“In-situ” Observation of the δ/γ Phase Transformation on the Surface of Low Carbon Steel Containing Phosphorus at Various Cooling Rates

Abstract

In this paper, the solid δ/γ phase transformation is observed “in-situ” on the surface of low carbon steels containing different phosphorus concentrations by the Confocal Scanning Laser Microscope (CSLM) at various cooling rates. The effects of phosphorus and cooling rate on the δ/γ phase transformation are discussed based on the experimental results and mathematical calculation. Phosphorus is shown to decrease the T_A4 temperature and to increase the T_A3 temperature at various cooling rates. This effect is enhanced by the redistribution of phosphorus from the γ phase to the δ phase during the transformation. Some retained δ phase is kept until the γ/α phase transformation in steel containing high phosphorus (0.2% P). The retained δ phase could retard the γ grain growth and promote the α phase precipitation. At the slow cooling rate, the γ cells appear first from the triple points or δ grain boundaries and then spread with finger-like patterns. While at the rapid cooling rate, the γ cells appear first from the δ grain boundaries with sword-like patterns, and spread sharply into both sides of initial δ grain boundary.