UT vs radiographic testing (RT)

Ultrasonic testing (UT) and radiographic testing (RT) are both volumetric NDT methods, but they do not produce the same information and are not universally interchangeable.

The correct question is not “Which is better?” but which method is required for this product, joint geometry and expected imperfection type?

How the methods work

UT introduces high-frequency sound into the material and interprets reflected signals. Conventional manual UT, phased-array UT and TOFD are all ultrasonic techniques, although their equipment and data presentation differ.

RT passes X-rays or gamma radiation through the component and records differences in transmitted radiation on film or a digital detector.

Practical comparison

Neither column means “always superior”.

Planar vs volumetric discontinuities

UT is often highly effective for planar discontinuities such as lack of fusion or cracks when the sound beam is correctly oriented. RT is traditionally strong for volumetric/density changes such as porosity and slag-type indications and provides a directly viewable image.

But real weld flaws are not neatly divided into two categories. Technique, geometry, wall thickness and orientation can change detectability substantially.

For a welded pipe seam, is one test enough?

That depends on the governing pipe specification.

Steel-tube product standards can prescribe specific seam NDT routes, sometimes with ultrasonic, radiographic or other methods and particular coverage. For example, the ISO 10893 series contains separate standards for radiographic and ultrasonic examination of welded-tube seams.

Therefore do not replace an RT requirement with UT — or UT with RT — merely because both are “volumetric”. The applicable product standard/API/EN requirement and project specification control whether methods are alternatives, complementary, or both mandatory.

What about seamless pipe?

Seamless pipe has no longitudinal or helical weld seam to radiograph. That does not mean NDT disappears. UT can still be required for pipe-body imperfections, laminations, wall thickness or other specified conditions.

A requirement written specifically as “RT of weld seam” simply does not map onto a seamless product in the same way.

Standards for weld testing

For general fusion welds, ISO 17640:2018 specifies manual ultrasonic testing techniques within its scope. Radiographic techniques are covered by:

• ISO 17636-1:2022 for X-/gamma-ray techniques with film;
• ISO 17636-2:2022 for digital radiography.

These are technique standards. Acceptance criteria may come from another standard or the governing fabrication/product code.

When both methods may be justified

Both can be required where the governing specification seeks complementary detection capability, where qualification demonstrates the need, or where a critical fabrication code explicitly demands it.

Using two methods is not automatically better if one is applied badly; inspection coverage, procedure qualification, calibration, personnel competence and acceptance criteria matter as much as the method name.

Procurement takeaway

When an RFQ says “100% NDT”, ask:

• UT, RT or both?
• parent material or weld?
• which standard/technique?
• what coverage?
• what acceptance level?
• manual, automated, film or digital?
• what reporting and TPI involvement?

That turns a vague quality statement into an auditable inspection requirement.

References and verification sources

• ASNT — Ultrasonic Testing
• ASNT — Radiographic Testing
• ISO 17640:2018 — manual UT of fusion welds
• ISO 17636-1:2022 — film radiography of welds
• ISO 17636-2:2022 — digital radiography of welds