The material which offers design engineer the best combination of low cost, design flexibility, good machinability, high strength to weight ratio & good toughness, wear resistance is Austempered Ductile Iron (ADI) [2]. Austempered Ductile Irons are an interesting class of materials because of their unique microstructure and interesting properties. When subjected to austempering treatment, ductile iron transforms to a microstructure consisting of ferrite and stabilized austenite rather than ferrite and carbide as in austempered steels. Technical literature oftendescribes this matrix as bainite (although it does notcontain carbides) [2] [7]. The matrix is also referred to asausferrite microstructure (high carbon austenite +ferrite) [14]. The structure of Austempered Ductile Iron is obtained by exactlycontrolled process of heat treatment of nodular castiron.Because of the presence of stabilized austenite, ADI exhibits excellent combination of strength and ductility, together with good fatigue and wear properties [1] [8]. Compared to the conventional grades of Ductile Iron, ADI delivers twice the strength for a given level of ductility in the form of elongation. But achieving excellent mechanical properties of the ADI material is not an easy task as they depend on austempered microstructure which is a function of its processing window [4]. The optimum combination of high carbon austenite and bainitic ferrite of ADI makes it possible to compete against steel forgings and other engineering alloys in terms of mechanical properties, physical properties, weight saving and all that at low cost [14]. ADI offers superior combination of properties because it can be cast like any other member of the ductile iron family, thus offering all the production advantages of a conventional ductile iron casting [6] [18]. The ductile Iron casting issubsequently subjected to the austempering process to produce mechanical properties that are superior to conventional ductile iron and forged steel[15][19]. Due to its vast area of applications,extensive work is being carried out now-a-days to study the processing and characterization of this material [14] [19].

To produce ADI, ductile iron must undergo austempering process. While the steps for austempering ductile Iron are essentially the same as

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carbon austenite and ferrite [17]. Austempering process consists of following-

• Heating to a temperature to produce austenite.

• Quenching rapidly to avoid pearlite formation or martensite. The quench temperature is called as austempering temperature.

• Holding at the selected austempering temperature for a time sufficient to transform the austenite to the desired end product; bainite for steel and ausferrite for ductile iron[9][19].

EXPERIMENTAL PROCEDURE

Experimental material was cast in the electric induction furnace. The basic charge was formed by pig iron, steel scrap and additives for the control of chemical composition. FeSiMg7 modifier was used for modification and FeSi75 inoculant was used for inoculation [7]. Ferrite-pearlitic nodular cast iron was used asbasic material for isothermal heat treatment. The austenitization temperature was 920°C andthe holding time at this temperature was 30 minutes.The isothermal transformation of austenite wasrealized in AS 140 salt bath at the temperatures 420,380, 320 and 250 °C and the holding time at this temperaturewas from 30 to 300 minutes (by 30 min. step)[9][14].Preparation of cast iron specimens is difficult due to the need to properly retain the graphite phase. The specimens were subjected to coarse grinding using motor driven emery belt. Coarse grinding is required to planarize the specimen and to reduce the damage created bysectioning. The planar grinding step is accomplished by decreasing the abrasive grit/ particle sizesequentially to obtain surface finishes that are ready for polishing. The machine examplegrinding/polishing pressure, speed, and the direction of grinding/polishing. The other steps wererough polishing using abrasive papers of successively finer grades. In order to ensure that theprevious rough grinding damage is removed when grinding by hand, the specimen should berotated 90 degrees and continually ground until all the scratches from the previous grindingdirection are removed. If necessary the abrasive paper can be replaced with a newer paper toincrease cutting rates. Then fine polishing was done in a cloth polishing mill using aluminapowder as polishing agent. The purpose of final polishing is to remove only surface damage. Itshould not be used to remove any damage remaining from cutting and planar grinding. If thedamage from these steps is not complete, the rough polishing step should be repeated. Finally thesamples were etched for microstructure study. The purpose of etching is to optically enhancemicro-structural features such as grain size and phase features. Etching selectively alters thesemicro-structural features based on composition, stress, or crystal structure. The most commontechnique for etching is selective chemical etching. Chemical etching selectively attacks specificmicro-structural features. Here etchant used was nital (2% conc. Nitric acid in methanolsolution) and washed thoroughly and dried. Then the microstructures were taken for different heat treated specimens using Scanning Electron Microscopy (SEM) with required magnifications [4][6].

From the microstructural point of view the basic material (after casting) is ferrite-pearlitic nodular cast iron (Fig. 2) with 57% content of ferrite in a matrix, the size of graphite within 15 and 60 μm and count of graphitic nodules 205 mm2. Graphite occurs only in a perfectly-nodular (80%) and imperfectly-nodular (20%) shape [14].

Sagar A. Patil1*, Sanket S. Vathare2 2

without copper austempered at 350°C were studied under scanning electron microscope with 550X magnification. (a) Without copper (b) With copper.

ADI is a heat treated Ductile Iron with a unique micro-structure consisting of Ausferrite which consists of high carbon Austenite and Bainitic ferrite with graphite nodules dispersed in it.ADI is a ductile iron that has been heat treated by the austempering process to make it tougher than regular ductile iron of half the strength. ADI is comparable in strength to heat treated wrought steels, has exceptional wear and fatigue resistance and has the ability to be work hardened [10][16]. As the carbon rich austenite phase is stable in Austempered Ductile Iron it enhances the bulk properties. Furthermore, while the austenite is thermodynamically stable, it can undergoa strain-induced transformation when locally stressed, producing islands of hard martensite that enhance wear properties. This behaviour contrasts with that of the metastable austenite retained in steels, which can transform to brittle martensite [6][20].

The content of retained austenite is slightlydecreased with increasing holding time in all sets ofspecimens. The shape, size and count of graphiticnodules in the specimens after isothermal heattreatment are not changed in comparison with thespecimen of basic material (after casting)[10][14].

The austenitizing temperature should be chosen so that the component is in the austenite + graphite (γ + G) phase field. Elements like Silicon raise the UCT while Manganese will lower it. If the austenitizing temperature is below the UCT or in the subcritical range (γ + α + G), then proeutectoid ferrite will be present in the final microstructure, resulting in a lower strength and hardness material. Once the ferrite forms, the only way to eliminate it is to reheat above the UCT. Fig.6shows the microstructure of an austempered material that was austenitized below the UCT [19].

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Graphite is the stable form of pure carbon in cast iron. Its important physical properties are low density, low hardness and high thermal conductivity and lubricity. Graphite shape, which can range from flake to spherical, plays a significant role in determining the mechanical properties of cast irons. The compact spheroid interrupts the continuity of matrix much less than graphite flake and this result in higher strength and toughness compared with a similar structure of gray cast iron [3]. Fig.7 (a) and (b) show that graphite flakes act like cracks in the iron matrix, while graphite spheroids act like “crack arresters", giving the respective irons dramatically different mechanical properties [3].

In iron-carbon alloys, though graphite is more stable phase than cementite but kinetically,it is easier to produce cementite than graphite. When graphite forms directly from the liquid, it iscalled primary graphitization, but when the graphite forms from the dissociation of cementite, itis called second stage graphitization. The formation of graphite from liquid takes place in anarrow range of temperature interval (1153-1147°C for plain carbon) and also the formation ofgraphite from austenite (738-727°C), i.e., under the condition of small under cooling. Thisrequires the alloy to be cooled slowly [5]. Nucleation of graphite requires much more energy and large amount of diffusion ofcarbon to the nuclei and iron atoms away from it to get 100% carbon to segregate. So formationof graphite from liquid in pure iron-carbon alloy is almost a rare reaction, unless the factors forgraphite formation are favorable. Commercial cast iron melt always contains fine particles ofinclusion, even particles of graphite. Nucleation of graphite on some of these inclusions needsless energy than that required or nucleation of cementite and thus graphite crystal can be formedkinetically even at temperatures below 1147oC [10].Graphite thus can form not only from liquid or austenite (738-727°C), the metastablecementite under favorable conditions, such as high temperatures can decompose to austenite +graphite (above738°C) or ferrite + graphite (below 738°C). The former is a faster process, butcomplete graphitization does not occur as 0.68% C remains dissolved in austenite, whereas thelatter is the slower process, but complete graphitization occurs [11] [16].

The alloying elements that are typically added for hardenability purposes include: Cu, Ni and Mo. Manganese additions are not recommended because of the tendency of Mn to segregate to the regions in between the graphite nodules. Manganese delays the austempering reaction, which can result in the

Sagar A. Patil1*, Sanket S. Vathare2 2

carbon austenite [19]. Nickel and Copper segregate to graphite nodule sites and do not form detrimentalcarbides [5]. According to the experimental work of P. W. Shelton, A. A. Bonner [12] and Olivera Eric [13] alloying with copper increases elongation and impact energy but decreases thestrength of ADI. As shown in Fig. 8, it was seen that a straight forward addition of copperto a conventionally produced and heat treated ADI had not produced the well defined coppercoated graphite nodules those were hoped for. In some cases the copper was associated withgraphite nodules, but not as a thin film. There were also isolated pools of copper present in themicrostructure comparable in size with graphite. The distribution and size of graphite spheroidswas similar however for both the materials, with no evidence that the copper addition influencedeither. Where there was porosity, a concave graphite surface or irregularity in the shape of thegraphite nodule, the copper was often attracted to those features [18].

been maximized. Ni additions of up to 2 % are typically made. Beyond that, the price becomes an important consideration. Lastly, Molybdenum is a potent hardenability agent. Unfortunately, it segregates highly to the intercellular/interdendritic locations between the graphite nodules. Molybdenum is strong carbide former. Fig.9 shows a photomicrograph of Molybdenum carbides that were present in ADI with a Mo addition. The formation of Mo carbides is undesirable, especially if a component is to be machined after heat treatment [19].

From the study of microstructure of properties of austempered ductile iron the following conclusions could be drawn- 1. The specimens with higher temperature of isothermal transformation of austenite have the matrix created by upper bainite and retained austenite and the specimens with lower temperature of isothermal transformation ofaustenite have the matrix created by lower bainiteand retained austenite. 2. The content of retained austenite is decreased withincreasing holding time at the temperature ofisothermal transformation. 3. The shape, size and count of graphitic nodules arenot changed in dependence on the temperature ofisothermal transformation of austenite and independence on the holding time at thistemperature.

This work has been supported by Atlanta Metallurgical Services, Kolhapur and Vivekanand

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[1] W.F. Smith, “Structure and Properties of Engineering Alloys”, second ed. McGraw-Hill, 1993. [2] Singh V, “Physical Metallurgy”, Standard Publishers Distributors, 1999. [3] Mullin J.D, “Ductile iron data for design engineers”, Rio tinto iron & titanium inc., Montreal, Quebec, Canada, 1990. [4] Rajan T.V, Sharma C.P, Sharma A, “Heat treatments-Principles and techniques”, PHI, New Delhi, 2006. [5] Davis J.R& associates, “Cast iron”, ASM Society Hand book, 1996. [6] Keough J. R, Hayrynen K. L, “Austempered cast irons”, Applied Process, Inc., Livonia, Michigan, 2004. [7] Hughes I.C.H, “Austempered Ductile Irons - Their Properties & Significance”, Materials & Design, Vol-6, No-3, 1985, 124-126. [8] Labreeque C & Gagne M. “Reviewed of Ductile Iron: Fifty years of Continuous Development”, Canadian Metallurgical Quarterly, 37, 5, 1998 343-378. [9] Kim Yoon-Jun, Shin Hochoel, “Investigation into mechanical properties of austempered ductile cast iron (ADI) in accordance with austempering temperature”, Materials Letters, Vol. 62, 2006, 357-360. [10] Kustuv A, Taran Y, Uzlov J, Krimmel A, Evsyukov M, “Formation of bainite in ductile iron”, Materials Science and Engineering, A 273-275, 1999, 480-484. [11] Taran Y.N, Uzlov K.I, Kustuv, Bainitic reaction kinetics in austempered ductile iron, A.Y, J Phys. (Fr.) IV 7, 1997, C 5-429. [12] P.W. Shelton, A.A.Bonner, “The effect of copper addition to the mechanical properties of austempered ductile iron (ADI)”, Material Processing Technology, 173, 2006, 269-274.

[15] Wilson Luiz Guesser, FábioKoda, Jairo Alberto Blanco Martinez, Carlos Henrique da Silva, “AUSTEMPERED DUCTILE IRON FOR GEARS”, SAE International, 2012-36-0305, 1-8. [16] Adel Nofal, “Advances in the Metallurgy and Applications of ADI”, Journal of Metallurgical Engineering (ME) Volume 2 Issue 1, January 2013, 1-18.

[18] HasanAvdusinovic, AlmaidaGigović-Gekic, “HEAT TREATMENT OF NODULAR CAST IRON”, 13th International Research/Expert Conference, “Trends in the Development of Machinery and Associated Technology”, 2009, 1-4. [19] Kathy L. Hayrynen, “The Production of Austempered Ductile Iron (ADI)”, World Conference on ADI, 2002, 1-6. [20] “Austempered ductile iron”, http://www.aditreatments.com.

Undergraduate student, Mechanical Engg., PVPIT Budhgaon