The objective of this study is to investigate carbides in low alloyed ductile cast iron containing less than 3mass%Cr or 2mass%Mo, respectively. All of the carbide in the ductile cast iron containing 3%Cr was Fe3C, and the carbide in the ductile cast iron containing 2mass%Mo was Fe3Mo3C, Mo2C and Fe3C. The calculated phase diagram shows that the stable carbides at the room temperature were Cr7C3 and Cr3C2 in the cast iron containing Cr, or MoC in that containing Mo. Fe3C or Mo3Fe3C which solidified from the molten metal transforms to stable carbide by the in situ nucleation mechanism with decreasing of temperature. The transformation from the unstable to the stable carbide proceeds from the concentration of Cr or Mo in the unstable carbide. Therefore, it is considered that the cooling time of the keel block used in this study was not so long as the stable carbide such as Cr7C3 and Cr3C2, or Mo2C precipitate.
