Peritectic transformation and austenite grain formation for hyper-peritectic carbon steel

Abstract

Three binds of solidifying regions, (L+δ), (L+δ+γ) and (L+γ) have been observed in an unidirectional solidified hyper-peritectic steel sample quenched from a temperature above the solidus temperature. The primary δ phase in the (L+δ) region solidifies to a dendritic structure, and the fine columnar γ grains are exist in the boundary part of the (L+γ) region near to the (L+δ+γ). The columnar shape of the fine γ grains is caused by the shape of δ-dendrite. A transition from the fine γ grains to coarse columnar γ grains occurs in the (L+γ) region when a volume fraction of liquid lowers. The structure of coarse columnar γ grain have no relation with dendrite structure. The fine columnar γ grains disappear as a result of growth of coarse columnar γ grain. The measured temperature ranges for the (L+δ+γ) region agree with those calculated under a non-equilibrium condition. The end temperatures of peritectic reaction and solidification calculated are somewhat lower than the equilibrium temperatures, and are independent of solidification rate.