Acicular Ferrite Identification in Heat-Affected Zone of a Ti-Killed High Strength Low Alloy Steel

It is frequently found in the literature that an acicular ferrite (AF) dominant microstructure has unsteady or even low-impact toughness. It is necessary to analyze whether they are qualified for real AF. Herein, a Ti-killed high strength low alloy steel is prepared and subjected to heat-affected zones simulation tests to induce various AF-like microstructures. The morphological differences of these microstructures are compared, and their crystallographic characteristics are analyzed. It is found that even within a single prior austenite grain, the transformed microstructure can be heterogeneous. Local areas having clear lath boundaries and a chaotic or interlocking morphology should be classified as AF. Adjacent laths in these areas are from different close-packed plane and Bain groups. In contrast, local areas having vague lath boundaries and a granular-bainite-like morphology should be classified as granular bainite. Neighboring laths in these areas are from the same Bain group, and a relatively low density of high-angle grain boundaries can be found. The heterogeneous microstructure consisting of granular bainite and AF has a wider grain size distribution and a higher area fraction occupied by large grains, which can lead to the scatter of Charpy impact energy. Criteria for the identification of AF are also proposed in this research.