Chawla, V: Fracture Behaviour

There is extensive work on the fracture of steel with ferrite and austenite structure, but little on fracture behaviour of ADI, whose microstructure also comprises austenite and ferrite. The present work is aimed in this direction. The role of austempering tempeature on the fracture behaviour of Ni-Mo austempered ductile iron has been studied. In present work, the fractographic and fractomicrographic features of the fractured specimens austempered at 270°, 330°, 380° C for 1 hr and broken in tensile test and impact test, respectively, have been studied. The matrix of ADI is a complex mixture of austenite, bainitic ferrite needles with or without carbide particle depends upon the austempering temperature. The matrix of ADI can withstand a certain amount of deformation before fracture during tensile or impact testing, however the graphite nodules in the matrix cannot deform and hence are barrier to matrix deformation and give rise to crack initiation. The crack propagation is influenced by the orientation of bainitic ferrite with respect to load direction, also by the presence of carbide particles inside the needles. Thus producing characteristic features.