In-situ observation of phase transformation during rapid cooling processes

Inclusions contributing to acicular ferrite nucleation were investigated from a crystallographic point of view to understand the formation mechanism for acicular ferrite microstructure in low alloy steel laser weld metals. The sample was low carbon Ti-B weld metals with an oxygen content of 480 ppm. In this sample, intragranular acicular ferrite formation was observed from some inclusions and acicular ferrite had Kurdjumov-Sachs orientation relationship with austenite matrix. The multi-phase inclusions contributed to nucleation of acicular ferrite. They were surrounded by a Ti-enriched layer. It was confirmed by selected area diffraction and EDS analysis that the Ti-enriched layer was TiO. The acicular ferrite had Baker-Nutting orientation relationship with TiO layer on the inclusion surface. The lattice 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.