Visual weld inspection basics

Visual inspection is the first and most widely used method of checking welds. Done properly, it is not just a glance at the finished bead: it covers the joint before welding, the work while it happens, and the completed weld against the acceptance criteria in the governing code. It catches a large share of problems at low cost, but it cannot see inside the weld, so it is often paired with other testing.

Why visual inspection matters

Many weld problems start before the arc is struck: a poorly fitted joint, the wrong filler metal, damp electrodes or missing preheat. Catching these early is far cheaper than cutting out and redoing a finished weld. Visual inspection also finds surface defects such as cracks, undercut, overlap, porosity and undersized welds, which are among the common welding defects that lead to rejection. On many projects, every weld gets a visual inspection and a portion also gets volumetric testing.

Before, during and after welding

Visual inspection works best as three linked checks rather than one final look. Each stage catches problems that would be expensive or impossible to find later, and the final check feeds a simple decision: accept, repair and reinspect, or send on for further testing.

Before welding

The inspector confirms that the work is set up to succeed:

• Drawings, specifications and the applicable code are available and current.
• A qualified welding procedure is in place for the joint, and welders are qualified for the process and position.
• Base metal matches the specification, with heat numbers or mill certificates where required.
• Filler metals and shielding gas are correct and consumables are stored as the manufacturer and procedure require.
• Joint preparation matches the procedure: bevel angle, root face, root opening and alignment.
• Joint surfaces are clean and free of paint, oil, rust, moisture and heavy mill scale.
• Tack welds are sound and of the correct size.
• Equipment works properly and preheat requirements are understood.

During welding

Inspection while work is in progress is where many process problems are caught:

• Preheat and interpass temperatures are measured and kept within the procedure range.
• Welding settings, such as current, voltage and travel speed, are within the procedure.
• Each pass is cleaned of slag before the next is laid.
• The root pass is checked, and back gouging is done where required.
• Shielding is protected from wind and drafts.
• The weld sequence follows the plan to control distortion.

After welding

The finished weld is checked against the acceptance criteria in the code or specification:

• No cracks of any kind, including at craters and weld ends.
• Fillet weld size and length meet the drawing, using fillet gauges.
• Groove welds are filled flush or with acceptable reinforcement.
• Undercut, overlap, porosity and surface profile are within the limits allowed.
• Arc strikes and spatter are removed if required.
• Dimensions and distortion are within tolerance.
• Welds are identified or marked as the project requires, and results are recorded.

Tools of the trade

Visual inspection relies on simple tools used carefully: good lighting, a flashlight held at a low angle to show surface shape, magnifiers, mirrors for hard-to-see spots, fillet and multipurpose weld gauges, a straightedge and tape, and temperature-indicating crayons or a contact thermometer for preheat. Viewing conditions matter. An inspector needs adequate light and to be close enough to the weld to see it clearly, so access by ladder or lift may need to be planned.

A worked example: checking a fillet weld

Fillet welds are the most common welds on structural steel, and checking them shows how visual inspection works in practice. Suppose a drawing calls for a fillet weld of a stated leg size along both sides of a stiffener plate.

1. Confirm the requirement. Read the weld symbol on the drawing: size, length, which side, and whether the weld is continuous or intermittent. Note any special finish or testing.
2. Clean and light the weld. Slag and heavy spatter must be removed first. Shine a light along the weld at a low angle so ripples, undercut and pores show up as shadows.
3. Look for cracks first. Check the full length, paying attention to the start and stop of each weld and the crater at the end. Any crack is normally rejectable regardless of size.
4. Measure the size. Use a fillet gauge on both legs at several points, including where the weld looks smallest. An undersized stretch matters even if the rest of the weld is oversized.
5. Check the profile. Look for excessive convexity, concavity, undercut at the toes and overlap. Measure undercut depth where it appears rather than estimating.
6. Check length and location. Confirm the weld runs the full specified length and wraps corners if required.
7. Record and mark. Note the result, any rejected areas and their measurements, and mark rejected areas on the steel so they are not missed.

Common mistakes to avoid

• Inspecting only after painting or galvanizing, when coatings hide cracks and undercut.
• Measuring one spot and assuming the whole weld is the same size.
• Judging welds by appearance alone. A smooth, even bead can still be undersized or lack fusion at the root.
• Skipping the "before" checks. Fit-up and joint preparation problems are much cheaper to fix than finished welds.
• Working in poor light or from too far away because access was not planned.
• Recording "acceptable" without stating which code and edition the weld was judged against.

Acceptance criteria

Whether a weld passes depends on the governing standard. In Canada, CSA W59 is widely used for welded steel construction, while pressure equipment, pipelines and other specialized work follow their own codes. Acceptance limits for undercut, porosity and weld profile can also differ depending on whether the structure is statically or cyclically loaded. The inspector should always work to the code and edition named in the contract. Check the current edition before relying on any specific limit.

Records and qualifications

Good inspection is documented: what was inspected, against which criteria, the result, any repairs, and the reinspection. These records support the fabricator's quality program and give owners evidence that work met the specification. Inspectors are often required to hold a recognized certification. In Canada, CSA W178.2 covers certification of welding inspectors; project specifications state what level or credential they require.

How ehZee can help

ehZee can act as an independent set of eyes on welding work, from checking procedures and joint preparation to inspecting finished welds and keeping clear records. Learn more about our welding inspections service, and how we coordinate further testing when visual inspection is not enough.

Common questions