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 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 kinetics of the nitrogen reaction with liquid iron-chromium alloys

The isotope exchange technique was employed to study the interfacial reaction kinetics of nitrogen with liquid iron-chromium and iron-chromium-sulfur solutions. Chromium was found to increase the rate of the nitrogen exchange reaction. The increase in the rate occurs, at least in part, through promotion of N₂ dissociation on the chromium surface sites. Although mass transport effects in the liquid are eliminated through the use of the isotope exchange technique, it was found that a correction for gas phase mass transfer was required for the determination of the interfacial reaction rate constant due to the faster exchange rates encountered in liquid iron solutions containing chromium. Formerly with the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA     

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.     

Influence of chromium and nickel on the dissociation of CO2 on carbon-saturated liquid iron

At 1600 °C, under conditions where the rate was not significantly affected by liquid-phase or gasphase mass transfer, the rate of dissociation of CO₂ was determined from the rate of decarburization of iron-based carbon-saturated melts containing varying amounts of chromium and nickel. The rate was determined by monitoring the change in reacted gas composition with an in-line spectrometer. The results indicate that neither chromium nor nickel had a strong effect on the kinetics of dissociation of CO₂ on the surface of the melt. Sulfur was found to significantly decrease the rate, as is the case for alloys without chromium or nickel, and the rate constant is given by $$k = \frac{{k^0 }}{{1 + K_s a_s }} + k_r $$ where k ⁰ denotes the chemical rate on pure iron, K _s is the adsorption coefficient of sulfur, a _s is the activity of sulfur corrected for Cr, and k _r represents the residual rate at a high sulfur level. The rate constants and adsorption coefficient were determined to be: $$\begin{array}{*{20}c} {k^0 = 1.8 \times 10^{ - 3} mol/cm^2 s atm} \\ {k_r = 6.1 \times 10^{ - 5} mol/cm^2 s atm} \\ {K_s = 330 \pm 20} \\ \end{array} $$ Experiments run at lower carbon contents showed that only a very small quantity of chromium was oxidized, immediately forming a protective layer. However, this oxidation occurred at a higher carbon content (2 pct) than what was expected from the thermodynamics.     

Kinetics of the reaction of CH4 gas with liquid iron

In iron bath smelting and other processes that use coal, the effective use of volatile matter can improve the energy efficiency of the process. The reaction of simulated volatile (CH₄) with iron was studied. The rate of carburization of liquid iron by CH₄ gas was measured between 1400 °C and 1700 °C under conditions for which the effect of mass transfer can be corrected with reasonable accuracy. The rate was measured for partial pressures of CH₄ in Ar in the range of 0.02 to 0.06 atm and sulfur contents in the metal from 0.0006 to 0.5 mass pct. The results indicate that the rate of carburization may be controlled by the dissociation of CH₄ on the surface. Sulfur was found to decrease the rate, and the residual rate phenomenon was observed for high sulfur contents. The rate constant may be represented by the following equation: $$ k_C = \frac{{k^\circ }}{{1 + K_S a_S }} + \frac{{K_S a_S k_r }}{{1 + K_S a_S }}$$ wherek ^o ,k _r,K _s, anda _s are the bare surface rate constant, residual rate constant, adsorption coefficient for sulfur, and activity of sulfur in the metal, respectively. The second term in the rate equation represents the rate of dissociation on the adsorbed sulfur. The rate constants and adsorption coefficient were determined as functions of temperature to be $$\begin{gathered} log k^\circ = \frac{{ - 12,000}}{T} + 2.95 (mole/cm^2 s atm) \hfill \\ log k_r = \frac{{ - 14,000}}{T} + 3.45 (mole/cm^2 s atm) \hfill \\ log K_S = \frac{{ - 1800}}{T} + 1.04 \hfill \\ \end{gathered} $$      

The nitrogen reaction between carbon saturated Iron and Na2O-SiO2 slag: Part II. Kinetics

The kinetics of the nitrogen reaction between carbon saturated iron and Na₂O-SiO₂ slags and between Na₂O-SiO₂ slags and an inert gas phase were investigated at 1200 °C. For the nitrogen transfer from the iron alloy to slag, the overall mass transfer coefficient of nitrogen was calculated to be 2.2 × 10⁻⁴ cm/sec. For nitrogen transfer from Na₂O-SiO₂ slag to argon gas, it is shown that the rate controlling process is mass transfer in the slag phase, and the mass transfer coefficient of nitrogen is 9 × 10⁻⁴ cm/sec. Experiments were also conducted to demonstrate nitrogen removal from hot metal by Na₂CO₃ treatment at 1200 °C. In these experiments, 325 grams of Na₂CO₃ was added to the 6.5 kg of Fe-C-N(-Si) alloy. When the metal contained silicon, nitrogen was transferred from the iron alloy to slag after the silicon was oxidized. When the iron alloy contains no silicon, nitrogen removal was faster. In both cases the nitrogen reversion occurred because of the decrease of slag volume and slag basicity. Furthermore, the presence of silicon in the metal retarded nitrogen reversion. is on leave of absence from the Department of Metallurgical Engineering and Materials Science, Faculty of Engineering, The University of Tokyo Formerly with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University     

Studies of the interfacial kinetics of the reaction of nitrogen with liquid iron by the15N-14N isotope exchange reaction

The rate of dissociation of N₂ on high purity liquid iron and iron-sulfur alloys between 1550 and 1650 °C has been studied by means of the¹⁵N-¹⁴N exchange reaction. It is shown that the rate constants at given sulfur concentrations are consistent with those for the absorption of nitrogen into iron-sulfur alloys, indicating a common rate determining step. The rate constant for high purity liquid iron, in units of mol cm^?2 s^?1 aim^?1, is given by: logk _f = ?340/T ? 1.38. The rate constant is found to be independent of carbon concentration up to about 4.3 wt pct and to be closely consistent with ideal chemisorption kinetics. The results are combined with those of previously published studies to give rational equations for the apparent rate constants for Fe-S and Fe-O alloys. Consistent values for the adsorption coefficients at 1600 °C for sulfur and oxygen are deduced to be about 130 and 220, respectively, for a standard state of the 1 wt pct ideal solution.     

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.     

Solubility of nitrogen in liquid Fe-Cr-Ni alloys containing manganese and molybdenum

The nitrogen solubility in liquid Fe-Cr-Ni alloys containing Mo or Mn was determined by the Sieverts’ method. The first and second order mutual interactions among nitrogen, chromium, nickel, molybdenum, and manganese in iron were determined as a function of temperature. The heat and entropy of solution in these alloys were correlated as functions of the logarithm of the activity coefficient of nitrogen at {dy1873} K independent of the composition of the alloys. An equation was derived to predict the nitrogen solubility in liquid multicomponent iron alloys for the range from logf_N, 1873K = 0 to -1.4 as, log (wt pct N)_T = (-247/T - 1.22) - (4780JT - 1.51) (logf_n, 1873K)-(1760/T -0.91) (logf_N,{dy1873}K )².     

Prediction of the effects of surface-active elements on gas-liquid metal kinetics

Equations have been developed for calculating the fraction of free surface, 1-θ_T, when two or more surface-active elements (S, O,Se,etc.) are present in liquid iron or its alloys. It is shown that equations of the form k =A Vl-θ_T -B well describe the essentially linear plots which represent the variation of rate constant k with concentration of surface-active elements in more than 30 researches on absorption and desorption of nitrogen in stirred liquid iron and its alloys. To normalize the effect of variation in metallodynamic properties from researcher to researcher, a second relation has been developedwhich describes a dimensionless rate constant for nitrogen k _N ^F (with range0 to 1) given by k _N ^F = CVl-θ_T-D, where C and D are small constants. The relation k _N ^F = 1.19 Vl-θ_T-0.19 is a fair representation of the absorption and desorption behaviors in the 1550 ° to 1600 ° range for all the many iron and iron alloy cases examined. Although these two relations are largely empirical they and the simple linear graphs involved provide potentially valuable new methods of rate prediction. There is some evidence that the behavior of hydrogen with liquid Fe, Cu, and Ni is analogous.     

Surface tension of liquid Fe-Cr-O alloys at 1823 K

The surface tension of liquid Fe-Cr-O alloys has been determined by using the sessile drop method at 1823 K. It was found that the surface tension of liquid Fe-Cr-O alloy markedly decreases with oxygen content at constant chromium content, and the surface tension at a given oxygen content remains almost constant, regardless of the chromium content. When the surface tension of liquid Fe-Cr-O alloys is plotted as a function of oxygen activity, with an increase in the chromium content, the surface tension shows a much steeper decrease with respect to oxygen activity. The surface tension of liquid Fe-Cr-O alloys at 1823 K is given as follows: σ=1842-279 ln (1+K _O a _O). Here, assuming a Langmuir-type adsorption isotherm, the adsorption coefficient of oxygen, K _O(Fe-Cr), as a function of chromium content, was shown to be K _O=140+4.2 × [wt pct Cr]+1.14 × [wt pct Cr]².     

Solubility of nitrogen in liquid Fe-Cr-Ni alloys containing manganese and molybdenum

The nitrogen solubility in liquid Fe-Cr-Ni alloys containing Mo or Mn was determined by the Sieverts’ method. The first and second order mutual interactions among nitrogen, chromium, nickel, molybdenum, and manganese in iron were determined as a function of temperature. The heat and entropy of solution in these alloys were correlated as functions of the logarithm of the activity coefficient of nitrogen at 1873 K independent of the composition of the alloys. An equation was derived to predict the nitrogen solubility in liquid multicomponent iron alloys for the range from logJn, 1873K = 0 to −1.4 as, log(wt pct N)_T = (-247/T-1.22)-(4780/T-1.51) (logf N, 1873K)- (1760/T-0.91) (logfN,1873K)².     

Thermodynamics and kinetics analyses of ZrB2 formation in molten aluminium alloys

Smelter grade aluminium can be used as a source for electrical conductor grade aluminium after the transition metal impurities such as zirconium (Zr), vanadium (V), titanium (Ti) and chromium (Cr) have been removed. Zirconium (Zr), in particular, has a significant effect on the electrical conductivity of aluminium. In practice, the transition metal impurities are removed by adding boron-containing substances into the melt in the casthouse. This step is called boron treatment. The work presented in this paper, which focuses on the thermodynamics and kinetics of Zr removal from molten Al–1?wt-%Zr–0.23?wt-%B alloy, is part of a broader systematic study on the removal of V, Ti, Cr and Zr from Al melt through boron treatment carried out by the authors. The thermodynamic analyses of Zr removal through the formation of ZrB₂ were carried out in the temperature range of 675–900°C using the thermochemical package FactSage. It was predicted that ZrB₂ is stable compared to Al–borides (AlB₁₂, AlB₂) hence would form during boron treatment of molten Al–Zr–B alloys. Al–Zr–B alloys were reacted at 750?±?10°C for 60 minutes, and the change in the chemistry and microstructure were tracked and analysed at particular reaction times. The results showed that the reaction between Zr and AlB₁₂/B was fast as revealed by the formation of boride ring at the early minutes of reaction. The presence of black phase (AlB₁₂), i.e. the original source of B, after holding the melt for 60 minutes advocated that the reaction between Zr and AlB₁₂/B was incomplete, hence still not reached the equilibrium state. The kinetics data suggested a higher reaction rate at the early minutes (2 minutes) of reaction compared to at a later stage (2–60 minutes). Nevertheless, a simple single-stage liquid mass transfer controlled kinetic model can be used to describe the overall process kinetic. The analysis of integrated rate law versus reaction time revealed that the mass transfer coefficient (k_m) of Zr in molten alloy is 9.5?×?10^?4?m?s^?1, which is within a typical range (10^?3 to 10^?4?m?s^?1) observed in other metallurgical solid–liquid reactions. This study suggests that the overall kinetics of reaction was predominantly controlled by the mass transfer of Zr through the liquid aluminium phase.     

The Rate of the Phosphorous Reaction Between Liquid Iron and Slag

In the current study, the rates of dephosphorization and rephosphorization of liquid iron with simulated steelmaking slags were investigated at 1873 K (1600° C). The experiments were conducted in an induction furnace with supplemental heating to maintain a consistent temperature within both the metal and slag phases. An integrated form of the rate equation was used to evaluate the results, assuming mass transfer in both the slag and metal was rate controlling. The results of the current and previous studies indicate that the mass transfer parameter, the slag-metal surface area, and the overall mass transfer coefficient (A*k ₀), decreased as the reaction proceeded. It is proposed that initially when the rate and oxygen flux are high, the interfacial energy decreases, and the interfacial fluid velocity increases causing disruption of the slag metal interface. The consequent increases in interfacial area and interfacial fluid flow cause A*k ₀ to be high initially and then decrease as the oxygen flux decreases.     

Effects of O,Se, and Te on the rate of nitrogen dissolution in molten iron

The effects of oxygen, selenium, and tellurium on the rate of nitrogen dissolution into molten iron have been investigated at 1973 K using an isotope-exchange reaction and the results are summarized as follows. The rate constant of nitrogen dissolution measured at lower oxygen concentration ([mass pct O] < 0.015) is larger than previously reported ones under an atmospheric pressure and agrees well with the value from desorption rate under reduced pressures. Selenium and tellurium retard the nitrogen dissolution into liquid iron more significantly than oxygen, and the degree of the retarding effect is in the order of tellurium, selenium, and oxygen. The adsorption coefficients are calculated to be K_O = 144, K_Se = 1120, and K_Te = 1640 with respect to [1 mass pct solute] from present results. A model that surface active elements and nitrogen are adsorbed on the same site at the interface and the dissociation reaction of nitrogen molecule on the site represented by the equation is the rate-determining step reasonably expresses the retarding effect of the surface active elements on the reaction rate on the assumption that all sites at the metal surface have a uniform adsorption energy for each solute.     

Rate of reduction of Cr2O3 by carbon and carbon dissolved in liquid iron alloys

The rate of reaction of Cr₂O₃ with carbon or carbon dissolved in liquid iron alloys, and the decarburization of Fe-Cr-C alloys in Ar-O₂ gas mixtures has been investigated. The rate of reduction of dense Cr₂O₃ on the surface of Fe-Cr-C alloys was controlled by the diffusion of carbon to the surface of the melt. The chemical diffusion coefficient derived from the results (8.5 × 10^-5 cm²/s) is in agreement with previous work. The decarburization of Fe-Cr-C alloys in Ar-O₂ gas mixtures was apparently carried out by the Cr₂O₃ which formed on the surface of the melt by the reaction of the dissolved Cr with the oxygen gas and the rate of decarburization was controlled by the diffusion of carbon to the surface. The rate of reduction of Cr₂O₃ by various types of carbon was also investigated at temperatures from 1300 to 1600°C; the initial rate appears to be controlled by gas phase mass transfer of the CO away from the surface of the reactants.     

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.     

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.