Influence of the microstructure on the crack resistance of cast irons

The morphology of the surface formed after crack propagation for five nodular and four lamellar graphite cast irons both with ferritic and ferritic-pearlitic microstructure has been studied. Single edge notched three point bending specimens were fatigue precracked and then tested. The critical stress intensity factor K C in plane stress loading conditions has been determined. The crack paths and the morphology of the fracture surfaces were investigated by light microscopy and scanning electron microscopy. The influence of the microstructural components on the type of fracture and the K c values has been studied. It is established that the presence of a plastic deformation zone in the crack tip vicinity is typical for the nodular cast irons where relatively ductile type of fracture takes place. The graphite and pearlite constrain the plastic deformation in the crack tip vicinity thus reducing the K C values. The graphite plays a principal role in this phenomenon. The behaviour of the graphite and the pearlite is entirely different in the brittle type of fracture typical for the lamellar graphite cast irons - both these phases favour the crack propagation.