Showing papers in "Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science in 2001"

Thermal and carbothermic decomposition of Na 2 CO 3 and Li 2 CO 3

Abstract: In order to elucidate the decomposition mechanism of Na2CO3 and Li2CO3 in mold-powder systems employed in the continuous casting of steel, decompositions of Na2CO3 and Li2CO3 were investigated using thermogravimetric (TG) and differential scanning calorimetric (DSC) methods at temperatures up to 1200 °C, under a flow of argon gas. For the case of pure Na2CO3, the thermal decomposition started from its melting point and continued as the temperature was increased, but at a very slow rate. For Li2CO3, however, the decomposition occurred at much faster rates than that for Na2CO3. When carbon black was added to the carbonate particles, the decomposition rates of both Na2CO3 and Li2CO3 were significantly enhanced. From mass-balanced calulations and X-ray diffraction (XRD) analyses of the reaction products, it is concluded that decompositions of Na2CO3 and Li2CO3 with carbon black take place according to the respective reactions of Na2CO3 (1) + 2C (s) = 2Na (g) + 3CO (g) and Li2CO3 (l) + C (s) = Li2O (s) + 2CO (g). It was found that liquid droplets of Na2CO3 were initially isolated due to carbon particles surrounding them, but, as the carbon particles were consumed, the liquid droplets were gradually agglomerated. This effected a reduction of the total surface area of the carbonate, resulting in a dependence of the decomposition rate on the amount of carbon black. For the case of Li2CO3, on the other hand, hardly any agglomeration occurred up to the completion of decomposition, and, hence, the rate was almost independent of the amount of carbon black mixed. The apparent activation energies for the decomposition of Na2CO3 and Li2CO3 with carbon black were found to be similar and were estimated to be 180 to 223 kJ mole−1.

Weld metal composition change during conduction mode laser welding of aluminum alloy 5182

Abstract: Selective vaporization of volatile elements during laser welding of automotive aluminum alloys affects weld metal composition and properties. An experimental and theoretical study was carried out to seek a quantitative understanding of the influences of various welding variables on vaporization and composition change during conduction mode laser welding of aluminum alloy 5182. A comprehensive model for the calculation of vaporization rate and weld metal composition change was developed based on the principles of transport phenomena, kinetics, and thermodynamics. The calculations showed that the vaporization was concentrated in a small high-temperature region under the laser beam where the local vapor pressure exceeded the ambient pressure. The convective vapor flux driven by the pressure gradient was much higher than the diffusive vapor flux driven by the concentration gradient. The computed weld pool geometry, vaporization rates, and composition changes for different welding conditions agreed well with the corresponding experimental data. The good agreement demonstrates that the comprehensive model can serve as a basis for the quantitative understanding of the influences of various welding variables on the heat transfer, fluid flow, and vaporization occurring during conduction mode laser welding of automotive aluminum alloys.

Cathodic deoxygenation of the alpha case on titanium and alloys in molten calcium chloride

Abstract: The oxygen-enriched alpha case on titanium and alloys was successfully deoxygenated to satisfactory levels by electrolysis in molten CaCl2, in which the cathode was made from the metal to be refined. The oxygen distribution in the metal before and after electrolysis was characterized by microhardness tests, scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX). The electrolysis has been carried out at voltages sufficiently below that for the decomposition of CaCl2, and the results obtained suggest that the alpha case deoxygenation follows a simple oxygen ionization mechanism in which the oxygen in the metal is simply ionized at the cathode/electrolyte interface, dissolves in the molten salt, and then discharges at the anode. It is shown that by applying the electrochemical method, the alpha cases on both commercially pure titanium (CP Ti) and the Ti-6Al-4V alloy can be effectively deoxygenated. In particular, due to the removal of oxygen, the original alpha case (single phase) on the Ti-6Al-4V alloy has been converted back to the two-phase microstructure.

Deformation and fracture in martensitic carbon steels tempered at low temperatures

Abstract: This article reviews the strengthening and fracture mechanisms that operate in carbon and low-alloy carbon steels with martensitic microstructures tempered at low temperatures, between 150 °C and 200 °C. The carbon-dependent strength of low-temperature-tempered (LTT) martensite is shown to be a function of the dynamic strain hardening of the dislocation and transition carbide substructure of martensite crystals. In steels containing up to 0.5 mass pct carbon, fracture occurs by ductile mechanisms of microvoid formation at dispersions of second-phase particles in the matrix of the strain-hardened tempered martensite. Steels containing more than 0.5 mass pct carbon with LTT martensitic microstructures are highly susceptible to brittle intergranular fracture at prior austenite grain boundaries. The mechanisms of the intergranular fracture are discussed, and approaches that have evolved to minimize such fracture and to utilize the high strength of high-carbon hardened steels are described.

Thermodynamic description for concentrated metallic solutions using interaction parameters

Abstract: The integral molar excess Gibbs energy of a multicomponent system is expressed in terms of interaction parameters, from which the analytical formulae of the activity coefficients of the solutes and solvent, as Eqs. [23] and [24], were deduced. This approach, named the e approach, is able to describe quantitatively the thermodynamic properties of multicomponent systems. It features thermodynamic consistency, high accuracy, and a rather small influence of the higher-interaction parameters on the thermodynamic properties of metallic solutions. A simple modification to the first-order interaction parameters extends the e approach, to be applicable to systems with strong interactions between components at both low and concentrated levels.

Bubble formation during horizontal gas injection into downward flowing liquid

Abstract: Bubble formation during gas injection into turbulent downward-flowing water is studied using high-speed videos and mathematical models. The bubble size is determined during the initial stages of injection and is very important to turbulent multiphase flow in molten-metal processes. The effects of liquid velocity, gas-injection flow rate, injection hole diameter, and gas composition on the initial bubble-formation behavior have been investigated. Specifically, the bubble-shape evolution, contact angles, size, size range, and formation mode are measured. The bubble size is found to increase with increasing gas-injection flow rate and decreasing liquid velocity and is relatively independent of the gas injection hole size and gas composition. Bubble formation occurs in one of four different modes, depending on the liquid velocity and gas flow rate. Uniform-sized spherical bubbles form and detach from the gas injection hole in mode I for a low liquid speed and small gas flow rate. Modes III and IV occur for high-velocity liquid flows, where the injected gas elongates down along the wall and breaks up into uneven-sized bubbles. An analytical two-stage model is developed to predict the average bubble size, based on realistic force balances, and shows good agreement with measurements. Preliminary results of numerical simulations of bubble formation using a volume-of-fluid (VOF) model qualitatively match experimental observations, but more work is needed to reach a quantitative match. The analytical model is then used to estimate the size of the argon bubbles expected in liquid steel in tundish nozzles for conditions typical of continuous casting with a slide gate. The average argon bubble sizes generated in liquid steel are predicted to be larger than air bubbles in water for the same flow conditions. However, the differences lessen with increasing liquid velocity.

Experimental study of phase equilibria and thermodynamic optimization of the Fe-Zn-O system

Abstract: The Fe-Zn-O phase diagram in air was studied over the temperature range from 900 °C to 1500 °C. The compositions of the phases in quenched samples were obtained by electron probe X-ray microanalysis (EPMA). This experimental technique is not affected by zinc losses resulting from vaporization of zinc at high temperatures. The model for the spinel solid solution was developed within the framework of the compound-energy formalism (CEF). The choice of parameters of the CEF and the sequence of their optimization can have a major influence on the predictions in multicomponent phases. These choices can only be made rationally by reference to the specific model being represented in the CEF. This is discussed for the case of the two-sublattice spinel model. In the limiting case, the proposed model reduces to the model by O’Neill and Navrotsky for spinels. When the CEF is used in combination with the equation of Hillert and Jarl to describe the magnetic contribution to thermodynamic functions of a solution, it is necessary to assign certain values of magnetic properties to all pseudocomponents and to magnetic interaction parameters to obtain the most reasonable approximation of the magnetic properties of a solution. It was shown how this can be done based on very limited experimental data. The same equations can be used when the Murnaghan or the Birch-Murnaghan equation is combined with the CEF to describe the pressure dependence of thermodynamic functions. The polynomial model was used to describe the properties of wustite and zincite, and the modified quasichemical model was used for the liquid slag. All thermodynamic and phase-equilibria data on the Fe-O and Fe-Zn-O systems were critically evaluated, and parameters of the models were optimized to give a self-consistent set of thermodynamic functions of the phases in these systems. All experimental data are reproduced within experimental error limits. These include the thermodynamic properties of phases (such as specific heat, heat content, entropy, enthalpy, and Gibbs energy); the cation distribution between octahedral and tetrahedral sites in spinel; the oxygen partial pressure over single-phase, two-phase, and three-phase regions; the phase boundaries (liquidus, solidus, and subsolidus); and the tie-lines.

Physical modeling of the deformation mechanisms of semisolid bodies and a mechanical criterion for hot tearing

Abstract: The mechanical response of a semisolid body to an applied, uniaxial strain rate has been expressed as a function of strain by modifying an existing analysis based on an idealized representation of the microstructure. An existing mechanical criterion for hot tearing of the semisolid body has been adapted to the deformation mechanisms. The resulting hot tearing model shows that the strength of the body depends on the strain, the viscosity of the intergranular fluid, the solid fraction, the isothermal compressibility of the fluid, the surface tension of the liquid, the limiting liquid-film thickness for viscous flow and a parameter m, which describes microstructure. The effect of each parameter on the mechanical response and the onset of hot tearing has been examined for ranges of values relevant to aluminum alloys and the direct-chill (DC) casting process. The parameter testing has shown that the mechanical response predicted by the model agrees well with some experimental data for both the mechanisms of fracture and the parameters that govern the process. An adjustment of unknown model parameters to experimental data would permit use of the model as a constitutive law and a fracture criterion for numerical modeling of hot tearing during the solidification of Al alloys by DC casting.

A kinetic study that compares the leaching of gold in the cyanide, thiosulfate, and chloride systems

Abstract: Due to the increasing environmental and public concerns over cyanidation, there has been a large amount of research into viable alternative lixiviants. This article presents a detailed kinetic study of gold leaching in cyanide, ammonia/thiosulfate, and chloride/hypochlorite solutions. The gold leach rates were measured using a rotating electrochemical quartz crystal microbalance (REQCM). This instrument allows the mass of a gold sample to be measured in situ, with a sensitivity of less than 10 ng. It will be shown that for the cyanide system, the leach rate of a gold/silver alloy is substantially higher than that of pure gold; for the gold/silver alloy, the reaction is diffusion controlled. For the thiosulfate system, reaction rates which are substantially higher than those for cyanide can be achieved with freshly prepared solutions containing copper(II), although the leach rate decreases as the copper(II) reacts with thiosulfate. A steady-state copper(II) concentration is obtained once the rate of copper(II) reduction by thiosulfate matches the rate of copper(I) oxidation by oxygen; at these steady-state concentrations of copper(II), the leach rates are slower than those obtained for cyanide. It will also be shown that reaction rates similar to the cyanide system can be readily achieved in chloride media at pH 3 with 2.5 mM hypochlorous acid; under these conditions, the reaction is limited by the diffusion of HClO.

Turbulent flow of liquid steel and argon bubbles in slide-gate tundish nozzles part i: model development and validation

Abstract: The quality of continuous-cast steel is greatly affected by the flow pattern in the mold, which depends mainly on the jets flowing from the outlet ports in casting with submerged tundish nozzles. An Eulerian multiphase model using the finite-difference program CFX has been applied to study the three-dimensional (3-D) turbulent flow of liquid steel with argon bubbles in slide-gate tundish nozzles. Part I of this two-part article describes the model formulation, grid refinement, convergence strategies, and validation of this model. Equations to quantify average jet properties at the nozzle exit are presented. Most of the gas exits the upper portion of the nozzle port, while the main downward swirling flow contains very little gas. Particle-image velocimetry (PIV) measurements are performed on a 0.4-scale water model to determine the detailed nature of the swirling velocity profile exiting the nozzle. Predictions with the computational model agree well with the PIV measurements. The computational model is suitable for simulating dispersed bubbly flows, which exist for a wide range of practical gas injection rates. The model is used for extensive parametric studies of the effects of casting operation conditions and nozzle design, which are reported in Part II of this two-part article.

Behavior of alumina-magnesia complex inclusions and magnesia inclusions on the surface of molten low-carbon steels

Abstract: It is well known that alumina inclusions on the surface of molten Al-killed steel quickly attract each other to form clusters. On the other hand, alumina-magnesia complex inclusions on the surface of molten low-carbon steel with a high oxygen content have a much weaker tendency to form clusters. In the present work, the reason for the different behaviors of the two types of inclusions was analyzed in detail. A confocal scanning laser microscope was used to carry out the experiment of in-situ observation of the two types of inclusion on the molten pool. The first type of inclusion was 93 mass pct alumina-7 mass pct magnesian, obtained in a Mg-added Al-killed steel. The second type of inclusion was nearly pure magnesia, obtained in a Mg-killed steel. The attractive force between a pair of inclusions, for both cases, was found to be approximately 1O−17 to 10−16 N and one-tenth of that between a pair of alumina inclusions. The various effects of contact angle, surface tension, and oxygen content of the steel melt on the attractive force are discussed in detail from the viewpoint of the capillary force.

Characterization of Phases Formed in the Iron Carbide Process by X-Ray Diffraction, Mossbauer, X-Ray Photoelectron Spectroscopy, and Raman Spectroscopy Analyses

Abstract: Iron carbide was prepared by iron ore reduction and iron cementation using Ar-H2-CH4 gas mixture with and without sulfur. Phases formed in the reduction/cementation process were examined by X-ray diffraction (XRD), Mossbauer, and Raman spectroscopy. The sample surface was also analyzed by X-ray photoelectron spectroscopy (XPS). XRD and Mossbauer analyses showed that iron oxide was first reduced to metallic iron, and then, metallic iron was carburized to cementite. Addition of a small amount of H2S to the reaction gas retarded the cementite formation but made the cementite more stable. XPS analysis showed that the surface of samples converted to iron carbide using sulfurcontaining gas consisted of mainly Fe3C and a small amount of graphitic carbon. Raman spectra of a sample produced in the iron carbide process showed the G and D bands, which are characteristic for carbon-carbon bonds. The intensity ratio of G/D bands depended on the sulfur content in the reducing/carburizing gas.

Effects of surface active elements on weld pool fluid flow and weld penetration in gas metal arc welding

Abstract: This article presents a mathematical model simulating the effects of surface tension (Maragoni effect) on weld pool fluid flow and weld penetration in spot gas metal arc welding (GMAW). Filler droplets driven by gravity, electromagnetic force, and plasma arc drag force, carrying mass, thermal energy, and momentum, periodically impinge onto the weld pool. Complicated fluid flow in the weld pool is influenced by the droplet impinging momentum, electromagnetic force, and natural convection due to temperature and concentration gradients, and by surface tension, which is a function of both temperature and concentration of a surface active element (sulfur in the present study). Although the droplet impinging momentum creates a complex fluid flow near the weld pool surface, the momentum is damped out by an “up-and-down” fluid motion. A numerical study has shown that, depending upon the droplet’s sulfur content, which is different from that in the base metal, an inward or outward surface flow of the weld pool may be created, leading to deep or shallow weld penetration. In other words, it is primarily the Marangoni effect that contributes to weld penetration in spot GMAW.

Review of experimental data and modeling of the viscosities of fully liquid slags in the Al2O3-CaO-‘FeO’-SiO2 system

Abstract: A general model based on the Urbain formalism has been developed, which enables the viscosities of liquid slags to be predicted for all compositions in the Al2O3-CaO-‘FeO’-SiO2 system in equilibrium with metallic iron. Available experimental viscosity data have been analyzed and critically reviewed. The Urbain formalism has been modified to include compositional dependent model parameters. Experimental data in unaries, binaries, ternaries, and the quaternary system have been described by the model over the whole compositional and temperature ranges using one set of model parameters. This viscosity model can now be applied to various industrial slag systems.

Investigative study into the hydrodynamics of heap leaching processes

Abstract: Three mathematical models (mixed side-pore diffusion (MSPD) and profile side-pore diffusion (PSPD) with uniform or distributed pore lengths) are derived in dimensionless form to simulate the transport of solutes through the flowing channels and the stagnant pores of an unsaturated heap. Model parameters are determined from experimental tracer residence-time distributions using the least-squares minimization approach. It is shown that the residence-time distribution curves display a long tail resulting from the very slow mass transfer (or diffusion) into the 1- to 6-cm-long stagnant pores, which take up 5 times more space than the flowing liquid. The very large coefficients of determination (R 2>0.99) confirm the validity of all models and especially that of the PSPD model with a distributed pore length. The effects of five factors (agglomeration, addition of binder, particle size, solution flow rate, and bed height) are examined. Data from experimental residence-time distributions prove that the advection time is directly proportional to the column height and inversely proportional to the flow rate. The two estimated parameters (stagnant liquid holdup and pore length) are only marginally affected by any change in crush size, agglomeration technique, or operating conditions. This, in turn, suggests that the model can predict and/or simulate the hydrodynamic behavior in taller columns and, possibly, heaps.

Effects of clogging, argon injection and continuous casting conditions on flow and air aspiration in submerged entry nozzles

Abstract: The inter-related effects of nozzle clogging, argon injection, tundish bath depth, slide-gate opening position, and nozzle-bore diameter on the steel flow rate and pressure in continuous-casting slide-gate nozzles are quantified using computational models of three-dimensional (3-D) multiphase turbulent flow The results are validated with measurements on operating steel continuous slab-casting machines and are presented for practical conditions with the aid of an inverse model Predictions show that initial clogging may enhance the steel flow rate due to a potential streamlining effect before it becomes great enough to restrict the flow channel The clogging condition can be detected by comparing the measured steel flow rate to the expected flow rate for those conditions, based on the predictions of the inverse model presented here Increasing argon injection may help to reduce air aspiration by increasing the minimum pressure, which is found just below the slide gate More argon is needed to avoid a partial-vacuum effect at intermediate casting speeds and in deeper tundishes Argon flow should be reduced during shallow tundish and low casting speed conditions (such as those encountered during a ladle transition) in order to avoid detrimental effects on flow pattern Argon should also be reduced at high casting speed, when the slide gate is open wider and the potential for air aspiration is less The optimal argon flow rate depends on the casting speed, tundish level, and nozzle-bore diameter and is quantified in this work for a typical nozzle and range of bore diameters and operating conditions

Modeling of the effects of surface-active elements on flow patterns and weld penetration

Abstract: A mathematical model was developed to calculate the transient temperature and velocity distributions in a stationary gas tungsten arc (GTA) weld pool of 304 stainless steels with different sulfur concentrations. A parametric study showed that, depending upon the sulfur concentration, one, two, or three vortexes may be found in the weld pool. These vortexes are caused by the interaction between the electromagnetic force and surface tension, which is a function of temperature and sulfur concentration, and have a significant effect on weld penetration. For given welding conditions, a minimum threshold sulfur concentration is required to create a single, clockwise vortex for deep penetration. When two metals with different sulfur concentrations are welded together, the weld-pool shape is skewed toward the metal with a lower sulfur content. Detailed physical insights on complicated fluid-flow phenomena and the resulting weld-pool penetration were obtained, based on the surface tension-temperature-sulfur concentration relationships.

Turbulent flow of liquid steel and argon bubbles in slide-gate tundish nozzles: Part II. Effect of operation conditions and nozzle design

Abstract: A three-dimensional (3-D) finite-volume model, developed and validated in Part I of this two-part article, is employed to study steady-state two-phase turbulent flow of liquid steel and argon bubbles through slide-gate tundish nozzles. Parametric studies are performed to investigate the effects of gas injection, slide-gate orientation, casting speed, gate opening, bubble size, port angle, and port shape on the flow pattern and characteristics of the jet exiting the nozzle port. Argon gas injection bends the jet angle upward, enhances the turbulence level, and reduces the size of the backflow zone. Gas injection becomes less influential with increasing casting speed. The off-center blocking effect of the slide gate generates an asymmetric flow that changes with the gate orientation. The 0-deg gate orientation creates the worst biased flow between the two ports. The 90-deg orientation generates significant swirl and directs the jet slightly toward one of the wide faces. The 45-deg orientation generates both types of asymmetry and, thus, appears undesirable. The horizontal jet angle indicates asymmetric flow in the horizontal plane. It increases with decreasing gate opening and decreasing gas injection rate and ranges from 3 to 5 deg. Most jet characteristics reach their maximum or minimum values near the critical opening of 60 pct (linear).Larger bubbles exert a greater influence on the flow pattern. The vertical jet angle becomes steeper with a steeper port angle and more slender port shape. These results will be useful for nozzle design and for future modeling of flow in the mold.

Largest-extreme-value distribution analysis of multiple inclusion types in determining steel cleanliness

Abstract: There is the need for new inclusion rating methods able to assess the cleanliness of modern steel and to predict the characteristic size of maximum defects in a component. These critical tasks can be extremely difficult, because of the presence of different inclusion types. Therefore, new statistical tools have to be developed for addressing these problems. A new method for the analysis of extreme defects is proposed in this article and is successfully applied to different steels containing multiple inclusions. In particular, the analysis, which allows the assessment of the relative density of the different inclusions, is based on inclusion samplings carried out on different control areas.

Modeling study of the influence of turbulence inhibitors on the molten steel flow, tracer dispersion, and inclusion trajectories in tundishes

Abstract: In order to optimize steel flow and maximize the contact time of the inclusions with the slag layer inside the tundish, a proper flow-control arrangement must be designed, considering the shape, the dimensions of the prototype, and the plant operating conditions of the tundish. Physical and mathematical modeling has been used in this study, in a complementary fashion, to evaluate the influence of turbulence-inhibiting devices on the velocity fields, tracer dispersion, small- and large-particle trajectories, flow-pattern characteristics, and grade changes in a large-volume tundish. From the water model and mathematical simulation results, a flow-control system with the best performance was identified; this system must contribute to improving the productivity and cleanliness of the continuous-cast steel.

The soda-ash roasting of chromite minerals: Kinetics considerations

Abstract: Soda-ash roasting of the chromite mineral is commonly used worldwide for the production of watersoluble sodium chromate. The formation of sodium chromate during the soda-ash roasting reaction depends on the oxygen partial pressure and availability of oxygen at the reaction front. The effects of temperature, oxygen partial pressure, charge composition, and their roles on the overall roasting reaction were studied in order to analyze the reaction mechanism. The influence of process parameters such as the addition of alkali and process residue as the filler material on the overall reaction rate is discussed. The rate-determining steps for the soda-ash roasting reaction are analyzed. The importance of the binary Na2CO3-Na2CrO4 liquid phase during the reaction in determining its speed is also examined. It is shown that the experimental results for the roasting reaction can be best described by the Ginstling and Brounshtein (GB) equation for diffusion-controlled kinetics. From the measured kinetics data, the apparent activation energy for the roasting reaction was calculated to be between 180 and 190 kJ · mol−1 in the temperature range from 1023 to 1210 K and between 35 and 40 kJ · mol−1 above 1210 K.

Geometry of laser spot welds from dimensionless numbers

Abstract: Recent computer calculations of heat transfer and fluid flow in welding were intended to provide useful insight about weldment geometry for certain specific welding conditions and alloys joined. However, no generally applicable correlation for the joining of all materials under various welding conditions was sought in previous work. To address this difficulty, computer models of fluid flow and heat transfer were used for the prediction of weld pool geometry in materials with diverse properties, such as gallium, pure aluminum, aluminum alloy 5182, pure iron, steel, titanium, and sodium nitrate under various welding conditions. From the results, a generally applicable relationship was developed between Peclet (Pe) and Marangoni (Ma) numbers. For a given material, Ma and Pe increased with the increase in laser power and decrease in beam radius. For materials with high Prandtl number (Pr), such as sodium nitrate, the Pe and Ma were high, and heat was transported primarily by convection within the weld pool. The resulting welds were shallow and wide. For low Pr number materials, like aluminum, the Pe and Ma were low in most cases, and low Pe made the weld pool deep and narrow. The cross-sectional areas of stationary and low speed welds could be correlated with welding conditions and material properties using dimensionless numbers proposed in this article.

Coal char reactivity and structural evolution during combustion—Factors influencing blast furnace pulverized coal injection operation

Abstract: A fresh char was prepared and reacted with oxygen under conditions similar to those prevailing in the raceway region of the blast furnace (BF) during pulverized coal injection (PCI), using a well-characterized drop-tube furnace (DTF). Char combustion under the present conditions was found to be controlled by the combination of pore diffusion and chemical reaction. Both the char density and size gradually decrease with burnoff, while the char surface area increases up to a burnoff of 40 to 50 pct due to the formation of a large amount of meso- and micropores, which were observed by high-resolution field-emission scanning electron microscopy (FESEM) and gas adsorption measurements. Despite the obvious increase in surface area, the char combustion reactivity decreases with burnoff. This is due to the loss of the intrinsic reactivity of char during combustion, as confirmed by fixed-bed (FB) measurements of fresh char and chars partly burnt in a DTF. The structural characterization by quantitative X-ray diffraction analysis (QXRDA) shows that the amorphous concentration (f am ) of the char decreases during combustion, while the aromaticity (f ar ) and the average crystallite size (L 002) of the char increase. The char becomes more ordered during combustion, which is in accordance with the observations made using high-resolution transmission electron microscopy (HRTEM). The char structural ordering observed was found to be responsible for the loss of char intrinsic reactivity during combustion. Based on the QXRDA, a char structure model has also been suggested to explain the char structural evolution observed during combustion. The implications of char structural evolution for char combustion during a PCI operation are also discussed.

Dilution control in gas-tungsten-arc welds involving superaustenitic stainless steels and nickel-based alloys

Abstract: Fusion welds were prepared between a superaustenitic stainless steel, (the AL-6XN alloy) and two Ni-based filler metals (IN625 and IN622) using the gas-tungsten-arc welding (GTAW) process. Fusionzone compositions over the full range of dilution levels (0 to 100 pct) were produced by varying the independent welding parameters of arc power and volumetric filler-metal feed rate. Microstructural characterization of the welds was conducted via light optical microscopy, with quantitative chemical information obtained through electron-probe microanalysis (EPMA). The dilution level of each weld was determined from the EPMA data as well as through geometric measurements of the weld cross-sectional areas. The dilution level was observed to decrease with increasing filler-metal feed rate and decreasing arc power. These effects are quantitatively interpreted based on a previously proposed processing model. The model is used to demonstrate that, in terms of welding parameters, the dilution level can be correlated exclusively to the ratio of the volumetric filler-metal feed rate (V fm) to arc power (VI), i.e., the individual values of V fm and VI are not important in controlling the dilution and resultant weld-metal composition. Good agreement is obtained between experimental and calculated dilution values using the model. It is also demonstrated that the melting enthalpies of the filler metal and substrate have only a minor influence on dilution at dilution levels in the range from 40 to 100 pct. This knowledge facilitates estimates of dilution levels in this range when the substrate and fillermetal thermal properties are not accurately known. The results presented from this study provide guidelines for controlling the weld-metal composition in these fusion-zone combinations.

The soda-ash roasting of chromite ore processing residue for the reclamation of chromium

Abstract: Sodium chromate is produced via the soda-ash roasting of chromite ore with sodium carbonate. After the reaction, nearly 15 pct of the chromium oxide remains unreacted and ends up in the waste stream, for landfills. In recent years, the concern over environmental pollution from hexavalent chromium (Cr6+) from the waste residue has become a major problem for the chromium chemical industry. The main purpose of this investigation is to recover chromium oxide present in the waste residue as sodium chromate. Cr2O3 in the residue is distributed between the two spinel solid solutions, Mg(Al,Cr)2O4 and γ-Fe2O3. The residue from the sodium chromate production process was analyzed both physically and chemically. The compositions of the mineral phases were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). The influence of alkali addition on the overall reaction rate is examined. The kinetics of the chromium extraction reaction resulting from the residue of the soda-ash roasting process under an oxidizing atmosphere is also investigated. It is shown that the experimental results for the roasting reaction can be best described by the Ginstling and Brounshtein (GB) equation for diffusion-controlled kinetics. The apparent activation energy for the roasting reaction was calculated to be between 85 and 90 kJ·mol−1 in the temperature range 1223 to 1473 K. The kinetics of leaching of Cr3+ ions using the aqueous phase from the process residue is also studied by treating the waste into acid solutions with different concentrations.

The effect of particle size and morphology on the in-flight behavior of particles during high-velocity oxyfuel thermal spraying

Abstract: A one-way coupled mathematical model is formulated to simulate the effects of particle size and morphology on the momentum and thermal energy transfer of particles during high-velocity oxyfuel (HVOF) thermal spraying. First, computational fluid dynamic techniques are implemented to solve the Favre-averaged mass, momentum, and energy conservation equations in the gas phase. The gas dynamic data are then used to model the behavior of particles in the gas field. The concept of sphericity is used to incorporate the effect of particle morphology into the model. The calculated results show that the particle velocity and temperature, before impinging onto the substrate, are strongly affected by particle size, morphology, and spray distance. Smaller particles are accelerated to a higher velocity but slowed down rapidly due to their smaller momentum inertia, while the larger particles are accelerated with some difficulty. The same tendency is observed regarding the effect of particle size on its thermal history.

Prediction of laser-spot-weld shape by numerical analysis and neural network

Abstract: The finite element method (FEM) and neural network were applied for predicting the bead shape in laser spot welding of type 304 thin stainless steel sheets. The parameters of pulsed Nd:YAG laser spot welding such as pulse energy, pulse duration, sheet metal thickness, and gap between sheets were varied for various experiments and numerical simulations. The penetration depth and nugget size of spot welds measured for specimens without gap were compared with the calculated results to verify the proposed finite element model. Sheet metal thickness, gap size, and bead shape of the workpiece without gap were selected as the input variables for the back-propagation learning algorithm of the neural network, while the bead shape of the workpiece with and without gap was considered as its output variable. Various combinations of stainless steel sheet metal thickness were considered to calculate the laser-spot-weld bead shape of the workpiece without gap, which was then used as the input variable of neural network to predict the bead shape for various gap sizes. This combined model of finite element analysis and neural network could be effectively applied for the prediction of bead shapes of laser spot welds, because the numerical analysis of laser spot welding for the workpiece with gap between two sheets is highly limited.

Enthalpy of formation of B2-Fe1-xAlx and B2-(Ni,Fe)(1-x)Alx

Abstract: The enthalpy of formation of the ordered B2 phases in the Fe-Al and Fe-Ni-Al systems was measured with very good accuracy using a special, laboratory-built differential-solution calorimeter. The measurements were performed at 1073 K as a function of composition, with an accuracy of about 1 pct. The enthalpy of formation of B2-FeAl is most negative for the composition Fe0.50Al0.50 (−36.29 kJ/mol). Compounds with Al contents less than about 40 at. pct show a deviation from the linear dependence of the enthalpy of formation with composition which prevails for Al contents larger than 40 at. pct. Upon replacing Fe by Ni while maintaining a constant Al content, the enthalpy of formation of B2-(Fe,Ni)Al compounds becomes more negative. With decreasing Al content and for a constant Fe/Ni ratio, the enthalpy of formation of the ternary phase becomes less negative.

Modeling of viscosities of the partly crystallized slags in the Al2O3-CaO-FeO-SiO2 system

Abstract: A viscosity model of the partly crystallized slag in the Al2O3-CaO-‘FeO’-SiO2 system has been developed in conjunction with the thermodynamic computer package F*A*C*T. Proportions of solids crystallized out of the liquid phase and compositions of the remaining liquid phase predicted by F*A*C*T are used in the viscosity model. Various heterogeneous viscosity models have been tested using large experimental dataset in the Al2O3-CaO-‘FeO’-SiO2 system in reducing conditions close to the equilibrium with metallic iron. The Roscoe equation with new empirical parameters was found to provide reasonable agreement with experimental data. Examples of model application to industrial nonferrous smelting slag systems are presented. This model can also be applied to coal ash slags.