Fracture Characteristics of Austempered Spheroidal Graphite Aluminum Cast Irons

The fracture characteristics of austempered spheroidal graphite aluminum cast iron had been investigated. The chemical content of the alloy was C3.2, Al2.2, Ni0.8 and Mg0.05 (in mass percent, %). Impact test samples were produced from keel blocks cast in CO2 molding process. The oversized impact samples were austenitized at 850 and 950 °C for 2h followed by austempering at 300 and 400 °C for 30, 60, 120 and 180 min. The austempered samples were machined and tested at room temperature. The impact strength values for those samples austempered at 400 °C varied between 90 and 110 J. Lower bainitic structures showed impact strength values of 22 to 50 J. The fractures of the samples were examined using SEM. The results showed that the upper bainitic fracture revealed a honey Comb-like topography, which confirmed the ductile fracture behavior. The lower bainitic fractures of those samples austempered for short times revealed brittle fracture.