Effect of Graphite Size on the Growth of Spheroidal Graphite Cast Iron

Abstract

  To ascertain the growth theory of irreversible graphite migration, two spheroidal graphite irons of coarse and fine graphite nodules from the same melt were heated cyclically in still air and the relation between growth characteristics and change of graphite phase was investigated. Furthermore effect of graphite nodule size on heat resistance of spheroidal graphite iron was discussed from the standpoint of irreversible diffusion occurring during dissolution and separation of graphite. The results obtained were as follows. 1) Growth characteristics of s.g. irons from the same melt depended significantly upon the nodule size. Growth rate of the coarse graphite iron at the maximum was about twice as high as that of the fine graphite iron. 2) By heating the coarse graphite iron number of graphite nodules and ratio of fine nodules in the graphite size distribution increased remarkably. 3) By heating the fine graphite iron ratio of fine nodules decreased and graphite nodules were enlarged. 4) Referring to the growth theory of irreversible graphite migration, the difference of growth characteristics of s.g. irons with different graphite size was attributed mainly to the fact that the distance of carbon diffusion in the austenitic matrix was longer in the coarse graphite iron than that in the fine graphite iron. 5) The effect of graphite size reduction in improving heat resistance of s.g. iron was interpreted by the theory of irreversible graphite migration.