Common Weld Defects
Recognize porosity, undercut, lack of fusion, slag inclusions, overlap and cracks, and trace each one to its cause and fix.
Table of contents
A bad weld usually tells you what went wrong, if you know how to read it. The look and location of a flaw point to heat, speed, angle, cleanliness or shielding. Whether a given flaw is acceptable depends on the code, and an inspector decides that, but you should catch most of them yourself before anyone else looks.
What You Will Be Able to Do
- Name the common weld flaws by sight.
- Trace each flaw to a likely cause.
- Decide what to change on the next pass.
- Know when a flaw must be removed and rewelded.
- Check your own weld before you move on.
Discontinuity or Defect
Inspectors use two words carefully. A discontinuity is any interruption in the weld or base metal, such as a pore or some undercut. A defect is a discontinuity that goes beyond what the governing code allows. A small pore may pass on one job and fail on another. Cracks are the exception: nearly every code rejects them.
Porosity
What it looks like: Small round holes on or in the weld, either scattered, in a cluster or in a line. Long tube-shaped pores are called piping porosity or wormholes.
What is happening: Gas gets trapped as the weld freezes. Surface pores usually mean there are more inside.
| Cause | Fix |
|---|---|
| Oil, rust, paint, mill scale or moisture on the metal | Clean back from the joint |
| Lost shielding gas from a draft, leak or empty cylinder | Block the draft, check flow and hoses |
| Dirty or spatter-clogged nozzle | Clean the nozzle |
| Wet electrodes or flux | Store and dry consumables properly |
| Arc too long or too much stickout | Shorten the arc or stickout |
Undercut
What it looks like: A groove melted into the base metal along the toe of the weld, left unfilled.
Why it matters: Undercut makes the base metal thinner right at the toe. It also creates a stress riser, a sharp notch where stress concentrates and cracks can start under repeated loads.
| Cause | Fix |
|---|---|
| Too much amperage or voltage | Turn it down |
| Traveling too fast | Slow down so the puddle fills the toe |
| Wrong angle, with the arc aimed at one plate | Correct the work angle |
| Not pausing at the edge of a weave | Pause briefly at each toe |
Lack of Fusion
What it looks like: Weld metal that sits on the base metal or on the previous bead without melting into it. Often it is hidden inside the joint and only shows up on testing. It is also called incomplete fusion.
| Cause | Fix |
|---|---|
| Too little heat | Raise the current |
| Traveling too fast | Slow down |
| Arc not aimed at the sidewall or previous bead | Point the arc into the joint |
| Dirt, oxide or slag on the surface | Clean between passes |
| Letting the puddle roll ahead of the arc | Keep the arc on the leading edge of the puddle |
Incomplete Joint Penetration
What it looks like: The weld did not reach the required depth, so an unwelded gap remains at the root. On a groove weld that must go all the way through, you see the unmelted root faces from the back side.
Why it matters: The joint is smaller than the design calls for, and the gap acts like a crack. It is a rejectable defect on a complete joint penetration weld.
| Cause | Fix |
|---|---|
| Root face too thick or gap too tight | Correct the joint preparation |
| Too little heat | Raise the current |
| Traveling too fast | Slow down |
| Electrode too large for the groove | Use a smaller size |
Slag Inclusions
What it looks like: Slag trapped in the weld or between passes. This happens only with slag-making processes, such as stick, flux-cored and submerged arc. It can look like a dark line on the surface, or it can be buried and show up on X-ray or ultrasonic tests.
| Cause | Fix |
|---|---|
| Slag not removed between passes | Chip and brush every pass, fully |
| Slag trapped under a ropy or undercut bead | Grind the bead smooth first |
| Too little heat | Raise the current |
| Slag running ahead of the arc | Correct the angle and speed |
Slag does not burn out reliably. Remove it all with a chipping hammer and wire brush, and grind out any that will not come free, before you weld the next pass.
Overlap
What it looks like: Weld metal that has rolled over the toe and is lying on the base metal without fusing to it. It looks like a lip and is also called cold lap or rollover.
| Cause | Fix |
|---|---|
| Traveling too slowly, puddle too big | Speed up |
| Too little heat | Raise the current |
| Wrong angle | Correct the angle |
Overlap acts like a crack at the toe, and most codes reject it.
Cracks
What it looks like: A sharp line in the weld or the metal beside it. A crack may be very fine and easier to see with a flashlight held at a low angle.
| Type | Where | Usual cause |
|---|---|---|
| Crater crack | At the end of a bead, often star-shaped | Crater not filled before stopping |
| Longitudinal | Along the center of the weld | Weld too small or thin for the stress, poor fit, or high sulfur |
| Transverse | Across the weld | High stress, hydrogen |
| Toe crack | At the toe | Stress or hydrogen |
| Underbead | In the base metal under the weld, often hidden | Hydrogen in a hard heat-affected zone |
The heat-affected zone (HAZ) is the base metal next to the weld that did not melt but was changed by heat.
A crater crack forms after you stop welding, when an unfilled crater shrinks as it cools. To prevent it, pause or back up slightly at the end of a bead to fill the crater. Use a crater-fill setting if your machine has one.
Hydrogen cracks can appear hours or days after welding, so a weld that looked good when you finished may crack later. Low-hydrogen filler, dry consumables and correct preheat prevent them.
Do not weld over a crack. Grind or gouge it out completely, confirm it is gone, then reweld it following your repair procedure. Under load, a small crack can grow.
Other Flaws
| Flaw | What it is | Usual fix |
|---|---|---|
| Excess spatter | Metal droplets stuck around the weld | Adjust voltage, clean the metal, check gas |
| Burn-through | Hole melted through thin metal or a root | Less heat, more speed, tighter gap |
| Underfill | Weld face below the plate surface | Add another pass |
| Excess reinforcement | Crown too high | Check travel speed and the procedure limit |
| Arc strike | Mark where the arc touched outside the joint | Strike inside the joint |
| Tungsten inclusion | Bits of tungsten in a TIG weld | Keep the tungsten out of the puddle |
An arc strike looks minor, but it quenches the metal fast and can start a crack. Strike in the joint and not on the plate beside it.
Reading Bead Shape
Bead shape tells you a lot before you find a flaw.
| What you see | What it usually means |
|---|---|
| Tall, overbuilt bead piled on the joint | Traveling too slowly, or current too low |
| Thin, narrow bead with sharp ripples or undercut | Traveling too fast |
| Very wide, flat bead with spatter | Too much heat or too long an arc |
| Crown that sits on top with sharp toes | Too cold or arc too short |
| Wavy edges and uneven width | Unsteady speed or body position |
A good weld has consistent width and height, an even ripple pattern, smooth blending at both toes, filled craters and no visible flaws.
Checking Your Own Weld
Run this check on every pass. Grinding out a flaw before it is buried costs you minutes. Cutting out a finished weld can cost hours.
- Let it cool enough to touch with tools.
- Remove all slag and spatter.
- Look along the toes with a flashlight held low.
- Check for pores, cracks, undercut and overlap.
- Check that craters are filled.
- Measure the size with a weld gauge.
- Fix any flaw before the next pass.
Common Mistakes
- Welding over a flaw and hoping. The next pass rarely fixes it, and may bury it.
- Grinding a crack smooth. It looks better, but the crack may still be there.
- Treating every flaw as a machine problem. Check cleanliness and technique first.
- Changing several settings at once. You will not know what helped.
- Skipping the crater. A crater crack can form after you have walked away.
Check Your Understanding
A new bead has a row of pinholes across it, and yesterday's work was fine. There is a fan running nearby. What do you check first?
Check for a draft blowing away your shielding gas, then check flow, hoses, and the nozzle. Move the fan or block the draft before you reweld.
Your stainless repair has a thin groove along one toe. What is that, and why does it matter?
That is undercut. It thins the base metal and leaves a notch that can start cracks. Lower your heat, slow down, or correct your angle, and repair it if it is over the limit.
You finished a multi-pass weld and find a dark line along the joint on the cap. What probably happened, and what is the fix?
Slag was likely trapped from an earlier pass. Grind it out to clean metal, then reweld it. Clean every pass fully from now on.
A weld passed inspection the day it was made, but a crack showed up two days later. What could cause that?
Hydrogen cracking. Hydrogen from moisture or dirty metal can cause delayed cracks, especially in a hard heat-affected zone. Check that the rods were dry and preheat was correct.
Key Takeaways
- A discontinuity becomes a defect when it goes beyond what the code allows.
- Porosity comes from gas: dirt, moisture or lost shielding.
- Undercut, overlap and lack of fusion come from heat, speed and angle.
- Slag inclusions mean the last pass was not cleaned. Clean every pass.
- Cracks are rejected by almost every code. Remove them completely.
- Inspect each pass, because a flaw you find early is cheap to fix.