Measurement of aluminum or silicon activity in Fe-Ni (< 30 %) alloys using mullite and ZrO2-based solid electrolyte galvanic cell

Using mullite (3Al₂O₃·2SiO₂)-tube type and ZrO₂ (MgO mole fractions of 9 %)-plug type solid electrolyte, the activities of Al in the Fe-15, 29% Ni-0.002 to 0.84% Al (mass contents in%) alloys were measured at 1873 K in an Al₂O₃ crucible, and those of Si in the Fe-13, 28% Ni-0.13 to 2.3% Si alloys were measured at 1823 K in a mullite crucible with an excess of SiO₂. The interaction coefficients of and were determined as ?0.0173 ± 0.0052 and 0.00021 ± 0.00007, respectively, and those of and were determined as 0.0094 ± 0.0049 and ?0.00008 ± 0.00004, respectively. The oxygen activities obtained by the EMF measurements were compared with those obtained from the contents of analyzed oxygen. The oxygen activities in the Fe-Ni-Al alloys were found to be supersaturated. The interaction coefficients of and were determined as 0.0027 ± 0.0012 and 0.00012 ± 0.00004, respectively, from the contents of analyzed oxygen in Fe-Ni-Si alloys.     

Nitride capacities in CaO–SiO2 and CaO–SiO2–Al2O3 melts

Nitride capacities, , defined by (mass-% N) · in the CaO–SiO₂ and CaO–SiO₂–AI₂O₃ melts were measured in the temperature range of 1 723 to 1 923 K by a gas-slag equilibration technique using CaO, Al₂O₃, and Mo crucibles. Nitrogen content in slag, (mass-% N), was proportional to oxygen partial pressure, , to the power of ?3/4 at constant nitrogen partial pressure, . The values for increased linearly with increasing temperature and increased with the content of nitride formers, SiO₂ and Al₂O₃, but the effect of SiO₂ on value was found to be greater than that of Al₂O₃. The activity coefficients of Si_3/4N in the CaO–SiO₂ melts tended to increase with increasing the content of SiO₂.     

Activities of FexO in CaO + CaF2 + SiO2 + FexO quaternary slags by disposable electrochemical oxygen probes

The activities of Fe_xO in CaO + CaF₂ + SiO₂ + Fe_xO quaternary slags were measured by means of solid-oxide galvanic cell. The Fe_xO activities in the slags are influenced by CaF₂ as well as SiO₂. At constant Fe_xO mole fractions, e.g., , at low SiO₂ mole fractions i.e., , the substitution of CaF₂ for CaO has an effect of raising the Fe_xO activity. At higher SiO₂ mole fractions, e.g., , however, such an effect becomes insignificant.     

Thermodynamics of iron oxide in FexO‐dilute CaO + Al2O3 + SiO2 + FexO slags at 1873 K

The ferrous and ferric ion capacities, and , for the slag of CaO + Al₂O₃ + SiO₂ are determined at 1873 K by means of the distribution equilibrium of iron between Fe_xO dilute slags of the system and Pt + Fe alloys under a controlled atmosphere. Compared with the values of for CaO + Al₂O₃ binary slag at a constant ratio of Al₂O₃ to decreases with an increase in SiO₂ content. Nearly linear relationships are observed between the logarithm of the ferrous and ferric ion capacities and the optical basicity. Applying these capacities, the relationship between (%Fe^T) and PO₂ under iron saturation, as well as the iron redox equilibrium in slag in relation to the optical basicity were discussed.     

Conventional chemical potentials of elements in dilute ferrous solution

Conventional thermochemical standard data are defined for dissolved elements i in iron. The standard partial enthalpy and entropy and in ideal dilute solution are described as simple temperature functions a_i and b_i are constants and denotes the partial molar heat capacity, which may be considered constant within a certain temperature range. From and further functions are derived, e.g. the conventional standard chemical potential and the modified Planck function β_i. The constants a_i, b_i, are evaluated and compiled for α-, γ, δ-iron as well as for liquid iron, as far as experimental data are available. In addition to the standard data the first order interaction coefficients for elements dissolved in liquid iron at 1873 K have been revised and tabulated.     

The oxygen pressure of dilute FexO‐CaO‐Al2O3 slag in equilibrium with liquid iron

The relationship between slag composition and oxygen partial pressure of dilute Fe_xO‐CaO‐Al₂O₃ slag in equilibrium with iron the activity of which is below 1 was extrapolated to give the oxygen partial pressure of a slag equilibrated with iron of the activity 1. In the process of extrapolation, the oxygen partial pressure and the activity of iron were unified to give a single thermodynamic quantity which is defined as: , where K(x) is the equilibrium constant for the formation reaction of Fe_xO and is considered to be a function of x. The dependence of on the slag composition was obtained experimentally. On the other hand, it was found that the plot of against log a_Fe has the slope ?1 as expected from the theoretical consideration. The relation has been employed to estimate the composition of slag in equilibrium with liquid iron. Thus, the obtained composition is used to calculate the values of and of the slag in equilibrium with liquid iron.     

Thermodynamics of nitrogen in CaO-TiO2-TiO1.5 slags

The nitride capacities in CaO-TiO₂-TiO_1.5 slags, and nitrogen distribution ratio between these slags and liquid Cu (L_N = (mass-% N)/[mass-% N]) were measured by a gas-slag-metal equilibration technique, using a Mo crucible in the temperature range of 1823 to 1923 K under controlled partial pressures of oxygen and nitrogen . The values for , (mass-% Ti³⁺)/(mass-% Ti⁴⁺) ratio, and solubility of TiN in CaO-TiO₂-TiO_1.5 slags were obtained as a function of slag composition (X_CaO = 0.24 ～ 0.39) and temperature. Activity coefficients of TiN were evaluated from the values for activity coefficients of Ti in liquid Cu which were calculated from the results of TiN saturation experiments.     

Thermodynamic behaviour of Cu in molten Fe‐C‐S alloys at high temperatures

The activity of Cu in molten Fe‐C‐S alloys at high temperatures was measured by a thermochemical equilibration technique. The effects of the contents of Cu, C, and S in the molten alloy as well as temperature on the activity coefficient of Cu in pure and carbon‐saturated iron were determined by measuring the distribution ratio of Cu between molten silver and Fe‐C‐S alloy melts. In molten pure iron and carbon‐saturated iron, the activity coefficient of Cu under infinite dilution, was measured as 11.0 and 28.5, respectively, at 1823K. The interaction parameters between Cu and Cu, S, C in the molten iron at 1823K were estimated: . The dependence of in molten Fe‐C alloy on temperature was determined as follows: pure iron: , carbon‐saturated iron: . Combining the effects of temperature and those of alloying components, the activity coefficient of Cu in the molten Fe‐C‐S alloy could be estimated as follows: .     

Structural statements,coordination numbers and water vapour solubility in aluminate slags

The IR emission of liquid CaO-Al₂O₃ slags is caused by IR active molecule vibrations of and -complexes. In the liquid CaO-Al₂O₃ slags the equilibrium dominates. At lower temperatures (1000°C) the formation of condensed -complexes was observed (glassy state). By the addition of B₂O₃ to CaO-Al₂O₃ slags B-O-Al associations were formed. Since the emission bands of the aluminate complexes overlap, a curve fitting with Gaussian functions was carried out to separate the bands. Furthermore, this quantitative evaluation leads to statements concerning the ratios between the different molecules. With the increase in temperature these associations are dissociated. In CaF₂-CaO-Al₂O₃ slags with higher contents of CaF₂, Al-O-F complexes with the coordination number 4 are present. Water vapour is dissoluted in liquid CaO-Al₂O₃ as hydroxide-ions (diffuse IR-emission-spectrum of ^?O-H^?…O^2?-molecule units).     

CaO-activities and structure of CaO-Al2O3-melts

By using structure chemical results taken from IR-spectroscopic analyses, the CaO-activity was calculated. In liquid state the aluminates dissociate into Ca²⁺ and into the complexes and . By application of the Temkin-theory the CaO-acitivity can be calculated and compared with measured values.     

Equilibrium between ammonia-hydrogen mixtures and the ε phase of iron

Foils made from pure iron with a porous structure were nitrided in mixtures of pure and entirely decomposed ammonia until constant weight was reached. The relationship between nitriding potential, temperature and concentration was measured in the ε region between 350 and 725°C and at concentrations between and . This relationship can be mathematically represented, if the ε phase is considered to be a subregular solution of nitrogen in hexagonal iron. From the Gibbs free energy of the nitrogen in the gas phase, the numerical relation and the parameters L′ = ? 600 000 + 775.6 T J/mole L″ = 1270000 ? 1423 T J/mole are deduced. The results of a calculation of the (γ + ε)/ε phase boundary using this equations are in accordance with an experimental measurement.     

Control of the aluminium-oxygen reaction in pure iron melts

Modified electrochemical oxygen sensors based on CaO-doped ZrO₂ and Y₂O₃-doped ThO₂ were developed and applied to study the aluminium-oxygen reaction in pure liquid iron with special regard to the provision of reliable experimental conditions and the attainment of true chemical equilibrium. The sensors represent valuable tools particularly for EMF long-term measurements over several hours. At 1600 °C, values of the constant and of the interaction parameters and were determined. When Fe-O-Al melts are covered with a slag phase at an activity a(Al₂O₃) < 1, e.g., CaO-saturated CaO-Al₂O₃ slag, the Al-O relationship in the iron may nevertheless be controlled by an equilibrium oxide phase at an activity a(Al₂O₃) = 1, presumably represented by inclusion particles consisting of pure Al₂O₃.     

Diffusion of the hydroxide-ion in liquid CaO-Al2O3-slags by application of Fourier-Transform-lnfrared-Spectroscopy

Water vapour is soluted in liquid CaO-Al₂O₃-slags as OH^?-ion. To determine the diffusion coefficient a capillary technique is used. Roughly 5 cm long platinum capillaries filled with liquid slag are equilibrated at 1500 and 1600 °C and bar different times and then are quenched in liquid D₂O. By dividing these capillaries into roughly 3 mm long segments enough sample substance is won to obtain FT-IR spectra of the OH-band. From measurement of the integral extinction of the OH-band and OH^?-concentration profile through the capillary is obtained. FT-IR-spectroscopy is very suitable for this capillary technique because it needs only very little sample substance.     

Reduction of Fe1-xO in liquid state by H2+Ar mixtures

The investigations of reducing Fe_1-xO in liquid state with H₂+Ar mixtures, in top blowing stream of gas reactor, by using thermobalance method have been carried out. To make the system more homogeneous pure iron crucibles were used in the experiment. The experiments were carried out in the range of gas flow rate V = (5+80) · 10^?6 m³/s and in 1723 K. Reduction rate depends on hydrogen partial pressure and flow rate of gas mixture. The logarithm of apparent reduction rate constant is a linear function of logarithm of gas flow rate. In experimental conditions of this work, the initial rate of reduction satisfies the relation . Thus it has been shown that the reduction process is controlled by the mass transfer in the gaseous phase. Sherwood numbers obtained from observed values of reduction rates and empirical equation of the gas-phase mass transfer rate are correlated by equation . The values of mass transfer coefficient in gas phase calculated from Sherwood numbers are consistent with the values of apparent reduction rate constant and with estimated rate constant of chemical reaction of liquid F_e1-xO reduction.     

High nitrogen steel powder for near net shape products

This study has been performed to relate the powder properties to the process parameters of the plasma-rotating-electrode-process (Prep). A regression equation for average powder diameter was acquired from the experimental data as . A model is developed to describe the nitrogen pick-up by the Prep. On the basis of this model an expression for nitrogen content of the powder produced is deduced as: The powder has a solidification structure of austenitic dendrites. The CrN precipitates, the amount of which is dependent upon the nitrogen content, can be found in the interdendritic areas. During the solution annealing the Cr₂N precipitation reaction occurs in both of the inter- and intradendrite.     

On the reaction mechanism of the carburization and decarburization of α-iron in CH4/H2-mixtures

The surface reaction of the carburization and decarburization of α-iron in CH₄/H₂-mixtures has been investigated by a method using the radioactive isotope ¹⁴C. Experimentally two different kinetic equations have been confirmed and discussed in detail: In accordance to ¹) the kinetic equation (1) is explained by a slow reaction: . For the kinetic equation (2) the adsorption, respectively the desorption of CH₄, is considered to be the most slow partial step. The correlation of the carburization with the adsorption is supported by the relations: Thus can be regarded as E_a, the activation energy of the adsorption. Its value was determined at 109 kJ/mol being equivalent to 1.1 eV, which is characteristic for a chemisorption. Further it was established, that the rate of the decarburization according to equation (2) increases with decreasing temperature, i.e. the (apparent) activation energy proved to be negative. This is explained by the fact that before its rate-controlling desorption the CH₄-molecule first of all must be formed step-by-step and by the assumption that for thermodynamical reasons the surface concentration of CH₄ increases very much with decreasing temperature. The range of validity of the kinetic equations (1) and (2) can be limited to each other as follows:

• – The kinetic law (1) is established especially at higher H₂-pressures and at surfaces with low, but effective impurities. The impurities simultaneously result in a high decline of the reaction rate.
• – The kinetic law (2) is valid especially at lower H₂-pressures.

     

Solubility of calcium in Fe-Ca-Xi melts

The equilibrium experiments on liquid Fe-Ca-X_i were carried out in a high pressure induction furnace in dense CaO crucibles. The solubility of calcium, saturated with CaO, in iron was determined as a function of temperature under 10 bar argon pressure. In pure iron the maximum solubility of calcium at 1 600°C is 0.03 wt.%. The alloying elements silicon, aluminium and nickel remarkably increase the calcium solubility in iron, whereas manganese causes an almost negligible increase. Chromium significantly reduces calcium solubility. The opposite influence of chromium and nickel on calcium solubility is balanced in an 18/8 steel. The interaction parameters of the alloying elements in the solvent iron with respect to Raoultian standard state are as follows:      

Control of the chromium-oxygen reaction in pure iron melts

Oxygen activities in Fe-O-Cr melts, equilibrated with chromium oxide, were determined as a function of chromium content by EMF measurements using ZrO₂(CaO) plug-type sensors with a Cr-Cr₂O₃ reference. Plug-type sensors with or without Cr₂O₃ saturator were used in the experiments. From the experimental results values for the activity coefficient f_O and the interaction parameters and were evaluated. Raoultian chromium activity features a slight positive deviation from ideal solution behaviour in the temperature range from 1550 to 1650°C. Taking into account the calculated interaction parameters, the equilibrium constant K_Cr-O was redetermined at 1550 to 1650°C. In comparison with previous studies a better agreement was obtained with existing thermochemical data.     

Thermodynamics of oxygen,nitrogen and sulfur in liquid iron equilibrated with CaO-TiOx and CaO-Al2O3-TiOx melts

Equilibria between CaO-TiO_x or CaO-Al₂O₃-TiO_x melts and liquid iron with respect to O, N and S were studied at 1873 K as a function of Ti (or Al) content in metal, using a CaO or an Al₂O₃ crucible. The Al-O and Ti-O relations in liquid iron were studied, and nitride and sulfide capacities defined by and , respectively, were obtained from nitrogen and sulfur distribution ratios, coupled with the analyzed oxygen content in liquid iron or the P_O2 determined by Ti/TiO₂ equilibrium. Based on these results, activities of Al₂O₃ and activity coefficients of TiO₂ and TiO_1.5 were evaluated.     

Equilibria of Ce-Al-O and Nd-Al-O in molten iron

Equilibrium of Ce-Al-O or Nd-Al-O system in molten iron was studied by separately melting radioactive isotope ¹⁴¹Ce or ¹⁴⁷Nd in an Al₂O₃ crucible filled with pure iron to form ¹⁴¹CeAlO₃ or ¹⁴⁷NdAlO₃ around its inner wall. The content of dissolved Ce or Nd in iron was determined by means of electrolysis in anhydrous electrolyte and radioassay. The concentration of dissolved Al in iron was obtained by photometry and followed by calculation. Activity of oxygen in liquid iron was directly measured by solid electrolyte sensors made of ZrO₂(MgO) tube. By extrapolation of data obtained, the temperature dependence of the equilibrium constant of RE-Al-O in molten iron, , or of interaction coefficient, may be described as: