OBTAINING THE SPHEROIDAL GRAPHITE IN CAST IRON BY ELECTROLYSIS OF THE SLAG OF THE MGO MGF

Transcription

1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 9, Issue 2, February 2018, pp , Article ID: IJMET_09_02_081 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed OBTAINING THE SPHEROIDAL GRAPHITE IN CAST IRON BY ELECTROLYSIS OF THE SLAG OF THE MGO MGF 2 CAF 2 NAF SYSTEM IN THE ELECTROSLAG PROCESSING OF MOLTEN CAST IRON A.N. Frumkin Institute of Physical Chemistry and Electrochemistry of the Russian Academy of Sciences, Laboratory of Highly Concentrated Disperse Systems, 31, Leninsky Prospect, Moscow, , Russia; Lomonosov Moscow State University, Faculty of Global Studies 1, building 51, Leninskiye gory, Moscow, , Russia ABSTRACT The results of electrochemical research of the processes occurring in the molten metal-slag system during electroslag treatment of cast iron have been presented. It is established that electrolysis of slags is of practical importance for electroslag processing. Upon electroslag remelting at high current densities, electrolysis of slags allows for transfer of various (mainly alloying and modifying) elements into the metal from the slag. The role of electrolysis in the electroslag process has been shown. Author has proposed and implemented a new method of electrolysis, according to which, the surface of slag is isolated from the oxidizing atmosphere by the layer of carbonaceous materials. A method for producing high-strength nodular cast iron using electrolysis of slag containing magnesium oxides and fluorides has been suggested and tested. The possibility of obtaining the spheroidal graphite in cast iron by electrolysis of slag for the first time has been proved experimentally. Key words: alloys, casting, heat treatment, electrochemistry, high-strength nodular cast iron, electroslag treatment Cite this Article:, Obtaining the Spheroidal Graphite in Cast Iron by Electrolysis of the Slag of the MGO MGF2 CAF2 NAF System in the Electroslag Processing of Molten Cast Iron, International Journal of Mechanical Engineering and Technology 9(2), 2018, pp INTRODUCTION The last few decades have seen the development of electroslag processes (ESP), during which the alloying elements are introduced [1, 2] and the properties of alloys are significantly improved [3]. ESP is applied for obtaining special alloys [4], alloying [5], and utilization of valuable alloys [6] editor@iaeme.com

2 In the foundry industry, obtaining high-quality cast irons, including cast iron with spheroidal graphite, is of special interest. The goal of the research was to determine the possibility of influencing the chemical composition and the properties of cast iron microstructure by using electrochemical treatment. 2. MATERIALS AND METHODS The materials and methods of the research were identical to the ones commonly used for similar research. The author used cast iron with typical gray cast iron composition: % of С, % of Si, 0.6% of Mn, up to 0.1% of S, up to 0.1% of P. A pilot furnace has been designed by the author (Fig. 1). The slag of the MgO MgF 2 CaF 2 NaF system consisted of commonly used chemical compounds. Figure 1 Furnace for electroslag treatment of cast iron: sectional view (a), and a picture of pilot electroslag furnace (b). 3. RESULTS Electroslag treatment of cast iron was invented by the author in the late 1970s [7]. Over the past years, a surge of interest has been observed as to the methods of production of highquality alloys for castings, and the electroslag method is one of such methods. The interested enterprises have appeared [4, 8]. During , research of electroslag treatment of cast iron was resumed at the Institute of General and Inorganic Chemistry of the Russian Academy of Sciences for the purposes of studying the electrochemical processes occurring in the metal-slag system. We were actuated to conduct this research due to the fact that there is an interest in electroslag treated cast iron melted in gas cupolas. The author of the article is the author of both the gas cupola and the electroslag treated cast iron [9]. Electrolysis of slags may be of practical importance for electroslag treatment processes. During electroslag remelting under high current densities, electrolysis makes it possible to transfer various elements (mainly alloying and modifying ones: V, Nb, W, Ti, Mg, Cr, Ni, Си, Mo, В) from slag into metal [9 11]. At that, the attention of most researchers has been editor@iaeme.com

3 Obtaining the Spheroidal Graphite in Cast Iron by Electrolysis of the Slag of the MGO MGF2 CAF2 NAF System in the Electroslag Processing of Molten Cast Iron attracted by remelting processes, where the end result is influenced by two factors: slag treatment of melt, and rapid solidification in water-cooled mold. Such process is hardly applicable for cast iron, maybe, except for continuous casting. The goal of the present research was further study of the role of electrolysis during the ESP with regard to electroslag treatment of molten cast iron with the subsequent casting. Experiments were conducted in a single-phase single-electrode furnace. The flux used for the first experiments had the following composition: 43% of CaF 2, 43% of MgF 2, 9% of MgO, 5% of NaF. The results of experiments are given in Table 1. Melting Table 1 Changes in chemical composition of cast iron during electroslag remelting under flux containing 43% of CaF 2, 43% of MgF 2, 5% of NaF, 9% of MgO Current and mode Charge/ Sample 2-53 Alternate 3.3/ Alternate 3.3/ Direct (metal cathode) Alternate (with introduction of 2-3% of carbon scrap into the slag) Content of elements, % C Si Mn Loss, Charge/ Loss, % Charge/ % of Sample of Sample initial initial 3.2/ / / / / / / / / / 0.63 Loss, % of initial Mg content after treatment Traces Shape of graphite and structure of cast iron P, C (40%) and separate inclusions of graphite G2, Gf1 P, C (40%) inclusions of graphite G2, Gf1 P, C (40%) inclusions of graphite (separate flake inclusions) G2, Gf8 P85, G4, Gd2, Gf2 Note: Mg is not recovered in sufficient quantities, spheroidal graphite is not formed. P pearlite; C cementite; G graphite content; Gf graphite form; Gd graphite distribution. G2 2% of graphite, G4 4% of graphite; Gf1 linear laminar graphite; Gf2 bumpy laminar graphite; Gf8 flake graphite; Gd2 uneven distribution. The provided data show that electroslag treatment of cast iron under this flux results in high losses of elements, Mg is not recovered in sufficient quantities, spheroidal graphite is not formed. After annealing, flake graphite was formed in experiment Certain spheroidal inclusions were observed. This cast iron cannot be designated as ductile cast iron. Significant losses of elements during electroslag treatment under oxide-fluoride melts occur due to high transport characteristics of these melts. Fe 2+ ions are recharged (Fe 2+ Fe 3+ ) on the metalslag and slag-gas interface surfaces. This process, associated with the consumption of electric current and transfer of oxygen from the atmosphere into metal, may be considered unfavorable. Active oxidation is confirmed by the analysis of slag after melting. It showed the content of up to 10.7% of SiО 2, and % of FeO. High content of SiО 2 and low residual editor@iaeme.com

4 content of FeO indicate an active process [Si] + (2FeO) = [2Fe] + [SiO 2 ]. At that, the role of electrolysis is completely imperceptible. During deoxidation of slag (experiment 18-4) there was no loss of carbon, and the loss of manganese was reduced. The loss of Si remained pretty high. However, the role of electrolysis was notable (experiment 04-5). It should be noted that the content of S reduced from 0.12 to 0.02%. Also, flux contained many fluoride compounds producing hazardous emissions (Table 1). In order to determine the role of electrolysis, experiments in electroslag unit with a "witness" have been conducted, as shown in Fig. 2. At that, in Compartment 1, the same processes occur as in Compartment 2, as well as the processes associated with electrolysis, whereas in the "witness", only electroslag processes not associated with electrolysis occur. Figure 2 Unit for studying the role of electrolysis during electroslag treatment of molten cast iron Alternate current experiments showed practically the same changes in the chemical composition of the "witness" and that of the sample. Direct current treatment conducted for various periods of time at the same temperature showed that electrolysis influenced mass transfer for all core elements of cast iron. In all cases, when passing direct current (metal cathode), a reduction of the elements losses and an increase in the sulfur losses are observed. Certain increase in the carbon losses is apparently associated with anodic oxidation of sulfur and general behavior of anodic processes. The use of direct current in the presence of oxides of the respective element in the slag allows the transition from the loss of element to the pick-up of element. Thus, chemical composition can be adjusted by adding the calculated quantity of the respective oxides according to current efficiency thereof into the slag:, (1) editor@iaeme.com

5 Obtaining the Spheroidal Graphite in Cast Iron by Electrolysis of the Slag of the MGO MGF2 CAF2 NAF System in the Electroslag Processing of Molten Cast Iron where M Me is the weight of material being treated; М SiO2 is the molecular weight of SiO 2 ; Si req is the required increase in Si content as to cast iron; A Si is the atomic weight of Si; ƞ Si is the current effervescive of Si; К a y is the active recovery coefficient. The required increase in carbon content as to cast iron Si req is calculated from equation:, (2) where Si M and Si sh are the required Si content in the cast iron and in the original cast iron, Y Si is the loss of Si during electroslag treatment without electrolysis. Based on the experimental data, it was determined that for Si: К a y = 0.4. Current effervescive, as shown during the experiments conducted in the cells at a low content of Si in flux, amounts to 15-20%. Therefore, ƞ Si К a y = 0.08, i.e. the efficiency of recovery of Si does not exceed 10%. At that, the electrical power consumption for reducing the loss from 15% to 0% of initial content amounts to 140 kw per 1 t/h. Considering the fact that during electrolysis of the systems containing (due to the loss) the oxides of Fe, the Fe current yield will primarily occur, it will become apparent that electrolysis under these conditions is unprofitable. The author has suggested and applied a new electrolysis method, according to which the upper surface of slag is isolated from the oxidizing atmosphere by a layer of carbonaceous materials. The electrolysis diagram is shown in Fig. 3. Pieces of broken electrodes of various sizes are floating on the surface of the slag; from the top, they are blocked from the influx of oxygen by 2 to 3 rows of carbonaceous materials. Thus, local areas with a reducing atmosphere are created on the metal-gas interface surface, so that mixed valence ions recharging processes may not occur. Figure 3 Electroslag treatment in oxygen-limited mode: a test cell, b electrochemical mechanism of phases interaction. The experiments have proven that losses of elements are practically eliminated; furthermore, Si is transferred into metal. This is explained by the transfer of SiO 2 from the lining of the unit, where the experiments were conducted, into slag. At that, cast iron is carbonized, and the loss of sulfur increases editor@iaeme.com

6 Based on the results of the experiments, it was considered expedient to deoxidize slag using heterogeneously-injected carbon, to eliminate losses of primary elements of cast iron, and, if necessary, to perform carbothermic recovery of elements by introducing the respective oxides into the slag. However, the mechanism of electrolysis new mode involves not only deoxidation of the slag. Under this mode, oxygen is "not allowed" to reach the slag-gas interface surface, so that an unfavorable process of mixed valence ions recharging may be liquidated. It is expedient to carbothermally recover all elements which, under the conditions of electrical slag treatment of cast iron, have higher potential, i.e. its absolute value lower than Si. Such elements М include V, Mn, Cr, Na, Zn, К, Р, Sn, Со, Ni, Cu. It is expedient to transfer the elements located below Si at the G Т plot (Ti, Al, Mg, Ca) into cast iron using electrolysis. Electrolysis may prove to be the most effective for extracting modifying elements, in particular Mg. The author has performed electrolysis of MgO MgF 2 GaF 2 system in oxygenlimited mode. The flux with high content of MgO has been chosen, as this material is most easy to obtain on an industrial scale. By way of introducing carbon into molten slag at a ratio of 1:1 to the weight of the slag, oxygen has been blocked from reaching the slag-gas surface. The results are shown in Table 2. Table 2 Electrolysis of magnesium-containing slag in oxygen-limited mode Experimental conditions Chemical composition of cast iron Shape of graphite Initial cast iron Flake Flux: 40% of Mg, 40% of MgF 2, 20% of CaF 2, and carbon scrap 1:1 of the flux weight Under direct current Spheroidal Under pulse current Spheroidal The presented data prove that the best results are obtained when using a direct pulse current electrolysis. Apparently, this may be explained by a decrease in polarization during no-current periods. The structure of the obtained nodular cast iron is shown in Fig.4. For the first time, the spheroidal shape of graphite has been obtained by electrolysis of slag. Figure 4 Microstructure of high-strength cast iron obtained by electrolysis of slag, x editor@iaeme.com

7 Obtaining the Spheroidal Graphite in Cast Iron by Electrolysis of the Slag of the MGO MGF2 CAF2 NAF System in the Electroslag Processing of Molten Cast Iron High-strength nodular cast iron has been produced in the form of as-cast iron, without heat treatment, under regular cooling rates during a non-remelting electroslag process with nonconsumable electrode. 4. CONCLUSIONS The electroslag treatment proposed by the author produces significant effect on the composition, structure, and properties of cast iron. Electrolysis of slag as to the MnO MgF 2 CaF 2 system proves to be the most effective. The possibility of obtaining spheroidal graphite in cast iron by means of electrolysis of slag has been experimentally proved for the first time ever. The author proposed and implemented a new method of electrolysis, in accordance with which the upper surface of the slag is isolated from the oxidizing atmosphere by a layer of carbonaceous materials. REFERENCES [1] Halfa, H. and Reda, A.M. Electroslag Remelting of High Tecnological Steels. Journal of Minerals and Materials Characterization and Engineering, 3, 2015, pp [2] Bandyopadhyay, T.R., Rao, P.K. and Prabhu, N. Behavior of Alloying Elements during Electro-Slag Remelting of Ultrahigh Strength Steel. Metallurgical and Mining Industry, 4(1), 2012, pp [3] Bandyopadhyay, T.R., Rao, P.K. and Prabhu, N. Imrovement in Mechanicak Properties of Standard 15CDV6 Steel by Increasing Carbon and Chromium Content and Inoculation with Titanium during ESR. International Scholarly Research Network, [4] Losertova, M. Technology of special alloys. VSB Technical University of Ostrava. Alloys.pdf. [5] Basu, S. and Kumar, D. Use of Electro-Slag Refining for Novel in-situ Alloying Process in Steel in Steel, 2nd International on Emerging Trends in Engineering and Technology (ICETET 2014), London (UK), May 30-31, 2014, pp [6] Mattar, T. Electro-Slag Remelting of AISI M41 High Speed Tool Steel Scrap. 6TH International Tooling Conference, pp [7] Grachev, V.A., Gorelov, N.A., Chernykh, A.A., et al. Electroslag furnace for the treatment of cast iron. Certificate of authorship No (Application as of , published on , BI 43(45) ). [8] Ren, Y., Zhang, L. and Yu, L. Precipitation of (Zr, Hf)C in FeCrAlYHfZr alloys during electroslag remelting process. Metallurgical Research and Technology, 114(1), 2017, pp. 8. [9] Grachev, V.A. Basic physics and chemistry of cast iron smelting. Moscow: The A.N. Frumkin Institute of Physical Chemistry and Electrochemistry of the Russian Academy of Sciences, 2016, pp [10] Leninskikh, B.M. and Manakov, A.I. Physical chemistry of oxide and oxide-fluoride melts. Moscow: Nauka, 1977, pp [11] Manakov, A.I. Electrochemical recovery of elements from oxide and oxide-fluoride melts. Abstract of dissertation for the academic degree of doctor of technical sciences. Kiev, 1975, pp editor@iaeme.com