Effect of Magnesium on the Solid-State Nucleation and Growth of Graphite During Annealing of White Iron

It is well-established that growth of spheroidal graphite occurs in several stages such as directly in the liquid, in the liquid through a solid austenite shell during the eutectic reaction, and in the solid because of decreased carbon solubility during cooling. The various mechanisms active during these stages of cast iron solidification have been the subject of much research over the recent years. To further understand graphite growth mechanisms, irons of similar composition but with three level of Mg (low-< 0.010%, medium-0.017%, and high-0.047%) were cast in copper molds to produce small disks with white microstructure. The disks were annealed at 950 degrees C to promote graphite growth, and then quenched in water after various holding times. The time evolution of nucleation and growth of graphite was analyzed through characterization of the microstructure by optical and scanning electron microscopy, on polished and deep-etched specimens. Unique structural features were observed in some unusual graphite aggregates, such as hemispherical graphite and hopper-like crystals. A mechanism for the growth of graphite in solid state is offered. The theory of multi-mechanism growth of graphite spheroids is confirmed.