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Burn-through: what it is and how to spot it

Fillet Gauge editorial teamSources checked 3 min read

Burn-through is a hole, or a collapsed root, where the arc melted all the way through the joint and the weld metal fell out of the back. It happens mostly on root passes and on thin material. From the face it often shows as a sunken spot in the weld; from the back as an open hole or drooping metal.

A V-groove butt weld in cross-section where weld metal hangs down out of the back of the joint like a drip, an open cavity passes through the root area, and the face has sunk below the plate surface
Too much heat at the root: the pool fell through. Original drawing, not to scale.
Use this diagram

How to recognize burn-through

  1. Check the back of the joint. An open hole through the root, or a lump of weld metal hanging below the back surface like an icicle, is the clearest sign.
  2. Read the face above it. Where metal fell through, the face over that spot is often sunken or concave, sometimes with a hole you can see through.
  3. Look at thin parts and root passes. Sheet metal, tube walls and the open root of a pipe weld are where it happens most.
  4. Note the repair history. A burned-through root is often repaired before the next pass. A lumpy, uneven patch on the root side may mark one.

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Welding Inspection and Discontinuities

A convex fillet weld replica is checked with a leaf-type fillet gauge. The drawing calls for a single fillet size. The vertical leg fits the 3/8 in blade and the horizontal leg the 5/16 in blade. What size is recorded?

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How it is found and recorded

Visual inspection finds it from either side when the back is accessible. A mirror or borescope helps inside pipe. On a finished weld with no access to the back, radiography shows burn-through as a dark, rounded, irregular area at the root, often with lighter metal hanging around it. Record its location and size. Many documents treat it with the root profile requirements, or call for repair outright, so read the governing document for that joint.

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Every answer comes with a full explanation.

Common mix-ups

  • Burn-through or incomplete joint penetration. They are opposites. Incomplete penetration means not enough melting at the root. Burn-through means too much.
  • Burn-through or excess root reinforcement. Excess reinforcement is a full, sound root bead that sticks out too far. Burn-through has a hole or collapsed, uneven metal.
  • Burn-through or underfill. A low face can come with burn-through, but underfill alone has no hole through the root.

Why it forms

Too much heat in one spot for the thickness. Typical causes are too high a current, too slow a travel speed, too wide a root opening, too thin a root face, dwelling at one point, and on thin sheet, any heat at all that isn't controlled. Good fit-up, a consistent root face and opening, and the right heat input for the thickness prevent it. Backing bars or rings also hold the pool up.

Burn-through on the CWI exam

On Part A, burn-through fits Visual Examination and Welding Application and Control, and heat input sits in Welding Related Calculations, at least 6% of the exam. On Part B it belongs to Welding Inspection and Discontinuities, at least 34% of the practical. On Part C, look in your own copy for the root profile requirements and the repair rules.

Practice it

See how current, voltage and travel speed add up with the heat input calculator. Then read its opposite at the root, incomplete joint penetration, and the low-face condition it often leaves, underfill. The weld discontinuities guide lists every type.

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Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

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