Migration of δ/γ Interface in Low Carbon Steels during Continuous Cooling

The δ-ferrite to γ-austenite phase transformation process of low carbon steel was observed in-situ under a confocal scanning laser microscope and the influence of manganese and chromium on the migration of δ/γ interphase boundary during the δ to γ phase transformation was studied. It was found that the δ/γ interphase boundary becomes unstable with the decrease of temperature, from planar to curved morphology during the δ to γ phase transformation of Fe-0. 08C steel and Fe-0. 08C-0. 8Mn steel. But in Fe-0.08C-0.8Cr steel, the δ/γ interphase boundaries are stable with planar morphology during the whole δ-ferrite to γ-austenite transformation. The destabilization of δ/y interphase boundary in Fe-0. 08C and Fe-0. 08C-0. 8Mn steels results from high degree of supercooling and sub-boundaries in γ, respectively. The stabilization of δ/γ interphase boundary in Fe-0. 08C-0. 8Cr steel results from the slow diffusion rate of carbon atom induced by the addition of chromium.