Showing papers in "Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science in 2005"

The effect of cooling rate on the solidification of INCONEL 718

Abstract: The superalloy INCONEL 718 (IN718) is a commonly used material in aerospace and turbine components. The advantage of this type of material with sluggish precipitation-hardening kinetics is that IN718 is readily weldable. Both wrought and cast parts are used and welded together. While the alloy has been studied previously, new production processes such as laser treatment demand better knowledge of the solidification process in IN718, especially at high cooling rates. In this investigation, the solidification process was studied over a wide range of cooling rates by three different experimental techniques: differential thermal analysis (DTA), mirror furnace (MF), and levitation casting. The solidification sequence and the reaction temperatures were identified. The microstructure and the change in growth morphology were also studied. Segregation measurements were performed, and the distribution of Nb was analyzed in detail for the different types of samples, because of its strong impact on the solidification sequence and microstructure. New observations are that the latent heat decreases and the effective partition coefficient increases with increasing cooling rate. The diffusion rate also seems to be enhanced in the first part of primary solidified dendrites. It is suggested that the new observations can be explained by an increased number of lattice defects formed in the solid as the cooling rate increases.

Solid oxide membrane process for magnesium production directly from magnesium oxide

Abstract: The solid oxide membrane (SOM) process is an emerging technology for the environmentally friendly extraction of high-energy-content metals such as magnesium, tantalum, and titanium directly from their respective oxides. This paper reports on the recent success of the SOM process for magnesium production from magnesium oxide dissolved in fluoride-based fluxes in the temperature range 1150 °C to 1300 °C. This process employs an inert oxygen-ion-conducting stabilized zirconia membrane to separate the inert cathode in the flux from the anode. When the applied electrical potential between the electrodes exceeds the dissociation potential of magnesium oxide, oxygen ions are pumped out of the melt and through the zirconia membrane to the anode where they are oxidized. Reduced magnesium evolves at the cathode as a vapor and is condensed in a separate chamber yielding a high-purity product. The SOM cell has been electrochemically characterized, and key concepts related to MgO dissociation, leakage current, and mass transfer relevant to the SOM process are explained.

Removal of B from Si by solidification refining with Si-Al melts

Abstract: To discuss the removal of B by solidification refining of Si with a Si-Al melt, the segregation of B between solid Si and the Si-Al melt was investigated by use of the temperature-gradient-zone melting (TGZM) method. The segregation ratio of B at its infinite dilution was determined to be 0.49 (1473 K), 0.32 (1373 K), and 0.22 (1273 K), respectively. With the obtained segregation ratio, the activity coefficient of B in solid Si at its infinite dilution relative to pure solid B was determined by the following equation: $$RT ln \gamma _{B(s) in solid Si}^ \circ = 117,000 ( \pm 2300) - 47.6 ( \pm 1.6)T ({J \mathord{\left/ {\vphantom {J {mol}}} \right. \kern- ulldelimiterspace} {mol}})$$ Calculated results of directional solidification of the Si-Al alloy revealed the removal fraction of B to be as much as 90 pct. The effective removal of B by a solidification refining process with a Si-Al melt is clarified.

Boron removal by titanium addition in solidification refining of silicon with Si-Al melt

Abstract: In order to effectively remove B from Si for its use in solar cells, a process involving B removal by solidification refining of Si using a Si-Al melt with Ti addition was investigated. For clarifying the effect of Ti addition on B removal from the Si-Al melt, TiB2 solubilities in Si-64.6 at. pct Al melt at 1173 K and Si-60.0 at. pct Al melt at 1273 K were determined by measuring the equilibrium concentrations of B and Ti in the presence of TiB2 precipitates. The small solubilities of TiB2 in the Si-Al melt indicate the effective removal of B from the Si-Al melt by Ti addition. Further, solidification experiments of Si-Al alloys containing B by Ti addition were performed, and the effect of Ti addition on the solidification refining of Si with the Si-Al melt was successfully confirmed.

Effect of CaO-Al2O3-MgO slags on the formation of MgO-Al2O3 inclusions in ferritic stainless steel

Abstract: A thermodynamic equilibrium between the Fe-16Cr melts and the CaO-Al2O3-MgO slags at 1823 K as well as the morphology of inclusions was investigated to understand the formation behavior of the MgO-Al2O3 spinel-type inclusions in ferritic stainless steel. The calculated and observed activities of magnesium in Fe-16Cr melts are qualitatively in good agreement with each other, while those of aluminum in steel melts exhibit some discrepancies with scatters. In the composition of molten steel investigated in this study, the log (X MgO/X Al 2O3) of the inclusions linearly increases by increasing the log [a Mg/a Al 2 ·a O 2 ] with the slope close to unity. In addition, the relationship between the log (X MgO/X Al 2O3) of the inclusions and the log (a MgO/a Al 2O3) of the slags exhibits the linear correlation with the slope close to unity. The compositions of the inclusions are relatively close to those of the slags, viz. the MgO-rich magnesia-spinel solid solutions were formed in the steel melts equilibrated with the highly basic slags saturated by CaO or MgO. The spinel inclusions nearly saturated by MgO were observed in the steel melts equilibrated with the slags doubly saturated by MgO and MgAl2O4. The spinel and the Al2O3-rich alumina-spinel solid solutions were formed in the steel melts equilibrated with the slags saturated by MgAl2O4 and MgAl2O4-CaAl2O4 phases, respectively. The apparent modification reaction of MgO to the magnesium aluminate inclusions in steel melts equilibrated with the highly basic slags would be constituted by the following reaction steps: (1) diffusion of aluminum from bulk to the metal/MgO interface, (2) oxidation of the aluminum to the Al3+ ions at the metal/intermediate layer interface, (3) diffusion of Al3+ ions and electrons through the intermediate layer, and (4) magnesium aluminate (MgAl2O4 spinel, for example) formation by the ionic reaction.

Temperature and seeding effects on the precipitation of scorodite from sulfate solutions under atmospheric-pressure conditions

Abstract: Arsenic is a major contaminant in the nonferrous extractive metallurgy. In the past 20 years, many studies have shown that it can be precipitated as relatively stable crystalline scorodite (FeAsO4·2H2O) by precipitation under ambient or elevated pressures. In the present study, an extensive program of scorodite precipitation tests under ambient pressure has shown that the rate of scorodite formation increases dramatically by a small increase in temperature from 85 °C to 100 °C. The beneficial effects of temperature are attributed to the higher thermodynamic stability of scorodite at elevated temperatures, but also to higher rates of secondary nuclei formation and crystal growth. In any case, irrespective of the precipitation temperature, the leachability of all scorodite precipitates observed in toxicity characterization leaching procedure (TCLP) tests is below 5 mg/L As. Another parameter examined in this study was seeding. It was observed that the higher the initial concentration of seed, the faster the precipitation. Precipitation of well-crystallized scorodite can be effected equally well on heterogeneous seed such as hematite (Fe2O3) or gypsum (CaSO4·2H2O) added externally or formed in situ.

Carbon structure of coke at high temperatures and its influence on coke fines in blast furnace dust

Abstract: A thermal annealing study of three industrial cokes was carried out in a horizontal tube furnace at a range of temperatures up to 1600 °C under N2. Evolution of the carbon structure of cokes was established by determining the stack height (L002) of aromatic carbon layers on the basis of the 002 carbon peak in their X-ray diffraction (XRD) spectra by using the classical Scherrer’s approach. The heat-treatment temperature is shown to have a strong impact on the growth of crystalline order of coke carbon by demonstrating a linear correlation between the carbon crystallite height (L002) and the annealing temperature. The intensity of the thermal effects on the growth of the crystalline order of coke carbon is influenced by the coke ash chemistry, particularly with the iron content of the coke. The carbon structure of blast furnace (BF) dust samples was also analyzed by using XRD and scanning electron microscopy (SEM). Under a similar range of heat-treatment temperatures, growth of the carbon crystallite (L002) of coke, in both the laboratory and the industrial BF, was found to be of the same order of magnitude. The correlation between the carbon structure (L002) of coke and the annealing temperature is used to ascertain the temperature of the origin of coke fines in a BF. The carbon structure of coke is shown to have a significant influence on the coke behavior in a BF such that highly ordered coke displayed lower reactivity as well as higher proportion of coke fines in the dust. The carbon structure of coke fines in BF dust has been shown as an indicator of the crystallite dimension (L002) of the coke in a BF, and has a potential to assess coke performance, particularly of the coke fine generations from different thermal regimes of a BF and also their subsequent consumption.

A mold simulator for continuous casting of steel: Part II. The formation of oscillation marks during the continuous casting of low carbon steel

Abstract: The restrictions on quality for low carbon continuously cast slab products require that surface defects be kept to a minimum. Currently, the steel industry has developed a wealth of experience on how to apply slabs with oscillation marks to very demanding applications. However, these practices circumvent the problem, rather than solving it. By understanding the formation mechanism of oscillation marks, one can then develop casting practices that can minimize their effect on slab surface quality. The techniques developed in this study allowed a more detailed examination of the mold heat-transfer interactions during continuous casting, such that the variations of heat flux due to irregular solidification near the meniscus could be measured. It is shown that the mechanisms proposed in the literature are not individually sufficient for the formation of an oscillation mark, but that several are necessary and must occur in sequence for an oscillation mark to form. A mechanism is proposed for the formation of oscillation marks that is shown to be in agreement with the trends observed and reported in the literature. Additionally, it is shown that the success of practices used in industry to reduce the severity of oscillation marks can be explained using this proposed hypothesis.

Modeling of trajectory and residence time of metal droplets in slag-metal-gas emulsions in oxygen steelmaking

Abstract: In basic oxygen steelmaking, the major portion of the refining is realized through reactions between metal droplets and slag. The residence time of metal droplets in the slag crucially influences the productivity. A model for the prediction of trajectory and residence time of metal droplets in slags has been developed based on mechanics and chemical kinetics principles. When there is no decarburization, analysis of the ballistic motion of metal droplets in the slag predicts very short residence times (<1 second). This result demonstrates that when decarburization is very weak, the metal droplets spend a very short time in the slag. This could explain in part the poor kinetic behavior in the end stage of the blow. During active decarburization metal droplets normally become bloated, resulting in a decreased apparent density. Accounting for this, the ballistic model predicts residence times ranging from 10 to 200 seconds, which are much more in keeping with practical experience and previous laboratory studies. Excellent agreement between the model and laboratory measurements, combined with reasonable predictions of industrial residence times, shows that this model can be used to provide a much improved understanding of theoretical aspects of oxygen steelmaking.

Effect of subsurface thermocouple installation on the discrepancy of the measured thermal history and predicted surface heat flux during a quench operation

Abstract: Water quenching plays an important role in metallurgical and materials manufacturing operations to control both the temperature of the product during processing and its final microstructure. In order to control a water-quench process, the surface heat-transfer coefficient or heat flux must be quantified accurately. A common procedure to do this is to use an inverse heat conduction (IHC) model to estimate the heat-transfer boundary condition (heat flux or heat- transfer coefficient) based on the measured thermal history during the quench operation at a known interior location in the sample. Traditionally, thermocouples (TCs) have been extensively used during quench tests to measure the sample temperature history. This article will examine the effect of the hole used to insert the thermocouple into the sample and its orientation with respect to the quenched surface, on the perturbation in the thermal field around the TC measurement point during water-quench operations characterized by boiling heat transfer. The effect of some other factors on the perturbation of the thermal field at the TC measurement point during water-quench operations such as the diameter of the thermocouple hole, thermocouple distance from the quench surface, sample thermal conductivity, and quench intensity were also investigated. A two-dimensional (2-D) axisymmetric IHC model developed at the University of British Columbia is used to estimate the error in the predicted heat fluxes based on the thermal history measured at the thermocouple measurement point. The study showed, for some quench conditions, that the thermocouple hole must be included in the IHC analysis as an independent body with its own thermophysical and geometrical characteristics. Validation of these model-predicted results was done using water-quench experiments performed on samples of steel and aluminum plates at the University of British Columbia. Using the Biot number (Bi), a simple criterion is developed to determine when the TC hole needs to be included in the heat-transfer analysis.

Inclusion control of ferritic stainless steel by aluminum deoxidation and calcium treatment

Abstract: A thermodynamic equilibrium between aluminum and oxygen and inclusion morphology in the Fe-16Cr stainless steel were investigated to understand the fundamentals of the aluminum deoxidation technology for ferritic stainless steels. Further, the effect of calcium addition on the changes in chemistry and morphology of inclusions was discussed. The measured results for the aluminum-oxygen equilibria exhibit relatively good agreement with the calculated values, indicating that an introduction of the first-and second-order interaction parameters, recently reported, is reasonable to numerically express the aluminum deoxidation equilibrium in a ferritic stainless steel. In the composition of dissolved aluminum content greater than about 60 ppm, pure alumina particles were observed, while the alumino-manganese silicates containing Cr2O3 appeared at less than 20 mass ppm of dissolved aluminum. The formation of calcium aluminate inclusions after Ca treatment can be discussed based on the thermodynamic equilibria among calcium, aluminum, and oxygen in the steel melt. In the composition of steel melt with relatively high content of calcium and low aluminum, the log (\(X_{CaO} /X_{Al_2 O_3 } \)) of inclusions linearly increases by increasing the log [aCa/aAl2·aO2] with the slope close to unity. However, the slope of the line is significantly lower than the expected value in the composition of steel melt with relatively low calcium and high aluminum contents.

Ultrasonic Processing of Materials

Abstract: Irradiation of high-energy ultrasonic vibration in metals and alloys generates oscillating strain and stress fields in solids, and introduces nonlinear effects such as cavitation, acoustic streaming, and radiation pressure in molten materials. These nonlinear effects can be utilized to assist conventional material processing processes. This article describes recent research at Oak Ridge National Labs and Purdue University on using high-intensity ultrasonic vibrations for degassing molten aluminum, processing particulate-reinforced metal matrix composites, refining metals and alloys during solidification process and welding, and producing bulk nanostructures in solid metals and alloys. Research results suggest that high-intensity ultrasonic vibration is capable of degassing and dispersing small particles in molten alloys, reducing grain size during alloy solidification, and inducing nanostructures in solid metals.

Seeding of single-crystal superalloys: Role of constitutional undercooling and primary dendrite orientation on stray-grain nucleation and growth

Abstract: An experimental study, together with a two-dimensional numerical simulation of solute segregation, was conducted to investigate (1) the mechanism for stray-grain nucleation following seed melt-back and initial withdrawal and (2) the role of the primary dendrite disposition of the seed crystal in relation to the mold wall during growth. It is proposed that the factors contributing to stray-grain nucleation during initial withdrawal are (1) the magnitude of local, solute-adjusted undercooling and (2) the rapidly changing curvature of the solidification front close to the mold walls during the initial solidification transient. Based upon the calculated local undercooling and experimentally observed stray-grain morphologies, it was concluded that stray grains nucleate near the mold wall around the seed perimeter and behind the columnar dendrites that advance into the bulk liquid ahead of the melt-back zone. These grains then compete during growth with the dendrites originating from the seed. Therefore, the morphological constraints arising from the inclination of the primary dendrites from the seed crystal with respect to the mold wall (converging/diverging/axial 〈001〉) determines the probability of the stray-grain nuclei developing into equiaxed/columnar grains following competitive growth.

Hydration of mechanically activated granulated blast furnace slag

Abstract: Ground granulated blast furnace slag (GGBFS) is known to possess latent hydraulic activity, i.e., it shows cementitious properties when in contact with water over a long period of time. Results are presented in this article to show that, in sharp contrast to published literature on the hydration of neat GGBFS, the complete hydration of slag is possible in a short time (days), even without a chemical activator. This is achieved if the slag used for hydration is mechanically activated, using an attrition mill. The nature of the hydration product of the mechanically activated slag depends not only on the initial specific surface area (SSA) of the slag but also on the surface activation, as manifested by the change in the zeta potential (ξ) of the slag during the milling process. Depending upon the SSA and the ξ, the hydration product changed from nonreacted slag with high porosity (slag SSA −29 mV) to hydrated slag with a compact structure (SSA=0.3 to 0.4 m2/g, ξ=−29 to −31 mV), and, finally, to fully hydrated slag with high porosity (SSA>0.4 m2/g, ξ ∼ 26 mV). Unlike the poorly crystalline hydration product formed by the nonactivated slag, even after prolonged hydration for years, the hydration product of mechanically activated slag was crystalline in nature. The crystallinity of the product improved as the duration of the mechanical activation increased. The calcium-silicate-hydrate (C-S-H) phases present in the slag hydration product, characterized by a Ca/Si ratio of 0.7 to 1.5, were similar to those found for the hydraulic cement binder, except for the presence of Mg and Al as impurities. In addition, the presence of a di-calcium-silicate-hydrate phase (α-C2SH), which normally forms under hydrothermal conditions, and a Ca-deficient and Si-Al-rich phase (average Ca/Si mole ratio < 0.1 and Si/Al ∼ 3) is indicated, especially in the hydration product of slag that was activated for a longer time.

Kinetics of metal/slag reactions during spontaneous emulsification

Abstract: An approach for accommodating the interfacial area changes in kinetic equations for heterogeneous reactions in the presence of spontaneous emulsification has been proposed. The kinetics were analyzed by incorporating time-averaged interfacial areas in the rate equations. The approach was found to be applicable for the experimental data and to satisfactorily describe the reaction kinetics. In the case of a high-temperature reaction between 2.35 g Fe-5 wt pct Al alloy metal droplets with CaO-SiO2-Al2O3 slag at 1650 degrees C, it was found that the kinetics follow a first-order relationship with respect to aluminum in the metal, and it was concluded that they were controlled by mass transport in the metal phase. The calculated metal mass-transfer coefficient k(m) was 1.7 x 10(-6) m/s.

A mold simulator for the continuous casting of steel. Part I. The development of a simulator

Abstract: Surface defects, such as oscillation marks, ripples, and cracks that can be found on the surface of continuously cast steel, originate in the continuous casting mold. Therefore, a detailed knowledge of initial solidification behavior of steel in a continuous casting mold is necessary because it determines the surface quality of continuously cast slabs. In order to develop an understanding of the initial solidification of continuous cast steels, a “mold simulator” was designed and constructed to investigate heat-transfer phenomena during the initial phase of strand solidification. The mold simulator was used to obtain solidified steel shells of different grades of steel under conditions similar to those found in industrial casting operations. The resulting cast surface morphologies were compared with industrial slabs and were found to be in good agreement, indicating that it is possible to simulate the continuous casting process by a laboratory scale simulator.

A quasi-chemical viscosity model for fully liquid slags in the Al2O3-CaO-‘FeO’-SiO2 system

Abstract: A structurally based viscosity model for fully liquid silicate slags has been proposed and applied to the Al2O3-CaO-‘FeO’-SiO2 system at metallic iron saturation. The model links the slag viscosity to the internal structure of melts through the concentrations of various anion/cation structural units (SUs). The concentrations of structural units are equivalent to the second nearest neighbor bond concentrations calculated by the quasi-chemical thermodynamic model. This viscosity model describes experimental data over the entire temperature and composition range within the Al2O3-CaO-‘FeO’-SiO2 system at metallic iron saturation and can be extended to other industrial slag systems.

Transient fluid flow and superheat transport in continuous casting of steel slabs

Abstract: The turbulent flow of molten steel and the superheat transport in the mold region of a continuous caster of thin steel slabs are investigated with transient large-eddy simulations and plant experiments. The predicted fluid velocities matched measurements taken from dye-injection experiments on full-scale water models of the process. The corresponding predicted temperatures matched measurements by thermocouples lowered into the molten steel during continuous casting. The classic double-roll flow pattern is confirmed for this 132×984 mm slab caster at a 1.52 m/min casting speed, with about 85 pct of the single-phase flow leaving the two side ports of the three-port nozzle. The temperature in the top portion of the molten pool dropped to about 30 pct of the superheat-temperature difference entering the mold of 58 °C. About 12 pct of the superheat is extracted at the narrow face, where the peak heat flux averages almost 750 kW/m2 and the instantaneous peaks exceed 1500 kW/m2. Two-thirds of the superheat is removed in the mold. The jets exiting the nozzle ports exhibit chaotic variations, producing temperature fluctuations in the upper liquid pool of ±4 °C and peak heat-flux variations of±350 kW/m2. Employing a static-k subgrid-scale (SGS) model into the three-dimensional (3-D) finite-volume code had little effect on the solution.

Mineralogical Characterization of a Copper Anode and the Anode Slimes from the La Caridad Copper Refinery of Mexicana de Cobre

Abstract: A detailed mineralogical study was carried out to characterize a copper anode, the anode-face slimes, the slimes on the bottom of the refining cell, and the autoclave-leached slimes from the La Caridad refinery of Mexicana de Cobre. The objective was to identify possible Pb-Sb-Bi and As-Sb-Bi interactions that could control the Sb and Bi concentrations of the electrolyte. Although some Pb, As, Sb, and Bi can be found in solid solution in the copper crystals of the anode, these elements are mostly present as Cu-Pb-As oxide and Cu-Pb-As-Sb-Bi oxide inclusions at the grain boundaries. During electrorefining, the Pb, As, Sb, and Bi in solid solution dissolve. Part of the Pb, As, Sb, and Bi in the oxide inclusions also dissolves, but part reacts in situ to form PbSO4 and Pb5(AsO4)3(OH,Cl). Some of the dissolved elements reprecipitate as PbSO4, SbAsO4, Sb-As oxide, Sb-As-Bi oxide, Pb5(AsO4)3(OH,Cl), and an oxidate phase of mainly Cu-Ag-AsO4-SO4 composition. Thus, high As contents facilitate the precipitation of Sb and Bi from the electrolyte. Although Pb-Sb oxide and Pb-Bi oxide species were only rarely detected, a high Pb content in the anode may retard the dissolution of the Cu-Pb-As-Sb-Bi oxide inclusions, thereby retaining some Sb and Bi in the raw anode slimes. Autoclave leaching dissolves part of As, Sb, and Bi, but the SbAsO4 and Sb-As-Bi oxide species remain in the leach residue. The Pb is converted almost entirely to PbSO4, which is present as subhedral crystals in the autoclave leach residue.

MgOHCl thermal decomposition kinetics

Abstract: Experiments to determine the kinetics of the thermal decomposition of MgOHCl were performed. It was found that the decomposition of MgOHCl commenced at 649 K, and it directly converted into MgO and HCl without undergoing any intermediate step. Decomposition vs time data showed that the thermal decomposition of MgOHCl was a first-order process with respect to the amount of MgOHCl remaining, and the mass transfer of the product HCl gas away from the interface was likely the rate-limiting step. It was also found that the time required to completely decompose MgOHCl into MgO, more than 20 minutes, at the operating temperatures of electrolytic magnesium production processes, 600 °C±50 °C, was significantly longer than the time required, less than 1 minute, to digest the solid magnesium chloride containing feed material into the molten salt electrolyte in these processes. Such delay in the decomposition would mean that any MgOHCl produced during heating and digestion of the feed would not be decomposed by the heat of the electrolyte and thus the persistent MgOHCl would dissolve into the molten salt electrolyte with potentially severe negative consequences on electrolysis cell operation.

Interaction between refractory crucible materials and the melted NiTi shape-memory alloy

Abstract: Attempts have been made to quantify the amount of contaminants absorbed by liquid metal from commercial ZrO2-, Al2O3-, and SiC-base crucibles used for vacuum melting of Ni-45 wt pct Ti alloy. The molten alloy was held under vacuum for 90 minutes at 1450 °C to become homogenized. Reactions between the liquid metal and the crucible were investigated by visual observation, chemical analysis, scanning electron microscopy (SEM) image processing, and X-ray mapping. The relative degree of contamination declined in the following sequence: commercially pure SiC>SiC-5 wt pct Al2O3-5 wt pct SiO2>slurry cast alumina>recrystallized alumina>zircon type A>oxygen deficient high-purity zirconia. Thermodynamic calculations showed a difference between the equilibrium and the experimental data, indicating that except for commercially pure SiC crucible, the amount of the crucible elements entering the melt is greater than the calculated equilibrium values. This discrepancy seems to be due to the immersion into the melt of the undissolved chemical compounds formed due to the reactions between the crucible and the liquid phase.

The reduction of iron oxides by volatiles in a rotary hearth furnace process: Part I. The role and kinetics of volatile reduction

Abstract: With iron ore reduction processes using coal-ore pellets or mixtures, it is possible that volatiles can contribute to reduction. By simulating the constituents of the individual reducing species in the volatiles, the rates for H2 and CO were investigated in the temperature and reduction range of interest; hydrogen is the major reductant and was studied in detail. The kinetics of the reduction by H2 has been found to be a complex mechanism with, initially, nucleation and growth controlling the rate. There is a catalytic effect by the existing iron nuclei, followed by a mixed control of chemical kinetics and pore diffusion. This results in a topochemical reduction of these iron oxide particles. Up to 1173 K, reduction by H2 is considerably faster than by carbon in the pellet/mixture or by CO. It was also found that H2S, which is involved with the volatiles, does not affect the rate at the reduction range of interest.

Energy-input-based finite-element process modeling of inertia welding

Abstract: A coupled thermal and mechanical finite-element (FE) model has been developed to describe the inertia welding of RR1000 nickel-base superalloy tubes using the DEFORM 7.2 FE package. The energy input rate is derived from measurements of torque, angular rotation speed, and upset taken from actual inertia welding trials. The model predicts the thermal history of the joint as well as the deformation pattern and final residual stresses. The thermal variation has been validated by a microstructural study of the weld region of the trial joints. Thermal profile predictions have been made for three welds having the same initial kinetic rotational energy but different levels of flywheel inertia and rotational velocity. The concomitant residual stress predictions have been compared with nondestructive neutron diffraction residual stress measurements. The implications of the results for inertia welding are discussed.

A comparative study on the surface integrity of plastic mold steel due to electric discharge machining

Abstract: The violent nature of the electric discharge machining (EDM) process leads to a unique structure on the surface of a machined part. In this study, the influence of electrode material and type of dielectric liquid on the surface integrity of plastic mold steel samples is investigated. The results have shown that regardless of the tool electrode and the dielectric liquid, the white layer is formed on machined surfaces. This layer is composed of cementite (Fe3C) and martensite distributed in retained austenite matrix forming dendritic structures, due to rapid solidification of the molten metal, if carbon-based dielectric liquid is used. The intensity of cracking increases at high pulse durations and low pulse currents. Cracks on the EDM surfaces have been found to follow the pitting arrangements with closed loops and to cross perpendicularly with radial cracks and continue to propagate when another discharge takes place in the neighborhood. The amount of retained austenite phase and the intensity of microcracks have found to be much less in the white layer of the samples machined in de-ionized water dielectric liquid. The number of globule appendages attached to the surface increased when a carbon-based tool electrode material or a dielectric liquid was used during machining.

Mixing time and correlation for ladles stirred with dual porous plugs

Abstract: Bulk mixing times up to a degree of 95 pct were measured in three different, cylindrical-shaped water model ladles (D=0.60 m, 0.45 m, and 0.30 m, respectively) in which, water was agitated by air introduced through two tuyeres/nozzles placed diametrically opposite at the base of the vessels at ±1/2 R positions. To this end, the electrical conductivity measurement technique was applied. A range of gas flow rates and liquid depths were investigated (viz. 0.7≤L/D≤1.2 and 0.002≤ɛ m (watt/kg)≤0.01) and these were so chosen to conform to the practical ladle refining conditions. In the beginning, extensive experimental trials were carried out to assess the reliability of the measurement technique. In addition, some experiments were carried out to determine the location of the probe in the vessel such that measured mixing times could be interpreted as the bulk mixing times.

Analysis of the source of dynamic interfacial phenomena during reaction between metal droplets and slag

Abstract: The dynamic interfacial phenomena during high-temperature reaction between an Fe-Al alloy droplet and a CaO-SiO2-Al2O3 slag were analyzed by evaluating the thermocapillary, solutocapillary, and electrocapillary effects. The magnitudes of these effects were determined using the local equilibrium model and utilizing kinetic data to determine local composition, temperature, and electrical potential. The electrocapillary effect was found to be dominant. It contributed approximately 85 pct of the maximum interfacial depression while the solutocapillarity contributed 15 pct. The thermocapillary effect was found to be negligible. In this work, the local gradient of interfacial tension along the interface due to the solutocapillary effect was estimated, i.e., Delta(gamma m-s) to 440 mN/m over a 1- to 2-mu m distance.

Kinetics of scrap melting in liquid steel

Abstract: The melting rate of steel bars with various sizes, shapes, and initial temperatures in a 70 kg liquid steel bath (1650 °C) was measured to investigate the kinetics involved in steel scrap melting. Our measurements revealed that a solidified shell was formed around the original bar immediately after it was immersed into the liquid steel. This shell and an associated interfacial gap generated between it and the original bar were found to be critical to the melting kinetics. We also found that the total melting time decreased linearly with increasing initial bar temperature. The melting process was simulated using a two-dimensional phase-field model that considered heat convection with a constant heat-transfer coefficient. Our simulations were in good agreement with our experiments and showed that the heat conduction associated with the interfacial gap was one of the most important physical aspects controlling the melting of steel scrap.

Degradation of MgO Refractory in CaO-SiO2-MgO-FeOx and CaO-SiO2-Al2O3-MgO-FeOx Slags under Forced Convection

Abstract: Experiments based on exposure of MgO to slags under forced convection flow conditions allowed the identification of different degradation mechanisms and the assessment of the role of Al2O3 in the degradation process. Slag with no alumina present resulted in direct dissolution. Samples immersed in alumina containing slag underwent indirect dissolution, with a spinel forming at the MgO-slag interface. At 1530 °C, the spinel was not effective in reducing the corrosion rate, as the scattered spinel grains were easily removed from the MgO surface. At 1500 °C, the loss of MgO was reduced due to the formation of a more cohesive spinel layer. Mechanical erosion then appears to play a greater role. Strength of the bond between the spinel and underlying MgO needs to be considered in strategies to reduce degradation of MgO refractories.

The physical chemistry of thermal decomposition of South African chromite minerals

Abstract: Natural chromite minerals form extended solid solutions with binary spinels of FeCr2O4, MgCr2O4, FeAl2O4, and MgAl2O4. The decomposition of natural spinels strongly depends upon the chemical potential imposed in the forms of temperature, pressure, and pH difference in aqueous media (e.g., during natural weathering). In this investigation, we studied the thermal decomposition behavior of South African chromite ores in order to relate the influence of oxygen potential with the likely product phases formed. The decomposition is also a generic step in the understanding of the formation of sodium chromate during soda-ash roasting and the reduction of chromite ores for ferrochrome alloy making. The phase equilibria in South African chromite minerals were investigated by isochronal thermal analysis and isothermal heat treatment of chromite mineral in air, argon, and Ar-5 pct H2 atmospheres over a temperature range from 473 to 1473 K. The effects of the oxygen partial pressure and temperature on the phase constituents of the heat-treated product are discussed by referring to the results of X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray detector (EDX), and electron probe microanalysis (EPMA). The structure of phases formed and the morphology of phase-separated regions in chromite appear to be strongly dependent on the oxygen partial pressure. The mechanism of the decomposition of complex spinel phases is described under the influence of oxygen partial pressure and temperature.

Investigations on the carbothermic reduction of chromite ores

Abstract: Experimental studies were carried out on the reducibility of two different chromite ores using different reducing carbonaceous reducing agents in the temperature range 1173 to 1573 K. “Friable lumpy” ores and “hard lumpy” ores were used in the experiments. Petroleum coke, devolatilized coke, (DVC) and graphite were used as reducing agents. It was found that iron was practically completely reduced before the commencement of the reduction of chromium in the ore. The reduction of iron was controlled by diffusion. The activation energy for this process was estimated to be 130 kJ/mole. The reduction of chromium was controlled by either chemical reaction or nucleation. Rate of reduction was highest when raw petroleum coke was used as the reducing agent. The DVC was less effective compared to raw coke, whereas the rate of reduction was lowest when graphite was used as the reducing agent.