Microscopic Observation of Inclusions Related to Microstructure Evolution in Low Carbon Steel Ti-B Weld Metal

Microstructures of low carbon submerged arc weld metals of a Ti–B system with different aluminum contents were analyzed. The inclusions contributing to formation of acicular ferrite were directly sliced into thin foils and crystallographic analyses were performed.In the sample with low Al/O ratio (0.48–0.73), acicular ferrite microstructure was observed. Meanwhile, bainite microstructure was observed in the specimen with an Al/O ratio of 1.52. The inclusions contributing to formation of acicular ferrite were surrounded by a narrow Ti-enriched zone which was analyzed 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.