Effect of Si content on the deterioration of vibration fracture resistance of ferritic SG cast iron under an aqueous environment

Ferritic spheroidal graphite cast irons generally exhibit intergranular fracturing even if the specimens are strengthened by silicon in solid solution when tested under wet conditions. The maximum area fraction of intergranular fracture is closely related to the microstructural feature and ambient aqueous environment that caused the deterioration in vibration fracture resistance. In spite of the increasing silicon content, experimental evidence confirms the overall D-N curves can be generalized into three characteristic regions. Intergranular cracks initiated in the vicinity of the nodular graphite; the existence of nodular graphite acting as porosity should be considered as a dominant microstructural factor on the initiation of intergranular fractures. It should be noted that the intergranular cracks can be prevented, resulting in better vibration fracture resistance, when a specimen is covered with oil film. Based on experimental results, when the silicon content is increased, the vibration fracture resistance in general will be improved for all specimens whether they are tested in oil mist, air or aqueous environments. In particular, when the deflection amplitude is fixed, the vibration fracture resistance can be significantly raised for a high silicon sample (4.4Si).