Abstract
Inclusions contributing to acicular ferrite nucleation were investigated from a crystallographic point of view in low carbon low alloy steel weld metals. The samples from ESW and SAW deposits with various cooling rates were prepared in this study. In those samples, intragranular acicular ferrite formation was observed from inclusions. The inclusions contributing to acicular ferrite formation were multi-phase type consisting of amorphous phase, spinel type and MnS. They were surrounded by a Ti-enriched layer. It was confirmed by selected area diffraction patterns and EDS analyses that the Ti-enriched layer was TiO. The acicular ferrite had Baker-Nutting orientation relationship (OR) 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. However, the morphologies of ferrite growth which nucleated from inclusions were different in both samples. Whereas ferrites nucleated from TiO were enough grown up in ESW, the size of nucleated ferrite in SAW was a few hundred nm in sizes. In the early stage of nucleation from TiO, ferrite had small deviation from Kurdjumov-Sachs (K-S) OR in both ESW and SAW. However, there was the difference in the growth stage of ferrite. The ferrite orientations were gradually changed to fit to K-S OR in ESW. On the other hand, the nucleated ferrite in SAW stopped growth and the newly nucleated ferrite which had K-S OR to prior austenite was formed in adjoining because of large super cooling due to small heat input.
