Formation of Ultrafine Graphite Nodules in Ductile Iron and its Effects on Mechanical Properties

The evolution of ductile iron with martensitic matrix during annealing at 700 & DEG;C was studied in the present study. The results showed that the ultrafine graphite nodules with a count of 8500 nod/mm2 and an average diameter of about 1 & mu;m were obtained in ductile iron through annealing treatment. The martensite in ductile iron was first decomposed into granular cementite, and then, the granular cementite aggregated and gradually transformed into ultrafine graphite nodules. With the annealing time extended to 90 min, the martensite was completely transformed into ferrite and ultrafine graphite nodules. These results implied that martensite in ductile iron was a good starting microstructure for forming the high graphite nodules in ductile iron by solid phase transition. The precipitation of ultrafine graphite nodules improved strength and plasticity of ductile iron, which was mainly because ultrafine graphite nodules refined ferrite grains, weakened stress concentration, and hindered crack propagation during deformation. The present study provided a new method to increase graphite nodule count in ductile iron.