EVALUATION OF DUCTILE IRON PRODUCED USING ROTARY FURNACE WITH VARIABLE COMPOSITIONS OF MAGNESIUM ADDITION

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1 EVALUATION OF DUCTILE IRON PRODUCED USING ROTARY FURNACE WITH VARIABLE COMPOSITIONS OF MAGNESIUM ADDITION OYETUNJI Akinlabi Dept of Metallurgical and Materials Engineering, The Federal University of Technology Akure Ondo State NIGERIA OMOLE S.O. Dept of Metallurgical and Materials Engineering, The Federal University of Technology Akure Ondo State NIGERIA Abstract: Automobile scrap of engine block was the scrap used and initially analyzed with optical light emission spectroscopy for its chemical composition. Forty kilograms (40 kg) of the engine block (grey cast iron) was charged with 0.5 kg graphite (in powder form) and 2 kg calcium tri-oxo carbonate (iv) salt (CaCO 3 ) (flux) into a rotary furnace of 100 kg capacity fired with diesel fuel. The charges in the rotary furnace were heated to a tapping temperature of 1420 o C using inserted thermocouple with temperature controller to monitor the temperature kg calcium carbide (CaC 2 ) was added to the charge in the furnace after 1420 o C melt, before tapping for desulphurization. Three consecutive tappings were made designated A, B and C poured into the prepared steel ladle via sandwich process. 3 %w/w magnesium (Mg), 2.5 %w/w Mg and 1.5 %w/w Mg by weight of the metal treated in the ladle was used to treat melts in ladles A, B and C respectively. Thereafter, the treated melts was poured into already prepared moulds where solidification took place and the as-cast ductile iron of diameter 20 mm by 200 mm length was produced. The as-cast cast iron was also analyzed with mass spectrometric analyzer machine to determine its chemical compositions and its confirmed that the % Carbon and Silicon were within the range of cast iron. Then, tensile, hardness and micro structural analysis were carried out on the as- cast ductile iron using standard methods and machines. The microstructures revealed that as-cast ductile iron from steel ladles A and B contained nodules both un-etched and when etched with 2 % Nital while cast produced from melt in ladle C revealed a flake graphite cast iron. Keywords- Evaluation, sandwich, mechanical properties, metallographic examination, ductile iron, rotary furnace I. INTRODUCTION Ductile iron also known as Spheroidal Graphite Iron (SGI) and Nodular iron consists of evenly distributed graphite spheroids (nodules) in a steel matrix, which may be completely ferritic or pearlitic in the ductile grades, martensitic in the strong and hard grades, or bainitic in the strong and tough grades, its microstructure has been found to occupy lofty position in the globe of engineering materials applications [1]. It is now replacing cast iron (grey and white iron) or cast steel as a result of many advantages it possesses. Some of these advantages are; good castability, low manufacturing and material cost as well as their good machinability. These advantages have led to expansions of its applications throughout the manufacturing industry [2]. Ductile iron has significantly good combination of tensile strength, ductility and toughness, along with good wear resistance and hardenability [3]. Because of even distribution of graphite nodules and the variations in the matrix structure, the mechanical properties of ductile cast irons vary in a wide range of values. Ductile iron is made by treating liquid iron of suitable composition with magnesium before casting [4]. The magnesium promotes the precipitation of graphite in the form of discrete nodules instead of interconnected flakes. As a result of the nodules formed, the material has high ductility, thereby allowing casting to be used in critical applications such as automotive transmission components e.g. sprocket, crankshaft and connecting rod [4]. The family of ductile cast iron covers a wide range of mechanical properties, replacing successfully both cast and forged steel and malleable cast irons in many applications (such as wheels, gears, crankshaft in cars and trucks)[5]. Matrix plays a key role determining the overall properties combination and allowing obtaining high ductility values (up to more than 18% rupture elongation) and high strength (up to 850 MPa and, considering austempered ductile iron up to 1600 MPa) with a good wear resistance [5]. Matrix names are usually used to designate ductile cast iron types. All the reviewed works studied, worked on ductile iron but did not use rotary furnace to melt the charges but rather used either induction furnace or electric arc furnace. This induction/ electric arc furnace can not be afforded by most small and medium scale foundry industries that need to produce ductile iron. This is a big problem which this research work had tried to solve through the use of rotary furnace with sandwich method for the benefit of small and medium scale foundry industries; this is the rationale behind this work. The aim of this work is to produce ductile iron using rotary furnace and grey cast iron scraps as base metal and then evaluate the ductile iron produced. 276

2 Materials (b) Mould Preparation Wooden cylindrical pattern of diameter 20 mm by 200 mm Wooden cylindrical pattern made from mahogany tree of length as shown in Fig. 1 was used to prepare sand mould diameter 20 mm by 200 mm length was used as pattern to made of silica sand. The moulding sand used was prepared by prepare the mould in which molten metal was poured. Engine adding 12 % bentonite as a binder and 8 % water into silica block (grey cast iron scraps) from automobile engine was used sand in line with work of Hassan et al [7]. The mould was as base metal and magnesium alloy for treating the metal, prepared using cope and drag by ramming the prepared sand graphite (in powder form), calcium carbide (CaC 2 ) used as round the pattern, to create an impression or cavity upon desulphuriser and calcium trioxo carbonate (iv) salt (CaCO 3 ) which the molten metal was poured after the withdrawn of used as flux, 100 kg capacity rotary furnace was used for pattern from the sand. This was done in accordance with melting, diesel fuel for firing, and inserted thermocouple with temperature controller to monitor the temperature. standard moulding procedure. Equipment Mass spectrometric analyser machine for its chemical composition, Rotary furnace was used to melt the grey cast iron which is the base metal to be converted to ductile iron. Buehler metaserv 2000 grinder and polisher were used to prepare the metallographic samples. AP 2000MTI Metallurgical microscope with Daheng digital camera having a maximum magnification of X800 was used to take the photomicrographs. Instron universal tester (for tensile test), and Rockwell hardness tester was used to measure the hardness value of the samples. Methods The following are the methods used in carrying out the research work. (a) Chemical Analysis of the Scraps. The scraps of automobile engine block were analyzed using optical light emission spectroscopy to determine the average chemical composition of the elements present in them. Cast to confirm their properties before proceeding for further tests. Three sparks test was performed on each sample at the two heads and at the middle points, and average of the three readings were taken and recorded as shown in Table 1, This test was done in accordance with ASTM E50-00 (2005) [6] standard. The chemical composition of the inoculant was already given by certificate that accompanied the products from the manufacturer as shown in Table 2. TABLE 1. COMPOSITION OF THE SCRAP USED Balance Table 2: Composition and Size of Mg-Fe-Si Inoculant Elements % Compositions Size of The Inoculant Si 42% Mg 5.00% 19 mm Ce 0.56% Ca 1.02% Al 0.62% La 0.45% Fig 1: Cylindrical Pattern with Dimensions in mm (c) Charge Preparation The scrap used was analyzed using optical light emission spectroscopy to determine the average composition of the elements present in the scrap before used. 40 kg of engine block (grey cast iron) scrap of the following compositions was used: 3.97 %C, 1.94 %Si, %S, and %P, 0.5 kg of graphite and 2.0 kg of calcium carbonate were charged into the rotary furnace of 100 kg capacity. Meanwhile, a theoretical charge calculation shown in Table 3 were prepared from using both Gaussian elimination technique and long year experience and these compositions were arrived at for the production of various compositions of ductile iron. They were done in accordance with Oyetunji s work [8]. Table 3: Theoretical Charge Calculation MATERIALS QUANTITY CHARGED kg %C %Si %S %Mg Grey Cast Iron Graphite Calcium Carbide From Ladle additions First 0.45 _ 1.26 _ 0.15 Tapping(MgFeSi) Second _ 1.05 _ Tapping(MgFeSi) Third _ 0.6 _ tapping(mgfesi) 20% loss in the furnace. Total First Tapping in the base metal. Second Tapping Third Tapping

3 (d) Melting and Casting The prepared charges with different weights were charged into 100 kg rotary furnace for heating and melting. They were heated to a tapping temperature of 1420 o C, and 0.25 kg CaC 2 was added to desulphurise some sulphur in the furnace before tapped into the steel ladle (lined with refractory material), built to conform to standard sandwich process of adding magnesium (Mg) into melt. 3 %w/w Mg, 2.5 %w/w Mg and 1.5 %w/w Mg of weight of base metal for treatment in the ladle were used for three consecutive tapping designated A, B and C respectively. The treated metal was then poured into the prepared mould to produce a cylindrical bar of 20 mm diameter by 200 mm length in accordance with Hassan et al s work [7]. Samples Preparations The as-cast ductile iron was prepared for mechanical, chemical composition analysis and metallographic examinations. (a) Mechanical Properties Examination ( and Hardness) The mechanical examination tests were done to measure both the tensile and hardness properties of the as-cast ductile iron. The as-cast ductile iron samples were machined using lathe machine with coolant into various standard tests specimens for tensile and hardness tests. The standard tensile test specimens were prepared according to ASTM E370[9].Standard specification with a gauge length of 30 mm [10] while the hardness samples were cut with power hacksaw machine into 20 mm long [10]. (1) Test specimen was machined using lathe machine from the as-cast product to conform to Instron universal tester specification of 30 mm gauge length which fits into instron machine. The tensile test specimen shown in Figure 2 was mounted on the Instron universal testing machine at the jawsone end stationary and the other movable. The machine was then operated, which pulled at constant rate of extension and the tensile test performed in accordance with ASTM E8-09/E8M-09 standards [11] according to works done by Oyetunji and Alaneme, Hassan et al [12] and [7] respectively. This tensile test was done on the two samples (A and B) confirmed to be ductile iron and the test result was as shown in Tables 4-5, and Figs.3-4. Fig 3: Stress-Strain Curve For Sample A Fig 4: Stress-Strain Curve For Sample B Table 8: Analysis Of Sample A Extension at Extension at extension at Yield Yield (Zero (Zero Slope) Slope) Diameter Area (cm^2) Length Fig. 2: Standard Test Sample 278

4 Table 9: Analysis Of Sample B Extension at Yield (Zero Slope) extension at Yield (Zero Slope) Diameter (2) Hardness Test Hardness specimen was cut using hacksaw machine from the as-cast product into 20 mm diameter and 20 mm height samples that conform to specification which fits into digital Rockwell hardness tester machine. The surface of the entire hardness test specimen shown in Figure 3 in which an indentation was to be made were ground using 200 nm grinding papers to make them flat and smooth. A diamond indenter under the application of 60 KN was pressed into the surface. The hardness test was performed with the aid of digital Rockwell hardness tester in accordance with Oyetunji and Alaneme [12], and Hassan et al works [7] and the machine operational procedure in line with ASTM E18-08b [13] and ASTM E [14] standards. This test was performed on the two samples (A and B) confirmed to be ductile iron and their results were presented in Table 6 Fig. 3: Hardness Test Sample extension at Area (cm^2) Length Table 7: Chemical composition of analyzed ductile iron designated A produced Balance Table 8: Chemical composition of analyzed ductile iron designated B produced Balance Table 9: Chemical composition of analyzed iron designated C produced Balance. (c) Metallographic Examination The preparation of the samples for metallographic examination entails grinding, polishing and etching. They were discussed under these headings: i. Grinding Metallographic samples of diameter 20 mm by 20 mm height were cut out using power hacksaw machine from the as- cast bars. The surface of the entire cut metallographic samples shown in Figure 2 in which the light beam will be focused from the microscope was to be made were ground using Buehlera metaserv2000 grinder of these different grits of grinding paper: 60,120, 400 and 600 grits in order to prepare the surface of the samples to make them flat and smooth. During grinding, at regular intervals when changed from one grit to the next grit of emery paper the samples were rotated through 90 o. The grinded samples were washed cleaned with water to remove the grounded particle and entire grinded process was done in line with work of Zipperian [15]. ii. Polishing After successful grinding operation, the grinded samples were polished on Buehler metaserv2000 polisher using 800 grits and 1200 grits of polishing papers attached to the rotating disc of the machine. This removed all rough surfaces left by grinding and final polishing was done using polishing cloth of 3 microns with prepared suspension of diamond paste suspension. The process was completed when mirror-finish appearance was obtained on all the samples which were in accordance with Zipperian [15] and Bello et al. [16]. Table 6: Average Hardness Value for Ductile iron produced Sample Hardness Value (HRc) A 24 B 25.6 (b) Chemical composition analysis The three as-cast ductile irons designated A, B, C produced were analyzed using optical light emission spectroscopy to determine the average chemical composition of the elements present in the as-cast to confirm their properties before proceeding for further tests. Three sparks test was performed on each sample at the two heads and at the middle and average of the three readings were taken and recorded as shown in Tables 7-9, This test was done in accordance with ASTM E50-00 (2005) [14] standard. iii. Etching All the polished samples with the exception of one sample (control sample) were subjected to etching process to assist in revealing the structures at the surface layer of the specimens when view under the microscope. This was done by applying an etchant - 2% Nital (2% of Nitric acid and 98% ethanol) solutions using cotton wool and left for 15 seconds. The samples were swabbed after etching and methylated (alcohol) spirit was used to remove water stain on the surface before viewing. The structures of both un-etched and etched samples were viewed under the microscope. The phases present were observed clearly, Photomicrographs were obtained using AP2000MTI Metallurgical Microscope with Daheng Digital Camera of maximum magnification of X800. They were done 279

5 in accordance with Zipperian [15] and Oyetunji and Alaneme works [12]. The entire results of the metallographic examination were shown in Plates 1-6. Plate 5: Microstructure of Sample C (Unetched) X200 Plate 1: Microstructure of Sample A. (unetched) X200 Plate 6: Microstructure of Sample C (Etched With 2% Nital) X200 RESULTS The tensile analysis is shown in Figures 3 and 4. The results of micrographs obtained are as shown in Plates 1-6. The hardness values are as presented in Table 6. The results of the composition analysis are as presented in Tables 7-9. The products from various melts and castings were satisfactory good with no defect. Plate 2: Microstructure of Sample A. (etched) X200 Plate 3: Microstructure of Sample B (Un-etched) X200 Plate 4: Microstructure of Sample B (Etched In 2% Nital) X200 DISCUSSION The result obtained from the composition of the used scrap analyzed in Tables 1, 5-7 indicated that the carbon percent of 3.97 has been oxidized to 2.81 % C in sample A and 2.64 % C in sample B. This represents a total carbon loss of 29.2 % and % respectively in spite of the 0.5 kg graphite added to minimize the rate of oxidation of the carbon. This oxidation was due to the nature of the mode of operation of rotary furnace in which the fuel, charges and the products of combustion are all in contact with themselves. The results obtained in both Tables 8-9 and Figures 3-4 from tensile analysis showed high percentage elongation for both castings A and B (over 18% and 16% for A and B respectively) this is as a result of high silicon achieved in the final casting. This high silicon content was achieved because significant amount of silicon was contributed from the Mg-Fe- Si added as nodulariser. The ultimate tensile strength obtained for sample A is MPa while for sample B is MPa which is significantly alright. The average hardness obtained in sample A was 24.0 HRc and in sample B was 25.6 HRc as shown in Table 4. This is beyond the minimum hardness limit (20HRc) for ferritic - pearlitic ductile iron [17]. According to Tiedje[17] after the treatment, residual Mg should be in the range wt-% when high purity Fe, C and Si are used to produce the base cast iron. That implies at least residual magnesium of 0.03% must be present in the casting as at the time it finally solidified, so as to be able to 280

6 REFERENCES sustain the nodules formed in the casting. As a result of the low purity of the material (scrap) used and the processing method in the rotary furnace high sulphur content up to 0.07 % was still found in the casting. However this high sulphur content did not prevent the formation of nodules in both castings A and B because there was sufficient residual Mg content to sustain the nodules in the casting contrary to some literature that the sulphur must be reduced to about 0.01% before nodularisation effect can take place. A has a residual Mg content of % while B has % Mg. C has a residual Mg of % in the presence of high sulphur of % so retention of nodules was not achieved in C as shown in Tables 5-7. Microstructure of sample A has more nodules than that of B as can be seen on the microstructure. Plates 1-6 show the as-cast structure of samples A, B, and C of X200 magnification in both etched (2% Nital) and unetched. According to the microstructures obtained for samples A and B (Plates 1 4) it revealed the presence of nodules which is an indication that production of ductile iron was achieved in samples A and B, also the structures of A and B contained pearlite and ferrite matrixes when etched. C contained no nodule but flake graphite in an un-etched state and pearlite and ferrite in an etched condition which is an indication of grey cast iron (Plates 5 and 6). This implies that composition of magnesium necessary to form nodules was not achieved in casting designated C. This magnesium was not enough to de-sulphurise the melt to the level that nodularisation can take place and sustained the nodules needed in the casting. This is evidenced by the residual magnesium content ( % Mg) found in the casting C. This agrees with work done by Tiedje [18] that the residual Mg should be in the range wt-% before nodules can be formed and sustained in the casting. CONCLUSION The following conclusions were drawn from this work: a) Ductile iron produced via this route is ferriticpearlitic ductile iron as revealed by the microstructures of A and B castings produced from the two melts. b) The grade produced according to BSS 2789 was 268/18 for sample A, and 321/16 for sample B. It means that sample A has UTS of 268 MPa and percentage elongation of 18 while sample B has UTS of 321 MPa and percentage elongation of 16. c) The result has demonstrated that at higher residual magnesium content above 0.05 %, even with higher sulphur content (above 0.02 % S) nodules can still be sustained in the casting. The preferred spelling of the word acknowledgment in America is without an e after the g. Avoid the stilted expression, One of us (R. B. G.) Thanks... Instead, try R. B. G. thanks. Put sponsor acknowledgments in the unnumbered footnote on the first page. [1] Heidarian B. Nilli-Ahmadabadi M. and Moradi M. (2010). Mechanical Properties of Thixo-formed Austempered Ductile Iron. Transaction of Non ferrous metals Society of China 20(2010) Elsevier Pp [2] Hassan S.B, Salihu S.A and Obi A.I. (2008) Effect of using fatty- base vegetable oil for Austempering of ductile cast iron. Proceeding of the 25 th Annual conference/agm Nigerian Metallurgical Society.Pp 3-4. [3] Fatahalla N. and Reafrey A. (2003). Effect of Microstructure on Properties of ADI and Low Alloyed Ductile Iron. Journal of Materials, Vol.38 Elsevier, London No.2. Science. [4] Brown, J. R. (1994). Foseco Foundry man handbook, Butter worth Heinemann U.K. [5] Dicocco V. Lacoviello F, and Cavallini M (2010); Damaging Micro mechanisms Characterisation of a Ferritic Ductile Cast Iron, Journal of Engineering Fracture Mechanics 77 Elsevier Pp 2-3. [6] ASTM E (2005) Standard Practices for Apparatus, Reagents, safety consideration for Chemical Analysis of Metals, Ores, and Related Materials. [7] Hassan Jafari, Mohd Hasbullah Idris, Ali Ourdjini, Majid Karimian, Gholamhassan Payganeh (2010). Influence of Gating System, Sand grain size, and Mould coating on Microstructure and Mechanical Properties of thin- wall Ductile Iron. Journal of Iron and steel research international 2010, 17(12) Pp [8] Oyetunji, A. (2007) Modelling Mechanical properties of grey cast iron. Ph.D Thesis submitted to the Department of Metallurgical and Materials Engineering, Federal University of Technology, Akure-Nigeria. [9] ASTM E370 Standard Test Methods and Definitions for Mechanical Testing of Steel [10] Oyetunji, A. (2010). Modelling the Impact Properties of Grey Cast Iron Produced from CO 2 and Steel Moulds Journal of Applied Science Research, ISINET Publication Pakistan. Vol. 6(5), Pp [11] ASTM E 8-09/ E 8M-09 Standard Test Methods for Tension Testing of Metallic Materials. [12] Oyetunji, A. and Alaneme K. K. (2005). Influence of the Silicon Content and Matrix Structure on the Mechanical Properties of Al-Si Alloy. West Indian Journal of Engineering. Vol.28. No.1, Pp [13] ASTM E 18-08b- Standard Test Methods for Rockwell Hardness of Metallic Materials. 281

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