STT and RMD are both controlled short-circuit MIG processes that tame the short-circuit cycle to cut spatter, control heat input, and produce clean open-root welds. STT (Surface Tension Transfer) is Lincoln Electric’s waveform. RMD (Regulated Metal Deposition) is Miller Electric’s. They solve the same problem in slightly different ways: a standard short-circuit arc is violent and unpredictable at the moment the wire shorts to the pool, and these processes use a programmed current waveform to make that moment calm and repeatable.
The payoff is a short-circuit arc you can run open-root on pipe and plate without burning through, with far less spatter than conventional short-circuit MIG and enough heat control for thin material. Both have pushed into work that used to belong to stick (SMAW) root passes and TIG (GTAW) roots. If you weld pipe, chances are you have run or will run one of these.
Why Conventional Short-Circuit MIG Falls Short for Roots
Standard short-circuit transfer (covered in detail in our MIG transfer modes guide) repeats a short-circuit event 90 to 200 times per second. The wire touches the pool, current spikes, the electromagnetic pinch separates the droplet, and the arc re-ignites. That re-ignition is the problem. On a constant-voltage machine, current is still high and rising at the instant the bridge of molten metal breaks. The break is explosive, it throws spatter, and the timing varies cycle to cycle.
For most fab work that is fine. For an open root, it is not. You want each droplet placed predictably, the heat held down so the root face does not melt away, and a stable arc that bridges the gap. Conventional short-circuit MIG gives you none of those reliably, which is why root passes were a stick or TIG job for decades.
How STT Works: Surface Tension Transfer
Lincoln developed STT in the early 1990s as a current-controlled, not voltage-controlled, short-circuit process. Per Lincoln Electric’s STT process literature, the power source senses the state of the short circuit electronically and adjusts current through each phase of the cycle independently. The defining move is what happens at the necking point.
The cycle runs roughly like this:
- The wire approaches the pool at a low background current. The droplet wets into the puddle.
- As the short forms, current is briefly reduced to let the wire seat against the pool without bouncing.
- A pinch current is applied to squeeze the liquid bridge and start the neck.
- A fast electronic sensor detects the neck forming (a sudden change in the rate of resistance rise). At that instant, current is cut to a low level.
- With current low, surface tension does the separation. The molten droplet is pulled into the pool by the pool’s own surface tension instead of being blown apart by high current. That is where the name comes from.
- A peak current pulse then re-establishes the arc and heats the wire and pool for the next droplet.
Because current is low at the moment of separation, there is almost no explosive break and almost no spatter. The independent controls (background, pinch, peak, and the tail-out ramp) let a welder or a saved procedure tune the arc to the joint.
How RMD Works: Regulated Metal Deposition
Miller’s RMD takes a related but distinct path. Per Miller Electric’s RMD process documentation, RMD uses a precisely controlled, staged current waveform that breaks the short-circuit cycle into discrete phases and anticipates the short rather than only reacting to it. Miller’s literature describes seven phases in the RMD cycle (wet, pinch, clear, blink, ball, background, and pre-short), each tuned so the metal transfers the same way every time.
The practical idea is the same as STT: lower the current at separation so the droplet leaves quietly, then control the re-ignition so it does not spatter. The difference is in how the waveform is shaped and triggered. RMD leans on a predictable, repeating profile so that the droplet size and timing stay uniform across changing arc conditions. Miller markets that consistency as a tolerance advantage, meaning the arc holds steady through the gap and stick-out changes you get hand-welding an open root.
Both processes deliver a calm, low-spatter short-circuit arc. The marketing language differs more than the result does.
STT vs RMD: Side by Side
| Characteristic | STT (Lincoln) | RMD (Miller) |
|---|---|---|
| Full name | Surface Tension Transfer | Regulated Metal Deposition |
| Manufacturer | Lincoln Electric | Miller Electric |
| Process family | Controlled short-circuit GMAW | Controlled short-circuit GMAW |
| Control method | Current sensing, cuts current at the neck | Staged, anticipated current waveform |
| Spatter | Very low | Very low |
| Heat input | Low, adjustable | Low, adjustable |
| Primary use | Open-root pipe and plate, thin material | Open-root pipe and plate, thin material |
| Host machines | Power Wave, Invertec STT | PipeWorx, Dimension, XMT 350 |
| Gas (carbon steel root) | Often 100% CO2 or 75/25 Ar/CO2 | Often 75/25 Ar/CO2 or 90/10 |
Confirm gas selection against the machine’s stored procedure and the welding procedure specification (WPS) for your job. Both vendors publish recommended gas and wire combinations, and a coded job will specify them.
Where These Processes Earn Their Keep
Open-Root Pipe Passes
This is the headline application. On a beveled open-root joint, the welder needs to fuse the root faces and lay a sound inside bead without melting the gap open or leaving a suck-back. STT and RMD do this well because the low separation current keeps the puddle controllable and the consistent droplet placement bridges the gap. Production pipe shops use these roots followed by a flux-cored or pulse-spray fill and cap, which is faster than running the whole joint with stick.
For weld qualification, the relevant code is usually AWS D1.1 (Structural Welding Code, Steel) for structural steel or the API and ASME pipe codes for pressure piping. Per the American Welding Society, GMAW-S (short-circuit gas metal arc welding) historically carried restrictions in structural code because of the cold-lap risk in plain short-circuit transfer. Controlled short-circuit processes are still GMAW-S electrically, so qualify the procedure to the code that governs your work and do not assume an STT or RMD root is automatically prequalified. Check the current edition of the governing code.
Thin Material and Sheet
The same heat control that prevents root burn-through helps on thin gauge. Where conventional short-circuit MIG can blow through and pulse runs hot for the thinnest stock, the low average current of STT and RMD lets you tack and weld light material with less distortion. Our guide to MIG welding thin metal covers the broader heat-control techniques that pair with these waveforms.
Poor Fit-Up and Gap Bridging
Open-root work and field repair rarely give you perfect fit-up. The controlled cycle bridges a wider gap than plain short-circuit because the puddle stays stiff between droplets and freezes fast. If gap bridging is a recurring headache on your joints, the fundamentals in our MIG welding gaps and fit-up guide apply directly, and a controlled short-circuit process widens the window further.
Lower Skill Barrier Than TIG
A TIG root on pipe demands a steady hand, filler coordination, and time. STT and RMD let a competent MIG welder produce a consistent root with less training, which is the real reason shops adopted them. You still need to qualify and you still need technique, but the learning curve to a passable root is shorter than GTAW.
How They Differ From Pulse MIG
People mix these up with pulse, so be clear: STT and RMD are short-circuit processes, and the wire physically touches the pool every cycle. Pulse MIG is a spray process where droplets fly across the arc without ever shorting. Our pulse MIG explainer covers that mechanism in full.
The split matters because it drives application. Pulse gives you spray-quality fill and cap at higher deposition with all-position capability, but its lowest heat settings are still warmer than a controlled short-circuit root and it does not bridge an open root as forgivingly. STT and RMD own the cold root. Pulse owns the hot fill. A lot of pipe procedures use one for the root and the other for the rest of the joint, on the same machine if it runs both processes.
| Trait | STT / RMD | Pulse MIG |
|---|---|---|
| Transfer mode | Controlled short circuit | Pulsed spray |
| Wire touches pool | Yes, every cycle | No |
| Best at | Open root, thin gauge, gap bridging | Fill and cap, deposition, all-position spray |
| Relative heat input | Lowest | Medium |
| Shielding gas | CO2 or Ar/CO2 mixes | 90/10 or richer argon |
Equipment and Cost
Neither process runs on a hobby CV machine. Both need an inverter power source with the waveform loaded and a wire feeder matched to it.
- Lincoln STT lives on the Power Wave platform and the dedicated Invertec STT units. On a Power Wave, STT is one of many stored waveforms alongside pulse and other modes.
- Miller RMD is a process option on machines like the PipeWorx 400, the Dimension series, and the XMT 350 with RMD software. The PipeWorx is built around this kind of pipe-welding workflow, switching between RMD root and a fill process at the gun.
Expect industrial pricing. These are professional pipe and fabrication machines, not entry-level hobby gear, and a complete setup with feeder and consumables runs into the thousands. Prices move, so get a current quote from a Lincoln or Miller distributor rather than trusting a number that goes stale.
For wire and consumables that suit root work, see our MIG gun consumables guide. Contact tip condition and a clean liner matter more on a controlled-waveform process because the arc control depends on consistent wire feed.
Which Should You Choose?
If you are buying into a brand ecosystem, that usually decides it. Lincoln shops run STT. Miller shops run RMD. Both are mature, both produce excellent open roots, and the day-to-day difference for the welder is small once the procedure is dialed in.
If you are choosing on the merits:
- Pick STT if you want the original current-sensing approach, you run Lincoln Power Wave equipment, or your existing procedures and welders are already built around it.
- Pick RMD if you run Miller pipe machines, want the staged-waveform consistency Miller emphasizes for hand-welding through gap and stick-out changes, or you want the PipeWorx root-to-fill workflow on one machine.
For the actual decision, weigh the hardware you already own, what your distributor supports locally, and what your welding procedures are qualified to. The process physics are close enough that brand support and procedure qualification should drive the call more than the waveform name on the box.
Safety and Procedure Note
This article is a process reference, not a welding procedure specification. Arc welding involves electric shock, intense ultraviolet and infrared radiation, fumes, and fire hazards. Follow your machine manufacturer’s manual, your shop’s safety program, and OSHA requirements for welding, cutting, and brazing under 29 CFR 1910 Subpart Q, including ventilation and respiratory protection per 29 CFR 1910.252 and 29 CFR 1910.253 for fuel-gas and compressed-gas safety where applicable. For coded or structural work, qualify the procedure and the welder to the governing code (such as AWS D1.1 for structural steel, AWS D1.2 for structural aluminum, or the relevant ASME or API pipe code) and verify the current edition. Do not treat any settings or gas recommendation here as a substitute for a qualified WPS.