The Nature of the Tensile Fracture in Austempered Ductile Iron with Dual Matrix Microstructure |
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Authors: | Volkan Kilicli Mehmet Erdogan |
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Affiliation: | (1) Materials Division, Metallurgy Education Department, Faculty of Technical Education, Gazi University, 06500 Besevler-Ankara, Turkey |
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Abstract: | The tensile fracture characteristics of austempered ductile irons with dual matrix structures and different ausferrite volume
fractions have been studied for an unalloyed ductile cast iron containing (in wt.%) 3.50 C, 2.63 Si, 0.318 Mn, and 0.047 Mg.
Specimens were intercritically austenitized (partially austenitized) in two phase region (α + γ) at various temperatures for
20 min and then quenched into a salt bath held at austempering temperature of 365 °C for various times and then air cooled
to room temperature to obtain various ausferrite volume fractions. Conventionally austempered specimens with fully ausferritic
matrix and unalloyed as-cast specimens having fully ferritic structures were also tested for comparison. In dual matrix structures,
results showed that the volume fraction of proeutectoid ferrite, new (epitaxial) ferrite, and ausferrite bainitic ferrite + high-carbon
austenite (stabilized or transformed austenite)] can be controlled to influence the strength and ductility. Generally, microvoids
nucleation is initiated at the interface between the graphite nodules and the surrounding ferritic structure and at the grain
boundary junctions in the fully ferritic microstructure. Debonding of the graphite nodules from the surrounding matrix structure
was evident. The continuity of the ausferritic structure along the intercellular boundaries plays an important role in determining
the fracture behavior of austempered ductile iron with different ausferrite volume fractions. The different fracture mechanisms
correspond to the different levels of ausferrite volume fractions. With increasing continuity of the ausferritic structure,
fracture pattern changed from ductile to moderate ductile nature. On the other hand, in the conventionally austempered samples
with a fully ausferritic structure, the fracture mode was a mixture of quasi-cleavage and a dimple pattern. Microvoid coalescence
was the dominant form of fracture in all structures. |
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