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Underbead crack: what it is and how to spot it

Fillet Gauge editorial teamSources checked 4 min read

An underbead crack is a crack in the heat-affected zone just beneath or beside a weld bead, usually caused by hydrogen. It often lies roughly parallel to the fusion line and may not reach the surface at all. Hydrogen takes time to gather, so it can appear hours after welding: delayed or cold cracking.

A V-groove butt weld in cross-section with dashed heat-affected zone boundaries and short jagged cracks inside the heat-affected zone on both sides, below the plate surface
Hidden in the hardened heat-affected zone, often with nothing showing on top. Original drawing, not to scale.
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How to recognize an underbead crack

  1. Know where to look. The crack sits in the base metal right next to the weld, in the coarse, hardened zone the arc heated but did not melt.
  2. Expect nothing at the surface. Many underbead cracks are fully buried. The ones that do break out show as fine lines near the toe, which is where hydrogen cracks and toe cracks overlap.
  3. Think about time. A weld that looked clean at the end of a shift can show cracks the next morning. When a steel is susceptible, a delayed final inspection is the safeguard.
  4. Check the conditions. Hardenable steel, thick sections, no preheat, damp electrodes or flux, and high restraint together make this crack likely.

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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

Visual inspection catches only the ones that reach the surface. Magnetic particle testing finds surface and near-surface cracks in ferromagnetic steel. Ultrasonic testing is the usual method for buried heat-affected zone cracks, because they are planar. Radiography can miss them unless the beam is lined up with the crack. Where a governing document requires a waiting period before final inspection for some steels, the reason is this crack.

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Common mix-ups

  • Underbead crack or a centerline crack. Centerline cracks are in the weld metal and form as it freezes. Underbead cracks are in the heat-affected zone and form after cooling.
  • Underbead crack or lamellar tearing. Both sit in the base metal under a weld. Lamellar tearing runs in steps parallel to the plate surface in thick rolled plate. An underbead crack follows the hardened zone beside the fusion line.
  • Underbead crack or incomplete fusion. Incomplete fusion is an unbonded line at the fusion boundary itself, with the shape of the groove face.

Why it forms

Hydrogen cracking needs three things at once: hydrogen in the weld area, a hard, susceptible microstructure in the heat-affected zone, and tensile stress. Remove any one and the crack doesn't form. Low-hydrogen electrodes stored and handled correctly cut the hydrogen. Preheat and interpass temperature slow the cooling, which softens the microstructure and lets hydrogen escape. Lower restraint and good fit-up cut the stress. Carbon equivalent gives a quick read on how hardenable a steel is.

Underbead cracks on the CWI exam

On Part A, underbead cracking is the classic link between Visual Examination and Base Materials, Heat Treatments and Metallurgy, at least 6% of the exam, where hydrogen, preheat and hardness are covered. On Part B it fits Welding Inspection and Discontinuities, at least 34%, and NDE Application. On Part C, the code's preheat requirements and the timing of final visual inspection are where to look, read in your own copy.

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Work out how hardenable a steel is with the carbon equivalent calculator. Then read the crack family it belongs to: transverse cracks and toe cracks. The weld discontinuities guide has every type in one table.

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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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