Effect of cooling rate on drop-tube processed commercial grey cast iron

This study focuses on the fundamental of solidification of commercial grey cast iron as a function of the externally applied cooling rate. Grey cast iron powders were prepared using the drop-tube method, which is a good analogue for commercial production via high pressure gas atomization. The as-solidified droplets were collected and sieved into size ranges from > 850 μm to < 53 μm diameter, with estimated cooling rates of 500 K s-1 to 75,000 K s-1, with each sieve fraction being prepared for metallographic characterization. The microstructure and phase composition of the powders were analyzed using XRD, optical and scanning electron microscopy, with the results being compared against a control sample subject to slow cooling in the drop-tube crucible; which has typical grey cast iron microstructure with extensive flake graphite in a largely ferrite matrix. In contrast, flake graphite was absent in virtually all the drop-tube samples, even in those with the most modest cooling rates. Microstructural analysis revealed that as the cooling rate increased there was less fragmentation of the primary austenite/ferrite dendrites and the volume fraction of primary dendritic material increased. Hence, as the particle fractions get smaller (D < 106 μm) there is a distinct microstructural evidence of a martensite phase which is related to its better mechanical properties (microhardness) as the sample sizes decrease.