A Study of Mechanism for Acicular Ferrite Formation in Low Carbon Steel Ti-B Weld Metals

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 deposites with various cooling rates.were prepared in this study. In those samples, intragranular acicular ferrite formation was observed from some inclusions. The inclusions contributing to acicular ferrite nucleation 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 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.

Moreover acicular ferrite grew to the plane to satisfy the Kurdjumov–Sachs orientation relationship with austenite matrix.