Most welding defects come down to a handful of causes: the wrong procedure or settings, poor joint preparation, contamination, or technique. Some defects, like undercut or surface porosity, are visible on the finished weld. Others, like lack of fusion or internal cracks, are hidden and need non-destructive testing to find. Knowing what causes each one helps you prevent it, not just catch it.
Discontinuity or defect?
Inspectors draw a line between a discontinuity and a defect. A discontinuity is any interruption in the normal structure of the weld, such as a small pore. It becomes a defect only when it exceeds the acceptance criteria in the governing code or specification. A weld with a few tiny pores may be fully acceptable for one application and rejectable for another. That is why every inspection starts with knowing which standard applies, such as CSA W59 for welded steel construction in Canada, or the code named in the project specification.
Defects inside the weld
These are the defects that cause the most concern because they are often hidden and reduce the strength of the joint.
Cracks
Cracks are generally treated as the most serious defect, and most codes do not accept them at all. They concentrate stress and can grow under load or vibration.
- Hot cracks form as the weld solidifies. They are often linked to high sulphur or phosphorus in the base metal, a bead that is too deep and narrow, or high restraint.
- Cold cracks (hydrogen-induced cracks) can appear hours or even days after welding. They need hydrogen in the weld, a susceptible hard microstructure and stress. Damp electrodes or flux, missing preheat on thicker or higher-strength steel, and fast cooling all contribute. This delay is one reason some specifications require inspection to wait a set time after welding.
- Crater cracks form at the end of a weld when the welder stops abruptly and leaves a shrinking crater. Filling the crater before breaking the arc prevents them.
Lack of fusion and incomplete penetration
Lack of fusion means the weld metal did not properly melt into the base metal or into a previous pass. Incomplete penetration means the weld did not reach through the full depth of the joint where the design called for it. Both reduce the strength of the joint and are usually hidden inside the weld.
Common causes:
- Current or heat input too low for the material thickness.
- Travel speed too fast.
- Wrong electrode or torch angle.
- Joint preparation that is too tight: small root opening, small bevel angle or large root face.
- Mill scale, rust or slag left between passes.
Porosity
Porosity is gas trapped in the weld as it solidifies, leaving round or elongated holes. It can be scattered, clustered or in a line. Causes include:
- Contamination: oil, paint, moisture, rust or galvanizing on the joint.
- Damp electrodes or flux.
- Poor shielding gas coverage from drafts, low gas flow, too much gas flow, leaks or a dirty nozzle.
- An arc length that is too long.
Wind is a frequent cause of porosity on site, which is why gas-shielded welding outdoors usually needs screens or enclosures.
Slag inclusions
With flux-based processes such as stick welding, flux-cored and submerged arc welding, slag can be trapped between passes or at the sides of the joint. This usually happens when slag is not fully cleaned between passes, when the previous bead has a poor profile that traps slag, or when the welding angle and travel speed let slag run ahead of the puddle.
Defects you can see
Weld profile
Several defects relate to the shape of the weld and are found during visual weld inspection:
| Defect | What it looks like | Usual causes |
|---|---|---|
| Undercut | A groove melted into the base metal at the toe of the weld and left unfilled | Current too high, travel too fast, wrong angle |
| Overlap | Weld metal rolled over the base metal without fusing to it | Travel too slow, current too low, wrong angle |
| Excess reinforcement | Weld face built up much higher than needed | Too much filler, slow travel, poor bead placement |
| Underfill or concavity | Weld face below the surface of the base metal or undersized fillet | Not enough passes or filler, travel too fast |
| Spatter and arc strikes | Metal droplets and small burn marks outside the joint | Settings, technique, careless striking of the arc |
Arc strikes look minor, but on some steels they can create small hard spots that crack. Many specifications require them to be ground out and checked.
Distortion
Welding heats and cools metal unevenly, which causes shrinkage and pulls parts out of shape. Distortion is not a defect in the weld itself, but it can put an assembly out of tolerance. It is controlled through weld sequence, balanced welding on both sides, minimum necessary weld size, fixturing and presetting parts.
Preventing defects
Most defects are prevented by the basics: a qualified welding procedure that suits the job, qualified welders following it, clean and properly prepared joints, dry consumables stored correctly, the right preheat, and protection from wind and weather. Our guide to welder and welding procedure qualification explains how these are set up. Hidden defects are found with methods covered in non-destructive testing methods.
How ehZee can help
ehZee provides independent welding inspection, from reviewing procedures and joint preparation to inspecting finished welds and coordinating testing where it is needed. See our welding inspections service for how we work with fabricators, contractors and owners.
Common questions
Can a cracked weld be repaired?
Often yes. The crack is usually removed by grinding or gouging, the excavation is checked to confirm the crack is gone, and the area is rewelded using an appropriate procedure, then reinspected. The cause should be found first so the repair does not crack again.
Is some porosity acceptable?
Depending on the code and type of joint, a limited amount of small porosity may be accepted. The governing standard sets the limits on size, spacing and total amount.
Why do some cracks appear after the weld passed inspection?
Hydrogen-induced cracking can be delayed by hours or days. For higher-risk work, specifications may require a waiting period before final inspection for this reason.