Activities of the Constituents in Spinel Solid Solution and Free Energies of Formation of MgO, MgO Al2O3.

Abstract

Spinel (MgO · Al 2 O 3 ) is known as one of the most harmful non-metallic inclusions in steel. However, the technology to avoid spinel formation has not yet been established due to lack of thermodynamic information on the spinel itself. In the present work, the free energies of formation of MgO The activity of stoichiometric MgO · Al 2 O 3 exhibits negative deviation from ideality in the spinel solid solution. Phase stability diagram of spinel formation in liquid steel was assessed as a function of aluminum, magnesium and oxygen contents in iron at 1 873 K. The predicted phase boundaries of MgO · Al 2 O 3 , MgO and Al 2 O 3 agreed well with previous observations by one of the present authors.KEY WORDS: Spinel; non-metallic inclusion; free energy of formation; steelmaking; activity.constituents in steelmaking slags 24) was used. MgO and Al 2 O 3 are the component oxides of spinel, and their free energies of formation are necessary to determine DG°f ,MgO · Al 2 O 3 and the activity in the spinel solid solution. Reliable data on the free energy of Al 2 O 3 formation have been reported in the literature. [6][7][8][9]24) On the other hand, the reported value of DG°f ,MgO shows discrepancy with the order among DG°f of other alkaline earth oxides such as CaO, SrO and BaO. 25,26) Therefore, DG°f ,MgO is also directly measured in the present work. Experimental Principle Free Energy of Formation of MgOFor the measurement of DG°f ,MgO , experiments were conducted by equilibrating liquid copper containing graphite in a MgO crucible with CO-Ar atmosphere. The following equilibrium reaction should be established:The free energy change of Eq. (1), DG°1, can be related to its equilibrium constant, K 1 :Where, a is the activity of each component relative to the pure substance, and P CO denotes the partial pressure of CO (atm). Since the activities of MgO and carbon are unity due to the use of a pure MgO crucible and a pure graphite block, Eq. (3) can be simply derived from Eq. (2) Hultgren et al. 30) and Nayeb-Hashemi and Clark.31)The literature data on g Mg are summarized in Fig. 1(A). The agreement of the activity coefficient among literature values at the high concentration range of Mg seems to be excellent. On the other hand, there are some discrepancies in the low concentration range of Mg. Since the accuracy of g Mg at low concentration range is the most important for the precise determination of DG°1, the present authors 32) have assessed the thermodynamic properties of liquid Cu-Mg alloy with mass spectrometry. Their values are also plotted in Fig. 1(A). It is seen from Fig. 1(A) that the values of RT ln g Mg evaluated by the authors agree well with the values observed by Garg et al. and recommended by Hultgren et al. Therefore, the values of RT ln g Mg given by the authors have been employed in the present work. It has also been reported that the temperature dependence of RT ln g Mg is very small as shown in Fig. 1(A). 32) Therefore, the value of K 1 can be determined from the equilibrium concentration of Mg in l...

Factors to Determine Inclusion Compositions in Molten Steel during the Secondary Refining Process of Case-Hardening Steel

“…The used parameters include the measured contents of Al, Ca and Mg as well as the values shown in Table 2 and the first and second order interaction coefficients listed in Table 3. [30][31][32][33][34][35] It is noticed that the effect of Mo on the activity coefficient of the other components was not taken into account due to the lack of thermodynamic data.…”

“…To determine the most stable phase of the three species of (1) (5), (7), 38) and (8). 33) In the calculations, the activities of all the oxides were assumed as unity and Al content was taken as 0.035 mass%. The steel samples in this research contained Ca of around 6 ppm and at least 2 ppm.…”

Distribución y crecimiento de inclusiones en la solidificación de barras de acero para rodamientos con cromo y alto contenido de carbono

“…(2)~(6) (Sigworth and Elliott, 1974;Kubaschewski et al, 1993;Itoh et al, 1997;Fujii et al, 2000;Zhang et al, 2015b), among which Eq. (2) …”

Thermodynamic Model for Prediction of Slag-Steel-Inclusion Reactions of 304 Stainless Steels

“…[12][13][14][15][16][17]23,24) A large number of thermodynamic models based on thermodynamic and phase equilibrium data for a given system has been developed, 25) which can be used to predict the relationship between compositions of slag, steel, and inclusions in relatively simple systems, such as Al-O, [26][27][28] AlMg-O, [29][30][31] Al-Ca-O, [32][33][34] Al-Ti-O, [35][36][37] Al-Si-Ca-O, 38,39) Al-Si-Mn-O, [40][41][42] etc. However, the thermodynamic models can hardly be applied to a multicomponent system due to complicated calculations and lack of thermodynamic data.…”