Cool Tech: Tandem GMAW
By Mark Norfolk on Thursday, July 29th, 2010
Wind towers are composed of multiple sections which are fabricated in production facilities, then bolted together on-site. Each section is comprised of several sub-sections joined together. The sub-sections are made by rolling steel plate, then welding the longitudinal seam. The sub-sections are then joined together via a circumferential seam weld. Submerged arc welding (SAW) is commonly used on the longitudinal and circumferential welds on the tower sections. Welding is generally mechanized and is done in the flat position. This process is well established for joining steel, has high deposition rates (up to 100-lbs/hr), and is tolerant to variations in joint preparation and fit-up. However, the process is generally only suited for flat position welds, and cannot be applied out-of-position. This requires rolling of the large diameter tower sections, while keeping the SAW torch stationary. The flux required for this process can also lead to several issues including:
- Improper storage and handling can cause moisture pick-up in the flux and lead to hydrogen induced cracking,
- Variations in flux chemistry can cause fluctuations in weld mechanical properties, including poor toughness,
- The flux produces a slag coating on the welds which needs to be removed. This introduces the potential for slag inclusions,
- Disposing of slag and used flux.
Tandem gas metal arc welding (T-GMAW) is a gas shielded, flux free process, which has the potential for replacing the SAW process for longitudinal and circumferential seams on tower sections. With this process two independently controlled electrodes are fed through a single welding torch. Interactions between the two welding arcs promote improved process stability and allow significant increases in deposition rate and travel speed since both arcs operate in the same weld pool.
EWI has successfully applied T-GMAW to the welding of thick section v-grooves in the horizontal, vertical, and overhead positions. Welds have been produced in the horizontal (2G) position at an average deposition rate of 25-lb/hr. Welds have also been made in the vertical position (3G), with an upward progression, at deposition rates over 8-lbs/hr and the overhead (4G) position at an average deposition rate of 15-lb/hr. The use of T-GMAW for conventional bevel groove joints, narrow gap joints, and high productivity high-speed fillet welds illustrates the effectiveness and flexibility of this variant of the widely deployed GMAW process.
Posted by Mark Norfolk