NUCLEATION BEHAVIOR DURING SOLIDIFICATION OF CAST-IRON AT HIGH UNDERCOOLING

During the solidification of cast iron, a modest melt undercooling is a common observation, but the identity of the nucleation catalyst limi ting the amount of undercooling has been elusive. In order to examine the influence of primary phases on the heterogeneous nucleation behavi or of secondary phases in both hypoeutectic and hypereutectic cast iro ns, solidification catalysis experiments were carried out with differe ntial thermal analysis (DTA) and droplet samples. To prevent decarburi zation and desiliconizaiton a SiO2-Al2O3-CaO slag containing a droplet dispersion has been developed for cast irons. With the droplet sizes of less than 100 mu m, large maximum undercoolings below the liquidus of 200-620 degrees C were obtained in both hypoeutectic and hypereutec tic alloys. The maximum undercooling level (Delta T-max/T-I) reached u p to 0.40 for a Fe-2.85wt.%C-1.91wt.%Si alloy. The undercooling for he terogeneous nucleation of secondary phases below the stable eutectic e xtended up to 430 degrees C for hypoeutectic alloys. The undercooling for a hypereutectic alloy was 20 degrees C. Based on the undercooling results, for hypoeutectic alloys the primary austenite iron (gamma-Fe) is a poor nucleant for both graphite and cementite (Fe3C). For hypere utectic alloys, primary graphite is very catalytic for gamma-Fe. These results were examined by a lattice disregistry model and relative int erfacial energy between nucleating solid and catalyst with respect to liquid. At large undercoolings, a variety of solidification microstruc tures have been observed. (C) 1997 Elsevier Science S.A.