Effect of the Austempering Process on Thin Wall Ductile Iron

Transcription

1 Journal of Materials Science and Engineering A 1 (211) Formerly part of Journal of Materials Science and Engineering, ISSN Effect of the Austempering Process on Thin Wall Ductile Iron Johny Wahyuadi Soedarsono 1,2, Tresna Priyana Soemardi 3, Bambang Suharno 1, Rianti Dewi Sulamet-Ariobimo 3,4, Elwado Zulfikar Damanik 1 and Wahyu Dwi Haryono 1 1. Department of Metallurgy and Materials, Faculty of Engineering, University of Indonesia, Jakarta, Indonesia 2. State Polytechnic of Jakarta, Jakarta, Indonesia 3. Department of Mechanical Engineering, Faculty of Engineering, University of Indonesia, Jakarta, Indonesia 4. Mechanical Engineering Department, Faculty of Industrial Technology, Trisakti University, Jakarta, Indonesia Received: November 22, 21 / Accepted: December 26, 21 / Published: July 1, 211. Abstract: The needs of material that consumed low of energy during its production and saved energy during its used enhance the use of austempered ductile Iron (ADI), especially when ADI is made as thin wall austempered ductile Iron (TWADI). This research is conducted to see effect of thin wall ductile iron (TWDI) chemical composition to microstructure, UTS, and elongation of TWADI. TWDI plates were produced by vertical casting. The dimension of the plate is mm with the thickness of 1 mm. Number of plate produced in 1 mould are 5. The difference of the plates lay on its position during the pouring. Chemical composition was checked before the liquid treatment process. All the plates then austempered in fluidized bed furnace with austenisation temperature 96 C for 3 min and austempered temperature 35 C for 1 min. This research found that the chemical composition of TWDI did not prevent austempering process to increase the ultimate tensile strength (UTS) and elongation. Key words: TWDI, TWADI, fluidised bed. 1. Introduction The use of un-renewable energy sources, such as oils and coals, during this time have put this world into an energy crisis. This problem will not be solve only by the searching of renewable energy sources that can replaced the un-renewable ones, but the most important thing is how the energy is being used. From this side there are two things to be done, that are: using the energy economically and trying to reduce weight of equipments. Austempered Ductile Iron (ADI) fulfils all of these requirements. ADI production processes save around 5% of energy consumption compare to steel and aluminium [1, 2]. ADI is made from ductile iron or Ferro Casting Ductile (FCD) which endured austempering process. Both of them, FCD and ADI, are Corresponding author: Johny Wahyuadi Soedarsono, Ph.D., professor, research fields: casting and corrosion. jwsono@metal.ui.ac.id. not classified to light weight material. Although that, ADI can compete with aluminium in weight problem due to the design flexibilities that it has. This is proven by the used of ADI as a material of Ford Mustang suspension arm rather than aluminium [3]. Apart from the design flexibilities, weight problem can also be solves by using a new casting technique known as thin wall casting. With this casting technique a lighter ADI can be obtained. Although ADI is made from FCD, the Ductile Iron Society notes slight differences between the chemical composition of FCD and austempered FCD [1]. The chemical composition of austempered FCD has to be alloyed appropriately to ascertain the growth of desire microstructure during the austempering process [4]. Apart from chemical composition, the microstructure of austempered FCD must meet specified condition to ensure a high quality ADI.

2 Effect of the Austempering Process on Thin Wall Ductile Iron 237 Fig. 1 Hollow connecting rod ADI - Martinez [7]. Recommendation for requirement of thin wall ductile iron (TWDI) to obtain thin wall austempered ductile iron (TWADI) has not been made because the thin wall technique is still undergoing research and development. The current recommendation is based on the guidelines for normal casting thickness. The use of ADI as thin wall has been mentioned in very limited references [5]. Fraś and Górny in their paper mentioned that TWADI can be obtained by short-term heat treatment of the casting without addition of alloying elements [6]. Mourad et al. reported the effect of austempering temperature on TWDI characteristics using samples with thickness of 2, 4, 6, and 8 mm [5]. In his work, Mourad found that microstructure resulting from austempering temperature of 35 ο C was finer than 4 ο C. In the austempering temperature of 35 ο C, tensile strength and hardness increased with the decreasing of wall thickness. Gagne et al. reported their worked in finding the effect of silicon and wall thickness in ADI [7]. The result of their work showed that low silicon thin wall ADI castings exhibited ultimate tensile strength exceeding 11 MPa and elongation higher than 1%. Martinez et al experimented with TWADI as a material in a hollow connecting rod shown in Fig. 1 [8]. After succeeded with hollow connecting rod, they also used TWADI to develop of hollow front up rights. Their research showed that the used of TWADI in hollow connecting rod for a two-cylinder car engine and a front upright for racing car fulfil the requirement. The objective of this research is to observe the effect of the austempering process on TWDI while producing TWADI plate. 2. Experimental Work The experimental work followed the diagram as shown in Fig. 2. The plate used for this research was cast in a vertical system. The design of the plate and its gating system are shown in Fig. 3. Plate dimensions were 15 mm 75 mm with 1 mm thickness for all five plates. Plates were arranged parallel to each other. The moulds were made from furan sand. The melts used in this work were produced in a one ton medium frequency induction furnace. The charging materials consisted of return scrap, steel scrap, carburizer, and ferrosilicon. The melts were held in the furnace at 1,52 ο C before tapping. Fe-Si-Mg with 6%Mg is used as the nodulation agent in sandwich method. Inoculation was carried out during nodulation Fig. 2 Research scheme.

3 238 Effect of the Austempering Process on Thin Wall Ductile Iron Fig. 3 Casting design. using S7 with 7%Si. Temperatures during pouring ranged from 1,334 ο C to 1,312 ο C. The austempering process was conducted in fluidized bed furnace. The austenitization temperature was 96 ο C hold for 3 min and the austempering temperature was 35 ο C hold for 1 min. The fluidizing gases used were air and nitrogen. Chemical composition was inspected using spectrometry before the liquid treatment process. Microstructure examination was conducted based on JIS G552. Identification of microstructures and anomalies were made based on the ASM Metal Handbook [9], ASM Handbook [1, 11] as well as paper made by C. Ecob [12] and G.M. Goodrich [13]. Quantitative analysis on graphite characteristic was completed using Nikon imagine software NIS Element Br 3.1. Tensile specimens were prepared according on JIS Z221 No. 13B and the tests were conducted based on the JIS Z2241 method. 3. Results and Discussion 3.1 Chemical Composition Chemical composition of elements used in this research noted in Table 1. Recommendation made by the Ductile Iron Society [1] regarding typical composition and control ranges indicate the following for composition of elements in this research then the result will be: carbon content was.5% higher than the maximum standard, manganese content was 15.7% lower than the minimum standard, and copper content was lower 58% lower than the minimal standard (Fig. 4). Compared to recommendations made by Harding [4] (Fig. 4), composition of carbon, silicon, manganese, magnesium, copper, nickel, and molybdenum did not exceed the recommended standards. Carbon and silicon content exceeded the maximum levels for.5% and 2.4% respectively but manganese content fell below the minimal level for 3.5%. Magnesium content 13.3% exceeded the maximum level whereas copper, nickel, and molybdenum fell below the minimum level by 97.2%, 98.6%, and 98.5% respectively. Compared to the FCDI standard [12], significant differences were recognized for manganese, phosphorus, copper, chrome, nickel, magnesium, and molybdenum, whereas the differences were more subtle for carbon, silicon, and sulphur. The greatest difference was for chrome content in amount of 391%. Based on the data provided above, chemical composition of TWDI used in this research correlated most closely with the standards recommended by the Ductile Iron society rather than Harding s standards. The significant differences that appear when comparing composition levels with recommendations made by the Ductile Iron Society occur in minor elements only. Therefore the chemical composition of TWDI fulfils certain basic requirements of the chemical composition of FCD needed to produce ADI. Copper is usually added in FCD made to ADI during production of ADI to improve hardenability, ductility, and toughness when the austempering temperature is below 35 [1, 4]. Nickel and molybdenum perform the Table 1 Chemical composition weight(%). Element/Pouring C Si Mn P S Cu Ni Cr Mg P

4 Effect of the Austempering Process on Thin Wall Ductile Iron 239 %Mo %Mg %Ni %Cr %Cu %S %P %Mn %Si %C Std. Min Harding Min FCDI Research Mourad Harding Maks Std. Maks Fig. 4 Chemical composition comparison. same function as copper, small additions of either one or a combination of both will prevent the formation of pearlitic matrix. The lack of these elements is not detrimental to properties of TWADI in this research because the casting thickness was very thin and the austempering temperature used in this work was 35 ο C. Although the excess of magnesium content will result in high nodularity and free of skin effect, special care should be taken because of the tendency towards carbide formation. Since Magnesium is a strong carbide promoter, magnesium content should be limited to minimum and maximum content standards. It should be noted that a lack of carbon content will disturb CE values with silicon and reduce the amount of graphite formed. The difference between the carbon content in FCDI and TWDI although not significant upgrade the nodule count in this research. Due to the thickness of TWDI the nodule count was significantly higher in TWDI than FCDI. This research produced a higher nodule counts than Mourad [5] although not significant. The difference of silicon content in this research compared to the composition of FCDI was inconsequential due to the low level of manganese. Manganese has an inverse effect compared to silicon. Compared to silicon content in Mourad, the higher silicon content existing in this research makes the graphite distribution even. High sulphur content enhanced the amount of magnesium used in the research. CE value of the composition in this research was 4.66% which is 3.6% higher than CE content in Mourad s research (CE: 4.5%) and 1.3% higher than the maximum limitation given by Martinez [8]. The CE value maintains a strong relationship with cast ability and ductility. 3.2 Microstructures Generally, the microstructure of TWDI is nodular graphite in a ferrite matrix as seen in Fig. 5. There is no trace of carbides or skin effects. Primary and exploded graphite were found in the microstructures as seen in Figs. 5d and 5e. Due to high silicon and sulphur content no carbides formed. As an explanation, high silicon content suppresses the reaction of graphite forming while high sulphur content controlled the magnesium. Magnesium bonds with sulphur to form MgS and in this latter magnesium will not promote carbide formation. Skin effects did not form due to high magnesium content.

5 24 Effect of the Austempering Process on Thin Wall Ductile Iron Microstructure of TWDI non etched a. 1mm position 1 b. 1mm position 2 c. 1mm position 3 d. 1mm position 4 e. 1mm position 5 Microstructure of TWDI etched a. 1mm position 1 b. 1mm position 2 c. 1mm position 3 d. 1mm position 4 e. 1mm position 5 Fig. 5 Microstructure of TWDI. Finally, although part of the magnesium bonded with sulphur some magnesium still reminded and acted as a nodulant. Primary graphite usually forms when in the CE value is higher than 4.3% due to an imbalance of the cooling rate to CE values. Exploded graphite appears because of high CE values. Based on the recommendations from the Ductile Iron Society the microstructure resulting from this research may be made into ADI. Nodularity and nodule count of TWDI can be seen in Table 2. The resulting nodularity ranged from 85-9% because of high Mg content, no waiting time before pouring, and slight temperature differences between the mould and molten metal. There is a slight tendency (less than 1%) for nodularity to increase as the position or level of the plate increases. High nodule count between 1,113 to 1,689 nodule/mm 2 was achieved in our research. There is an evident correlation between nodule counts with position of the plates. Nodule count increases as the level of a plate gets higher. The tendency is strong, especially between positions 4 and 5. From position 4 to 5 noted an increase in nodule count of 26% due to cooling rate. Cooling rate in position 5 was higher than it was for position 4 due to its larger surface heat transfer area. At certain rates, nodule count increase if the cooling rates increase. The nodularity and nodule counts that resulted from this research fulfil the standard requirements recommended by the Ductile Iron Society. Microstructures resulting from the austempering process showed transformation in the matrix. Part of the matrix transformed from ferrite to ausferrite as seen in Fig. 7 and was formed in all plates (Fig. 8). A change in nodule counts (Table 2), was also noted and requires further research. 3.3 Tensile Testing Ultimate tensile strength (UTS) from all TWDI plates (Fig. 9), did not fulfil the minimum UTS and elongation required by the standard. There is no special Table 2 Chemical composition weight(%). Examine object Nodule counts (nodule/mm 2 ) Nodularity (%) TWDI TWADI TWDI TWADI Plate 1 mm position Plate 1 mm position Plate 1 mm position Plate 1 mm position Plate 1 mm position

6 Effect of the Austempering Process on Thin Wall Ductile Iron 241 Nodule Count - nodule/mm y = 1.25x x x x y = -.125x x x x + 8 y = x x x x y = x x x x + 56 Position of Plate Nodularity - % NC - TWDI NC - TWADI N - TWDI N - TWADI Poly. (N - TWDI) Poly. (N - TWADI) Poly. (NC - TWDI) Poly. (NC - TWADI) Fig. 6 Correlation of position of plate to graphite characteristic. FCD ADI : A1, A2, A3 ADI: A4, A5 Fig. 7 Matrix comparison. a. 1mm position 1 b. 1mm position 2 c. 1mm position 3 d. 1mm position 4 e. 1mm position 5 Fig. 8 Microstructure of ADI etched. UTS - kg/mm y = -7.75x x x R 2 =.9956 y = x x x R 2 =.9979 Position of Plates TWDI TWADI Poly. (TWADI) Poly. (TWDI) UTS - kg/mm y = -7.75x x x R 2 =.9956 y = x x x R 2 =.9979 Position of Plates TWDI TWADI TWDI Y-Block Poly. (TWADI) Poly. (TWDI) Fig. 9 Correlation of position of plate to ultimate tensile strength (UTS). correlation between UTS and position of the plates. High nodule counts and nodularity did not guarantee high UTS. In this research, low UTS occurred due to the matrix and anomalies formed in the graphite. As for elongation (Fig. 1), the test results using JIS Z221 No. 13B did not indicate elongation in all of the plates but test piece made from Y-block using JIS Z221 No. 4 showed elongation of 25%. The specimen geometry was suspected to be responsible for this. Tensile testing result for TWADI (Figs. 9 and 1), however showed UTS increases from 44 to 39%. The same things also happened regarding elongation with increased between 5 to 1%. Matrix changes are responsible for these results.

7 242 Effect of the Austempering Process on Thin Wall Ductile Iron e - % y = -.125x x x R 2 =.8929 y = R 2 = #N/A e - % y = -.125x x x R 2 =.8929 y = 25 R 2 = #N/A y = R 2 = #N/A Position of Plate TWDI TWADI Linear (TWDI) Poly. (TWADI) TWDI TWDI Y-Block Poly. (TWADI) Position of Plate TWADI Linear (TWDI) Linear (TWDI Y-Block) Fig. 1 Correlation of position of plate to elongation (e). 4. Conclusions The conclusions reached from this research are that TWADI can still be produced although the chemical composition of TWDI does not fulfil the recommended standard made by the Ductile Iron Society. The UTS and elongation of TWDI are determined by all aspects of its microstructure, while nodularity and nodule counts will increase as position or level of the plate increases. The austempering process increased the UTS of TWDI from 44% to 39% and elongation from 5% to 1% due to changes in the matrix. Acknowledgment The authors wish to thank The Goverment of Republic of Indonesia for the research grants, PT. Geteka Founindo and PT. ASSAB Indonesia for permission to use their foundry. The authors also wish to thank Prof. Hiratsuka and Dr. Bimo from Iwate University for their helps in measuring the graphite nodule properties, Mr. Chris Ecob from Elkem and Dr. Richard Harding from Birmingham University for the discussions. Reference [1] J.R. Keogh, Austempered Ductile Iron, 1998, available online at: [2] Anonymous, ADI Solutions Aid Vehicle Design, Transport, FTJ March, 24, [3] R.A. Harding, The production properties and automotive application of austempered ductile iron, Kovove. Mater. 45 (27) [4] M.M. Mourad, K.M. Ibrahim, M.M. Ibrahim, A.A. Nofal, Optimizing the properties of thin wall austempered ductile iron, 68th World Foundry Congress, Chennai, India: WFO, 28, pp [5] E. Fraś, M. Górny, thin wall austempered ductile iron, Archives of Foundry Engineering 9 (29) [6] M. Gagne, C. Labrecque, M. Popescu, M. Sahoo, Effect of silicon content and wall thickness on the properties of austempered ductile irons, AFS Transactions 114 (26) [7] R.A. Martinez, R.E. Boeri, J.A. Sikora, Application of ADI in high strength thin wall automotive parts, Paper, 22 world conference on ADI., Louisville, Kentucky, USA: Ductile Iron AFS, 22. [8] ASM, Atlas of Microstructures of Industrial Alloys, Handbook Vol. 7, American Society for Metals, Metals Park, Ohio, USA, 1972, pp [9] ASM, Metallography and Microstructures, Handbook Vol. 9, American Society for Metals, Metals Park, Ohio, USA, 2, pp [1] ASM, Cast Iron, ASM Specialty Handbook, American Society for Metals, Metals Park, Ohio, USA, 1996, pp [11] C.M. Ecob, A review of common metallurgical defects in ductile cast iron, causes and cures, in: Proceeding of the 9th Asian Foundry Congress, Hanoi, Vietnam, 25, pp [12] G.M. Goodrich, Explaining the peculiar: cast iron anomalies and their causes, Modern Casting April (1998) [13] R.D. Sulamet-Ariobimo, Austempered ductile iron as an alternatif material, National Seminar of Mechanical Engineering Department, Jakarta, Indonesia, Trisakti University, 2, pp. A8 (1/17-17/17).