Effect of cooling rate on grain size of ferrite in a carbon steel

Abstract

Ferrite grain refinement by accelerated cooling has been studied in a carbon steel. The size of ferrite grains dα formed by continuous cooling transformation from polygonal austenite has been measured as a function of cooling rate and austenite grain size dγ. In the cooling rate range studied (q= 0·05–5 K s−1), dα was found to be proportional to q−0·26dγ0·46. The mechanism of grain refinement by accelerated cooling is discussed, and it is shown that this occurs in the transformation where the ratio of nucleation to growth rate increases with a decrease in temperature. The austenite grain size dependence of ferrite grain size is shown to become progressively large as the nucleation mode changes from homogeneous to grain surface to edge to corner. A theoretical estimation of ferrite grain size formed by continuous cooling transformation was attempted on the basis of nucleation and growth rates. In the alloy studied, ferrite grain size was theoretically estimated to be proportional to q−0·17dγ0·33. This was in close agreement with the dependence obtained in the present experiment.MST/466