Every rejected weld has a cause, and most causes repeat. A welder who leaves slag between passes will do it again tomorrow. A wet electrode stored overnight in the open will produce porosity in the morning. Knowing the common welding defects detected by inspection, what produces them and which methods find them, lets inspectors and supervisors fix the cause rather than repairing the same defect week after week. The sections below are written for welding inspectors, QC engineers, welding supervisors and project managers who want fewer repairs.
What is a welding defect?
A welding defect is an imperfection in a weld that exceeds the acceptance criteria of the applicable code or specification. Not every imperfection is a defect: codes allow small amounts of some features, such as minor porosity or slight undercut. An imperfection becomes a defect, and must be repaired, when it falls outside those limits.
The most common weld defects
| Defect | What it is | Typical causes | How it is usually found |
|---|---|---|---|
| Porosity | Gas pockets trapped in the weld | Damp consumables, contaminated joints, poor gas shielding, wind | Visual for surface pores, RT for internal |
| Slag inclusions | Non-metallic material trapped between passes | Poor cleaning between passes, wrong technique | RT, UT |
| Lack of fusion | Weld metal not fused to the base metal or previous pass | Low heat input, wrong angle, travel too fast | UT is particularly effective, RT may miss it |
| Incomplete penetration | Root not fully penetrated | Root gap too small, low current, poor fit-up | RT, visual from inside where accessible |
| Cracks | Fractures in the weld or heat-affected zone | Hydrogen, high restraint, wrong preheat, unsuitable filler | MT, PT for surface; UT, RT for internal |
| Undercut | Groove melted into the base metal at the weld toe | Excessive current, wrong angle, high travel speed | Visual, measured with gauges |
| Overlap | Weld metal rolling over the surface without fusing | Low travel speed, wrong angle | Visual |
| Excess reinforcement or root protrusion | Too much weld metal above or inside the joint | Poor technique, wrong gap | Visual, dimensional checks |
| Burn-through | Hole melted through the root | Excessive heat, large gap, thin material | Visual, RT |
| Misalignment | Pipe or plate edges offset at the joint | Poor fit-up | Visual, gauges before welding |
| Arc strikes | Accidental arcing on the base metal surface | Careless electrode handling | Visual, sometimes MT |
Cracks deserve special attention
Cracks are the most serious defect because they can grow in service. Hydrogen-induced cracking can appear hours after welding, which is why some specifications require a delay before final NDT on susceptible materials. Hot cracking during solidification relates more to joint design, filler choice and the chemistry of the material. Cracks are normally rejected outright, regardless of size.
Which inspection methods find which defects
Visual inspection finds surface defects such as undercut, overlap, surface porosity and profile problems, and it should happen on every weld. Radiography is strong on volumetric defects like porosity and slag. Ultrasonic testing is stronger on planar defects such as lack of fusion and cracks. Magnetic particle and penetrant testing find surface-breaking cracks. Choosing the right method for the defects that matter most on a given joint is covered in when NDT is used in fabrication and pipeline work.
Preventing defects instead of repairing them
Most defects trace back to a short list of controllable causes:
- Consumable control. Low-hydrogen electrodes kept in heated ovens and holding containers, filler wires kept clean and dry.
- Joint preparation and fit-up. Correct bevel, root gap and alignment before the first pass, as described in pipe fit-up quality checks before welding.
- Welding parameters. Current, voltage and travel speed within the procedure range.
- Cleaning between passes to remove slag and oxides.
- Shielding protection from wind on site, with screens or shelters.
- Preheat and interpass control where the procedure requires them.
These controls belong in the in-process checks described in welding inspection from fit-up to final acceptance.
Use defect data to fix causes
Track defects by type, welder, procedure and location every week. If one welder produces lack of fusion repeatedly, that is a training issue. If porosity rises across a whole crew after a windy day, that is a shelter issue. If cracks appear on one material, check preheat and consumables. Data turns repairs into corrections. It also improves selection, because the defects a welder produces in a test coupon are the defects they will produce on site, which is why assessing welders before industrial deployment matters.
Welding quality support from Apex Core Engineering
Apex Core Engineering supplies certified welding inspectors, NDT personnel and qualified welders for industrial projects. Welders are trade-tested before deployment, and the company’s training arm, Mansol Technical Training Institute (MTTI), runs welding technology programs aimed at fabrication, pipeline and heavy engineering work. The company’s quality and safety systems are listed on its certifications page.
Frequently asked questions
What is the most common welding defect?
Porosity and slag inclusions are among the most frequently found, but the most common defect on a given project depends on the processes, materials and site conditions.
Is every weld imperfection a defect?
No. Codes allow some imperfections within limits. Only imperfections outside the acceptance criteria are defects that must be repaired.
Why is lack of fusion hard to find?
Because it is often a thin, planar flaw that radiography can miss depending on its orientation. Ultrasonic testing is usually better at detecting it.
How are weld defects repaired?
By removing the defect, confirming removal, re-welding to an approved repair procedure and re-inspecting with the method that found the original defect.
Talk to Apex Core Engineering about welding inspection and qualified welders.