Common Weld Defects: Types, Causes and Detection

Common Weld Defects: Types, Causes and Detection covers the discontinuities that form in fusion welds, why they occur, and how each is found before it causes a service failure. Every weld contains discontinuities. What matters is whether their type, size, and location exceed the acceptance limits of the governing code.

How weld imperfections are classified

ISO 6520-1 groups weld imperfections into six families: cracks (group 100), cavities including porosity (200), solid inclusions such as slag (300), lack of fusion and penetration (400), imperfect shape like undercut (500), and miscellaneous imperfections (600). This classification maps directly to how a defect behaves and how it is detected. Rounded volumetric flaws such as gas pores tolerate far more stress than sharp planar flaws such as cracks or lack of fusion.

Volumetric versus surface-breaking defects

The most useful split for choosing an inspection method is whether a defect breaks the surface or sits buried in the weld volume. Surface-breaking defects such as undercut, surface cracks, and surface porosity are found by visual, liquid penetrant, or magnetic particle methods. Volumetric defects such as internal porosity, slag, and incomplete penetration require radiographic or ultrasonic testing. Planar defects such as lack of fusion and cracks are the most dangerous and the hardest to find, because a tight crack poorly aligned with a radiographic beam can be missed entirely.

Porosity and slag inclusions

Porosity is gas trapped during solidification, usually from moisture, contamination, or lost shielding gas. It is volumetric and rounded, so it is comparatively benign, but clustered or linear porosity can still fail acceptance. Slag inclusions are non-metallic residues left when slag from one pass is not cleaned before the next bead is deposited. Both are found reliably with radiography, which shows them as dark spots or elongated shadows.

Lack of fusion and incomplete penetration

Lack of fusion occurs when the weld metal fails to coalesce with the base metal or a previous pass, typically from insufficient heat input, too fast a travel speed, or poor torch angle. Incomplete penetration is failure to fill the joint root, driven by tight root gaps, low current, or excessive root face. Both are planar and remove load-bearing area, so they are structurally serious. Ultrasonic testing detects them well because the sound beam reflects strongly off the flat unfused interface.

Undercut, spatter, and geometric defects

Undercut is a groove melted into the base metal at the weld toe that is not filled, caused by excessive current, too long an arc, or wrong electrode angle. It is a surface defect and a stress raiser, so it is limited tightly in fatigue-loaded joints. Spatter, the ejected droplets around the bead, is cosmetic but can mask real defects. Both are caught by systematic visual examination. A structured welding inspection program starts with visual acceptance before any volumetric method is applied.

Cracking: hot solidification and cold hydrogen-assisted

Cracks are the least tolerable defect because they are sharp, planar, and often propagate in service. Hot cracks form during solidification when low-melting films of sulphur or phosphorus segregate to grain boundaries under shrinkage restraint. Cold cracks, also called hydrogen-assisted or delayed cracking, need three factors together: diffusible hydrogen from moisture or contamination, a hard susceptible microstructure in the heat-affected zone, and tensile stress. They can appear hours after welding, so on higher-strength steels acceptance inspection is often delayed up to 48 hours. Control means dry consumables, correct preheat, and low-hydrogen processes.

Defect versus discontinuity and detection method

A discontinuity becomes a defect only when it exceeds code limits. ISO 5817 defines quality levels B, C, and D (B being the most stringent) for steel welds, and cracks, lack of fusion, and lack of penetration are not acceptable at any of these levels. Structural codes such as AWS D1.1 set their own acceptance criteria for production welds, while ASME BPVC Section IX governs qualification of welding procedures and welders rather than service acceptance limits. The same small pore may pass level D yet reject at level B. The table below links each defect to its usual cause and the method that finds it.

Surface cracks in ferromagnetic steel are found fast with magnetic particle testing, while buried volumetric flaws are documented with radiographic testing. Logging these inspection results, acceptance rulings, and repair welds against each asset is easier in one maintenance system. Book a Fabrico demo to see how weld inspection history can be logged alongside equipment records.

Frequently Asked Questions

What is the difference between a discontinuity and a defect?

A discontinuity is any interruption in the normal structure of a weld. It becomes a defect only when it exceeds the acceptance criteria of the governing code or specification.

Which weld defect is the most dangerous?

Cracks and lack of fusion are the most severe because they are sharp, planar, and reduce load-bearing area while acting as crack initiation sites. Cold hydrogen cracks are especially insidious because they can appear hours after the weld has cooled.

Why is ultrasonic testing preferred over radiography for lack of fusion?

Lack of fusion is a tight planar flaw. Radiography relies on a thickness change along the beam path, so a flat defect not aligned with the beam may be invisible. An ultrasonic beam reflects strongly off the flat unfused face, making it far more reliable for planar defects.

How can hydrogen cracking be prevented?

Control the three contributing factors: keep diffusible hydrogen low with dry, low-hydrogen consumables and clean joints; apply the correct preheat and interpass temperature to slow cooling and soften the heat-affected zone; and reduce residual stress through joint design and sequencing.