Welding
Weld defects: causes and remedies
In short
The diagnosis links the ticked symptoms (at the arc, in the pool, on the bead) to likely causes, ranked by the number of symptoms they explain. Each cause comes with its remedies, taken from TWI Job Knowledge; defects carry their ISO 6520-1 designation where one exists.
Calculation
Example calculated with the tool’s default values.
Inputs
- Symptoms
- Porosity, pinholes (ISO 6520 2011–2017)
Results
- Most likely cause
- Poor gas shielding (air entrained)explains 1 of 1 symptom(s)
Worked example
Every value in this section belongs to the example, calculated with the tool’s default values.
Other results of the example
- First remedy
- Find and seal leaks in the gas line.
- Possible causes
- 3
- Diagnostic aid: a cause is only confirmed by changing one parameter at a time and checking the next bead.
Step-by-step calculation
Ticked symptoms
porosity, pinholes
Ranking
causes sorted by number of symptoms explained
gaz-air (1), hydrogene (1), revetement (1)
1. Poor gas shielding (air entrained)
| Explains: porosity, pinholes |
|---|
| Find and seal leaks in the gas line. |
| Reduce excessive gas flow (turbulence); 1 % air in the gas is enough for porosity. |
| Shelter from draughts; avoid pool turbulence. |
| Use a filler with enough deoxidants. |
2. Moisture, grease or oil (hydrogen)
| Explains: porosity, pinholes |
|---|
| Bake electrodes and flux per the manufacturer. |
| Clean and degrease the workpiece and wire. |
3. Coating (primer, zinc) in the joint
| Explains: porosity, pinholes |
|---|
| Clean the joint edges just before welding; check the weldable primer maximum thickness. |
| Avoid joint geometry that forms a cavity (under the web of a T). |
How to use it
- Tick what you see at the arc: spatter, deflected arc, unstable arc.
- Tick what you see in the pool: not wetting the edges, too large, boiling, slag hard to remove.
- Tick what you see on the bead: porosity, cracks, undercut, lack of fusion, incomplete root penetration…
- Read the most likely cause.
Frequently asked questions
What causes porosity in welding?
According to TWI, three causes stand out: poor gas shielding (leaks, excessive flow, draughts; 1 % air in the gas is enough for porosity), moisture, grease or oil (hydrogen), and a coating (primer, zinc) in the joint. Remedies: seal the gas line, shelter, bake electrodes, degrease, clean the joint edges just before welding.
Why does my weld crack?
A crack along the bead centreline (ISO 6520 1011) is linked to solidification cracking: bead shape, restraint or gap; aim for a depth-to-width ratio between 0.5 and 2 and fill the crater. A crack beside the weld after cooling is linked to cold cracking: hydrogen, hard structure and stress; preheat, use baked low-hydrogen electrodes, clean. Most codes require removing the crack, about 5 mm beyond its ends, and rewelding.
What causes undercut in welding?
TWI links undercut (ISO 6520 5011) to an arc voltage that is too high with too little metal. Remedies: reduce travel speed and/or current, keep the correct arc length, angle the torch to push metal into the toe.
How do I reduce MIG spatter?
Causes quoted by TWI: voltage too low or current too high, wire speed, polarity, pure CO2 (more spatter than an argon/CO2 mix), a dirty earth, a worn contact tip, or a coating in the joint. Remedies: rebalance voltage and wire speed, check the wire polarity, clean the earth clamp, replace the contact tip.
What about incomplete penetration or lack of fusion?
Incomplete root penetration: root face too thick, gap too small or angle too small, current too low or travel too fast, or poor fit-up; keep the preparation and tack so the gap does not close. Lack of sidewall fusion: preparation too narrow, arc too long, wrong torch angle, or magnetic arc blow; widen the preparation, keep a short arc, move the return clamp.
Limits
Diagnostic aid, indicative results: a cause is only confirmed by changing one parameter at a time and checking the next bead. Causes and remedies are taken from the quoted public TWI Job Knowledge pages; the ranking only counts the symptoms that each source links to the cause.
Sources
- TWI Job Knowledge 42 : porosités : Causes et prévention (link)