The effects of Mg and S on the spheroidization of graphite were investigated. The process of the graphitization was controlled in an annealing atmosphere under of various partial pressures of hydrogen diluted with argon. The results obtained are as follows:
(1) The spheroidization of graphite by Mg was almost independent of the rate of graphitization.
(2) The spheroidizing effect of S is, on the contrary, considerably affected by the rate of graphitization, which means that there exists an incubation time required for the spheroidization.
(3) The incubation time is shortened as the annealing temperature increases in the temperature range between 1173 and 1253 K.
The spheroidizing mechanisms of Mg and S may be as follows. Mg becomes gas bubbles in molten cast iron and is frozen in by casting. S causes micropores during annealing at temperatures higher than its boiling point. C atoms precipitate into these voids or pores to form spheroidal graphite.
