AUSTEMPERED DUCTILE IRONS WITH SUPERIOR MECHANICAL PROPERTIES DESIGNED BY UTILIZING A COMPUTATIONAL DESIGN APPROACH

Date

2023-9-7

Author

Yalçıner, Caner

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Given its unique microstructure providing the opportunity to have high strength, ductility, and toughness, austempered ductile iron (ADI) finds applications in various industries, including automotive, agriculture, energy, defense. This typical microstructure consists of acicular bainitic ferrite, graphite nodules, and retained austenite phases. Fine acicular ferrite formed during austempering treatment provides high strength and hardness to the material. Nevertheless, it is essential to note that the size of this ferrite phase plays a significant role in this context. Graphite nodules bring in ductility and arrest the cracks within the structure, thereby preventing their propagation. However, achieving a high nodularity percentage and ensuring the homogeneous distribution is imperative. Moreover, retained austenite in the microstructure undergoes stress-induced transformation when subjected to applied stress, enhancing material’s wear resistance. However, this behavior depends on the stability level of the retained austenite phase. This stability level is determined by chemical composition and morphology of retained austenite phase. Taking all these effects into consideration, it is believed that it is possible to develop new Austempered Ductile Iron (ADI) materials with superior mechanical properties compared to conventional ones, provided that the necessary adjustments are made to the microstructure. This study aims to design two distinct ADIs: one with ultra-high strength and moderate toughness, and another with high strength and high toughness. The design processes involve modifications in chemical compositions and heat treatment scenarios, utilizing the CALPHAD (CALculation of PHAse Diagrams) approach. These production parameters are specified to yield samples with varying microstructures, including different volume fractions of retained austenite with distinct chemical compositions, and varying sizes of ausferrite phases. Simultaneously, the goal is also to achieve a high level of nodularity and ensure a uniform distribution of the graphite phase. The mechanical properties of these samples are assessed through Brinell hardness tests, tensile tests, and impact tests at room temperature. In conclusion, aside from achieving the study objective, this thesis demonstrates the utility and consistency of the computational design methodology employed within its scope.

Subject Keywords

Austempered Ductile Iron, Austempering Treatment, CALPHAD Approach, Integrated Heat Treatment and Alloy Design, Computational Materials Design

URI

https://hdl.handle.net/11511/105434

Collections

Graduate School of Natural and Applied Sciences, Thesis