THE EFFECT OF BASE METAL AND CORE ROD CARBON CONTENT ON UNDERWATER WET WELD POROSITY

摘要

Porosity is a common defect observed in underwater wet welding. Several research programs have been developed to understand how pores form in order to mitigate the problem. No superficial pores and a limited number of internal pores (based on size) are important requirements to classify underwater wet welds according to the American Welding Society - AWS D3.6M standard. The main objective of this work is to study the effect of base metal and core rod carbon content on weld metal porosity. A pressure chamber with 20 atmospheres capacity was used to simulate depth with fresh water. To perform the welds, a gravity feeding system able to open an electric arc and deposit the weld automatically was used. Beads-on-plate were made using Direct Current Electrode Negative (DCEN) configuration on two base metals with different carbon contents (C2 - 0.1 wt. pct. and C7 - 0.7 wt. pct.) at 50 meters water depth. Commercial E6013 grade electrodes were used to deposit the welds. These electrodes were produced with core rods with two different carbon content (E2 - 0.002 wt. pct. and E6 - 0.6 wt. pct.) and painted with varnish for waterproofing. Samples were removed from the beginning, middle and end of the BOP welds and prepared following metallographic techniques including macroetching and image analysis for weld porosity. A data acquisition system was used to record current, voltage and welding time at 1.0 kHz rate. The porosity measurements indicated an increase of about 85% and 70% when E6 electrodes were used instead of E2 electrode on C2 and C7 steel plates, respectively. Simultaneously, the increase in porosity was followed by an increase in short circuiting events, an increase in weld bead penetration and a decrease in welding voltage. These observations seem to confirm, a direct effect of carbon content of the core rod on weld metal porosity and that porosity is associated with the CO reaction that can occur during metal transfer in that molten droplets carry gas bubbles to the welding pool. On the other hand, the increase of carbon content in the base metal was seen to decrease the porosity in the weld metal. This result can be related with the decrease in penetration observed when changing C2 to C7 plates. The smaller participation of carbon from the base metal in the weld pool reactions should then reduce the CO formation and, consequently, the amount of pores in the weld.