Liquid Oxysulfide Research Articles

Formation and Evolution of Inclusions in Si-killed Resulfurized Free-cutting Steel

In order to control the machinability and mechanical property of resulfurized free-cutting steel, the high temperature experiments at laboratory and thermodynamic analysis were carried out to investigate the formation and evolution of inclusions in the processes of refining and solidification. Furthermore, the migration behavior of sulfur was also assessed between the inclusions and the liquid steel. The results showed that the liquid oxysulfide inclusion (Mn,Si)x(O,S)y with trace amount of sulfur formed after FeS addition at 1600°C. As sulfur segregated in the residual liquid steel during solidification, sulfur rapidly migrated into (Mn,Si)x(O,S)y from the liquid steel at the initial stage of solidification. However, sulfur migrated out from (Mn,Si)x(O,S)y at the middle stage of solidification because MnS precipitated from the supersaturated liquid steel. After the steel solidified completely, sulfur continued to migrate out from the liquid oxysulfide inclusion (Mn,Si)x(O,S)y. When the temperature decreased to 1250°C, the liquid inclusion (Mn,Si)x(O,S)y transformed to two solid phases of (Mn,Si)xOy and MnS. Finally, the complex inclusion composed of (Mn,Si)xOy and MnS formed.

Erratum to: Thermodynamics of MnO-SiO2-Al2O3-MnS Liquid Oxysulfide: Experimental and Thermodynamic Modeling

YE-JIN KIM, Graduate Student, HENRI GAYE, formerly Professor, HAE-GEON LEE, Professor, and YOUN-BAE KANG, Assistant Professor, are with the Graduate Institute of Ferrous Technology (GIFT), Pohang University of Science and Technology, Pohang, Republic of Korea. Contact e-mail: ybkang@postech.ac.kr DAE-HEE WOO, formerly Graduate Student, Graduate Institute of Ferrous Technology (GIFT), Pohang University of Science and Technology, Pohang, Republic of Korea, is now with Technical Research Laboratories, POSCO Ltd, Kwangyang, Cheonnam, Republic of Korea. The online version of the original article can be found under doi: 10.1007/s11663-011-9500-y. Article published online October 16, 2013.

Experimental investigation of phase equilibria and thermodynamic modeling of the CaO–Al2O3–CaS and the CaO–SiO2–CaS oxysulfide systems

Phase equilibria of the CaO–Al2O3–CaS and the CaO–SiO2–CaS systems were experimentally investigated using equilibration and quenching techniques. Equilibrium phases were analyzed by means of electron probe X-ray microanalysis, X-ray diffraction analysis and differential thermal analysis. Solubility limits of all solid phases in these liquid oxysulfide phases were successfully constructed in the temperature range investigated in the present study (1500–1600°C). In order to supplement understanding the phase equilibria, a thermodynamic modeling of these liquid oxysulfides was conducted by taking into account strong chemical short-range ordering (SRO) in the framework of the modified quasichemical model in the quadruplet approximation. As for the solubility of CaS in the liquid oxysulfides, the solubility increases with increase in CaO in the case of the CaO–Al2O3–CaS system, whereas it decreases with increase in CaO in the case of the CaO–SiO2–CaS system. Such opposite behavior is explained by differences in the effect of the SRO in different liquid phases. It is shown that consideration for the SRO in the thermodynamic modeling is essential in order to properly describe the Gibbs energy of the liquid oxysulfide phase. Using the thermodynamic model and the database developed in the present study, liquidus projections of these oxysulfide systems are proposed for the first time.

Thermodynamics of MnO-SiO2-Al2O3-MnS Liquid Oxysulfide: Experimental and Thermodynamic Modeling

The activities of MnO and MnS in a MnO-SiO2-Al2O3(or AlO1.5)-MnS liquid oxysulfide solution were investigated by employing the gas/liquid/Pt-Mn alloy chemical equilibration technique under a controlled atmosphere at 1773 K (1500 °C). Also, the sulfide capacity, defined as CS = (wt pct S)(pO2/pS2)1/2, in MnO-SiO2-Al2O3 slag with a dilute MnS concentration was obtained from the measured experimental data. As XSiO2/(XMnO + XSiO2) in liquid oxysulfide increases, the activity coefficient of MnO decreases, while that of MnS first increases and then decreases. As X(AlO1.5) in liquid oxysulfide increases, the activity coefficient of MnS increases, while no remarkable change is observed for the activity coefficient of MnO. The behavior of the activity coefficient of MnS was qualitatively analyzed by considering MnO + AxSy(SiS2 or Al2S3) = MnS + AxOy(SiO2 or Al2O3) reciprocal exchange reactions in the oxysulfide solution. The behavior was shown to be consistent with phase diagram data, namely, the MnS saturation boundary. Quantitative analysis of the activity coefficient of the oxysulfide solution was also carried out by employing the modified quasichemical model in the quadruplet approximation.

Reduction of iron-silicon-oxysulfide by CO gas injection

The reduction of liquid oxysulfide in the Fe-Si-S-O system by CO gas injection has been studied by monitoring the exit gas composition. The reduction rate of oxygen was calculated from the volume of evolved CO2. Sulfur-bearing species such as COS were close to the detection limit of the mass spectrometer, which indicated that the reduction of sulfur was very limited. The volume of evolved CO2 reached steady values 1 minute after CO injection. The reduction reaction was controlled by a chemical reaction. The observed maximum reduction rate of oxygen at 1250 °C was 8.3×10−6 g-O/cm2 s, which was within the range of the reduction rates in other melts such as iron oxide and iron silicates.

The solubility of the liquid oxysulfide phase in liquid Fe-O-S alloys

The solubility of the liquid oxide phase in liquid Fe-O alloys has been measured for the temperature range of 1378 to 1740 °C. Also the solubility of the liquid oxysulfide phase in liquid Fe-O-S alloys has been determined for the composition range of 0.08 to 0.30 wt pct oxygen and 0 to 0.5 wt pct sulfur. The oxygen content of liquid iron saturated with the liquid oxide phase is log O = −6358/T + 2.76. The standard free energy for the formation of the oxide phase is: xFe(l) + O(pct) = FexO(l); ΔG° = 242.4 − 0.0829T + 166,990/T(kJ). The equation for the standard free energy in the temperature range of 1550 to 1650 °C may be written as: −117.5 + 0.0496T (kJ). The effect of composition on temperature of saturation of liquid Fe-O-S alloys with the oxysulfide phase is:T(K) = −6358/(log pct O − 2.76) - (pct S)x [554 + 135.0(log O − 2.77)]. The relationship applies for the composition range of 0.15 to 0.30 wt pct oxygen and 0.0 to 0.5 wt pct sulfur and temperatures from 1480 to 1680 °C.

The diffusivity of sulfur in wüstite and its solubility in wüstite and γ iron in equilibrium with each other and a liquid oxysulfide

The solubility of sulfur in wustite in equilibrium with γ iron and liquid oxysulfide was found to be 0.011 wt pct at 1050°C. The sulfur solubility in γ iron in equilibrium with wustite and liquid oxysulfide was also determined at 1050°, 1150°, and 1250°C and found to be 135, 165, and 160 ppm respectively. These values are considerably lower than the sulfur-solubility in y iron in the binary Fe-S system saturated with pyrrhotite. The diffusivity of sulfur in wustite was determined by oxidizing Fe-S alloys in mixtures of CO and CO2, and analyzing the entire sample for sulfur afterwards. From the amount of sulfur diffused through the growing wustite layer into the gas phase, the diffusivity of sulfur in wustite was evaluated, and found to be 4.1 × 10−8 and 9.6 × 10−7 cm2/s at 1050° and 1250°C respectively. These values are of the same order as the self-diffusivity of iron in wustite in equilibrium with iron at the same temperatures.

Sulfides and oxides in Fe-Mn alloys: Part III. Formation of oxysulfides during freezing of steel

The previous study of the phase relations (Part I) is now extended to the solid-liquid regions of Fe-Mn-S and Fe-Mn-S-O systems. The differential thermal analysis measurements made with 1 to 5 pet Mn-Fe alloys containing sulfur and oxygen substantiated the positions of the univariant curves in pet Mn vsT plots derived from other available data and theoretical considerations. It is found that if sufficient oxygen and sulfur are present in the Fe-Mn alloys, there may be a liquid oxysulfide phase present at temperatures above 900°C, depending on the manganese content. The higher the manganese content in solution, the higher is the temperature below which no liquid phase is present,e.g. for 10 ppm, 900°C and for 10 pet Mn, ∼1200°C. The formation of oxides, sulfides, and liquid oxysulfides in the interdendritic regions during solidification, as predicted from the phase relations in the Fe-Mn-S-O system, was verified by microscopic examination using scanning electron microscopy and metallographic techniques.

Sulfides and oxides in Fe−Mn alloys: Part II. Precipitation of solid sulfides and liquid oxysulfides during oxidation of iron alloys

Experimental results are presented showing that oxidation in air or in mixtures of CO2 and CO of iron alloys, containing Mn (0 to 0.34 pct) and/or S (150 ppm) at temperatures between 900° and 1300°C, results in the formation of a liquid oxysulfide phase at and near the metal/oxide interface. Electron microprobe analysis showed that this liquid consists of Fe (Mn), S and O. Oxidation of Fe−S alloys at rates slower than the rate of diffusion of sulfur in wustite does not cause any precipitation of a liquid oxysulfide at the interface. From indirect evidence, the sulfur diffusivity in wustite is estimated to be of the same order of magnitude as that in iron. The solubility of sulfur in wustite in equilibrium with iron and a liquid oxysulfide at 1250°C is about 140 ppm. It is found that the presence of a thin layer of liquid oxysulfide at the iron/wustite interface does not have a perceptible effect on the parabolic rate of oxidation of the metal.

Sulfides and oxides in Fe−Mn alloys: Part I. Phase relations in Fe−Mn−S−O system

This is a critical review of available equilibrium data between phases involving Fe, Mn, S, and O. Using the Morey-Williamson theorem, and that modified by Darken, the sulfur and oxygen potential diagrams are constructed for the Fe−S−O system involving nine univariant and three invariant equilibria. The solubility of sulfur in wustite-saturated iron is evaluated; the sulfur content of iron in equilibrium with wustite and liquid oxysulfide reaches a maximum of 143 ppm at 1200°C; this is about one half of that corresponding to the solidus of the Fe−S system. An estimate is made of the phase equilibria in the Fe−Mn−S−O quaternary system involving gamma iron and Mn(Fe)O phases. There is a eutectic invariant at ∼900°C, and the liquid miscibility gap invariant is estimated to be at ∼1225°C. From the expected phase relations and equilibria, it is deduced that if sufficient oxygen and sulfur,i.e. Mn(Fe)O and Mn(Fe)S phases, are present in Fe−Mn alloys, there may be a liquid oxysulfide phase present at temperatures above 900°C, depending on the concentration of manganese in solution. The higher the manganese content in solution, the higher is the temperature above which a liquid phase is present,e.g. for 10 ppm Mn, 900°C and for ∼90 pct Mn, ∼1225°C. A mechanism is suggested for the precipitation of sulfides and oxysulfides near the surface of steel during heating in an oxidizing atmosphere. In the Appendix by Darken and Gurry, results are given of the melting temperatures of mixtures of wustite and pyrrhotite in equilibrium with iron, from an investigation carried out about thirty years ago.