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. Within an austenite grain of Ti-killed steels, the transformed microstructure can be heterogeneous. Heterogeneous microstructure has a wider grain size distribution and more large grains, hampering impact toughness. Separating acicular ferrite areas from granular bainite areas is necessary for a thorough characterization of the microstructure. Criteria for acicular ferrite identification have been proposed in this research.image (c) 2024 WILEY-VCH GmbH