Weld discontinuities explained for the CWI exam

Discontinuity versus defect

A discontinuity is simply a break in the expected makeup of the weld. It becomes a defect only when it's bigger, longer or more frequent than the governing code allows. That's why the same flaw can pass on one job and be rejected on another. The code, contract or specification decides, and the inspector checks the flaw against those limits.

For study, that means two separate skills. One is identifying and explaining the flaw, which is what this article covers. The other is looking up the limit in your code, which is a navigation skill you practice with the book open. We give no limits here, because they depend on the code and the job. For D1.1, D1.1 visual acceptance criteria, explained shows how to find the governing row fast.

The five flaws to know

Porosity: gas that got trapped

Porosity is gas that formed in the molten pool and froze in place as voids. It shows up as round holes, sometimes scattered, sometimes in a cluster or a line. Wormholes are elongated voids from gas escaping upward as the pool freezes.

Typical causes:

• Contamination on the joint: oil, paint, rust, moisture.
• Damp electrodes or flux, or a wet low-hydrogen electrode.
• Shielding lost to wind, a clogged nozzle, low gas flow or a long arc.
• Too fast a travel speed, freezing the pool before gas escapes.

Prevention follows the cause: clean the joint, store and handle consumables properly, protect the arc from drafts and slow down enough for the pool to degas. Surface porosity is visible. Buried porosity needs radiography to see well.

Undercut: a groove at the weld edge

Undercut is a trench cut into the base metal along the edge of the weld and left unfilled. It looks like a small trench beside the toe. It reduces the thickness at a spot where stress already concentrates, so it matters most on parts under fatigue loading.

Common causes are excessive current, a long arc, a poor electrode angle, a fast travel speed, or pausing too little at the edge of a weave. Fixes are the reverse: lower the heat, shorten the arc, correct the angle and dwell briefly at each side. Undercut is a visual and measured item, so it links straight to Part B work. The measurement workflow guide shows how to take and document a reading, and the weld gauge reading trainer gives you practice.

Lack of fusion and incomplete penetration

These two are often mixed up on the exam, so keep them apart:

• Lack of fusion is weld metal that never bonded to the base metal or to the previous bead. It can sit on the sidewall or between passes, and it's planar, like a thin crack.
• Incomplete joint penetration is weld metal that didn't reach the root where the design needed it to.

Causes include low heat input, a travel speed that's too fast, a wrong electrode angle, poor joint fit-up, a bevel too narrow to reach, and dirty surfaces. Because these flaws are planar and often buried, ultrasonic testing with an angle beam is a strong candidate. Radiography finds them less reliably unless the beam lines up with the flaw.

Cracks: the flaw inspectors fear most

Cracks are narrow, sharp-ended breaks, and they spread under load. Most codes reject a crack no matter its size. They show up in several forms:

• Hot cracks form while the weld is freezing, often from high sulfur or a bad bead shape.
• Hydrogen-induced (cold) cracks form after cooling, from hydrogen, residual stress and a hard microstructure together. Preheat, dry low-hydrogen consumables and controlled interpass temperature are the usual countermeasures. Welding metallurgy for the CWI exam explains why each one works.
• Crater cracks form where an arc was stopped without filling the crater.

Surface cracks are found with penetrant testing or magnetic particle testing. Magnetic particle works only on ferromagnetic metals, so a surface crack on stainless steel points to penetrant. Buried cracks call for ultrasonic or radiographic testing.

Overlap: metal sitting on top

Overlap is weld metal that has spilled over the base metal without bonding to it, usually at the toe. People confuse it with undercut because both appear at the edge. The difference is direction: undercut is a groove that removes base metal, and overlap is a lip that adds unfused metal on top.

Causes are too slow a travel speed, too low a current for the electrode size, or a poor technique that lets the pool roll over cold metal. Overlap hides a notch under the lip, so it's treated seriously. It's visible, and a close look or a surface method confirms it.

To test yourself on these and their look-alikes, take the weld discontinuities quiz: 15 original drawings, each with the tell that gives it away. For the rest of the list, from slag inclusions and tungsten inclusions to underbead cracks and lamellar tearing, the weld discontinuities guide has one page and drawing per type.

Which method finds which flaw

Use a simple rule. Ask where the flaw sits and what shape it has.

The NDE methods guide goes deeper on what each method can and cannot see, and the Part A guide shows how discontinuities sit among the exam's domains.

Study them by cause

Don't memorize a list of names. Build one flashcard per flaw with three lines: cause, appearance, method. Then shuffle and answer from the cause side, because that's how exam questions tend to arrive. The free 10-question Part A check shows whether you can pair a flaw with its cause and its method, and which areas to study first. Take the free check before you build your cards, so you spend time where you need it.

Flaws sit next to paperwork on Part A, so pair this with the WPS, PQR and WPQ explainer. If a check shows gaps, the rebuild plan turns them into a study list. Causes often trace back to the arc process, so see welding processes for the CWI exam.

FAQ

What's the difference between undercut and overlap?

Undercut is a trench cut into the base metal beside the weld, so it removes material. Overlap is weld metal lying on the base metal without bonding, so it adds a lip with a notch under it.

What causes porosity?

Gas trapped as the pool freezes. The usual sources are contaminated or damp joints and consumables, lost shielding and a travel speed that's too fast.

Is every discontinuity a defect?

No. A discontinuity becomes a defect when it exceeds the limits of the governing code or specification. The same flaw can be acceptable on one job and rejected on another.

Which NDE method finds subsurface flaws?

Radiography suits volumetric flaws such as porosity and slag, and ultrasonic testing suits planar flaws such as lack of fusion and buried cracks. Penetrant and magnetic particle testing only reach the surface or just below it. Test yourself on the NDE methods quiz.