δ-pearlite Reaction by Carburization in Fe-Cr Binary Alloy

The eutectoid transformation of delta-ferrite -> gamma-austenite + carbide by carburization was studied comprehensively with Fe-Cr binary alloy. It was observed that delta-pearlite consisted of gamma-austenite and M23C6 carbide formed with unidirectional lamellar structure parallel to the carburization direction, and consequently the gamma-austenite transformed to a'-martensite on quenching. The reconstruction of the parent gamma-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 8-pearlite. The growth rate of 8-pearlite followed parabolic kinetics, and thus, the interlamellar spacing of 8-pearlite was gradually increased according to the Zener-Hillert model. Furthermore, the 8-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 M23Ce at the eutectoid reaction front.

Automated summary

The eutectoid transformation of delta-ferrite to gamma-austenite and carbide was studied in Fe-Cr binary alloy. The growth of gamma-austenite and M23C6 carbide in pearlite was found to have a preference growth direction along [011], with a cube-on-cube orientation relationship within the 8-pearlite structure. The formation of 8-pearlite followed parabolic kinetics, with interlamellar spacing gradually increasing over time. The dominant mechanism for 8-pearlite formation was determined to be partitioning local equilibrium with long-range Cr diffusion through the interface.