Surface effects and flow conditions in small volume melts with varying sulphur content

The surface effects and flow behaviour in melts with a volume of a few cubic millimetres were investigated using 1 mm thick X5CrNi18‐9 (AISI 304) stainless steel strips after partial remelting by a TIG (tungsten inert gas) burner. In the industrially produced material the S mass content was varied between 20 and 100 ppm. The surface excess of sulphur indicates that the surface saturation was not achieved compared to the values in the Fe‐S system. The thickness of the layer with high sulphur enrichment depends on the S activity and varied between approximately 260 and 460 nm and achieves the bulk S content at a distance of approximately 1.2 μm from the surface. This layer is noticeably thinner than the diffusion boundary layer and considerably thinner than the flow boundary layer. The topography of the rapidly solidified liquid pool makes it possible, taking previously estimated temperature coefficients of surface tension into account, to explain the flow behaviour in the melt with regard to the S content and to the temperature gradients. The results obtained indicate that the shearing force caused by surface tension gradients (Marangoni effect) controls the flow conditions in small volume melts.     

Interfacial Tension between CaO‐Al2O3 Slag and Iron Containing Nonmetal or Tramp Elements

The interfacial tension between CaO‐Al₂O₃ slag and iron was studied for the iron containing nonmetal or tramp elements. The sessile drop technique was used to measure interfacial tensions under argon atmosphere at 1853K. Oxygen in liquid iron markedly reduced the interfacial tension. This effect was taken into consideration when analysing the interfacial tension between the slag and Fe‐S, Fe‐C, Fe‐Sn, Fe‐Sb and Fe‐Cu alloys. When adding S, Sn, Sb, or Cu to the iron, the interfacial tension was observed to decrease while carbon had no significant effect.     

Time Dependent Effects in Surface Tension Measurements of an Industrial Fe‐alloy

The surface tension of an industrial FeC‐alloy containing Si, Mn and S in the few atomic per cent range was measured by the oscillating drop method in ground based electromagnetic levitation and under reduced gravity conditions on board a parabolic flight. The results from the parabolic flight exhibited a large variation between different heating cycles and a discrepancy with the ground based experiments with regard to the value of the surface tension at the liquidus temperature and the temperature coefficient. The variation of the surface tension temperature coefficient is interpreted as resulting from the loss of volatile surface active components such as S and Si in subsequent heating cycles. The results demonstrate the importance of the time scale of the measurement and ambient atmosphere for surface tension measurements of industrial alloys containing volatile surface active components. For comparison with the experimental values the surface tension and segregation were modelled using different models. As a result of the analysis, the surface tension as a function of temperature best representing the starting industrial alloy is obtained as σ(T) = 1.52–1.53 10^‐4 (T–1752K) Nm^‐1.     

碱性环境中黄铁矿表面反应机理研究

采用SEM、XPS检测手段和前线轨道理论计算,对碱性环境下的黄铁矿表面反应机理进行研究。结果表明,黄铁矿中的S、Fe、Cu原子由晶格跃迁到表面,并氧化形成硫酸盐、氢氧化物、单质S或S8。随着处理时间的增加,黄铁矿表面氧化产物明显增多,在处理4 h后,黄铁矿表面已经完全被氧化。黄铁矿中Cu原子进入晶格后,改变了HOMO轨道中Fe、S原子电子得失能力（反应活性）,但对LUMO轨道的原子性质变化影响不大。在氧化过程中,Fe、S原子更易与氧作用,其次为Cu原子。这也可以解释黄铁矿表面先出现铁、硫化合物,之后才发现有铜的产物的现象。     

Effect of chromium–sulphur interactions on decarburisation of levitated Fe–C alloy droplets by CO2–Ar gas mixtures

This study provides an evaluation of the influence of chromium and sulphur on the decarburisation kinetics of Fe–Cr–C alloy droplets when exposed to argon gas mixtures containing carbon dioxide as the oxidant. For droplets containing the same amount of surface active sulphur, it was found that the decarburisation rate of iron–chromium droplets was substantially faster than that of chromium free alloys. This enhanced rate can be explained in terms of the attractive interaction between chromium and sulphur, which results in a decrease in the number of surface sites occupied by sulphur. An appropriate rate equation has been developed to characterise this behaviour. The findings from this work have beneficial implications with respect to the preferential oxidation of carbon rather than chromium when stainless steel is refined with argon–carbon dioxide gas mixtures.     

Effects of Interaction Between Surface Active Elements on the Surface Tension of Iron

Abstract

Surface tension data at 1650°C have been measured for the Fe + S + O ternary system and the two binary systems, Fe + O and Fe + S using the levitated drop technique. The results indicate that the reductions in surface tension due to oxygen and to sulphur are essentially additive in the ternary system. Excess concentrations (Γ) for both oxygen and sulphur have been calculated for both binary and ternary systems; an empirical rule for calculating Γ values for the ternary system from Γ values for the binaries has been proposed. The temperature dependencies (dγ/dT) for both ternary alloys and for a stainless steel were found to be positive. Surface analysis of the samples has been determined using electron spectroscopy (ESCA); the results indicated a considerable enhancement in the concentration of the surface active elements, O and S in the surface layers. The effects of additions of deoxidizers and desulphurizers like Al and Ce upon the surface tension have been investigated. The significance of the surface tension upon the welding characteristics of steels is discussed and the models for their interdependency evaluated in the light of the data obtained in this investigation.

Résumé

Les mesures de tensions superficielles ont été effectuées à 1650°C pour le système ternaire Fe-S-O et les systemes bin aires Fe-O et Fe-S au moyen de la technique de lévitation. Les résultats indiquent que les réductions de la tension superficielle sont essentiellement additifs dans le système ternaire. La concentration en excès (Γ) en oxygène et en sulfure a été calculée pour les systèmes bin aires et ternaires. Une règle empirique du calcul des valeurs de Γ pour le système ternaire a été établie à partir des valeurs des systèmes binaires. Les dépendances avec la température (dγ/dT) des alliages ternaires et de l'acier inox se sont révélées positives. L'analyse superficielle des échantillons a été éffectuée par spectroscopie électronique (ESCA). Les reésultats ont montré un enrichissement considérable dans la concentration des éléments actifs, O et S dans les couches superficielles. L'effet de désoxydation et désulfuration (Al et Ce) sur la tension superficielle a été étudié. L'effet de la tension superficielle des aciers sur la soudabilité est discuté et le modèle de leur interdépendance est évalué à la lumière des résultats de cette recherche.     

Density and Surface Tension of Liquid Fe-Mn Alloys

The density and surface tension of liquid Fe-Mn alloys were determined by using the sessile drop method at 1823  K (1550 °C). The density of liquid Fe-Mn alloys decreased with increasing Mn content. When the molar volume was plotted with respect to the mole fraction of Mn, a linear relationship was obtained. Consequently, it was found that there is no excess volume in liquid Fe-Mn alloy. The surface tension of liquid Fe-Mn alloys was found to decrease with increasing Mn content. The current experimental data were higher than the reported results but close to the calculated results using Butler’s equation.     

Effect of Surface Tension on Gas Atomization of a CrMnNi Steel Alloy

The aim of the current research is the experimental investigation of the mass median particle size d₅₀ as a function of surface tension for liquid Cr–Mn–Ni steel alloy with 16% Cr, 7% Mn, and 9% Ni. To modify the liquid steel design sulfur was add to the Cr–Mn–Ni steel in five steps up to a 1000 mass ppm. The surface tension of the liquid steel alloy was measured using maximum bubble pressure method and yttria stabilized capillary in a temperature range from 1701 to 1881 K. In addition, the same steel charges were sprayed to steel powder using a vacuum inert gas atomization using pure argon gas. The increase of sulfur in Cr–Mn–Ni steel will decrease the surface tension to 0.91 N m^?1. The temperature coefficient of surface tension is positive for all investigated Cr–Mn–Ni alloys due to a sulfur content ≥100 mass ppm. The final mass median particle size d₅₀ decreases from 54.3 µm for AISI 304 reference steel alloy to 17.1 µm for Cr–Mn–Ni steel alloy (16‐7‐9 S10) with the highest sulfur content and the lowest surface tension of all investigated liquid steels. It is concluded from the present work that surface tension is the decisive factor in adjusting d₅₀ at a constant spraying parameters.     

In‐Situ Observation of the Formation of MnS during Solidification of High Sulphur Steels

A Confocal Scanning Laser Microscope equipped with a gold image furnace was used to directly observe the precipitation of MnS during solidification of high sulphur steels under isothermal conditions in the temperature region 1440 to 1480°C on the free surface of the steel melt. For the case of Al‐killed steels, below 1480°C MnS particles were found to precipitate with Fe forming simultaneously around them. This MnS containing structure continued to grow rapidly (264 μm/s) as a surface film. The film gradually changed, as the level of S in the melt decreased, into a eutectic structure (with lamella spacing of 2 μm) as predicted by thermodynamics. In Si‐ killed steels there was significantly lower tendency to form MnS both in terms of time until precipitation occurred and growth rate.     

The thermodynamics of the Fe–Co–S ternary system

The thermodynamic properties of the Fe–Co–S ternary system were determined by equilibrating the system with a known sulphur pressure established by using a gas mixture of H₂S/H₂ at 1473, 1548 and 1623 K. The iso-activity contours for Fe and Co were established by application of the Gibbs Duhem equation using Schuhmann’s method for ternary systems. For this purpose a computer program was used to calculate the activities of iron and cobalt. The cobalt activity coefficients in dilute solutions of cobalt in the iron–cobalt–sulphur system, which are of industrial interest, were determined. From the experimental data, the partial heats of solution for iron, cobalt, and sulphur and the integral heats of solution for the Fe–Co–S ternary systems were derived.     

Thermodynamic evaluation of surface tension of liquid metal‐oxygen systems

Thermodynamic equations have been derived to evaluate the surface tension of liquid metal‐oxygen systems. On the basis of these equations, the effect of the oxygen on the surface tension of liquid metals has been evaluated by using the fundamental information on the oxygen solubility in the metals, the surface tension and the molar volume of pure liquid metals and oxides. The calculated results from these equations agree with the literature values of the surface tension of liquid Fe‐O, Co‐O, Ni‐O, Cu‐O and Si‐O systems.     

Surface tension and wettability studies of liquid Fe-Ni-O alloys

Surface tensions of iron-nickel alloys were measured as a function of oxygen potential at 1550 °C using the sessile drop technique. The surface tension of pure liquid nickel and iron-nickel alloys was measured at a total pressure of 1 atmosphere under varying CO₂/CO ratios. An increase in the oxygen potential in the gas phase was found to correspond to a decrease in surface tension of pure nickel and iron-nickel alloys, indicating that oxygen is surface active in both liquid nickel and iron-nickel alloys. At low oxygen potentials, nickel additions to liquid iron were found to cause small decreases in alloy surface tensions; however, at higher oxygen potentials, the surface tension of the alloy exhibited a minimum value as nickel was added to iron. The adsorption coefficients of oxygen in liquid iron-nickel alloys and pure liquid nickel were determined from the surface-tension data using Belton’s analysis, and were found to be similar to those calculated from kinetic studies. Wettability of iron-nickel alloys on an alumina substrate was studied through contact-angle measurements. At a constant alloy nickel content, the contact angle between the alloy and alumina decreased with increased oxygen potential in the gas phase.     

Influence of phosphorus addition on the surface tension of liquid iron and segregation of phosphorus on the surface of Fe-P alloy

This article presents a study of the surface tension and phosphorus surface segregation in Fe-P alloys. The surface tension was measured by the sessile drop technique. The result of the dynamic surface tension for the low phosphorus content alloys shows that the alloy surface vaporization has a clear effect on the surface tension and causes a positive surface tension temperature coefficient. However, from this article, it is evident that phosphorus in liquid iron acts as a surface active element similar to arsenic. The surface segregation was determined using Auger electron spectroscopy. The result on the surface analysis of as-solidified sample indicates that the adsorption of impurity elements, such as oxygen, carbon, and nitrogen, can conceal phosphorus segregation on the free surface. Phosphorus segregation was also examined in the samples as-cleaned by Ar⁺ and then treated 30 minutes at 650°C. Phosphorus was found to segregate extensively on the surface of the alloys. On the basis of the analysis of the published data, the surface active intensity sequence of some nonmetallic elements was arrayed, and the surface active intensity of fluorine and boron in liquid iron was estimated.     

Thermodynamics of surfaces and adsorption in the Fe-C-S-O system

Surface tension experiments have been conducted on Fe-C-S-O alloys using the sessile drop technique at 1823, 1873, and 1923 K in a purified argon atmosphere. The experimental data indicate lowering of surface tension values for alloys with low carbon and oxygen contents relative to the corresponding tenary Fe-S-O system of comparable compositions. The data exhibit an increasing trend of the surface tension values as a function of carbon and oxygen contents and temperature of the melt. Modified Butler’s equations have been used in conjunction with the consistent thermodynamic relations for a dilute quaternary system to calculate the surface tensions and adsorption functions of the system. The calculated values are in excellent agreement with those of the experimental data for compositions having lower oxygen at 1823 K, however, the values for the alloys containing higher oxygen contents depart from those of the experimental data. The calculated surfact tension values agree within 150 mN/m with those of the experimental data at 1873 K. The agreement, in general, has been favorable despite the constraints involved in the calculations as discussed in the text.     

Microstructure and Deformation Behaviour of Iron‐Rich Iron‐Aluminium Alloys with Ternary Carbon and Silicon Additions

The microstructure, hardness, yield stress, fracture strain, and brittle‐to‐ductile transition temperature of Fe‐Al alloys with Al contents of 12‐18 at.% Al, which are in the range of the so‐called K‐state with possible short‐range ordering reactions, and with ternary additions of carbon and silicon were studied with respect to the effects of possible impurities on the hardening of Fe‐Al alloys. It was found that perovskite‐type Fe₃AlC carbide particles precipitate even in alloys with low C and Si contents; they are controlled by prior heat treatments and strongly affect the deformation behaviour.     

Application of a thermodynamic database to the calculation of surface tension for iron-base liquid alloys

Thermodynamic models based on Butler's equation for surface tension of liquid alloys has been discussed. In alloys, in which activities of components deviate largely from Raoult's law, the calculated surface tensions are found to be affected by the selection of the ratio of the coordination number in the surface phase to that in the bulk phase. Then, the surface tension of liquid Fe-Al, Fe-Co, Fe-Cr, Fe-Mn, Fe-Mo, Fe-Ni, Fe-Si, Fe-Ti and Fe-W binary alloys and liquid Fe-Cr-Ni ternary alloys have been calculated from thermodynamic data in a database constructed by Kaufman. The calculated results reproduce the concentration dependence of the surface tension in those alloys reported so far, but their absolute values are dependent upon the selection of surface tension values for pure elements.     

Calculation of Oxygen and Sulfur Average Velocity on the Iron Surface: A Two‐dimensional Gas Model Study

In the present work, a two‐dimensional (2D) gas model is derived and used to simulate the average velocity of individual atoms of the surface active elements oxygen and sulfur on the Fe(100) surface. The average velocity of oxygen and sulfur atoms was found to be related to the vibration frequencies and minimal energy barrier. The calculated results are based on data from density functional calculations combined with thermodynamics and statistical physics. The calculated average velocity of oxygen on the Fe (100) is lower than that of sulphur. This is because of the stronger interaction between oxygen and the first iron layer. We conclude that our simple 2D gas model may be useful for simulating and understanding the complex interfacial phenomena in the steelmaking refining process from an atomic point of view.     

Mechanical properties of iron processed by severe plastic deformation

In the present study, the mechanical properties of Fe processed via severe plastic deformation (equal-channel angular pressing (ECAP)) at room temperature were investigated for the first time. The grain size of annealed Fe, with an initial grain size of about 200 μm, was reduced drastically during ECAP. After eight passes, the grain size reaches 200 to 400 nm, as documented by means of transmission electron microscopy (TEM). The value of microhardness during pressing increases 3 times over that of the starting material after the first pass and increases slightly during subsequent pressing for higher-purity Fe. Examination of the value of microhardness after eight passes as a function of post-ECAP annealing temperature shows a transition from recovery to recrystallization, an observation that resembles the behavior reported for heavily deformed metals and alloys. The tensile and compression behaviors were examined. In tension, a drop in the engineering stress-engineering strain curve beyond maximum load was observed both in the annealed Fe and the ECAP Fe. This drop is related to the neck deformation. The fracture surface, examined by scanning electron microscopy (SEM), shows vein patterns, which is different from the dimples found on the fracture surface of annealed Fe. In compression, an initial strain-hardening region followed by a no-strain-hardening region was observed in the ECAP Fe. The yield strength in tension of the ECAP Fe was observed to be higher than that in compression. The strengthening mechanisms and softening behavior are discussed.     

Iron Aluminium Alloys with Strengthening Carbides and Intermetallic Phases for High‐Temperature Applications

An overview of materials developments of iron aluminium alloys with strengthening precipitate phases is given. The discussion is focussed on recent studies on Fe‐Al‐based alloys with strengthening precipitates, such as κ‐phase Fe₃AlC_x, MC‐carbide and Laves phase. Alloys of the following alloy systems were investigated: Fe‐Al‐C, Fe‐Al‐Ta, Fe‐Al‐Ni, Fe‐Al‐Ti‐Nb, and Fe‐Al‐M‐C (M = Ti, V, Nb, Ta). The investigations were centred on microstructure, constitution, and mechanical properties of such Fe‐Al‐based alloys with Al contents ranging from 10 to 30 at. %. Mechanisms and problems are discussed and perspectives are outlined.     

Interfacial phenomena and mass transfer in extractive metallurgy

Interfacial phenomena play an important role in pyrometallurgical processes and knowledge of them and of physical properties involved is necessary for understanding the mechanisms and the kinetics of such reactions. A large number of measurements, performed at Irsid under equilibrium conditions, are presented in this review. Several experimental techniques were used and more particularly: – Sessile drop method for measurement of liquid metals surface tension and contact angle liquid metal/solid oxide; – measurement of the contact angle between a liquid slag drop and its liquid metal substrate from which the interfacial tension can be derived; – direct determination of the interfacial tension from X-ray pictures of metal drops immersed in the slag. The systems studied consisted, for the metal phase, of binary and ternary Fe alloys containing C, Mn, Si, O, S and, for the slag phase, binary and ternary mixtures made from CaO, SiO₂, Al₂O₃, MnO, iron oxides, CaF₂ and Na₂O. A strong effect of O and S potentials was observed. For non-equilibrium conditions, however, the dynamic interfacial tension between liquid metal and slag decreases sharply when an intense mass transfer occurs through the interface. The potential consideration of interfacial turbulence phenomena (Marangoni effect) in metallurgical reactions is also discussed.