The study was designed to investigate the effects of the charge, melting conditions, nodularizing and inoculation on the ductile iron castings properties. Results showed that the temperature and holding time of the melt in an induction furnace and the intensity of spheroidizing effect on the carbon and residual magnesium contents in the ductile iron castings. The same grade of ductile iron may be obtained using different chemical compositions. The castings of ductile iron will be ferritic as-cast only when large amount of pig iron in the charge and in addition some-steps inoculating treatment are used

A. Dobrovolskis

S. Bockus

English

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METALURGIJA 45 (2006) 1, 13-16 13S. BOCKUS et al.: EFFECT OF MELTING TECHNIQUES ON DUCTILE IRON CASTINGS PROPERTIESReceived - Primljeno: 2005-03-01Accepted - Prihvaćeno: 2005-09-01Original Scientifi c Paper - Izvorni znanstveni radISSN 0543-5846METABK 45 (1) 13-16 (2006)UDC - UDK 621.74:669.13.7=111 CKUS, A. DOBROVOLSKISS. Bockus, Kaunas University of Technology, Kaunas, Lithuania, A. Dobro-volskis, Join-Stock Company “Kauno ketaus liejykla”, Kaunas, LithuaniaEFFECT OF MELTING TECHNIQUES ON DUCTILE IRON CASTINGS PROPERTIESINTRODUCTIONDuctile iron is one of the most important engineering ma-terials, in view of its excellent castability, signifi cantly better mechanical properties and low cost [1, 2]. It represents the fastest growing segment of the iron market. In the medium term, it is to be expected that the market share of nodular iron will level off at 40 % to 45 % [3]. Achieving the full potential of ductile iron requires superior metallurgical pro-cess control, as well as the highest levels of skill in melting the ductile iron base, spheroidizing and inoculation [4].The fi rst step of the production of ductile iron castings is the careful selection of the charge materials. Manganese and chromium have the most infl uence on all mechanical properties [5]. For this reason, their concentration in metal is of particular importance. These elements arise in the charge from steel scrap, iron units and returns. It is recom-mended practice to purchase steel scrap so that the average Cr content remains below 0,1 percent. Ideally, the same advice would be given for Mn but, unfortunately, all steel scraps contain Mn, the majority being at the 0,5 percent level. The amount of steel scrap in the charge must be that The study was designed to investigate the effects of the charge, melting conditions, nodularizing and inocula-tion on the ductile iron castings properties. Results showed that the temperature and holding time of the melt in an induction furnace and the intensity of spheroidizing effect on the carbon and residual magnesium contents in the ductile iron castings. The same grade of ductile iron may be obtained using different chemical composi-tions. The castings of ductile iron will be ferritic as-cast only when large amount of pig iron in the charge and in addition some-steps inoculating treatment are used.Key words: ductile iron, melting, inoculation, propertiesUtjecaj postupaka taljenja na svojstva odljevaka od nodularnog lijeva. Studija je namijenjena istraživanju utjecaja uloška, uvjeta taljenja, nodulariziranja i cijepljenja na svojstva odljevaka od nodularnog lijeva. Rezultati su pokazali kako temperatura i vrijeme zadržavanja taljevine u indukcijskoj peći te intenzitet nodulariziranja utječu na sadržaj ugljika i zaostalog magnezija u odljevcima od nodularnog lijeva. Ista vrsta nodularnog lijeva može se dobiti korištenjem različitog kemijskog sastava. Odljevci od nodularnog lijeva bit će feritni u lijevanom stanju samo ako se koristi velika količina sirovog željeza u ulošku i, osim toga, određeni korak inokulacijske obrade.Ključne riječi: nodularni lijev, taljenje, inokulacija, svojstvaamount which will give castings that are as free of carbides as possible [4]. It is particularly important for the produc-tion of ferritic ductile iron [6]. Charge materials result in the average size of graphite spheroids. The amount of steel scrap affects metallic matrix structure too. It increases the pearlite formation [7]. The graphite structure is affected by the carbon content too. If initial metal does not contain the amount of the carbon enough then graphite particles are of compact form [8]. The carbon equivalent affects the grain size too [9]. The metallic matrix structure is affected not only by carbon equivalent but by C/Si ratio too. Increasing this ratio in ductile iron decreases the proportion of ferrite and increases the proportion of pearlite [10].All equipment in use for melting steel, grey or malleable iron can be used to melt ductile iron base. Each type has its advantages and drawbacks in a situation. The vast majority of ductile irons cast today are melted in cupolas and induc-tion melting furnaces [11]. Ductile iron is particularly prone to the formation of primary carbides during solidifi cation [12]. One of reason for this susceptibility is high superheat temperatures [13]. Increasing of holding time in a furnace increases the number of primary carbides too [14].The addition of magnesium or magnesium alloy to cast iron with the purpose of changing graphite shape from fl ake to spheroidal is an essential processing step for manufactur-METALURGIJA 45 (2006) 1, 13-1614S. BOCKUS et al.: EFFECT OF MELTING TECHNIQUES ON DUCTILE IRON CASTINGS PROPERTIESing ductile iron. The choice of a treatment method (open ladle, sandwich, tundish cover, in-mould, plunger, converter, injection and others [15]) for an individual foundry must be based on the circumstances present in foundry.Inoculation is a necessary step for production of ductile iron castings. It may take place at different process in com-bination or separately. Inoculation is a procedure whereby the microstructure and properties of cast iron are controlled by increasing the number of nucleating sites for eutectic solidifi cation. Several variables infl uence the effectiveness of inoculation: charge materials, temperature of the melt, inoculant, time, casting method, section size, mode of inocu-lation, and others. It is known about 1500 inoculants. Most inoculants are ferrosilicons. Inoculation has a vital role to play in the continuing progress of ductile iron [16].This very brief survey shows that, although considered as a mature technology, recent process and product devel-opments open new avenues to this family of materials. In this work, efforts have been focused on the study of change in chemical composition of the melt from its temperature and holding time in a furnace and on the effect of inocu-lation on the ductile iron castings matrix microstructure and properties.EXPERIMENTAL PROCEDUREThe metal was melted in the standard line frequency induction furnace of capacity the 10 t. We used pig iron, mild steel; return scrap, ferrosilicon FeSi75 and carbu-rizing agent as the charge materials. Variables studied included composition, section size, base iron pre-con-ditioning, inoculation type and practice. The average composition of the melts was 3,76 % C, 2,6 % Si, 0,21 % Mn, 0,05 % Cr. Chemical composition of the cast iron has been determined by spectrometer Spektrolab, liquid metal temperature has been measured by device Mikron and thermometer W50A with S type thermocoupl (Pt and 10 per cent Rt-Pt). The spheroidization process was been carried out in the special ladle [18] at about 1500 °C by tundish treatment. Spheroidizing was performed by means of 2,1 % of FeSiMg7. After spheroidizing, the melt was poured into the casting ladle. We investigated the infl uence of the time of spheroidizing treatment in the ladle on the graphite content in the liquid ductile iron. Three kinds of sphero-idizing process have been studied: very fast (with evaporation and burning of the magnesium) (4 seconds), medium (15 seconds) and very slow (100 seconds).Inoculation was performed by means of SB5 (68 % Si, 1,5 % Al, 1,5 % Ca, 2,3 % Ba). The metal stream, ladle and in-mould inoculation methods were applied. During in-mould inoculation the inoculants was placed in a reaction chamber within the gating system of the individual mould. Standard test pieces for microstructural and mechanical properties testing were machined from the samples which were cast separately in green sand moulds under representa-tive spheroidization and inoculation treatments. Test pieces were prepared in accordance with LST EN1563:2001. Effect of section size on the metallographic structure was carried out with 6 - 30 mm-thick sections. During the tests the metallic matrix was only examined because it is known [6] that graphite spheroids are larger and usually less well formed in heavy sections than in thin ones.RESULTS AND ANALYSISOn the basis of experimental fi ndings, the initial chemical composition of the melt depends on its temperature and hold-ing time in a furnace. Figure 1. shows the change of the carbon content of melt held in a furnace. It can be seen that carbon contain decreases from 4,00 to 3,72 percent (at temperature of 1480 °C) or to 3,61 percent (at temperature of 1580 °C) if melt is held in the furnace 2,5 hours. But, as Litovka et al [14] have observed, the effectiveness of spheroidizing process METALURGIJA 45 (2006) 1, 13-16 15S. BOCKUS et al.: EFFECT OF MELTING TECHNIQUES ON DUCTILE IRON CASTINGS PROPERTIEShad decreased with increasing holding time in the furnace of melt. The decrease in effectiveness of spheroidizing process of cast iron contained less amount of carbon has been also observed by [8]. Therefore, it is introduced the inoculation in the furnace and in the ladle before spheroidizing treatment.Carbon content depends also on the intensity of the spheroidizing. Figure 2. shows the decreasing of it with increasing of spheroidizing time.The spheroidizing intensity effects also on content of the residual magnesium. This content is the highest (about 0,05 percent) when the spheroidizing continuance is about 15 seconds (Figure 3.). When the spheroidizing continuan-ce is very short (4 s) or very long (100 s) then residual magne-sium contents are 0,024 and 0,027 percent, respectively.The effect of the amount of pig iron in the charge on the mechanical properties of castings (tensile strength and elongation) is summarized in Figure 4. It is evident, that ferritic ductile iron requires larger amount of pig iron. Areas 1 and 2 represent the stable processes. Kapilevich et al. [18] have also obtained the similar results. They reported that the amount of pig iron in the charge of fer-ritic ductile iron grade 400 as-cast must be higher then 53 percent as in the present work. But there are some differ-ences between their and our results with the other grades. They recommended 34,5 percent of pig iron for the grade 500 and 31 percent for the grade 600. We did not get the stable results with that amounts of pig iron.Figure 5. presents the effect of chemical composition on ductile iron grades. It is seen that the same ductile iron grade can be produced using different chemical composi-tions. These results are very useful for the manufacturer for producing the desirable ductile iron grade. For instance, ductile iron with Rm = 500 MPa can be manufactured using metal No 2 (Figure 5.), which containing 0,0375 % remain magnesium Mgrem or No 3, which containing 0,032 % Mgrem. Ductile iron with Rm = 700 MPa we can get with metal No 4, which containing 0,058 % Mgrem or with metal No 5, which containing 0,052 % Mgrem. Data of Figure 6. show that the variant No 1 is an optimum chemical composition of metal for getting ductile iron with Rm < 450 MPa. The variant No 2 is suitable for getting ductile iron with Rm = (450-500) MPa, variant No 3 - for getting Rm = (500-600) MPa, variant No 4 - for getting Rm = (600 - 700) MPa and variant No 5 - for getting ductile iron with Rm > 700 MPa.The chemical composition of the ductile iron varies with the matrix structure. For instance, the amount of ferrite in the structure of the ductile iron No 1 (Figure 5.) METALURGIJA 45 (2006) 1, 13-1616S. BOCKUS et al.: EFFECT OF MELTING TECHNIQUES ON DUCTILE IRON CASTINGS PROPERTIESis (100 - 94) %. Ductile iron No 2 has (80 - 94) % ferrite in its structure, No 4 has (35 - 45) %, and ductile iron No 5 has (20 - 30) % ferrite in its structure. The effect of the section size on the amount of ferrite in the ductile iron castings is shown in Figure 6. It clearly shows that it is very diffi cult to produce thin section of ferrite ductile iron as-cast. This‘ is associated with high solidifi cation rate and formation of carbides.The manufacture of thin section ferritic ductile iron castings presents all the processes common for other cast-ings plus one more, complex inoculation. Figure 7. shows the various methods of the melt treatments have been used during these investigations. The results of industrial experiments show that the most effective treatment technique of the melt for the pouring 6 - 12 mm-thick ferritic as-cast sections consists of four steps. It is j-variant (Figure 7.): inoculation in the treatment ladle, spheroidizing in the treatment ladle, inoculations the treatment ladle stream during fi lling a pouring ladle and in the mould or k-variant: inoculation in the treatment ladle, spheroidizing in the treatment ladle, REFERENCES [1] B. I. Imasogie, A. A. Afonja, A. Ali, Scandinavian Journal of Metallurgy 30 (2001) 2, 91 - 102. [2] N. Fatahalla, S. Bahi, O. Hussein, Journal of Materials Science 31 (1996) 21, 5765 - 5772. [3] K. Urbat, Proceedings, Conference: Balance Opening - the European Foundry Industry 2004 and beyond economic facts and consequences of the EU enlargement, Kielce, Poland, 2003, p. 24 - 35. [4] V. A. Chaykin, V. V. Ishutin, Lit’jo i metallurgija 4 (2002), 71 - 75. [5] S. J. Karsay, Ductile Iron Production Practices, American Foundry-men’s Society, Des Plaines, Illinois, 1994, p. 78 - 88. [6] C. F. Yeung, H. Zhao, W. B. Lee, Materials Characterization 40 (1998) 4 - 5, 201 - 208. [7] V. J. Krestyanov, Liteynoje Proizvodstvo 11 (1998), 7 - 8. [8] I. A. Shaprikov, Liteynoje Proizvodstvo 11 (1992), 6 - 7. [9] G. Rivera, R. Boeri, J. Sikora, International Journal of Cast Metals Research 16 (2003) 1 - 3, 23 - 28. [10] L. A. Solncev, et al., Liteynoje Proizvodstvo 6 (1976), 3 - 4. [11] N. M. Sytnik, Metal Science and Heat Treatment 33 (1991), 317 - 322. [12] G. M. Goodrich, AFS Transactions 105 (1997), 669 - 683. [13] C. Labrecque, M. Gagne, Canadian Metallurgical Quarterly 37 (1998) 5, 343 - 378. [14] V. I. Litovka, A. A. Sheyko, B. G. Zeliony, Liteynoje Proizvodstvo 4 (1983), 13 - 14. [15] Y. S. Lerner, Foundry Trade Yournal 177 (2003), 19 - 21. [16] R. Carl, Jr. Loper, Proceedings, 65th World Foundry Congress, World Foundrymen Organization, Gyeongju, Korea, 2002, p. 169 - 179. [17] S. Bockus, A. Dobrovolskis, Materials Science (Medziagotyra) 8 (2002) 4, 372 - 374. [18] A. N. Kapilevich, et al. Liteynoje Proizvodstvo 3 (2001), 4 - 6.inoculations in the stream while pouring and in the mould. However, heavy ductile iron castings need only two steps, for example, a-, c-, d- or l-variant.Based on research results, ferritic as-cast ductile iron castings with 6 - 12 mm wall thickness have been intro-duced into production.CONCLUSIONSIt was found by industrial experiments that the initial chemical composition of the melt was changed from its temperature and holding time in an induction furnace. High temperature and long holding time decreases carbon content but increases silicon one. Chemical composition of the melt changes from intensity of spheroidizing process too. Very intensive and very slow spheroidizing process decreases car-bon and residual magnesium contents in the ductile iron.It was observed that microstructure of ductile iron cast-ings depends on the charge, chemical composition and sec-tion size. The castings of ductile iron will be ferritic as-cast only when more than 50 percent of pig iron in the charge is used. However, the producing of as-cast ferritic thin-sec-tion ductile iron castings needs in addition several steps inoculating treatment. The same grade of ductile iron may be obtained using different chemical compositions. Results of the present investigation give some recommendations how to produce the desirable grade of ductile iron.

Effect of Melting Techniques on Ductile Iron castings Properties

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