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Showing papers in "Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science in 2000"


Journal ArticleDOI
TL;DR: In this article, a modified quasichemical model for short-range ordering in liquid and solid solutions is presented, where the energy of pair formation is expanded in terms of the pair fractions rather than the component fractions, and coordination numbers are permitted to vary with composition.
Abstract: Further improvements to the modified quasichemical model in the pair approximation for short-range ordering in liquid and solid solutions are presented. The energy of pair formation is expanded in terms of the pair fractions rather than the component fractions, and coordination numbers are permitted to vary with composition. A formalism is introduced whereby the quasichemical equations are shown to be equivalent to the equations of an associate model if the pairs are formally treated as fractional associates. The model is applied to the liquid phase in a new optimization of the KCl-MgCl2 system.

557 citations


Journal ArticleDOI
TL;DR: In this article, the role of hydrogen traps in steel is described. And the trap model corresponds to Oriani's trap model in equilibrium, which is slightly different from the Trap model of McNabb and Foster.
Abstract: This article describes the role of hydrogen trapping in steel. Trapping increases the solubility of hydrogen and decreases the diffusivity. Traps are characterized by their nature, i.e., reversible or irreversible, saturable or unsaturable. A dislocation core is a saturable, reversible trap, while voids and crack are unsaturable, reversible traps. A trap model based on saturable, reversible traps is developed, which is slightly different from the trap model of McNabb and Foster. In equilibrium, the trap model corresponds to Oriani’s trap model. Kumnick and Johnson found experimentally that the trap density increases as the plastic strain increases. Using their trap data, it is shown that equilibrium between hydrogen in lattice sites and trap sites can be assumed when strain rates are used as in standard tensile tests.

218 citations


Journal ArticleDOI
TL;DR: In this paper, three modes of inclusion removal from molten steel in the tundish, i.e., flotation to the free surface, collision and coalescence of inclusions to form larger ones, and adhesion to the lining solid surfaces, are taken into account.
Abstract: Three-dimensional fluid flow in continuous casting tundishes with and without flow control devices is first studied. The results indicate that flow control devices are effective to control the strong stirring energy within the inlet zone, and other zones are with much uniform streamline. By dividing tundish into two zones with different inclusion removal mechanisms the inclusion removal is calculated. Three modes of inclusion removal from molten steel in the tundish, i.e., flotation to the free surface, collision and coalescence of inclusions to form larger ones, and adhesion to the lining solid surfaces, are taken into account. The Brownian collision, Stokes collision, and turbulent collision are examined and discussed. The suitable coagulation coefficient is discussed, and a value of 0.18 is derived. Calculation results indicate that, besides flotation, collision of inclusion and adhesion to the lining solid surfaces are also important ways for inclusion removal from molten steel in tundish especially for the smaller inclusions. The flotation removal holds 49.5 pct, and the adhesion removal holds 29.5 pct for the tundish with flow control devices; the collision effect is reflected in improving flotation and adhesion. Finally, industrial experiment data are used to verify the inclusion removal model.

173 citations


Journal ArticleDOI
TL;DR: In this article, a new model for critical fracture stress given from the measured critical strain has been proposed to estimate the cracking condition in continuously cast steels, which can take into account the brittle temperature range and strain rate.
Abstract: To estimate the cracking condition in continuously cast steels, a new model for critical fracture stress given from the measured critical strain has been proposed, which can take into account the brittle temperature range and strain rate. The effects of brittle temperature range and strain rate on critical strain for internal crack formation were analyzed. When the brittle temperature range and strain rate were increased, the possibility of internal crack formation increased due to the decreasing critical strain. To describe the thermomechanical property model of the mushy zone between zero strength temperature (ZST) and zero ductility temperature (ZDT), the yield criterion for porous metals, which can take into account δ/γ transformation, was used. Using the fitting equation for the measured critical strain and the microsegregation analysis, the thermomechanical behavior of the mushy zone could be successfully described by the proposed model, which incorporates the effects of microsegregation of solute elements and δ/γ transformation on hot tear during solidification at the given range of steel compositions and strain rates. A cracking criterion based on the difference of deformation energy in the brittle temperature range is proposed to explain the cracking phenomenon of whole carbon range.

155 citations


Journal ArticleDOI
TL;DR: In this article, the peritectic reaction is characterized by the formation of the γ-austenite phase at the junction of the liquid and the grain boundary of δ-ferrite crystals and subsequent propagation of the three-phase point, liquid/γ/δ, along the liquid/α boundary.
Abstract: In situ dynamic observation of the progress of a peritectic reaction and transformation of Fe-(0.14 pct C)- and Fe-(0.42 pct C)-peritectic Fe-C alloys has been successfully made with a combination of a confocal scanning laser microscope and an infrared image furnace. The peritectic reaction is characterized by the formation of the γ-austenite phase at the junction of the liquid and the grain boundary of δ-ferrite crystals and subsequent propagation of the three-phase point, liquid/γ/δ, along the liquid/δ boundary, whereas the peritectic transformation occurs by the thickening of the intervening γ toward both the liquid side and the δ side. The rates of the peritectic reaction for the two peritectic alloys are found to be much faster than the rate that would be controlled by carbon diffusion, suggesting that either massive transformation to γ or solidification as γ controls the rate. This is also the case for the Fe-0.14%C transformation in the hypoperitectic alloy. However, the rate of the peritectic transformation in the Fe-0.42%C alloy is determined by carbon diffusion.

143 citations


Journal ArticleDOI
TL;DR: In this paper, a modified Rayleigh criterion has been developed which accounts for directional permeability and orientation of the growth front relative to the gravity vector, and the results indicate a dependency of freckling on growth front angle likely related to the anisotropic in permeability.
Abstract: The evaluation of a numerical criterion to provide quantitative insight on freckling conditions is critical to the successful manufacture of large superalloy castings. Of the criteria reported in the literature, those based on the Rayleigh number seem best suited to predict the onset of freckle formation. However, in their current form, these criteria cannot explain why freckles develop predominantly at the surface of single crystal (SX) castings and at midradius in VAR/ESR ingots. An experimental Bridgman-type furnace has been built to directionally solidify freckle-prone superalloys, CMSX-11B, RENE88, NIM80A, WASPALOY, MAR-M247, and a variation of IN718 with high silicon content, at various angles to the vertical. Under typical industrial solidification conditions (thermal gradient between 500 and 4000 K m−1 (5

141 citations


Journal ArticleDOI
TL;DR: In this article, it is suggested that interfacial chemical reactions induce Marangoni and natural convection at the slag-metal interface, which gives rise to interfacial waves due to a Kelvin-Helmholtz instability.
Abstract: The equilibrium interfacial energy between a liquid iron alloy and a liquid slag is a key physical parameter in the design of steel-refining processes as high interfacial energies are desired to avoid emulsification of slag in steel and the creation of casting defects. During a chemical reaction between a liquid iron alloy droplet and a liquid slag, it is possible to observe by X-ray photography a number of dynamic interfacial phenomena such as droplet flattening, interfacial turbulence, and spontaneous emulsification that can potentially lead to serious processing problems. These dynamic phenomena have been studied during reactions between Fe-Al and Fe-Ti alloys and silica-containing slags, and the presence of significant interfacial disturbance has been observed during the times of high reaction rate between the slag and the metal. It is suggested that interfacial chemical reactions induce Marangoni and natural convection at the slag-metal interface. This interfacial flow gives rise to interfacial waves due to a Kelvin-Helmholtz instability. The waves grow, become unstable, and lead to spontaneous emulsification of slag in steel and steel in slag. Experiments using industrial samples and controlled laboratory tests have indicated that this phenomenon may be more common than once thought and could lead to some serious problems in the processing of steel alloys containing high quantities of aluminum and/or titanium.

134 citations



Journal ArticleDOI
TL;DR: In this paper, the effect of solid suspension on the viscosity of molten slags was investigated using a Brookfield DVII+ viscometer, and the results of the experiment were in good agreement with the results reported in previous work.
Abstract: This article reports an experimental investigation into the effect of solid suspension on the viscosity of molten slags. Up to about 20 vol pct of spinel (MgAl2O4) particles of three size ranges (fine: 0.10 to 0.21 mm; medium: 0.21 to 0.44 mm; and coarse: 0.44 to 0.99 mm) were added to a CaO-MgO-Al2O3-SiO2 melt at 1646 (±10)K. A Brookfield DVII+ viscometer was used. The viscosity determined for the solid-free melts was in good agreement with the results of published work. The viscosity for the solid-containing melt was found to increase with the addition of the particles. With more than 10 vol pct solid particles, particularly the fine and the coarse ones, the melt showed an apparent “Bingham” behavior, i.e., the shear stress increased linearly with the shear rate but had a residual shear stress (up to 3 Pa depending on the amount and size of solid added) at zero shear rate. The viscosity of the solid-containing slag, η, was found to fit an Einstein-Roscoe type equation, η=η 0 (1−af)−n, where η 0 is the viscosity of the solid-free melt, f is the volume fraction of solid particles in the melt, and a and n are parameters taking the value of 4.24, 3.29, and 3.56 and 1.28, 2.36, and 2.24 for the fine, medium, and coarse particles, respectively, for best fit.

90 citations


Journal ArticleDOI
TL;DR: In this article, metal foams with porosities greater than 90 pct were prepared by a novel powder metallurgy route using a polymeric vehicle, and the resulting polymer-metal foam was closed cell, but particles were not retained in the windows.
Abstract: Metal foams with porosities greater than 90 pct were prepared by a novel powder metallurgy route using a polymeric vehicle. Coarse titanium powder and fine carbonyl iron powder were tested. The powders were blended with each component of a two-part polyol-isocyanate foaming system, and the resulting suspensions were mixed and allowed to expand. Although the resulting polymer-metal foam was closed cell, particles were not retained in the windows. Upon pyrolysis to remove the resin, the windows opened and the final sintered metal foam was reticulated. Such foams present very low sintered density and are correspondingly weak after sintering but offer a fine reticulated structure with cell diameters in the region of 100 to 200 µm. They may have applications in the areas of catalysis, biomaterials, and composites.

84 citations


Journal ArticleDOI
TL;DR: In this article, the thermal field and the grain structure of a cored superalloy turbine blade, which has been directionally solidified with the liquid metal cooling (LMC) process, has been simulated in three dimensions using a cellular automaton coupled with finite-element (CAFE) model.
Abstract: The thermal field and the grain structure of a cored superalloy turbine blade, which has been directionally solidified with the liquid metal cooling (LMC) process, has been simulated in three dimensions using a cellular automaton (CA) coupled with finite-element (CAFE) model. The cooling induced by the liquid aluminum bath has been replaced by a heat-transfer coefficient, whose temperature- and time-dependence has been adjusted on the basis of natural convection simulations and dimensionless analyses. The simulated grain structure and crystallographic texture have been compared with the microstructure, and the electron back-scattered diffraction (EBSD) results were obtained for a real blade. In both the experiment and the simulation, it has been found that the grains do not exhibit a well-defined texture, even near the top of the blade, mainly as a result of a concave liquidus surface. In order to improve the texture and decrease the number of stray crystals, the LMC process was then optimized by changing several parameters. The baffle geometry, the liquid bath level, and the thermal conductivity of the ceramic mold were found to be the dominant parameters. Using the optimized design, the effect of the withdrawal rate on the resulting grain structure was investigated.

Journal ArticleDOI
TL;DR: An analytical solution for the temperature-rise distribution in arc welding of short workpieces is developed based on the classical Jaeger's moving heat-source theory to predict the transient thermal response as mentioned in this paper.
Abstract: An analytical solution for the temperature-rise distribution in arc welding of short workpieces is developed based on the classical Jaeger’s moving heat-source theory to predict the transient thermal response. It, thus, complements the pioneering work of Rosenthal and his colleagues (and others who extended that work), which addresses quasi-stationary moving heat-source problems. The arc beam is considered as a moving plane (disc) heat source with a pseudo-Gaussian distribution of heat intensity, based on the work of Goldak et al. It is a general solution (both transient and quasi-steady state) in that it can determine the temperature-rise distribution in and around the arc beam heat source, as well as the width and depth of the melt pool (MP) and the heat-affected zone (HAZ) in welding short lengths, where quasi-stationary conditions may not have been established. A comparative study is made of the analytical approach of the transient analysis presented here with the finite-element modeling of arc welding by Tekriwal and Mazumder. The analytical model developed can determine the time required for reaching quasi-steady state and solve the equation for the temperature distribution, be it transient or quasi-steady state. It can also calculate the temperature on the surface as well as with respect to the depth at all points, including those very close to the heat source. While some agreement was found between the results of the analytical work and those of the finite-element method (FEM) model, there were differences identified due to differences in the methods of approach, the selection of the boundary conditions, the need to consider image heat sources, and the effect of variable thermal properties with temperature. The analysis presented here is exact, and the solution can be obtained quickly and in an inexpensive way compared to the FEM. The analysis also facilitates optimization of process parameters for good welding practice.

Journal ArticleDOI
TL;DR: In this paper, a model for the prediction of the interfacial heat transfer coefficient during the unidirectional solidification vertically upward of an Al-7 wt pct Si alloy cast onto a water cooled copper chill is presented.
Abstract: A model is presented for the prediction of the interfacial heat-transfer coefficient during the unidirectional solidification vertically upward of an Al-7 wt pct Si alloy cast onto a water cooled copper chill. It has been experimentally determined that the casting surfaces were convex toward the chill, probably due to the deformation of the initial solidified skin of the casting. The model was, therefore, based upon a determination of the (macroscopic) nominal contact area between the respective rough surfaces and, within this region, the actual (microscopic) contact between the casting and the chill surfaces. The model produced approximate agreement with both experimentally determined values of the heat-transfer coefficient and the measured curvature of the casting surface and showed a reasonable agreement with measured temperatures in the casting and the chill also. A common experimental technique for the experimental determination of the heat-transfer coeffcient involves the assumption of one-dimensional heat transfer only. An implication of the approach adopted in this model is that the heat transfer in the region of the casting-chill interface may be two-dimensional, and the subsequent error in the experimentally determined values is discussed.

Journal ArticleDOI
TL;DR: In this paper, the reduction of titania using methane-containing gas was investigated in a laboratory fixed-bed reactor in the temperature range 1373 to 1773 K. The reduction product is titanium oxycarbide, which is a solid solution of TiC and TiO.
Abstract: Reduction of titania using methane-containing gas was investigated in a laboratory fixed-bed reactor in the temperature range 1373 to 1773 K. The reduction product is titanium oxycarbide, which is a solid solution of TiC and TiO. At 1373 K, the formation rate of TiC is very slow. The rate and extent of reaction increase with increasing temperature to 1723 K. A further increase in temperature to 1773 K does not affect the reaction rate and extent. An increase in methane concentration to 8 vol pct favors the reduction process. A further increase in methane concentration above 8 vol pct causes excessive carbon deposition, which has a negative effect on the reaction rate. Hydrogen partial pressure should be maintained above 35 vol pct to depress the cracking of methane. Addition of water vapor to the reducing gas strongly retards the reduction reaction, even at low concentrations of 1 to 2 vol pct. Carbon monoxide also depresses the reduction process, but its effect is significant only at higher concentrations, above 10 vol pct.

Journal ArticleDOI
TL;DR: In this paper, phase equilibrium and distribution of minor elements between copper matte and SiO2-saturated FeOx-SiO 2-MgO-based slag containing 5 to 10 wt pct MgO have been investigated at 1573 K under the SO2 partial pressures of 10.1, 50.7, and 101.3 kPa.
Abstract: As part of a fundamental study of copper smelting processes using oxygen or oxygen-enriched air as a blowing gas, phase equilibrium and distribution of minor elements between copper matte and SiO2-saturated FeOx-SiO2-MgO-based slag containing 5 to 10 wt pct MgO have been investigated at 1573 K under the SO2 partial pressures of 10.1, 50.7, and 101.3 kPa. The copper and sulfur solubilities in the slag were found to be independent of \(p_{SO_2 } \) when the matte grade was specified, and this behavior was ascribed to the constancy of \(\left( {{{p_{O_2 } } \mathord{\left/ {\vphantom {{p_{O_2 } } {p_{S_2 } }}} \right. \kern- ulldelimiterspace} {p_{S_2 } }}} \right)\) against \(p_{SO_2 } \) at a given matte grade. When the distribution ratio of a minor element (X) between the slag and matte phases was defined as Lxs/m=(wt pct X in slag)/{wt pct X in matter}, Lxs/m for arsenic, antimony, and bismuth at a given matte grade increased with increasing \(p_{SO_2 } \). On the other hand, the distribution ratio of silver at a given matte grade was almost constant against \(p_{SO_2 } \).

Journal ArticleDOI
TL;DR: In this paper, a mathematical model has been developed to analyze molten metal flow, considering the effects of argon gas injection and static magnetic-field application in the continuous casting process, and a homogeneous fluid model with variable density is employed to tackle the molten metal-argon gas flow.
Abstract: A mathematical model has been developed to analyze molten metal flow, considering the effects of argon gas injection and static magnetic-field application in the continuous casting process. The k-ɛ turbulence model is used to calculate the turbulent variables. A homogeneous fluid model with variable density is employed to tackle the molten metal-argon gas flow. The electromagnetic force is incorporated into the Navier-Stokes equation, and the effects of boundary conditions of the magnetic field on the velocity distribution near the mold wall are included. A good agreement between the numerically obtained flow-field results and measurements is obtained. The argon gas injection changes the molten metal flow pattern, mainly in the upper portion of the mold. By applying the magnetic field, values of the averaged velocity field in the bulk decrease significantly, and, especially at the top free surface, they become very small, which can cause meniscus freezing. When magnetic-field application and argon gas injection are used together, the external flow field out of the gas plume is significantly suppressed; nevertheless, flotation of gas bubbles is still active and is not affected directly by the magnetic field. Although the penetrating length of the gas plume is shortened, the argon gas bubbles in molten steel still cause fluctuation at the top free surface, which prevents the occurrence of freezing.

Journal ArticleDOI
TL;DR: In this article, an extensive research program focused on the characterization of various metallurgical complex smelting and coal combustion slags is being undertaken, which combines both experimental and thermodynamic modeling studies.
Abstract: An extensive research program focused on the characterization of various metallurgical complex smelting and coal combustion slags is being undertaken. The research combines both experimental and thermodynamic modeling studies. The approach is illustrated by work on the PbO-ZnO-Al2O3-FeO-Fe2O3-CaO-SiO2 system. Experimental measurements of the liquidus and solidus have been undertaken under oxidizing and reducing conditions using equilibration, quenching, and electron probe X-ray microanalysis. The experimental program has been planned so as to obtain data for thermodynamic model development as well as for pseudo-ternary liquidus diagrams that can be used directly by process operators. Thermodynamic modeling has been carried out using the computer system FACT, which contains thermodynamic databases with over 5000 compounds and evaluated solution models. The FACT package is used for the calculation of multiphase equilibria in multicomponent systems of industrial interest. A modified quasi-chemical solution model is used for the liquid slag phase. New optimizations have been carried out, which significantly improve the accuracy of the thermodynamic models for lead/zinc smelting and coal combustion processes. Examples of experimentally determined and calculated liquidus diagrams are presented. These examples provide information of direct relevance to various metallurgical smelting and coal combustion processes.

Journal ArticleDOI
TL;DR: In this paper, a nonlinear two-dimensional model based on the shallow-water approximation is proposed to analyze magnetohydrodynamic phenomena in aluminum reduction cells, where the background melt flow and instability are considered as coupled fully nonlinear parts of one nonsteady process.
Abstract: A new approach to numerical analysis of magnetohydrodynamic phenomena in aluminum reduction cells is proposed. The flows of aluminum and cryolite, as well as the instability of the interface between the two layers, are described using a nonlinear two-dimensional model based on the shallow-water approximation. There are two principal distinctive properties of our model. First, the background melt flow and instability are considered as coupled fully nonlinear parts of one nonsteady process. Second, while capturing the main physical mechanisms operating in the system, the model is computationally very efficient and allows simulations in a “real-time” regime.

Journal ArticleDOI
TL;DR: In this article, a generalized temperature boundary condition coupling strategy for the modeling of conventional casting processes was implemented via experiments and numerical simulations with commercial purity aluminum, aluminum alloy, and tin specimens in copper, graphite, and sand molds.
Abstract: A generalized temperature boundary condition coupling strategy for the modeling of conventional casting processes was implemented via experiments and numerical simulations with commercial purity aluminum, aluminum alloy, and tin specimens in copper, graphite, and sand molds. This novel strategy related the heat transfer coefficient at the metal-mold interface to the following process variables: the size of the air gap that forms at the metal-mold interface, the roughness of the mold surface, the conductivity of the gas in the gap, and the thermophysical properties of both the metal and mold. The objective of this study was to obtain, apply, and evaluate the effect of incorporating an experimentally derived relationship for specifying transient heat transfer coefficients in a general conventional casting process. The results are presented in two parts. Part I details the implementation of a systematic experimental approach not limited to a specific process to determine the heat transfer coefficient and characterize the formation of the air gap at the metal-mold interface. The heat transfer mechanisms at the interface were identified, and seen to vary in magnitude during four distinct stages, as the air gap formed and grew. A semiempirical inverse equation was used to characterize the heat transfer coefficient-air gap relationship, across the various stages, for experimental data from the literature and this study.

Journal ArticleDOI
TL;DR: In this paper, the effect of CaO or SiO2 addition on the interfacial chemical reaction rate of hydrogen reduction was empirically evaluated as a function of the ferrous-ferric ratio in the slag.
Abstract: Interfacial kinetics on the hydrogen reduction of liquid Fe t O in Fe t O-M x O y slag (M x O y = CaO, SiO2, Al2O3, and TiO2) has been studied at 1673 K. Because the rate of hydrogen reduction was very fast, the rate was controlled by gas-phase mass transfer under most of the experimental conditions. The effect of CaO or SiO2 addition on the interfacial chemical reaction rate of hydrogen reduction was empirically evaluated as a function of the ferrous-ferric ratio in the slag. The observed interfacial chemical reaction rates in Fe t O-CaO and Fe t O-SiO2 slags showed reasonable agreement with the estimated values. Most of the available literature data on the reduction rate of liquid iron oxide by solid carbon, hot metal, and reducing gases were also reviewed and compared with the results of the present work. It was found that the rate of hydrogen reduction of liquid iron oxide slag is much faster than that with other reducing agents such as solid carbon, carbon dissolved in the liquid iron, and CO gas.

Journal ArticleDOI
TL;DR: In this article, the velocity of the advancing interface first increased while approaching the particle, but became stagnant during engulfment and increased again after that, and it was concluded that the flow was induced by the local difference in temperature and oxygen content in front of the interface, particularly in the case of a higher oxygen content.
Abstract: The present study is concerned with the interaction phenomena of nonmetallic inclusions in front of a moving solid-liquid interface. The in situ observation was done in a high-temperature experiment by using a laser microscope. Alumina inclusions in an aluminum-killed steel with low oxygen content exhibited the well-known clustering behavior. The velocity of the advancing interface first increased while approaching the particle, but became stagnant during engulfment and increased again after that. Alumina-magnesia complex inclusions in a magnesium-added steel with high oxygen content were very finely dispersed in the molten pool. These inclusions escaped from the advancing interface during solidification, but gathered again at the retreating interface during remelting. The tiny inclusions were thought to behave just as tracer particles of a local flow. The velocity of particles was measured on a video image, and the significant acceleration or deceleration was found near the interface. It was concluded that the flow was induced by the Marangoni effect due to the local difference in temperature and oxygen content in front of the interface, particularly in the case of a higher oxygen content. However, the flow was weak in the case of a low oxygen content.

Journal ArticleDOI
Abstract: Both numerical analysis based on finite-element (FE) modeling and experimental evidence concerning the secondary oxide-scale failure at entry into the roll gap are presented and reviewed for a better understanding of events at the roll-workpiece interface, in turn, leading to better definition of the boundary conditions for process models. Attention is paid to the two limit modes leading to oxide-scale failure, which were observed earlier during tensile testing under rolling conditions. These are considered in relation to the temperature, the oxide-scale thickness, and other hot-rolling parameters. The mathematical model used for the analysis is composed of macro and micro parts, which allow for simulation of metal/scale flow, heat transfer, cracking of the oxide scale, as well as sliding along the oxide/metal interface and spallation of the scale from the metal surface. The different modes of oxide-scale failure were predicted, taking into account stress-directed diffusion, fracture and adhesion of the oxide scale, strain, strain rate, and temperature. Stalled hot-rolling tests under controlled conditions have been used to verify the types of oxide-scale failure and have shown good predictive capabilities of the model. The stock temperature and the oxide-scale thickness are important parameters, which, depending on other rolling conditions, may cause either through-thickness cracking of the scale at the entry or lead to entry of a nonfractured scale when the scale/metal interface is not strong enough to transmit the metal deformation.

Journal ArticleDOI
TL;DR: In this paper, the reduction of higher manganese oxides to MnO by carbon monoxide has been studied in the temperature range 700 °C to 1100 °C, and a topochemical pattern with a single shrinking core inside the ore particles has been observed in most cases.
Abstract: A step has been made in the direction of understanding the fundamental chemical processes taking place inside electric are furnaces producing manganese alloys. The reduction of higher manganese oxides to MnO by carbon monoxide has been studied in the temperature range 700 °C to 1100 °C. A topochemical pattern with a single shrinking core inside the ore particles has been observed in most cases. It has been found that the reduction of some manganese silicates (braunite minerals) is influenced by reaction interface kinetics, whereas the reduction rate of manganese oxides (bixbyite and hausmannite) is mostly determined by product shell pore diffusion. Sintering kinetics and the extent of natural porosity determine the product shell pore diffusivity. As the melting point of the reaction product is approached, rapid sintering leads to a decrease in diffusivity.

Journal ArticleDOI
TL;DR: In this paper, a mechanical model has been developed for the prediction of the depth of oscillation marks of the depression type, which is based on the beam bending theory and on viscoplastic material behavior.
Abstract: The surface of continuously cast slabs is characterized by the presence of oscillation marks. Direct linkage of the continuous casting process and hot rolling process requires that cast slabs should be free of surface defects. In the present work, a mechanical model has been developed for the prediction of the depth of oscillation marks of the depression type. It is based on the beam bending theory and on viscoplastic material behavior. The downward movement of the strand is taken correctly into account, which has not been done in previous models. Auxiliary parts of the model are the models for the determination of the temperatre field and of the fluid flow and pressure in the meniscus region and in the gap between strand and mold. The deflection of the shell is computed as a function of time and distance from the shell tip. The retained deflection, which corresponds to the depth of oscillation marks observed on the slab surface, is determined for different values of stroke, frequency, and casting velocity. The theoretical data are compared with the measured data as available in the literature.

Journal ArticleDOI
S. Bounds1, G. Moran1, Koulis Pericleous1, Mark Cross1, T.N. Croft1 
TL;DR: This work describes an attempt to model the formation of macrodefects explicitly as a function of the interacting continuum phenomena in arbitrarily complex three-dimensional geometries using a compatible set of finite volume procedures extended to unstructured meshes.
Abstract: High-integrity castings require sophisticated design and manufacturing procedures to ensure they are essentially macrodefect free. Unfortunately, an important class of such defects—macroporosity, misruns, and pipe shrinkage—are all functions of the interactions of free surface flow, heat transfer, and solidication in complex geometries. Because these defects arise as an interaction of the preceding continuum phenomena, genuinely predictive models of these defects must represent these interactions explicitly. This work describes an attempt to model the formation of macrodefects explicitly as a function of the interacting continuum phenomena in arbitrarily complex three-dimensional geometries. The computational approach exploits a compatible set of finite volume procedures extended to unstructured meshes. The implementation of the model is described together with its testing and a measure of validation. The model demonstrates the potential to predict reliably shrinkage macroporosity, misruns, and pipe shrinkage directly as a result of interactions among free-surface fluid flow, heat transfer, and solidification.

Journal ArticleDOI
TL;DR: In this paper, water modeling and mathematical simulation techniques were used to study the melt flow under the influence of turbulence inhibitors in a multistrand bloom caster tundish, and the results indicated that the TI&D arrangement retains the tracer inside the vessel for longer times, increasing the minimum residence time, and showed an improvement of fluid flow characteristics, yielding better tracer distribution among the outlets, lower values of back mixing flow, and higher values of plug flow.
Abstract: Water modeling and mathematical simulation techniques were used to study the melt flow under the influence of turbulence inhibitors in a multistrand bloom caster tundish. Three different cases were studied: a bare tundish (BT), a tundish with two pairs of baffles and a waved impact pad (BWIP), and a tundish equipped with turbulence inhibitor and a pair of dams (TI&D). Chemical mixing of tracer turbulence diffusion was also simulated and compared with actual experimental results. The TI&D arrangement showed an improvement of the fluid flow characteristics, yielding better tracer distribution among the outlets, lower values of back mixing flow, and higher values of plug flow. A mass transfer model coupled with k-ɛ turbulence model predicted acceptably well the experimental chemical mixing of the tracer in the water model. The water modeling and the numerical simulation indicated that the TI&D arrangement retains the tracer inside the vessel for longer times, increasing the minimum residence time. These results encourage the use of turbulence-inhibiting devices in bloom and billet casters, which pursue excellence in product quality.

Journal ArticleDOI
TL;DR: In this article, an experimental study of 304 stainless steel melts in direct contact with copper substrates under conditions approximating the meniscus region of a strip caster has highlighted the importance of interfacial heat transfer during the first 30 ms.
Abstract: An experimental study of initial solidification of 304 stainless steel melts in direct contact with copper substrates under conditions approximating the meniscus region of a strip caster has highlighted the importance of interfacial heat transfer during the first 30 ms of contact. The mechanisms governing initial heat transfer are strongly influenced by dynamic wetting phenomena. This has been illustrated experimentally by the effects of the buildup and melting of oxide films such as manganese silicates at the interface during successive immersions, by the role of surface active agents such as tellurium in the melt, and by the use of specially designed substrate textures to control contact areas.

Journal ArticleDOI
TL;DR: The alpha case thickness at the surface of a Ti-6Al-4V (wt pct) step wedge investment casting has been measured and successfully predicted using temperature-time results obtained from a heat flow simulation of the casting as discussed by the authors.
Abstract: The alpha case thickness at the surface of a Ti-6Al-4V (wt pct) step wedge investment casting has been measured and successfully predicted. The prediction uses temperature-time results obtained from a heat flow simulation of the casting. The temperature-time results were coupled to a simple model for diffusion of oxygen into the beta phase during continuous cooling. Oxygen concentration and microhardness profiles were measured from the surface in contact with the ZrO2 face coat of the shell mold into the interior of the casting. The oxygen content in the metal at the shell mold interface was between 5 and 9.5 wt pct in general agreement with a thermodynamic calculation for bcc Ti in contact with ZrO2. At the limit of the alpha case region, as determined by standard metallographic technique, the oxygen concentration was found to be no more than 0.02 wt pct above the level of oxygen in the bulk alloy. Using this information and one particular literature value for the activation energy for diffusion of oxygen, a nearly linear relationship was obtained between the measured and predicted alpha case thicknesses at various positions on the casting surface. Reduction of the prefactor of this diffusion coefficient by a factor of 7.5 produces excellent agreement between predicted and measured alpha case thicknesses. Such a reduction is not inconsistent with the scatter of literature values for the diffusion coefficient.

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TL;DR: In this paper, the authors investigated the influence of factors governing the rate of carbon dissolution from carbonaceous materials, especially coals, into Fe-C-S melts by using a carburizer cover technique in an induction furnace.
Abstract: Carbon dissolution from graphite and coals was investigated by using a carburizer cover technique in an induction furnace. The intent of the study was to investigate the influence of factors governing the rate of carbon dissolution from carbonaceous materials, especially coals, into Fe-C-S melts. The factors studied were the initial melt carbon and sulfur concentrations and the wettability between carbonaceous materials and the melt.

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TL;DR: In this paper, a finite-element software is developed to simulate metal-matrix composite infiltration by adaptation of a code originally developed for soil mechanics, which can be applied for isothermal infiltration of complex geometries and boundary conditions and aid mold design and process parameter optimization.
Abstract: In pressure infiltration processing of metal-matrix composites, molten metal is injected under external pressure into a porous preform of the reinforcing phase and solidified, either during infiltration or after the mold is filled. If infiltration is isothermal, the physics of the process are similar to drainage phenomena encountered in soil mechanics. Using this similarity, a finite-element software is developed to simulate metal-matrix composite infiltration by adaptation of a code originally developed for soil mechanics. Solutions are given for isothermal infiltration of porous preforms by a molten metal under any increasing function of the applied pressure vs time, taking into account capillary phenomena. Experimental validation is performed using SAFFIL alumina fiber preforms infiltrated with an aluminum matrix in a series of isothermal infiltration experiments in unidirectional and axisymmetric configurations. Numerical and experimental data show good agreement, both in terms of infiltration kinetics and porosity distribution. The simulation tool can, thus, be applied for isothermal infiltration of complex geometries and boundary conditions and aid mold design and process parameter optimization.