Analytical Investigation of Inclusions Related to Acicular Ferrite Formation in Steel Weld Metal

In the present work, the morphological development of acicular ferrite in situ, was observed during weld cooling. The nucleation and growing at inclusion, sympathetic nucleation and impingement event of acicular ferrite were directly shown in high time resolution. To clarify the evolution mechanism, the inclusions related to acicular ferrite formation were investigated from the crystallographic points of view. The foiled samples including inclusions were prepared with a focused ion beam (FIB) device and were observed with a transmission electron microscope (TEM) which was operated at an acceleration voltage of 200kV. Inclusions were surrounded by a narrow titanium oxide. It was identified as TiO. The Baker-Nutting orientation relationship was satisfied between the TiO and acicular ferrite. It was supposed that the TiO on the inclusion surface contributes to the heterogeneous nucleation of acicular ferrite, supplying low interface energy.