Liquid Fe-C Alloys Research Articles

Structural characteristics of liquid iron with various carbon contents based on atomic simulation

The liquid Fe-C alloy is the most important system in the metallurgical process and the molecular dynamics simulation were carried out to investigate the structure and properties of liquid Fe-C alloy using EAM and MEAM potential. The local structural order, transport properties and fluidity were analyzed. By comparison, the EAM potential is better than MEAM potential to simulate the properties of liquid Fe-C system. In the liquid state, the Fe-C alloy have a short-ranged ordered structure, accompanied by the transformation of Fe4C units to Fe6C units with the carbon content increasing. And the increase in carbon content could increase the complexity of the liquid structure. In terms of transport properties and fluidity, there is an inflection point when carbon content increases to 2 wt%. When the carbon content is less than 2%, the change of Fe atom transport performance is the main factor that leads to the increase of viscosity. On the contrary, when the carbon content is more than 2%, the transportation of C atom dominates.

Effect of smelting temperature and CO2 gas flow rate on decarburization kinetics between CO2 gas and liquid Fe-C alloy

ABSTRACT The effect of smelting temperature and CO2 flow rate to decarburization kinetics between CO2 and liquid Fe-C alloy were investigated. The decarburization rate caused by CO2 increases with smelting temperature. Compared with low carbon condition (near 0.45 wt pct), the effect of temperature on decarburization rate is more significant under high carbon condition (near 3.5 wt pct). At high carbon content, the decarburization rate increases linearly with CO2 flow rate. And a CO2 decarburization kinetics model considering the influence of bubble diameter, bubble generation frequency, rising velocity and rate controlling step was established. When CO2 transfer in gas phase boundary layer is rate controlling step, the calculated data are in good agreement with the experimental data. It was also found that the effect of CO2 flow rate on the decarburization rate was mainly affected by the reaction surface area, but less on gas phase mass transfer coefficient in current experimental situations.

Dissolution of carbon from coke and char in liquid Fe-C alloys

Purpose: The aim of this paper was to study dissolution of carbon from carbonaceous materials of different origin with different morphology, microtexture and microstructure in the liquid Fe-C alloys. Design/methodology/approach: The dissolution of carbon from coke, char and glassy carbon in the molten Fe-C alloy (initial carbon concentration 2.46 wt.%) at 1350°C was measured and compared with that from graphite. The dissolution of carbon from demineralised coke and char in the Fe-C solution was also examined to study the effect of mineral matter on the carbon dissolution. Findings: The concentration of carbon in the Fe-C solution dissolved from graphite was higher than that from coke and char. Demineralisation of coke and char had a significant effect on the carbon dissolution. The concentration of carbon dissolved from demineralised coke and char in the Fe-C alloy approached the solubility of graphite in this alloy under the same conditions. Results obtained in this work confirmed that ash has a strong effect on the carbon dissolution. Research limitations/implications: Investigations in this paper were conducted at 1350°C. At higher temperatures; (1) the degree of coke and char graphitisation increases changing the microstructure of carbonaceous materials; (2) the ash can melt, and (3) some of the metal oxides in the ash can be reduced by carbon to the metal phase, thereby weakening the effect of ash on the carbon dissolution. Demineralisation of coke was incomplete; it reached 70-80% with some effect on the carbon dissolution. The effect of ash composition and further coke demineralisation on the carbon dissolution at higher temperature will be investigated in the future study. Originality/value: This study demonstrated that dissolution of carbon from coke and char was strongly affected by ash. Reactions of dissolution of carbon from coke and char in liquid Fe-C alloy reached a steady state within 1-2 hours. In this state, the coke/char – metal system was far from equilibrium. The “apparent” activity which can be assigned to carbon in the steady state is below one for graphite with significant implications for metallurgical processes.

Dissolution rate of solid iron into liquid Fe-C alloy

Dissolution rate of solid iron into liquid Fe-C alloy was investigated using a sampling method at a fixed reaction area. The initial carbon concentration of the bulk liquid Fe-C alloy was set as 4.23, 3.29 and 2.17 wt% for the experiments at 1503, 1597, and 1688 K, respectively. The temperature dependence of the rate constant was obtained as follows: \(\ln k(m/s) = - 3.47 - \tfrac{{8905}} {T}\). The activation energy was estimated to be 74 kJ/mol. A dimensionless analysis was carried out separately to confirm the rate-determining step. The measured values showed reasonable agreement with the calculated values from the dimensionless analysis. Therefore, it is concluded that the mass transfer in the liquid boundary layer is the rate-determining step for the dissolution of solid iron in the liquid Fe-C alloy. The liquid boundary layer was estimated approximately 100 μm, which slightly decreased with increasing temperature. Present experimental results would be helpful to understand the mechanism of liquid iron formation in the ironmaking and steelmaking processes as well as in the reduction of iron-containing slags.

In-Situ observation of the carburization of solid iron by graphite

The carburization of solid iron by graphite was investigated by using confocal laser scanning microscopy to understand the initial carburization reaction mechanism of solid iron by solid carbon at high temperatures. As the carburization was initiated, a liquid layer was formed at the interface and grew parallel to the interface after an incubating period for the liquid phase formation. This required incubation time decreased with an increasing temperature due to the decrease in the solubility limit of carbon in γ-Fe. A moving interface model was used to interpret the carburization and melting behavior of solid iron by solid carbon with consideration for the diffusion of carbon in both solid and liquid phases. Using the moving interface model, the diffusion coefficient of carbon in the liquid Fe-C alloys was obtained. $$D_L (m^2 /s) = 5.1 \times 10^{ - 7} \exp \left( { - \frac{{53.9 \times 10^3 }} {{RT}}} \right)\{ 1458 - 1623K\} $$ The rate of carburization in the solid and liquid phases was evaluated. As the liquid phase was formed, the contribution of solid phase in the carburization decreased from 9.5% at 1.3 s to 5.4% by 20 s.

Molecular Dynamics Modelling of Liquid Fe-C Alloys

Properties and structure of liquid Fe-C alloys with carbon concentration up to 20 at% at temperatures up to 2500 K were calculated using molecular dynamics modelling. Interaction between Fe-Fe and Fe-C atoms was described using the embedded atom potential (EAM). The Morse potential was chosen for Fe-C pair interaction, while C-C pair interaction was presented in the form of the repul- sive potential. Parameters of potentials were adjusted using experimental data for density, internal energy, bulk modulus and distance between Fe and C atoms in liquid Fe-C alloys near the liquidus temperature. Calculated density and molar volume of Fe-C alloys at 1873 K decreased with increasing carbon concentration; a temperature coefficient of density of liquid alloys was, practically, independent of the carbon concentration. Carbon content of the alloys had a negligible effect on the distance between Fe-Fe and Fe-C atoms and on a sum of coordination numbers Fe-Fe and Fe-C. Distribu- tion of atoms in the first coordination sphere in the Fe-C alloys was close to statistical.

A MODIFIED MODEL USED TO DESCRIBE AUSTEN-ITE/MARTENSITE INTERFACE

A new method (electromagnetic steel-teeming method) using electromagnetic induc- tion heating in slide-gate was proposed to overcome the disadvantage of the pollution of the traditional nozzle sand on the molten steel. The basic idea of this new process is to melt part or whole of the new ladle well-packing material (i.e. Fe-C alloy with similar composition of the molten steel), which becomes the substitute of the traditional nozzle sand, and achieve smoothly molten steel-teeming. In the experiment, the state of the Fe-C alloy well-packed in the upper nozzle was examined when the ladle held molten steel using the self-designed electromagnetic steel-teeming simulation facility. The results showed that the Fe-C alloy in the upper nozzle were divided into liquid Fe-C alloy, solidified Fe-C alloy, liquid-sintered, solid-sintered and original layers. The solidified, liquid-sintered and solid-sintered layers could prevent the molten steel flow into the upper nozzle from destroing the slide plate. The blocking layers could be melted with induction heating to achieve electromagnetic steel- teeming. In addition, the position of the liquid/solid interface of the Fe-C alloy in the upper nozzle was investigated. The length of liquid Fe-C alloy increased with the increase of time after molten steel poured into the ladle and then kept in a certain length. Furthermore, the position of the interface increased with the increase of melting points which were related to the composition of the Fe-C alloy.

Analysis of Reaction Rate between Solid Carbon and Molten Iron by Mathematical Models

The factors affecting the carbon dissolution rate into liquid iron and the interdependency between these rate influencing factors were discussed with aid of a kinetic model. These rate influencing factors include temperature, carbon structure, interfacial active elements, metal composition, liquid agitation, wettability between solid and liquid, solid particle size, ash in carbonaceous materials and side reactions.Experiments were carried out on carbon dissolution from graphite and coke into liquid Fe-C alloy using resistance or induction furnaces at 1573-1873 K. The kinetic parameters describing carbon dissolution rate were obtained from the comparison of observed carbon dissolution rates and model calculations. The dissolution rates at higher temperatures or in an induction furnace were found more rapid than those at lower temperatures or in a resistance furnace. The mass transfer in liquid was found to be the major limiting step for the carbon dissolution process.

Kinetics of Reduction of FeO from Slag by Graphite and Coal Chars

The rates of reduction of FeO from iron-saturated FeO-CaO-Al2O3-SiO2 slags by graphite, coke, bituminous coal and anthracitic coal chars at temperatures in the range 1 673-1873 K have been measured using a sessile drop technique. The extents of reaction were determined using EPMA analysis of quenched samples, and on line gas analysis using a quadrupole mass spectrometer. The reaction rates have been shown to be dependent critically on carbon type. For the reaction geometry used in this investigation the reduction rates of graphite and coke are observed to be faster than with coal chars. This unexpected finding is shown to be associated with differences in the dominant chemical and mass transfer mechanisms occurring at the reaction interface. High reaction rates are observed to occur with the formation of liquid Fe-C alloy product and the associated gasification of carbon from the alloy. The rates of reduction by coal chars are determined principally by the chemical reaction at the carbon/gas interface and slag phase mass transfer.

THE STRUCTURE OF LIQUID Fe-C ALLOY NEAR THE MELTING POINT

The structure of liquid Fe-C alloy near the melting point has been studied at three different temperatures (1540, 1560 and 1580 °C). It is shown that the structural correlation length (or the cluster size), coordination number, distance between the nearest neighbour atoms and number of atoms in a cluster all decrease with increasing temperature. Small prepeaks before the first main peak were found in the structure factors of liquid alloy and the prepeaks became slightly obscure as the temperature increased. In general, the presence of the prepeaks corresponds to a medium range order structure. The prepeaks fade away with increasing temperature implying that the degree of superheat contributes to the disappearance of the medium range order.On a étudié la structure de l’alliage liquide Fe–C près du point de fusion, à trois températures différentes (1540, 1560 et 1580 °C). On montre que la longueur de corrélation de la structure (ou taille de la grappe), le nombre de coordination, la distance des atomes les plus proches voisins et le nombre d’atomes dans une grappe diminuent tous avec une augmentation de la température. On a trouvé de petits pré–pics avant le premier pic principal dans les facteurs de structure de l’alliage liquide et ceux-ci s’obscurcissaient légèrement à mesure que la température s’élevait. En général, la présence des pré–pics correspond à une structure d’ordre à moyenne portée. Les pré–pics s’estompent avec une augmentation de la température, impliquant que le degré de surchauffe contribue à la disparition de l’ordre à moyenne portée.

The Effect of the Liquid Fe-C Phase on the Kinetics in the Carburization of Iron by CO at 1523K.

In order to evaluate the effect of liquid Fe-C alloy produced during the carburization on the kinetics of iron carburization, the rates of carburization of iron by CO-Ar gas mixtures have been measured gravimetrically at 1 523 K and the growing rates of the liquid Fe-C layer thickness are also measured. The measurements were made under conditions for which the gas phase mass transfer was negligible. It is found that the weight gains due to the carburization reaction are almost proportional to t 2/3 within experimental scatters. For all runs except that of 1 % CO, the liquid phase is observed after the carburization time of 300 s. The liquid phase thickness is found to be almost constant for the first 3.6 ks. After 3.6 ks, the liquid phase thickness starts to increase almost linearly with time. The mechanism to maintain the constant thickness is briefly discussed. The carburization reaction rate is calculated based on the well established decarburization rates and is reasonably agreed with the experimental carburization rate. Compared the experimental results with the calculated liquid phase diffusion rates and the chemical reaction rates, it is confirmed that the rate-controlling step of the carburization reaction of iron with CO gas at 1 523 K is the chemical reaction CO + O ad →CO 2 at the surface after the liquid Fe-C phase is produced.

Carbon Solution in Liquid Iron and Iron Alloys

The dissolution of carbon into liquid iron and iron alloys was investigated using four independent experimental approaches, i.e., immersion of a graphite cylinder in a melt bath using an electric resistance furnace, immersion of a coke particle in a melt bath using an induction furnace, a molten droplet on a graphite plate using an induction furnace and sliding a metal droplet down a graphite spiral in a resistance furnace. The dissolution rate was analysed by assuming that this reaction is governed by the diffusion of carbon from the carbon-metal interface to the bulk liquid metal through a boundary layer. The experimental results for each case were well interpreted by both concentration and activity driven models. The variation in the carbon dissolution rate with the experimental method was explained by the Olsson relation or the penetration theory. An increase in carbon dissolution rate with temperature was observed. The carbon dissolution rate into liquid Fe-C alloy was not changed by addition of 1.9 wt% silicon to the melt, while a decrease in the dissolution rate of carbon was observed by adding 1 wt% sulfur to the melt.

On the precipitation of TiC in liquid iron by reactions between different phases

In order to study the in situ formation of carbide-reinforced metal matrix composites and the inoculation reactions of some low alloy steels, titanium carbide precipitation in liquid iron was investigated. TiC particles were formed directly in liquid iron solution through reactions between different phases. The high-temperature reactions studied in the present work are reactions between (a) solid graphite and liquid Fe-Ti alloys, (b) Fe-Ti and Fe-C liquid alloys, and (c) pure solid titanium and liquid Fe-C alloys. Small samples for these reactions have been processed using a mirror furnace facility in a precisely controlled manner. Samples of Fe-Ti alloys reacted with graphite were also processed by a high frequency induction furnace and other facilities. The precipitation behavior of titanium carbide in various reactions and the distribution of carbide particles in iron melts were investigated. Different growth morphologies of the carbide were observed under various conditions. The effects of convection on the reactions were observed and discussed, and by processing the samples with various orientations relative to gravity, various patterns of carbide distribution were obtained.

On the reaction between Fe-Ti and Fe-C liquids under microgravity

Reaction between liquid Fe-Ti and Fe-C alloys has been studied under microgravity on board an aeroplane during parabolic flights. The two alloys were melted in contact with each other and held for some seconds at a temperature of 1450 °. The samples were melted and solidified during the microgravity period. During the contact of the two liquid alloys, carbon and titanium atoms reacted in the melt and titanium carbides were formed. A carbide containing layer was formed at the interface region between the two liquids on the Fe-Ti side. The thicknesses of the carbide layers were meas-ured. Theoretical calculation of the precipitation kinetics was attempted, and good agreement between the calculated and experimental results was obtained. The effect of alloy composition on the growth rates of the carbide zones was revealed by both experiments and calculations. Supersaturation of the carbide in front of the reaction interface was calculated. Homogeneous nucleation of titanium carbide can readily occur according to the present calculations.

Reaction Kinetics of CO2-H2O Gas Mixtures with Liquid Fe-C Alloys.

The rate of decarburization of carbon-saturated liquid iron with CO2-H2O gas mixtures has been studied to bettter understand the reaction mechanism between liquid iron containing carbon with post-combusted gas in an iron bath smelting reactor. The measurements were done under conditions in which the effect of mass transfer is negligible. The experimental temperature was 1773–K and the sulfur content in the metal was fixed at 0.2 mass%. The rate was measured for a partial pressure of CO2 in the range of 0.5 to 1 atm for CO2-Ar, 0.7 to 0.9 atm for CO2-H2O and 0 to 0.95 atm for CO2-5%H2O-Ar gas mixtures. The measured rates agreed well with the predicted rates by assuming an ideal additivity of CO2 and H2O reaction rates individually measured in previous work. It was concluded that there were no special effects retarding or accelerating the reaction rate with liquid iron and the rates were simply additive. It was also found that iron oxide did not form on the surface as long as the surface concentration of carbon did not become very low. The critical carbon content at which FeO forms on the surface was expressed as a function of partial pressure of oxidant, sulfur content in the metal, mass transfer coefficient and temperature and was found to be in agreement with experimental observations.

The density of liquid iron-carbon alloys

The density of liquid Fe-C alloys at temperatures ranging from 1250 °C to 1550 °C was measured by the sessile drop profile method. An accurate method of digital image processing was de-veloped to capture, enhance, and determine the coordinates of the X-ray shadow image of the droplet. Laplace's equation was then solved to obtain the volume and density of the droplet. The density of iron-carbon alloys was then determined as a function of carbon content (from 0 to 4 wt pct) and temperature (1250 °C to 1550 °C). A least-squares analysis of our data points gives an equation for the density of liquid iron-carbon alloys as a function of temperatureT [K] and carbon content [pct C] [wt pct]: ρ[g/cm3] = (7.10 - 0.0732[pct C]) - (8.28 - 0.874[pct C]) × 10-4(T - 1823) These results will give a value to within ±1.5 pct of the data of Lucas, [3] Widawski and Sauerwald, [2] and the present work.

Application of the quasi-subsubregular solution model: The iron-carbon system

The quasi-subsubregular solution model introduced by Chuang et al. represents the most complex approach yet to dealing with both compositional and temperature nonregularity in metallic solutions. Evaluation of the model in relation to the Fe-C equilibrium diagram demonstrates that its applicability in dilute solutions is highly dependent on the accuracy of the experimental data from which its coefficients are derived. Lower-temperature equilibria between α-ferrite, aus-tenite, and pure graphite were modeled with reasonable accuracy; the modeling of higher-temperature equilibria in the system was less successful. In particular, experimental difficulties with the measurement of the thermodynamic properties of liquid Fe-C alloys limited the ability of the model to accurately predict the phase diagram. The model can also be used to determine infinite-dilution activity coefficients and to test the adherence of dilute-solution phases to Henrian behavior.

The kinetics of the nitrogen reaction with carbon saturated iron alloys.

The kinetics of the nitrogen reaction with liquid Fe-C alloys was investigated by using the isotope exchange technique. The effect of small amounts of S, P, Pb, Sn, Bi and Te in Fe-Csat, alloys on the rate constant of the interfacial nitrogen reaction was studied at 1450°C The rate constant for Fe-Csat.-S alloys was in agreement with the previous results for Fe-S alloys. It was also confirmed that the influence of carbon in Fe-C-S alloys on the rate of the nitrogen reaction is negligible. The effect of tin on the rate constant is small even when the tin content is as high as 0.8wt%. However Pb, Bi and Te have significant effects on the rate constant and the addition of small amounts of these elements decreases the rate significantly. Phosphorus also decreased the rate constant. According to the adsorption model, the interfacial reaction rate should approach zero at high concentration of surface active elements, but in the case of Fe-Csat.-P, Fe-Csat.-Pb and Fe-Csat.-Te alloys, there was evidence of a residual rate at high concentrations of P, Pb and Fe.

Sulfur solubility in iron-carbon melts coexistent with solid CaO and CaS.

Experimental data are presented for the solubility of sulfur in liquid Fe-C alloys coexistent with solid calcium oxide, calcium sulfide and carbon monoxide of known pressures at 1 600°C. The equilibrium constant is determined for the sulfur reaction in terms of the activities of sulfur and carbon in iron for composition ranges 0.005 to 0.4% S and up to 3% C. The extrapolation of the data to hot metal compositions predicts for unit activities of calcium oxide, calcium sulfide and carbon monoxide, the equilibrium sulfur content of 4ppm for 1 600°C and 8ppm for 1 400°C. In relation to ladle desulfurization of hot metal, selected data are presented on some slag-metal reaction equilibria.

Coupling phenomena in the transfer of elements between a gas phase and iron melts

The simultaneous transfer of Si and C from a gas phase containing SiO and CO to liquid Fe-C alloys has been investigated. It was found that, although the silicon content of the melt increased with time as expected, the carbon content initially decreased, in spite of the fact that the carbon potential of the gas phase was above that of the liquid alloy. These phenomena are interpreted in terms of irreversible thermodynamics which shows that the overall transfer reactions are comprised of coupled reactions: SiO(g) + CO(g) →Si + CO2(g) andC + CO2(g) → 2CO(g). It is also shown that the simultaneous transfer of carbon and oxygen from gas mixtures of CO and CO2 to liquid iron occurs via the coupled reactions CO2(g) →O + CO(g) and 2CO(g) →C + CO2. In each case there is a predominant, or driving reaction which promotes the other.