|M.Sc Student||Amran Yogev|
|Subject||Characterization and Modeling of Phase Transition Kinetics|
in Austempered Ductile Iron
|Department||Department of Materials Science and Engineering||Supervisor||Professor Emeritus Menachem Bamberger|
|Full Thesis text|
Austempered ductile iron (ADI) is a material that enables achieving a variety of excellent mechanical properties. The material is essentially a cast ductile iron that undergoes a specifically designed austempering heat treatment. The special microstructure of ADI, contains of graphite spheres embedded in ausferrite (microstructure of ferrite and carbon supersaturated austenite) matrix, offers design engineers an alternative to steel and aluminum alloys when high strength and low wear are required. In the present thesis, the transformation from austenite to ausferrite was investigated for two kinds of ADI alloys, with and without Cu addition, austenized at 900°C for 90 min and austempered for various times at two different temperatures, 400°C and 350°C, followed by water-quenching.
Current thesis focuses on understanding the phase transition involved in austempering and the influence of Cu addition and austempering temperature on the phase transition. The samples were investigated by optical microscopy, X-Ray Diffraction, Neutron Diffraction, Auger Electron Spectroscopy, Scanning Electron Microscopy, Transmission Electron Microscopy and Microhardness measurements. In addition, a modified quantitative kinetics model taking into account the nucleation sites of ferrite in ADI alloys was developed.
During the austempering process, the amount of the "primary" austenite (which transforms to martensite during water-quenching after austempering) decreases while the amounts of ferrite and carbon supersaturated austenite increase with the austempering time, until stabilization is reached. The carbon content in the austenite also increases with the austempering time, until stabilization is reached.
The addition of copper increases the fraction of the "primary" austenite in the alloy and the carbon content in the "primary" austenite after austenization. After austempering, the addition of copper decreases the final carbon content in the austenite, but increases the final austenite fraction.
Decrease of the austempering temperature decreases the austenite fraction, increases the final carbon content in the austenite and refines the ferrite needle.
A modified quantitative model, describing the kinetics of the ausferrite transformation, was developed. For the first time, the modified model takes into account the nucleation sites of ferrite in ADI.