Every weld discontinuity has a cause — and nearly every cause can be traced back to a process, a parameter, a consumable, or a technique. This library teaches you to diagnose the weld in front of you: symptom to discontinuity, mechanism to cause, cause to correction.
Look at the weld. Name what you see — a hole, a groove, a crack.
Classify the discontinuity. Porosity? Undercut? Lack of fusion?
Understand how it formed — gas trapped, arc melting, shrinkage.
Trace the mechanism to a controllable variable.
Change the variable — parameter, consumable, technique, prep.
Re-inspect. Confirm the defect is gone before the next weld.
Never change three variables at once. Change one, weld, inspect. When the defect disappears, you found the cause. If you change everything, you learn nothing.
Any interruption in the expected structure of a weld — a pore, a toe notch, a slightly convex bead. Nearly every weld contains small discontinuities. Most are harmless. Discontinuity is a description of what exists, not a verdict on whether it matters.
A discontinuity that the applicable code, specification, or service condition deems unacceptable — large enough, located badly enough, or present in a weld that cannot tolerate it. The same discontinuity can be a defect under one code and perfectly acceptable under another.
Rejecting welds that the code accepts costs time and money. Accepting welds the code rejects costs your reputation and the structure. The governing document — not a gut feeling — draws the line. See the full treatment in Weld Discontinuity vs Defect.
Surface defects — undercut, overlap, arc strikes, spatter, surface porosity — are visible to the eye. Internal defects — lack of fusion, slag inclusion, internal porosity, lamellar tearing — hide inside the joint and need NDT to find.
Every process has its signature defects: tungsten inclusions belong to TIG and plasma, slag inclusions to stick and FCAW, worm tracks to FCAW, incomplete penetration to any open-root weld.
Hydrogen cracking, hot cracking, and lamellar tearing live here — driven by steel chemistry, restraint, cooling rate, and through-thickness ductility, not by what the welder dials in.
Worn contact tips, damp electrodes, wrong tungsten, leaking gas hoses, dirty liners — the hardware between the machine and the arc causes a long list of defects.
Current, voltage, travel speed, stick-out, angle, weave — the welder's inputs. Undercut, overlap, burn-through, and profile defects almost always trace back here.
Wind stripping shielding gas, cold base metal, humidity in electrodes, wet joints — the shop conditions that turn good parameters into bad welds.
| Method | Finds | Best For |
|---|---|---|
| VT — Visual | Undercut, overlap, arc strikes, spatter, crater, surface cracks, profile | Every weld, first and always |
| PT — Penetrant | Fine surface cracks, surface porosity | Non-magnetic metals (aluminum, stainless) |
| MT — Magnetic Particle | Fine surface and near-surface cracks | Carbon and low-alloy steel |
| UT — Ultrasonic | Lack of fusion, slag, internal cracks, incomplete penetration | Thick sections, in-service inspection |
| RT — Radiographic | Internal porosity, slag, lack of fusion, cracks | Pressure vessels, pipe, critical structural |
| Dimensional | Profile, reinforcement height, fillet size, distortion | Fit-up and final acceptance checks |
Every defect page in this library frames acceptance the same way: the limits live in the governing document — AWS D1.1 for structural steel, ISO 5817 for fusion welds, API 1104 for pipelines, plus project specifications. Read the code that applies to your job.
17 references, organized by severity. Each page covers identification, mechanism, prevention, correction, process-specific causes, parameters, equipment, inspection, and FAQs.
The family that propagates — hot cracks, crater cracks, and the failures they cause.
CriticalWeld metal that never bonded to the base metal or previous pass.
CriticalA root that never fused through the joint — a built-in stress riser.
CriticalThe delayed crack that appears hours after the weld has cooled.
CriticalThrough-thickness failure in rolled plate — a metallurgy and design problem.
Gas pockets in the weld — the most common rejection on the shop floor.
HighFlux trapped between passes — a pass-to-pass cleaning failure.
HighMelted-out holes and sagging drops on thin material.
HighTungsten from the TIG electrode trapped in the weld.
HighShrinkage voids and star cracks where the weld ends.
A groove melted into the base metal at the weld toe.
MediumWeld metal rolled over the base without fusing — a hidden cold joint.
MediumReinforcement too high, too low, or shaped wrong for the code.
MediumAccidental arcs outside the joint — small marks, hard spots, real consequences.
MediumShrinkage and heat pulling the work out of shape.
Every defect points back to a process, a machine, or a number. Follow the trail.
Each defect page names the processes that produce it — and links to the full process references.
Most defects are a parameter problem. The calculators turn a suspicion into a number.
The fundamentals that stop defects before they start.
The questions welders ask before they open a defect page.
A discontinuity is any interruption in the weld's expected structure. It becomes a defect only when the applicable code, specification, or service condition says it exceeds the acceptable limit. The same discontinuity can pass one code and fail another.
Porosity — usually from contamination, low gas flow, wrong stick-out, or damp consumables. It is also one of the easiest to eliminate with a systematic gas-and-cleanliness check.
Cracking — hot, hydrogen, and lamellar — because cracks grow under load. Hydrogen cracking can appear hours after welding, which is why preheat and low-hydrogen practice matter on high-strength steel.
Visual inspection (VT) for surface indications; penetrant (PT) and magnetic particle (MT) for fine surface cracks; ultrasonic (UT) and radiography (RT) for internal indications. The code decides which method applies to which weld.
Most can — excavate to sound metal and re-weld to a qualified procedure. Codes usually limit the number and size of repairs. Some defects are not repairable at all, which is why prevention is the real fix.