UDK 669.14:532.613	ISSN 1580-2949
Professional article/Strokovni članek	MTAEC9, 48(3)415(2014)
INTERFACIAL TENSION AT THE INTERFACE OF A SYSTEM OF MOLTEN OXIDE AND MOLTEN STEEL
MEDFAZNA NAPETOST NA STIKU STALJEN OKSIDNI SISTEM
STALJENO JEKLO
Silvie Rosypalova, Rostislav Dudek, Jana Dobrovska, Ludovit Dobrovsky,
Monika @aludova
VŠB - Technical University of Ostrava, Faculty of Metallurgy and Materials Engineering, 17. listopadu 15/2172, 708 33 Ostrava, Czech
Republic silvie.rosypalova@vsb.cz
Prejem rokopisa - received: 2012-08-30; sprejem za objavo - accepted for publication: 2013-07-16
This paper is focused on a study of the interfacial tension between selected oxide and metal phases. The experimental research on the interfacial tension was performed in a horizontal resistive graphite Tamman furnace using an original method of measuring. This method consists of fixing both liquid phases in a horizontal position using a mandrel made of tungsten wire in a corundum cover. In this work the influence of the carbon content in the steel on the interfacial tension was studied. For this purpose a steel with 0.411 % of mass fraction of carbon and a steel with 2.64 % of carbon were used. Because of the wide variety of oxide systems used in industry, a characteristic system of casting powder was chosen for this study. This system contains dominant components, i.e., SiO2, CaO, Al2O3 and MgO, as well as a range of attendant mixtures, e.g., Fe2O3, TiO2 and Na2O. Simultaneously, the influence of SiO2 on the temperature dependence of the interfacial tension was observed. For this reason a concentration series with gradual additions of SiO2 was created. It was found that an increasing content of carbon in the steel significantly decreases the interfacial tension between the oxide system and the steel. The interfacial tension was found to decrease slightly with an increase in the content of SiO2 in the oxide system.
Keywords: steel, casting powder, interfacial tension
Članek predstavlja študijo medfazne napetosti med izbranim oksidom in kovinsko fazo. Določanje medfaznih napetosti je bilo izvršeno z originalno metodo v horizontalni uporovni grafitni Tammanovi peči. Ta metoda sestoji iz zadržanja obeh talin v horizontalnem položaju s trnom iz volframove žice in korundnega pokrova. V tem delu je bil preučevan vpliv ogljika v jeklu na medfazno napetost. Uporabljeno je bilo jeklo z masnim deležem 0,411 mas. % ogljika in jeklo z 2,64 % ogljika. Zaradi velike raznolikosti oksidnih sistemov, ki se uporabljajo v industriji, je bil za študij izbran livni prašek. Ta vsebuje glavne komponente, ki so SiO2, CaO, Al2O3 in MgO, ter vrsto primesi, kot so na primer Fe2O3, TiO2 in Na2O. Hkrati je bil opažen vpliv SiO2 na temperaturno odvisnost medfazne napetosti. Zato je bila pripravljena serija z naraščajočo vsebnostjo SiO2. Ugotovljeno je, da naraščanje vsebnosti ogljika v jeklu občutno zmanjša medfazno napetost med oksidnim sistemom in jeklom. Za medfazno napetost je ugotovljeno, da se nekoliko zmanjša, če se povečuje vsebnost SiO2 v oksidnem sistemu.
Ključne besede: jeklo, livni prašek, medfazna napetost
tallic phase. The interfacial tension was calculated using 1 INTRODUCTION	the following equation:
) ^20)-(.) + ^2s)-(.) -(0)-(,) • ^(s)-(,) • cos e (1)
2.1 Material
Interfacial phenomena play an important role in	(0)-(s
many metallurgical processes in which two immiscible	where CT(s)-(g)/(mN/m) is the the surface tension of oxide
liquid phases co-exist. Numerous steps in primary pro-	system, CT(o)-(g)/(mN/m) is the surface tension of the
cessing and the refining of materials include a mass	molten steel, and e is the wetting angle of the liquid
transfer through an interface, which significantly affects	phases. the rate of individual reactions. Surface and interfacial
tensions can accelerate these reactions, or completely	2 EXPERIMENTAL dampen them. It is, therefore, necessary to know the properties of the interface, which together with the other
physicochemical properties1 forms the properties of the	For the investigation of the interfacial tension a
resulting product. Although various methods2,3 have been	casting powder was chosen as a representative of an
developed for the research of interface phenomena, any	oxide system. Its composition is given in Table 1.
experimental investigation remains very difficult. For	Within this system we investigated the influence of
this reason the literature data for a determination of the	the carbon in the steel on the interfacial tension. For this
interface phenomena, particularly the interface pheno-	purpose we choose, as representatives of the metallic
mena for slag-metal, are not commonly available.	phase, the steel (I) containing 0.411 % of carbon, and the
The aim of this research was to study the interfacial	steel (II), containing 2.64 % of carbon. The chemical
tension in the system involving an oxide phase and a me-	compositions of both steels are given in Tables 2 and 3.
	component	C	Si	Mn	S	P	Cu
Steel (I)		0.41	0.375	0.344	0.011	0.016	0.076
	component	Ni 1	Cr	Mo	Ti	W	Fe
		0.218	12.392	0.027	0.013	0.084	75.6
Table 3: Chemical composition of steel (II), w/% Tabela 3: Kemijska sestava jekla (II), w/%							
	component	C	Si	Mn	S	P	Cu
Steel (II)		2.64	2.04	0.57	0.031	0.043	0.020
	component	Ni 1	Cr	Mo	Ti	W	Fe
		0.022	0.051	0.008	0.034	-	94.5
MgO
Al2O3
TiO2
Fe2O3
1.70
12.50
0.50
0.64
P2O5
Ct
CO2
0.10
4.10
7.20
4.40
MnO2
0.10
Table 2: Chemical composition of steel (I), w/%o Tabela 2: Kemijska sestava jekla (I), w/%
Steel (I)
component
component
C
Si
Mn
0.41
0.375
0.344
0.011
Ni
Cr
Mo
Ti
0.218
12.392
0.027
0.013
P
0.016
W
0.084
Cu
0.076
Fe
75.6
Table 3: Chemical composition of steel (II), w/% Tabela 3: Kemijska sestava jekla (II), w/%
Steel (II)
component
component
C
Si
Mn
2.64
2.04
0.57
0.031
Ni
Cr
Mo
Ti
0.022
0.051
0.008
0.034
0.043
W
Cu
0.020
Fe
94.5
The influence of SiO2 on the interfacial tension in an oxide/steel system was also investigated. For this research a concentration series was prepared with gradual additions of (3, 6, 9 and 15) % of SiO2 to the measured system of casting powder, within the concentration range from 37.1 % to 52.1 %.
2.2 Experimental methods
Differential Thermal Analysis (DTA) was used to obtain the melting temperature of the steel samples.4 The analysis was performed with the suse of the laboratory system Setaram SETSYS 18TM. The samples with masses of approximately 200 mg were analysed using a controlled rate of heating equal to 15 °C/min. A dynamic atmosphere of Ar (purity > 99.9999 %) was maintained in the furnace during the analysis.
For the calculation of the interfacial tension according to Equation (1) the surface tension and the wetting angle of the liquid phases were determined experimentally. The surface tension of the casting powder, the steel and the concentration series with the addition of SiO2 was measured using the method of sessile drop. This method is based on a recognition of the geometrical shapes of a drop of melt sessile on a non-wetting pad.5 A
graphite pad was used for the oxide systems and a corundum pad was used for the steel. The experimental measurements were performed in a horizontal resistance furnace under an Ar atmosphere (purity > 99.9999 %).
In order to determine the wetting angles of both liquid systems an original experimental methodology was used. This is schematically represented in Figure 1a. The principle of this methodology consists of fixing both liquid phases in a horizontal position.6
A sample of steel was put into a corundum pad of the appropriate shape. Afterwards, a tablet of oxide material was put on the steel. For the fixation of both phases in the horizontal position a mandrel formed by a tungsten wire in corundum protection was passed through the centre of the whole sample, as shown in Figure 1b. After the melting of both systems the wetting angle was calculated on the basis of the determined contours of the steel and oxide system, as can be seen in Figure 1c.
3 RESULTS AND DISCUSSION
The liquidus temperatures of the steels, determined by a DTA analysis, were 1455 °C for steel (I) and 1158 °C for the steel (II).
Figure 1: a) Schematic diagram of experimental method, b) a prepared sample before the experiment, c) image of molten phases Slika 1: a) Shematski prikaz eksperimentalne metode, b) pripravljen vzorec pred preizkusom, c) posnetek staljenih faz
F
P
Figure 2: Temperature dependence of interfacial tension of system oxide - steel (I)
Slika 2: Temperaturna odvisnost medfazne napetosti oksidni sistem -jeklo (I)
For the calculation of the interfacial tension according to Equation (1), first of all the surface tensions of the steel and the casting powder were measured. The surface tension of steel (I) was measured within the temperature interval 1465-1550 °C. The surface tension decreased with the temperature in the interval 1770-1680 mN/m. The surface tension of steel (II) was measured within the temperature interval 1185-1550 °C and the values were in the interval 1409-1205 mN/m. From the measured values it is evident that the surface tension of the steel containing 0.411 % of carbon is on average higher by 350 mN/m, compared to the steel containing 2.64 % of carbon. The surface-tension measurements of the oxide system were performed in the temperature interval 1180-1550 °C and the obtained values ranged within the interval 390-370 mN/m.
During the next stage of the experimental research the wetting angles were measured in the interface systems oxide/steel (I) and oxide/steel (II). The values of the wetting angles for steel (I) increased slightly with the temperature in the range of 2-2.4 rad; in contrast to that
Figure 3: Temperature dependence of interfacial tension of system oxide - steel (II)
Slika 3: Temperaturna odvisnost medfazne napetosti oksidni sistem -jeklo (II)
Figure 4: Dependence of interfacial tension on mass fraction of SiO2 at a temperature of 1500 °C
Slika 4: Odvisnost medfazne napetosti od masnega deleža SiO2 pri temperaturi 1500 °C
in steel (II) the values decreased slightly in the interval 2-1.75 rad.
From the experimental data the interfacial tension was calculated according to Equation (1). Figures 2 and 3 show the temperature dependences of the interfacial tension of steel (I) and steel (II).
From Figures 2 and 3 it is evident that the interfacial tension in both systems decreases with the temperature. The values of the interfacial tension of steel (I) varied from 2020 mN/m to 1930 mN/m. The values of the interfacial tension of steel (II) were in the interval 1540-1330 mN/m. From these temperature dependences it follows that the values of the interfacial tension are, in steel (II), on average lower by 25.5 % (505 mN/m) in comparison with the values of the interfacial tension of steel (I).
Moreover, the influence of SiO2 on the interfacial tension in both steels was determined experimentally. Figure 4 shows the dependence of the interfacial tension of the system oxide/steel (I) or oxide/steel (II) on the content of SiO2 at a temperature of 1500 °C. It is generally true that in the case when the melt contains the ions Si4+, they will have a decisive influence on its structure, since silicon is the most electronegative of all the elements of the system, and with oxygen it creates stable complexes with a strong covalent bond.
The ions Al3+ also play an important role in formation of polyanion networks. It is evident from Figure 4 that the trend of influence of the SiO2 content on the interfacial tension is the same in both steels. The inter-facial tension in the systems with additions of (0, 3, 6 and 9) % of SiO2 decreases slightly. This confirmed the theory about SiO2 functioning as a networks former. In the case of the system with the addition of 15 % of SiO2 the values of the interfacial tension increase slightly. This phenomenon may be caused by the formation of phases with shorter chains and a change of the coordination of the Al3+ cations
4 CONCLUSIONS
We can summarise the obtained results as follows:
•	The interfacial tension between the molten steel and the molten casting powder with dominant components of SiO2, CaO, Al2O3 and MgO decreases with temperature.
•	A higher content of carbon in steel decreases the values of the interfacial tension at the interface system involving molten oxide and molten steel.
•	The interfacial tension of the system involving molten casting powder and molten steel decreases with an increasing content of SiO2 in the concentration interval 37.1-46.1 %.
Acknowledgements
The work was carried out within the scope of the student projects SP 2012/25, entitled "Experimental study of inorganic heterogeneous systems", and SP 2012/196, entitled "Specific research in metallurgy, materials and process engineering". It was financed from the specific research resources of the Faculty of Metallurgy and Materials Engineering, VSB - Technical University of Ostrava, Czech Republic and of the project No. CZ.1.05/
2.1.00/01.0040 "Regional Materials Science and Technology Centre" within the frame of the operation programme "Research and Development for Innovations" financed by the Structural Funds and from the state budget of the Czech Republic.
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