Visual inspection (VT) catches more weld defects than any other method. It’s the first inspection performed on every weld, and a trained eye with the right gauges identifies about 75% of all defects without any special equipment. Every welder should inspect their own work before an inspector sees it, and knowing what to look for prevents costly rework.
VT is required on 100% of all production welds under AWS D1.1, ASME Section IX, and every other major welding code. It happens before, during, and after welding. A good inspector checks fit-up before the first pass, monitors interpass quality, and examines the finished weld against the acceptance criteria.
Before Welding: Pre-Weld Inspection
Inspection starts before the arc. Pre-weld VT verifies:
- Joint preparation: Groove angle, root face, root opening match the WPS and fit-up tolerance guide requirements
- Alignment: Hi-lo mismatch within limits
- Tack welds: Properly placed, sized, and free of cracks
- Cleanliness: No oil, rust, scale, paint, or moisture within 1 inch of the joint
- Preheat: Correct temperature verified with a pyrometer or temp stick
- WPS available: The correct WPS is at the workstation, and the welder has read it
Catching problems at this stage prevents defects. A root opening that’s 1/4 inch where the WPS calls for 1/8 inch will cause problems in the root pass. Fix it now, not after the weld is complete.
During Welding: In-Process Inspection
On multi-pass welds, in-process inspection catches defects before they get buried under fill passes:
- Root pass: Check root penetration (on pipe, look at the inside if accessible)
- Interpass cleaning: All slag removed before the next pass
- Interpass temperature: Within the WPS maximum
- Bead profile: Each pass flat to slightly convex, proper width
- Pass sequence: Matches the WPS (root, hot pass, fill, cap in order)
- Cracks: Check each pass for cracks, especially in the crater area
After Welding: Final VT Inspection
Final VT is the most detailed examination. The inspector evaluates every aspect of the finished weld against the code acceptance criteria.
What to Check
Cracks: The most serious defect. Check the weld face, toes, crater areas, and HAZ. Use raking light (flashlight at a low angle to the weld surface) to catch tight cracks that aren’t visible under direct lighting. Any crack, regardless of size or location, is a rejection under all codes.
Porosity: Surface porosity shows as small round holes in the weld face. Check the entire weld length. Scattered porosity, clustered porosity, and linear porosity each have different acceptance limits.
Undercut: A groove melted into the base metal at the weld toe. Measure depth with a bridge cam gauge or undercut gauge. Check both toes of the weld.
Incomplete fusion: The weld bead sits on the base metal without bonding. Look for a visible line between the weld and the base metal at the toes. Incomplete fusion at the root is harder to detect with VT alone.
Reinforcement height: Measure with a straightedge and ruler, or a bridge cam gauge. Both excessive reinforcement (too tall) and underfill (too thin) are rejectable.
Weld size: Use fillet gauges on fillet welds. Check leg size on both legs and verify the throat dimension. On groove welds, check cap width and reinforcement.
Profile: Convexity, concavity, and overlap. AWS D1.1 specifies acceptable profiles for both fillet and groove welds.
Arc strikes: Stray arc marks on base metal outside the weld zone. These create hard spots and potential crack initiation points. AWS D1.1 requires removal by grinding.
Spatter: While spatter itself isn’t a structural defect, excessive spatter indicates unstable arc conditions and can hide surface defects underneath.
AWS D1.1 Acceptance Criteria
| Defect | Statically Loaded (Table 6.1) | Cyclically Loaded (Table 6.1) |
|---|---|---|
| Cracks | Not permitted | Not permitted |
| Fusion | Complete fusion required | Complete fusion required |
| Crater cross-section | Must be filled to full weld size | Must be filled to full weld size |
| Undercut depth (non-tubular) | 1/32" max | 0.01" max (transverse to stress) |
| Porosity (visible) | Sum diameter per inch limited | Tighter limits per inch |
| Groove weld reinforcement (butt) | 1/8" max | 1/8" max, flush if specified |
| Fillet weld convexity (5/16" and under) | 1/16" max | 1/16" max |
| Fillet weld convexity (over 5/16") | 1/8" max | 1/8" max |
| Fillet weld leg size | Must meet drawing minimum | Must meet drawing minimum |
| Underfill | Not permitted on CJP groove welds | Not permitted |
VT Inspection Tools
Fillet Weld Gauges
A set of fillet gauges measures leg size and checks convexity/concavity. Place the gauge against the weld face with both arms touching the base metal. Daylight between the gauge and the weld face indicates an incorrect profile.
Bridge Cam Gauge
The most versatile single gauge. It measures:
- Fillet weld leg size
- Groove weld reinforcement height
- Undercut depth
- Root opening
- Misalignment (hi-lo)
Hi-Lo Gauge
Specifically designed for pipe root alignment. Hooks over the pipe wall and measures the offset between inside surfaces at the joint.
Flashlight and Mirror
A flashlight at a raking angle (10-15 degrees from the weld surface) casts shadows that reveal cracks, undercut, and surface irregularities invisible under direct lighting. A dental-style mirror lets you inspect the back side of welds and areas you can’t see directly.
Taper Gauge
A thin tapered strip that slides into the root opening to measure gap width. Graduated marks on the strip show the dimension.
Inspection Technique
Lighting: Good lighting is non-negotiable. AWS D1.1 requires a minimum of 15 foot-candles at the weld. Many inspectors carry a headlamp or work light in addition to a flashlight.
Distance: VT is performed at a maximum distance of 24 inches from the weld, per AWS D1.1. Closer is better. Use a magnifying lens (2x-5x) for suspected cracks.
Systematic approach: Inspect the full length of every weld. Don’t spot-check. Start at one end, move your eyes and light along the entire bead, checking toes, face, starts, stops, and craters.
Documentation: Record findings on an inspection report. Note the weld joint identification, WPS number, inspector name, date, and accept/reject status. For rejected welds, describe the defect, its location, and the repair required.
Weld Profile Requirements
Understanding acceptable weld profiles is essential for VT. AWS D1.1 Figure 5.4 shows acceptable and unacceptable profiles for both fillet and groove welds.
Fillet Weld Profiles
- Acceptable: Flat face with smooth toe transition, slightly convex, or slightly concave (as long as throat meets minimum)
- Unacceptable: Excessive convexity (exceeds limits), excessive concavity (throat below minimum), overlap (weld metal rolls over the toe without fusing), insufficient leg size
Groove Weld Profiles
- Acceptable: Slightly convex cap within reinforcement limits, smooth toe transition on both sides, uniform width
- Unacceptable: Excessive reinforcement (over 1/8 inch), undercut at either toe, overlap, underfill
Weld Profile by Position
Welds made in vertical and overhead positions tend to have different profiles than flat welds. Vertical up welds are often slightly convex with heavier buildup at the toes (from the pause technique). Overhead welds tend to be flatter or slightly concave. The inspector should account for position when evaluating profile but still enforce the acceptance criteria.
Inspector Qualifications
AWS D1.1 requires that weld inspectors be qualified. The most common credential is the Certified Welding Inspector (CWI) certification from AWS. CWI requirements include:
- Minimum experience (varies by education level): 2-15 years
- Passing a three-part exam: fundamentals, practical, and code application
- Vision test: 20/40 near vision (corrected), Jaeger J2 reading ability
- Recertification every three years
Not every person performing VT on a job needs a CWI. AWS D1.1 allows inspection by a “qualified person designated by the contractor.” But the responsible inspector (the one signing off on acceptance) must meet the code’s qualification requirements.
Common Mistakes
Inspecting under poor lighting. A 100-watt shop light 10 feet overhead doesn’t meet the standard. Use direct lighting at the weld surface.
Not using gauges. Eyeballing a fillet weld size is unreliable. Use the gauge every time. A 1/4 inch fillet looks surprisingly similar to a 5/16 inch fillet without a gauge in your hand.
Inspecting too fast. Rushing through VT misses tight cracks and shallow undercut. Take time to examine each area systematically.
Not checking the back side. On accessible groove welds, the root side reveals lack of penetration, melt-through, and root reinforcement problems. Use a mirror if direct viewing isn’t possible.
Confusing acceptable imperfections with defects. Not every mark on a weld is a defect. Ripple lines, slight discoloration, and minor variations in bead width are normal. Focus on the criteria that matter: cracks, porosity, undercut, and fusion.
Skipping arc strike inspection. Stray arc marks on the base metal (from accidentally touching the electrode to the plate outside the joint) create hard spots in the HAZ. AWS D1.1 requires these to be ground smooth and inspected for cracks. They’re easy to miss because they’re small and away from the weld.
Not verifying weld location against the drawing. A perfect weld in the wrong location is still wrong. Verify that every weld matches the drawing for joint identification, weld type, size, and position. Missing welds are also VT rejectable.
For more on specific defects and their causes, see the weld defects category. For additional NDT methods, read the dye penetrant testing, magnetic particle testing, and ultrasonic testing guides. Return to weld inspection or the welding techniques pillar for the complete list.