Weld Inspection for Steel Structures: UT, MT, RT Explained

Title: Weld Inspection for Steel Structures: UT, MT, RT, and What Each Test Catches

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Overview

Weld defects are the number one cause of steel structure failures. A single bad weld on a crane girder or bridge joint can crack and collapse under fatigue. Weld inspection — visual plus non-destructive testing (NDT) — is how fabricators and erectors prove a weld is sound before the structure goes into service.

Below is a practical guide to the five weld inspection methods used on steel structures, what each one detects, and when to use it.

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Inspection Methods

表格

Method	Code	Detects	Best For	Cost	Speed
Visual Testing (VT)	GB 50661 / AWS D1.1	Surface cracks, undercut, porosity, wrong size	Every weld, 100%	Lowest	Fastest
Ultrasonic Testing (UT)	GB/T 11345 / ISO 17640	Internal cracks, lack of fusion, slag, porosity	Full-penetration welds over 12 mm	Medium	Fast
Magnetic Particle (MT)	GB/T 26951 / ISO 9934	Surface and near-surface cracks	Fillet welds, surface of thick welds	Medium	Medium
Penetrant Testing (PT)	GB/T 18851 / ISO 3452	Surface-breaking cracks, pinholes	Non-magnetic steel, small parts	Low	Medium
Radiographic Testing (RT)	GB/T 3323 / ISO 17636	Internal defects, records for archive	Critical joints, code-required	High	Slow

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What Each Test Catches

VT — Visual Testing

Checks weld profile, size, undercut, overlap, porosity on surface, and spatter. Cheap and fast, but only sees the surface. Performed on 100% of welds.

UT — Ultrasonic Testing

Sends high-frequency sound waves through the weld. Internal defects — lack of fusion, slag inclusion, internal cracks — reflect the wave and show on screen. The standard method for full-penetration welds over 12 mm thick. No radiation, portable, one-man operation. Limitation: needs a smooth surface and skilled operator.

MT — Magnetic Particle

Magnetizes the weld area, then applies iron powder. Surface and near-surface cracks pull the powder into a visible line. Works only on ferromagnetic steel. Common for fillet welds and weld toes where fatigue cracks start.

PT — Penetrant Testing

Applies a red dye or fluorescent liquid to the surface; liquid seeps into surface-breaking cracks, developer draws it out. Works on any material, including stainless and aluminum. Used for small parts and where MT cannot work.

RT — Radiographic Testing

X-ray or gamma-ray film behind the weld. Internal defects show as dark spots on film. Gives a permanent record — required by code for critical bridges and pressure-bearing joints. Slow, needs radiation protection and a licensed operator.

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Advantages

Catches defects before failure — a 3 mm lack-of-fusion found in the shop costs ¥100 to fix; the same defect in service can collapse a roof.

Code compliance — GB 50661, AWS D1.1 and AISC 360 all require NDT on specified weld categories.

Multiple methods, full coverage — VT sees surface, UT/RT see inside, MT/PT catch surface cracks — together they cover every defect type.

Portable equipment — UT and MT units run on battery, work on scaffolding and in the field.

Documented proof — RT film and UT reports give the owner an inspection record for insurance and warranty.

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Disadvantages / Limitations

Operator dependent — UT results vary with the operator's skill and experience; a bad scan can miss real defects.

Surface prep required — UT and MT need smooth, clean weld surfaces; scale and paint cause false readings.

RT is slow and costly — radiation licensing, film, and protection add time and money.

Not all defects are detectable — UT misses small surface cracks, MT misses internal defects, RT misses tight cracks parallel to the beam.

Inspection after erection is hard — welds high on columns or inside box girders are difficult or impossible to reach.

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Typical Use Cases

Crane girder welds — full-penetration UT on flange-to-web welds, MT on weld toes, every joint on heavy-duty crane runways.

Bridge welds — RT or UT on main girder splices, MT on diaphragm connections, per JTG D64 and AASHTO.

Portal frame rigid joints — UT on full-penetration flange welds of beam-to-column moment connections.

Box girder and lattice column welds — UT on internal diaphragms and chord splices before box closing.

Repair and retrofit welds — MT on existing welds after fatigue cracking, UT to verify repair depth.

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Inspection Sequence

Weld appearance check (VT) — size, profile, undercut, porosity — 100% of welds.

Weld surface prep — grind weld toes smooth, remove scale and spatter.

Internal test (UT/RT) — per weld category in the code, typically 10–100% of full-penetration welds.

Surface crack test (MT/PT) — weld toes and fillet welds in fatigue zones.

Report and sign-off — NDT report with weld map, defect locations, and repair records.

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Bottom Line

No weld is perfect — the job of inspection is to find the defects that matter before they fail. Visual testing on everything, UT on full-penetration welds, MT at weld toes: that combination covers more than 95% of real-world weld failures at a reasonable cost. Skip the inspection to save money, and the first failure pays for a thousand inspections

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