Silicon is used as a deoxidizing (killing) agent in the melting of steel and as a result most steels contain a small percentage of Silicon.
Silicon is a graphitizer and if not counterbalanced by carbide-promoting elements, it favour solidification according to the stable iron-graphite system. Therefore, during solidification in the presence of silicon, carbon is precipitated as primary graphite in the form of flakes. Once primary graphite has formed, its shape cannot be altered by any method. It is these weak graphite flakes that break up the continuity of the matrix and the notch effect at the end of these flakes that accounts for the low strength and low ductility of gray iron.