2022 Volume 62 Issue 2 Pages 313-320
The eutectoid transformation of δ-ferrite → γ-austenite + carbide by carburization was studied comprehensively with Fe–Cr binary alloy. It was observed that δ-pearlite consisted of γ-austenite and M23C6 carbide formed with unidirectional lamellar structure parallel to the carburization direction, and consequently the γ-austenite transformed to α’-martensite on quenching. The reconstruction of the parent γ-austenite orientation revealed that the both of austenite and M23C6 phases have a preference growth along [011] direction while keeping a cube-on-cube orientation relationship within δ-pearlite. The growth rate of δ-pearlite followed parabolic kinetics, and thus, the interlamellar spacing of δ-pearlite was gradually increased according to the Zener-Hillert model. Furthermore, the δ-pearlite formation mechanism was discussed under PE (para-equilibrium), NPLE (non-partitioning local equilibrium), and PLE (partitioning local equilibrium) conditions. As a result, it was concluded that the PLE model with long-range Cr diffusion through the interface diffusion is dominant, which leads to the redistribution of Cr between austenite and M23C6 at the eutectoid reaction front.