Every weld defect is a message about the process that made it. A crack that appears a day after welding, a string of pores along one joint or a repeated undercut on the same welder’s work each point to a specific cause, and an inspector who reads the pattern can often prevent the next hundred joints from failing in the same way.
Most lists of weld defects stop at the names. This article goes a step further and groups common weld defects and causes by what they reveal: material and hydrogen problems, process control problems, technique problems and preparation problems. It is written for buyers and quality managers who receive welded fabrication from Indonesian yards and want to ask better questions when repair rates start climbing.
- Imperfection or defect? A necessary distinction
- Cracks: hydrogen, stress and hardness
- Porosity: the signature of contamination
- Lack of fusion and incomplete penetration
- Undercut, slag and arc strikes
- Lamellar tearing and material problems
- Reading repair patterns
- Frequently asked questions
Imperfection or Defect? A Necessary Distinction
Welding standards distinguish between an imperfection and a defect. An imperfection is any departure from the ideal weld, while a defect is an imperfection that exceeds the acceptance limits of the applicable code or specification. A small pore may be an imperfection that is acceptable, whereas the same pore in a different service or a different code may be a rejectable defect.
This matters because the label is not decided by the flaw alone. It is decided by the acceptance criteria, which is why the contract must name them clearly. An inspector who calls something a defect needs a clause to point to, and a vendor who disputes it needs the same clause to argue against.
Types of weld defects are also grouped by shape. Planar flaws such as cracks and lack of fusion are generally the most dangerous because they concentrate stress, while rounded flaws such as isolated pores are usually more tolerable. Codes reflect this by being stricter on planar flaws, a point worth remembering when comparing results from different methods.
Cracks: Hydrogen, Stress and Hardness
Cracks are the most serious category. Cold cracking, also called hydrogen-assisted cracking, appears after the weld has cooled, often in the heat-affected zone, and it can show up hours after welding. Its main ingredients are dissolved hydrogen, residual tensile stress and a hard, brittle microstructure, and removing any one of them reduces the risk.
That is why preheat, controlled interpass temperature, low-hydrogen consumables and proper storage appear in procedures. Each one attacks a different ingredient. Preheat slows cooling and lets hydrogen escape, low-hydrogen consumables reduce the hydrogen supply, and stress control reduces the driving force.
Hot cracking, or solidification cracking, forms in the weld metal as it solidifies. It is linked to high restraint, poor joint design, impurities such as sulfur and phosphorus, and parameters that produce a narrow, deep bead. Crater cracks form when the arc is broken off too quickly, and various toe and root cracks point back to moisture, hydrogen or an unsuitable filler.
Porosity: The Signature of Contamination
Porosity consists of gas trapped in the weld metal as it freezes. Isolated pores, clusters and elongated wormholes all share the same family of causes: moisture, contamination on the joint or the filler, inadequate shielding and unsuitable settings. A wet electrode coating or a surface carrying oil, paint or rust can be enough.
Where the pores appear gives clues. Pores scattered through one joint on a windy day suggest lost shielding. Pores at the start of every weld suggest a technique problem at arc initiation. Pores that appear only after a rain shower suggest that joints were not dried or that consumables absorbed moisture.
In humid coastal conditions, discipline matters more. Consumables should be stored and issued according to the procedure, joints should be dry and clean, and screens should protect gas-shielded processes in the open. A yard that treats porosity as bad luck rather than as a message will see it again.
Lack of Fusion and Incomplete Penetration
Lack of fusion means the weld metal has not properly bonded with the parent metal or with the previous bead. Incomplete penetration means the weld has not reached the root of the joint. Both are planar flaws, and both can be hard to see from the surface because the weld looks well formed on the outside.
Typical causes include current that is too low, travel that is too fast, a wrong electrode angle, a poor manipulation technique, an inadequate joint preparation or a root gap that is too tight. Fit-up errors are a frequent source. A joint that is not prepared as the procedure expects leaves the welder fighting the geometry.
Because the flaw can hide below a sound-looking cap, these defects are classic targets for volumetric NDT and for hold points at root pass. They are also an argument for in-process surveillance, since a welder who is observed following the procedure is less likely to produce them than one who is checked only at the end.
Undercut, Slag and Arc Strikes
Undercut is a groove melted into the parent metal at the toe of the weld that is not filled by weld metal. It reduces the effective thickness and creates a stress raiser. Common causes are excessive current, travel that is too fast, an incorrect electrode angle, a damp electrode and an arc that is too long.
Slag and flux inclusions are non-metallic material trapped in the weld, typically between passes or at the sidewalls. Poor cleaning between passes, uneven bead surfaces and bad overlap are the usual causes. Inclusions from rust or mill scale on the parent metal are also possible when the joint was not cleaned properly.
Arc strikes are small localized melts where an electrode or return clamp touched the surface outside the joint. They look minor but can leave hard zones that crack, so many specifications forbid them and require their removal. Inspectors look for them on the base metal around the joint as well as on the weld itself.
Lamellar Tearing and Material Problems
Not every defect starts in the welding. Lamellar tearing occurs in rolled steel plate when shrinkage stresses act through the thickness of the plate and open up weak planes caused by non-metallic inclusions, especially sulfides. It appears near the weld but originates in the plate.
The remedies involve joint design and material selection rather than welder technique. Reducing restraint, changing the joint arrangement or specifying plate with controlled through-thickness properties can all help. The case shows why material verification and design review sit alongside welding control in a good quality plan.
Material issues also include wrong grade, unsuitable chemistry and contamination. A weld made to a flawless procedure on the wrong steel still has a problem. This is part of why material traceability and review of certificates come before welding on the inspector’s list, as covered by the general classification of welding defects in public references.
Reading Repair Patterns
The most useful habit an inspector can teach a project team is to analyze repairs by pattern. Take the repair list and sort it three ways: by defect type, by welder and by joint position or day. Each sorting answers a different question.
If a single defect type dominates, look for a process cause such as consumables or parameters. If one welder accounts for a large share of repairs, the cause may be skill or technique, and a retest or retraining may be justified. If defects cluster on certain days, the cause may be weather, drying or consumable handling.
Be careful with conclusions. A pattern is a lead, not proof. Check it against the records for each repaired joint, and treat repairs as information: a repair rate that rises over weeks signals a drifting process, and the cheapest time to correct it is before the next batch of joints is welded.
G.I Quality Services lists welding inspection, visual inspection and non-destructive testing among its services, and these are the activities that generate the data for this kind of analysis. Ask any provider you appoint to report repair statistics as well as individual results.
Frequently Asked Questions
What are the most common weld defects?
Common ones include cracks, porosity, lack of fusion, incomplete penetration, undercut, slag inclusions and arc strikes. Which ones dominate depends on the process, the material, the conditions and the discipline of the shop.
What causes porosity in welding?
Porosity is caused by gas trapped in the weld metal. Typical sources are moisture, contamination from oil, paint or rust, inadequate shielding gas coverage and incorrect settings. Wet consumables and wind are common triggers.
What is the difference between a weld imperfection and a weld defect?
An imperfection is any departure from the ideal weld. A defect is an imperfection that exceeds the acceptance limits of the applicable code or specification, so it requires repair or rejection.
Why do some weld cracks appear hours after welding?
This is typical of hydrogen-assisted cold cracking. It develops after cooling when hydrogen, tensile stress and a hard microstructure are present together, which is why some specifications require a waiting period before final NDT.
Which weld defects are the most dangerous?
Planar defects such as cracks and lack of fusion are generally the most serious because they concentrate stress and can grow in service. Codes are usually stricter on these than on rounded flaws.
How can repair rates be reduced?
Analyze repairs by defect type, welder and day, then correct the process cause, whether it is consumable handling, parameters, preparation or technique. Early in-process inspection is usually more effective than extra inspection at the end.