What Is Radiographic Testing (RT)?

Radiographic testing (RT) uses X-rays or gamma rays to create an image of a weld or material cross-section on film or a digital detector. The radiation passes through the test piece; internal defects such as porosity, slag inclusions, lack of fusion, and cracks absorb radiation differently than sound metal, producing visible indications on the resulting image. RT is one of the most widely specified volumetric NDT methods for pressure piping under ASME B31.3, ASME Section V, and API 1104.

RT Radiation Sources

Source Type	Radiation	Energy Range	Best For
X-ray tube	X-rays	100-400 kV	Shop welds, thin to medium wall (up to ~50 mm)
Iridium-192	Gamma rays	0.31-0.61 MeV	Field welds, 10-70 mm wall thickness
Cobalt-60	Gamma rays	1.17-1.33 MeV	Thick sections, >50 mm wall
Selenium-75	Gamma rays	0.12-0.40 MeV	Thin-wall pipes, confined spaces

X-ray tubes produce higher-quality images with better contrast and are preferred for shop radiography. Gamma sources are portable, require no electrical power, and dominate field weld inspection on pipeline and plant piping projects.

Conventional vs Digital RT

Parameter	Film RT	Computed Radiography (CR)	Digital Radiography (DR)
Detector	Silver halide film	Reusable phosphor imaging plate	Flat panel detector (DDA)
Exposure time	Baseline	Similar to film	50-75% shorter
Image availability	After chemical processing (10-30 min)	After plate scanning (2-5 min)	Immediate (seconds)
Image quality	Excellent (reference standard)	Good to excellent	Excellent
Archiving	Physical film storage	Digital file	Digital file
Cost per shot	High (film + chemicals)	Medium (reusable plates)	Low (no consumables)
Governing standard	ASME V Article 2	ASME V Article 2, SE-2007	ASME V Article 2, SE-2698

Digital radiography is increasingly replacing film, particularly on large pipeline projects where rapid interpretation and electronic archiving reduce cycle time.

Acceptance Criteria

Acceptance criteria depend on the construction code. Key references:

Standard	Application	Key Acceptance Criteria
ASME B31.3	Process piping	Rejects cracks, incomplete penetration, slag >2.5 mm for t<19 mm
ASME Section VIII	Pressure vessels	Per ASME V Article 2 + Section VIII Div.1 UW-51
API 1104	Pipeline welding	Porosity limits based on cluster/individual pore size vs. wall thickness
EN 12517-1	European piping	Acceptance levels 1, 2, 3 linked to EN ISO 5817 weld quality

Radiographs are interpreted by certified Level II or Level III technicians per ASNT SNT-TC-1A or ISO 9712. Each film must include image quality indicators (IQIs/penetrameters) to verify sensitivity. ASME V requires a minimum 2-2T sensitivity (ability to detect a wire diameter equal to 2% of the material thickness).

RT Limitations

RT excels at detecting volumetric defects (porosity, inclusions) but is less effective for planar defects oriented parallel to the beam (tight cracks, lack of sidewall fusion). For crack-sensitive applications, ultrasonic testing or TOFD is often specified as a complementary or alternative method.

Radiation safety requires controlled exclusion zones, dosimetry, and licensed operators. Field RT on live plant sites typically requires night work to minimize personnel exposure—adding cost and schedule constraints.

Common Mistake

Specifying RT alone for crack-sensitive services (sour gas, high-cycle fatigue). RT can miss tight, planar cracks aligned with the beam direction. Combine RT with UT or TOFD for critical welds, or replace RT entirely with automated UT per ASME Code Case 2235.

RT results are documented alongside mill test certificates and form part of the weld data package reviewed during final pipe inspections.

Read the full guide to non-destructive testing