Weld Inspection & AWS Visual Inspection (VT)

Every weld is inspected. The only question is whether it is inspected by the welder who made it, by a lead hand in the shop, by a Certified Welding Inspector (CWI), or by a code-qualified third party. The earlier a bad weld is caught, the cheaper it is to fix - and the welder who learns to inspect their own work before the inspector arrives is the welder who stays employed. Visual inspection (VT) is the cheapest, fastest, and most widely-performed nondestructive examination method in the welding industry. Nearly every code-governed weld is visually inspected before any other test is performed, and a large fraction of welds are never inspected beyond VT.

This guide covers VT fundamentals from the welder's and the inspector's perspectives: what to look for, what tools to use, how to distinguish a discontinuity from a defect, the AWS D1.1 accept/reject criteria, and when a VT finding warrants escalating to a volumetric NDT method.

Discontinuity vs Defect - The Difference That Matters

These two words get used interchangeably on the shop floor, but they mean very different things to an inspector.

• Discontinuity - any interruption in the uniform structure of the weld or base metal. Porosity, slag, undercut, spatter, arc strikes, and cracks are all discontinuities. Being a discontinuity by itself does not mean a weld is rejected.
• Defect - a discontinuity that exceeds the acceptance criteria of the governing code. A defect is a rejectable condition. All defects are discontinuities; not all discontinuities are defects.

Every code (AWS D1.1, ASME IX, API 1104, etc.) defines its own accept/reject limits. A 1/16-inch surface pore in a 1/4-inch fillet might be perfectly acceptable under AWS D1.1 for a statically loaded structure, and an immediate rejection under the same code for a cyclically loaded structure. Knowing the governing code and its criteria is the inspector's job - but the welder who understands the difference writes fewer repair tickets.

Visual Inspection Tools

A VT inspector carries a small kit. You should carry the same kit in your tool bag.

The Fillet Weld Gauge

A pair of gauges (often on a split ring) used to measure fillet leg length, throat, and convexity. The most common style is the HiLo or the V-WAC gauge:

• Fillet leg gauges - slide onto the weld; each gauge is cut to a specific leg length (1/8, 3/16, 1/4, 5/16, 3/8, 1/2). The correct gauge slides onto the fillet without gapping at the toes.
• Convexity gauge - measures how convex the bead is above a straight line drawn across the legs. Excessive convexity is a discontinuity worth noting.

The Cambridge Gauge (Bridge Cam)

A single sliding-rule gauge that measures:

• Fillet leg length (two-sided)
• Fillet throat
• Crown height (reinforcement on groove welds)
• Undercut depth
• Butt-weld misalignment (Hi-Lo)
• Bevel angle
• Porosity size (via comparison)

If you buy one tool, buy a Cambridge gauge. It replaces six other gauges and lives in a shirt pocket.

Feeler Gauges

Used to measure gap between fit-up plates, root openings on groove joints, and misalignment. A 0.005 to 0.020 set covers most shop needs.

Pit / Undercut Gauge

A small depth gauge, typically 0 to 3 mm, used to measure the depth of surface porosity and undercut. Undercut depth is a common reject criterion - measuring it directly beats eyeballing.

Micrometer / Calipers

Used to measure base metal thickness, groove depth before welding, and crown/reinforcement after welding.

Flashlight

A cheap right-angle LED flashlight is the most-used tool in an inspector's kit. Ambient shop light never shows you what a 1000-lumen flashlight at a grazing angle does. Undercut, overlap, and slag lines that are invisible under overhead fluorescents jump out under a flashlight.

Mirror / Borescope

For inspecting the back side of pipe root welds, inside box sections, or anywhere the eyeball cannot reach directly. A dental-style mirror covers most needs; a borescope is for tight accesses.

Common Discontinuities

Porosity

Gas pockets trapped in the weld as it solidifies. Causes: shielding gas loss (drafts, empty cylinder, clogged nozzle, wrong flow), contaminated base metal (oil, rust, paint, moisture), wet flux (SMAW rods), too-long arc. Appears as small round holes in the bead surface (surface porosity) or as round voids inside the weld (internal porosity, only detectable by RT or UT).

• D1.1 statically loaded fillet welds (Table 8.1): the sum of visible piping porosity 1/32 inch or larger shall not exceed 3/8 inch in any linear 1-inch length, and shall not exceed 3/4 inch in any 12-inch length of weld. For fillet welds joining stiffeners to webs, additional frequency limits may apply - always check the current D1.1 table for your connection class.

Slag Inclusion

Chunks of flux or oxide trapped between weld passes or between the weld and base metal. A flux-covered process problem (SMAW, FCAW, SAW). Causes: improper inter-pass cleaning, wrong electrode angle, too-low amperage, slag flowing ahead of the puddle.

• Surface slag - visible on top of the weld, always a defect (just chip and wire-brush).
• Subsurface slag - detected by RT or UT, accept/reject per code.

Undercut

A groove melted into the base metal adjacent to the weld toe that is not filled by weld metal. Causes: too-high amperage, too-long arc, too-fast travel speed, wrong gun angle. Undercut is a stress riser and a fatigue initiator - codes limit it tightly.

• AWS D1.1 static: undercut shall not exceed 1/32 inch for material under 1 inch; 1/16 inch otherwise.
• AWS D1.1 cyclic: undercut shall not exceed 0.010 inch for fatigue-critical members.

Overlap (Cold Lap)

Weld metal that has flowed over the base metal at the toe without fusing to it. Looks like a rolled lip at the toe. Causes: too-slow travel, too-low amperage, wrong gun angle. Overlap is always a defect under AWS codes - there is no acceptable amount of overlap.

Cracks

Any crack is a rejectable defect under nearly every welding code. Types:

• Transverse - across the weld
• Longitudinal - along the weld axis
• Crater - at the stop point (caused by not filling the crater)
• Toe / HAZ - at the weld-to-base-metal interface, often hydrogen-induced
• Underbead / root - only detectable by NDT

Cracks do not grow by magic - they propagate under load. A 1/8-inch crack today is a 2-inch crack next month. Reject, gouge out, reweld, re-inspect.

Incomplete Fusion

The weld metal did not fuse to the base metal or to a prior pass. Often at the sidewalls of a groove or between passes. Causes: too-low heat input, wrong angle, dirty base metal, too-fast travel.

Incomplete Penetration (Lack of Penetration)

The weld did not reach the required depth at the root of a groove weld. In a full-penetration joint, the weld must go root-to-cap. Lack of root penetration is a rejectable defect for CJP welds and a fatigue-sensitive defect for partial-penetration welds.

AWS D1.1 Accept/Reject - Visual Summary

For a statically loaded non-tubular connection (D1.1 Table 8.1 simplified):

• Cracks - no cracks of any size.
• Weld/base metal fusion - complete fusion shall exist between adjacent layers and between weld metal and base metal.
• Crater - all craters filled to the full cross section of the weld.
• Weld profile - shall meet standards in the code (no undercut beyond limits, no overlap).
• Weld size - may be up to 1/16 inch less than specified for specified fillet sizes.
• Undercut - 1/32 inch max for material less than 1 inch, 1/16 inch max otherwise.
• Porosity (CJP groove, D1.1 Table 8.1) - total visible piping porosity 1/32 inch or larger shall not exceed 3/4 inch in any 12-inch length of weld. No single pore greater than 3/32 inch. Fillet welds use a tighter per-linear-inch limit (3/8 inch per 1 inch) in addition to the 3/4 inch per 12 inch rule.

Cyclically loaded structures have tighter limits - always reference the correct table for the joint type.

When to Escalate to NDT

VT catches surface discontinuities and gross weld-profile issues. It cannot see inside the weld. Escalate to a volumetric or surface-enhancement method when:

• PT (Penetrant Testing) - use when you suspect tight surface cracks that VT cannot resolve. Cheap, fast, works on any metal, stainless and non-ferrous included.
• MT (Magnetic Particle Testing) - use for surface and near-surface cracks in ferromagnetic material (carbon steel, low-alloy steel). Fast and sensitive, but limited to magnetic materials.
• UT (Ultrasonic Testing) - use when you need internal volumetric inspection on thick plate, structural groove welds, or critical structural members. A trained UT tech with a calibrated probe can find lack of fusion and internal cracks that no other method will catch without cutting.
• RT (Radiographic Testing) - use for full-volumetric coverage where a permanent film or digital record is required. Standard for pressure piping, pressure vessels, and critical structural connections.

A welder's or shop-level VT should not wait on NDT - go find the inspector when you see something you cannot measure, cannot call, or cannot repair confidently.

Inspection Timing

Inspection is not a one-time event at the end. A good QC plan includes:

• Before welding - joint fit-up, cleanliness, bevel angle, root gap, land, preheat temperature, WPS at the station, electrode identification, base metal identification, welder qualification certificate.
• During welding - interpass cleaning, interpass temperature, amperage, voltage, travel speed, bead sequence, backing / purging gas if applicable.
• After welding - final VT, dimensions, any required post-weld heat treatment verification, final NDT as specified.

Skipping pre-weld inspection is how a fit-up error rides through an entire production sequence and shows up at the end as a rejection the welder cannot salvage.

The CWI Path

The AWS Certified Welding Inspector (CWI) credential is the most widely recognized inspector cert in North America. The certification exam has three parts:

1. Fundamentals - closed-book, 150 questions on welding processes, metallurgy, destructive and nondestructive testing, safety, symbols.
2. Practical - open-book hands-on test using tools (fillet gauges, Cambridge gauge, pit gauge) on weld replica samples. You inspect real (or replica) welds and call out defects.
3. Code - open-book using whichever code book the test center uses (usually D1.1). Time-constrained code interpretation.

Prerequisites include documented work experience - 5 years of welding experience with a high school diploma, less with an engineering degree. Cost is in the range of $1,000 to $1,500 for the exam and required AWS membership. Renewal is every 3 years (9-year career, then recert by exam).

Many welders who lose the physical stamina for welding become CWIs. The pay is often higher, the conditions better, and the code knowledge you build as a welder transfers directly.

Day 1 Checklist

• Fillet gauge set OR Cambridge gauge in tool bag
• Right-angle LED flashlight
• Feeler gauges (0.005 to 0.020)
• Understanding of AWS D1.1 basic accept/reject for undercut, porosity, cracks, fusion
• Can distinguish discontinuity from defect
• Know when to call an inspector vs repair and move on
• Know which NDT method applies to which suspected flaw

The welders who advance fastest in this trade are the ones who can look at their own work critically and catch their own defects before anyone else does. Start building that eye now.