Effects of Mn and Al on acicular ferrite formation in SAW weld metal

The acicular ferrite (AF) formation behaviors of submerged arc weld metals with various Al/O ratios and Mn contents were investigated. The nucleation sites of AF consisted of inclusions surrounded by a TiO layer. The Al/O ratio affected the number of inclusions that acted as AF nucleation sites. At high Al/O ratios, the number of inclusions was low and the microstructure was coarse. We considered that the formation of AF was promoted by the presence of the TiO layer via two mechanisms: Good lattice matching with ferrite and formation of a Mn-depleted zone owing to the absorption of Mn. The Baker-Nutting orientation relationship between TiO and AF was observed. Moreover, we determined that Mn-depleted zones were formed around the inclusions surrounded by TiO. The Mn content of weld metals affected the formation of AF via the formation of Mn-depleted zones. When the Mn content of the weld metals was high, Mn-depleted zones were formed.