Fracture behavior and mechanism in austempered ductile iron
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The fracture behavior of copper-alloyed austempered ductile iron (ADI) was studied using metallography and fractography of selected samples. Three different grades of ADI were developed by austenitization at 900 °C for 60 min, followed by austempering for 60 min at either 270, 330, or 380 °C. The variation in austempered microstructure was determined by scanning electron microscopy of metallographically prepared samples, and structural parameters such as volume fraction of austenite, carbon content, and bainitic needle width were determined from the X-ray diffraction of powdered samples. The effect of austempering temperature on these structural parameters and on hardness, 0.2% proof stress, ultimate tensile strength (UTS), percent elongation, and impact strength was also studied. The fracture behavior under tensile and impact loading was determined by examination of the fractured surfaces and transverse cross sections near the fracture surface. The hardness, 0.2% proof stress, and UTS decrease and the impact energy increases as the austempering temperature is increased, and the morphology of the bainitic structure changes from lower to upper.
Keywordsaustempered ductile iron bainitic crack initiation and growth fracture behavior fracture mechanisms
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