Overfill, Underfill & Undercut

Among the many things that can go wrong in welding, overfill is one of the most common defects that welders face in their work.

By the same token, underfill is just as problematic. Meanwhile, undercut can harm the base material to the point where it’s left in a weakened state and cracks can occur.

So, what’s the difference between these three types of frequently faced difficulties – and how can you prevent them from happening?

Key issues of overfill (excessive reinforcement)

Overfill, or ‘excessive reinforcement’, simply occurs when metal welded in place exceeds the amount that is really required for the job and the weld metal stands excessively proud of the parent material. It’s most frequently found in butt and fillet welds.

It’s often the result of poor welding parameters or operating technique – including poor manipulation of the welding torch, slow travel speed, excessively high amperage, or using the wrong consumable or shielding gas – and causes a range of problems:

Places the toes of the weld under stress
Makes subsequent filling runs in multipass welds harder

Higher likelihood and occurrence of slag entrapment
Leaves poor surface finish and appearance

How can overfill be avoided?

Because most of the mistakes behind overfill are avoidable, specific techniques can eliminate it from your welding process. That means optimising and controlling welding parameters and your technique:

Using the right current and smaller electrode diameter
Using higher welding speed
Better preparation including proper cleaning

Correct alignment of materials to be welded
Careful weld bead placement to ensure no overfill
Using a different weld torch angle

Overfill can be fixed by surface grinding smoothly at the base of the metal to remove overlays.

Key issues of underfill and undercut

When you don’t deposit enough weld metal in the joint, some of the parent material may remain unfused and, as a result, the joint is underfilled. In most cases, a depression appears in the surface of the parent material at the weld boundary.

In the main, this issue is brought about by depositing too little weld metal in the joint preparation, resulting in underfill. This may be due to use of too low a welding current or too high a travel speed.

In multi-pass welds, it may also be due to incorrect placement of weld beads, or laying weld beads too thinly.

As with undercut, although the loss of section is usually small it is a potential stress raiser.

Undercut is slightly different - most of the time, it runs for only a small distance - but still causes stress during fatigue loading. It always runs parallel to the weld metal, appearing as a narrow groove on the base metal adjacent to the weld metal along the edge.

Undercutting also leads to a decrease in productivity and a reduction in speed. It is common and costly, even among welding professionals. Causes include:

Maintaining an arc length that’s too long
Using the incorrect electrode angle

Selecting the wrong shielding gas
Welding too fast with excessively high current

How can underfill and undercut be avoided?

Underfill is mainly eliminated using:

Careful bead placement
Better operator technique

Undercut, meanwhile, is normally cut out by:

Reducing your arc length and current
Reducing the welding speed
Ensuring you have the correct electrode angle and size
Using an optimal weaving technique, allowing enough dwell time at edges

Undercut can be corrected by placing a stringer bead along the entire length of the defect. If necessary, you can blend it with a grinder to achieve the desired weld profile.