Burn Thru Welding: Causes, Prevention & Root Pass Control

A fabrication shop gets a call: a structural plate assembly came back from inspection with gaping holes burned through the root of three joints, and the whole lot is on hold. Meanwhile, a pipe welder in a refinery turnaround is deliberately ‘burning thru’ the root of a 6-inch line with a perfect keyhole trailing the arc — the same phenomenon, controlled to the millimeter, is the difference between a reject and a weld that passes radiography. Understanding what burn thru welding really is — defect on one side, disciplined technique on the other — is what separates the two outcomes.

What ‘Burn Thru’ Actually Means in Welding

Ask five welders what “burn thru” means and you can easily get three different answers. One will describe a hole blown through a sheet-metal panel. Another will describe a pipe root pass with a glowing keyhole trailing the arc. A third will shrug and say it’s just another way of saying the puddle got too hot. So let’s settle the main question right away: burn-through is both a defect and a technique, and the only thing separating the two is control. Same physics, opposite outcomes. Whether you are trying to prevent it or deliberately use it, you first have to be precise about which one you mean.

Burn-Through as a Weld Defect

Used as a defect term, burn-through means the arc has melted away more base metal than the joint can hold, so the weld pool collapses through the joint and leaves an opening or a severely sunken, uncontrolled bead. The most widely cited formal definition comes from AWS API 1104, discussed on the AWS Welding Journal forum, which defines a burn-through (BT) as “a portion of the root bead where excessive penetration has caused the weld puddle to be blown into the pipe.”

Notice that this definition is written for pipeline work, where the consequence is specific: metal that should have stayed as a smooth root bead has instead sagged or sprayed into the pipe bore, disrupting flow and creating a stress raiser. In everyday defect guides, burn-through appears alongside porosity, undercut, lack of fusion, and cracks as one of the standard rejectable conditions inspectors look for.

Its appearance changes with the material:

• On thin sheet (duct, auto body, stainless fabrication): a visible hole or blown-out puddle where the base metal simply disappeared, often with a bluish heat tint ringing the opening.
• On pipe root passes: heavy, sagging internal reinforcement, or a collapse of the root ligament, where the remaining wall thickness between arc and bore goes to zero.
• On plate CJP joints: a drop-through or washout on the back side that may not be a full hole, but is deep enough that the joint no longer meets the specified penetration.

Burn-Through as an Intentional Root Technique

Now the other meaning. In pipe welding, “burning through the root” is a deliberate, skilled operation. The welder sets a root gap and a small root land, strikes the arc, and opens a small keyhole through the root face. The keyhole is the visual proof that the arc has fully penetrated the joint. The welder then washes molten metal into both sides of the gap and chases the keyhole around the pipe, closing it at the tie-in. Controlled burn-through like this is what open-root stick welding with 6010 or 7010 electrodes and TIG root passes are built around, because it is the only honest way to guarantee full penetration when you cannot see the back of the joint.

The key word is controlled. A defect puddle blows through because the welder did not manage heat, gap, or travel speed. A technique keyhole blows through because the welder opened it intentionally, sized it to a few millimeters, filled it consistently, and kept the resulting root reinforcement inside the acceptance limits of the procedure.

Burn-Through vs. Excessive Penetration vs. Melt-Through

These terms get used loosely in shops, and welding forum debates over the “correct” definition are endless. Practically, here is how experienced inspectors and welders generally separate them:

Term	What it generally describes	Is a hole required?
Burn-through	Root or face blown open; base metal melted away so the puddle passes through the joint	Usually yes, or a collapsed root
Excessive penetration	Penetration deeper than the procedure allows, often sagging or protruding, but the joint is still closed	No
Melt-through	Visible penetration on the reverse side of a single-groove weld, permitted when the code or drawing allows it	No, it can be acceptable
Drop-through	Root face sags below the joint line without fully opening	No
“Blowing a hole”	Shop slang for burn-through, especially on thin material	Yes

The practical takeaway for the rest of this article: every time you read “burn thru welding,” ask whether the context is prevention (thin metal, hot settings, poor fit-up, an unwanted hole) or deliberate execution (an open root, a keyhole, a qualified procedure that expects the arc through). Both share the same variables, amperage, travel speed, gap, thickness, and heat input, so the troubleshooting and the technique draw on the same knowledge. Getting that distinction straight up front is what keeps a defect from becoming a reject and keeps a technique from becoming a defect.

The Physics: Heat Input, Material Thickness, and the Root Ligament

Burn-through is rarely a mystery. Underneath every hole blown in a root, energy is entering the joint faster than the surrounding metal can carry it away or hold it back. Three things decide the outcome: how much heat you deliver per millimetre, how much metal is left to absorb it, and how joint geometry concentrates it.

The Heat Input Equation: Measuring What You Deliver

TWI defines heat input in arc welding as the energy supplied to the workpiece, and notes that heat input per unit length equals the arc energy multiplied by an arc efficiency factor. In practical shop terms, that translates into one formula worth memorising:

HI (kJ/mm) = (V × I × 60) / (S × 1000) × efficiency

Where V is arc voltage, I is welding current in amps, and S is travel speed in millimetres per minute. The 60 converts minutes to seconds, the 1000 converts joules into kilojoules, and the efficiency factor sits below 1.0 because not all of the arc’s energy reaches the workpiece — some is lost to radiation, spatter, and the electrode’s own melting.

Run the numbers once and the logic becomes tangible. At 120 amps, 22 volts and 250 mm/min with an illustrative efficiency of 0.8: (22 × 120 × 60) ÷ (250 × 1000) = 0.63 kJ/mm, about 0.51 kJ/mm after efficiency. Drop to 95 amps and push travel speed to 400 mm/min, and the heat per millimetre falls by roughly half. Same rod, same joint, completely different risk of blowing through.

Thickness: The Heat Sink and the Thin-Edge Rule

Base metal thickness acts as a heat sink. A thick plate pulls heat away from the puddle in every direction, so the molten pool stays shallow and localised even at aggressive settings. Thin sheet has nowhere to send the heat, so the puddle grows, sags, and opens into a hole. That is why sheet metal practice has always run on amperage limits: the old shop “thin-edge rule” of roughly one amp per thousandth of an inch of mild steel thickness is a starting ceiling, not a target, and it drops further near a sharp or already-melted edge, where almost no mass remains to support the pool.

Aluminium makes the point dramatically. Its high thermal conductivity pulls heat away fast, yet thin panels still blow instantly because there is so little material to lose. Welders working through Aluminum MIG Welding: The Complete Practical Guide will recognise the balance — high travel speed, tight fit-up, and sometimes pulse control to keep average heat down while still achieving penetration.

Root Land, Root Gap, and Joint Geometry

Joint geometry decides where your heat lands. The root land (or root face) is the small flat section at the bottom of a bevel; the root gap is the spacing between the two members. Together they control how quickly the arc can consume the last metal holding the joint closed.

• Large root gap: the arc focuses on a narrow bridge with a long unsupported span, and that bridge melts long before the sidewalls are ready.
• Thin or absent root land: there is not enough material to survive the arc, even at moderate settings.
• Heavy root land with no gap: the opposite problem — the arc struggles to get through, the temptation is to crank amperage, and burn-through arrives the moment the root finally gives.
• Poor fit-up and tack gaps: an unintended gap behaves like an intentional one you did not plan for, showing up as a sudden blowout halfway along the bead.

Small, consistent geometry beats perfect-looking settings every time. If the fit-up is wrong, no dial position will save the root.

The Root Ligament in In-Service Welding

Now add pressure. When welding on pipe still in service, the metal between your puddle and the flowing product inside is the remaining ligament. Two forces oppose each other: the flowing fluid cools the pipe wall from behind, which helps, but internal pressure pushes outward on a pool that has gone liquid. TWI’s in-service welding work puts it plainly — burn-through arises when a weld pool becomes large enough that the underlying ligament cannot contain the pressure of the flowing fluid in the pipe. Slow travel, a hot setting, and a wall thinned by corrosion turn a routine tie-in into an incident.

Risk factors (push toward burn-through)	Protective factors (pull away from it)
Excessive amperage for the thickness	Amperage matched to the thin-edge rule
Slow, hesitating travel speed	Steady travel that keeps heat input low
Wide or inconsistent root gap	Controlled gap with uniform tacking
Thin, missing, or burned-off root land	Consistent land width across the joint
Thin filler wire at high current (amp density)	Wire or rod diameter sized to the range
Preheat buildup on a small, enclosed part	Interpass temperature control and cooling breaks
Thinned or pressurised in-service wall	Cooling backflow plus a qualified heat limit
High-efficiency processes dumping energy fast	Pulsed or short-circuit modes with lower average heat

Read the table as a set of trade-offs rather than a checklist. Every protective factor buys margin that a risk factor will happily spend. Controlling burn-through is not about finding one perfect setting — it is knowing which side of the ledger your joint sits on, then adjusting one variable at a time until the two balance.

Main Causes of Burn-Through: A Complete Troubleshooting Map

Burn-through rarely has a single cause. More often it’s two or three small deviations stacking up until the base metal gives out, which is why chasing amperage alone rarely fixes it. Before you touch a dial, you need to know which suspect is actually guilty. Here is every major cause, what it looks like on the finished weld, and what to change first.

Excessive Amperage

Current is the volume knob of arc energy. Push it too high and the puddle outruns your ability to control it. On thin root passes, even 10–15 amps over the procedure range can turn a manageable puddle into a hole within two electrode diameters of travel. The telltale sign is a wide, flat bead with a concave or washed-out center and heavy, ragged edges: the metal melted faster than filler could replace it. On stick, the rod seems to “dig” rather than lay down. Fix it by dropping to the lower end of the recommended band for your electrode diameter and letting travel speed, not amperage, do the work. If you’re unsure where that band starts, our all position welding rod guide breaks down common electrode sizes and their practical current ranges.

Slow Travel Speed

Heat input rises as travel speed falls, because the arc dwells on the same spot longer. A welder compensating for an unfamiliar joint by creeping forward can double the energy delivered per inch without changing a single machine setting. The appearance is a lumpy, over-built bead with excessive reinforcement and a root that sags or opens up. The correction is counterintuitive: move faster and add filler, rather than slowing down and hoping the puddle catches up.

Long Arc or Wrong Voltage

Voltage controls arc length and width. A long stick arc spreads energy sideways and loses much of it to the atmosphere, but still drives a broad, shallow melt that can wash out a thin root. In wire processes, voltage too high for the wire feed speed gives a stubby, wide arc profile that hammers through the root. Watch for a crackling, unstable arc and spatter that scatters away from the puddle. Tighten arc length, correct your trim, and re-balance voltage against wire feed.

Overheated Workpiece and Preheat Buildup

A joint that has been preheated, or a part that already absorbed heat from earlier passes, starts with less capacity to sink energy. Chassis panels, sequential root passes, and multi-pass fills on small-diameter pipe all fall into this trap. The symptom is a weld that runs fine at the start of a pass and blows through midway. Let the part return to near interpass temperature before continuing, or reposition so heat isn’t concentrating in the same zone.

Excessive Root Gap

The root gap is where burn-through gets its worst reputation. A gap opening beyond the procedure’s fit-up tolerance leaves nothing under the arc but molten metal, and the puddle collapses through it. Fit-up variation from thermal cutting, plate distortion, or careless tacking is the hidden driver. A tack gap on one end that’s twice the specified land width will fail long before the nominal gap does.

Thin or Absent Root Land

The root land — the flat surface on the bevel face at the root — holds the molten pool until the welder chooses to open it. Remove it or thin it to a feather edge and the joint becomes an open gap from the first strike. Veteran pipe welders report that catastrophic “giant keyhole” failures on root passes almost always trace back to too little root land, or too large a gap combined with excessive heat, as Western Welding Academy’s root pass analysis emphasizes. Never blame your settings until you’ve measured the land.

Wrong Electrode or Filler Diameter

Counterintuitively, a wire that’s too small can cause burn-through rather than prevent it. The machine is set by amperage, but the arc delivers that amperage over a smaller cross-section, so amp density at the workpiece rises and the wire melts off in a hot, concentrated stream. Fix it by stepping up wire or rod diameter, or by re-balancing settings against the correct size.

Dirty or Coated Joint

Mill scale, oil, paint, galvanizing, and rust force welders to crank current to burn through contamination, which then burns through the base metal instead. The result is an erratic arc, porosity, and a rough, wandering bead. Clean properly and the amperage you previously needed disappears. On coated or alloy materials this gets worse fast: prep guidance in our aluminum cleaning article shows how dramatically surface condition changes arc behavior on heat-sensitive alloys.

Process Choice and Its Tendencies

Not all processes share equal risk:

• SMAW: high-duty-cycle rods like 7018 generate sustained heat; thin-root work usually favors 6010/6011 with lighter rods.
• GMAW: spray transfer deposits far more heat per second than short-circuit transfer, which is why short-circuit is preferred on thin sheet.
• FCAW: self-shielded flux core runs hot and penetrates aggressively, making thin-root burn-through a real concern — see aluminum flux core welding considerations.
• GTAW: least forgiving of poor fit-up because there’s no flux or slag to cushion the puddle, but the easiest to modulate with foot pedal control.

Interaction Effects: Why Two Small Problems Beat One Big One

This is the part troubleshooting guides undersell. A 1/16-inch excess root gap alone is survivable. Ten amps over on its own is survivable. Together they multiply: the gap removes the heat sink, the extra current melts what’s left, and travel speed never gets a chance to compensate. Always fix fit-up before touching the machine.

Cause	What You See	First Move
Excessive amperage	Wide, flat bead; washed root center	Drop to lower band for rod size
Slow travel	Heavy, lumpy reinforcement	Speed up, add filler
Long arc / high voltage	Erratic arc, scattered spatter	Tighten arc length, re-trim
Hot workpiece	Starts fine, blows through mid-pass	Pause for interpass temp
Excessive root gap	Root collapses, irregular keyhole	Re-fit, re-tack to procedure
Thin/no root land	Instant uncontrollable keyhole	Restore land per WPS
Wrong filler diameter	Hot, concentrated melt-off	Step up rod/wire size
Dirty joint	Porosity, wander, arc instability	Clean, then reset amps

Diagnose in this order: fit-up, cleanliness, then settings. If the appearance doesn’t match any single row, suspect an interaction — and fix the mechanical variable first, because most burn-through prevention guides agree that settings only work when the joint geometry supports them.

Prevention by Process: SMAW, GMAW/FCAW, GTAW, and TIG Roots

Burn-through never happens for the same reason twice. A setting that runs beautifully on 3/8-inch plate will eat straight through 16-ga sheet, and a root pass that works on a fit-up with a 1/16-inch land will blow apart on the same joint with no land at all. That is why prevention has to be process-specific. Each welding process puts heat into the joint in a different way, and each one has its own set of levers for pulling that heat back before the puddle goes through.

SMAW: Rod Size, Amperage Bands, and a Steady Arc

Stick welding gives you the fewest electronic safeguards and the most personal control. The electrode diameter is your first defense: a 3/32-inch rod on thin material melts off faster and concentrates current density into a smaller spot than a 1/8-inch rod at the same amperage, so stepping down a size often fixes a burn-through problem before you touch the machine dial. As a working band, 3/32-inch rods on sheet run roughly 40–70 amps, 1/8-inch rods on 1/8- to 1/4-inch plate sit around 90–130 amps, and open-root pipe passes with 6010 usually land in the 70–110 amp range depending on wall thickness. Treat those as starting points, not rules.

Technique matters as much as the dial. A tight arc (about one electrode diameter) keeps the heat focused where you want it; a long arc dumps voltage into the surrounding metal and widens the puddle. Drag rather than push on a root pass, keep the puddle short, and avoid wide weaves — a weave holds heat in one place long enough to wash the root edge away. If you are running open roots or all-position work, our All Position Welding Rod Guide walks through which electrodes hold a puddle best without collapsing it.

GMAW and FCAW: Transfer Mode Decides Your Margin

With wire processes, the transfer mode is the decision that sets your burn-through risk. Short-circuit transfer feeds wire that touches the puddle and shorts out dozens of times per second, so there is never a large, sustained spray pool sitting in the joint. That is exactly why short-circuit GMAW is favored on thin sheet, where the burn-through risk is highest. Spray transfer, by contrast, delivers a continuous stream of molten metal at higher voltage and current — superb for thick plate, far too hot for 20-ga.

The controls to watch are wire diameter, voltage, and trim (the voltage-to-wire-speed balance). Too much voltage relative to wire feed speed lengthens the arc and deepens penetration; drop the voltage or raise the wire feed and the puddle shortens. On flux-cored wires the same logic applies, though FCAW runs hotter overall and is better reserved for thicker sections. Our Aluminum MIG Welding: The Complete Practical Guide covers the short-circuit versus spray decision in detail for aluminum, where the margin is even thinner.

GTAW: The Narrowest Window of All

TIG roots leave nowhere to hide. Western Welding Academy’s analysis of TIG root failures puts it plainly: insufficient heat causes lack of fusion, while excessive heat causes burn-through, and the acceptable window between them is narrow. Prevention is therefore about consistency rather than dial position.

Key GTAW habits that hold the window:

• Purge first. Backing gas on stainless and alloy pipe prevents oxidation that weakens the root and changes how the puddle flows.
• Walk the cup at a steady pace. Pausing at the trailing edge of the keyhole dumps heat into one spot.
• Time the filler. Adding rod into the puddle actually cools it; backing off the filler is often what causes the burn-through, not too much current.
• Keep tungsten geometry tight. A balled, blunt electrode widens the arc cone and spreads heat.

For joint-level preparation that makes a TIG root forgiving, see Aluminum Cleaner for Welding: How to Prep Aluminum Right.

Pulse Welding: Heat Input on a Dimmer Switch

Pulsed processes cycle between a high peak current that penetrates and a low background current that lets the puddle cool without extinguishing the arc. The result is lower average heat input while still maintaining penetration — the single most effective electronic tool for controlling burn-through on thin or heat-sensitive material. Pulse is available on GMAW, GTAW, and some FCAW power sources, and it pairs naturally with the short-circuit or spray mode appropriate to your thickness.

Illustrative Starting-Point Settings

Process	Material	Filler	Starting point (tune to your WPS)
SMAW	16–18 ga steel	1/16–3/32 in 6013	40–65 A, DCEP, tight arc
SMAW	Open-root pipe	3/32–1/8 in 6010	70–110 A, drag, no weave
GMAW (short circuit)	18–20 ga steel	0.030 in ER70S-6	16–19 V, modest wire speed
GMAW (spray)	1/4 in and heavier	0.035–0.045 in	24–30 V, high wire speed
GTAW root	3/16–1/4 in wall	Matching ER filler	Lowest current that opens a stable keyhole
Pulsed GMAW/GTAW	Thin, fit-up-limited	Process matched	Peak for penetration, background for cooling

Whatever process you run, the pattern is the same: choose the smallest filler that will fill the joint, use the coolest transfer mode the thickness allows, keep the arc short and the travel speed steady, and prove the setting on a coupon before you touch production.

Controlled Burn Thru: The Intentional Root Pass Technique

On a pipeline crew, nobody apologizes for burn thru — they aim for it. On an open-root butt joint, the root pass is the one place where a welder deliberately melts straight through the base metal, opens a small hole ahead of the puddle, and fills it back in while moving. Done with control, that burn-through is the clearest evidence available that the weld fused the full thickness of the joint, face to inside. Done badly, it is the gaping hole that puts a lot on hold. The difference is fit-up, technique and procedure, not luck.

Anatomy of an Open Root: Gap, Land and Keyhole

Three terms carry most of the meaning here:

• Root gap — the opening left between the two beveled pipe ends before welding starts. For a 6010 open root, a 1/8 in (3.2 mm) gap with a small root land is the configuration most schools and shops teach as a starting point.
• Root land — the thin flat left at the bottom of the bevel, the last bit of un-melted edge the arc has to break through. Too much land and the arc wanders on top without reaching the inside; too little and the edge melts away instantly, dropping a blob of reinforcement into the pipe.
• Keyhole — the small, teardrop-shaped opening that forms just ahead of the puddle once the arc has burned through. Seeing it is the welder’s live confirmation of full penetration.

The geometry exists to make burn-through repeatable. The gap gives the arc access to both root faces, the land gives the puddle something to hang on, and the keyhole becomes a visual gauge the welder reads continuously. That is why the technique is often described as chasing the keyhole: you open it, wash molten metal into each sidewall, and follow it around the pipe.

Why Welders Chase a Keyhole Instead of Avoiding It

Uncontrolled burn-through is a defect because the result is unpredictable — a hole, a heavy sag, or root reinforcement outside tolerance. Controlled burn-through is a technique because the welder sets the limits in advance and holds to them. Codes acknowledge the distinction. API 1104 sets limits on internal root reinforcement and defines acceptable burn-through criteria for pipeline work, which is exactly why welders pursue a steady keyhole rather than tiptoeing shy of the root. Within those limits, visible, uniform penetration is an asset; outside them, it becomes rejectable. Terminology here is looser than people expect, as the ongoing discussion over a working definition of burn-through on the AWS forum makes clear.

Feature	Controlled keyhole (technique)	Burn-through (defect)
Appearance	Small, consistent teardrop tracked along the root	Irregular hole, dropout or heavy internal sag
Cause	Planned gap, land, amperage and travel speed	Excess heat, poor fit-up or uncontrolled gap
Root reinforcement	Uniform and within code limits	Excessive, asymmetric or missing
Inspection result	Full penetration, passes visual and radiography	Rejectable, requires gouge-out and repair

Modulating the Arc: Rod Feed, Body Position and Current

With a cellulosic 6010 root — the classic choice for open-root carbon steel, alongside 7010 in heavier wall — most welders use a whip-and-pause motion: dig into the root, pause to build the puddle and fill the keyhole, then step forward. The modulation tools are simple. Feeding rod faster cools the puddle and adds metal to hold the keyhole open longer; slowing the feed lets heat build and penetration deepen. Travel speed sets how long the arc sits on any point. Body position matters more than it sounds: a welder who cannot hold a steady arc length while moving around the pipe will fluctuate heat constantly, which is why fit-up and comfortable access are planned before the root ever starts. If you are still building fluency with stick roots, our Advantages of Stick Welding: Why SMAW Still Wins guide explains why the process remains the field default for roots.

TIG Roots: ER

Burn-Through on Thin Metal: Sheet, HVAC, and Automotive Repair

Thin metal is where burn-through stops being a theoretical risk and starts costing money. A shop flips a finished floor pan patch and finds a silver dollar of daylight where the seam used to be. An HVAC installer leans on the torch too long in 20-gauge duct and blows a hole that now needs a patch. On thick plate you can back the settings off and keep going. On sheet, the mistake and the damage happen in the same second.

The core problem is simple: at 0.040-inch thickness there is very little metal to absorb an arc. The pool forms faster than the surrounding material can carry the heat away, and the keyhole opens wider than your filler can bridge. Everything below is about buying those extra few seconds of margin.

Why Thin Sheet Blows Through So Fast

A guideline sheet welders repeat constantly is roughly one amp per 0.001 inch of thickness for mild steel. So 0.030-inch steel sits around 30 amps, 0.060-inch around 60. Treat it as a starting point, not a spec: joint design, position, travel speed, and heat sinking all move the real number. Aluminum needs more current to get the puddle moving, but it pulls heat away so fast that the surrounding panel stays cool and the arc energy stays concentrated at the joint, a combination beginners misread as “needs more amps.”

Three materials behave very differently at the same thickness:

• Mild steel is forgiving. Moderate conductivity lets heat spread slowly enough to hold a stable puddle but fast enough to avoid instant melt-through.
• Stainless steel has low thermal conductivity, so heat concentrates in the weld zone instead of spreading. At identical thickness it burns through far more readily and is also prone to sugaring and distortion.
• Aluminum has high conductivity, so heat races away from the arc, yet thin panels still blow through instantly because the melting point is low and the sheet is so thin. You are balancing two opposing problems at once.

Quick Note: Moving from steel to aluminum means relearning your settings rather than transferring them. The Aluminum MIG Welding: The Complete Practical Guide covers transfer modes, gas choices, and spool-gun setup that make thin aluminum controllable.

Backing, Chill Blocks, and Heat Sinks

The most effective mechanical fix for thin butt joints is backing. Copper and aluminum bars sit against the root of the joint and wick heat out of the molten pool before it can punch through. Copper is the classic choice because it conducts heat away quickly and does not readily fuse to weld metal, so the bead releases cleanly once cool. Chill blocks work on the same principle, clamped or weighted on the reverse side of the panel.

Backing also improves bead shape: the underside stays flat and supported instead of sagging into a grape-like drip. Where a backing bar will not fit, welders use:

• Plug welds to stitch a patch to a substructure, letting the underlying panel act as backing.
• Stitch or tack techniques: short 1/2-inch beads with cooling pauses, skipping around the seam instead of running one continuous pass.
• A smaller filler rod or thinner wire, which adds less heat-hungry mass but lets you feed faster into an opening keyhole to close it.
• Pulse settings, which drop to a background current between peaks and give the metal time to solidify.

Practical Settings Starting Points

These are illustrative ranges only, a bench-side starting point to be tuned through procedure qualification and test coupons:

Material	Thickness	Process	Starting Range
Mild steel	18–20 ga	GMAW short-circuit	28–40 A, 16–18 V
Stainless	18 ga	GMAW short-circuit	25–35 A, tighter trim, back purge where needed
Aluminum	18 ga	GMAW pulse or AC TIG	60–90 A with copper backing
Mild steel	16 ga	SMAW 1/16 in. rod	35–50 A, drag

Short-circuit transfer is the default preference on thin steel because it deposits metal in controlled bursts rather than a continuous spray. More background on transfer modes and trim balance is covered in Aluminum Flux Core Welding: What Actually Works in 2026 and the related process guides.

Mistakes Beginners Make on Sheet

• Staying in one spot. The arc dwells, the pool grows, and the hole opens before the hand reacts. Keep the arc moving even when it feels too fast.
• Cranking amps to compensate for a dirty joint. Contamination causes poor fusion, and more current fixes neither problem, it makes both worse. Clean first: see Aluminum Cleaner for Welding: How to Prep Aluminum Right.
• Running mild-steel settings on stainless. Stainless demands the lowest practical heat input and careful travel speed, which is why a quick pass with your steel setup is the most common way to blow a hole in an exhaust repair.
• No pulse on aluminum. Without pulse, thin aluminum either fails to fuse or burns through. There is a narrow window, and pulse widens it.
• Ignoring fit-up. An oversized tack gap on 20-gauge sheet is an invitation to blow through, no matter how good your settings are.

Real shop work follows the same pattern. Auto body: butt-weld a patch with copper behind it, weld 1-inch segments, and let the panel cool between passes. HVAC ductwork: small rod, low amperage, short stitches, so galvanized sheet never reaches full melt across a wide zone. Stainless: back-purge or use backing, and accept that slower travel with lower amps beats a quick pass that burns through. For thin, cosmetically critical aluminum wheels, the heat-control sequence in Alloy Wheel Welding: A Comprehensive Guide to Safe and Effective Aluminum Wheel Repair walks through the repair steps.

Note: Heat buildup compounds. On a long thin-metal seam, the panel around your last weld is already hot when you start the next one, so the burn-through threshold drops with every pass. Let the part cool, clamp heat sinks along the joint, or weld in a skip sequence.

Thin-metal burn-through is almost never a machine problem alone. It is fit-up, backing, cleanliness, and travel discipline working together. Dial the amperage to the thickness rule, put something conductive behind the joint, clean the metal, and keep the arc traveling, and 20-gauge steel becomes routine instead of risky.

Inspection, Acceptance Criteria, and Repair of Burn-Through

Finding burn-through is easy. Deciding whether it is a reject, a grind-and-blend, or a cut-out-and-re-weld is where codes, experience, and metallurgy meet. This section covers how the defect is detected, what the two codes most welders encounter say about it, and how to repair it without creating a second, worse problem.

How Burn-Through Gets Detected

Detection runs on a spectrum, from a welder glancing at the back of a test plate to a radiographer reading film on a pipeline girth weld:

• Visual observation. On open joints and test coupons, check the root side directly: a blown hole, a sagging frozen puddle, or an open keyhole is obvious. On closed structures, use a mirror, an endoscope, or a bright light while a helper watches for the arc breaking through.
• Borescope or remote visual inspection (RVI). On pipe and vessel work where the interior can’t be reached, a borescope through a nozzle, handhole, or open end shows the internal root profile. This is usually the fastest way to separate heavy-but-acceptable reinforcement from true burn-through.
• Radiography (RT). Film or digital radiography shows internal penetration profile clearly and remains the standard volumetric method on pipeline work. Burn-through reads as localized darkening: an irregular, blown-out root shape rather than smooth, uniform penetration.
• Ultrasonic testing (UT). Phased-array UT maps root condition where radiography is impractical, such as restricted areas or jobs needing radiation permits. It is strong on lack of fusion and on measuring remaining wall, but interpreting a blown-out puddle requires an experienced technician.
• Non-contact thermography. Research published in Sensors demonstrated that infrared thermography could detect burn-through defects in SS304 TIG welding, with onset associated with heat input around 0.74 kJ/mm on their test coupon (study). Treat that number as a material- and joint-specific reference point, not a universal threshold, but it reinforces the central point: heat input is the variable to control and record.

Quick Note: Before any grinding or repair, document the defect with a photo, a borescope image, and a written note of location and size. Once you grind it out, the evidence, and your ability to argue acceptance or justify the repair scope, is gone.

What the Codes Say: API 1104 and AWS D1.1

API 1104 (Welding of Pipelines and Related Facilities) applies to pipeline girth welds and contains explicit acceptance criteria for internal weld reinforcement and burn-through. The key principle: acceptance limits are tied to the pipe’s nominal wall thickness. A small, smooth, isolated root depression is tolerable up to a limit proportional to wall thickness, while a through-thickness hole or a crack is always unacceptable. Exact criteria change between editions, so work from the current edition specified in the project documents, not a memorized number.

AWS D1.1 (Structural Welding Code — Steel) addresses melt-through in complete-joint-penetration (CJP) groove welds. In structural work, melt-through is acceptable only where the detail requires full penetration or where it stays within the code’s stated limits; excessive penetration beyond those limits is a condition requiring repair. D1.1 also governs how that repair is executed: removal of the defect, inspection of the cleaned groove, and re-welding under the applicable procedure.

Note: The words inspectors use vary. “Burn-through,” “melt-through,” “excessive penetration,” and “blown root” often describe the same physical condition in casual conversation. Read the acceptance table in the governing code, not the word on the report. Disposition depends on where the weld falls against the measured limit, not on which term the inspector chose.

A Defect-Assessment Flow: Accept, Blend, or Remove

1. Locate and classify. Visual first, then borescope or RT to confirm internal profile. Is it a hole, a sag, or just heavy root reinforcement?
2. Measure. Note root penetration height, affected length, and remaining wall thickness at the site. On pipe, confirm the actual wall at the weld, not the nominal.
3. Compare to the governing code. API 1104 for pipeline girth welds, D1.1 for structural CJP work, or the project’s approved WPS/ITP. Apply the acceptance limits for that wall thickness.
4. Decide the disposition. Small, smooth, within-limit reinforcement: accept and record. A shallow depression or rough root that doesn’t breach the wall: blend by grinding if the code permits, then re-inspect. A through-thickness hole, a cracked sag, or burn-through on a pressure boundary: full removal and re-weld.
5. Re-inspect after repair. Every repair goes back through the same NDT method that found it. A repair that isn’t re-inspected isn’t a repair.

Finding	Typical Disposition
Smooth root reinforcement within code limit	Accept, document
Rough or saggy root, no wall breach, within limit	Grind smooth, re-inspect
Local depression approaching wall-thickness limit	Grind blend or repair per code
Through-thickness hole or blown keyhole	Full removal, re-weld, re-inspect
Cracked or oxidized root sag	Full removal, re-weld

Repair Without Creating a Worse Problem

The mechanics of repair are straightforward; the metallurgy is where people get into trouble.

Removal. Small defects come out with a grinder, a burr, or a carbide cutter, cut back to sound metal with feathered edges. Larger blowouts and thicker sections usually need air-arc gouging or carbon arc gouging to remove the defective weld and a bit of the underlying metal, followed by grinding to clean, bright steel. Back-gouging from the opposite side is the standard way to verify full fusion on a CJP joint before the second side is welded.

Re-welding. The repair weld goes in under the same procedure, preheat, and interpass requirements as the original, not a quick pass on a higher setting. If the base material is susceptible to hydrogen-induced cracking, re-applying preheat and controlling interpass temperature is non-negotiable.

The repeated-repair caution. Every weld cycle re-austenizes and re-transforms a band of base metal. The heat-affected zone beside the original weld has already seen one thermal cycle; a second and third pass can coarsen grain structure, reduce toughness, and build residual stress. Most codes limit the number of successive repair layers or require engineer approval after a given number of cycles. On alloy steels, this is where a repair that “looks fine” can quietly become a toughness liability.

Aluminum deserves its own mention. Aluminum gives no color change to warn you, and a burn-through on thin sheet leaves almost nothing to re-weld. Repair on thin aluminum, body panels, duct, wheels, is usually a cut-out-and-patch job rather than a fill-the-hole job. The practical setup for thin-panel work is covered in our guide to aluminum MIG welding, with wheel-specific practice in alloy wheel welding.

Quick Note: On in-service welding, burn-through risk is built into the procedure qualification itself. Fluid flow cools the wall while internal pressure acts on the molten pool, so the qualified procedure, including heat-input limits and the inspection plan, is the primary defense. TWI’s work on in-service welding procedure qualification and inspection methodology is the reference point for how that planning is done.

The bottom line: measure against the code, not your memory of the code; document before you grind; and treat every repair as a fresh thermal cycle that has to earn its place in the part’s service life.

Safety, Fumes, and the Human Side of Burn-Thru Incidents

Everything discussed so far treats burn thru welding as a quality problem or a technique choice. On the shop floor and in the field it is also a safety hazard. A hole burned through a wall, a floor, or a pressurized line does not just fail inspection — it starts fires, drops molten metal into spaces nobody can see, and can turn a routine weld into a process-safety incident.

Why a Burn-Through Is Literally a Fire Hazard

The regulation is explicit. OSHA 1910.252 requires that welding, cutting, or brazing on metal walls, partitions, ceilings, or roofs be done with precautions against ignition of combustibles on the opposite side. In practice that means someone checked what sits behind or below the joint before the arc strikes — insulation, wiring, stored paper, dust, hydraulic lines, or a neighboring room full of inventory.

The logic is simple: a burn-through is an unplanned opening, and sparks and molten slag follow the path of least resistance through it. The far side is usually the side nobody is watching. A welder focused on the puddle cannot see smoke starting two feet away.

Quick Note: A back-side check takes thirty seconds. Pull the panel, open the access hatch, or send a second person to the other side before you strike an arc on any joint where the root is close to unseen material. Fixing fit-up is cheaper than replacing burned insulation.

Hot Work Permits, Fire Watch, and Blowback Burns

Most facilities manage this with a hot work permit system that identifies the location, the combustibles present, the fire watch assignment, and the extinguisher rating required. The fire watch stays in place after welding stops — commonly 30 minutes — because smoldering insulation and dust can ignite long after the bead has cooled.

Burn-through is also a direct personal injury risk. When the root gives way, the arc blows out with a violent pop, and molten metal, spatter, and slag exit the back side. If your face, arm, or leg is on that side — or you are reaching around a pipe to inspect the root — you take a deep spatter burn instead of a surface one. Standing off the plane of the blowout path, wearing leathers that close at the collar, and using a hand shield on the root side are unglamorous habits that prevent most of these injuries.

Welding Fume: The Exposure Nobody Feels Right Away

The other human dimension is respiratory. In its welding fact sheet, OSHA states that prolonged exposure to welding fume may cause lung damage and several cancers, including lung, larynx, and urinary tract (OSHA welding fact sheet). Burn-through worsens the exposure: an uncontrolled puddle throws more spatter and fume, and chasing the hole shut keeps the welder on the arc longer.

Practical controls are not exotic:

• Ventilation first. Local exhaust at the joint or a clean-air supply hood beats any respirator for comfort and compliance.
• Respiratory protection where ventilation cannot reach — confined spaces, interior pipe work, and inside ducts.
• Material awareness. Stainless, galvanized, coated, and plated base metals produce fume with higher hazard potential than plain carbon steel. Stripping coatings before welding (see Aluminum Cleaner for Welding: How to Prep Aluminum Right for prep discipline) reduces what ends up in the air.
• Consumable selection. Low-hydrogen electrodes and wire, correctly stored, keep fume levels predictable; damp or contaminated rods do the opposite.

In-Service Lines: When Burn-Through Becomes a Process-Safety Event

The gravest case is welding on piping still in service. The remaining wall is the only thing separating the arc from whatever the line carries — hydrocarbon, steam, chemical, or water under pressure. A burn-through does not merely create a reject; it breaches a live line. Work on procedure qualification for in-service welding (TWI) shows why these welds get such cautious treatment: flowing fluid cools the wall and works in your favor, but pressure pushes against the molten pool, so the margin between a sound weld and a release is thin.

So in-service work runs on qualified procedures with hard limits on heat input, interpass temperature, pool size, and travel speed. Deposition rate is capped, root gaps controlled, and a spotter often watches the far side while an isolation plan stays ready. Burn-through here is a shutdown event — and on a hydrocarbon line, a potential fire or explosion.

Note: Treat burn-through as a safety control, not just a defect class. If a procedure has been qualified without a realistic assessment of what lies behind or inside the joint, the quality plan and the fire plan are both incomplete.

Handled well, the same discipline protects the weld and the people around it: check the back side, control the heat, manage the fume, and never let a live line become the surface where you discover your limits.

Common Mistakes, Practical Tips, and a Step-by-Step Fix Plan

By now you know what burn-through is, why heat input pushes a joint toward it, and how each process behaves at the root. So why do experienced welders keep punching holes in the same joint day after day? In most shops the answer isn’t missing information. It’s a short list of habits that quietly reintroduce the problem every time it appears to be “fixed.”

The Six Mistakes That Keep Holes Coming

1. Chasing settings instead of fixing fit-up. The machine is the obvious suspect, so the welder turns the amperage down and tries again. If the root gap has opened to 6 mm because the tacks pulled or the bevels were cut unevenly, no setting will save the joint. Fit-up comes first, always. A tight, consistent gap with a real root land buys you more margin than any dial adjustment.
2. Ignoring joint prep. Mill scale, oil, rust, primer and cut slag all change how the arc starts and where it points. An arc wandering on contamination digs in at one spot and blows through, then the welder compensates with more heat, which makes the next hole worse. Wire-brushing and grinding the bevel faces to clean, square metal costs two minutes. On aluminum, prep is even more unforgiving, which is why proper cleaning matters so much — see Aluminum Cleaner for Welding: How to Prep Aluminum Right.
3. Cranking amperage to compensate for a bad arc. Industry troubleshooting guides consistently list “too high current for the material thickness” as the number-one cause of burn-through, and it is frequently reached by a welder trying to burn through a contaminant, a gap or a sticky arc start rather than by accident (Unimig’s burn-through troubleshooting guide). Clean the joint, check your gas and polarity, then reset the amperage to the procedure range.
4. Running too large a gap. A gap that drifts past what the procedure allows turns the root into an unsupported bridge of molten metal. Alone it produces sag; combined with a hot setting it produces a hole. A big gap plus aggressive amperage is far worse than either problem by itself.
5. Not letting the part cool. On thin or intermittent stitch welds, each pass preheats the next area. Interpass temperature climbs, the heat sink effect disappears, and a setting that worked on the first tack burns through on the fourth. Space the tacks, wait, and check temperature with a contact thermometer or temp-stick rather than guessing.
6. Skipping procedure limits. Reusing a setting from a 6 mm plate on 3 mm material, or welding outside the ranges your procedure allows because “it worked yesterday,” guarantees inconsistency. Electrode choice compounds this: a filler that is too fine for the current density concentrates heat at the arc, so reviewing the All Position Welding Rod Guide before switching rods is cheaper than grinding out a rejected root.

Quick Note: If burn-through disappears when a different welder runs the same joint, your problem is technique or fit-up. If it fails identically for everyone, your problem is the procedure or the joint design.

A Step-by-Step Fix Plan

1. Diagnose the appearance. Photograph and classify the defect before touching a knob. A clean round hole points to too much heat or too slow a travel speed. A sagging, lumpy root usually means gap and land. A hole localized at one spot suggests a pause, a bad tack or a restart crater. Standard defect descriptions from published welding defect references are a useful cross-check when you report the problem.
2. Check fit-up. Measure the root gap at several points, confirm the root land exists, verify bevel angle and tack integrity, and confirm the joint is clean and dry.
3. Calculate heat input. Run the numbers using the TWI formula — (voltage × amps × 60) ÷ (travel speed × 1000) × arc efficiency — and compare the result with the limits in your procedure (TWI heat input vs. arc energy). Often the measurement alone reveals the fault.
4. Adjust one variable at a time. Typical order: increase travel speed slightly, then trim amperage in small steps, then correct gap or land. Amperage, travel speed and gap interact non-linearly, so changing all three at once tells you nothing about what actually worked.
5. Run a test coupon. Cut a scrap piece of the same thickness, in the same position, and weld it with the new settings. Inspect the root, and if you can, cut and inspect the cross-section. Never qualify a change on production material.
6. Document the result. Record the final amperage, voltage, travel speed, wire or rod size, gap, land and pass sequence in your WPS or PQR. Proven settings written down are what prevent recurrence when a different welder or a different shift takes over the same joint.
7. Requalify when you go outside the range. If the fix pushes you beyond the qualified parameters, stop and requalify the procedure rather than improvising on the job.

The same discipline applies to the thin-metal and aluminum work covered earlier in this article, where Aluminum MIG Welding: The Complete Practical Guide shows how much finer the adjustment window becomes.

Final Thoughts

Burn-through is a controllable variable, not an act of bad luck. The heat going into the joint is a number you can calculate, the joint geometry that supports that heat is something you can measure, and the technique that delivers it is something you can qualify and write down. On one side of the trade that control means keeping the puddle out of the pipe; on the other it means opening a keyhole and chasing it deliberately to the millimeter. Same physics, opposite intent, identical requirement: know your settings, respect your fit-up, change one thing at a time, and prove it on a coupon before it ever reaches production.

Frequently Asked Questions

What is burn thru in welding?

Burn thru (burn-through) is when heat input melts completely through the base metal, creating a hole or a sagging, blown-out puddle — most often at the root of a joint. It is a weld defect unless it is a deliberately controlled root-pass technique, where a small keyhole is opened and closed to guarantee full penetration.

What causes burn-through on a pipe root pass?

The usual combination is too much amperage for the wall thickness, a root gap that is too wide, a root land that is too thin (or absent), and travel speed too slow. Fit-up problems and excess preheat make it worse, and the keyhole grows faster than the welder can close it.

How do I fix burn-through on thin sheet metal?

Drop amperage, speed up your travel, use a smaller filler rod, and add a copper backing bar or chill block to pull heat away. Stitch or tack instead of running a continuous bead, and use short-circuit or pulsed transfer on MIG rather than spray.

Is controlled burn-through acceptable under welding codes?

Codes such as API 1104 define burn-through and set acceptance limits for root-bead conditions on pipeline welds, and procedures must be qualified. A controlled keyhole root is acceptable only when it stays within the approved procedure’s internal reinforcement and penetration limits.

What heat input causes burn-through?

There is no universal threshold — it depends on thickness, alloy, joint geometry, and cooling. Research on stainless TIG welds has reported heat inputs around 0.74 kJ/mm as the onset of burn-through for a specific coupon, which illustrates how specific the threshold is. Always qualify on your own material.

How is burn-through detected in inspection?

Visual inspection catches external blowholes and heavy root sag; radiography, ultrasonic testing, and borescope examination of pipe internals detect internal burn-through. Noncontact thermography has also been studied as a real-time detection method during welding.

Can burn-through be repaired?

Yes — the damaged area is ground out or gouged to sound metal and re-welded within the procedure’s limits. Repeated repairs in the same spot risk enlarging the heat-affected zone, so remove enough material to make the first repair count.

What is the difference between burn-through and excessive penetration?

Excessive penetration is more weld metal in the root than the acceptance criteria allow, but the joint is still closed. Burn-through is the extreme case where the puddle has actually breached the root face or blown a hole, losing the joint’s integrity.