The processes that take place at the liquid metal-sand mold interface during the casting and crystallization of metal were studied in this paper. The mold was made using the CO2 – method, with the addition of active components, such as: MgO, TiO2, AlK(SO4)2 and Na2SO4. At the liquid steel pouring temperature, these active components undergo decomposition in the mold, caused by heat release during pouring, crystallization and cooling of castings. Silicates, which have an important influence on casting surface quality, are formed as a result of the interaction between the mould material and liquid steel

A. Patarić

M. Mihailović

Z. Gulišija

Z. Janjušević

English

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197METALURGIJA 53 (2014) 2, 197-200Z. JANJUŠEVIĆ, Z. GULIŠIJA, M. MIHAILOVIĆ, A. PATARIĆPROCESSES AT THE INETRFACE MOLTEN METAL-SAND MOLDReceived – Prispjelo: 2013-07-13Accepted – Prihvaćeno: 2013-10-30Preliminary Note – Prethodno priopćenjeISSN 0543-5846METABK 53(2) 197-200 (2014)UDC – UDK 621.746.5.01:669.14:544=111Z. Ja njuševic, Z . G ulišija, M . M ihailovic, A . Pa tarić, I nstitute f or Technology o f Nu clear a nd O ther M ineral R aw M aterials, F ranchet d’Esperey 86, Belgrade, SerbiaThe processes that take place at the liquid metal-sand mold interface during the casting and crystallization of metal were studied in this paper . The mold was made using the C O2 – method, with the addition of ac tive components, such as: MgO, TiO2, AlK(SO4)2 and Na2SO4. At the liquid steel pouring temperature, these active components undergo decomposition in the mold, caused by heat release during pouring, crystallization and cooling of castings. Silicates, which have an important infl uence on casting surface quality, are formed as a result of the interaction between the mould material and liquid steel.Key words: molten steel, sand mold, interface INTRODUCTION The forming of a casting usually is monitored at the liquid metal-mold interface, i.e., during the cooling pe-riod, in the temperature range from the pouring tem-perature to room temperature. In this interval a remark-able interchange of heat occurs between the liquid met-al and the mold. The forming of a casting is followed by many processes which have substantial infl uence on the casting quality. At the liquid metal-mold interface, a lot of chemical and/or metallur gical reactions occur . The products of those reactions are set on both sides, it means at the side of casting as well as at the side of the mold.During consideration of interactions at the interface between the mold and the liquid metal, the following variables must be included: primary metal (M), alloying elements (M 1, M 2, ...M i), oxygen (O), oxides of mold material (RO), and temperature. Three types of chemi-cal reactions can occur between the participants in the process at the mould surface [1]: M1,Mi + 1/2O2= M1O,MiO  (1) M1,Mi + RO= M1O,MiO + R  (2) M1O,MiO + RO= M1O·RO,MiO·RO (3)The equilibrium of these reactions is determined by behavior of the interface between mold and liquid met-al.The reaction between metal and oxygen can be shown by general equation (3).The composition of an oxide which forms on the metal surface depends on the properties of the casting material. For a carbon steel, among the oxides iron ox-ides prevail, primarily iron (II)-oxide (FeO) with a melt-ing temperature of 1 370 °C. These oxides, highly su-perheated at the steel casting temperature, are very ac-tive and liquid, and react with the mould material, form-ing silicates. Formed silicates have certain physical and chemical properties: high surface tension, low viscosity (5-10 Pa·s), optimal melting temperature (800-900 °C), and high oxidation potential [2].Similar problems were studied, for example by Zhang et al [3], who presented an oxidation mechanism of CaOFeOxSiO2 slag with high iron content in a pure oxygen atmosphere. The phase equilibria for slag sys-tems containing iron oxides were studied by Turkdogan, Bodsworth and Bell [4], and Kongoli and Yazawa [5]. Muan and Osborn [6] reported the phase diagrams for the FeOMnO system in a low oxygen pressure, as well as for the FeOMnO-SiO 2 system in CO 2/H2 and air at-mospheres. Structural models for binary silicate sys-tems, including MnO-SiO 2 and CaO-SiO 2, as well as thermodynamics of Fe3O4–Mn3O4 spinels, are presented in the work of Pelton et al. [7, 8]. Semykina and a few other authors worked on the oxidizing kinetics of liquid FeO-MnO-CaO-SiO2 slags [9-11].EXPERIMENTALThe experiments were conducted in laboratory and industrial conditions, on steels from the manufacturing program of the Foundry of Steel Casts. The moulds were made according to the CO2 proce-dure. The mould mixture consisted of about 90 % quartz sand; 6 % casting water glass, density 1,55 g/cm3; 0,1 % sodium-sulfate; and the remainder of active compo-nents.As active components the oxides of titanium and  magnesium, TiO2 and MgO, were used. They transfer the binary Na2O - SiO 2 system, into the ternary Na 2O SiO2 MxOy system with higher viscosity and surface tension.  We also used sulfates of aluminum, sodium, and potas-198  METALURGIJA 53 (2014) 2, 197-200Z. JANJUŠEVIĆ et al.: PROCESSES AT THE INETRFACE MOLTEN METAL-SAND MOLDThe fourth zone is a layer of unchanged mould mix-ture with unchanged compounds and properties.Major changes, the subject of interest, occurred in the fi rst zone. In this zone, the liquid metal penetrated into the mold material immediately after pouring into the mold, causing intensive reactions of liquid phase with the mould surface. The contact of such intensity , which causes reaction on entire mold surface, results in maximal speed of reaction layer growth at the begin-ning of the casting solidifi cation.Since the process on metal-mould contact surface directly depends on temperature to which the mold is subjected, it comes out that penetration of liquid phase into the mold will be more intense with the temperature increase. Consequently, the process of chemical interac-tion with participation of liquid phase will be shifted towards the mold depth.Temperature measuring of the contact zone in so-lidifi cation period of liquid metal showed that contact zone warms up to high temperatures, 1 400 to 1 500 °C, Figure 2.These temperatures cause dehydration, oxidation, and formation of new compounds (for instance, silicates).  The formation of interaction products in the contact zone is signifi cantly determined by kinetics and temperature.  Consequence of both, oxidation of elements and forma-tion of silicates is determined by their concentration and  change of thermodynamic potential of oxides and sili-cates which characterize ener getic potential, reaction  course and stability of product formation.Photographs of microscopic structure in the transmit-ted polarized light taken from the fi rst contact layer, Fig-ure 3a, show that the shape of quartz grains is changed in the vicinity of the castings surface. Those grains are de-formed, cracked, embedded in reactive melt, and partially transformed into crystobalite. Frequently , those quartz  grains are impregnated by metallic minerals, Figure 3b,  or they are pasted together with hematite, Figure 3c, but  they rarely grew together, Figure 3d.sium, where sodium sulfate was a constantly present  component in the mould mixture, and aluminum sulfate  and aluminum potassium sulfate were added as active  components. Due to thermal ef fects in the mould during  processes of casting, solidifi cation and cooling down of  the ingot, these active components dissociate, releasing  gaseous reaction products and steam from chemically  bonded moisture, which additionally affects oxidation of metal fi brils, and thereby the formation of new com-pounds in the contact zone of metal and mould.Experimental ingots had a cylindrical shape, with dimensions of 0,038 x 0,060 m. The casting was used steel cast taken directly from manufacturing process of the foundry, with the chemical composition shown in Table 1.Table 1 Chemical composition of the steel / %C Si Mn Cr Mo S P 0,79 0,53 0,85 1,60 0,32 0,025 0,033Casting temperature was in the range 1 580-1 600 ºC, with initial temperature of the mold, and its external surface temperature equal to environment temperature (∼ 20 ºC). The studies of behavior at the metal-mold interface, during the production of castings were done using the following methods: visual inspection, measuring the sur face roughness, optical microscopy , quantitative chemical analyses, powder dif fraction, and electron-micro probe analysis. The other characteristics of the mold material, including mechanical, thermo-physical, and thermodynamic characteristics, were monitored, as well as the appearance and thickness of newly formed zones at the metal-mold interface.For better understanding of the processes at the con-tact surface, the temperatures were measured at distanc-es of 0, 1, 2, 3, 5, 10, 15, 20 and 25 mm from the sur -face. The interface region was studied using optical microscopy, scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS).RESULTS AND DISCUSSIONA section of the mould with visible changes in the mould mixture caused by reaction with the metal and the non-reacted part of the mould mixture is given in Figure 1, [12].By observing the section of the mold in Figure 1, four layers-zones are visible. Changes occurred in the fi rst three zones depending on temperature, and the fourth layer remained unchanged, as shown in the Fig-ure 1.The fi rst zone is dark in color and is a zone of pene-tration of a liquid metal into the mold pores.The second zone is light in color due to allotropic changes of quartz.The third grey zone is the area in which only small changes have occurred.Figure 1 Section of the mold with visible zones199METALURGIJA 53 (2014) 2, 197-200Z. JANJUŠEVIĆ et al.: PROCESSES AT THE INETRFACE MOLTEN METAL-SAND MOLDFor correct interpretation of some reactions which were carried out, the use of energy dispersive spectros-copy (EDS) became necessary . The SEM-EDS results are shown in Figure 4. CONCLUSIONS The following conclusions may be withdrawn from this investigation:0 100 200 300 400 500 60002004006008001000120014001600temperature /°Ctime/s25 mm20 mm15 mm10 mm5 mm3 mm1 mm2 mm0 mm0 100 200 300 400 500 6000100200300400500600700800900100011001200130014001500temperature /°Ctime/s25 mm20 mm15 mm10 mm5 mm3 mm2 mm1 mm0 mm0 100 200 300 400 500 60002004006008001000120014001600temperature /°Ctime/s25 mm20 mm15 mm10 mm5 mm3 mm2 mm1 mm0 mm0 100 200 300 400 500 60002004006008001000120014001600temperature /°Ctime/s25 mm20 mm15 mm10 mm5 mm3 mm2 mm1 mm0 mma) addition of MgO b) addition of TiO2c) addition of MgO+TiO2 d) addition of MgO+TiO2+AlK(SO4)2Figure 2 The temperature vs. time over the mold cross-section for diff erent additivesFigure 3 Microscopic photographs of the Ā rst contact zone in transmitted light 200  METALURGIJA 53 (2014) 2, 197-200Z. JANJUŠEVIĆ et al.: PROCESSES AT THE INETRFACE MOLTEN METAL-SAND MOLD-  Products of interaction between the melt and the mold are generally oxides whose physical and physicochemical properties essentially depend on the type of a defect in the structure and on their concentration. Interaction between the liquid steel and the mold takes place under the thermal shock, causing the formation of non-stoichiometric oxide composition, thus determining the course of subse-quent reactions in the contact zone.-  Chemical changes are visible in a more intensifi ed reaction zone of penetrating metal fi  brils with quartz and active components, with formation of a glassy phase.-  Penetrating compounds into the mold from liquid steel are: iron, manganese and chrome.-  Considering components that the mold mixture consists of, the major reaction signifi cance has the silicon and partially the potassium, while the sodi-um leaves the system.-  The composition of the reaction melt is rather com-plex, based on silicon, iron, manganese and potas-sium. It is predominantly presented by glassy phase with occurrence of the fayalite, the wustite, the spi-nel and a smaller share of other minerals.AcknowledgmentThe work was fi nancially supported by the Ministry of Education and Science of the Republic of Serbia, grant 34002.REFERENCES[1] A. N. Tsibrik, Fiziko-khimicheskie protsessy v kontaktnoi zone metal-forma, Naukova dumka, Kiev , 1977, pp 12-45.[2] B. Kočovski, Founding Theory, BIGZ, Belgrade, 1972, pp. 38-56.[3] L.-N. Zhang, L. Zhang, M.-Y. Wang, and Z.T. Sui, Trans. Nonferr. Metals Soc. China, 15 (2005), 938–943.[4] C. Bodsworth and H. B. Bell, Physical Chemistry of Iron and Steel Manufacture, Longman, London, 1972, pp. 343–345.[5] F. Kongoli and A. Yazawa: Metall. Mater . Trans. B, 32B (2001), 583–592.[6] A. Muan and E. F . Osborn, Phase Equilibria Among Oxi-des in Steelmaking, Addison-Wesley, Reading, MA, 1965, pp. 33–86.[7] A. D. Pelton, H. Schmalzried, and J. Sticher,Berichte Bun-sengesellschaft für Phys.Chemie, 83 (1979) 3, 241–252.[8] A. R. Serrano and A. D. Pelton, Metall.Mater.Trans.B, 26B (1995), 305–315.[9] A. Semykina, V. Shatokha, M. Iwase, and S.Seetharaman: Metall. Mater. Trans. B, 41B (2010), 1230–1239.[10] A. Semykina, J. Nakano, S. Sridhar , V.Shatokha, and S. Seetharaman, Metall.Mater .Trans.B, 42B (201 1), 471–476.[11] A. Semykina, J. Nakano, S. Sridhar , V.Shatokha, and S. Seetharaman, Metall.Mater .Trans.B, 41B (2010), 940–945.[12] Z. Janjušević, Contact surface Metal-Mould, Fondation Andrejević, Belgrade, 2008, pp 48.[13] S.I. Filipov, Teorya metallurgicheskih procesov, Metallur-giya, Moskva, 1985, pp. 23-89.[14] Yu. P. Vasin, A. Y. Rasulov, Okisliteli novye protivprigar -nye materialy, Chelyabinsk, 1969, pp. 6.[15] O. Kubichevski, B. Gopkins, Okiselenie metalov i splavov, Metallurgiya, Moskva, 1985, pp. 12-35.Note:  The responsible for English language is the lecturer from ITNMS, Beograd, SerbiaFigure 4 a-h SEM – EDS images

Processes at the inetrface molten metal-sand mold

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