Influence of Austempering Temperature on Fracture Toughness of a Low Manganese Austempered Ductile Iron (Adi)

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 (a + c) 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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“…These results are consistent with the published literature ( Ref 3,4,8,[15][16][17][18][19][20][21][22][23][24][25][26].…”

Section: Heat Treatments and Microstructuressupporting

confidence: 96%

The Nature of the Tensile Fracture in Austempered Ductile Iron with Dual Matrix Microstructure

Kılıçlı

Erdoğan

2009

J. of Materi Eng and Perform

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“…The obtained microstructure consisting of ausferritic ferrite free of carbide and stable, reacted, highly enriched retained austenite, attained the best combination of mechanical properties. (6) It has been demonstrated that the prolonged austempering time (beyond the time at which the Stage II reaction starts) results in a transformation of the stable, reacted, highly enriched retained austenite to bainite (ferrite and carbides).…”

Section: Discussionmentioning

confidence: 99%

“…A further contribution in the field of optimization of heat treatment parameters was given by Putatunda [6].…”

Section: Introductionmentioning

confidence: 99%

Austempering kinetics of Cu-Ni alloyed austempered Ductile Iron

et al. 2014

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The aim of the paper was to investigate the effect of austempering parameters (time and temperature) on the microstructure and mechanical properties of ADI alloyed with 1.5% Cu and 1.6% Ni (in wt.%) in order to establish the optimal processing window. It was shown that the strength, elongation and impact energy strongly depend on the amounts of ausferritic ferrite and retained austenite. A processing window was established according to the results of the kinetics of the isothermal transformation. The results show that the processing window for ADI alloyed with Cu and Ni at 350°C was relatively wide, while the processing window for the isothermal transformation at 400°C becomes narrower and shifted to the left. The processing window of ADI austempered at 300°C is also narrower, but shifted to the right towards the longer times compared to the processing window of ADI austempered at 350°C.

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“…Most studies of ADI focused upon the effects of austempering treatments Ausferrite obtained using higher austempering temperatures bear coarse upper bainitic ferrite adjacent to plenty of retained austenite This structure exhibits lower hardness, strength, and higher ductility. Conversely, needle‐like lower bainitic ferrite is formed at lower austempering temperature leading to higher hardness, strength, and reduced ductility.…”

Section: Introductionmentioning

confidence: 99%

Microstructural Evolutions and Properties of Partially Austenitizing and Austempered Ductile Irons

Chen

Tsai

2015

steel research int.

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In this study, commercial FCD700 ductile irons are austenitized in the intercritical region to observe their effects upon the microstructures and mechanical properties of austempered ductile irons (ADIs). The austenitizing temperature is chosen such that ferrite, austensite, and graphite coexist in the microstructures. Strength of 974 MPa, 16.4% elongation, and impact energy of 166 J are achieved by the ADI austenitized at 1103 K and subsequently austemperted at 673 K. These properties are in contrast to 790 MPa strength, 8.2% elongation, and 42 J impact energy in as-cast FCD700. The properties are also superior to the traditionally austempered ductile iron with 967 MPa strength, 12.7% elongation, and 124 J impact energy. Combinations of the optimum ductility and strength are achieved by 8-19% of proeutectoid ferrite and over 20% of retained austenite in the intercritically treated microstructures. Nanoscale twinned and lenticular martensite forms within the microsize blocky retained austentite islands during plastic deformation leading to the transformation induced plasticity. The intercritically austenitizing and austempering treatments provide unique combination of strength and elongation for commercial ductile irons.

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Influence of Austempering Temperature on Fracture Toughness of a Low Manganese Austempered Ductile Iron (Adi)

Cited by 24 publications

References 17 publications

The Nature of the Tensile Fracture in Austempered Ductile Iron with Dual Matrix Microstructure

The Nature of the Tensile Fracture in Austempered Ductile Iron with Dual Matrix Microstructure

Austempering kinetics of Cu-Ni alloyed austempered Ductile Iron

Microstructural Evolutions and Properties of Partially Austenitizing and Austempered Ductile Irons

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