Microstructural Evolution as a Function of Cooling Rate in Rapidly Solidified Commercial Grey Cast Iron

The influence of post-production re-engineering of microstructure through rapid solidification processing at constant elemental composition on microhardness of low alloyed commercial BS1452 grade 250 grey cast iron has been studied. In this investigation, the effect of cooling rate on rapidly solidified droplets cooled separately in Nitrogen and Helium in a 6.5 m high drop-tube are compared against conventionally solidified as-cast alloy. Powder sample sizes obtained ranges from ≥ 850 µm to ≤ 38 µm in diameter with corresponding estimated cooling rate of 200 K s-1 to 16,000 K s-1 in N2 and 700 K s-1 to 80,000 K s-1 in He gases respectively.

Microstructure evaluation were made by light optical and SEM, while XRD and TEM were employed for evolved phase’s identification and confirmation. DTA was used to determine the onset of the evolved metastable phase and deep cryogenic treatment of the droplets further transform fractions of the retained austenite to martensite. The microscopy result shows that the as-cast bulk sample reveals extensive graphite flakes randomly distributed in a ferritic – pearlitic dendritic matrix, meanwhile the same was absent in virtually all the droplets samples. However, with decreasing droplet size (i.e increasing cooling rate); there was a progressive phase transformation from the initial ferrite (α-Fe) phase fraction decreasing to retained austenite (γ-Fe) phase which further decreases as α'-Fe increases in smaller droplets with evidence of undercooling effect.

The relationship between cooling rate (R ̇) and the individual droplet diameter (D) in measuring microhardness values in the two media are governed by a power functions R ̇ = 6.40 x 10-3D-1.45 in N2 gas and R ̇ = 7.75 x 10-3D-1.60 in He gas. Hence, a cooling rate of ~ 200 K s-1 in N2 results in approximately double the measured hardness value of the as-cast (conventionally cooled) material. Meanwhile, Helium gas has five times better thermal conductivity compared to Nitrogen.