The effect of initial carbide morphology on abnormal grain growth in decarburized low carbon steel

Abstract

The effect of fine or coarse carbide particles on abnormal grain growth in a low carbon steel subjected to small deformation was studied. Grain growth was induced in a decarburizing atmosphere at 1450 ‡F (788 ‡C) in order to study the effect of the initial carbide particle morphology on the subsequent motion of the grain boundaries at temperature. In this manner, the primary influence of second phase particles during straining could be evaluated. Transmission electron microscopy revealed that the large carbide particles exhibit a larger dislocation density area around them than the small carbide particles after straining up to ten percent. At higher strains, cell formation begins and the dislocation distribution is more uniform. Both coarse and fine carbide particle samples demonstrated three stage abnormal grain growth at strains below 10 pct. An incubation stage, which decreases with increasing strain, is followed by an abnormal grain growth stage, in which certain selected grains grow at the expense of other grains. Finally an equilibrium stage is reached in which the largest grain size is obtained for the lowest strain. At the lowest strains coarse carbide material has a longer incubation time and coarser Stage III grain size than the fine carbide alloy.