2000 Volume 86 Issue 2 Pages 86-91
In order to investigate the formation process from δ dendrite to γ grain in a hypo-peritectic carbon steel, an unidirectionally solidifying carbon steel sample is quenched from a temperature above the solidus temperature. Three zones of primary dendrite, fine columnar grain and coarse columnar grain are observed in the sample. The dendritic zone consists of (L+δ) phase, the fine columnar grain is both phases of (L+δ+γ) and (δ+γ), and the coarse columnar grain is γ phase. The width of the fine columnar grain agrees with the primary arm spacing of the δ dendrite and it is the same result as that for a hyper-peritectic carbon steel. The γ grain boundary in the zone of the fine columnar grain forms at the skeleton of the δ dendrite, while that for a hyper-peritectic carbon steel forms at the position between the δ dendrites. Since transition from the fine columnar grain to the coarse one occurs immediately after the disappearance of δ phase, the δ phase prevents the coarsening of the fine columnar grain. A length of the zone of the fine columnar grain decreases with increasing carbon concentration. The length extends according to the addition of ferrite-stabilizing elements. The extended effect becomes stronger in the sequence of Nb>V>Mo, while Ni as an austenite-stabilizing element have no influence on the length.
