Types of weld discontinuities: what each one looks like
A weld discontinuity is any interruption in the normal structure of a weld or the metal around it: a hole, an inclusion, a crack, a missing bond or a wrong shape. It only becomes a defect when it breaks the limits of the code, specification or contract that governs the job. Naming it correctly comes first, because the name decides which rule you look up.
Check anything you rely on against your own licensed code book and the edition your AHJ enforces.
This page groups the 20 discontinuities a welding inspector meets most, gives the one tell that names each, and links to a page per type with an original cross-section or surface drawing, how to recognize and measure it, and the look-alikes that trip people up. We give no acceptance limits. They depend on the governing document, so read yours.
How to sort what you see
Ask three questions in order.
- Where is it? In the weld metal, at the toe or root, in the heat-affected zone, or out in the base metal.
- What shape is it? Round and smooth, angular and ragged, thin and flat, sharp-tipped, or a profile that is too high or too low.
- Which way does it run? Along the weld, across it, or parallel to the plate surface.
Those three answers narrow almost any discontinuity to one or two names. The same questions also pick the NDE method: surface flaws suit visual, penetrant and magnetic particle testing, round buried flaws suit radiography, and flat buried flaws suit ultrasonic testing. The NDE methods guide covers that choice in depth.
Try one question
Question 1 of the free 10-question check on CWI Part B. No signup. Then ask Tack, the AI study buddy, why the answer is right and the others are wrong.
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?
Press A–D or 1–4 to choose an answer while this question is on screen.
Choose an answer to begin
Inside the weld
| Discontinuity | Where it sits | The tell | Full page |
|---|---|---|---|
| Porosity | Weld metal, face or buried | Round, smooth gas voids: scattered, cluster, elongated or piping | Weld porosity |
| Slag inclusion | Between passes, against the sidewall | Angular, ragged pocket; dark on film | Slag inclusion |
| Tungsten inclusion | GTAW weld metal | A tiny chip that shows bright on film | Tungsten inclusion |
| Incomplete fusion | Groove face or pass boundary | A thin, flat unbonded line with the shape of the surface | Incomplete fusion |
| Incomplete joint penetration | Root of a groove weld | Root faces never melted; straight dark line on film | Incomplete joint penetration |
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Weld shape and profile
| Discontinuity | Where it sits | The tell | Full page |
|---|---|---|---|
| Undercut | Base metal at the toe | A notch melted into the plate and left open | Weld undercut |
| Overlap | On the plate past the toe | A lip of weld metal resting on the plate, unfused | Weld overlap |
| Underfill | Groove weld face or root | The face sits below the plate surface | Weld underfill |
| Excessive reinforcement or convexity | Groove crown, fillet face | Too high a crown or too round a fillet face | Excessive weld reinforcement |
| Burn-through | Root, thin material | A hole or collapsed root where the pool fell through | Burn-through |
Cracks
| Discontinuity | Where it sits | The tell | Full page |
|---|---|---|---|
| Longitudinal crack | Along the weld, often the centerline | A sharp line parallel to the weld axis | Longitudinal crack |
| Transverse crack | Across the weld, often into the plate | A sharp line at right angles to the weld | Transverse crack |
| Crater crack | The crater at a bead's end | Short, often star-shaped cracks in an unfilled crater | Crater crack |
| Toe crack | The toe, running into the plate | A tight line along the toe with no visible floor | Toe crack |
| Root crack | The root of a groove or fillet weld | A jagged line from the root, through the throat in fillets | Root crack |
| Underbead crack | Heat-affected zone under the bead | Hydrogen cracking, often buried, sometimes delayed | Underbead crack |
Base metal and surface
| Discontinuity | Where it sits | The tell | Full page |
|---|---|---|---|
| Lamination | Inside rolled plate | A flat mill split parallel to the surface | Lamination |
| Lamellar tearing | Through-plate under a T or corner weld | Step-shaped tears parallel to the plate surface | Lamellar tearing |
| Arc strike | Base metal outside the weld | A small melted, quenched scar | Arc strike |
| Spatter | Plate surface beside the weld | Raised droplets stuck on the plate | Weld spatter |
Discontinuities on the CWI exam
Discontinuities run through all three parts. On Part B, Welding Inspection and Discontinuities is at least 34% of the practical, the biggest domain, and you name and measure discontinuities on replicas with the Book of Specifications open; the Part B guide covers the format. On Part A, which is closed book, they fit Visual Examination and NDE Fundamentals, and their causes fit Welding Application and Control and the metallurgy domain. On Part C you look up the governing acceptance criteria in the code; for D1.1, D1.1 visual acceptance criteria explained shows how to reach the right row fast.
Practice them
Test your naming on the weld discontinuities quiz: original drawings, each with the tell that separates it from its look-alike. Measure profiles on the weld gauge trainer. For causes and prevention in one read, see weld discontinuities explained for the CWI exam.
Check the rest of Part B
Take the free 10-question check. It's weighted to the Part B outline, scored by domain and needs no signup, so you see whether discontinuities or another domain costs you the most points. The Part B course then gives you specification drills, two full-length timed practice exams and a worked explanation with every answer.
Sources
Show the list
- CWI Part B Practical Examination Information, American Welding Society, Rev. 03.01.2024. Retrieved 20 Oct 2026.
- AWS B5.1:2025, Specification for the Qualification of Welding Inspectors, American Welding Society, Table 8.1. Retrieved 20 Oct 2026.
- Defects / Imperfections in Welds series (Job Knowledge 40 to 47 and 67), TWI. Retrieved 25 Oct 2026.
- Discontinuity names follow AWS A3.0, Standard Welding Terms and Definitions. Every description here is our own wording.
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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