Strength and toughness of ductile cast iron

Ductile cast iron is used for various structural components which need high strength and toughness. The evaluation of fracture toughness and an understanding of fracture mechanism are necessary for the design and use of this iron. Ductile fracture changes to brittle fracture with a lowering of the testing temperature and the transition temperature is affected by loading rate, notch shape and specimen geometry. The average value of J(AC) and J(R), that is J(mid), is proposed to define the fracture toughness of this iron. J(mid) is considered to be a reasonable measure for the fracture toughness, irrespective of loading condition and the pearlite content in the matrix. On the other hand, newly developed AD1 with greater toughness and good fatigue properties is introduced, where microsegregation effect of alloying elements and special austempering treatment are applied. Refining and increasing the stability of the retained austenite phase and avoiding the precipitation of carbide are the key factors in toughening ADI.