Impact Fracture Characteristics of Cast Irons

Abstract

Fracture mechanisms of cast iron under impact loading are not understood so clearly as under static loading. In this study, the instrumented Charpy impact test has been carried out on an annealed grey cast iron with ferrite-pearlite aggregate structure and an annealed nodular cast iron with ferrite structure, and an analysis of impact fracturing behaviour and microfractography on the fractured specimens have been performed.
Impact values of grey cast iron are not affected by specimen size and notch effects, and these values are not changed by temperature. The fracture process of the grey cast iron can be divided into two stages from the observation of load-deflection curves; i.e., crack-initiation and crack-propagation stages. In the initiation stage, the flaky graphites are cracked and these cracks propagate unstably in the next stage. According to the result of microfractography, cleaved graphite cracks are connected by intergranular fracture and, in some cases, cleavage or tear fracture of ferrite.
In the nodular cast iron, a transition behaviour with temperature is observed. Especially, in unnotched specimens, this transition is observed clearly and the energy absorbed is mainly occupied by the work for crack-initiation. However, in notched specimens, the impact values are reduced largely and a load-deflection curve of the crack-propagation type is observed. From the result of microfractography, graphite nodules act as a ductile crack nucleating site. At low temperature, however, graphite nodules act as arrester of cleavage crack propagation.
Further, fracture toughness values of such a low strength brittle material as cast irons are evaluated from the unstable fracture load during impact; these values nearly coincide with the unit crack-propagation energy measured from the area under the load-deflection curve.