Common welding defects and how to fix them
A welding defect is an imperfection that compromises the strength, the seal or the look of a joint. Knowing each one by its appearance, and its cause, lets you fix it on the next bead instead of repeating the mistake.
The most common defects are porosity, undercut, lack of fusion, incomplete penetration, slag inclusion, spatter, cracks and distortion. Most come from three things: wrong settings, technique (angle, travel speed, arc length) and poorly prepared metal.
Quick bench diagnosis
| What you see | Likely defect | First fix |
|---|---|---|
| Dark line between passes after chipping slag | Slag inclusion | Clean each pass well; correct angle and travel speed |
| Bead "sitting" on top, edges not wetted | Lack of fusion | Raise the current; slow down; correct the angle |
| Root not fused on the back side | Incomplete penetration | Widen the root gap; raise the current; use a smaller electrode |
| Groove along the edge of the bead | Undercut | Lower current or voltage; pause at the edges; correct the angle |
| Droplets stuck all around | Spatter | Shorten the arc; lower the current; dry the electrode; check the polarity |
| Pinholes on the surface | Porosity | See Table 12.1 (causes of porosity) |
Table 12.2 of the e-book The Welder's Forge.
Porosity
Gas trapped in the metal as it freezes. Pores at the start of the bead are almost always dirt; surface pores in MIG mean a lack of gas. The full diagnosis is in weld porosity.
Undercut
A groove beside the bead that the weld metal did not fill. It looks cosmetic, but it concentrates stress and can start a fatigue crack. How to avoid it: lower the current or voltage, slow down a little, correct the angle and, if you weave, pause briefly at the edges.
Lack of fusion and incomplete penetration
These are among the most dangerous defects because they can hide under a nice-looking bead. The weld sticks to the surface but never fused with the plate, and lets go under the first load. How to avoid them: enough current, controlled travel speed, the right angle and, on thicker plate, a groove with a root gap. A shop rule from the e-book: a bead that sits on the surface without fusing is not a weld.
Slag inclusion
This happens with stick electrodes and flux-cored wire, when slag from one pass is trapped under the next. How to avoid it: chip and brush all the slag between passes, and keep the angle so the slag does not run ahead of the puddle. For stick technique, see how to stick weld.
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Spatter
Spatter does not weaken the joint by itself, but it means cleanup work and shows that something is off. Causes: long arc, high current, damp electrodes or wrong polarity. In MIG, pure CO₂ spatters more than 75/25, and so does voltage that is too high. Reference values are in the MIG welding settings chart.
Cracks
Cracks are the most serious defect because they grow under load and can lead to fracture. Cold cracks show up hours or days after welding and combine three factors: hydrogen (often from damp electrodes), harder steel and stress. How to avoid them: keep low-hydrogen electrodes dry, preheat when the steel calls for it, and avoid highly restrained joints.
On trailers, hitches, structures, scaffolding or anything that carries weight, do not accept a crack or lack of fusion: grind it out and weld it again. When in doubt, the part needs evaluation by someone qualified to judge it. Grinding also throws sparks: wear eye and face protection and clear flammables first.
Distortion (warping)
Metal expands when heated and contracts as it cools. If the heat concentrates along a line, that area shrinks more and pulls the part out of shape. How to avoid it: clamp the parts well, tack them, alternate sides and spread the heat. On thin sheet, use staggered stitch welds.
How a defect gets evaluated
In industry, ISO 5817 defines three quality levels (B, C and D), while AWS D1.1 and ASME Section IX give the acceptance criteria used for structures and pressure vessels. The first inspection is always visual. On demanding work, liquid penetrant, magnetic particle, ultrasonic or radiographic testing follow.
Frequently asked questions
What is the most dangerous welding defect?
Why is my weld ugly and covered in little balls?
How do I reduce spatter in MIG welding?
What is undercut in welding?
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