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

摘要

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 primaryδphase in the (L +γ) region solidifies to a dendritic structure, and the fine columnarγgrains are exist in the boundary part of the (L+γ) region near to the (L+δ+γ). The columnar shape of the fineγgrains is caused by the shape ofδ-dendnte. A transition from the fineγgrains to coarse columnarγgrains occurs in the (L +γ) region when a volume fraction of liquid lowers. The structure of coarse columnarγgrain have no relation with dendrite structure. The fine columnarγgrains disappear as a result of growth of coarse columnarγgrain. 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.