In recent posts, we’ve been talking about weld quality—why it matters and how small mistakes in fabrication can create big problems down the line. Today, we’re going to take a close look at a specific type of weld discontinuity that is extremely common, often goes unnoticed until after painting or final inspection, and can become very costly to repair: overlap.

The following information is taken directly from our guide on Weld Troubleshooting for Non Welding Engineers.  

What Is Overlap?

The American Welding Society (AWS) defines overlap as “the protrusion of weld metal beyond the weld toe or root.” In simple terms, overlap occurs when molten weld metal rolls over onto the base metal without fusing properly.

This discontinuity can happen in fillet welds as well as groove welds. It is essentially a form of incomplete fusion—but unlike undercut or porosity, there is no allowance for overlap under AWS D1.1 Structural Welding Code. Complete fusion between weld metal and base metal is required.

Why is this so critical? Overlap creates a mechanical notch. These notches act as crack initiation sites, making overlap dangerous not only in cyclically loaded structures (like bridges or cranes) but also in static structures. Left unaddressed, overlap can significantly reduce the performance and fatigue life of a welded component.

Why Overlap Happens

Overlap is most often caused by one of the following:

1. Improper welding procedures
2. Presence of mill scale or surface contaminants
3. Inadequate welder technique

Let’s break down each of these causes and the solutions that help eliminate them.

1. Improper Welding Procedures

Welding procedures that do not provide sufficient control of essential parameters can lead directly to overlap. If deposition rates are too high or travel speeds too slow, the weld puddle will roll ahead of the arc and fail to fuse at the toes.

Solutions:

• Reduce amperage or deposition rate – Lower the amperage in SMAW or reduce wire feed speed in GMAW, FCAW, MCAW, or SAW. This decreases the amount of molten metal and helps the arc stay ahead of the puddle.
• Increase travel speed – A faster travel speed prevents the puddle from running ahead of the arc. This is especially critical in out-of-position welidng as in the case of vertical or horizontal positions where gravity works against the welder.
• Increase heat input – In some cases, overlap results not from excess deposition rates, but from insufficient heat at the weld toe. Increasing voltage (or both voltage and amperage) can help provide the energy needed for proper fusion.

2. Presence of Mill Scale and Surface Contaminants

Ideally, all welding should be done on clean, bare metal. In reality, fabricators often deal with mill scale from hot-rolled steel. Mill scale acts as a barrier between the molten puddle and the base metal, contributing to incomplete fusion and overlap.

Solutions:

• Remove mill scale – Wire brushing or grinding prior to welding can eliminate this cause entirely. Even with adequate parameters, mill scale can block fusion.
• Change shielding gas – Shielding gases with higher carbon dioxide content (e.g., moving from 90/10 Ar/CO2 to 85/15 or 80/20) improve arc action on mill scale. But keep in mind: too much CO2 increases spatter and limits transfer modes.
• Change filler metal or process – If removing mill scale is not possible, using filler metals with higher deoxidizers (manganese, silicon) or switching to a process like SMAW or self-shielded FCAW may help, since these processes naturally tolerate more surface contaminants.

3. Inadequate Welder Technique

Even with the right procedure and clean base metal, poor technique can lead to overlap. Factors such as travel angle, weaving, and arc manipulation all play a role.

Solutions:

• Increase travel speed – Keeping the arc on the leading edge of the puddle ensures proper sidewall and toe fusion.
• Minimize electrode manipulation – Excessive weaving slows travel speed and creates larger puddles prone to overlap. If manipulation is necessary, check whether welding parameters themselves need adjustment.
• Follow the Welding Procedure Specification (WPS) – Deviating from established WPSs introduces inconsistencies. Welding procedures should also include base metal preparation instructions to prevent defects.

Why Simply Covering Up Overlap Doesn’t Work

One of the most dangerous mistakes in fabrication shops is attempting to “fix” overlap by welding over the discontinuity. This does not remove the defect—it conceals it. The only proper way to correct overlap is to grind away the excess weld metal at the toe. If grinding reduces the weld below the required size, additional weld metal must be deposited correctly.

Don’t Ignore Overlap

Overlap may seem minor compared to undercut, but it creates a dangerous stress riser that can lead to premature weld failure. The good news is that overlap is both easy to detect (visual inspection) and easy to prevent—provided procedures are sound, surfaces are prepared, and welders use correct technique.

Next Steps to Prevent Overlap and Other Weld Discontinuities

If you found this article useful, you’ll find much more detailed troubleshooting guidance for all other weld discontinuities  in our resource:

Weld Troubleshooting for Non-Welding Engineers

And don’t forget to grab your free Welding Quality Checklist to help your team catch costly issues before they reach your customers:

Download the Welding Quality Checklist

Need to create your Welding Quality Standards?

The Welding Quality Standard Template helps fabricators quickly create a professional welding quality standard that meets customer documentation requirements while also providing the framework to reduce rework, improve consistency, and boost productivity in welding operations.