Effect of rapid solidification on the microstructure and microhardness of BS1452 grade 250 hypoeutectic grey cast iron

Containerless solidification of low alloyed commercial grey cast iron in two different cooling media (N-2 and He) using a 6.5 m high vacuum drop-tube have been investigated. Both the conventionally cooled, as-cast alloy and the rapidly cooled drop-tube samples were characterized using SEM, XRD and Vickers microhardness apparatus. The estimated range of cooling rates are 200 K s(-1) to 16,000 K s(-1) for N-2 cooled droplets and 700 K s(-1) to 80,000 K s(-1) for He cooled droplets (in each case for 850 mm and 38 mm diameter droplets respectively). Microstructural analysis reveals that the as-received bulk sample displayed a graphitic structure while the rapidly cooled samples display decreasing amounts of alpha-Fe as the cooling rate increases. At moderate cooling rates a is replaced with gamma and Fe3C, while at higher cooling rates with alpha'. Microhardness increase with cooling rate but cannot be mapped uniquely onto cooling rate, suggesting undercooling also influences the mechanical properties. (C) 2016 Elsevier B.V. All rights reserved.