On the Isothermal Pearlite Transformation Kinetics of High-Carbon Hypoeutectoid Steels Affected by Nb Microalloying

The nucleation and growth of pearlite in high-carbon steels with and without Nb are studied based on the in situ observation, and the effect of Nb on isothermal pearlite transformation is revealed in detail. The results show that the pearlite nucleation rates of the studied steels decrease over the isothermal time. The addition of Nb increases the pearlite nucleation rate due to its refinement in prior austenite grains. In addition, the pearlite growth is found to be controlled by the grain boundary diffusion of carbon in this study. Nb reduces the pearlite growth rate due to the solute drag effect of solute Nb and/or the pinning effect of Nb-containing precipitates. Moreover, Nb delays the kinetics of isothermal pearlite transformation at an elevated austenitization temperature (1180 °C), while it accelerates the transformation at a lower austenitization temperature of 900 °C. This phenomenon is related to the competing effect between the promotion of Nb on pearlite nucleation and its hindrance on pearlite growth.