Toughening of ADI Austenitized in Intercritical Region

The current article investigates the influences of intercritical austenitizing temperatures on microstructures and mechanical properties of austempered ductile irons. A series of intercritical austenitizing temperatures ranging from 775 to 900 degrees C are used and austempering is performed at 300 and 400 degrees C on a conventional unalloyed FCD700 ductile iron. Experimental results show that mechanical properties, including strength, ductility, and the toughness, all increase with intercritical austenitizing temperatures till an optimum austenitizing temperature of 830 degrees C. At this optimum processing condition, strength of 974MPa, impact energy of 166 J, and 16.4% ductility is achieved. These properties are much higher than non-treated ductile iron with 790MPa strength, 42J impact energy, and 8.2% ductility. Microstructure refining via well distributed sub-micron ausferrite structure is essential for the increase of strength. Furthermore, the increased carbon content of retained austenite and austenite volume fraction combines to increase toughness through martensitic transformation giving rise to transformation induced plasticity (TRIP) effect.