A process from the occurrence of a delta dendrite(delta) to austenite(gamma) grain formation is investigated. When a hyper-peritectic carbon steel is unidirectionally solidified and quenched in water, three regions of delta dendrited, fine columnar grain and coarse grain are observed. The region of the delta dendrites is corresponding to (L+delta) phase. The regon of the fine columnar grain is (KL+delta+gamma) and (L+gamma) phases. The region of the coarse grain is (L+gamma) and gamma phases. The size of the narrow side of one fine columnar grain changes suddenly to the region of the coarse one when a liquid fraction in the (L+gamma) phases reduces below a certain value. On the other hand, in a hypo-peritectic carbon steel, three regions are observed, same as the hyper-peritectic carbon steel, The region of delta dendrites is correpsonding to (L+elta) phase, and fine columnar grain is (L+elta+gamma) and (delta+gamma) phases. The coarse grain is gamma phase. Furthermore, the region of the fine columnar grain changes suddenly the region of the coarse grain when the delta phase completely disappears. Likewise, the narrow size of the region of the coarse grain when the delta phase completely disappears. Likewise, the narrow size of the fine columnar grain coincides with the primary dendrite arm spacing. However, the gamma grain boundaries of the hypo-and hyper-peritectic carbon steels form at different location. That is ,they are formed in the skelecton of the delta dendrite and in the interspacing of the primary dendrite arm. Therefore, it seems that the liquid phase in the gyper-peritectic carbon steel and the delta phase in the hypo-peritectic carbon steel act as the prevention of the gamma grain growth.
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