço e excesso de \"splash\". Atualmente, existem diversas práticas de operação no VOD em que se tem uma redução contínua da pressão de vácuo em conjunto com alterações de outras variáveis durante o sopro de oxigênio, de forma a controlar a reação de descarburação sem perder em excesso, o cromo. O objetivo desse trabalho foi encontrar o tempo de sopro de oxigênio em função do carbono inicial para cada corrida, a partir do qual seja possível reduzir a pressão de vácuo sem causar danos ao processo. Isso porque a geometria da panela e o teor de carbono inicial empregados na Villares Metals S.A. possuem características distintas em comparação com outras plantas, que faz com que seu processo de descarburação seja singular. Nos resultados obtidos é possível confirmar que, operando com pressões de vácuo mais baixas durante o sopro de oxigênio, pode-se evitar perdas desnecessárias de cromo. Além disso, a relação encontrada entre o tempo de sopro e o carbono inicial através de uma equação cinética, pode ser utilizada na determinação do tempo aproximado para redução da pressão de vácuo para cada corrida. Assim se terá uma operação de descarburação mais eficiente em relação à oxidação dos elementos de liga e redução do consumo de oxigênio, de redutores e do tempo de processo, com segurança e economia. / The stainless steel melt shop production growth in the last years was the biggest comparing to the other important metals in the industry. It is noticeable the importance of the stainless industrial process due to its continuous development in order to seek better quality with lower costs and sustainable methods. One of the possibilities to produce stainless is by VOD process, which consists to remove the carbon content by oxygen injection and reducing the partial pressure of (CO) gas through vacuum application. Oxygen blowing is the first step of this process, where it is important to control the chamber pressure in order to avoid the chromium oxidation without steel overflow and excessive splash during process. Currently, different operation practices in the VOD have been applied reducing continuously the vacuum pressure simultaneously with other parameters during oxygen blowing, in the interest of controlling the decarburization reaction avoiding the loss of chromium. The objective of this study was to find the oxygen blowing time according to the initial carbon content for each heat, from which it is possible to reduce the vacuum pressure avoiding damages to the process. The reason of this is due to the ladle geometry and the initial carbon content applied on the Villares Metals process have special characteristics comparing to other plants, which make its process singular. On the results achieved, it is possible to confirm that, operating in lower vacuum pressure during oxygen blowing, it is possible to avoid unnecessary chromium losses. Moreover, the ratio found out between the blowing time and the initial carbon content through a kinetic equation, can be applied to determine the estimated time to reduce the vacuum pressure for each heat. Therefore, the decarburization process will be more efficient according to the alloy oxidation and save the oxygen consumption, deoxidizers and process time, with safe and reducing cost.

No description available.