Volume 23, Issue 5 (September 1995)
Fracture Toughness of Unalloyed Austempered Ductile Cast Iron (ADI)
The influence of microstructure on the tensile properties and the plane strain fracture toughness (KIc) of unalloyed austempered ductile cast iron (ADI) was examined in room temperature ambient atmosphere. The crack growth mechanism during fracture toughness tests was determined through detailed fractographic studies. Compact tension and round cylindrical specimens were prepared from unalloyed ADI and were given four different austempering heat treatments to produce four different microstructures. Tensile properties and fracture toughness of these four differently heat-treated materials as well as cast materials were determined as per relevant ASTM standards.
The results of the present investigation demonstrate that the fracture toughness of unalloyed ADI increases with increase in volume fraction of ferrite in the matrix and reaches a peak when the ferrite content of the matrix is around 65%. Both yield and ultimate tensile strength of the unalloyed ADI was found to increase with increase in volume fraction of ferrite in the matrix. The ductility of ADI, on the other hand, was found to increase with increase in volume fraction of austenite in the matrix. The crack growth mechanism was found to be predominantly by the microvoid coalescence. The crack path appears to connect the graphite nodules along the way.