Effect of phosphorus on bainite transformation in low-carbon steel

The effect of P on the kinetics of bainite transformation was investigated in Fe–0.08% C–3.0% Mn and Fe–0.08% C–3.0% Mn–0.05% P (mass%) alloys. Alloys transformed isothermally at 803 K for 3–1000 s were analysed using dilatation measurements, scanning electron microscopy, and field-emission electron probe microanalysis. The initiation of transformation in the 0.05% P alloy occurred later than in the P-free alloy, suppressing bainite transformation from the austenite grain boundaries. In addition, the transformation ratio during stasis in the 0.05% P alloy was lower than in the P-free alloy, and the C concentration in the untransformed austenite decreased. The driving force could not explain the suppression of bainite transformation because the presence of 0.05% P had a negligible effect on the thermodynamic stability of the bcc and fcc phases. These results indicate that the segregation of P at the grain boundaries affects the nucleation and growth of bainitic ferrite.