§3.1B · Under Visual examination
Undercut, overlap or underfill: the weld toe question
Undercut in welding eats into the plate right beside the weld, leaving a notch the weld never filled. Overlap is weld metal that spilled out past the edge of the weld and sits on the plate without bonding to it. Underfill is the third look-alike: a weld that ends up short of the plate surface.
- Exam
- CWI
- Clause
- §3.1B
- Tags here
- 7
- Parent
- Visual examination
3.1B.1Trouble at the edge of the weld
All three show up at or near the toe, where weld metal meets the plate. That's the spot cracks like to start from, so every visual gets a slow look along both toes.
Light helps. A flashlight held low, almost parallel to the plate, throws a shadow into undercut and under the lip of an overlap that light from straight above washes out. The same conditions can turn up on the root side of an open-root weld, so check it whenever you can reach it.
What causes each
Undercut is too much heat at the edge of the puddle: current or voltage up, travel quick, the electrode aimed at one member, or a weave that dwells in the middle and skips the toes. Overlap comes from the cold side: low current, a puddle that runs ahead of the arc, or more metal than the joint can take in one pass. Underfill is a shortage of metal: not enough passes, or not enough filler, to bring the face up level.
Technique and heat sit in the welding application and control clause. Measuring depth is a gauge job: see inspection gauges.
Loading changes the call
A notch concentrates stress. Cyclically loaded joints are less forgiving of it than statically loaded ones, so D1.1 judges toe conditions by loading type. A toe that passes on a building column can be a reject on a crane runway girder, where the load comes and goes all day.
3.1B.2Three conditions side by side
| Point | Undercut | Overlap | Underfill |
|---|---|---|---|
| Where | Base metal at the toe or root | Weld metal past the toe | Weld face or root surface |
| Metal | Missing from the plate | Extra, lying on the plate | Missing from the weld |
| Fusion | Fused, but the plate edge is gone | None under the rolled-over metal | Fused, just short |
| Measured with | Pit depth, bridge cam or V-WAC gauge | Eye and low light along the toe | Bridge cam or depth gauge on the face |
| Usual repair | Clean, weld up the groove, blend | Grind it off, then check the toe underneath | Add weld metal to bring the face up |
| D1.1 view | Depth limits by loading in Clause 8; edition-sensitive | Judged under the weld profile requirements | Judged against weld size and profile |
≠ marks a row where at least one of them differs.
3.1B.3What each looks like in section
3.1B.4At the toe
Naming, measuring and preventing the three conditions.
0 of 7 tagged · 0 accepted
Tag 01
Which type of weld discontinuity is characterized by the base metal being melted away at the weld toe?
Remarks on every option
- A Underfill is the weld face or root sitting below the plate surface: weld metal missing, not base metal melted away.
- B Overlap is weld metal rolled onto the plate past the toe without fusing. It's the reverse of undercut.
- C Reinforcement is weld metal above the surface, not a groove at the toe.
- D Correct: The arc melted the plate edge and the puddle never filled it back in. The missing metal is in the plate, which separates it from underfill.
Pick an option. The remarks on all 4 open here.
Tag 02
What should be verified when examining weld toe regions?
Remarks on every option
- A Correct: Cracks start from notches at the toe. Look for a smooth blend into the plate with no undercut, overlap or sharp angle.
- B Toes don't have a width to check. You're looking at how the weld blends into the plate.
- C Length gets checked, but not at the toes. The toes are about blend and notches.
- D Heat tint is color, not shape. What matters at the toe is a smooth blend.
Pick an option. The remarks on all 4 open here.
Tag 03
The term 'cold lap' refers to:
Remarks on every option
- A Cold lap isn't about preheat. It's a fusion problem at the toe.
- B That's cold (hydrogen) cracking: a crack, not unfused metal lying on the plate.
- C Nothing to do with lap joints or weather. 'Lap' here means metal lapped over the toe.
- D Correct: The metal looks tied in but only sits on the plate. It's 'cold' because the puddle edge wasn't hot enough to melt the surface it rolled onto.
Pick an option. The remarks on all 4 open here.
Tag 04
How should overlap be evaluated in a fillet weld?
Remarks on every option
- A Correct: Find how far the unfused weld metal rolls past the toe, then check it against the code's profile rules. Overlap leaves a notch at the toe.
- B Color tells you about heat, not whether metal at the toe fused.
- C Ripple count is a workmanship look. It doesn't show overlap.
- D Face width doesn't show overlap. What matters is unfused metal past the toe.
Pick an option. The remarks on all 4 open here.
Tag 05
What technique should be used to prevent cold lap when making a fillet weld?
Remarks on every option
- A Less current means less melting at the toes, so cold lap gets more likely.
- B Correct: The right angle and a steady speed keep the arc on the leading edge of the puddle, melting the toes instead of laying cold metal on them.
- C Speed alone isn't the fix. Too fast brings undercut and poor fusion; it's angle and steady speed together.
- D A wide weave tends to roll cooler metal onto the toes, which is exactly how cold lap forms.
Pick an option. The remarks on all 4 open here.
Tag 06
You lay a straightedge across a finished single-V groove weld in plate. Along the middle of the joint the weld face sits below the plate surface, yet both toes blend into the plate with no groove cut into it. What do you record?
Remarks on every option
- A Undercut is a groove melted into the base metal at the toe. Here the toes are intact and the low spot is in the weld itself.
- B Overlap is weld metal rolled over the toe onto the base metal without fusing. It sits proud of the plate, not below it.
- C Correct: Underfill is a weld face or root surface that ends below the adjacent base-metal surface, which is what the straightedge shows.
- D Incomplete joint penetration is a root condition: weld metal that doesn't reach through the joint thickness. A low face doesn't tell you that.
Pick an option. The remarks on all 4 open here.
Tag 07
Looking inside an open-root pipe weld with a mirror, you see the root bead flush with the inside wall, but a narrow groove has been melted into the pipe wall right beside it and left unfilled. What do you record?
Remarks on every option
- A Underfill would mean the root bead itself sits below the inside wall. Here the bead is flush; the missing metal is in the pipe wall.
- B Correct: Undercut isn't only a face-side condition. A groove melted into the base metal beside the root bead and left unfilled is undercut too.
- C Incomplete penetration leaves the joint root unfused or open. This root bead is fully through and flush with the wall.
- D Overlap is weld metal lying on the base metal without fusing. Here metal is missing, not piled up.
Pick an option. The remarks on all 4 open here.
Ten questions on a break
Our CWI prep app runs on iPhone and iPad. A few questions between jobs beats none at all.