Characterization of rapidly solidified commercial grey cast iron in drop-tube

This study presents containerless solidification of BS 1452 grade 250 commercial grey cast iron using 6.5m drop-tube apparatus. It gives a comparative summary of microstructural changes that occur between the rapidly cooled droplet particles as against its conventional slowly cooled control as-cast sample. The bulk as-received sample was melted and rapidly cooled during free fall in high vacuum containerless equipment. These rapidly solidified samples were collected and sieved into size ranges from >850 μm to <53 μm diameter, corresponding to estimated cooling rate of 500 K s-1 to 75,000 K s-1 with each sieve fraction being prepared for metallographic characterization. The analytical methods used include; light optical microscope and scanning electron microscopy, x-ray diffraction and differential thermal analyses. The result of this investigation reveals that the microstructure of the as-cast sample shows flake graphite randomly dispersed in ferrite matrix which is typical of slowly cooled grey cast iron. In contrast, flake graphite was absent in virtually all drop-tube samples even those with modest cooling rate. The evolved microstructure clearly shows the effect of cooling rate on the transformation from the conventional to rapidly solidified droplet particles in terms of microsegregation.