Acicular ferrite morphologies in a medium-carbon microalloyed steel

The influence of time and isothermal transformation temperature on the morphology of acicular ferrite in a medium-carbon microalloyed steel has been studied using optical and transmission electron microscopy (TEM). This study has been carried out with the analysis of the microstructures obtained with one- and two-stage isothermal treatments at 400 °C and 450 °C, following austenitization at 1250 °C. The heat treatments were interrupted at different times to observe the evolution of the microstructure at each temperature. The results show that a decrease in the isothermal transformation temperature gives rise to the development of sheaves of parallel ferrite plates, similar to bainitic sheaves, but intragranularly nucleated. These replace the face-to-edge nucleation that dominates the transformation at higher temperatures. The TEM observations reveal that the plates correspond to upper acicular ferrite and the sheaves to lower acicular ferrite. In this last case, cementite precipitates are present at the ferrite unit interiors and between the different platelets.