Characteristics of inclusions in rutile-type FCAW weld metal

The morphological and chemical nature of the inclusions in flux cored arc welding (FCAW) weld metal of E71T-1 grade were analyzed in SEM/TEM/scanning transmission electron microscope (STEM) equipped with energy-dispersive X-ray (EDX) to understand the sequence of inclusion formation and to verify the absence of the Mn-depleted region in close proximity to the inclusion surface. Basically, all the inclusions were nearly spherical in shape and chemically a mixture of Ti-rich oxide and (Si,Mn)-rich oxide partly covered with MnS phase. From the morphological features of each phase, the formation sequence of the inclusion can be predicted in the order of Ti-rich solid oxide, (Si,Mn)-rich liquid oxide, and MnS phase. The MnS phase always presented on the surface of the (Si,Mn)-rich liquid oxide and was fully confined within the outer circle of the inclusion implying that it was formed from (Si,Mn)-rich oxide. STEM/EDX analysis performed on the MnS phase and its neighbor did not show any Mn depletion over a distance of about 70 nm from the outer surface of the MnS phase.Based on these results, it could be concluded that as the MnS phase was formed from the liquid (Si,Mn)-rich oxide, the manganese concentration in the steel matrix cannot be varied.