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No description available.

Refractory materials must, in their everyday environment, withstand high stress levels which are a result of mechanical and thermal loadings. Any failure which results from these applied stresses can have serious financial and human consequences and therefore should be avoided. One key aspect to understanding the thermal shock behaviour of refractories is the mechanical behaviour at low temperatures. In this thesis the mechanical behaviour of a small range of multiphase refractories is explored. In particular the stress-strain response and its influence on the fracture behaviour is investigated. Experiments, performed on magnesia and magnesia spinel composites, indicate that non-linear stress-strain behaviour accompanied by permanent deformation upon unloading is a result of the release of microscale residual stresses by microcracking. A micromechanical constitutive model combining these features was developed using linear elastic composite theory and isotropic continuum damage mechanics. This non-linear stress-strain behaviour also gives rise to increasing toughness as crack propagation occurs. This increase in toughness results from an expansion which occurs when microscale residual stresses in front of the crack tip are relaxed by microcracking. A micromechanical model has been developed based upon the specifically developed constitutive model and previous work on transformation toughening. These models are capable not only of simulating experimental results, but can also indicate the microstructures which are most likely to exhibit extensive non-linear stress-strain behaviour and strongly rising toughness curves.

Mold behavior, mold-related quality and the columnar-to-equiaxed transition influenced by in-mold EMS were examined by performing trials at two steel companies, metallurgical examination of the billet samples and mathematical modelling. The thermal fields in the walls of billet molds (102 X 102mm and 127 X 178mm) and in the cooling water were monitored by a system of thermocouples as the EMS was switched on and off during the continuous casting of several heats. The effect of electro-magnetic stirring on mold heat extraction was found to be negligible. The mold temperatures and cooling water temperatures are strongly dependent on the mold/billet gap which is affected by dynamic distortion of the mold tube. In the case of the square mold, the time-dependent mold distortion resulted from boiling adjacent to the cold face due to low water velocity and poor water quality. In the rectangular mold, differential expansion of the wide and narrow faces of the mold led to periodic wall movement at the midface causing cycling in the mold and water temperature. Both effects completely dominated any potential influence of EMS on mold heat extraction. Cooling water velocities measured in separate experiments and the mold temperature profiles were input to a two-dimensional heat-flow model to establish mold heat-flux profiles. A steep taper of 2.6 %/m in the upper regions of the mold increased heat extraction compared to previously published heat-flux data in 0.8 %/m tapered-molds. However, due to the periodic wall movement in the rectangular mold, the heat flux declines to lower values periodically. The calculated heat flux profiles were employed in a one-dimensional transient heat flow model to predict superheat removal from the liquid pool under a variety of assumed fluid flow conditions. The major heat flow effect of EMS was inferred to be one of increasing the convective heat flow at the solidification front leading to earlier superheat extraction from the liquid steel. Solidification structures in billet samples collected during the trials were examined. The columnar-to-equiaxed transition in continuous casting takes place provided all the superheat is removed from the melt and there is a sufficient density of nuclei present in the pool. At superheats of <20° C in the tundish, high heat extraction in the mold and remelting of the mold generated nuclei facilitate the removal of the superheat well within the mold and the columnar-equiaxed transition is triggered after 10-15 mm of shell growth on both the inside and outside radius faces. At higher superheat in the tundish, the liquid pool leaves the mold with residual superheat which takes longer to remove because of the declining fluid flow. Even though all the superheat is removed lower in the machine, the columnar-equiaxed transition occurs only if dendrite debris generated in the vicinity of the mold has survived in their descent through the superheated liquid. The effect of carbon on the columnar-to-equiaxed transition appears to stem from its influence on facilitating dendrite arm remelting and the survival of the dendrite fragments till the pool reaches sub-liquidus temperature. EMS extracts more superheat by maintaining a steep temperature gradient in the thermal boundary ahead of the solidification front and achieves an earlier columnar-equiaxed transition. Electro-magnetic stirring appears not to affect either the average depth or the variation of depth of oscillation marks across a given face. However, the electro-magnetically driven flow dominates the turbulance at the meniscus due to the input stream and stabilizes a meniscus shape with the result that the oscillation marks are also of a well-defined shape unlike the unstirred billets. No influence of EMS was found on the formation of "hooks" or the fine equiaxed crystal zone near the surface. The influence of EMS on inclining the growing dendrites appears to not come into effect until about 1 mm of shell has formed. It appears that the existence of the momentum boundary layer where the velocity of the rotating steel falls to zero at the surface is the reason for the absence of the influence of EMS on the subsurface solidification. Rhomboidity and off-corner crack formation were found to depend, as reported by previous researchers, on mold distortion and its dynamic nature. The absence of any effect of EMS on these defects is due to its lack of effect on mold heat transfer and thus mold distortion. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Thesis (Ph.D.)--University of Wollongong, 2004. / Typescript. Includes bibliographical references: leaf 192-205.

Thesis (M.App.Sc.) - Carleton University, 2007. / Includes bibliographical references (p. 101-105). Also available in electronic format on the Internet.

Thesis (M.Eng.)(Mechanical)--University of Pretoria, 2005. / Title form opening screen (viewed March 20 2006). Summaries in English and Afrikaans. Includes bibliographical references.

No description available.

In the continuous casting of steels, clogging of the submerged entry nozzle has long been a persistent and costly issue. Previous modelling attempts have assumed that inclusions of different types exhibit the same degree of adhesion when colliding with the nozzle wall - an assumption not borne out by evidence in the literature. In this thesis, a dynamic clogging model is proposed which accounts for the effects of different degrees of inclusion-wall and inclusion-clog adhesion on clog formation and growth. The overall clogging model consists of several sub-models in order to account for the different physics. The melt flow and inclusion motion are modelled using an Eulerian-Lagrangian approach. The inclusion adhesion behavior is determined by the use of a stochastic model activated when an inclusion collides with a surface. A user defined sticking probability is used to determine if an inclusion sticks to a surface (Swall for wall collision or Sclog for clog collision) or instead bounces off. A macroscopic model is used to determine clog growth, where the volume of clog in a cell is tracked and used to determine when the clog grows into adjacent cells. Finally, a modified Kozeny-Carmen equation is used as a porosity model so that the presence of the clog affects and diverts the melt flow. The modified melt flow then alters subsequent inclusion deposition and clog growth. The model is used to investigate the effects of different degrees of inclusion adhesion on inclusion deposition and clog growth. Three scenarios are examined - 1) Inclusion deposition in a pilot scale nozzle, 2) Inclusion deposition in an industrial scale slide-gate controlled nozzle and 3) Clog formation and growth in a pilot scale nozzle. The deposition studies indicate that in a pilot scale nozzle, only a minority of inclusions ever collide with the nozzle (≈ 10%). In contrast, in the industrial scale nozzle there are far more inclusion collisions with the nozzle wall, ranging from 80% when the slide-gate is 20% open to 30% when the slide-gate is 100% open. Despite the differences in nozzle geometry and flow conditions, a similar effect on inclusion deposition is seen when Swall is varied. The effects of Swall can be divided into two regimes. When 0 ≥ Swall < 0.05 there is a sharp increase in the deposition ratio as Swall increases. When Swall > 0.05 there is a small and linear increase in the deposition ratio as Swall increases. This pattern is also seen in the study of clog formation and growth in a pilot scale nozzle. The effects of Swall or Sclog on clog volume can be divided into two regimes. As Swall or Sclog increases, there is a large increase in clog volume, until the sticking probability increases above 1E-2, then any further increase results in only a small increase in clog volume. In comparison to literature data the model successfully simulates the location of clog formation, the initial jump in clogging factor and the clogging factor growth rate in the later stages of clogging. However, the model underestimates the overall increase in clogging factor, resulting in a clogging factor at the end of the simulation which is half of that seen in the experiment. / Thesis / Doctor of Philosophy (PhD) / One of the ongoing challenges in the continuous casting industry is the occurrence of nozzle clogging. Over time, a buildup of material occurs within the submerged entry nozzle, called a clog. The clog leads to the partial or complete blockage of the nozzle, resulting in increased production costs. Since studying this phenomena experimentally is difficult due to the high temperature and opacity of the molten steel, modelling provides a useful alternative approach. However, previous modelling efforts regarding nozzle clogging have treated all inclusions as exhibiting the same adhesion behavior. This thesis aims to address this issue by presenting a dynamic nozzle clogging model which accounts for the effects of different degrees of inclusion adhesion. The model is used to study both inclusion deposition and clog formation. Results indicate that even a small amount of sticking probability results in a significant degree of inclusion deposition and clogging. The effect of sticking probability on clogging can be divided into two regimes, one where the clogging is very sensitive to the sticking probability and one where it is insensitive. Finally, the model was shown to run adequately even on coarser meshes (meshes with a smaller number of larger cells), indicating its utility in industrial applications, where it can be used to predict the location of clog formation and the clog growth rate.

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