Improvement of the toughness and ductility of the weld beads by inducing growth of acicular ferrite with TiO₂-nanoparticles during submerged arc welding

This work was focused to relate the mechanical properties of weld beads with size and superficial density of the acicular ferrite (AF) needles changing by impregnating two different sizes of TiO₂-nanoparticles at the bevel before the SAW process. Microstructure of weld metal with addition of TiO₂-nanoparticles with 21 and 127 nm sizes, respectively, shows an increase of the density (61.3 and 75.4%) and length of AF needles formed (156.4 and 68.8%), in comparison to the weld metal without TiO₂-nanoparticles, namely the WB-0. Moreover, chemical analyses at weld metal and inclusions of weld beads with TiO₂-nanoparticles reveal and increase on their Ti contents respect to those in the WB-0. Tensile and yield strength of the weld bead fabricated with the smallest TiO₂-nanoparticles diminished in comparison to other weld beads. Besides, it was found that the average Vickers microhardness of weld metal decreased by adding TiO₂-nanoparticles. On the other hand, their Charpy V-notch toughness values were found to be 30.1 and 102.6% bigger respect to those of the WB-0, by adding TiO₂-nanoparticles with 21 and 127 nm sizes, respectively. In general, it was found that by adding TiO₂-nanoparticles before the SAW process, the density and dimensions of the AF needles increase, which could be associated with the improvement of toughness and ductility of weld beads.