Abstract
The formation prcess of cast structure and ferritization prcess was investigated in spheroidal graphite cast iron containing 1.7～1.9%C, 1.4/2.0%si, and 0.2～0.3%Mn. Thermal analysis and quenching experiment revealed that the primary γ started crystallization at around 1364℃ and developed dendritically to about 88% of the specimen. Eutectic graphite crystallized in the residual liquid among dendrites and the area fraction of graphite (Fg) was about 0.4% just after eutectic solidification. Fg increased to 3.5% by the precipitation of secondary graphite during cooling to eutectoid temperature. The behaviors of Si and Mn during solidification were also clarified based on the partition coefficients of these elements to primary γ. Ferritization heat treatment was carried out on as-cast specimens with pearlite matrix by heating them at 740℃, 720℃, and 700℃ for 1h to 10h. The changes in fraction of ferrite during ferritization were examined using a series of specimens quenched during heat treatment. The higher the heating temperature, the higher was the ferritization rate. When heated at 740℃ and 720℃ for 10h, the fraction of ferrite exceeded 90%. Ferritization was retarded in the Mn-rich pearlite area where the melt finally solidified. Fg increased to 7.4% after full ferritization. The number and size of graphite nodules also increased with the precipitation of secondary graphite and ferritization treatment.
