How To Study The Effects Of Section Size On The Content Of Ductile Iron

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1 2/13 Archives of Foundry, Year 4, Volume 4, 13 Archiwum Odlewnictwa, Rok 4, Rocznik 4, Nr 13 PAN Katowice PL ISSN EFFECT OF SECTION SIZE ON PRODUCING DUCTILE IRON CASTINGS S. BOCKUS 1, A. DOBROVOLSKIS 2 1 Kaunas University of Technology, Kestucio g. 27, Kaunas, Lithuania 2 Join-Stock Company Kauno ketaus liejykla, Kalantos 49, Kaunas, Lithuania SUMMARY The study was designed to investigate the effects of thickness on the dependent variables of matrix structure, chemistry and molten metal processing variables. This investigation has shown that section size has a significant effect on the amounts of pearlite, cementite and residual magnesium in ductile iron castings. Key words: ductile iron, section size, inoculation, metallic matrix. 1. INTRODUCTION Ductile iron is one of the more recent developments in cast iron technology and has been around since As the name suggests, it was developed to overcome the brittle nature of grey and white irons. The advantages of ductile iron which have led to its success are numerous, but they can be summarized easily versatility and higher performance at lower cost. The reason for ductile iron phenomenal growth is due to the microstructural fact that the graphite in ductile iron is spheroidal instead of flakelike. Similar to grey iron the matrix may be ferritic, pearlitic or martensitic. The pearlite content is varied from to about 95 per cent by the use of different heat treatment processes. The apparent variation in the properties with the pearlite level reveals the remarkable consistency in the relationship between mechanical properties and pearlite content. As the percentage of pearlite increases, the yield, ultimate tensile strengths and hardness increase and the impact toughness decreases. [1]. Traditionally, castings have been produced with thick walls (greater than 7 mm), which results in a large percentage of softer, more ductile ferrite [2]. Mechanical properties are dependent on the 1 Habil. dr. Prof.; address: stasys.bockus@ktu.lt 2 Dr. Assoc. Prof.; address: ketus@takas.lt

2 processing parameters, including the chemical composition, cooling rate, inoculation, solidification rate and many others variables [3]. Reducing the weight of ductile iron castings by producing thin-wall parts is an important method for saving energy in vehicles. The lower weight of iron used also reduces the energy needed for melting. Both of these aspects lead to reduced carbon dioxide emissions into atmosphere. But the high cooling rate in thin-section ductile iron results in increased amount of carbides with corresponding loss in mechanical properties, specifically ductility and toughness [4]. Many ferritic matrix castings are obtained by heat treating operations which add significantly to the cost of production. The use of the most common grades of ductile iron as-cast eliminates heat treatment costs, offering a further advantage. Achieving the full potential of ductile iron requires superior metallurgical process control, as well as the highest levels of skill in melting the ductile iron base, spheroidizing and inoculation [5]. There are two key processing steps that have been used for production ductile iron [6]. The first step introduces a nodularizing agent (such as magnesium) that creates the condition for the graphite to precipitate and grow in a nodular shape. If insufficient Mg is added or if the molten metal is held for an extended period after the Mg has been added, the graphite will not precipitate in a nodular shape. The second significant processing step is the inoculation. The inoculant is usually a ferrosilicon that contains small amounts of calcium and/or aluminium or other special purpose elements. The principal purpose of inoculant is to prevent chill. More specially, the inoculant enhances graphite nucleation, preventing the formation of primary carbides. Inoculation may take place at different process steps in combination or separately [7]. There is only limited date available regarding the procedures of manufacturing different thickness-wall castings for ductile iron. In some practices, for example, during producing heavy section castings [8], the spheroidized iron is poured with no inoculation. As outlined by A. Javaid et al. [4], pre-conditioning of the base iron increases the nucleation potential, and so minimizes the potential for primary carbide formation in the final iron. Techniques used to produce thin-wall castings will require increased control over melting and metal treatment [9]. The aim of this research is to evaluate the section size affecting the manufacturing technology and properties of ductile iron castings. 2. EXPERIMENTAL PROCEDURE Melts were prepared in the standard line frequency induction furnace of capacity the 1 t. Variables studied included composition, section size, base iron preconditioning, inoculation type and practice. The average composition of the melts was 3.76% C, 2.6% Si,.21% Mn,.5% Cr. The spheroidization process was been carried out in the special ladle [1] at about 15 C by tundish treatment. We used Ø13 mm wire modifier M7213 which contains 53 g/m magnesium and 94 g/m silicon. This is done primarily to reduce the violence of the reaction that occurs when the molten iron contacts the magnesium and to improve the working conditions. Inoculation was

3 Pearlite, % 21 performed by means of SB5 (68%Si, 1.5% Al, 1.5%Ca, 2.3%Ba) or Germalloy (7-78% Si, % Al,.3-1.5% Ca). The samples for metallographic examination and mechanical property evaluation were obtained from 4-, 8-, 12- and 24-mm sections of cross-shape block castings poured in CO 2 bonded silica sand molds. 3. RESULTS AND ANALYSIS The effect of section size on the pearlite content is given in Fig. 1. It appears that for identical conditions (i.e. four sections for the same casting of five experiments) the thinner sections contain more pearlite than thicker sections Section Thickness, mm Fig. 1. Effect of the section thickness on the content of pearlite. Rys. 1. Wpływ grubości ścianki na udział perlitu w osnowie. The influence of the section size on the cementite content is shown in Fig. 2. It is apparent that only thinner sections (<12 mm) sizes show a significant effect on the cementite content. The presence of the carbides, even in small amounts, drastically decreases the elongation values in thin sections [4]. The formation of primary carbides can be prevented by using a large amount of pig iron in the charge, high purity charge materials, the pre-conditioning of the base iron by addition of SiC and by inoculation in the stream and in the mold [11]. In inoculation experiments, two types of inoculant were investigated in terms of their effectiveness in decreasing the pearlite content in 12 and 24 mm sections. Comparison of the effectiveness of these inoculants is shown in Fig. 3. Inoculant type has a significant effect on the pealite content in 24 mm sections but no effect in 12 mm

4 Pearlite, % Cementite, % 22 sections..5% SB5 containing Si, Al, Ca and Ba was the most effective in terms of decreasing the pearlite content in 24 mm sections Section thickness, mm Fig. 2. Effect of the section thickness on the content of cementite.. Rys. 2.. Wpływ grubości ścianki na udział cementytu w osnowie Section Thickness, mm Germalloy.4 %SB5.5 %SB5 Fig. 3. Rys. 3. Effect of type of inoculant and section size on the pearlite content. Wpływ rodzaju modyfikatora I grubości ścianki na udział perlitu w osnowie.

5 Pearlite, % 23 The effect of carbon equivalent (CE) on the pearlite content in 4, 8 and 12 mm sections is given in Fig. 4. The composition of the melts made during investigation ranged from % carbon and % silicon. The increase in carbon equivalent results in a significantly decrease in the pearlite content in 12 mm sections, whereas the pearlite content in 4 and 8 mm sections do not significantly vary despite changes in the carbon content. The results of our industrial experiments show that silicon influences more significantly on the pearlite content than carbon Carbon Equivalent, % 12 mm 8 mm 4 mm Fig. 4. Rys. 4. Effect of carbon equivalent and section size on the pearlite content. Wpływ równoważnika węgla i grubości ścianki na udział perlitu w osnowie. 4. CONCLUS IONS The present investigation has shown that the section size of ductile iron castings has very strong effect on the contents of pearlite and cementite. It can be concluded that the effect of chemical composition shows a strong effect on the pearlite content in the thicker section. The type of inoculant shows a strong effect on the pearlite content in the thicker sections, while this effect in the 12 mm sections is much smaller. REFERENCES [1] M. Hafiz: Mechanical properties of SG-iron with different matrix structure. Journal of Materials Science, vol. 36, No 5, 1, pp

6 24 [2] K.K. Screms, J.A. Hawk, O.N. Dogan, A.P. Druschitz: Statistical Analysis of the Mechanical Properties of Thin Walled Ductile Iron Castings, presented at the 3 SAE World Congress [3] R. O Rourke: Cast Iron: The Engineering Metal. Advanced Materials & Processes, vol. 159, No 1, 1, pp [4] A. Javaid, K.G. Davis, M. Sahoo: Mechanical Properties in Thin-Wall Ductile iron Castings. Modern Casting, vol. 6,, pp [5] S.J Karsay: Ductile Iron Production Practices. Am. Foundrym. Soc. Inc. Des Plaines, Illinois, [6] G.M. Goodrich: Cast Iron Microstructure Anomalies and Their Causes. AFS Transactions, vol. 15, 1997, pp [7] C. Labrecque, M. Gagne: Review ductile iron: fifty years of continuous development. Canadian Metallurgical Quarterly, vol. 37, No 5, 1998, pp [8] Z. Ignaszak: The risk of ductile iron post-inoculation for heavy section castings. Materials Science (Medziagotyra), vol. 9, No 3, 3, pp [9] V.J. Krestyanov: Some conditions for producing nodular iron with a high complex of as-cast mechanical properties. Litejnoe Proizvodstvo, 1991, No 7, pp [1] S. Bočkus, A. Dobrovolskis: Feature of Ductile Iron Producing Technology. Materials Science (Medziagotyra), vol. 8, No 4, 2, pp [11] S. Bočkus, A. Dobrovolskis: Melting and Modification Aspects of Producing Ductile Iron Castings. Proceedings of the Estonian Academy of Science. Engineering, vol. 1, No 1, 4, pp STRESZCZENIE WPŁYW GRUBOŚCI ŚCIANKI NA STRUKTURĘ ODLEWÓW Z ŻELIWA SFEROIDALNEGO Opiniowany referat jest fragmentem szerokiej pracy badawczej, zmierzającej do określenia wpływu grubości ścianek na strukturę osnowy i właściwości mechaniczne surowych odlewów z żeliwa sferoidalnego. Niniejsza część badań pozwoliła wykazać istotny wpływ grubości ścianek odlewów na udział perlitu i cementytu oraz resztkową zawartość magnezu w odlewach, szczególnie w ściankach o większej grubości. Recenzent: prof. zw. dr hab. inż. Czesław Podrzucki.