Weld Inspection & NDT: Visual, MT, PT, UT and What Gets Accepted

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Alright, Weld Inspection & NDT: Visual, MT, PT, UT and What Gets Accepted. Don't let the plain title fool you. Here's the part that actually matters on the job: Visual inspection by a qualified inspector against the code's acceptance criteria catches most problems, and NDT such as MT and UT is used where the drawings and the quality plan require it to find what the eye cannot. Learn it well and you will know what the inspector is looking for before the inspector arrives, which is the whole secret to passing.

A weld is accepted or rejected against written criteria, not against an opinion. This lesson covers who inspects, what they check, which nondestructive testing (NDT) methods find what, and what happens when something fails.

Who inspects and under what rules

Two inspection programs run on a structural job. Quality control (QC) belongs to the contractor doing the welding: a QC inspector, usually an AWS Certified Welding Inspector (CWI) qualified under AWS QC1 or an equivalent the code accepts, checks the work as it happens. Quality assurance (QA) belongs to the owner: under IBC Chapter 17 structural steel welding is a special inspection, and AISC 360 Chapter N spells out what the QA special inspector verifies before, during, and after welding, splitting tasks into those the inspector must observe on a sampling basis and those the inspector must perform on every item. NDT technicians qualify separately, typically to ASNT SNT-TC-1A Level II in their method.

Visual inspection is the backbone

Visual testing (VT) is required on all welds under D1.1 and is done in three windows: before welding (WPS matching the joint, welder qualified, fit-up within tolerance, surfaces clean, preheat verified, consumables controlled), during welding (within the WPS ranges, interpass cleaning and temperature, position and technique), and after welding, against the acceptance criteria. The D1.1 criteria for statically loaded structures that a field welder should know:

• Cracks — none, anywhere, of any size.
• Fusion — complete between weld metal and base metal and between passes; craters filled.
• Fillet size — measured with a gauge. A fillet may be undersize by 1/16 in. for welds up to 3/16 in., by 3/32 in. for 1/4 in. welds, and by 1/8 in. for 5/16 in. and larger, but only if the undersize portion is no more than 10 percent of the weld length.
• Undercut — for material under 1 in., no deeper than 1/32 in., with up to 1/16 in. allowed for a total of 2 in. in any 12 in.; for thicker material, 1/16 in. for any length.
• Porosity — fillet welds may have visible piping porosity totaling no more than 3/8 in. in any inch and 3/4 in. in any 12 in.; CJP groove welds in butt joints carrying tension across the weld may have none.
• Arc strikes outside the weld are not permitted; they are ground out and checked.

NDT methods and when each is used

• Magnetic particle testing (MT) finds surface and near-surface discontinuities in ferromagnetic steel with a yoke and iron powder. It is the go-to method for fillet and PJP welds, backgouged roots, weld access holes in heavy shapes, and repaired areas.
• Penetrant testing (PT) finds surface-breaking discontinuities in any material and is used where MT cannot be, such as stainless and aluminum.
• Ultrasonic testing (UT) sends sound into the weld and reads reflections from internal flaws. It is the standard volumetric test for CJP groove welds in structural steel, applies to material from 5/16 in. thick upward, and D1.1 provides its own acceptance tables. AISC 360 Chapter N calls for UT on CJP groove welds 5/16 in. and thicker, at a sampling rate for ordinary buildings that increases if rejects are found and at 100 percent for higher risk category buildings.
• Radiographic testing (RT) is common on pipe and rare on building steel because of geometry, access, and radiation control. For high-strength quenched-and-tempered steels, D1.1 delays NDT at least 48 hours after welding so delayed hydrogen cracks have time to appear.

Discontinuity versus defect and what causes each

A discontinuity is any interruption in the weld; it becomes a defect only when it exceeds the acceptance limit. The common ones and their field causes:

• Porosity — moisture, scale, paint, galvanizing, lost shielding gas, wind, wet consumables.
• Undercut — too much current, too fast, wrong angle, too long an arc.
• Overlap — too cold, too slow, too much wire for the heat.
• Incomplete fusion or penetration — arc not reaching the sidewall, oxide on the groove face, tight root, too little current.
• Slag inclusions — poor interpass cleaning, bad bead placement in a narrow groove.
• Cracks — hydrogen (no preheat, wet rod, restraint), crater cracks, lamellar tears in thick plate.

Repairs and reinspection

Defects are removed by grinding or gouging to sound metal, never welded over. A crack is removed along with sound metal beyond each end as the code requires, confirmed gone by MT or PT, and then rewelded under a WPS. Repairs are reinspected by the same method that found the defect. Repairs beyond what the code allows or repeated repairs in one location go to the engineer for approval. Every repair is written up on a nonconformance report with location, cause, disposition, and verification. Headed shear studs have their own routine: a pre-production bend test, a check of every stud for a full 360-degree flash, and a 15-degree bend of any stud with flash missing.

Going Deeper (Intermediate)

The inspector cannot see a weld through fireproofing, deck, or drywall. Field crews call for VT before anything covers a weld, and foremen keep a weld map with each welder's stencil beside each joint so UT indications trace to a person and a WPS. On T-joints and corner joints with backing, UT has geometric blind spots; a good technician scans from more than one face and writes the limitations on the report.

Advanced / Pro-Level

Phased-array UT is accepted under D1.1 with a written procedure and qualified technicians and is becoming common on heavy splices because it images the weld rather than reading a single beam. Seismic projects under D1.8 take demand-critical welds to 100 percent UT. Pros also watch for k-area cracking after welding continuity and doubler plates into rolled columns, and the best QC inspectors track reject rates by welder, joint type, and cause, and fix the cause rather than the symptom.

Practice Challenge

A 5/16 in. fillet weld is 30 in. long. The inspector's gauge reads 1/4 in. along 6 in. of it; the rest is full size and otherwise sound. Accept or reject? (Answer: reject. A 5/16 in. weld may be undersize by up to 1/8 in., but only along 10 percent of the length, and 6 in. of 30 is 20 percent. The undersize section is cleaned, another pass is added under the WPS, and the weld is reinspected.)

In Practice

A welder who knows the undercut and porosity limits, carries a gauge, and fixes a cold cap before walking away rarely sees a reject tag. A welder who treats the criteria as a mystery learns them one gouged-out weld at a time.

Common Mistakes to Avoid

• Covering welds with deck, fireproofing, or finishes before VT
• Welding over a defect instead of removing it to sound metal
• Calling a discontinuity a defect, or the reverse, without checking the written criteria
• Skipping interpass cleaning and leaving slag inclusions for UT to find

Takeaway: Visual inspection by a qualified inspector against the code's acceptance criteria catches most problems, and NDT such as MT and UT is used where the drawings and the quality plan require it to find what the eye cannot.

Educational content only — not engineering, design, inspection, or safety advice, and not a substitute for hands-on training, a qualified welding procedure, or a licensed professional. Weld to the WPS and the drawings, follow AWS D1.1, AISC, and OSHA requirements as adopted and applied on your project, and follow the manufacturer's instructions for your equipment and consumables.