Fatigue propagation of long cracks in ferritic thin wall ductile iron castings

The present work focuses on the fatigue propagation of long cracks in ferritic ductile iron parts of varied thickness. Samples were taken from thin wall ductile iron plates and standard 'Y' blocks with nodule counts ranging from 250 to 2200 nod/mm(2) and ferrite grain sizes from 70 to 150 mu m. Fatigue crack propagation threshold Delta K-th and Paris law constants C and m were measured, and the influence of the microstructure refinement (established through nodule count and grain size) on the characteristics of the fatigue fracture surface was analysed. The results show that, once the crack is initiated, the fatigue crack growth threshold diminishes as the microstructure is refined, while the rate of growth remains practically unaltered. The crack path preferentially intersects graphite nodules and is mostly transgranular through the ferrite grains. Nevertheless, at low levels of Delta K, a low percentage of intergranular fracture was also observed.