In this study, white cast iron powders were made from cast iron melts by means of gas and water atomization methods and the annealing behavior of these powders was studied by SEM observation of the annealed structure. Furthermore, the annealing behavior of cast iron poured into a metal mold (10 mm φ) was also studied. The experimental results are summarized below.
  The graphitization rates of these cast iron powders were extremely higher than those of cast iron poured into a metal mold and graphitization began at the surface of the cast iron powder. The carbide of gas-atomized cast iron powder was completely decomposed in 120 s at 1273 K. On the other hand, carbide of cast irons poured into metal mold completely decomposed in 4.3 ks at the same temperature. However, the graphitization rate of fine powder produced by crushing cast irons poured into metal mold was very high and the graphitization time of these was similer to that of gas-atomized cast iron powder. It was confirmed that the graphitization time of cast iron powder is affected by the modulus (Volume/Area) of powder, the former being directly proportional to the latter. The results suggest that graphite preferentially precipitates at free surfaces of cast iron powder.