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Explaining Porosity Formation in Underwater Wet Welds

[+] Author Affiliations
Faustino Pérez-Guerrero

Instituto Mexicano del Petróleo; Colorado School of Mines, Golden, CO

Stephen Liu

Colorado School of Mines, Golden, CO

Paper No. OMAE2007-29696, pp. 249-257; 9 pages
doi:10.1115/OMAE2007-29696
From:
  • ASME 2007 26th International Conference on Offshore Mechanics and Arctic Engineering
  • Volume 4: Materials Technology; Ocean Engineering
  • San Diego, California, USA, June 10–15, 2007
  • Conference Sponsors: Ocean, Offshore and Arctic Engineering Division
  • ISBN: 0-7918-4270-3 | eISBN: 0-7918-3799-8
  • Copyright © 2007 by ASME

abstract

Macroscopic porosity in underwater wet welds is one of the main defects that deteriorate the mechanical properties of the wet welded joints. It is well established that weld metal porosity is a function of pressure, thus water depth. However, the mechanism of porosity formation is not well understood, therefore the problem is not yet mitigated to acceptable levels, particularly at water depths close to and beyond 100 m. To purposely produce porous welds similar to those obtained in wet welding, bead-on-plate (BOP) welds were deposited in air with gas metal arc welding (GMAW) with no shielding gas, with autogenous gas tungsten arc welding (GTAW) and GTAW with cold wire feed using insufficient shielding gas (8 CFH). During welding with both processes, oxygen from the atmosphere readily reacts with the alloying elements in the molten tip of the wire and in the weld pool. Under these conditions, droplets that detach from the wire electrode will generally contain a gas bubble, which is transported into the weld metal. These two welding processes were selected because there is no slag produced in the process. Slag slows down the cooling while giving enough time for degassing to occur, as in the case of shielded metal arc welding (SMAW) in air. Even with insufficient shielding gas, the autogenous GTAW welds did not exhibit porosity because there was no metal addition in the form of droplets. However, when a wire was fed into the arc, droplets detached from the wire in the oxidizing atmosphere transported gas into the weld pool, manifested as external and internal weld metal porosity. Similarly, the GMAW BOP welds exhibited internal porosity. When quenched in water, the droplets that detached from the electrode in these oxidizing conditions exhibited internal voids. Metal transfer analysis performed on the GMAW BOP welds associated short circuiting mode with large droplets and high porosity contents (10 pct.). Conversely, small droplets are expected to transport less gas and produce less porosity. Proof of concept welds using the pulsed current GMAW (GMAW-P) process resulted in higher droplet detachment frequency, smaller droplets and a low number of short circuiting droplets. Even though a few short circuiting events were still present, the GMAW-P process drastically reduced porosity to only 0.2 pct. Chemical reaction between oxygen and carbon generates CO gas at the bottom surface of the droplets in flat welding position, this gas ascends and is partially trapped inside the droplet. However, when the welding torch and base metal are rotated 90 degrees or in horizontal welding position, the CO gas generated escapes. Consequently there is no CO bubble in the pendant droplet or porosity in the weld metal. Wet welds were made with pulsed current using AWS E6010 electrode at a pressure equivalent to 50 m water depth. Porosity was reduced from 3.9 with constant current to 2.5 pct with pulsed current. Even when porosity was reduced with pulsed current, higher pulse frequency needs to be tested along with different peak and background current values to further reduce porosity. Flux covered electrodes with ferro-manganese, ferro-titanium and boron additions were extruded for wet welding. These electrodes produced wet welds with an average porosity of 1.2 pct., which could be further reduced to 0.85 pct. by better control of the arc at the beginning side of the weld.

Copyright © 2007 by ASME

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