EFFECT OF CARBON AND SILICON ADDITION ON MECHANICAL PROPERTIES AND MICROSTRUCTURE IN NODULAR CAST IRON

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1 EFFECT OF CARBON AND SILICON ADDITION ON MECHANICAL PROPERTIES AND MICROSTRUCTURE IN NODULAR CAST IRON Mehmet EKICI a, Ugur OZSARAC b, Salim ASLANLAR b, Faruk VAROL b a Yalova University, Vocational School of Yalova, Yalova, Turkey b Sakarya University, Technology Faculty, Department of Metallurgical and Materials Engineering, Sakarya, Turkey Abstract In this study, the effect of carbon and silicon alloying elements with different weight per cent on mechanical properties and microstructural appearances in nodular cast iron alloys was investigated. The nodular cast iron ingot and scrap materials were charged into the induction furnace and when the temperature was risen to 1510 o C, the spheroidizing and inoculation processes were applied to molten metal. Having completed pouring of the molten metal into patterns by means of Furan method, the solidification was occurred in room temperature. Two groups of specimens were obtained containing 3.65 wt. % C, 2.30 wt. % Si and 3.25 wt. % C, 3.65 wt. % Si. The microstructure of specimens were investigated and uni-axial tensile test and the Charpy impact test were performed and their micro-hardness measurements were done in order to characterize the mechanical behaviours of nodular cast iron alloys having different alloying elements. Keywords: Nodular cast iron, alloying element, tensile test 1. INTRODUCTION Spheroidal graphite cast iron is dispersed in the form of spheres of graphite particles in ferritic-pearlitic matrix. Presence of spherical rather than lamellar graphite structure, as distinct from grey cast iron material gives high ductility and strength [1-3]. Spherical graphite cast iron exhibit a wide range of mechanical properties depending on the shape and chemical composition, microstructure and the distribution of matrix graphite spheres[4]. Studies of the samples to spheroidal graphite cast iron material were examined and the number of spheres is increased by increasing the carbon content of the material. It changes from 2.11 wt. % C to 3.97 wt. % C while the volume fraction of spherical graphite changes from 384 spheres/ mm 2 to 725 spheres /mm 2. Erosive wear test was applied to these samples. Erosive wear rate, such as soft ferrite matrix does not change with the increase in the number of spheres, lower bainite and martensite showed that the wear-hard matrix structures [5]. The amount of carbon and silicon-carbon affects equivalence of spheroidal graphite cast iron, and then it also plays an important role in the carbon segregation and graphitization values of carbon equivalence (CE = C + 1/4 % Si + 1/2 % P), greater than 4.3 promotes the growth of graphite and formation of spheres [4]. In case of carbon equivalent to 4.6 which is larger than the compositions and 2.54 cm from the thicker sections, it is expected to swim the upper part of the graphite spheres casting floats to the surface (flotation) and to cause for carbon segregation. When silicon alloying elements than the liquid metal is added at the end of the process as ferrosilicon (inoculation), graphite is more effective in the control of number of spherical graphite. 2. EXPERIMENTAL STUDIES In this study, a medium frequency induction furnace was used to produce spheroidal graphite cast iron with a capacity of 1800 kg. The induction furnace was charged with 800 kg ductile iron, 650 kg steel scrap and 350 kg of scrap materials as given in Table 1.

2 Tab.1 The chemical composition of ductile cast iron and steel scrap Sample %C %Si %Mn %S %P %Cu %Ni %Cr %Al %Mg Ductile cast iron Steel scrap Spheroidizing and inoculation have been made of molten metal in 1500 o C as shown in Table 2. Spheroidizing process was carried out in sandwich-type crucible; grafting procedure was carried out in the casting ladle. For spheroidizing process, spherodizing and inoculation materials were used as 1 wt % and 0.3 wt %, respectively. Tab. 2 The chemical composition of materials Material % Si % Al % Mg % Ba % Ca Particle Size, mm Inoculant (FeSi75) Spheroidizer The smelted liquid metal was poured into the Y-Block sand molds at o C temperatures and allowed to cool to room temperature. Square cross-sectional specimens were cut from Y-Block sand cast materials and then the cylindrical samples were prepared from them. Two different type nodular cast iron materials were casted according to given composition as shown in Table 3. Tab. 3 The chemical composition of casted nodular cast iron samples Material C Si Mn P S Cr Ni Cu Mg Carbon equivalent KGDD 1 3,66 2,38 0,325 0, ,050 0, , KGDD 2 3,34 3,72 0,27 0,020 0,010 0,042 0,045 0,092 0, RESULTS AND DISCUSSION In this study, the tensile test specimens prepared in accordance with the standards from spheroidal graphite cast iron materials, which have different ratios of carbon and silicon alloying element to determine the mechanical properties of samples. Tensile tests were made on a single axis, at a constant speed and at a constant temperature until the material breaks. Tensile and yield strength of materials have been identified. At the same time elongations of samples were also recorded. The results of all samples were shown in Figure 1.

3 Fig. 1 Tensile test results of nodular cast iron specimens From the tensile test results, an increment in yield and maximum tensile strength as 54 MPa and 32 MPa, respectively was observed in the sample containing 3.34 wt % C and3.72 wt % Si. However, a significant reduction has been observed in per cent elongation proving the decrease in toughness. In iron casting samples containing 3.66 wt % C and 2.38 wt % Si element is seen as the optimum alloying element value for the yield strength and tensile strength. As shown in Figure 2, according to the results of the hardness test the sample containing 3.34 wt % C and 3.72 wt % Si, HBN hardness is greater than that of the other alloy s. Fig. 2 Hardness results of samples As shown in Figure 3, notch impact test was applied to the samples at different temperatures. In between 60 C and -30 C temperatures, both samples breaking energies has not been much change. However, the temperature of 0 C fracture energies of the samples is increased. Fracture energy of element of the sample containing 3.72% C and 3.34% Si was observed to be better. In general, it can be said that the fracture energies for each sample by looking at the work areas are generally between 0 C to +60 C to determine.

4 Fig. 3 Impact test results of samples As can be seen from Figure 4, in general, depending on the content of the alloying element carbon and silicon, respectively, samples of different phase structure of ferrite, pearlite and ferrite phases completely in the image of the microstructure obtained from bovine eye type. In this study, the obtained from the 3.66% C % of the samples that contain Si alloying elements were found as 420küre/mm 2. Containing 3.72% C- 3.34% Si alloy element in the sample number of spheres per unit area is observed that 391 sphere/mm 2. Looking at the results both on sphere volume fraction of graphite in the sample rates are close to each other, but sample containing 3.66% C % Si volume fraction of 34% pearlitic matrix and graphite observed as 13%, ın the sample containing 3.34% C % Si, pearlitic matrix is not considered at all. It is observed that 86% of the ferrite phase structure of the sample and graphite volume ratio is 14%.

5 Fig. 4 a)the microstructure of nodular cast iron sample KGDD 1 (x200), b) Phase fractions of KGDD 1 sample, c) The microstructure of nodular cast iron sample KGDD 2 (x200), d) Phase fractions of KGDD 2 sample Fig. 5 Amount of phases observed in nodular cast iron samples 4. CONCLUSION This study shows that microstructure has been found to occur of ferrite, pearlite and graphite globes, when the scope of work spheroidal graphite cast iron castings is analysed. In conditions of spilled Samples volume fraction of ferrite increases with increasing;% C and Si content. In general, with increasing of the amount of pearlite in the internal structure of spheroidal graphite cast iron materials, the hardness values are increasing. This is the material that contains the pearlitic / ferritic structure 3.66 % C % Si alloy. In addition, the increasing the number of spheroidal graphite cast irons sphere reduced the mechanical strength of the material. According to the results obtained from the fracture energy, determination of spheroidal graphite cast iron material usage is recommended that at least at room temperature.

6 REFERENCES [1] Morrogh, H., Production of nodular graphite structures in gray cast irons, AFS TRANSACTION, 56, 72-90, 1948 [2] Goodrich, G.M., Metallurgy of cast irons, Iron castings engineering handbook, AFS, Prited in the United States of America, 47-61, 2003 [3] Karsay, S, I., Ductile iron, production pratices, American Foundrymen s Society, Inc, USA, 24-77, 1985 [4] Velez J. M. A, D.K. Tanaka B, A. Sinatora B, Tschiptschin A.P., (2001). Evaluation of Abrasive Wear of Ductile Cast Iron in a Single Pass Pendulum Device, Wear, 251, s [5] Rio Tinto&Titanium Inc., The Sorelmetal Book of Ductile Iron, 2004 [6] Cast Irons, A.S.M. Specialty Handbook, 1996 [7] LOPER, C.R., Preconditioning Effect from Crystalline Recarburisers and Their Use in Safety Cast Components Production, II Foundry Technical Forum 2005, Bilbao/Spain, September 2005