Rate of nitrogen desorption from liquid iron-carbon and iron-chromium alloys with argon

The rate of nitrogen desorption from inductively stirred liquid iron, iron-carbon, and iron-chromium alloys with argon carrier gas has been measured by the sampling method for a wide range of nitrogen, carbon, and chromium contents mainly at 1600 °C. The results obtained by the present work and other data of previous investigators are used to clarify the reaction mechanism of nitrogen desorption from liquid iron. The rate of nitrogen desorption from liquid iron and iron alloys is second order with respect to nitrogen content in the metal under the present condition, and mutual relationships among interfacial chemical reaction, liquid-phase mass transfer, and gas-phase mass transfer are elucidated. The effects of oxygen and sulfur on the rate of nitrogen desorption are given byk _' ^{c ’} = 3.15?_N ² [1/(1 + 300a₀ + 130a_s)]. Carbon dissolved in iron increases the rate of nitrogen desorption, and chromium decreases it. The effects of these alloying elements can be explained by the change of the nitrogen activity in the metal.     

Rate of nitrogen desorption from liquid iron-carbon and iron-chromium alloys with argon

The rate of nitrogen desorption from inductively stirred liquid iron, iron-carbon, and iron-chromium alloys with argon carrier gas has been measured by the sampling method for a wide range of nitrogen, carbon, and chromium contents mainly at 1600 °C. The results obtained by the present work and other data of previous investigators are used to clarify the reaction mechanism of nitrogen desorption from liquid iron. The rate of nitrogen desorption from liquid iron and iron alloys is second order with respect to nitrogen content in the metal under the present condition, and mutual relationships among interfacial chemical reaction, liquid-phase mass transfer, and gas-phase mass transfer are elucidated. The effects of oxygen and sulfur on the rate of nitrogen desorption are given byk _' ^{c ’} = 3.15ƒ_N ² [1/(1 + 300a₀ + 130a_s)]. Carbon dissolved in iron increases the rate of nitrogen desorption, and chromium decreases it. The effects of these alloying elements can be explained by the change of the nitrogen activity in the metal. This paper is based on a presentation made at the G. R. Fitterer Symposium on Nitrogen in Metals and Alloys held at the 114th annual AIME meeting in New York, February 24–28, 1985, under the auspices of the ASM-MSD Thermodynamic Activity Committee.     

The kinetics of the nitrogen reaction with liquid iron-Sulfur alloys

An experimental apparatus was designed and constructed which permits utilization of both the modified Sieverts' technique and the isotope exchange technique for study of the kinetics of ni-trogen reaction with liquid iron alloys. Experimental results from the two techniques are used in con-junction with the current knowledge in the field to readdress several persisting questions dealing with the nitrogen reaction. Results from this study have confirmed that N₂ dissociation is the rate lim-iting step in the intrinsic chemical reaction mechanism of nitrogen absorption. In addition, the ex-istence of a residual nitrogen absorption rate at high sulfur concentrations has been reaffirmed and possible reasons for the phenomenon are discussed. Lastly, results of an investigation into the in-fluence of carbon on the nitrogen reaction rate in liquid Fe-C-S alloys indicate that carbon has a negligible effect on the rate. Formerly with the Department of Metallurgical Engineer-ing and Materials Science, Carnegie-Mellon University, Pittsburgh, PA     

A Kinetic Study on the Control of Nitrogen in Molten Slag and Metal

Nitrogen can easily contaminate molten steel during the steelmaking process and due to the low nitrogen capacity in slag, it is difficult to remove entrapped nitrogen from liquid steel. Degassing is often done to the steel at secondary steelmaking to lower the nitrogen content, but the control can often be kinetically limited by the steel grade and also the slag composition. Thus, a fundamental understanding of nitrogen dissolution into molten slag and metal including the rate of nitrogen dissolution can help in controlling nitrogen content in the final product.The kinetics of nitrogen dissolution in the molten calcium aluminate based slags and in molten steel with various element additions was investigated by measuring the 14N-15N isotope exchange reaction using a mass spectrometer at 1873 K.Results show that effect of elements on the rate constant of nitrogen dissolution such as Ni in Fe is relatively minimal similar to molybdenum. The surface rate constant of nitrogen dissolution in liquid Fe-10%Ni alloy was found to be 3.77×10-5 (mol/cm2·s·atm).The rate constant of nitrogen dissolution in the CaO-Al2O3-CaF2 slag was found to be wedge shaped, which decreased with increasing CaF2 to about 20 mol% followed by an increase through the rest of the CaF2 composition range. This was related to the effect of CaF2 on the structure of Al-O bonds for this slag.     

Surface and interfacial properties of Fe-30%Cr-S alloys in contact with alumina

The surface tension of liquid iron-30% chromium alloys at 1823 K was determined as a function of sulfur content using the sessile drop technique. The surface tension of iron-30% chromium alloys decreases with increasing sulfur content and the results can be described by the following equation for sulfur mass contents of greater than 21 ppm: γ = 1592 – 178ln(1 + 145a_S)mN/m. The contact angle between the droplet and an alumina substrate was also measured and found to decrease from 142° at 21 ppm sulfur activity to 80° at a sulfur activity of 0.53 (related to the standard state with a mass content of 1%). From these measurements the interfacial energy between liquid iron-30% chromium alloys and alumina was calculated using Young's equation. The interfacial tension can be described by the following equation: γ = 2026 – 519ln(1 + 29a_S)mN/m.     

Interfacial rates of reaction of CO2 with liquid iron silicates,silica-saturated manganese silicates,and some calcium iron silicates

Measurements of the rates of dissociation of CO₂ have been made by the¹⁴CO₂-CO isotope exchange technique on liquid iron silicates, calcium iron silicates, and silica-saturated manganese silicates as functions of temperature and imposed equilibrium CO₂/CO ratio. It is shown that the rates of reduction of the liquid iron silicates and an iron oxide-rich slag in CO-CO₂ atmospheres are consistent with the rates of isotope exchange, indicating a common rate determining step. The dependences of the apparent first order rate constant on the oxygen activity for the dissociation of CO₂ on silica-saturated iron silicates and an equimolar “FeO”-CaO-SiO₂ melt are found to be closely consistent with the ability to transfer two charges to the adsorbing or dissociating CO₂ molecule, as was previously found for liquid iron oxide and lime-saturated calcium ferrites. Apparent rate constants at fixed oxygen activity are found to increase generally with the basicities of the melts. Formerly Postdoctoral Fellow, Department of Metallurgy, University of Newcastle, New South Wales, Australia     

Kinetic studies on the dissolution of nitrogen in CaO-Al2O3, CaO-SiO2, and CaO-CaF2 melts

The rate of nitrogen dissolution in CaO-Al₂O₃, CaO-SiO₂, and CaO-CaF₂ melts was measured by ¹⁴N-¹⁵N isotope exchange reaction. The rate constant of nitrogen dissolution in CaO-based oxide melts, which is defined as first order with respect to nitrogen partial pressure, was found to be much smaller than that in molten iron-based alloys investigated in our previous work. The activation energies for nitrogen dissolution in 40 mass pct CaO-60 Al₂O₃ and 50 CaO-50 SiO₂ melts are 224 and 581 kJ/mol, respectively. For CaO-Al₂O₃ and CaO-SiO₂, dependence of the rate constant on composition is very similar to that of nitride capacities. Moreover, it was confirmed that the rate constant was not affected by oxygen or nitrogen partial pressure.     

A study of the reaction of CO on liquid iron alloys

In all previous studies of the CO reaction on liquid iron the rate was controlled by liquid phase mass transfer. In this study an isotope exchange technique was used to eliminate liquid phase mass transfer as a possible rate-controlling mechanism. The rate in the present work is over an order of magnitude faster than previously measured. The isotope exchange rate for C¹³O¹⁸ in normal CO on liquid iron was not a function of temperature or sulfur content up to 0.43 pct S, but was a function of gas flow rate, indicating that the rate is controlled by gas phase mass transfer. the mass transfer parameter for the specific experimental condition was determined and the calculated rate for gas phase mass transfer of C¹³O¹⁸ in CO controlling the rate is in agreement with the experimental results. At low temperatures (1523 K) and sulfur contents greater than 0.015 pct the rate may be controlled by mixed control, gas phase mass transfer, and chemical kinetics in series. The chemical rate constant is estimated to be 1.5×10⁻⁵ moles/cm² sec atm for these conditions. S. ANTOLIN, formerly Granduate Student at Carnegie Mellon University     

On the interfacial rate of reaction of CO2 with a calcium ferrite melt

Measurements of the rate of dissociation of CO₂ have been made by the¹⁴CO₂-CO isotope exchange technique on calcium ferrite melts with Ca/Fe = 0.30 at 1300 °C. Studies have also been made of the interfacial rates of oxidation of calcium ferrite melts with an average CaO content of 19.45 wt pct (CaJFe ≃0.33) in CO₂-CO atmospheres at 1362 °C. It is shown that the rates of oxidation are consistent with the rates of isotope exchange, indicating a common rate determining step. Measurements of the equilibrium Fe³⁺/Fe²⁺ ratio as a function of the CO₂/CO ratio for 19.3 wt pct CaO melts at 1360 °C and for 28.7 and 18.6 wt pct CaO melts at 1300 °C are found to be in close agreement with the deductions of Takeda, Nakazawa, and Yazawa. Combination of the equilibrium data with the results of the isotope exchange studies indicate that the apparent first order rate constant for the dissociation of CO₂ is inversely proportional to the square of the Fe³⁺JFe²⁺ ratio of the melt, as has been previously found for liquid iron oxide, lime-saturated calcium ferrites, silicasaturated iron silicates, and an equimolar “FeO”-CaO-SiO₂ melt.     

Kinetics of chromium oxide reduction from a basic steelmaking slag by silicon dissolved in liquid iron

The reduction of chromium oxide from a basic steelmaking slag (45 wt pct CaO, 35 wt pct SiO₂, 10 wt pct MgO, 10 wt pct A1₂O₃) by silicon dissolved in liquid iron at steelmaking temperatures was studied to determine the rate-limiting steps. The reduction is described by the reactions: (Cr₂O₃) + Si = (SiO₂) + (CrO) + Cr [1] and 2 (CrO) +Si = (SiO₂) + 2 Cr [2] The experiments were carried out under an argon atmosphere in a vertical resistance-heated tube furnace. The slag and metal phases were held in zirconia crucibles. The course of the reactions was followed by periodically sampling the slag phase and analyzing for total chromium, divalent chromium, and iron. Results obtained by varying stirring rate, temperature, and composition defined the rate-limiting mechanism for each reaction. The rate of reduction of trivalent chromium (reaction [1] above) increases with moderate increases in stirring of the slag, and increases markedly with increases in temperature. The effects of changes in composition identified the rate-limiting step for Cr⁺³ reduction as diffusion of Cr⁺³ from the bulk slag to the slag-metal interface. The rate of reduction of divalent chromium does not vary with changes in stirring of the slag, but increases in temperature markedly increase the reaction rate. Thus, this reaction is limited by the rate of an interfacial chemical reaction. The reduction of divalent chromium is linearly dependent on concentration of divalent chromium, but is independent of silicon content of the metal phase.     

Mass transfer between a horizontal,submerged gas jet and a liquid

The rate of reaction between a horizontal, submerged gas jet and a liquid has been measured in a model system under conditions where mass transfer in the gas phase is rate limiting. The gas was 1 pct SO₂ in air, and the liquid was a 0.3 pct solution of hydrogen peroxide in water. SO₂ absorption rates were measured as a function of jet Reynolds number (10,000 < N_Re < 40,000) and jet orifice diameter (0.238 < d₀ < 0.476 cm). The product of the gas phase mass transfer coefficient and the interfacial area per unit length of jet trajectory, k_SO2 α was found to increase linearly with increasing Reynolds number and to be a strong function of the orifice diameter. The ratio of k_so2 α to volumetric gas flow rate was shown to be independent of Reynolds number for a given orifice diameter. Extrapolated values of k_so2 α are lower than the coefficients measured for vertical CO jets blown upward through liquid copper. Extrapolation of the measured mass transfer data to the jet conditions in copper matte converting and in the gaseous deoxidation of copper has indicated that the gas utilization efficiencies in these processes should approach 100 pct if gas phase mass transport is rate controlling.     

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     

Mass Transfer in Slag Refining of Silicon with Mechanical Stirring: Transient Interfacial Phenomena

Experiments have been carried out to study the rates of mass transfer between liquid silicon and CaO-SiO₂ slag with impeller stirring at 1823 K (1550 °C). The occurrence of transient interfacial phenomena related to the mass transfer of calcium has been observed; the evidence suggests that the reduction of calcium oxide at the interface leads to a rapid, temporary drop in the apparent interfacial tension. At low apparent interfacial tension, mechanical agitation facilitates the dispersion of metal into the slag phase, which dramatically increases the interfacial area; here, it has been estimated to increase by at least one order of magnitude. As the reaction rate slows down, the apparent interfacial tension increases and the metal recoalesces. The incidental transfer of calcium very likely promotes the transfer of boron by increasing the interfacial area. Mechanical mixing appears to be an extremely effective means to increase the reaction rate of boron extraction and could feasibly be implemented in the industrial slag refining of silicon to improve reaction rates.     

Interfacial reaction between CO2-CO gas and molten iron oxide containing P2O5

The interfacial reaction between CO₂-CO gas and molten iron oxide containing P₂O₅ was investigated by the ¹³CO₂-CO isotope exchange technique at 1773 K with CO₂/CO=1.0. The apparent rate constant rapidly decreased with the addition of P₂O₅ up to 2.86 mol pct PO_2.5 and the Fe³⁺/Fe²⁺ ratio kept constant at approximately 0.2. By the classic site blockage model, in which the reaction only occurs on the vacant sites, and the modified site blockage model, in which the reaction occurs on the vacant sites and the sites occupied by phosphorus simultaneously, the effect of the addition of P₂O₅ was analyzed and the reaction mechanism of CO₂ dissociation was discussed. It may be concluded that the dissociation of the adsorbed CO₂ molecule is reasonable as a rate-determining step and that the effect of phosphorus on the interfacial reaction is caused by the decrease in the number of active sites with the increase of phosphorus content as a surface active element in molten iron oxide.     

Effects of Ti,Zr, V,and Cr on the rate of nitrogen dissolution into molten iron

The rate of nitrogen dissolution into molten iron-M (M: Ti, Zr, V, and Cr) alloys at temperatures from 1873 to 2023 K has been measured using an isotope exchange reaction. The rate of nitrogen dissolution into molten iron is shown to increase by adding an element with a stronger affinity for nitrogen than Fe, such as Ti, Zr, V, and Cr. Of these, Ti increased the reaction rate most and the rate constant for the Fe-0.08 mass pct Ti alloy was 1.5 times larger as that for pure iron at 1873 K. The correlation of rate constant with thermodynamic interaction parameter with nitrogen was observed. This effect is discussed in terms of the change in the activity of the vacant site on the surface of the molten alloy.     

Determination of the Rate of CO2–CO Reaction with Magnetite by Isotope Exchange Technique

To clarify the mechanism and get the accurate kinetic parameters of CO₂–CO reaction with magnetite, ¹³CO₂–CO isotope exchange technique was used to determine the rate constants of CO₂ dissociation on the surface of magnetite from 1073 to 1373 K. The real interfacial rate constant was estimated by considering the gas phase mass transfer along a plate. The relationship of the rate constant and CO₂/CO ratio was expressed by the formula of k_c = k₀ (CO₂/CO)^?1?m, m represents the effect of CO₂/CO ratio on the activity of oxygen in iron oxide. The apparent activation energy of the reaction between CO₂–CO gas and magnetite was calculated to 161, 175 ± 15, 198, and 197 ± 16 kJ mol^?1 for CO₂/CO ratios of 4.0, 5.0, 6.0, and 10.0, respectively. This dependence relationship may be caused by the decrease of free electron in magnetite phase with the increase of CO₂/CO ratio.     

Kinetics of CO-CO<Subscript>2</Subscript> reaction with CaO-SiO<Subscript>2</Subscript>-FeOx melts

Measurements of the rate of interfacial reaction between CO₂-CO mixtures and CaO-SiO₂-FeOx slags have been made using the ¹³CO₂-CO isotope exchange technique. Ranges of slag compositions from 0 to 100 wt pct ‘FeO’ and CaO/SiO₂ between 0.3 and 2.0 were examined in the experiments. For each slag, the dependence of the apparent rate constant on temperature and equilibrium oxygen potential was studied. The relationship between the rate constant and oxygen potential was found to be in the form k _a=k _a ^o (a_o)^-α. The parameter a, with values between 0.5 and 0.9, was dependent on the slag composition. The activation energy of the reaction was independent of iron oxide content and dependent on slag basicity.     

Kinetics of the Reaction between CO2—CO and Liquid Copper

Direct determination of the rate of the reaction between CO₂—CO and pure liquid copper in the interfacial reaction regime and an examination of the effect of oxygen has been made by using the ¹⁴C isotope exchange technique. The first order rate constant for the dissociation of CO₂ on clean, oxygen-‘free’ surfaces is found to be described by the equation:

log K⁰(mol/cm².s.atm) = ? 3510/T ? 2.36

between 1150 and 1400°C, to within an uncertainty of +60%. The first order rate constant for the dissociation (k₂) and that for the formation (k₁) of CO₂ on oxygen-doped surfaces are related through the equation:

k1 = k²a⁰

where a_o the oxygen activity in liquid copper with the standard state defined by pCO₂Sol;pCO = 1. Full consistency is found between experiments with ¹⁴C-labelled C0₂ and CO. The effect of a₀ on the rate of dissociation of CO₂ is in agreement with the site-blocking model and the rate constant (k₂) is found to be consistent with the equation:

with a₀ between 0.015 and 50 at 1200°C. In comparison, the magnitude of the reaction rate is significantly greater than those suggested by earlier studies. Possible causes for the discrepancy are discussed.     

Interaction of nitrides of group IV–V transition metals with chromium

The interaction of the nitrides TiN, ZrN, HfN, VN, and TaN with chromium is studied in the temperature range 1600–1900°C in pure nitrogen and argon by x-ray, metallographic, and microdurometric analyses. The temperature T of the liquid phase that forms at the nitride-chromium interface is found to be below the melting point of pure chromium coressponds to the melting point of the eutectic composition of the solid solution and intermetallic compounds of the nitride-forming metal with chromium (with the exception of vanadium). The wetting temperature and the angles of contact of nitrides of group V transition metals are less than those of nitrides of group IV metals; they increase with the number of the nitride-forming metal in the group. The growth of the lattice parameter of most of the nitrides obtained after interaction with chromium is attributable to Cr atoms occupying vacant sites of the metalloid sublattices of the nitride. Intermetallic compounds such as MeCr₂, the nitride Cr₂N, and solid solutions of chromium in nitrides as well as solid solutions of the nitride-forming metal in chromium are formed in the liquid-phase interaction. Titanium nitride is the most stable of the nitrides studied. Institute for Materials Science Problems. Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya. Nos. 3–4, pp. 90–96, March–April, 1997.     

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.