STT Technology

STT Technology is an innovative welding method based
on the transfer of metal by virtue of Surface Tension Transfer™.

Developed by the Lincoln Electric Company, the STT process
is based on the advanced concept of using Waveform Control Technology™.

semiautomatic welding MIG
— high spatter

Main features of STT process are:

  • The welding current is set automatically, depending
    on the requirements of the weld arc.
  • The time required to effect changes in the weld puddle
    is measured in microseconds.
  • The arc parameters are optimized continuously during the entire process of forming and transferring each drop of molten metal from the electrode to the weld puddle.  

The STT process ensures that the current is adjusted automatically
to the instantaneous heat requirements of the arc.

semiautomatic welding MIG–STT — nearly no spatter

STT welding cycle follows:

  1. At the start of the cycle, when the electrode shorts, the current is reduced immediately, eliminating the incipient short. This low–level current
    is maintained for a short time so that the surface tension forces can begin transferring the drop to the puddle, forming a solid mechanical bridge.
  2. A high level of pinch current is then applied to accelerate the transfer of the drop. The necking down or squeezing of the shorted electrode is monitored. When a specific value is reached, the pinch current is reduced quickly
    to a low value before the fuse separates. When a short breaks, it does so
    at a low current, which produces very little spatter.
  3. Next, the current returns to a high level to establish arc length and form
    the next droplet before reducing to a medium or background value.
  4. After the droplet starts forming, the current goes down to medium
    or background value. 
view of an STT–welded bead
from inside the pipe

This instantaneous control over the arc current, the first type that works in accordance with the physics of the weld arc, eliminates the highly violent and explosive energy of the usual welding process, which in turn gets rid of such problems as a rough arc and significant splatter.

Precision control of the transfer of metal in the weld zone, which takes place
in a gas atmosphere provides for:

  • good penetration and fusion,
  • superb back bead,
  • high production rate,
  • minimal spatter,
  • significant fume reduction,
  • lack of weld deformation and burn–throughs due to the precise control
    of heat input,
  • low hydrogen weld metal — crack prevention technology
semi–automatic pipe welding with STT process

Welding with the STT process permits welding with a fixed, including zero, root gap with the aid of an external automatic system.

Despite the fact that the linear weld speed here is less than with the traditional automatic welding technology, the STT technology enables the welder to work without a line–up clamp and internal backup, ceramic or copper (avoiding copper inclusions), when working with small diameter pipe.

The advantages of STT welding technology combined with the high degree
of flexibility of the process made it ideal for use in automatic welding systems developed by CRC–Evans for externally welding of the root pass.

Furthermore, the STT process provides for minimum diffusible hydrogen in the metal and minimal spatter, which considerably broadens the range of steels
that can be welded and significantly reduces the likelihood of hydrogen cracking.

automatic pipe welding
with STT process