13 ijaers feb 2016 28 review of enhancement in mechanical properties using austempered ductile iron

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International Journal of Advanced Engineering Research and Science (IJAERS)

[Vol-3, Issue-2, Feb- 2016] ISSN: 2349-6495

Review of Enhancement in Mechanical Properties using Austempered Ductile Iron (ADI) C S Wadageri1, R V Kurahatti2 1

Department of Mechanical Engineering, MMEC, Belagavi, Karnataka, India 2 Department of Mechanical Engineering, BEC, Bagalkot, Karnataka, India

Abstract— Austempered ductile cast iron (ADI) has arisen as significant engineering material in recent years. It has exhibited outstanding mechanical properties from perspective of structural applications. These include high strength with good ductility, good wear resistance, fatigue strength and fracture toughness. Hence its vast usage as an economical substitute for conventional materials in several structural applications especially in the automotive industry has drawn attention of researchers. Lot of research is been reported in this area. Brief review on the same is presented in this paper. Keywords— ADI, Fatigue properties, Strain hardening, Stepped Austempering I. INTRODUCTION When ductile iron is subjected to an austempering treatment, a range of microstructures is obtained depending on heat treatment parameters such as austenitizing time and temperature and austempering time and temperature [1-5]. This results in austempered ductile iron (ADIs) of different grades ranging from high strength-low ductility types, to low strength-high ductility ones, which have been found to be economical substitutes for high strength steels in several applications. Besides, it has the advantage of strength-to-weight ratio, toughness, wear resistance, fatigue resistance, lower material cost, lower production cost, better machinability, and higher damping capacity. ADI is considered a very promising engineering material, and is an economical substitute for wrought or forged steel in several structural applications in the automotive industry like crankshaft, transmission gears, connecting rods and in defense like cannon shells, aircraft landing gears, etc. The influence of heat treatment parameters on the microstructure has been extensively studied using optical microscopy, electron microscopy, and X-ray diffraction. Austempering treatment generally consists of (a) fully austenitizing the iron at suitable austenitizing temperature, (b) quenching to an austempering temperature and holding it for an appropriate length of www.ijaers.com

time for the isothermal transformation, and (c) air cooling to room temperature. Quenching from austenitizing temperature to austempering temperature should be rapid enough to avoid the formation of ferrite and pearlite in order to maximize toughness and ductility. The austempering reaction in ADI occurs in two stages. In the first stage, austenite matrix transforms to a mixture of acicular ferrite and carbon-enriched stabilized austenite, termed appropriately as ausferrite. The second stage consists of decomposition of the carbon-enriched austenite to a ferrite-carbide aggregate. The best combination of properties in ADI is obtained when austempering treatment is stopped when the first stage is nearly complete, but the transformation has not progressed well into the second stage. The austempering transformation in ADI differs from the analogous reaction in steels where austenite decomposes uniformly into the ferrite carbide aggregate called bainite in a relatively shorter period of time [6]. If the austempering time is too short, the first stage of the austempering reaction will be incomplete and the unreacted parent austenite will transform to martensite, resulting in poor toughness and ductility. Conversely, prolonged holding at austempering temperature will result in austenite enrichment to the point where it becomes less stable, and the second stage of the reaction occurs, resulting in more stable ferrite and carbide phases. This results in loss of toughness and ductility. The optimum austempering time is, therefore, the period between the end of stage I and the beginning of stage II. This is called the processing window. The processing window can be enlarged by adding alloying elements, such as nickel, molybdenum, or copper. Proper austempering thus produces a unique bainitic structure in ADI that consist of high carbon or transformed austenite and acicular ferrite with graphite nodules dispersed in it. Higher austempering temperature produces a coarser ferrite but a lower volume fraction of ferrite. This accompanied by a lower yield strength. Lower austempering temperatures Page | 61


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