Modification of mechanical properties of submerged arc welds by adding Mn₂O₃- or TiO₂-NPs directly to the beveled surface of low-carbon steels

This work was aimed to analyze the changes in the mechanical properties of submerged arc welds due to the growth of the acicular ferrite (AF) phase promoted by addition of Mn₂O₃- or TiO₂-nanoparticles (Mn₂O₃- or TiO₂-NPs). Such Mn₂O₃- or TiO₂-NPs were uniformly deposited along the V-groove, formed from joining two beveled surfaces of low-carbon steel plates to be welding. Mechanical properties of the weld beads with and without NPs were determined through Tensile, Vickers microhardness and Charpy V-Notch (CVN) impact tests; meanwhile their microstructural features changes were followed by Optical and Scanning Electron Microscopies (OM and SEM). Tensile Strength, Yield Strength and Vickers microhardness values were lower for weld beads obtained with addition of Mn₂O₃- or TiO₂-NPs compared to those obtained without NPs additions. Namely, 13.5, 17.5 and 11.4%, respectively, lower by adding Mn₂O₃-NPs, and 15.8, 24.6 and 12.4%, respectively, lower by adding TiO₂-NPs. In contrast, the CVN impact energy values were determined to be about 60 or 30% higher by adding Mn₂O₃- or TiO₂-NPs, respectively. OM and SEM micrographs of weld beads with Mn₂O₃- or TiO₂-NPs show a clear increase in the surface density and length of the AF needles, in comparison to the weld bead without NPs. Namely, it was determined that in average, the length of the AF needles increases 131 and 155% by adding Mn₂O₃- or TiO₂-NPs, respectively. Moreover, the increase in the CVN impact energy and the surface density of AF needles can be correlated with the decrease in the average dimple diameter and with the cleavage sheets width, respectively. Thus, it can be pointed out that the addition of Mn₂O₃- or TiO₂-NPs has a direct influence in the size of the AF needles, which caused the improvement of the corresponding weld bead toughness.