Formation of D-type Graphite at Center of Cast Iron

Abstract

  Graphite shapes in gray cast iron are determined by the course of solidification. Therefore, it is important to study the causal factors of graphite shapes. Traditionally, it has been reported that D-type graphite readily appears on the surface or thin walled portion of a casting. This phenomenon can be explained by thermal undercooling. On the other hand, there are cases in which D-type graphite appears in the center region of a casting. Such cases can not be fully explained only by thermal undercooling. In this research, we considered constitutional undercooling as one of the mechanism of D-type graphite formation in the center region. We conducted thermal analysis of iron melt equivalent to FC250 in the shell mold to obtain cooling curves for the surface and center region, respectively. We also observed microstructures in each of the regions.

  We found that A-type graphite was formed in the middle of the surface and the center while D-type graphite was formed along with A-type graphite in the center region. We also noticed that the temperature gradient became milder as the measured point got closer to the center. Therefore, we assumed that a certain range of the center region of a casting has an inclination for constitutional undercooling. We also assumed that the growth speed of a eutectic cell is accelerated during the constitutional undercooling of the iron melt. Hence, we concluded that the graphite shapes in the center region tend to become D-type by the effect of constitutional undercooling.