Acicular ferrite morphologies in a medium-carbon microalloyed steel

The influence of time and isothermal transformation temperature on the morp hology of acicular ferrite in a medium-carbon microalloyed steel has been s tudied using optical and transmission electron microscopy (TEM). This study has been carried out with the analysis of the microstructures obtained wit h one- and two-stage isothermal treatments at 400 degreesC and 450 degreesC , following austenitization at 1250 degreesC. The heat treatments were inte rrupted at different times to observe the evolution of the microstructure a t each temperature. The results show that a decrease in the isothermal tran sformation 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 transformati on at higher temperatures. The TEM observations reveal that the plates corr espond 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.