Quick answer: STT stands for Surface Tension Transfer, a MIG (GMAW) welding process developed by Lincoln Electric. Instead of letting each short circuit end in a small explosion of spatter, the machine watches every droplet and controls the current through each stage of the transfer. The result is short-circuit MIG with much less spatter, lower heat input and better fusion. It’s used for open-root pipe welding, thin sheet, stainless steel and galvanised steel.
STT sounds exotic, but it’s simply MIG welding with a far smarter power source. Here’s how it works, when it’s worth using, and how it compares to the alternatives.
What does STT mean in welding?
STT = Surface Tension Transfer. It’s a registered trade name of Lincoln Electric for its waveform-controlled short-circuit GMAW process. The name describes the key idea: the molten droplet is pulled into the weld pool mainly by the pool’s surface tension, not by brute electrical force.
In code and procedure documents you’ll often see it described as GMAW-S with controlled short-circuit transfer, or as a “modified short-circuit” process. Other manufacturers make similar processes under their own names (see the comparison below).
How STT works: the current waveform
In ordinary short-circuit MIG, the wire touches the puddle, the current spikes, and the molten bridge bursts. That burst is spatter. STT’s power source reads the arc voltage thousands of times a second and changes the current at each moment of the droplet’s transfer:
- Background current (roughly 50–100 A) keeps the arc alive and gently heats the base metal. This is your fine heat control.
- Short: the wire touches the pool, and the current drops briefly so the droplet wets in cleanly.
- Pinch current rises to squeeze the molten bridge and pass the metal into the pool.
- Just before the bridge breaks, the machine senses it and cuts the current to around 45–50 A, so the bridge separates gently instead of exploding. This step is what removes most of the spatter.
- Peak current re-establishes the arc, pushes the puddle down, and sets the arc length.
- Tail-out brings the current smoothly back to background. This is your coarse heat control.
Then the cycle repeats, many times a second.
STT settings: what the knobs do
STT machines work differently from a normal MIG welder. There is no voltage knob. Instead you set:
| Control | What it does |
|---|---|
| Wire feed speed | Deposition rate, meaning how much metal goes in |
| Peak current | Arc length |
| Background current | Fine heat input |
| Tail-out | Coarse heat input |
| Hot start | Extra current at the start of the weld |
Because heat is set by current rather than voltage, changes in wire stick-out don’t change the heat much, which makes STT more forgiving in awkward positions. On modern Lincoln machines these settings are usually picked synergically: you choose wire and gas, then trim.
Typical consumables:
- Mild steel: solid wire such as ER70S-6, often .035”–.045”, shielded with 100% CO₂ or argon/CO₂ blends. STT runs smoothly on straight CO₂, which is cheap and gives good root penetration.
- Stainless steel: stainless wire with suitable argon-rich gas.
- Silicon bronze: used for brazing galvanised steel with very little zinc burn-off.
How to dial in STT settings: a step-by-step starting point
Lincoln doesn’t publish one universal settings chart for STT, because the right values depend on the pipe or plate, the joint, the wire, the gas and the qualified procedure. What’s consistent is the order in which experienced welders tune it:
- Pick the synergic program first. Choose the wire type, wire diameter and gas on the machine (for example ER70S-6, .045”, 100% CO₂). The machine loads sensible starting values for peak, background and tail-out.
- Set wire feed speed for the joint. This sets how much metal goes in. Start at the program’s suggestion and adjust it to suit your root gap and travel speed. Don’t use it to control heat.
- Set the arc length with peak current. Too little peak and the wire stubs into the puddle. Too much and the arc gets long and the puddle pushes around. Aim for a short, crisp arc.
- Fine-tune heat with background current. Raise it if the root edges aren’t fusing. Lower it if the keyhole grows or the inside bead sags.
- Use tail-out for bigger heat changes, for example moving from a cool pipe to a pipe that has heated up, or changing wall thickness.
- Add hot start only if starts are cold. Then grind starts and stops anyway.
- Run a test coupon and cut or bend it. Check root fusion and the inside profile before welding the real job, and record what worked.
Tip: change one setting at a time, in small steps. With STT, a small change in background current makes a visible difference.
What STT is used for
- Open-root pipe welding. This is STT’s biggest use. It makes a sound root pass on open-gap joints much faster than TIG, including vertical-down. Lincoln holds patents specifically for STT pipe root passes.
- Thin sheet metal, such as automotive and HVAC work, where low heat input means less burn-through and less distortion.
- Stainless steel and nickel alloys in petrochemical, power and food-processing plants.
- Galvanised steel and silicon bronze, for example furnace ducting, because there’s less coating damage.
- Robotic and automated welding as well as manual work.
Tips for STT root passes on pipe
These are general tips. Always follow the qualified welding procedure (WPS) for your job.
- Fit-up matters more than settings. Keep the root gap and landing (land) consistent all the way around. STT tolerates gaps well, but not big changes in gap.
- Vertical-down progression is common for STT roots on carbon steel pipe, and it’s much faster than uphill stick or TIG. Check what your procedure specifies.
- Watch the keyhole. A small, steady keyhole at the leading edge means full penetration. If it grows, reduce background current or speed up. If it closes, slow down or add a little heat.
- Adjust heat with background and tail-out, not wire speed. Wire speed sets how much metal you add, and the current settings set how hot the puddle runs.
- Keep a consistent stick-out and gun angle through the clock positions. Although STT is forgiving, big changes still show up in the root.
- Clean between passes and grind starts and stops so the hot pass fuses cleanly.
Common STT problems and fixes
| Problem | Likely cause | Fix |
|---|---|---|
| Suck-back or concave root on the inside | Too much heat at the top of the pipe | Reduce background current or tail-out; speed up travel |
| Lack of fusion at the root edges | Too little heat or travel too fast | Raise background current; slow down slightly; check the gun angle |
| Burn-through | Gap too wide or too much heat | Tighten fit-up; reduce peak and background current |
| Excess spatter | Wrong wire or gas selection in the synergic menu; worn contact tip | Check the program matches the wire and gas; replace the tip |
| Porosity | Draughts, low gas flow, dirty pipe | Shield from wind, check flow and hoses, clean the bevel |
Advantages and disadvantages of STT
| Advantages | Disadvantages |
|---|---|
| Very low spatter and fume compared with standard short-circuit MIG | Needs an STT-capable Lincoln Power Wave system, which is expensive |
| Low heat input, so less distortion and burn-through | Not something you’ll find on hobby welders |
| Excellent for open roots and gaps | Still a short-circuit process: for fill and cap passes on thick metal, pulse or spray MIG, flux-core or stick deposit faster |
| Much faster than TIG for pipe roots (Lincoln quotes up to about 4× travel speed) | Different controls (no voltage knob) take some relearning |
| Runs on cheap 100% CO₂ for mild steel | Like all gas-shielded processes, it needs wind protection outdoors |
| Easier to train than TIG root welding |
STT vs other welding processes
| STT | Standard short-circuit MIG | TIG (GTAW) | Stick (SMAW) | |
|---|---|---|---|---|
| Spatter | Very low | Moderate to high | None | Moderate, plus slag |
| Heat input | Low | Low to medium | Low to medium | Medium to high |
| Open-root pipe | Excellent and fast | Risk of cold lap and lack of fusion | Excellent but slow | Good, the traditional method |
| Thin sheet | Excellent | Good | Excellent | Poor |
| Equipment cost | High | Low | Medium | Low |
| Skill to learn | Medium | Easiest | Hardest | Medium |
See our full guide to GTAW (TIG) welding and the overview of welding processes.
STT vs pulse MIG
People often confuse STT with pulsed MIG, because both use a computer-controlled current waveform. They solve different problems:
| STT | Pulse MIG (GMAW-P) | |
|---|---|---|
| Transfer type | Controlled short-circuit: the wire touches the puddle every cycle | Spray-type droplets: the wire never touches the puddle |
| Heat input | Lowest of the wire processes | Low to medium, higher than STT |
| Best for | Open root passes, gaps, thin sheet, galvanised steel | Fill and cap passes, out-of-position welding on thicker metal, aluminium |
| Deposition rate | Lower | Higher |
| Gap bridging | Excellent | Poor, can blow through an open root |
| Typical gas (steel) | 100% CO₂ works well | Argon-rich mixes (e.g. 90/10) |
That’s why a common pipe procedure uses STT for the root and then pulse MIG, flux-core or stick for the fill and cap. Many Lincoln Power Wave machines can run both processes, so you switch programs rather than machines.
STT vs RMD vs CMT
Other manufacturers make their own controlled short-circuit processes:
- Miller RMD (Regulated Metal Deposition): Miller’s version of the same idea, also widely used for pipe roots.
- Fronius CMT (Cold Metal Transfer): this physically pulls the wire back at each short circuit, combining mechanical wire motion with current control for very low heat. It’s popular for thin sheet, aluminium and welding steel to aluminium.
- Lincoln STT controls the transfer purely through the current waveform, with no mechanical wire retraction.
All three aim for the same result: short-circuit transfer without the spatter and cold lap. Which one you use usually depends on the brand your shop or procedure already uses.
STT welders: what equipment do you need?
STT runs on Lincoln’s industrial Power Wave platform. For example, the Power Wave STT Process Module (K2902-1) adds STT to Power Wave S350 and S500 power sources, alongside other advanced processes. You’ll also need a compatible wire feeder and gun. Older stand-alone machines such as the Invertec STT are still found in the used market.
This is professional, industrial equipment that typically costs several thousand dollars for the module alone, before the power source. For home and farm work, a standard MIG or a stick welder makes far more sense.
FAQ
What does STT stand for in welding?
Surface Tension Transfer, Lincoln Electric’s controlled short-circuit MIG (GMAW) process.
Is STT welding the same as MIG?
It’s a type of MIG (GMAW). It uses the same wire, gun and gas, but the power source controls the current through each droplet transfer, so it behaves very differently from standard short-circuit MIG.
Is STT good for thin material?
Yes. Its low heat input makes it one of the best wire processes for thin sheet and stainless steel without burn-through or warping.
Can STT be automated?
Yes. STT is used in both semi-automatic (hand-held) and robotic welding.
Does STT reduce spatter?
Yes, a lot. Reducing the current just before the molten bridge breaks prevents the small explosion that causes most short-circuit spatter.
Can STT be used for the entire weldment?
It can, but it’s rarely the best choice for thick sections. STT is a low-heat short-circuit process, so it’s ideal for the root and for thin material. On thicker pipe and plate, fill and cap passes are usually done with pulse MIG, flux-core or stick because they deposit metal faster. On thin sheet, STT can easily do the whole joint.
Is STT the same as RMD?
No, but they’re close cousins. STT is Lincoln Electric’s process and RMD (Regulated Metal Deposition) is Miller’s. Both control the current through each short circuit to cut spatter and improve root fusion. They aren’t interchangeable programs: each runs only on its own brand’s machines.
What does “surface tension” mean in STT?
When the wire touches the weld pool, the liquid pool’s surface tension pulls the molten droplet off the wire, much like a drop of water being drawn into a puddle. STT lowers the current at exactly that moment, so surface tension (not a burst of current) finishes the transfer. That’s where the name comes from.
Can you get STT on a hobby or home MIG welder?
No. STT is only available on Lincoln’s industrial Power Wave systems (and older Invertec STT machines). Some newer hobby MIG welders have “smooth” or “low-spatter” short-circuit modes, but they’re not the same process.
Is STT better than TIG for pipe roots?
For speed, usually yes, because STT roots are much faster and easier to train. TIG remains the standard where codes require it, for very critical alloys, or where the best root profile matters most. Many procedures use an STT root followed by flux-core or stick fill and cap passes.
How this guide was researched: the process description and phase values come from Lincoln Electric’s STT process documentation; the equipment details come from current Lincoln and supplier listings (October 2026). STT, Power Wave and Invertec are trademarks of Lincoln Electric. See how I test and review.