Fatigue Damaging Micromechanisms in Ductile Cast Irons

Ductile iron discovery in 1948 gave a new lease on life to the cast iron family. In fact these cast irons are characterised both by a high castability and by high toughness values, combining cast irons and steel good properties. Ductile cast irons are also characterised by high fatigue crack propagation resistance, although this property is still not widely investigated. In the present work we considered three different ferritic-pearlitic ductile cast irons, characterised by different ferrite/pearlite volume fractions, and an austempered ductile cast iron. Their fatigue crack propagation resistance was investigated in air by means of fatigue crack propagation tests according to ASTM E647 standard, considering three different stress ratios (R = Kmin/Kmax = 0.1; 0.5; 0.75). Crack surfaces were extensively analysed by means of a scanning electron microscope both considering a traditional procedure and performing a quantitative analysis of 3D reconstructed surfaces, mainly focusing graphite nodules debonding mechanisms and considering the microstructure influence.