Mechanism of Acicular Ferrite Formation in Low Carbon Steel Ti–B Weld Metals with Different Oxygen Levels

Inclusions contributing to acicular ferrite nucleation were investigated from a crystallographic point of view to understand the formation mechanism for acicular ferrite microstructure in mild and low alloy steel weld metals.The samples with different oxygen content in Ti–B weld metal were prepared in this study. In those samples, intragranular acicular ferrite formation was observed from some inclusions and acicular ferrite had Kurdjumov–Sachs orientation relationship with austenite matrix. The inclusions contributing to acicular ferrite nucleation were multi-phase type consisting of amorphous phase and MnS. They were surrounded by a Ti-enriched layer. It was confirmed by selected area diffraction patterns and analysis of EDS that the Ti-enriched layer was TiO. The acicular ferrite had Baker–Nutting orientation relationship with TiO layer on the inclusion surface. The misfit was 3.0% at the interface between the acicular ferrite and TiO.Therefore, it is considered that the TiO on the inclusion surface contributes to the heterogeneous nucleation of acicular ferrite by small lattice misfit.