The rate of absorption of nitrogen into liquid iron containing oxygen and sulfur

The rate of nitrogen absorption into and desorption from liquid iron containing sulfur and/or oxygen was measured by employing a constant-volume technique with a highly sensitive pressure transducer. Critical evaluation of the results demonstrated conclusively that the chemical rate at high oxygen or sulfur contents is second order with respect to nitrogen content in the metal and probably controlled by the dissociation of the nitrogen molecule on the surface. The effect of sulfur on the rate is complex because of the influence of 1) liquid-phase mass transfer at low sulfur contents, 2) the chemical rate on vacant iron sites at intermediate sulfur contents, and 3) the rate on the adsorbed sulfur layer or the limiting rate at high sulfur contents. However, at intermediate concentrations the limiting case for the adsorption isotherm for sulfur is adhered to and the rate is inversely proportional to the sulfur concentration. For Fe-O melts the rate is inversely proportional to the oxygen content except at low oxygen levels where mass transfer affects the rate. The rate for Fe-S-O melts can be calculated reasonably well from the results for the Fe-S and Fe-0 alloys, assuming that oxygen does not effect the adsorption of sulfur andvice versa and that there is nearly complete coverage of the surface with oxygen and sulfur atoms.     

The rates of desulfurization of liquid iron by solid CaO and CaO-saturated liquid iron oxide at 1600‡c

The rates of desulfurization of Fe-O-S melts by CaO crucibles and by CaO-saturated liquid iron oxide have been measured at 1600 ‡C. It was found that irons containing 1.62 wt pct and 0.64 wt pct sulfur and 0.070 wt pct oxygen are desulfurized by a reaction with the containing CaO crucible which does not involve the formation of a CaS product layer. The rate of desulfurization reaction is controlled by diffusion of sulfur in the iron melt, and a value of 6.7 ±1.7 × 10^-5 cm² per second was obtained for the diffusion coefficient of sulfur in liquid iron. Iron containing 0.088 wt pct sulfur and 0.070 wt pct oxygen is not desulfurized by solid CaO. The rate of desulfurization of liquid iron containing 0.088 wt pct sulfur and 0.070 wt pct oxygen by CaO-saturated liquid iron oxide is significantly greater than that calculated on the assumption of diffusion control in the metal phase, and evidence is presented in support of speculation that the reaction rate is enhanced by Marangoni turbulence at the slag-metal interface. The addition of 4 wt pct CaF₂ to the CaO-saturated liquid iron oxide has no influence on the rate of desulfurization of the melt. A. Saelim formerly Lecturer, Faculty of Engineering, Prince of Songkla University, Thailand     

The solubility of copper in lime-saturated and calcium ferrite-saturated liquid iron oxide

The solubility of copper in lime-saturated and calcium ferrite-saturated liquid iron oxide has been measured at 1300 °C by equilibrating copper-gold alloys with the melts in CO-CO₂ atmospheres of oxygen potentials 10^?7, 10^?8, 10^?9, and 10^?10 atm. It was found that copper exhibits Henrian behavior in the oxide melts and that the solubility is given by $$wt pct Cu = 17.6a_{CuO_{0.5} } $$ The copper capacity of the melts, 17.6, is approximately one-half of that of silica-saturated iron silicate melts at the same temperature. The results are compared with those of previous studies, and the difference between the solubilities of copper in silica-saturated and lime-saturated melts are discussed in terms of ionic interactions in the melts.     

Nitrogenation and decarburization of stainless steel

The rate of nitrogenation of iron alloys by nitrogen bubbling was determined. The rate of nitrogen pickup in iron with high oxygen activities was controlled by a chemical reaction at the gas bubble-metal interface. For an 18-8 type stainless steel and for iron containing between 50 and 400 ppm oxygen, the rate is controlled by a chemical reaction and liquid-phase mass transfer in series. The rate equation for this case was developed. The rates calculated from existing rate data and the fluid dynamic properties of the system were in good agreement with the experimental results. When argon-oxygen mixtures are bubbled through shallow (7.5 cm) stainless-steel melts, the rate of oxidation of chromium is considerably faster than that of carbon. It is suggested that oxygen primarily oxidizes chromium and iron and as the oxides are carried through the bath by argon bubbles they oxidize the carbon.     

The chemical diffusivity of oxygen in liquid iron oxide and a calcium ferrite

Measurements have been made of the chemical diffusion coefficient of oxygen in liquid iron oxide at temperatures from 1673 to 1888 K and in a calcium ferrite (Fe/Ca = 2.57) at temperatures from 1573 to 1873 K. A gravimetric method was used to measure the oxygen uptake during the oxidation of the melts by oxygen or CO₂-CO mixtures. The rate was shown to be controlled by mass transfer in the liquid melt. The chemical diffusivity of oxygen in liquid iron oxide at oxygen potential between air and oxygen was found to be 4.2±0.3 × 10⁻³ cm²/s at 1888 K. That in iron oxide at oxidation state close to iron saturation was established to be given by the empirical expression log D=−6220/T + 1.12 for temperatures between 1673 and 1773 K. For the calcium ferrite (Fe/Ca=2.57) at oxygen potential between air and oxygen, the diffusivity of oxygen was found to be given by log D=−1760/T−1.31 for temperatures between 1673 and 1873 K. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS.     

Fundamental studies on the deoxidation of low carbon steels with ferrosilicon

Laboratory size (4.6 kg) low carbon-iron melts were deoxidized using plain ferrosilicon, ferrosilicon with calcium aluminate flux, and ferrosilicon with calcium silicate flux. The dissolved oxygen and total oxygen contents in these heats were measured as a function of time and temperature using oxygen probes and quenched pin samples taken from the melts. The dissolved oxygen values, as measured by the oxygen probes, indicated that the iron-silicon deoxidation reaches equilibrium within five minutes of reaction time. However, the total oxygen (dissolved oxygen+oxygen in oxide inclusions), as measured from the quenched pin samples, took almost twenty minutes to reach a steady state. The two aforementioned features were common to all the experimental heats. Below 1978 K the ferrosilicon heats with calcium silicate or calcium aluminate flux had lower steady state total oxygen values as compared to the plain ferrosilicon heats; the difference was more significant at lower quenching temperatures. Also, below 1978 K, the dissolved oxygen-temperature relationship of flux heats was identical to their steady state total oxygen-temperature relationship, which indicated that the addition of fluxes leads to elimination of oxide inclusions. The performance of calcium aluminate and the calcium silicate fluxes in removing the oxide inclusions was found to be identical in these laboratorysize heats. U. B. PAL, formerly Senior Metallurgist, Melting and Primary Operations, Allegheny Ludlum Corporation, Technical Center, Brackenridge, PA 15014     

The rate of absorption of hydrogen into iron and of nitrogen into Fe-Cr and Fe-Ni-Cr alloys containing sulfur

The rate of absorption of hydrogen into liquid iron and of nitrogen into liquid Fe-Cr alloys containing various levels of sulfur was measured by using a constant-volume Sieverts apparatus employing a sensitive pressure transducer. The rate for the absorption of hydrogen was measured by using H2 containing a small amount of H₂S(<0.2 pct) such that the activity of sulfur on the surface of the melt was the same as in the bulk metal. The hydrogen-absorption rate for Fe-S melts containing up to 0.72 pet sulfur was virtually independent of sulfur content and controlled by liquid-phase mass transfer. The liquidphase mass-transfer coefficient for hydrogen in liquid iron, calculated from the results, was comparable to that for nitrogen transfer in liquid iron. The rate of absorption of nitrogen into Fe-Cr melts with low-sulfur contents was controlled by liquid-phase mass transfer. For melts containing significant amounts of sulfur it was controlled by both mass transfer and the chemical rate of the dissociation of nitrogen on the surface in series. Equations were developed to calculate the chemical rate from the measured rate, correcting for mass transfer. The chemical rate decreased with increasing sulfur content as expected, because sulfur is strongly adsorbed on the surface and increased with chromium content at constant sulfur activity, possibly because available Cr sites promote nitrogen dissociation. Formerly with United States Steel Corporation, Monroeville, PA     

The solubility of nickel and cobalt in iron silicate slags

The solubility of nickel and cobalt in silica saturated iron silicate slags in equilibrium with nickel-gold and cobalt-gold alloys has been investigated under controlled oxygen pressures in the temperature range of 1250 to 1350°C. It was found that the metal solubility increased with 1) decreasing temperature, 2) increasing oxygen pressure, and 3) increasing metal content of the alloy. The solubility of cobalt in the slag was found to be much higher than that of nickel. The solubility of the metal in the slag from its alloy can be explained by a simple oxidation process.     

Thermodynamics of Arsenic in FeOx-CaO-SiO<Subscript>2</Subscript> Slags

The distribution of arsenic between calcium ferrite slag and liquid silver (wt pct As in slag/ wt pct As in liquid silver) with 22 wt pct CaO and between iron silicate slag with 24 wt pct SiO₂ and calcium iron silicate slags was measured at 1573 K (1300 °C) under a controlled CO-CO₂-Ar atmosphere. For the calcium ferrite slags, a broad range of oxygen partial pressure (10^–11 to 0.21 atm) was covered, whereas for the silicate slags, the oxygen partial pressure was varied from 10^–9 to 3.1 × 10^–7 atm. The measured relations between the distribution ratio of As and the oxygen partial pressure indicates that the oxidation state of arsenic in these slags is predominantly As³⁺ or AsO_1.5. The measured distribution ratio of arsenic between the calcium ferrite slag and the liquid silver was about an order of magnitude higher than that of the iron silicate slag. In addition, an increasing concentration of SiO₂ in the calcium-ferrite-based melts resulted in decreases in the distribution of arsenic into the slag. Through the use of measured equilibrium data on the arsenic content of the metal and slag in conjunction with the composition dependent on the activity of arsenic in the metal, the activity of AsO_1.5 in the slags was deduced. These activity data on AsO_1.5 show a negative deviation from the ideal behavior in these slags.     

Fracture toughness of f.c.c. nickel and strain ageing b.c.c. iron in the temperature range 77–773 K

Ductile initiation fracture toughness J_IC of b.c.c. Armco iron and f.c.c. nickel has been measured in the temperature range 77–773 K. Armco iron exhibits dynamic strain ageing (DSA) in the temperature range 383–573 K while nickel of the purity used does not evince DSA. Load vs load line displacement (LLD) plots during fracture toughness testing of Armco iron show serrations in the temperature range 383–573 K similar to those observed in the tensile stress-strain curves. DSA is found to have a beneficial effect on the fracture toughness J_IC. A marked increase in tensile strength and fracture toughness occurs in Armco iron in the DSA regime. The strain hardening exponent, known to have a bearing on the plastic zone size and the void growth rate, seems a clear parameter in terms of which the observed J_IC variation with temperature can be understood. Remarkably, the variation of n with temperature is found to closely follow the observed trend in _IC. However, the slope of the J-R curve, dJ/da, decreases in the DSA regime with a minimum at 423 K. The decrease has been related to the fracture propagation process which is shown to occur by an alternate fast fracture and the ductile dimpled mode in the DSA regime. In the case of nickel, free from DSA, J_IC or dJ/da are found to be largely unaffected by the test temperature. At room temperature f.c.c. nickel, at closely matching strength levels, possesses higher fracture toughness as compared to b.c.c. Armco iron. The crystal structure effect is more pronounced at temperatures below the ambient. At 77 K, the fracture toughness of iron is drastically reduced due to the onset of cleavage while nickel, not prone to a change in the fracture mode, maintains the same level of J_IC, as at the ambient.     

Reduction of Chromite in Liquid Fe-Cr-C-Si Alloys

The kinetics and the mechanism of the reduction of chromite in Fe-Cr-C-Si alloys were studied in the temperature range of 1534 °C to 1702 °C under an inert argon atmosphere. The rotating cylinder technique was used. The melt consisted of 10 and 20 wt Pct chromium, the carbon content varied from 2.8 wt Pct to saturation, and the silicon content varied from 0 to 2 wt Pct. The rotational speed of the chromite cylinder ranged from 100 to 1000 rpm. The initial chromium to iron ratios of the melts varied between 0.11 and 0.26. In Fe-C melts, the effect of rotational speed on the reduction of chromite was very limited. Carbon saturation (5.4 wt Pct) of the alloy caused the reduction to increase 1.5 times over the reduction observed in the unsaturated (4.87 wt Pct) alloy at a given rotational speed. The addition of chromium to the carbon-saturated Fe-C alloy increased the reduction rate. The addition of silicon to the liquid phase increased the reduction rate drastically. The reduction of chromite in Fe-Cr-C melts is hindered because of the formation of, approximately, a 1.5-mm-thick M₇C₃-type carbide layer around the chromite cylinders. This carbide layer did not form when silicon was present in the melt. It was found that the reduction rate is controlled by the liquid-state mass transfer of oxygen. The calculated apparent activation energies for diffusion were 102.9 and 92.9 kJ/mol of oxygen in the Si-O and C-O systems, respectively.     

Slag corrosion of gamma aluminium oxynitride

Corrosion of γ‐aluminium oxynitride (AION) by CaO‐MgO‐“FeO”‐Al₂O₃‐SiO₂ melts corresponding to blast furnace slag was examined from 1693 to 1753 K under static and forced convection conditions. An intermediate layer was observed between the unreacted oxynitride and slag. After a certain time interval, the rate of the growth of this layer was found to be equal to the rate of the dissolution of the layer. Slag corrosion of AION is a strongly thermally activated process, the overall activation energy being 1002 kJ/mol. The rate of corrosion was found to be significantly enhanced by the addition of ?FeO?.     

Kinetics of decarburization of iron-chromium melts in highly oxidizing atmosphere

The kinetics of decarburization in Fe-Cr-C melts were studied to determine the rate con-trolling step for the process. The experiments were carried out under nitrogen-oxygen at-mosphere in a resistance-heated vertical-tube furnace. The liquid melt was held in a freshly prepared magnesite crucible. Sampling and chemical analysis of the metal phase led to time-carbon concentration curves for the system. An iron oxide layer just below the impinging are a and a general boil were observed. Results obtained by varying param-eters such as temperature, partial pressure of oxygen, flowrate of the oxidizing gas and amount of melt determined the limiting reaction mechanism. The rate has been found to be almost independent of flow rate and partial pressure of oxygen (between 1.0 to 2.0 l/min. and 0.5 to 1.0 atm of oxygen). The amount of melt and temperature have a marked effect on the reaction rate. The apparent activation energy has been found to be 48.0 ± 5.4 K cal/mol. The carbon oxidation reaction has been proposed to occur predominantly at CO bubble/metal interface. On the basis of the experimental results and discussions reaction involving reduction of oxides by carbon has been proposed to be the rate controlling step. Formerly a graduate student of IIT, Kharagpur     

Role of a silicate phase in the reduction of iron and chromium and their oxidation with carbide formation during the manufacture of carbon ferrochrome

The reactions of reduction of chromium and iron from chromospinelide and the reactions of carbide formation from the reduced metals are separated in space in experiments performed on ore grains with an artificially applied silicate shell. It is found that the silicate layer that isolates spinelide fro direct contact with carbon takes part in the reactions of both reduction and carbide formation. Free carbon extracts oxygen anions from the layer at the contact surface with the formation of CO, and the forming anion vacancies transfer “excess” electrons to the iron and chromium cations in the spinelide lattice and reduce them. Free and carbide-fixed carbon extracts iron and chromium cations from the silicate layer, and carbides form on the surface. The cation vacancies and electron holes (high-charge cations) that form in the silicate phase under these conditions are involved in the oxidation of the metal reduced in spinelide and cause its dissolution in the silicate phase and the precipitation of lower carbides on the surface of the silicate phase. The structure that is characterized of carbon ferrochrome forms on the surface of the silicate phase. Carbide formation is slower than reduction because of higher energy consumed for the formation of high-charge cations and the transfer of cations from the spinelide volume to the outer surface of the silicate phase. In the absence of a silicate layer, a carbide shell blocks the contact of carbon with oxides, which leads to the stop of reduction and, then, carbide formation. In the presence of a silicate (slag) shell around a spinelide grain, the following two concentration galvanic cells operate in parallel: an oxygen (reduction) cell and a metal (oxidation) cell. The parallel operation of the two galvanic cells with a common electrolyte (silicate phase) results in a decrease in the electric potentials between spinelide inside the silicate phase and carbon and carbides on its surface, and each of the processes is significantly facilitated and accelerated. In other words, the production of carbon ferrochrome is accelerated.     

Distribution of Bi Between Slags and Liquid Copper

The distribution of Bi between liquid copper and calcium ferrite slag containing 24 wt pct CaO, iron silicate slag with 25 wt pct SiO₂, and calcium iron silicate slags was measured at 1573 K (1300 °C) under controlled CO-CO₂ atmosphere. The experimental results showed that bismuth distribution is affected by the oxygen partial pressure, and bismuth is likely to exist in slags in the 2+ oxidation state, i.e., as BiO. The distribution ratio between calcium ferrite slag and metal was found to be close to that of iron silicate slag. The Bi distribution ratio was found to decrease with increasing SiO₂ and Al₂O₃ content in slag. Increasing temperature was found to decrease the Bi distribution ratio between slag and metal. Using the measured equilibrium data on Bi content of the metal and slag and composition dependence of the activity of Bi in liquid copper, the activity and hence activity coefficient of BiO in the slag was calculated. The close value of activity coefficient of BiO in both slags at the same oxygen partial pressure indicates that the CaO-BiO and SiO₂-BiO interactions are likely to be at the same level, or the FeO _x -BiO interaction is the predominant interaction for BiO in the slag. Therefore at a constant FeO _x content in the slag, the CaO-BiO and SiO₂-BiO interactions doesn’t affect \( \gamma_{\text{BiO}} \) significantly.     

Spreading and interlayer formation at the copper-copper oxide/polycrystalline alumina interface

Spreadability and reaction layer growth rates of copper-oxygen alloys on polycrystalline alumina were measured above the melting point of copper to better understand the direct bonding process. Spreading was measured as a function of composition and temperature by monitoring the diameter of molten droplets as a function of time. As the oxygen content of the melt increased from 0 to 3 wt pct, the spreading diameter increased linearly, at fixed time and temperature. Constant diameters were observed for oxygen compositions between approximately 3 and 6 wt pct. The diameters again increased linearly for oxygen concentrations greater than 7 wt pct. This behavior was explained by reference to the copper-oxygen binary phase equilibrium. An interfacial product was identified to be the complex oxide, CuA10₂. A detailed investigation of the interlayer growth kinetics was performed to understand the fundamental phenomena controlling the spreading rates. The growth rate of the CuAlO₂ phase and the spreading rate were simultaneously measured for alumina in contact with a copper-2 wt pct oxygen alloy drop as a function of temperature. The reaction layer thickening was found to be diffusion controlled, with an apparent activation energy of 309 kJ/mol, and the spreading rate did not correlate with the thickening rate. Formerly Research Associate, Center for Welding and Joining Research, Department of Metallurgical and Materials Engineering     

Measurement of the surface tensions of Fe-saturated iron silicate and Fe-saturated calcium ferrite melts by Padday’s cone technique

Padday’s cone technique allows determination of the surface tension of a liquid from measurement of the maximum excess force exerted on a cone during its immersion in, or withdrawal from, the liquid and knowledge of the density of the liquid. As the technique does not require rupture of the meniscus, an equilibrium measurement is obtained. The surface tensions of Fe-saturated iron silicate melts and Fe-saturated calcium ferrites, measured at 1410 °C using iron cones, are in good agreement with values in the literature obtained using the hollow cylinder technique. The maximum excess forces exerted on Pt-Rh cones immersed in liquid iron oxides at 1460 °C in the composition range from saturation with iron toX _Fe2_O3 = 0.205 have been measured. The surface tensions of these melts cannot be determined unambiguously from the experimental measurements because of an uncertainty in the densities of the melts. However, it is shown that the literature values for the surface tension of liquid iron oxide are incorrect because of an error in the experimental procedures used.     

Solubility of water in lime-alumina-silica melts

The water solubility in fused silicates of the CaO-SiO₂ and CaO-SiO₂-Al₂O₃ systems has been measured using a vacuum fusion technique. The melts were equilibrated with nitrogen as “carrier gas” containing an accurately known water content. The solubility of water increased on the addition of lime to the melts of both systems. The effect of alumina is initially, to decrease the solubility. Additions above about 20 wt pct at a constant basicity of 0.6 however raised the water solubility of lime-silica melts. From the plot of oxygen density against the hydrogen concentration of the silicate melts of the system lime-silica-alumina studied in this investigation, it may be concluded that free hydroxyls take up interstitial positions in the silicate network. Wustite additions to lime-silica melts of basicity equal to 1 do not appear to influence the water solubility, although the rate of gas absorption from the furnace atmosphere is accelerated significantly. P. L. SACHDEV, formerly Graduate Student, Institut für Eisenhüttenwesen R. W. Technische Hochschule Aachen, West Germany A. MAJDIČ, formerly Senior Engineer, Institut für Eisenhüttenwesen R. W. Technische Hochschule Aachen     

The kinetics of oxidation of iron-silicon and iron-aluminum alloy melts by pure oxygen or oxygen-bearing gases

The kinetics of oxidation of Fe−Si and Fe−Al melts by pure oxygen, and that of pure Fe by He−O₂, N₂−O₂, or Ar−O₂ mixtures have been investigated by a modified Sieverts' method at 1600°C. Considerable decrease in the oxidation rate has been observed for the alloy melts containing a few percent of Si or Al since formation of a silica- or alumina-rich oxide layer on the melts prevents further progress of the exothermic chemical reaction. The oxidation rate for melts high in Al has been considered to be limited by the diffusion of ions through the oxide layer. Addition of diluents to O₂ markedly and continuously decreases the oxidation rate of a pure Fe melt. The latter rate has been show to be controlled by the diffusion of O₂ across the gaseous boundaries at gas/melt interfaces.