Oxide Formation Mechanisms in High Manganese Steel Welds

Oxide inclusions in high-Mn steel welds were analyzed and almost all of which were found to belong to the MnO-Al₂O₃-SiO₂ system. In this study, the inclusions were categorized based on MnS morphology into the following two types: (1) aluminosilicate with a MnS patch, or (2) aluminosilicate with a MnS shell. The most frequently detected was type 1, the formation mechanism of which was investigated using commercially available thermochemical computing software, FactSage™ (ver. 6.3). The thermodynamic calculations predicted that galaxite (MnAl₂O₄), tephroite (Mn₂SiO₄), and MnS could precipitate during solidification. However, because of the fast cooling rate in welding processes, galaxite and tephroite phases were unable to fully crystallize, but rather were supercooled as glassy phases. In order to confirm the validity of the thermodynamic calculations, the composition of the observed inclusions was compared with the MnO-SiO₂-Al₂O₃ ternary phase diagram, resulting in remarkably good agreement. Furthermore, it was found that the type of the oxide inclusions was dependent on their location (i.e., MnS shell- and MnS patch-type oxides were detected at the dendritic core and interdendritic boundary, respectively). Both types of oxides were occasionally found in one oxide, near the interdendritic boundary. This indicates that the morphology variation originates from the redistribution of solute due to fast solidification.