Discontinuity
Underfill
A groove weld whose face or root sits below the surface of the adjoining base metal, so the joint has less cross-section than the surrounding material it is supposed to match.
TitleUnderfill
ClassShape / profile
ViewIndicative
What it looks like
- A shallow depression along the weld face where the cap does not reach plate level
- On the root side, a concave sunken bead sometimes called suck-back on pipe welds
- A straightedge laid across the joint shows daylight over the weld centre
- On film, a lighter band along the weld because there is less metal for the beam to pass through
Why it happens, and how it is stopped
Causes
- Insufficient filler metal deposited for the joint volume, often one pass short of a full cap
- Travel speed too high on the final pass, spreading too little metal over too much length
- Excessive root opening consuming more filler than the welder allowed for
- Excessive grinding of the weld face after welding, removing sound metal below plate level
- Surface tension pulling a hot root bead back into the joint before it solidifies
Prevention
- Deposit a cap pass sized to bring the weld face at least level with the base metal
- Check fit-up before welding so the actual joint volume matches what was planned
- Control root heat input and travel so the root bead stays convex rather than sinking
- Blend rather than remove metal when dressing a weld face, and check with a straightedge
How it is found
Which examination methods can see it, and what it is about the discontinuity that makes them able to.
- VTA straightedge, weld gauge, or simple visual comparison against the plate surface measures the shortfall directly.
- RTThe reduced thickness lets more radiation through, so underfill prints as a lighter band along the weld and reveals root suck-back on pipe.
- UTThickness measurement over the weld confirms how much section is actually present where the surface is hard to gauge.
How inspectors think about it
- The core question is whether the joint still has the cross-section the design assumed, which is why underfill is measured against base metal thickness rather than against the bead's own appearance.
- Root underfill usually draws more concern than face underfill, because the root is normally the highest-stressed part of a groove weld.
- Unlike most discontinuities, underfill is straightforward to correct by adding weld metal, so identifying it early saves far more than finding it after final acceptance.
General reasoning only. The accept/reject limits for your work come from the code or specification governing the job, which this site does not reproduce.
Related discontinuities
Excessive reinforcementA weld face built up far higher than the joint needs, producing an abrupt change of section at the toes that concentrates stress despite the extra metal looking like extra strength.Incomplete joint penetrationA groove weld that did not reach through the joint as the design required, leaving unfused root faces and an effective throat smaller than the drawing calls for.UndercutA groove melted into the base metal at the weld toe and left unfilled by weld metal, reducing the section of the parent material and leaving a sharp notch exactly where stress concentrates.Crater crackA small star-shaped or linear crack in the depression left where the arc was broken, formed because the last shallow pool of metal cooled fast and shrank into a cavity it could not fill.