Comparison of Mechanical Properties of Austenitic Ductile Cast Iron with Ferritic/Pearlitic Ductile Cast Iron

Abstract

Austenitic ductile iron belongs to the family of ductile iron which performs better properties than conventional ductile iron due to its soft matrix. Ductile iron (DI) has got enormous applications in different fields such as wind turbines, automotive components, spun pipe and fittings etc. As mechanical properties are primarily dependent on the matrix structure, different heat treatments are performed to the ductile iron to achieve required matrix. Alloying additions are made to stabilize the matrix structure at all the temperatures. In the present work, investigation has been conducted on mechanical properties of Ferritic/PearliticDuctile Iron and Austenitic Ductile Iron and comparison has been made by varying different heat treatments. Here, Austenitic phase is stabilized by adding Nickel as alloying element.
Stress relieving and austempering heat treatments are performed to Ferritic/Pearlitic Ductile Iron. Stress relieving treatment is carried out by heating the specimen to 600˚C, furnace cooling to 290˚C followed by air cooling to room temperature. Austempering treatment is carried out by heating the sample to 925˚C, quenching in salt solution maintained at 475˚C followed by air cooling to produce ausferritic matrix. Annealing treatment is done to Austenitic Ductile iron by heating the specimen to 1000˚C and then furnace cooled to room temperature to produce coarse grain structure. The microstructures of all the heat treated and as cast specimens are viewed under optical microscope and planes obtained for different phases are determined using X-Ray diffractometer. Morphological quantification like nodularity, nodule count are determined by following ASTM E2567-13a standard. Mechanical properties like tensile tests and hardness are conducted on (UTM) INSTRON-1995 and Vickers hardness tester respectively. Impact energies at room temperature and -20˚C are determined from Charpy impact tester. Fractures surfaces are viewed under scanning electron microscope in order to find out the type of fracture the specimen has undergone.
The results showed that the microstructures of as cast and stress relieved Ferritic/Pearlitic Ductile iron have 98% modularity with approximately83% ferrite as its matrix and 17% graphite, leading to higher amount of ductility and impact toughness. Whereas ADI consists of 93 % upper bainite and7% graphite nodules, resulting in higher hardness with considerable amount of ductility due to 95% nodularity. The redistribution of Carbon in austenite was shown 1.104%.
Austenitic Ductile iron consists of uniform 88 % austenitic matrix and 12% graphite nodules embedded in it with around 96 % nodularity. Austenitic Ductile Iron showed lower hardness and tensile strengths with higher elongations when compared to all the as cast and heat treated Ferritic/PearliticDuctile iron. As cast and stress relieved Ferritic/PearliticDuctile iron showed lower impact energies with quasi cleavage mode of fracture at room temperature but at -20˚C the values were decreased leading to slight brittleness in the material. Austempered Ductile iron showed brittle fracture at -20˚C from quasi cleavage fracture at room temperatures. Whereas austenitic ductile to brittle transition has occurred.