Study of arc stability in underwater shielded metal arc welding at shallow depths

摘要

The main objective of this work was to study the effects of changes in hydrostatic pressure and electrode coating composition on the shielding metal arc behaviour during underwater wet welding. Thus, wet welding operations were performed by an automatic device using a hyperbaric chamber to simulate depths of 5.0, 12.5, and 20.0 m. A covered electrode already developed in W & RTL was used as reference and compared with others with different amounts of CaCO{sub}3, TiO{sub}2, and aluminium added to their coatings. Hence, effects of welding condition and electrode coating on weld metal were evaluated through visual inspection of the weld beads, measurement of porosity level, and the results related to welding current and voltage signals. The welding arc signals were analysed through indexes calculated from instantaneous values of current and voltage, and fast Fourier transforms frequency spectrum. The mixtures containing CaCO{sub}3 additions exhibit fewer defects, while those containing aluminium additions have presented a great amount of pores and other discontinuities in the weld metal. As expected, all consumables showed a higher deposition rate, as welds were performed at greater depths (pressures), confirming the effect of arc constriction and the consequent increase in current density. Moreover, a region of transition in the metal transfer mode was identified around a depth of 12.5 m, probably from short-circuit to globular. Also, a clear tendency of increase in arc stability for those welds performed at greater depths could be noticed, as well as better weld metal quality. On the other hand, those consumables with CaCO{sub}3 added to the coating exhibited the best arc stability for welds performed at shallow depths.