What is the STT Welding Technology?

What is the STT Welding Technology?

For many years, pipe fabricators have been searching for a faster, easier method to make single-sided low hydrogen open root welds. It is difficult even for skilled welders to weld open root pipe and inflexible positioning makes pipeline welding more difficult, time consuming and expensive.

Higher strength pipe steels are driving a requirement to achieve a low hydrogen weld metal deposit. GTAW has been the only available process capable of achieving the quality requirements but GTAW root welds are very expensive to make. The GMAW process has been avoided because of problems with sidewall fusion and lack of penetration.

Lincoln Electric has developed and proven the Surface Tension Transfer (STT) process to make single-sided root welds on pipe. The patented STT® is a modified GMAW short circuit welding process, designed for open root welding. This unique process simplifies open root welding, requiring less operator experience to implement. STT® is a proven pipeline process, produces excellent quality welds, and improves productivity over more traditional SMAW & GTAW root welding methods. These results are due to the patented design of this process, eliminating common problems such as burnthrough and lack of fusion commonly associated with standard short-circuit transfer.

Advantages of STT® Welding Technology


  • Provides reliable root pass, complete back bead, and ensured sidewall fusion.


  • Reduces burn through, cracking, and other weld defects.


  • Uses 100% CO2, the lowest cost gas, when welding carbon steel.


  • Allows operator to control the heat put into the weld puddle.


  • Provides the capability of welding stainless steel, alloys, and mild or high strength steels without compromising weld quality.
  • Capable of welding out of position.


  • High quality open root welds at faster travel speeds than GTAW.
  • 2 times faster than SMAW
  • 4 times faster than GTAW


  • More forgiving process than conventional short arc welding with CV machines
  • Requires less operator skill

Comparing STT® to GTAW/SMAW

Conventional CV GMAW welding processes produce a flat internal bead, or “suck back” where the bead shrinks back into the root due to high weld puddle temperatures. Another disadvantage with CV GMAW welding on pipe is decreased tolerance for arc deviation. To obtain a weld without defects, the arc must be on the leading edge of the puddle. If the arc is too far back on the puddle, the arc will not penetrate completely. If the arc is too far ahead, the electrode shoots through the gap and causes “whiskers” to form on the inside of the pipe.

GTAW welding produces good pipe welds, but travel speeds are slow and heat input is high. Stick welding with cellulose electrodes provides good fusion characteristics, but leaves deep wagon tracks (requiring more labor for grinding), a very convex root weld, and a high hydrogen deposit.

The STT process produces a low hydrogen weld metal deposit in open root joints with easier operation, better back beads, better sidewall fusion and less spatter and fumes than other processes. STT differs from the traditional GMAW short-arc welding process because the arc current is precisely controlled independently from the wire feed speed. Changes in the electrode extension do not affect the heat input. Additionally, the arc current is carefully regulated to reduce puddle agitation and to eliminate violent “explosions” that occur during the traditional short-arc GMAW process. In comparison with traditional short-arc GMAW processes, STT is a controlled short circuit transfer GMAW process that offers the operator increased control over the welding process and produces less spatter and fumes.

Recommended Equipment

Power Source Wire Feeder Accessories
Power Wave S350, R350, 455M/STT®, S500® or R500 w/ Digital STT Module or Advanced Module Power Feed® 84 or 25M SuperArc Wire ®
Magnum Pro Gun ®