抄録
The effect of bismuth on the eutectic solidification of cast iron was investigated by thermal analysis followed by microscopic observation for the purpose of analyzing the mechanism of suppressing mottling. Bismuth addition to gray iron increases both the eutectic cell number and the chill depth, decreases length of graphite flake, and changes graphite flakes morphologically to undercooled ones firstly accompanied by mesh-like and/or cluster type graphite and finally accompanied by plate-like cementite. Bismuth particle exists in the pearlite matrix. In mottled cast iron having lower carbon equivalent value, smaller addition of bismuth is necessary for obtaining the required supercooling to stabillize cementite in the structure, partly and fully.
The process of both cell formation and abnormal graphite precipitation was discussed by observing the microscopic structures of specimens quenched from various stages of eutectic solidification. Bismuth prevents eutectic cell formation and produces refined graphite substructure and hexagonal structure. This means that bismuth is adsorbed on the graphite surface. Moreover it has been clarified that bismuth particles are suspended in liquid iron during eutectic solidification in quenched specimens.
