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International Journal of Advanced Engineering Research and Studies E-ISSN2249–8974 Research Article MECHANICAL PROPERTIES OF METALLIC AND NONMETALLIC CHILLED AUSTEMPERED DUCTILE IRON (ACDI) Yogesha K.B*, Joel Hemanth a * Address for Correspondence Assistant Professor, Department of Mechanical Engineering, School of Engineering and Technology, Jain University, Bangalore-560112, Karnataka, India. a Dean, Professor and Head, Department of Mechanical Engineering, Rajiv Gandhi Institute of Technology, Bangalore-560032, Karnataka, India ABSTRACT This paper presents the results obtained and deductions made from a series of ultimate tensile strength (UTS) and hardness tests involving austempered chilled ductile iron containing 0.1%Mo and Cu contents varying from 0.5 to 4.2%. By using metallic (copper) and non-metallic (graphite) chills, the effect on ultimate tensile strength (UTS) and hardness of varying the chill rate was also examined. All the tests were carried out in conformance with AFS (American Foundryman’s Society) standards. It was found that austempered chilled ductile iron is highly dependent on the location on the casting from where the test samples are taken and also on the Cu and Mo content combination of the material, as well as the rate of chilling. It was found that the hardness and tensile strength is highly dependent on the rate of chilling. Finally comparative studies of using different chills and without using any chills have been made to see that effect of chilling is influencing the properties of composite under investigation. KEYWORDS metallic and non-metallic chill; Ultimate tensile strength; hardness 1. INTRODUCTION 1.1. Chilled ductile cast iron Chilled cast iron belongs to a group of metals possessing high strength, high hardness, high toughness and high wear resistance [1, 2]. They are widely used in the manufacture of wear shoes, wear liners, rollers, brake shoes, crushing jaws, grinding mill liners, cam surfaces, wearing plates and other machine components and equipments requiring such material characteristics. For the design engineer, there are sufficient data available on the mechanical properties of ductile iron and austempered ductile iron. However, there is a dearth of information on the tensile strength and hardness of austempered chilled ductile iron [3]. This has promoted a series of experiments to determine the tensile strength and hardness of austempered chilled ductile iron containing 3.37%C, 2.5%Si, 0.1% Mo, Cu contents varying from 0.5 to 4.2% and other alloying elements as indicated in Table 1. The reason behind the selection of this series of cast iron for the present investigation is that a wide range of mechanical properties can be obtained with different microstructures. Strengthening of ductile iron includes mould cooling to get a bainitic structure in highly alloyed irons. The presence of copper as an alloying element improves the machinability of the cast iron. In addition, chilling, in case of cast iron promotes directional solidification and eliminates shrinkage, porosity, cracks, hot spots and other casting defects and thus, resulting in production of sound castings. Also, controlled chilling eliminates ferrite (too soft) structure but promotes bainitic structure. Therefore, a simple copper, graphite and without using any chills are used in the present investigation. 1.2 Current position of ductile iron: Ductile iron was developed approximately 60 years ago and has recently replaced millions of tones of steel castings and grey iron castings. The major users of ductile iron components include automotive and machine tool industry. The reason for its popularity is due to its advantages over steel castings and grey iron IJAERS/Vol. I/ Issue II/January-March, 2012/240-243 castings. For the past few years, attempts were made all over the world to further improve the properties of ductile irons [4].The specific applications of ductile iron are: (1) rolling mill rolls; (2) gears; and (3) crank shafts, etc. The strength, hardness and wear resistance of ductile iron can be increased through proper heat treatment of ductile irons. The presence of copper as an alloying element improves the machinability of the cast iron. Moreover, a low-grade cast iron can be converted into one of superior qualities by chilling ductile iron and austempering with little addition of alloying elements. 1.3 Austempered ductile iron It is known that steel components, either hardened or forged can be heat-treated and alloyed to get a beneficial change in matrix structure and hence a range of useful properties. The common steels in use are the ones with ferritic, pearlitic, martensitic and austenitic matrices. The demand for a tougher, harder and stronger matrix led to the development of bainitic matrix steels. Such a matrix was developed through a spheroidised heat treatment known as ‘austempering’, in which the matrix gets transformed from austenite to bainite directly without entering the martensitic stage at any time [5]. The beneficial use of this heat treatment had encouraged many researchers to apply this to the ductile iron so that it could become a tougher, more wear resistant and stronger material [6]. Ductile iron subjected to an austempering treatment is often referred to as Austempered Ductile Iron [ADI].in most international forums. The austempering heat treatment involves the following stages: 1. Heating ductile iron castings to a temperature above the upper critical temperature and soaking for times varying from 1 to 5 h depending on the section thickness and the amount of eutectic carbides. 2. Quenching the above in a fluidised salt bath maintained at a temperature in the range between martensite and fine pearlite formation temperature at rates fast enough to avoid the transformation of austenite to pearlite. International Journal of Advanced Engineering Research and Studies 3. Holding in the salt bath long enough to form the desired mixture of various micro constituents of bainite. 4. Cooling in air to room temperature. 1.4 Mechanical properties of cast iron The mechanical properties of a metal depend very much on its microstructure. This is especially true for cast iron. The structure depends on the interaction between the effects of the elements present and the cooling rate during and after solidification in the mould [7]. The properties of cast iron are affected by the structure of the austenitic dendrites, graphite flakes and eutectic ceils, as well as the formation of different alloying additions to the base metal. The alloying elements added should therefore be selected on the basis of their effect in controlling both nucleation and growth of graphite and primary austenite [8]. 1.5 Tensile properties of cast iron It is generally acknowledged that most of the mechanical qualities of cast iron can best be represented by its tensile strength. Since cast iron is a very brittle material, its yield strength can in practice be taken to be equal to its ultimate tensile strength (UTS). Optimum tensile strengths can be obtained in cast iron by increasing the amount of graphite-free area (in most cases, primary austenitic dendrites), refining eutectic cell size and establishing the pearlitic matrix structure [9]. Treating the base metal with commercial inoculants and neutralizing certain elements which inhibit graphite nucleation can result in the formation of more eutectic cells. By increasing the eutectic cell count, the effective span and stress concentrating effect of the graphite can be reduced, thus improving the tensile strength [10]. 1.6 Hardness of cast iron The chemical composition of cast iron is one of the most important factors that influence the hardness of the casting. The composition of iron determines the quality and character of the graphite and the metallurgical characteristics of the metallic matrix for some specified set of cooling conditions. The cooling rate is also an important factor which affects hardness, especially when the metallic and non metallic chills are placed at end the mould cavity [3]. It is well known to the foundryman that fully sound castings under ordinary circumstances cannot be produced by simple techniques since cast irons are subjected to micro porosity and shrinkage porosity during solidification [11]. This defect can be minimized by making solidification directional by setting up steep temperature gradients directed towards the riser, and this can be achieved by employing chill [12]. Typical benefits of employing chills for the solidification of cast iron are the densification of the section and the promotion of directional solidification, resulting in sound castings [13]. 1.7 Relevance of this research Ductile iron has the potential to replace costlier material in many significant engineering applications. The requirements concerning safety and reliability are always on the increase and, therefore, the mechanical properties are ever more crucial. Since there is presently no published data on tensile strength and hardness of metallic and non-metallic IJAERS/Vol. I/ Issue II/January-March, 2012/240-243 E-ISSN2249–8974 chilled austempered ductile iron, the present research is intended to fill the void. 2. Experimental procedure 2.1. Fabrication of the material Ductile iron alloys of five different compositions were produced by casting at 14500C into the form of ingots, denoted as alloys A, B, C, D and E with chemical compositions as indicated in Table 1.Apart from the usual alloying elements such as Si, Mn, S and P, copper was also added in compositions ranging from 0.5% to 4.2% to enhance machinability as well as to act as a grain refiner. Table 1: Chemical composition details of the different alloys tested alloy designation composition (wt. %) 2.2. Casting procedure To make the mould for casting, a teak wood pattern of size 225 mm x 150mm x 25 mm (AFS standard) was employed with standard pattern allowances. The moulds were prepared using silica sand with 5% bentonite as binder and 5% moisture. The melts were carried out using a medium frequency coreless induction furnace. A base metal analysis, generally of 3.37% C and 2.5% Si were produced and superheated to 15000C prior to tapping into a pre-heated 20-kg capacity handshake, containing modularizing alloy using sandwich techniques. Prior to casting an inoculating addition (ferro-Si to promote uniform structure) and other alloying elements were stirred into the molten metal using a graphite spoon. The treated metal was poured directly into the mould at a pouring temperature of 14500C, which was cooled from one end by a copper chill and graphite chill set in the mould (as shown in Figure 1). Ingots were cast employing different chills and without using any chills in order to study the effect of volumetric heat capacity on the chill. The length and breadth of the chills were kept constant at 175 and 35 mm, respectively. This type of casting production routine was found to result in chilled ductile iron of consistent chemical analysis and good nodularity. 2.3. Heat treatment Austempering heat treatment comprises of two stages, namely heating the plate castings to the austenising temperature of 9500C, followed by quenching and holding in the induction furnace maintained at a temperature of 200-4000C for 6 to7 hrs. Figure1. The mould for producing chilled ductile iron. International Journal of Advanced Engineering Research and Studies 2.4 Tensile tests The tensile test specimens were prepared according to AFS (American Foundryman’s Society) standards [7]. The specimens were taken from various locations in the casting, namely 25 mm, 75 mm, 1.50 mm and 225 mm from the chill end, the last being right at the riser end. The longitudinal axes of these specimens were parallel to the longitudinal axis of the copper chill during casting. The ultimate tensile strength (UTS) of these tensometer specimens were measured using an Instron Tension Testing Machine. 2.5 Hardness tests Hardness tests were conducted on each of the specimens prepared for microstructural examination. As specified in the AFS standards [7], a Brinell Hardness Tester with 5 mm ball indenter and 300 kg load was used on the polished specimens. Each hardness result was obtained from an average of at least three repetitions on the same sample. 3.0 Results and discussion 3.1 Ultimate tensile strength (UTS): The experimental results of the ultimate tensile strength tests on castings using various chilled like copper, graphite, and without chills are tabulated in Table 2.Comparing the results in this table it can be seen that changing the chill materials and without using any chill does have a significant effect on the UTS of the casting. If other factors are kept constant, using the copper chills generally causes a marked increase in the UTS. This implies that increasing the rate of chilling results in an increase in the UTS of the material. However, the chilling effect is more significant on copper chilled casting compared to other chilled castings and also without using any chilled castings. When a graphite chill is substituted with a copper chill, probably because these two chills have different thermal conductivity a small difference in their volumetric heat capacities (VI-K) relative to their actual VI-KS. It can also be clearly seen that if all other factors are kept constant, graphite chilled casting and also without using any chills invariably has the lowest UTS. This shows that increasing the copper content results in an increase in the UTS of the material. Moreover, the farther away from the chill the specimen is taken from, the lower is the UTS, with the exception of the specimens taken right at the riser end. This could be because the farther away from the chill the specimen is, the lower is the rate of chilling. The graph showing the comparison results of Ultimate Tensile Strength in MPa with copper chill, graphite chill and without using any chills (as shown in graph 2). Graph 2: Graph showing the comparison results of Ultimate Tensile Strength in MPa IJAERS/Vol. I/ Issue II/January-March, 2012/240-243 E-ISSN2249–8974 Table 2: Comparison results of Ultimate Tensile Strength (σu) in MPa 3.2 Hardness Test: The experimental results of the hardness tests done on castings chilled using different metallic and nonmetallic chills and without using any chills are tabulated in Table 3. It is a well-known fact that for metallic materials, hardness increase as tensile strength increases. The results obtained for hardness are therefore not surprising. Comparing the results in Table 3, it can be seen that, as in the case of UTS, changing the different chill does have a significant effect on the hardness of the casting. The graph showing the comparison results of hardness tests with copper chill, graphite chill and without using any chills (as shown in graph 3). Table 3: Comparison results of hardness (BHN) test: Graph 3: Graph showing the comparison results of hardness (BHN) test 4.0 CONCLUSIONS Tensile strength and hardness of austempered chilled ductile iron is highly dependent on the chilling rate as well as Cu and Mo combinations. An increase in the rate of copper chilling compare to graphite chill and without using any chill and increase in the Cu content of the material both result in an increase in tensile strength and hardness due to an increase in the nodule count especially at the chill end. The mechanical properties of austempered chilled ductile iron are also significantly affected by bainite content of the material. Therefore the vast areas of applications of austempered chilled ductile iron are in the manufacture of components where the periphery of the part needs very good surface properties. The following are examples: 1. gears and pinions; 2. crank shafts; 3. drive shaft yoke and related components to replace forged and case hardened steels. 4. in gearboxes low noise may be important; and International Journal of Advanced Engineering Research and Studies 5. the high damping-capacity of cast irons leads to less vibration and transmission noise than occurs with steels. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. Hemanth J, Seah KWH. WEAR 1996; 192:134. Hemanth J, Seah KWH. J Mater Des 1995; 16:175. K H W Seah, J Hemanth and S.C Sharma, Tensile strength and hardness of sub-zero Chilled cast iron, Materials & Design Volume 16 Number 3 1995, 175 Blackmore PA, Harding RA. First International Conference on ADI. Chicago, 1984:197. Voight RC, Loper CR. First International Conference on ADI. Chicago, 1984:84. Dodd J, Gundlach RB. BCIRA Conference on development for future foundry prosperity. University of Warwick, 1984. AFS Cast Metals and book, 4th edition, Des Plaines, Ilhnois, 1957, 82. Doelman, R. L. et ul. The beneficiation of inoculated cast iron with silicon carbide as a major source of silicon and carbon. Trans AFS 1980, 88,787. Gilbert, G. N. T. The elastic properties of flake graphite irons. British Foundryman, July 1968, 264. Ruff, G. F. and Wallace, J. F. Effects of solidification structures on the tensile properties of grey iron. Truns AFS 1977, 85, 179 Wallace, J. F. and Evans, E. B. Trans. AFS 1958, 52,49 Ruddle, R. W. The Institute o/Metals Publication, London, 1957. Grant, J. W. and Morrison, J. C. The British Foundryman, 1972, 65, May, 172. IJAERS/Vol. I/ Issue II/January-March, 2012/240-243 E-ISSN2249–8974