Phase transformation, microstructural evolution and property modification in rapidly solidified grey cast iron

The phase transformation and microhardness changes of hypoeutectic grey iron subjected to rapid solidification in Helium was studied to showcase the relationship between the evolved microstructure and microhardness of this important engineering material. Droplets samples were prepared, using containerless processing via drop-tube technique. These rapidly solidified samples were collected and sieved into 8 size ranges from 850 µm to 53 µm diameters with corresponding estimated cooling rate range of ~900 K/s to ~78,000 K/s respectively. The emerged phases and evolved morphologies were characterized using optical and scanning electron microscopy and X-ray diffraction analysis; while Vickers microhardness tester was used to measure the hardness of the various samples. As the cooling rate increases, the undercooling increased and martensitic or acicular ferrite structure was observed in the very small droplets which confirms the progressive increase in microhardness of the samples from the as-cast to decreasing droplets sizes.