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Univerzitet u Zenici
  Doctoral theses defended at the University in Zenica

Date and place of thesis defense:
19.03.2010. Faculty of Metallurgy and Materials Science
Candidate:
mr. sc. Hasan Avdušinović
Mentor:
prof. dr. Sreto Tomašević

Thesis title:

Investigation Of Optimal Parameters For Production Of Austempered Ductile Iron Castings With Different Wall Thickness

Summary:
Austempered ductile iron is a material that has high tensile strength (800-1600 MPa), good ductility, elongation up to 16 % and good resistance to wear, as a result of the microstructure that is obtained in the casting after heat treatment (ausferrite). The microstructure consists of acicular ferrite and carbon-stabilized saturated austenite. The presence of saturated austenite in the microstructure gives a high ductility and toughness and fine grainy structure of the entire matrix provides high tensile properties. Investigation of properties of austempered ductile iron within this thesis has included three cycles:
1. Preliminary investigations have included determining the heat treatment parameters of samples (10x35x200mm) in laboratory conditions to achieve ausferrite microstructure. Heat treatment was carried out on ten samples including variation of the basic parameters of heat treatment (austenitisation time and temperature, austempering time and temperature). Metallographic analysis and results of mechanical tests of treated samples showed that the austenitisation temperature has the largest influence on the final microstructure and properties of the samples. With increasing austenitisation temperature the appearance of pearlite in the microstructure is observed and deviation of the mechanical properties. Parameters of the heat treatment that are most suitable for the chemical composition of samples and laboratory conditions in which the experiments were carried out are: Ta=850°C, ta=60min, Tip=370°C and tip=90min.
2. Major investigations - Within the major research studies have been extended to the samples of size 10x35x200mm, 20x35x200mm and 30x35x200mm, and the task was to define the optimal value of the interval of austempering time for all three investigated wall thickness (processing window). Comparing to the preliminary investigation only austempering time was different during major research. Analysis of the results of metallographic and mechanical testing showed that the optimal interval for austempering time was between 80 to 100 minutes. Samples for all three thickness treated in this interval had a full ausferritic microstructure and the values of mechanical properties required by the standards. All samples treated in this interval showed increase of the value of mechanical properties with very good ductility.
3. Heat treatment of representative casting - After the major research and analysis of the obtained results heat treatment of representative castings is carried out with the aim of certification of obtained optimal parameters on the casting with different wall thickness (8 to 20 mm). Heat treatment is carried out on three castings and the results of metallographic analysis have shown full ausferrite microstructure of the castings. Mechanical tests have shown that the tensile force of heat treated castings increased by over 80 % compared to the tensile force before heat treatment.

Key words:
Austempered ductile iron, ADI, austenitisation, austempering, ausferrite, saturated austenite, acicular ferrite


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