Showing papers in "Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science in 2006"

State of the art in the control of inclusions during steel ingot casting

Abstract: This article extensively reviews published research on inclusions in ingot steel and defects on ingot products, methods to measure and detect inclusions in steel, the causes of exogenous inclusions, and the transport and entrapment of inclusions during fluid flow, segregation, and solidification of steel cast in ingot molds. Exogenous inclusions in ingots originate mainly from reoxidation of the molten steel, slag entrapment, and lining erosion, which are detailed in this article. The measures to prevent the formation of exogenous inclusions and improve their removal are provided, which are very useful for the clean steel production of ingot industries.

Inclusion removal by bubble flotation in a continuous casting mold

Abstract: Fundamentally based computational models are developed and applied to quantify the removal of inclusions by bubbles during the continuous casting of steel. First, the attachment probability of inclusions on a bubble surface is investigated based on fundamental fluid flow simulations, incorporating the turbulent inclusion trajectory and sliding time of each individual inclusion along the bubble surface as a function of particle and bubble size. Then, the turbulent fluid flow in a typical continuous casting mold, trajectories of bubbles, and their path length in the mold are calculated. The change in inclusion distribution due to removal by bubble transport in the mold is calculated based on the computed attachment probability of inclusions on each bubble and the computed path length of the bubbles. In addition to quantifying inclusion removal for many different cases, the results are important to evaluate the significance of different inclusion-removal mechanisms. The modeling approach presented here is a powerful tool for investigating multiscale phenomena in steelmaking and casting operations to learn how to optimize conditions to lower defects.

Oxide films, pores and the fatigue lives of cast aluminum alloys

Abstract: In the absence of gross defects such as cold shuts, the fatigue properties of castings are largely determined by the sizes of microstructural defects, particularly pores and oxide films. In contrast, the effects of grain size, second-phase particles, and nonmetallic inclusions are insignificant. The authors review the fatigue properties of castings made by gravity die casting, sand casting, lost-foam casting, squeeze casting, and semisolid casting, and compare A356/357 alloys with 319-type alloys. The application of fracture mechanics enables the properties to be rationalized in terms of the defects that are characteristic of each casting process, noting both the sizes and types of defect. The differences in the properties of castings are entirely attributed to their different defect populations. No single process is inherently superior. For defects of the same size (in terms of projected area normal to the loading direction), oxide films are less detrimental to fatigue life than pores. Areas of current controversy are highlighted and suggestions for further work are made.

An overview of the effects of bifilms on the structure and properties of cast alloys

Abstract: A short overview of the role of bifilms (double oxide films) in cast Al alloys includes control of (1) grain size, because of the suppression of convection; (2) dendrite-arm spacing (DAS) as a result of isolated regions of liquid devoid of nuclei that consequently undercool; and (3) the mechanism of the modification of Al-Si alloys by Na and Sr, which is explained for the first time by the deactivation of bifilms as substrates for Si nucleation and growth.

Modeling of microwave heating of particulate metals

Abstract: Recent studies have shown that metal powder compacts can be heated to high temperatures using microwaves. While microwave heating of ceramics is well understood and modeled, there is still uncertainty about the exact mechanism and mode of microwave heating of particulate metals. The current study describes an approach for modeling the microwave heating of metal powder compacts using an electromagnetic-thermal model. The model predicts the variation in temperature with time during sintering. The effect of powder size, emissivity, and susceptor heating on the heating rate has also been assessed. These predictions have been validated by the experimental observations of the thermal profiles of Sn-, Cu-, and W-alloy compacts, using a 2.45 GHz multimode microwave furnace.

A method to study the history of a double oxide film defect in liquid aluminum alloys

Abstract: Entrained double oxide films have been held responsible for reductions in mechanical properties in aluminum casting alloys. However, their behavior in the liquid metal, once formed, has not been studied directly. It has been proposed that the atmosphere entrapped in the double oxide film defect will continue to react with the liquid metal surrounding it, perhaps leading to its elimination as a significant defect. A silicon-nitride rod with a hole in one end was plunged into liquid aluminum to hold a known volume of air in contact with the liquid metal at a constant temperature. The change in the air volume with time was recorded by real-time X-ray radiography to determine the reaction rates of the trapped atmosphere with the liquid aluminum, creating a model for the behavior of an entrained double oxide film defect. The results from this experiment showed that first oxygen, and then nitrogen, was consumed by the aluminum alloy, to form aluminum oxide and aluminum nitride, respectively. The effect of adding different elements to the liquid aluminum and the effect of different hydrogen contents were also studied.

Modeling of slag eye formation over a metal bath due to gas bubbling

Abstract: When gas is bubbled through a molten metal, overlying slag is pushed to the side forming an open “eye” of exposed metal. Eye sizes were measured in room-temperature modeling over a wide range of conditions including the fluids to simulate slag and metal, gas flow rates, and depths of both fluids. A mechanistic model for eye size was developed from fundamental fluid flow considerations. The model expresses a dimensionless eye area in terms of a density ratio of the fluids and a Froude number. The model is consistent with the present experimental results and those of others in different liquid systems. Finally, previously published correlations for eye size have been critically evaluated.

Acidity, valency and third-ion effects on the precipitation of scorodite from mixed sulfate solutions under atmospheric-pressure conditions

Abstract: The present study is focused on the precipitation of scorodite from mixed sulfate media at 95 °C under atmospheric pressure. In particular, this study explores the effects of acidity (pH), valency [Fe(II)/Fe(III), As(III)/As(V)], and solution composition (third cation/anion) on the yield, crystallinity, and stability (leachability) of scorodite precipitates. Thus, it was found that the precipitation of crystalline scorodite can be achieved without stringent pH control once the precipitation has started. Nonetheless, the selection of the initial pH is critical to avoid the formation of an amorphous precipitate. A leachability as low as 0.5 mg/L As at pH 5 and 22 °C (TCLP-like test) is obtained when the initial molar ratio Fe(III):As(V) is increased to 3:1, but the precipitation yield is very low. When Fe(II) is used as excess iron, the precipitate solubility drops to 0.2 mg/L As with a yield exceeding 80 pct in 2.5 hours. The stability of the product is not measurably affected by the presence of Cu2+, Zn2+, Ni2+, Co2+, Mn2+, SO 4 2− , and NO 3 − . The presence of PO 4 3− , however, leads to the formation of crystalline phosphate-containing scorodite precipitates of somewhat reduced stability. In most cases, the TCLP leachability of the precipitate was found to be between 1 and 3 mg/L As, and never exceeded the regulatory limit of 5 mg/L As.

Microstructure-based fatigue life prediction for cast A356-T6 aluminum-silicon alloys

Abstract: Fatigue life prediction and optimization is becoming a critical issue affecting the structural applications of cast aluminum-silicon alloys in the aerospace and automobile industries. In this study, a range of microstructure and porosity populations in A356 alloy was created by controlling the casting conditions and by applying a subsequent hot isostatic pressing (“hipping”) treatment. The microstructure and defects introduced during the processing were then quantitatively characterized, and their effects on the fatigue performance were examined through both experiment and modeling. The results indicated that whenever a pore is present at or near the surface, it initiates fatigue failure. In the absence of large pores, a microcell consisting of α-Al dendrites and associated Si particles was found to be responsible for crack initiation. Crack initiation life was quantitatively assessed using a local plastic strain accumulation model. Moreover, the subsequent crack growth from either a pore or a microcell was found to follow a small-crack propagation law. Based on experimental observation and finite-element analysis, a unified model incorporating both the initiation and small crack growth stages was developed to quantitatively predict the dependency of fatigue life on the microstructure and porosity. Good agreement was obtained between the model and experiment.

Effect of ferrosilicon addition on the composition of inclusions in 16Cr-14Ni-Si stainless steel melts

Abstract: The silicon deoxidation equilibrium between the 16Cr-14Ni-1.5Mn-Si melts and the CaO-SiO2-8MgO-5CaF2 (basicity=1.8) slag at 1743 K was investigated to understand the effect of aluminum and silicon contents on the composition of inclusions. Therefore, the ferrosilicon alloys with different aluminum content were chosen based on the preceding objective. In addition, the phase stability diagram of the inclusions was computed using commercial thermodynamic software based on the Gibbs energy minimization principles. The content of MnO in the inclusions sharply decreases with increasing silicon content when the steel melts were deoxidized by the ferrosilicon alloys containing high aluminum (FeSi-H). The content of SiO2 in the inclusions slightly increases with increasing silicon content when the FeSi-L is used, while a maximum value is shown at [Si]=1.5 pct when the FeSi-H is used. The content of MgO in the inclusions increases by increasing the content of silicon, regardless of the kinds of ferrosilicon alloys. The use of the FeSi-L as a deoxidizer could suppress the formation of Al2O3 in the inclusions, while the content of Al2O3 increases with increasing silicon content when the FeSi-H is used. When the FeSi-H is used as a deoxidizer, the inclusions are the glassy type with the composition of Mn-silicates at [Si]≤1.3 pct, while the Mg(Ca)-silicates with the composition of the forsterite phase are observed in the steel composition of [Si]=3.3 pct. When the steel melts were deoxidized by the FeSi-L alloys, the inclusions are the glassy-type Mn-silicates at [Si]=0.8 pct, while the Mn-silicates containing the cristobalite phase are observed at [Si]=1.5 to 2.4 pct. In the composition of [Si]=3.3 pct, the Mg-silicates with the composition of the rhodonite phase are observed. The log(X SiO2/X MnO) of the inclusions linearly increases by increasing the log [a Si · a O / a Mn] with the slope close to unity when the FeSi-L is used as a deoxidizer, while the slope of the line is about 2 times greater than that of the expected value when FeSi-H is used. The log (X MgO/X MnO) of the inclusions linearly increases by increasing the log [a Mg/a Mn] with slopes greater than the expected value of unity.

Sulfide capacity of high alumina blast furnace slags

Abstract: Sulfide capacities of high alumina blast furnace slags were experimentally determined using the gas-slag equilibration technique. Two different slag systems were considered for the current study, namely, CaO-SiO2-MgO-Al2O3 quaternary and CaO-SiO2-MgO-Al2O3-TiO2 quinary system. The liquid slag was equilibrated with the Ar-CO-CO2-SO2 gas mixture. Experiments were conducted in the temperature range of 1773 to 1873 K. The effects of temperature, basicity, and the MgO and TiO2 contents of slags on sulfide capacity were studied. As expected, sulfide capacity was found to increase with the increase in temperature and basicity. At the higher experimental temperature, titania decreases the sulfide capacity of slag. However, at the lower temperature, there was no significant effect of titania on the sulfide capacity of slag. Sulfide capacity increases with the increase in MgO content of slag if the MgO content is more than 5 pct.

A model for accurate predictions of self-diffusivities in liquid metals, semimetals, and semiconductors

Abstract: By combining the modified Stokes-Einstein formula with the authors’ model for the melting-point viscosity, the authors present a model for accurate predictions of self-diffusivity of liquid metallic elements. The model is expressed in terms of well-known physical quantities and has been applied to various liquid metallic elements for which experimental data are available. The results of calculations show that agreement with experimental data is excellent; the uncertainties in the calculations of the self-diffusivities in various liquid metallic elements are equal to the uncertainties associated with experimental measurements. Also, the authors propose an expression for the temperature dependence of self-diffusivity in liquid metallic elements in terms of melting-point temperature. Using the model, self-diffusivity data are predicted for liquid iron, cobalt, nickel, titanium, aluminum, magnesium, silicon, and so forth.

Alloy Shrinkage Factors for the Investment Casting Process

Abstract: This study deals with the experimental measurements and numerical predictions of alloy shrinkage factors (SFs) related to the investment casting process. The dimensions of the A356 aluminum alloy casting were determined from the numerical simulation results of solidification, heat transfer, fluid dynamics, and deformation phenomena. The investment casting process was carried out using wax patterns of unfilled wax and shell molds that were made of fused silica with a zircon prime coat. The dimensions of the die tooling, wax pattern, and casting were measured, in order to determine the actual tooling allowances. Several numerical simulations were carried out, to assess the level of accuracy for the casting shrinkage. The solid fraction threshold, at which the transition from the fluid dynamics to the solid dynamics occurs, was found to be important in predicting shrinkage factors (SFs). It was found that accurate predictions were obtained for all measured dimensions when the shell mold was considered a deformable material.

Modeling of the turbulent flow in induction furnaces

Abstract: Experimental results show that heat- and mass-transfer processes in recirculating turbulent flows, which comprise several vortexes of the mean flow, are significantly influenced by low-frequency large scale flow oscillations. The large eddy simulation (LES) model reproduces with good conformity not only these oscillations together with the dynamics of the macroscopic coherent structure, but also the turbulent energy transfer. Numerical studies, presented in this article, confirm the possibility of using LES for successful simulation of heat- and mass-transfer processes in metallurgical applications.

The role of oxides in the formation of primary iron intermetallics in an Al-11.6Si-0.37Mg alloy

Abstract: Recent research suggest that the iron-rich intermetallic phases, such as α-Fe Al15(Fe,Mn)3Si2 and β-Fe Al5FeSi, nucleate on oxide films entrained in aluminum casting alloys. This is evidenced by the presence of crack-like defects within these iron-rich intermetallics. In an attempt to verify the role of oxides in nucleating iron-rich intermetallics, experiments have been conducted under conditions where in-situ entrained oxide films and deliberately added oxide particles were present. Iron-rich intermetallics are observed to be associated with the oxides in the final microstructure, and crack-like defects are often observed in the β-Fe plates. The physical association of the Fe-rich intermetallic phases with these solid oxides, either formed in situ or added, is in accordance with the mechanism suggesting that iron-rich intermetallics nucleate upon the wetted sides of double oxide films.

A mesoscale cellular automaton model for curvature-driven grain growth

Abstract: A two-dimensional mesoscale cellular automaton algorithm is developed to simulate curvature-driven grain growth during materials processing. In the present model, a deterministic switch rule for the grain growth is adopted, and thus the kinetics of the grain growth can be simulated quantitatively. In addition, the grain-boundary energy is dependent on the misorientation between neighboring grains. At mesoscale, the simulations show that each grain displays a unique growth behavior. The growth behavior of individual grains can be categorized into four types: (1) grains with monotonically increasing equivalent diameters, (2) grains that first grow and then begin to shrink, (3) grains with almost constant diameters, and (4) grains that decrease in size. Furthermore, an oscillation grain-growth behavior is observed in the present mesoscale cellular automaton simulations. This simulated individual grain-growth behavior has not been reported in the literature.

Electrical and electronic conductivity of CaO-SiO2-FeOx slags at various oxygen potentials: Part I. Experimental results

Abstract: The oxygen potential dependences of total electrical and partial electronic/ionic conductivities for ‘FeO’-CaO-SiO2 slags have been studied both experimentally and theoretically in the present work. In the first part of this two-part article, the experimental results are presented for slags with 30 wt pct ‘FeO’ and CaO/SiO2 wt ratios of 0.5, 1.0, and 2.0. For each slag composition, measurements of total electrical conductivity and electronic transference were made over a range of oxygen potentials and temperatures. The results were used to calculate the partial conductivities. A maximum was achieved in total and electronic conductances as a function of equilibrium CO2/CO. The CO2/CO corresponding to the maximum was shifted to lower values with increasing slag basicity (CaO/SiO2 ratio). The other effect of basicity was increasing total and partial conductivities, with a magnitude that depends on oxygen potential and temperature. The activation energies for ionic and electronic conductances were in similar ranges and decreased with the basicity.

The dissolution of sphalerite in ferric sulfate media

Abstract: The dissolution of sphalerite, (Zn,Fe)S, in ferric sulfate media was investigated using closely sized fractions of crushed sphalerite crystals. Linear kinetics were observed, and the rate increased in proportion to the surface area, as the average particle size of the sphalerite decreased. The predominant reaction products are ZnSO4, FeSO4, and elemental sulfur. The leaching rate increases with increasing temperature, and the apparent activation energy is 44 kJ/mol. The relatively high apparent activation energy suggests that the rate is chemically controlled, a conclusion supported by the insensitivity of the rate of the rotation speed that was observed in complementary rotating disk experiments. The rate increases as the 0.3 to 0.4 power of the Fe(SO4)1.5 concentration, and is nearly independent of the pulp density, in the presence of a stoichiometric excess of ferric sulfate. In 0.3 M Fe(SO4)1.5 media, the rate increases with increasing acid concentrations >0.1 M H2SO4, but is insensitive to more dilute acid concentrations. In the absence of ferric ions, the rate increases rapidly with increasing H2SO4 concentrations, and relatively rapid rates are observed in solutions containing >0.5 M H2SO4. The rate decreases with increasing initial concentrations of ZnSO4, MgSO4, or FeSO4 in the ferric sulfate leaching solution, and this emphasizes the importance of maintaining the dissolved iron in a fully oxidized state in a commercial leaching operation.

Electrical and electronic conductivity of CaO-SiO2-FeOx slags at various oxygen potentials: Part II. Mechanism and a model of electronic conduction

Abstract: The experimental results obtained for ionic and electronic conductivity of ‘FeO’-CaO-SiO2 melts have been analyzed considering the mechanism of each conduction process. The Nernst-Einstein equation was employed to calculate diffusion coefficients of Fe2+ and Ca2+ cations from ionic conductance. A “diffusion-assisted charge transfer” model was developed to explain the dependence of the electronic conductivity on the oxidation state of iron in the slag. The model considers the electronic conduction as a two-step process: in one step, ferrous ions diffuse from their initial position to a proper distance from ferric ions; in the next step, an electron is transferred between Fe2+ and Fe3+. The optimum distance of the iron ions for electron hopping was found to be approximately 4 A, in great consistency with the values reported for electron transfer between Fe2+ and Fe3+ in aqueous solutions and solid glasses.

Effects of additives and temperature on dissolution rate and diffusivity of lime in Al2O3-CaO-SiO2 based slags

Abstract: The dissolution rate of dense lime specimens in calcium aluminosilicate based melts was measured at 1430 °C to 1600 °C in air, using a rotating disk/cylinder technique. The measured dissolution rates were strongly dependent on the rotation speed with the results indicating mass transfer in the slag phase to be a rate-limiting step. At a given rotation speed, the slag chemistry and temperature had strong effects on the dissolution rate. The diffusivity of CaO in the slag was calculated from the dissolution rate and solubility data, using known mass-transfer correlations. Addition of CaF2 MnOx, FeOx, and TiO2 to the slag increased the CaO diffusivity, while SiO2 had an opposite effect. Addition of CaF2 had the strongest effect and increased the diffusivity by a factor of 3 to 5 in the temperature range of 1500 °C to 1600 °C. The deduced activation energy for diffusion of CaO in these slags ranged from about 53 to 246 kJ/mole, depending on the concentration of additives used.

Preparation of enriched cerium oxide from bastnasite with hydrochloric acid by two-step leaching

Abstract: Roasted with sodium carbonate, bastnasite (Ln(Ce)CO3F) was converted to calcine containing rare earth oxides (REO), among them cerium, which existed mainly as CeO2. The calcine was first leached with diluted hydrochloric acid, which resulted in a sludge with the enriched cerium (IV) dioxide. The sludge was further leached with a concentrated hydrochloric acid, adding hydrogen peroxide as a reducing agent; in this manner, the enriched cerium tri-chloride (CeCl3) was prepared. The optimal technological parameters are suggested as follows: first, the hydrochloric acid concentration, the leaching temperature, the ratio of solid to liquid, and the leaching time are 1 mol/L, 60°C, 1:20, and 90 minutes, respectively; second, the hydrochloric acid concentration, the dosage of hydrogen peroxide in every 5 g of the sludge, the ratio of solid to liquid, the leaching temperature, and the leaching time are 6 mol/L, 6 mL, 1:20, 50°C, and 90 minutes, respectively. As a result, the cerium-enriched rare earth (RE) solution, containing over 95 pct cerium oxide, is obtained, which is in turn available for use in preparing a kind of polishing powder containing high cerium. The total recovery of cerium was 91 pct (85.3 pct, in the second step).

A thermal and microstructure evolution model of direct-drive friction welding of plain carbon steel

Abstract: A model of direct-drive friction welding has been developed, which can be used to predict the time-temperature histories, the resultant microstructure, and the microhardness distribution across the weld interface of direct-drive friction-welded AISI/SAE 1045 steel bars. Experimentally measured power and axial displacement data were used in conjunction with a finite-element transient thermal model to predict the time-temperature history within the heat-affected zone (HAZ) of the weld. This was then used with a microstructure evolution model to predict the volume fraction of the subsequent microconstituents and the microhardness distribution across the weld interface of welds produced using three significantly different welding conditions: one with optimal conditions, one with a long burn-off time, and one with high axial pressure and rotational speed but short burn-off time. There was generally good agreement between the predicted and the measured time-temperature histories, volume fraction of the resultant microstructures, and microhardness distribution in the HAZ of AISI/SAE 1045 steel friction welds produced using these three significantly different welding conditions.

Mathematical Simulation of Fluid Dynamics during Steel Draining Operations from a Ladle

Abstract: Fluid flow dynamics during ladle drainage operations of steel under isothermal and nonisothermal conditions has been studied using the turbulence shear stress transport k-e model (SST k-ω) and the multiphase volume of fluid (VOF) model. At high bath levels, the angular velocity of the melt, close to the ladle nozzle, is small rotating anticlockwise and intense vertical-recirculating flows are developed in most of the liquid volume due to descending steel streams along the ladle vertical wall. These streams ascend further downstream driven by buoyancy forces. At low bath levels, the melt, which is close to the nozzle, rotates clockwise with higher velocities whose magnitudes are higher for shorter ladle standstill times. These velocities are responsible for the formation and development of a vortex on the bath free surface, which entrains slag into the nozzle by shear-stress mechanisms at the metal-slag interface. The critical bath level or bath height for this phenomenon is 0.35 m (in this particular ladle design) for a ladle standstill time of 15 minutes and decreases with longer ladle standstill times. At these steps, the vertical-recirculating flows are substituted by complex horizontal-rotating flows in most of the liquid volume. Under isothermal conditions, the critical bath level for vortex formation on the melt free surface is 0.20 m, which agrees very well with that determined with a 1/3 scale water model of 0.073 m. It is concluded that buoyancy forces, originated by thermal gradients, as the ladle cools, are responsible for increasing the critical bath level for vortex formation. Understanding vortex mechanisms will be useful to design simple and efficient devices to break down the vortex flow during steel draining even at very low metal residues in the ladle.

Capillary puddle vibrations linked to casting-defect formation in planar-flow melt spinning

Abstract: Planar-flow melt spinning (PFMS) is a single-stage rapid manufacturing/solidification technique for producing thin metal sheets or ribbons. Molten metal is forced through a nozzle onto the substrate where it freezes and is spun as ribbon product. A puddle of molten metal held by surface tension (capillarity) forms between the nozzle and substrate. An important measure of product quality is the uniformity of thickness along and across the ribbon. At small length scales, local thickness changes or surface defects are present that are undesirable. This work examines the cross wave, a well-defined periodic surface defect, seen when casting aluminum-silicon alloys. The presence of the defect is related to processing conditions and puddle dynamics. Motions of the puddle menisci are captured using high-speed video and analyzed for frequency content. A high frequency vibration of both menisci corresponds to the observed frequency of the surface defect. A scaling analysis reveals these motions to be capillary in nature and comparisons are made with two model problems of vibrating capillary liquids.

Investigation into the role of the boudouard reaction in self-reducing iron oxide and carbon briquettes

Abstract: With increasing impetus to utilize more waste byproducts from ironmaking processes, a greater understanding of processes that utilize these wastes is required. In this article, the kinetics of reaction of briquettes consisting of ultrafine iron oxide and carbon are examined representing a progression in the understanding of methods to utilize fine iron oxide and carbon resulting from all stages of processing. More specifically, this article examines the role of the Boudouard reaction and its level of control in such reaction systems for the temperature range of 1000 °C to 1200 °C. Various techniques were used to examine this role, including investigating the effect of the surface area of the carbon used, the effect of a known Boudouard reaction catalyst, and the effect of temperature.

Influence of phosphorus element on direct laser sintering of multicomponent Cu-based metal powder

Abstract: This article presents a detailed investigation on the influence of the phosphorus element upon the laser sintering of a multicomponent Cu-based metal powder system consisting of Cu, Cu-10Sn, and Cu-84P Powder systems containing 0, 10, 15, and 20 wt pct CuP were sintered in atmosphere at room temperature using the following optimal processing parameters: laser power of 350 W, scan speed of 004 m/s, scan line spacing of 015 mm, and layer thickness of 025 mm It was found that the relative density of the sintered sample with 15 wt pct CuP increased by 24,4 pct as compared with the sample without phosphorus addition A further increase in the CuP content (≥20 wt pct), however, resulted in a poor densification with a serious delamination The exact metallurgical roles of the phosphorus element in the laser sintering process were addressed as follows First, the phosphorus could prevent the sintering system from oxidation by forming CuPO3, thereby improving the wetting characteristics and the sintering kinetics Second, the phosphorus could decrease the surface tension of molten materials, leading to a successive transition from highly discontinuous sintered tracks to fairly coherent ones with increasing the phosphorus content Third, the phosphorus could lower the melt viscosity, thereby improving the microstructural homogeneity of the laser-sintered samples

Water-cooled probe technique for the study of freeze lining formation

Abstract: Furnace protection by water-cooled freeze linings becomes increasingly important as the metal producing industry attempts to achieve higher process intensities. Systematic investigations of the growth and the resulting microstructure and compositional profile of freeze linings are necessary to understand the behavior of freeze linings, their relation with the industrial process, and their interaction with the wall cooling system. We have developed a technique based on the submergence of a water-cooled probe into a liquid slag bath. Freeze linings of two industrial nonferrous slags have been produced using this technique and their growth, microstructural, and compositional profiles as a function of submergence time were determined. Thermodynamic equilibrium for the investigated slag systems was calculated and compared with the observed microstructures. The freeze linings form in approximately 15 minutes. Close to the water cooling, the freeze linings are predominantly amorphous in structure. With increasing distance from the water cooling, the proportion of crystalline phases increases and bath material is entrapped in the microstructure. Cellular crystals are observed close to the bath. The freeze linings exhibit an approximate homogeneous composition. The results demonstrate that the technique is a successful tool in obtaining information on the growth, microstructure, and composition of freeze linings in industrial water-cooled furnaces.

Dissolution of carbon from alumina-carbon mixtures into liquid iron: Influence of carbonaceous materials

Abstract: Due to their excellent thermal shock and wear resistance at high temperatures, alumina-carbon based refractories are used extensively in the steel industry. A clear understanding of factors affecting the dissolution of carbon from refractories is of crucial importance, as carbon depletion from the refractory can significantly deteriorate refractory performance and metal quality. Atomistic simulations on the alumina-graphite/liquid iron system have shown that nonwetting between alumina and liquid iron is an important factor inhibiting the penetration of liquid metal in the refractory matrix and limiting carbon dissolution. This study investigates the role played by the carbonaceous material in the dissolution of carbon from the refractory composite. Two carbonaceous materials, namely, petroleum coke and natural graphite, respectively, containing 0.35 and 5.26 pct ash, were used in this study. Substrates were prepared from mixtures of alumina and carbon over a wide concentration range. Using a sessile drop arrangement, carbon pickup by liquid iron from alumina-carbon mixtures was measured at 1550 °C and was compared with the carbon pickup from alumina-synthetic graphite mixtures. These studies were supplemented with wettability measurements and microscopic investigations on the interfacial region. For high alumina concentrations (>40 wt pct), carbon dissolution from refractory mixtures was found to be negligible for all carbonaceous materials under investigation. Significant differences however were observed at lower alumina concentrations. Carbon dissolution from alumina-petroleum coke mixtures was much lower than the corresponding dissolution from alumina synthetic graphite-mixtures and was attributed to poor wettability of petroleum coke with liquid iron, its structural disorder, and the presence of sulfur. Very high levels of carbon dissolution, however, were observed from alumina-natural graphite mixtures, with carbon pickup by liquid iron from mixtures with up to 30 wt pct alumina reaching saturation. A sharp reduction to near zero levels was observed in the 30 to 40 wt pct alumina range. Along with implications for commercial refractory applications, these results are discussed in terms of material characteristics, interactions between ash impurities and alumina, and formation of complexes in the interfacial region.

Blast furnace burden softening and melting phenomena: Part III. melt onset and initial microstructural transformations in pellets

Abstract: The softening and melting of the ferrous burden in the blast furnace occur in the cohesive zone, affecting the furnace’s performance. This work was designed to better understand these phenomena. In this article, the onset of melting and the evolution of the microstructure before softening were analyzed using a confocal scanning laser microscope (CSLM). Experiments were performed with industrial or synthetic pellets prereduced to wustite, placed over a metallic iron liner, and heated at either 1°C/min or 3°C/min under argon. The samples showed exudation of liquid, which coated the liner at temperatures above 1150°C. The melt onset for FeO-SiO2 synthetic samples occurred at similar temperatures, regardless of the experimental conditions. The melt onset temperature for the basic samples was lower than the acid samples. A sudden increase of the amount of liquid occurred first for the acid samples. A transition in the microstructure of the burden from heterogeneous “as-received” to a partially molten homogeneous structure with wustite particles permeated by an intergranular (slag) phase was observed in the industrial samples. These results suggest that the initial liquid does not directly influence softening; rather, it acts to improve mass transport and to push the system to a semisolid.

Analysis of interfacial area changes during spontaneous emulsification of metal droplets in slag

Abstract: In some metal/slag reactions involving spontaneous emulsification, there is a significant increase of interfacial area, which in turn affects the global rate. In previous work by the authors, the reaction between Fe-Al alloy droplets and CaO-SiO2-Al2O3 slag was investigated. Re-evaluation of the data has shown that at an initial reaction rate above 9 × 10−7 mol min−1 mm−2, the maximum change in interfacial area increases linearly with the initial rate and with the change of free energy due to chemical reaction. There were found to be two sources of interfacial area increase: (a) flattening of the original droplet, which was independent of initial rate; and (b) separation of smaller droplets, which was dependent on the initial rate.