Common welding defects and what causes them

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:

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.

Tracing a defect back to its cause

Finding a defect is only half the job. If the cause is not fixed, the repair weld and the next hundred welds will have the same problem. A typical case shows how an inspector works through it.

A fabricator building steel frames on an open yard finds clusters of porosity on fillet welds made with a gas-shielded process. Welds made inside the shop with the same wire and the same welders are clean. The inspector works through the likely causes in order:

1. What is different about the bad welds? Location: outdoors. Time: afternoons, when the wind usually picks up. That points at shielding gas coverage.
2. Check the equipment. Gas flow at the nozzle, hose and fitting leaks, spatter build-up in the nozzle and the condition of the diffuser. Record readings, not just "OK".
3. Check the material and consumables. Is the steel primed, oily or wet from overnight dew? Is the wire rusty from being left on the feeder outside?
4. Check technique. Contact tip distance, torch angle and travel speed against the welding procedure.
5. Change one thing and confirm. Here, wind screens are put up and the joints are wiped dry before welding. Porosity stops. The repair procedure and the corrective action are both recorded.

The same thinking applies to cracks, lack of fusion and slag. Look for the pattern first: one welder, one joint type, one position, one batch of consumables, or one time of day. A defect that follows a welder suggests technique or training. A defect that follows a joint suggests preparation or procedure. A defect that follows the weather suggests shielding, moisture or preheat.

What to record when a defect is found

• The weld identification, location on the drawing and welder identification.
• The type of defect, its size and position, measured rather than estimated.
• The acceptance criterion it was judged against, including the code and clause.
• The repair method, the procedure used and who did the repair.
• The result of reinspection or retesting after repair.
• The suspected cause and the corrective action taken to stop it recurring.

Over a project, these records become a useful picture of where quality problems come from. A rising repair rate on one crew or one joint type is an early warning worth acting on.

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