An analysis of the microstructure of the heat-affected zone of an ultra-low carbon and niobium-bearing acicular ferrite steel using EBSD and its relationship to mechanical properties

We describe here the microstructure of simulated thermal cycles with the different peak temperatures associated with the heat-affected zone (HAZ) in a 600 MPa acicular ferrite steel. The microstructure, crystallographic features, and the effective grain size of different regions in the HAZ were examined by transmission electron microscopy (TEM) and electron back scattering diffraction (EBSD) analysis. The study indicated that the effective grain size decreases with decrease in peak temperature and attains the smallest value at a peak temperature of 900 °C. However, EBSD analysis indicated that the effective grain size of the sub-zones is small, which contributes to the excellent low temperature toughness in the sub-zones of HAZ. Two hardness peaks were observed at temperatures of 800 °C and 1200 °C because of grain refinement. The study convincingly demonstrates that EBSD can accurately identify the effective grain size contribution on complex and fine-grained microstructures, and is helpful in predicting the grain refinement mechanism.