It is particularly interesting to note that the fine spherical Mg-Al-O-based inclusions were formed in the presence of 300 ppm of Mg in the weld metal. There has been limited discussion in the literature on the application of Mg as an alloying element in steels; however, it has been noted to produce nitride and oxide precipitates, which may be useful in refining the grain structure in the heat-affected zone (Ref. 53). The use of MgO in welding flux is common, due to the strong deoxidizing role of Mg in the weld metal (Ref. 54), although the concentration of Mg in weld metal is seldom ever reported due to its low solubility in steel. To the authors’ knowledge, this work represents the first time that Mg has been observed to play a significant role in the structure of fine oxides in a weld metal, as indicated by the AES observations in Fig. 6 and the thermodynamic modeling in Fig. 4. Since the chemistry and microstructural features contributing to the properties of the weld metal were heavily influenced by the flux used here (Ref. 27), it is worth examining how this can be optimized further in consumables for other processes. Conclusions Fine Mg-bearing inclusions with a core/shell structure have been observed in a carbon steel weld metal. Flux cored arc welding was used to produce a weld metal that contained primarily bainitic ferrite with a fine packet size in the as-deposited metal and mainly nonaligned ferrite in the reheated zones. Spherical inclusions with an average diameter of 311 nm were observed with a shelled structure that was mainly rich in aluminum, magnesium, and oxygen in the core, vs. magnesium and oxygen in the outer shell, which was suggested to be halite based on thermodynamic calculations for the weld metal chemistry. 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