Weld Discontinuities, Causes, and Corrections

What Is a Weld Discontinuity?

A discontinuity is any interruption in the physical structure of a weld. Not every discontinuity is automatically a defect — a defect is a discontinuity that exceeds the acceptance limits set by the governing code. In structural welding, AWS D1.1 defines those limits. Understanding the difference matters: you must be able to identify what you see, know what caused it, and fix it before the weld is rejected or, worse, fails in service.

FCAW is widely used in building and construction because it is fast and adaptable to field conditions. However, that flexibility comes with a specific set of discontinuities that you must learn to recognize and correct.

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Eight Common FCAW Discontinuities

1. Porosity

Porosity appears as rounded voids — either on the surface (visible) or inside the weld (subsurface, found by testing). In FCAW, the most common cause is inadequate shielding gas coverage: a low flow rate, a damaged nozzle, a drafty work environment, or a gas hose leak. Contaminated base metal (mill scale, moisture, paint, grease) also produces gas that is trapped as the weld solidifies. Correction: Verify flow rate at the nozzle (not just at the regulator), clean and degrease the base metal, shield the work area from wind, and inspect the nozzle for spatter buildup that disrupts the gas stream.

2. Lack of Fusion (LOF)

Lack of fusion means the weld metal and the base metal — or adjacent weld passes — did not fuse together. It feels like a cold, layered seam. Causes include too-low heat input (low voltage or fast travel speed), improper gun angle, or inadequate joint preparation. Correction: Increase voltage slightly, slow travel speed, adjust gun angle to direct heat into the joint faces, and ensure joint fit-up is within tolerance.

3. Undercut

Undercut is a groove melted into the base metal along the weld toe that is not filled by weld metal. It reduces the effective throat and creates a stress riser. In FCAW the most frequent cause is excessive voltage or arc length combined with fast travel. Correction: Reduce voltage (typically 1–2 V at a time), slow travel speed slightly, and maintain proper gun angle so the arc does not wash metal away from the toe.

4. Overlap (Cold Lap)

Overlap — sometimes called cold lap — occurs when weld metal rolls over the base metal surface without fusing to it. The bead looks wide and convex with a distinct, unfused toe line. The usual cause is too-slow travel speed or too-low heat input, allowing liquid metal to pile up ahead of the arc. Correction: Increase travel speed and confirm voltage and wire feed speed are matched to the joint and position.

5. Excessive Spatter

Spatter consists of metal droplets ejected from the arc that land on the base metal or weld surface. While spatter itself is not always a structural defect, it can mask undercut, trap slag, and indicate that arc parameters are off. Causes include incorrect voltage-to-wire-feed-speed ratio, dirty base metal, or a partially blocked nozzle. Correction: Fine-tune the voltage/wire feed balance, clean the nozzle, apply an approved anti-spatter compound to the base metal, and verify gas flow.

6. Burn-Through

Burn-through is a hole or complete melt-through of the base metal, most often on thin material or at the root of a joint. FCAW causes include excessive heat input — too-high wire feed speed (amperage), too-slow travel, or pausing over one spot. Correction: Increase travel speed, reduce wire feed speed, use a weave technique rather than dwelling, and verify joint fit-up (gaps that are too wide accelerate burn-through).

7. Worm Tracks

Worm tracks (also called piping porosity or elongated porosity) appear as elongated voids or surface channels running parallel to the weld axis. They look like worm trails in the weld face. In FCAW, the primary cause is moisture in the flux core or significant shielding-gas disruption. Correction: Store wire indoors and keep it on the spool (not on a wet floor); replace suspect wire; verify gas flow; protect the weld area from drafts.

8. Incomplete Joint Penetration (IJP)

IJP occurs when weld metal does not extend through the full thickness required by the joint design. Causes include too-fast travel, too-low heat input, or a joint geometry that prevents the arc from reaching the root. Correction: Adjust wire feed speed and voltage upward, slow travel, and confirm joint preparation dimensions before welding.

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Cause-and-Correction Logic

When you see a discontinuity, ask three questions: What does it look like? Where is it? What changed just before it appeared? A systematic approach prevents chasing the wrong variable. Change one parameter at a time, run a test bead on scrap, and evaluate before changing another variable.

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Safety During Diagnostic Welding

When running intentional error passes to study discontinuities, all normal FCAW PPE applies: auto-darkening helmet (minimum shade 10 for FCAW), flame-resistant clothing, leather gloves, and steel-toed boots. Maintain ventilation — FCAW flux produces fume even on short test beads. Before handling hot coupons, use pliers or tongs; allow samples to cool or quench in water per your shop's procedure before placing them on plastic-topped tables. If a wire feed malfunction creates an uncontrolled arc, release the trigger immediately and perform lockout/tagout on the machine before inspecting the wire drive or nozzle assembly.

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Code Acceptance

AWS D1.1 sets visual acceptance criteria for discontinuities such as undercut depth, porosity frequency, and overlap. Any weld submitted for inspection under D1.1 must meet those limits — your corrective action is not complete until a visual check (and any required nondestructive testing) confirms the repaired weld is within code tolerance.