Radiographic Testing (RT) — Complete Field Guide

How the physics works

Radiography uses penetrating ionizing radiation — gamma photons from Ir-192 (380 keV avg.), Se-75 (215 keV avg.), Co-60 (1.25 MeV avg.), or X-ray bremsstrahlung from a tube — passed through the part to expose film, an imaging plate, or a digital detector array on the far side. Density differences in the image map to thickness and density differences in the part: a slag inclusion, a porosity void, a lack-of-fusion gap, or a crack absorbs less photons than the surrounding steel and shows up dark on the developed film. ASME BPVC Section V Article 2 sets the procedure rules, IQI placement, and density requirements [1]; ASTM E94 covers radiographic examination practice [2]; ASTM E1742 governs film handling [3]. The image quality indicator (IQI) — a wire set or hole set placed source-side of the part — proves the technique can resolve a specific small-feature size, expressed as 2-1T or 2-2T sensitivity per ASME V T-276 [1].

When to choose this method

Choose RT when the code mandates film or digital radiography (most ASME B31.3 normal-service welds permit either UT or RT, but client specifications still default to RT for the permanent image record), when you need to detect porosity and slag that UT amplitude criteria struggle with, and when weld geometry — small-bore branch connections, complex fittings — defeats shear-wave UT. RT also wins for legal/insurance-driven jobs where a film image survives 30 years in a file cabinet.

RT is wrong for in-service work on populated facilities where evacuating the radiation zone is impractical, for thick-wall vessels (>3" steel) where exposure times stretch past an hour per shot, for parts in service where the asset cannot be decoupled from process flow, and for any weld where the dominant defect is planar and parallel to the beam — RT misses tight lack-of-fusion that runs perpendicular to the film. Crews near radiation work face exposure limits under 10 CFR 20 [4]; on a manned platform or refinery turnaround, the lost-production cost of clearing 100 feet around every shot kills the schedule.

Materials & geometries

Method coverage depends on couplant, surface, and section thickness. Compatible forms include:

Procedure

Source and energy selection

Source choice flows from material and thickness. ASME V Article 2 Mandatory Appendix VI [1] lists permitted energy ranges. For steel 0.25" to 2.5" wall, Ir-192 (380 keV gamma) is the workhorse — 50-100 Ci activity, half-life 74 days, sealed source in a Sentinel 880 or QSA Global 880 Delta camera. For thinner walls (0.1" to 1" steel), Se-75 (215 keV) gives finer contrast and a smaller exclusion zone but costs 3× more per Curie. For thick sections (>2.5" steel), switch to Co-60 (1.25 MeV) or a 300+ kV X-ray tube.

Source activity must be within usable life — Ir-192 below about 20 Ci becomes uneconomic for routine work because exposure times balloon. Source-changer paperwork (the leak-test certificate, the manufacturer's decay sheet, the NRC or Agreement State license number) ships with every camera and gets photocopied into the job package.

Geometry and SFD calculation

Source-to-film distance (SFD) controls geometric unsharpness Ug = F × t / SFD, where F is source size and t is film-to-source-side-of-part distance. ASME V T-274.2 [1] caps Ug at 0.020" for materials under 2" and 0.030" for thicker. For a typical Ir-192 source (F = 0.080" effective), a 0.5" wall pipe weld with the film against the back wall requires SFD ≥ 14 inches to stay under Ug = 0.020". The technician runs this math at every setup — most modern RT software (Industrex, Carestream, Vidisco) bakes it into the exposure calculator.

Single-wall single-image (SWSI) is standard for vessels and large-bore pipe. Double-wall single-image (DWSI) and double-wall double-image (DWDI) cover small-bore pipe per ASME B31.3 §344.5.2. DWDI elliptical shots on pipe <3.5" OD require a minimum of two exposures 90° apart; smaller pipe (<2.5") often needs three or four exposures to cover the weld circumference.

IQI selection and placement

IQI sensitivity is the proof of technique. ASME V T-276 [1] sets a 2-2T requirement for most pressure-equipment work — the technique must resolve the 2T hole on a 2T-thickness IQI placed source-side on the part. ASTM E747 wire IQIs are equally acceptable; the wire diameter that must be visible is given in ASME V Table T-276. For a 0.5" wall steel pipe shot from outside, ASTM E747 set B wire 4 (0.013" diameter) must be visible on the developed film.

IQI placement is source-side except when the geometry makes that impossible — then film-side with the letter F prominent in the image, and the sensitivity requirement tightens by one thickness step. The IQI sits on a shim of base-metal-equivalent thickness when it cannot sit directly on the weld reinforcement.

Exposure and density control

Film density on processed radiographs must fall between 1.8 and 4.0 H&D under SE-1 viewing, per ASME V T-282 [1]. A densitometer reading at the IQI shim location verifies this on every film before release. Under-density (<1.8) loses small-defect contrast; over-density (>4.0) demands a viewer with higher brightness than most field shop boxes deliver. Exposure factor (curies × minutes) is calibrated against a step-wedge or against the manufacturer's exposure chart for the specific film type (Industrex AA400, MX125, M100).

Digital radiography (DR) detectors and computed radiography (CR) imaging plates substitute pixel grayscale for film density. ASTM E2698 [5] sets DR equivalency rules — the technique must demonstrate the same IQI sensitivity as the equivalent film class and the same signal-to-noise ratio at the region of interest. DR exposure is shorter (often 1/3 to 1/10 of film) but capex is 5-10× higher per detector.

Processing and film reading

Film processing follows ASTM E999 [6] — automatic processors (Kodak Industrex M37, Agfa Structurix) running a developer-fixer-wash-dry cycle at controlled temperature and replenishment rate. Manual tank processing is permitted but rarely used today outside remote field locations. Every batch of processed film gets a processed-film image quality check using a step-wedge film and the lab's own density verification before any production films are released for reading.

Reading is done in a darkened room with a high-intensity viewer (Industrex M-400, X-Tek HV-1000) and a densitometer. The Level II interpreter scans the entire image, marks every indication on a film-reading record, and assigns disposition per the governing code. ASME Section V does not set acceptance — the construction code does. ASME B31.3 §344.5.3 acceptance limits for normal fluid service are listed in Table 341.3.2 [7]; API 1104 §9 covers cross-country pipeline weld acceptance [8].

Reporting and archive

The report names the procedure, source isotope and activity at exposure time, SFD, exposure time, film/detector type, IQI sensitivity achieved, density at the region of interest, technician name and certification, and every reportable indication with location and disposition. Films are archived per the client's document control — typically 5 years minimum for ASME work, longer for API 1104 pipeline construction.

Source utilization gets logged separately for NRC/Agreement State compliance — every Curie-minute of exposure, the radiographer's film badge dose, and the daily survey-meter readings outside the exclusion zone. 10 CFR 34 [9] mandates these records for industrial radiography and the licensee must retain them indefinitely for the source license.

Equipment

Source projectors and X-ray tubes

The Sentinel 880 Delta and QSA Global 880 Sigma dominate the Ir-192/Se-75 projector market in the US. Both use a depleted uranium shield, a tungsten S-tube, and a Selectorr crank-out drive. Service life is roughly 10 years before regulator-mandated full rebuild. Co-60 cameras (QSA 660A, Tech-Ops 660A) are heavier (>50 lb) and reserved for thick-section work.

X-ray tube systems for portable field RT include the ICM SiteX C-1607 (160 kV, 7 mA) and the YXLON SMART 200 (200 kV). Crawlers for pipeline RT (RTI X-Stream, Jireh Bug-O) carry an X-ray tube inside the pipe to shoot single-wall radiographs of butt welds at production rates.

Film, CR, and DR detectors

Film classes are governed by ASTM E1815 [10] and ASME V T-260. Class I (Industrex M100, Structurix D2) is fine-grain ultra-high-definition. Class II (M125, D4) is general-purpose. Class III (AA400, D7) is faster but lower resolution — used for survey and screening. Lead intensifying screens (front 0.005", back 0.010") sandwich the film in the cassette.

CR imaging plates (Carestream HPX-1, Fuji ST-VI) replace film with a photostimulable phosphor plate read in a scanner. DR flat-panel detectors (Vidisco FoxRayzor, GE DXR250) deliver digital images in seconds. Both must demonstrate ASME V Article 2 IQI sensitivity to be code-acceptable.

Survey instruments and personnel monitoring

Survey meters (Ludlum 9DP, Mirion DMC 3000) verify the radiation field at the boundary of the high-radiation area. 10 CFR 34.41 [9] requires a survey at the source position before storage at the end of every exposure to confirm the source returned to the shielded position — survey-meter failure here has caused multiple over-exposure incidents in US industrial radiography history.

Personnel dosimeters — OSL film badge plus a direct-reading pocket dosimeter or electronic personal dosimeter — are mandatory under 10 CFR 20. Annual dose limit for radiation workers is 5 rem (50 mSv) whole body. Most radiographers run well under 1 rem/year with proper geometry and shielding discipline.

Codes & standards that govern this method

Procedures and acceptance criteria are anchored in published codes:

Acceptance criteria

For new-construction ASME B31.3 normal fluid service welds, ASME B31.3 Table 341.3.2 [7] caps cumulative porosity at 1.5% of weld area for any 6-inch length, single porosity at no greater than the lesser of 1/4 t or 5/32", slag inclusions at no greater than t/3 long with a cumulative limit, and rejects all cracks, lack of fusion, and incomplete penetration regardless of dimension. For ASME VIII Division 1 vessels, ASME V Article 2 Mandatory Appendix VIII references UW-51 with similar limits. API 1104 §9 [8] applies to cross-country pipelines and is generally tighter than B31.3 for incomplete penetration but allows slightly more porosity. Severe-cyclic-service piping under B31.3 cuts every limit roughly in half. The film interpreter assigns repair/accept on a per-indication basis; any indication classified rejectable goes back to the welder for excavation and re-RT after the repair weld.

How this compares to other methods

Choosing between methods is rarely about capability alone — cost, throughput, and code coverage all weigh in:

Cost range

Typical RT pricing in the US market runs $75–$280 USD per exposure (single weld shot, Gulf Coast crew, 2025), with most jobs landing around $140 USD per exposure (single weld shot, Gulf Coast crew, 2025). Mobilisation, access, and certification level shift the band.