Macrostructural preparation and sintering of batch

Physical analysis suggests an integral sintering characteristic K _si, which may be used not only to assess batch preparation at a particular plant, since it includes the structural characteristics of the granules and the batch bed, but also for the analysis and optimization of sinter production.     

Some fundamental aspects of mixing in metallurgical reaction systems

It is a well-established fact that mixing time (t _m ) in ladle metallurgy systems may be related to the specific power input (P _t ) by a relationship of the following form:t _m = Const.P _t ^−1/3. On the basis of arguments developed, using turbulence theory, it is shown that this relationship previously developed on an empirical basis does have a fundamental basis. The theoretical derivation given in the paper accounts for both the numerical values of the exponent and of the multiplying factor. A. MURTHY, formerly with Massachusetts Institute of Technology, is with Eastman Kodak Company in Rochester, NY.     

Transfer of iron to Cu–Ni alloys from the lining of the vacuum induction furnace

In Ni–Cu alloys, iron must be excluded in many cases. Iron may enter the alloy from the batch or the furnace lining. Since the Fe₂O₃ content in refractories may be as much as 2.5%, it is important to assess the increase in iron content in alloys on account of interaction with the furnace lining. In the present work, the influence of the Fe₂O₃ content in the crucible and the volume of the crucible on the iron content in the final alloy is studied. Thermodynamic analysis and experimental data indicate that the nickel and copper in Ni–Cu alloys may reduce iron that is present in the lining. When using low-iron batch, iron from the crucible is transferred almost completely to the melt. The increase in iron content in Ni–Cu alloys is investigated as a function of the capacity of the vacuum induction furnace and the Fe₂O₃ content in the periclase crucibles, with complete transfer of the iron from the lining to the melt. With increase in furnace capacity, less iron enters the melt from the crucible. With more than 200 kg of metal, the increase in iron concentration mainly depends not on the furnace capacity but on the Fe₂O₃ content in the refractory. In order to produce Ni–Cu alloys with <0.01% Fe, refractories with Fe₂O₃ content no higher than 0.5% must be used. To produce Ni?Cu alloys with <0.05% Fe, the use of lining refractories with Fe₂O₃ content no higher than 2.5% is recommended.     

THE APPLICATION OF FRACTURE MECHANICS TO CEMENTED TUNGSTEN CARBIDES

Abstract

The difficulties encountered in the measurement of the toughness of cemented tungsten carbides are discussed and the benefits that might be expected from an application of fracture mechanics to the problem are described. A simple method for the measurement of the fracture-toughness parameter, K_IC, for the more brittle grades of carbide is considered. The method involves indenting a beam-shaped specimen with a Knoop diamond to produce a crack, and loading the pre-cracked specimen to failure in four-point bending. Results from two grades of cemented carbide are presented and show that a standard error of the mean K_IC of ～3% can be obtained from a set of 10 measurements, with a minimum of specimen preparation and no special testing equipment. The results show also that the toughness of the cemented carbide can be affected by grain-size variations within the same batch of material and by the pressing direction during manufacture.     

Oxidation kinetics of zinc vapor in CO:CO2 mixtures: Part II. Application of plug flow concepts

In the first of two articles on the subject, it was shown that the oxidation kinetics of Zn vapor in CO:CO₂ mixtures cannot be understood on the basis of previously proposed rate expressions. In this second article, an alternative interpretation, which appears capable of reconciling the apparent discrepancies of the past literature, is proposed. Evidence is provided to the effect that competing reactions occur in the system Zn−CO−CO₂ under the majority of conditions investigated. Analysis on the basis of a simple “plug flow” model reveals that oxidation by CO₂ can take place indirectly by a combination of the reactions Zn_(g)+CO_(g)→ZnO_(s)+C_(s) and C_(s)+CO_2(g)→2CO_(g) Zinc is also oxidized by direct reaction with both CO and CO₂. It is proposed that the distinctive morphologies observed (coarse, intermediate, and fine) can be related to the degree of direct and/or indirect oxidation occurring in the system.     

The mechanism of elution of gold cyanide from activated carbon

Numerous articles have appeared on the mechanism of the adsorption of gold cyanide onto activated carbon. In contrast, little information is available on the mechanism of elution of the adsorbed gold. It is the objective of this article to formulate such a mechanism on the basis of batch and column elution tests without analyzing adsorbed species on the carbon directly. The presence of spectator cations (M ⁿ⁺) enhances the formation of M ⁿ⁺{Au(CN) ₂ ⁻ } _n ion pairs on the carbon, which in turn suppress the elution of gold cyanide. The dynamics of removal of these cations determine the horizontal position of the gold peak in an elution profile. When the concentration of cations in the eluant is high and no cyanide is present in the solution or on the carbon, very little desorption of gold is observed. The quantitative effect of the concentration of spectator cations on the equilibrium for desorption of aurocyanide can be estimated from the elution profiles for gold and cations. Free cyanide in the eluant, which causes some competitive adsorption of cyanide with aurocyanide, therefore plays a minor role at the elevated temperatures used in industry. A more important effect of cyanide is its reaction with functional groups on the carbon, the products of which passivate the surface for adsorption of aurocyanide, and thereby cyanide promotes the elution of aurocyanide. The degree of passivation, which is determined to a large extent by the temperature of pretreatment, also affects the elution of cations and the degradation/adsorption of cyanide itself. Reactivation of the carbon surface occurs when the adsorbed/decomposed cyanide is removed by the eluant. At high temperatures of pretreatment, such as used in practice, it is not necessary to include a reactivation term in the mathematical model for elution.     

Application of thermodynamic and kinetic principles in the reduction of metal oxides by carbon in a plasma environment

The application of plasma technology to metal oxide reduction is discussed with reference to established thermodynamic and kinetic principles. ΔG°-T diagrams for the corresponding metal oxide, metal carbide, and C-CO reactions are presented and the important role played by thep _CO/P _{CO 2} ratio examined. On the basis of these theoretical considerations, supported by some earlier experimental results conducted on the reduction of iron and chromium oxide concentrates in the form of taconite and chromite by carbon within a plasma reactor, the tendency to form either elemental metals or carbides is discussed. It is also suggested that the reduction of taconite by carbon takes place in two stages within the plasma medium. In the first stage, ferric oxide is reduced to wustite by carbon, and in the second stage wustite is reduced to metal. It is also postulated that in the first stage of reduction, ferric oxide may also be reduced to wustite through an exchange reaction between ferric oxide and iron, without CO evolution. The rate controlling step for the first stage of taconite reduction is thought to lie at the gas/slag interface generated within the plasma environment, while the second stage of reduction is controlled by carbon gasification by CO₂. Formerly Postdoctoral Fellow with Mineral Research Center, University of Minnesota     

A strain energy-based approach to the low-cycle fatigue damage mechanism in a high-strength spring steel

Low-cycle fatigue tests were conducted using smooth, cylindrical specimens under a strain-controlled, fully reversed condition for a high-strength spring steel heat treated to different strength levels. The variation of the cyclic deformation substructure was observed with a transmission electron microscope (TEM). The results indicate that the average plastic strain energy dissipated per cycle (ΔW _ps ) is an important parameter upon which a consistent evaluation of the cyclic stress-strain, the strain-life, and the plastic strain energy-life relationships is made feasible. Furthermore, the total plastic strain energy dissipated prior to failure (W _f ), determined on the basis of ΔW _ps , is proven to be another important parameter, from the variation of which the extent of local damage accumulation can be evaluated. Confirmed by the results of TEM observations, a strain localization-induced damage mechanism is proposed and discussed.     

Yield and Fracture Phenomena in Powder-Forged Fe—0·2C and Their Prediction by NDT Methods

Abstract

On the basis of thermodynamic and kinetic considerations, the role played by surface oxide layers in the sintering of metallic powders is evaluated. For metals with stable oxides, the influence of the surface layer is determined by the relative sintering and diffusional fluxes x _s/x _d to the interparticle neck region. When x _s/x _d>1 sintering is retarded by the oxide layer. Conversely, unretarded sintering occurs when x _s/x _d <1. For metals with oxides which are unstable with respect to dissolution in the metal at the sintering temperatures, the sintering process is preceded by an incubation period. Calculated values of the relative neck growth rates and the incubation periods provide the bases for predicting the effects of these surface layers. Predictions of the present analysis are shown to be consistent with experimental observations. PM/0174     

Structure and deformation behavior ofT 1 precipitate plates in an Al- 2Li- 1 Cu alloy

A high-resolution TEM study was performed in order to determine the structure, transformation mechanisms, and deformation behavior ofT ₁ precipitate plates in an Al-2Li-lCu alloy aged to peak strength. It is shown that a possible structure for theT ₁ plates may be an A₁BA₂C... stacking of close-packed planes with the A, planes mostly Al, the B and C planes containing a mixture of Cu and Al, and the A₂ planes mostly Li. Furthermore, the structural transformation necessary to achieve the A₁BA₂C ... stacking from the ABC ... matrix planes may be accomplished by the nu-cleation and propagation of a pair of Shockley partial dislocations on every third and fourth {111}_α matrix plane. After straining 3 pct, theT ₁ plates are sheared by dislocations, which cause a disruption in atomic order within the precipitates. Experimental evidence also indicates that the binding between Li and Cu in Al-Li-Cu alloys may be a major factor in determining microstructural evolution in this system, and comparison of diffraction data from hexagonal precipitates on {111}_α planes in a variety of Al alloys indicates that they may have similar structures. Formerly a Staff Scientist, ALCOA Laboratories.     

Reduction kinetics of Fe<Subscript>2</Subscript>MoO<Subscript>4</Subscript> fine powder by hydrogen in a fluidized bed

Iron molybdate (Fe₂MoO₄) powders with an average particle size of 100 μm were reduced by hydrogen using a fluidized-bed batch reactor in the temperature range of 923 to 1173 K. The extent of the reaction was followed as a function of time by gas chromatography. The fluidizing-gas velocity was set at about 1.5 times the minimum fluidization velocity. The ratio of the height of the static bed to its diameter is about 1. Under the prevailing experimental conditions, it was found that the chemical reaction was the rate-controlling factor. The activation energy for this process was 158±17 kJ/mol. The crystal size of the Fe₂Mo powder produced at lower temperatures was in the nanometer range, indicating the possibility of mass production of alloys and intermetallics in the nanorange, using a fluidized bed.     

Microstructural characteristics of 5083 Al alloys processed by reactive spray deposition for net-shape manufacturing

The superplastic forming of Al-Mg alloys is of considerable interest due to its potential for structural applications. In the present study, reactive spray-deposition processing is implemented to reduce grain size to the superplastic range and, thus, to obtain superplasticity for net-shape manufacturing. A typical Al-Mg alloy (5083) was selected for this study and was spray deposited using different N₂-O₂ atomization gas mixtures. The as-deposited microstructures reveal an equiaxed grain morphology, a limited dynamic recovery structure, and a distribution of intermetallic phases and oxide dispersoids. The grain size decreases with increasing atomization gas oxygen content and is affected by changes to the superheat temperature (ST) and by small additions of Zr. In some deposits, γ-Al₂O₃ crystallites less than 20 nm were distributed as stringers along grain boundaries; this observation was found to be strongly dependent on the oxygen content. The oxidation kinetics during reactive spray deposition were studied on the basis of the Mott-Cabrera theory of oxidation. A linear growth rate of dX/dt=2A ₀·exp (−Q/kT)·exp (k₀·P ^1/2/kT) was obtained, which suggests that the rate decreases rapidly with decreasing temperature or oxygen pressure and that, when the temperature (or oxygen pressure) is low, the rate decreases steadily with decreasing oxygen pressure (or temperature). Calculations of the width of oxide stringers as a function of oxygen content and ST show good agreement with the experimental observations. Finally, the volume fraction of oxide phases was estimated on the basis of kinetic considerations.     

Reassessment of Al-Ce and Al-Nd binary systems supported by critical experiments and first-principles energy calculations

The present study reinvestigates the Al-Ce and Al-Nd phase diagrams and reoptimizes their thermodynamics using the CALPHAD method. First-principles energy calculations play an important role in terms of sublattice formalism and phase-stability prediction, demonstrating that they should be effectively integrated into experimental investigations and thermodynamic assessments. Specifically, current experimental results and theoretical calculations show that Al₂Nd (or Al₂Ce) should be treated as a stoichiometric compound phase rather than as the solution phase that was proposed in previous studies. Further, a new compound, AlCe₂, is found stable at high temperatures (648 °C to 775 °C) in the Al-Ce system. It forms through a peritectic reaction of liquid and AlCe phases at 775 °C, and decomposes into AlCe and βAlCe₃ at 648 °C and below. Since the AlCe₂ phase is not retained at room temperature by quenching experiments, it is suggested that AlCe₂ may be isostructural with the previously known compound AlNd₂ (oP12). Based on current differential thermal analysis (DTA) measurements and theoretical calculations, it is also proposed that there is an α/βAl₃Ce polymorphous transition occurring at 973 °C in the Al-Ce system and an α/βAl₃Nd polymorphous transition occurring at 888 °C in the Al-Nd system. The βAl₃RE phase may be isostructural with βAl₃Y (hP12). Finally, the previously described βAl₁₁RE₃ phase (rare earth elements (RE)=La, Ce, Nd, or Pr) is proposed to have a stoichiometry of Al₄RE (tI10), based on direct evidence from differential scanning calorimetry (DSC) measurements.     

Oxidation kinetics of zinc vapor in CO∶CO2 mixtures: Part I. Comparison with past literature

The methods employed and the results obtained during a recent investigation of the oxidation of gaseous zinc by CO∶CO₂ mixtures are described. The kinetic data are tested against the predictive models derived and employed by previous investigators of this reaction. Although the experimentally determined reaction rates are highly reproducible, no consistent agreement can be found with the models presented in the prior literature. Efforts to account for the kinetics on the basis of the oxygen potential of the gas mixture are also unsuccessful. It is concluded that previous attempts to understand the kinetics in terms of the reaction Zn _(g) +CO _2(g) ⇒ZnO _(s) +CO _(g) are oversimplistic. A novel interpretation which appears to resolve the discrepancies of the literature is detailed in the second of two articles on the subject.     

Preparation of sodium aluminate from the leach liquor of diasporic bauxite in concentrated NaOH solution

A new process to produce monosodium aluminate hydrates (MAH) by fast crystallization from the leach liquor of a diasporic bauxite in concentrated NaOH solution is presented. The crystallization of MAH was carried out easily compared to the precipitation of gibbsite and the effect of agitation, initial concentration of sodium aluminate, seed amount and the presence of red mud were systematically studied in a batch crystallizer. The apparent kinetics of crystallization followed a second order rate law with an apparent activation energy for MAH crystallization of 38.0 kJ/mol which implies a surface-diffusion controlled mechanism. X-Ray diffraction and scanning electron microscopy identified the structure of MAH as Na₂O·Al₂O₃·2.5H₂O with a flake crystal morphology. The molar ratio α of Na₂O to Al₂O₃ in the MAH products was < 1.2 after a simple wash by dilute sodium aluminate.     

Cavitation damage and creep crack growth

The effect has been investigated of prior damage on the creep crack propagation characteristics of 0.5 pct Cr, 0.5 pct Mo, 0.25 pct V steel at 823 K. On a macroscopic basis, the parametersK ₁ andC ^* both appear to correlate withda/dt although the parameterC ^* is unable to distinguish between virgin and damaged specimens. Rupture lives in the predamaged specimens are reduced by up to 60 pct when compared to virgin samples. Microscopically, it is found that the nature of the cavitation damage suggests that surface and grain boundary diffusion processes may have a minimal part to play, crack growth being controlled by the growth of cavities which is in turn controlled by the deformation of the surrounding matrix. A number of microscopic models are compared with the experimental data and it is suggested that a model which gives the best correlation with results is one proposed on the basis of matrix deformation. Formerly of the Department of Metallurgy, Manchester University Formerly of the Department of Metallurgy, Manchester University     

Removal of antimony from copper by injection of soda ash

The removal of Sb from molten copper is of importance in the development of processes which can smelt copper concentrates directly into copper in a single furnace. A promising method is injection of oxygen and sodium carbonate in a modified anode furnace. This study encompassed a thermodynamic analysis of the impurity removal reactions and an experimental investigation of antimony removal from molten copper in a 15 kW induction furnace. The results showed that the reaction was controlled by diffusion of Sb in the metal phase. The reaction between metal and injected flux can be divided into two subprocesses-. (1) “transitory contact” reaction to the injected flux particles as they rise through the melt and (2) “permanent contact” reaction across the interface between the metal bath and the supernatant slag layer. On the basis of the experimental work, the overall volumetric mass transfer coefficient (cm³/s) at 1473 K was expressed in terms of the two subprocesses as follows:(k _d A) _ov = (k _d A) _pc +(k _d A) _tc = 1.25Q _g ^0.29 + 0.28(H Q _f ) whereQ _g is the injection gas flow rate in normal liters per minute,H is the depth of injection in centimeters, andQ _f the rate of flux injection in grams per second.     

Fractal fracture and transformation toughening in CuNiAl single crystal

The fracture toughness J _Ic (ρ) and fractal dimension D _f of fractured surfaces of CuNiAl single crystal have been measured at temperatures in the range 20 °C to 120 °C, in which thermoelastic martensitic transformation takes place. The parent phase has higher fracture toughness than the martensite phase due to stress-induced transformation. The relationship between J _Ic (ρ) and D _f has been studied. It was found that log J _Ic (ρ) is linearly related to D _f if the failure involves a single mechanism. The slope of the plot may either be positive or negative for brittle or ductile failure, respectively. The difference in the J _Ic (ρ)-D _f correlation can be understood in terms of the micromechanism of fracture.     

Carbonation Behavior Assessment of RH Slag Batch after Aqueous Extraction at Environmental Pressure

In order to assess CO₂ sequestration amount and carbonation degree for RH slag at surrounding pressure, carbonation process of RH slag batch in lab is investigated, and the parameters of carbonation degree and CO₂ sequestration amount are the targets, and the relationship between both and relevant factors, such as CO₂ flow, gas bubble size etc. is originally discussed. The carbonation degree increases when temperature increases before 60 °C, then decreases. Particle size has a positive effect on carbonation degree, and carbonation degree for 0.5 L/min is bigger than those for 0.1 L/min and 1.0 L/min. When small gas bubble generator is adopted, carbonation degree and CO₂ sequestration amount is improved. The maximum carbonation degree and CO₂ sequestration amount is 34% and 178.65 g/kgslag, respectively when 38 μm RH slag batch is carbonated for 90 min at 60 °C under the conditions that CO₂ flow is 0.5 L/min and bubble size equals 5 mm. CaCO₃ and MgCO₃ phases exists through XRD analysis, showing that carbonation process is effective. Carbonation degree model is established assuming carbonation reaction occurs on the active surface of RH slag batch. This model fits very well by comparison between experimental results and model results.