Peritectic Reaction andδ-γTransformation Mechanism in Carbon Steels

Abstract

Fe-C binary alloys and commercial carbon steel were quenched at various temperatures above and below the peritectic temperature. Petitectic reaction and the mechanism of subsequent transformation from δ phase to γ phase were investigated on the basis of solidification structure and measurement of carbon concentration by electron probe microanalysis in the quenched specimen. The result obtained are summarized as follows.
When the peritectic reaction occurs, primary δ dendrites are isolated from surrounding liquid because γ phase is formed on the surface of the primary δ crystals.
The γ phase nucleates successively in the untransformed primary δ crystals as temperature falls below the peritectic temperature and then conjugated concentration relationship between the δ and γ phases holds locally true. On the other hand, the γ phase formed on the surface grows into the surrounding residual liquid.
As the γ phase elements nucleated in the untransformed δ region begin to interconnect with each other, the diffusion path for solute is progressively established, and carbon diffuses gradually through the γ phase formed on the surface to that nucleated in the untransformed δ region.