To Study Effect of Austempering Time and Temperature On Spheriodal Graphite Iron

Transcription

1 Volume-5, Issue-5, October-2015 International Journal of Engineering and Management Research Page Number: To Study Effect of Austempering Time and Temperature On Spheriodal Graphite Iron Aashish Tuli 1, Kiranjot Kaur 2, Ramesh Kumar 3 1,2,3 Assistant Professor at Chandigarh Group of Colleges, Landran, Mohali, Punjab, INDIA ABSTRACT The Spheroidal Graphite Iron is known for its good elongation or ductility (from the beginning of the casting process only) at the part of low tensile strength.so an experimental research was conducted on ADI (with an alloy composition having relatively higher percentage of Cu) for improving the tensile strength, hardness and elongation with the variation of austempering temperature and different time periods. The variables, austempering temperature were 350 C and 400 C and holding time periods of 0.5hr and 3hrs. Experimental resultsproved that with an increase of austempering temperature and time, both tensile strength and hardness decrease whereas elongation increases.light microscopy was utilized to investigate the bainitic transformation, while tensile tests were conducted for determining mechanical properties. By austempering at 350 C for.05hr and 3h, a microstructure comprising a mixture of free bainitic ferrite and carbon rich retained austenitewas observed.also, appearance of martensitic structure is the characteristic of austempering at400 C is. This appearance of martensite afteraustempering at400 C remains the main cause for much lower tensile properties than at 350 C. Keywords--- SG Iron, ADI, Austempering, Light Microscopy. I. INTRODUCTION ADI has found an increasing application due to its excellent mechanical properties such as high strength, more toughness, high machinability (all that at, low cost). These excellent mechanical properties of ADI material are due to its microstructure that comprises of ausferrite which consists of high carbon austenite and bainitic ferrite with dispersed graphite nodules. This austempered microstructure is a function of the austempering time and temperature. Thus, obtaining desired mechanical properties selection and control of proper austempering time, temperature and alloying additions is important. Therefore, an attempt was made in the present work to study the effect of austempering temperature and time on the mechanical properties of ADI like tensile strength, elongation and hardness by carrying out austempering treatment of specimens at 350 C and 400 C for 0.5hr and 3hrs. 213 Copyright Vandana Publications. All Rights Reserved. II. LITERATURE SURVEY Austempered Ductile Iron (ADI) offers the design engineer best combination of low cost of casting, flexibility in design, good machinability, high strength to weight ratio & good toughness, wear resistance [1]. ADI is an interesting class of materials because of its special microstructure and properties. When ADIis subjected to austempering treatment, it transforms to a microstructure consisting of ferrite and stabilized austenite, and because of the presence of stabilized austenite, ADI exhibits seemingly good strength, ductility, good fatigue and wear properties [2,3]. Developed austempered ductile iron used for high tensile strength was studied by Ravishankar K.S[6] and found that two step austempering resulted in a microstructure consisting of a bainitic structure, the relativeproportion dependedupon first step time. Tensile strength and yield strength increase with increasing first steptime and the ductility as well as tensile strength decrease. The improved fracture toughness obtained through two stepaustemperingwas due to fine ferrite particle size, increased carbon content of retained austenite, and theincreased stability of retained austenite. M.A.Yescas alsoconcluded that improved properties of ADIare a function of their chemical composition and austempering variables [7].

2 III. 3.1 Material EXPERIMENTAL WORK The composition of alloy in SG Iron sample used for testing is given below table 4.1. Table 4.1 Material Composition. Alloys C Si Mn S P Mg Cu % Tensile Test Specimen The specimens used for determining the mechanical properties had grip diameter 20mm, gauge diameter 14mm, gauge length of 70mm, grip length of 50 mm for 100 tonneutm testing (Figure 4.1). Figure 3.1 Tensile specimen specifications 3.3 Heat Treatment Spheroidal Graphite Iron are primarily heat treated so that there would be certain amount of pearlite andferrite inthe matrix of the ductile iron.the heat treatment comprises two stage processing: Austenitizing andaustempering a) Required no. of specimens (for each observation 2 samples and a total of 4) were heated to thetemperature of900 C in muffle furnace for 1 hrso that the specimen got properly homogenized. b) A salt bath was prepared by taking salt mixture in a saltbath(containing 50% NaNO 3 and 50% KnO3)furnace. The salt remains in the liquid state in the temp range of C.The Salt water temperature had been monitored by digital temperature meter. c) After the specimens were properly homogenized at 900 C these were taken out of thefurnace and immediately put in the salt bath furnace where the containers with the saltmixture were kept at 350 C and 400 C. d) In the salt bath the specimens were held for 0.5 hrs and 3hrs, as given in table 4.2. In this time the austenite gets converted to required microstructures. Theobjective behind choosing the maximum temperature of 350 C is that heat treatingwithin this temperature will give lower bainitic ferrite which is acicular in structure sothat the properties developed in the materials are excellent. e) During transfer of the samples to the salt bath or cooling to room temperature, because of slight oxidation of a surface of cast iron, there is a possibility of scale formation onthis surface. To avoid these difficulties the surface of the specimens were polished to removethe scales from the surface. After the scale removal the specimens were ready for thefurther experimentations. Table 3.2 The Austempering Temperature & Time. Austenite Temperature Austempering Temperatures Holding time in Salt Bath hr. 3 hr hr. 3 hr. IV. RESULT AND DISCUSSION The results of the experiment performed in the laboratory are given below in the table 5.1and carefully depicted in figures 5.1, 5.2 and 5.3 which helps to describe 214 Copyright Vandana Publications. All Rights Reserved.

3 effect of different austempering time and temperature on the mechanical properties of SGI. Austenite temperature (0C) 900 Austempering temperature (0C) Table 4.1 Results of all specimens Austempering Tensile strength time in MPa (in hrs) Elongation in % Hardness in HRB Copyright Vandana Publications. All Rights Reserved.

4 Also, it was seen that the graphite nodules in specimens of as-cast SG ductile iron were non-uniformly distributed. More than 90% spheroidisationwas clearly visible and average nodule size variedfrom 28-67μm, and an average nodule distribution of about 98 per mm 2 (Figure 5.4). Effect of austempering temperature and time on microstructure of ADI is shown in Figure 5.5a to 5.5d. ADI when austempered at 300 C for 0.5h resulted in large amount of martensite formation from the unreacted austenite during cooling at room temperature (Figure 5.5a). Themartensite formation after shorteraustempering time is initiateddue to lower carbon content of few austenitic regions. However whenaustempering time is increased to 3h the martensite amount decreases with the simultaneous increase in amount of bainitic ferrite and rich carbon austenite (Figure 5.5b) andaustemperingat 400 C for 0.5hr resulted in a microstructure containing martensitealongwith some unstable austenite (could be seen in the dark areas), stable retained austenite (could be seen in the white areas), whereas martensite (looking platelike)remains the major microstructural constituent (Figure5.5c). At 3hr ofaustempering dark blocks containing high fraction plate-like bainite may be seen (Figure 5.5d). It should be mentioned thatsuchmartinsitic structure is the distinguishable characteristic for austempering at 400 C. 216 Copyright Vandana Publications. All Rights Reserved.

5 V. CONCLUSION The results indicate that after austempering of SG iron a correlation between microstructure and mechanical properties exists. Austempering at 320 C for 0.5hr produces a typical ADI microstructure comprising of free bainitic ferrite with stablecarbon rich retained austenite. The whole range of austempering time at 400oC is distinguished by the presence of blocky austenite and themartensite was formed duringsubsequent cooling to the room temperature. It is also concluded that with decrease inaustempering temperature, the tensile strength and hardness increase gradually. Also, when the austempering time and temperature increases, the elongation increases. REFERENCES [1] Hughes I.C.H, Austempered Ductile Irons - Their Properties & Significance,1985. [2] W.F. Smith, Structure and Properties of Engineering Alloys, second ed. McGraw-Hill, [3] Labreeque C & Gagne M. Reviewed of Ductile Iron: Fifty years of ContinuousDevelopment, Canadian Metallurgical Quarterly, [4] O. Eri, M. Jovanovi, L. Sidjanin and D. Rajnovi Microstructure And Mechanical Properties Of CunimoAustempered Ductile Iron, [5] B. Harding, Austempering Ductile Iron Castings- Advatages, Production, Properties and Specifications, BCIRA report, 56(1991)356. [6] Ravishankar K.S, K.RajendraUdupa, P.Prasad Rao., Development of austempered ductile iron for high tensile and fracture toughness by twostep austemperingprocesss, 68th World foundry congress, [7] Yescas. M.A. Bhadeshia.H.K.D.H, Mackay.D.J., Estimate of the amount retained austenite in austempered ductile irons using neuralnetworks, Material Science and engineering, A322, pp , [8] Mullin J.D, Ductile iron data for design engineers, Rio tinto iron & titanium inc., Montreal, Quebec, Canada, [9] Kim Yoon-Jun, Shin Hochoel, Investigation into mechanical properties of austempered ductile cast iron (ADI) in accordance with austempering temperature, Materials Letters, Vol. 2, 2006, [10] Yunlongbai, Yikumluan, Song Nannan, Kang Xiuhong, Li Diamond and YiyiLi., Chemical composition, microstructure and mechanicalproperties of roll core used ductile iron in centrifugal casting composite rolls, J. Mater, Sci, Technol, 28(9), [11] Cho.G.S, Lee.K.H, Ikenaga.A., Effects of alloying elements on the microstructure and mechanical properties of heavy section Ductile iron,j.mater, Sci. Techno, [12] Nakae Hideo, Jung Sanghoon, Kitazawa Takayuki., Eutectic solidification mode of speroidal graphite cast iron and graphitization, Chinafoundry, [13] Wenbang Gong, Guodong Chen and Gangyu Xiang., Calculation of carbon content of austenite during heat treatment of cast irons, Chinafoundry, [14] B.D. Cullity, Elements of X-Ray Diffraction,2nd Edition,Addison-Wesley Reading, MA, 1978, p.324 [15] B. Harding, Austempering Ductile Iron Castings- Advatages, Production, Properties and Specifications, BCIRA report, 56(1991)356. [16] P.A. Blackmore and R. A. Harding, in Proc. 1st Int. Conf. on Austempered Ductile Iron, Chicago, IL, American Foundrymen`s Society, 1984, p Abbreviations Used: SGI: Spheroidal Graphite Iron ADI: Austempered Ductile Iron 217 Copyright Vandana Publications. All Rights Reserved.