The molten pool melts completely through thin material — a hole, or a sagging drop on the root side. Heat input that exceeds what the plate can sink, and a repair that is usually harder than the original weld.
Burn-through is a full-thickness failure: the arc melts through the base metal, leaving a hole — or the pool sags through the root and freezes as a drop. It belongs to thin material, typically below about 1/8 inch (3 mm), where the plate simply cannot sink the heat fast enough.
It is one of the few defects visible on both sides of the joint, and one of the most expensive to repair on small parts — a burned hole in a thin sheet often costs more to patch than the weld was worth. The fix is not more skill; it is less heat, a smaller wire, and a backing bar.
Melt-through, blow-through, drop-through, excessive penetration (root side).
Always turn the piece over — burn-through confesses on the root side, where nobody is looking.
Burn-through is a heat-sink failure: the pass delivers more energy than the plate can conduct away.
Thin plate is a heat-sink problem. The pool can only stay liquid if the plate conducts the heat away — but a thin plate has almost no section to conduct into. Every amp and every second of travel concentrates heat in the same spot, and the pool grows in all directions faster than the plate can take it.
Once the root is open — a gap in the joint, or material melted through — the pool is supported only by its own surface tension. Arc force pushes down, pool weight pulls down, and when the two exceed the surface tension, the pool falls through. A copper backing bar or ceramic tape changes the game by supporting the pool and quenching it from below — which is why burn-through is largely a fit-up and technique problem, not a mystery.
Thin work is won before the arc: fit-up, backing, and transfer mode decide the outcome.
Fit the joint tight — control the root gap, tack more than you think you need, and back anything under 1/8 inch with a copper bar or ceramic tape.
Short-circuit transfer only on thin metal, a steady travel speed, and the arc pointed forward so the heat leads the pool instead of drilling it.
Let multipass thin work cool below hand-hot before the next pass — two passes back to back on thin sheet is how drop-through happens.
Spray transfer on thin sheet burns through almost instantly — it is the wrong transfer mode, not a technique you can out-weld. If the joint is under 1/8 inch, short-circuit only.
A burn-through repair runs on less heat than the original pass — or it burns again.
| Step | Action | Why |
|---|---|---|
| 1 | Grind the hole clean — remove ragged, oxidized edges to sound metal. | Oxide and torn edges will not weld; they feed the burn. |
| 2 | Close the gap from the back first if it is accessible — low heat, small wire. | A root-side fill halves the heat the top pass needs. |
| 3 | Use a copper backing bar or ceramic tape for the final pass. | It supports the pool and quenches it from below. |
| 4 | For holes too large to close, cut a patch and weld it in at low heat, per the code. | A patch is a controlled repair; welding over a big hole is a bigger burn-through. |
| 5 | Inspect both sides — VT, plus PT or RT where the spec requires. | A repair that fails the root side fails the job. |
A burn-through repair runs on the same heat that caused the burn — lower it. Same settings, same hole, twice the cost. If the code limits repair size or count, know the limit before you start grinding.
Each process burns through differently — and has its own way out.
Short-circuit transfer only under 1/8 inch, with 0.023–0.030 inch wire and a steady travel speed. Spray or globular transfer on thin sheet burns through before you can react. On lap joints, spot welding is often the right answer.
Pulsed current is the TIG answer on thin material — peak current high enough to wet the root, background low enough to let the plate recover between pulses. Smaller filler and faster travel keep the pool small.
High deposition is the enemy — FCAW puts down metal faster than thin plate can sink the heat. On sheet steel, drop to MIG short-circuit instead; where FCAW is mandatory, use the smallest wire and the lowest settings.
The wrong process for most thin work — arc force and heat are hard to control under 1/8 inch. Small electrodes at low current with fast travel can work in a pinch, but TIG or short-circuit MIG is the better tool.
Heat input is the number — then the support structure under the pool.
| Parameter | Check | Typical Fix |
|---|---|---|
| Current | Too high for the thickness | Drop into short-circuit range; on TIG, pulse it |
| Voltage / transfer mode | Spray or globular on thin metal | Short-circuit only under 1/8 inch |
| Travel speed | Too slow concentrates heat in one spot | Speed up; keep the pool small and moving |
| Wire size vs thickness | Wire deposits heat faster than the plate sinks it | 0.023–0.030 inch on sheet steel |
| Root gap / fit-up | Gaps remove the pool's support | Tight fit-up, more tacks, close the root |
| Backing | No support under the root | Copper bar or ceramic tape on thin work |
The number that matters is heat input — use the Heat Input Calculator to see how much energy the pass delivers per inch, the Material Thickness guide to pick the right transfer mode and wire, and the Welding Calculator for a starting point on thin material.
The tool that prevents burn-through is the one that separates heat from force — pulse capability.
A power source with pulse capability — TIG or MIG pulse — is the real tool for thin material: it separates the heat needed to wet from the heat that burns. On the consumable side, the smallest wire the feeder can run smoothly and the right contact tip are the difference between control and chaos.
Check both sides — the root side is where burn-through confesses.
Acceptance depends on the code and project: AWS D1.1, ISO 5817, API 1104 and project specifications each set their own limits for this discontinuity. Check the governing document before judging a weld acceptable — there is no universal pass/fail rule.
The questions welders actually ask about burn-through — answered straight.
With short-circuit transfer, a small wire (0.023–0.030 inch), and a steady hand, welders handle 24-gauge sheet. Below about 1/8 inch you must be on short-circuit — never spray. Below about 1/16 inch, TIG pulsed or spot welding is usually the better process.
Small holes and drop-throughs can often be filled from the root side at low heat, then finished from the top over a copper backing bar. Holes too large for that need a patch per the applicable code — cutting out and re-welding is sometimes the cheaper, cleaner repair.
It supports the molten pool from below and conducts heat away so fast that the weld freezes against it instead of burning through. Copper does not fuse to steel, so the root comes out clean — it is the standard answer for thin sheet and open-root joints.
The pool got heavier than surface tension could hold — too much heat for the thickness (current or voltage too high, travel too slow) or a root gap too wide to support it. Lower the heat, tighten the fit-up, and back the joint.