Radiographic Testing (RT): The Complete 2026 Guide — Physics, Equipment, Procedure & Codes

Radiographic Testing (RT): The Complete 2026 Guide

Radiographic testing (RT) is the gold-standard volumetric non-destructive testing method for detecting internal discontinuities in welds, castings, forgings and pressure-retaining components. By passing ionising radiation — X-rays from an electrical source, or gamma rays from a sealed isotope such as Iridium-192, Selenium-75 or Cobalt-60 — through a test piece and recording the attenuated beam on film, computed radiography (CR) phosphor plates or a digital detector array (DDA), an inspector obtains a permanent two-dimensional shadowgraph that reveals porosity, slag, lack of fusion, lack of penetration, cracks and geometric defects at densities measurable to 0.5 % of the parent metal. Radiography is the only NDT method whose result is a stand-alone, court-admissible image: clients can re-interpret a digital radiograph ten years later. For these reasons it remains mandated by ASME Section V Article 2, ASME B31.3 Chapter VI, API 1104 Section 11, AWS D1.1 Clause 10 and ISO 17636-1/-2 for critical welded fabrication across oil & gas, aerospace, power generation, nuclear and shipbuilding. This complete guide — written by ASNT Level III practitioners — covers the physics, equipment, procedure, codes, certification, applications, common defects, advantages, cost and recent innovations driving RT in 2026.

Physics & Principles of Radiographic Testing

Radiography exploits the differential absorption of high-energy electromagnetic photons (X-rays or gamma rays) as they pass through a material. The intensity of the transmitted beam follows Beer–Lambert attenuation: I = I₀ · e^-μt, where μ is the linear attenuation coefficient (a function of material density, atomic number Z and photon energy E) and t is the thickness traversed. A discontinuity — a void filled with air or slag — has a lower μ than the surrounding steel, so the transmitted intensity at that location is locally higher. When the beam strikes silver-halide film, a phosphor imaging plate (CR), or the scintillator of a flat-panel detector (DR), that local intensity difference is recorded as a darker (more exposed) image. Three quantitative parameters govern image quality:

Geometric unsharpness (U_g) = F · t / d, where F is focal spot size, t is object-to-film distance and d is source-to-object distance. Code-compliant radiography typically demands U_g ≤ 0.5 mm for thicknesses < 50 mm and ≤ 0.76 mm above 100 mm.

Contrast — the density difference between defect and parent — is governed by photon energy. Lower energies (Ir-192 at 360 keV, Se-75 at 320 keV) give higher contrast but penetrate less; higher energies (Co-60 at 1.17/1.33 MeV, linear accelerators at 4-25 MeV) penetrate up to 300 mm of steel but lose contrast.

Definition is verified using an image quality indicator (IQI) — a wire or hole-type penetrameter placed source-side on the part. ASME V Article 22 (T-276) requires the smallest wire (e.g. wire 1T at 6 mm steel) to be visible at the area of interest. The radiographic density of the image must fall between 2.0 and 4.0 H&D units for film, or signal-to-noise ratio > 130 for digital detector arrays per ISO 17636-2 class B. Energy selection follows the half-value layer table: choose Se-75 below 30 mm steel, Ir-192 from 20-75 mm, Co-60 above 50 mm, and high-energy X-ray (linac) for thick castings above 150 mm.

Radiographic Testing Equipment

RT equipment falls into four categories — radiation sources, image-capture media, accessories, and radiation-safety hardware. A typical service crew running an Ir-192 crawler shoot on a 20-inch pipeline carries the gear listed below.

Equipment Class	Examples / Spec	Indicative Cost
Industrial X-ray tube	Constant-potential 160 kV / 300 kV / 450 kV portable; 1.5-4.5 mm focal spot	$25,000 - $120,000
Linear accelerator	4, 6, 9 or 15 MeV — for steel > 150 mm and concrete	$500,000 - $2,500,000
Iridium-192 (Ir-192) source	10-100 Ci, half-life 74 d, average energy 380 keV	$12,000 - $25,000 per source change
Selenium-75 (Se-75) source	5-80 Ci, half-life 120 d, average energy 320 keV — better contrast on thin walls	$15,000 - $30,000
Cobalt-60 (Co-60) source	10-50 Ci, half-life 5.27 y, energies 1.17 / 1.33 MeV — heavy castings	$30,000 - $80,000
Exposure device (camera)	QSA Sentinel 880, Tech Ops 660, GammaMat M — ANSI N432 / ISO 3999 certified	$15,000 - $40,000
Source pigtail & guide tube	10 m flexible drive tubes, source stops, collimators 60°/120°	$2,500 - $8,000
Film (industrial)	Agfa D2/D4/D7, Kodak AA400/T200 — class I/II/III per ISO 11699-1	$8 - $25 per 14x17" sheet
Computed radiography (CR)	Carestream HPX-1 plus, GE CRxFlex — phosphor plates reusable 1,000+ cycles	$45,000 - $90,000 system
Digital detector array (DDA / DR)	Varex 2530HE, GE DXR250, YXLON XRD 1622 — 100 μm pixel pitch, 4-16 bit	$60,000 - $250,000
IQI / penetrameter set	ASTM E747 wire (1A-6A), ASME hole-type 5-80, ISO 19232	$200 - $600 per set
Lead screens & cassettes	0.125 mm front / 0.25 mm back lead intensifying screens	$80 - $300 each
Densitometer	X-Rite 301, MacBeth TD-504 — for film density 0.05 - 4.5	$1,200 - $3,500
Film viewer	20,000 cd/m² for density 4.0 readability per ASME V T-282	$1,500 - $6,000
Survey meter	Ludlum 19, Thermo RadEye G — calibrated annually, range 0.01 μSv/h - 999 mSv/h	$1,500 - $4,500
Personal dosimetry	OSL + electronic dosimeter + audible alarming dosimeter (triple-D rule)	$150 - $600 / technician / year

A full crawler/dual-wall pipeline crew runs $180,000 - $250,000 in capital; an aerospace DR cell can exceed $400,000.

Step-by-Step Radiographic Testing Procedure

1. Pre-job planning & safety setup. Review the written procedure (qualified per ASME V T-150). Calculate exposure (Source-to-Film Distance, time, source strength) using the gamma-ray exposure chart for the current source activity in Ci. Survey the work area, post and barricade to the 0.02 mSv/h boundary (high-radiation area) and the 0.0025 mSv/h boundary (restricted area) per 10 CFR 20 / IAEA SSR-6. The Radiation Safety Officer (RSO) issues an exposure permit; the radiographer verifies dosimeters are zeroed and the audible alarm threshold is set at 2 mSv/h.
2. Surface preparation. Remove weld spatter, scale and slag that could mimic defects. Mark the part with lead-foil location markers (per ASTM E747) at every 250 mm or per shot length, plus the part number, weld number, side identifier and Pb arrows for direction. Place the IQI source-side at the area of interest unless source-side access is impossible (then film-side with the letter "F").
3. Source / detector positioning. Choose the geometry: single-wall single-image (SWSI) for accessible welds, double-wall single-image (DWSI) for 3"-3.5" pipe, double-wall double-image (DWDI) elliptical for ≤ 3" pipe. Position the source at the calculated SFD so geometric unsharpness U_g ≤ code limit. Verify U_g with the formula above before exposure.
4. Exposure. Crank the source out of the exposure device using the drive cable; do not exceed the calculated time. Re-confirm with the survey meter that the source is fully shielded before approaching the camera. Each exposure is logged in the radiographer's daily diary (source serial #, time, Ci, SFD, kV).
5. Image processing. For film: develop 5 min at 20 °C, stop 30 s, fix 5-10 min, wash 20 min, dry. Verify density 2.0-4.0 with a calibrated densitometer at three points across the IQI and weld. For CR: scan the imaging plate within 4 h of exposure (latent fade). For DR: image is available in < 15 seconds.
6. Image evaluation. Read on a calibrated viewer at < 100 lux ambient with the inspector dark-adapted > 5 minutes. Verify: (a) required IQI sensitivity wire/hole visible; (b) density / SNR within limits; (c) part identification, location markers and IQI all present in the image; (d) no processing artifacts (streaks, fog, pressure marks). Mark and size every indication using a transparent scale.
7. Acceptance / rejection per code. Apply the governing acceptance criteria (e.g. ASME B31.3 Table 341.3.2, API 1104 Section 9, AWS D1.1 Clause 10.21). Reject if any single indication exceeds maximum allowable length, cumulative indications exceed thickness ratio, or any crack-like indication exists at all.
8. Documentation & reporting. Issue a radiographic report listing source, kV, mA, SFD, exposure time, IQI used, density, indications with location and disposition, signature of Level II reader and Level III reviewer. Archive digital radiographs in a DICONDE-compliant archive (IS&O retention typically 10 years; nuclear retention permanent).
9. Post-job decommissioning. Survey the exposure device, lock and seal the source, return camera to the source storage bunker, complete the post-shoot dosimetry log, transmit area survey data to the RSO.

Standards & Codes Governing Radiographic Testing

RT is one of the most heavily codified NDT methods because it involves both quality acceptance and ionising-radiation safety. The hierarchy of governing documents is:

ASME Boiler & Pressure Vessel Code, Section V Article 2 ("Radiographic Examination") — the primary procedural standard in North America. Defines IQI selection (Table T-233), required image quality, density (2.0 - 4.0 transmitted film density; SNR ≥ 130 for DDA per Mandatory Appendix VIII), location marker placement, surface preparation, and back-scatter verification (lead letter "B" on the film cassette). Article 2 is invoked by reference from Section VIII Div 1 (pressure vessels), Section III (nuclear), B31.1 (power piping) and B31.3 (process piping).

ASTM E94 / E94M — Standard Guide for Radiographic Examination. The companion practice document that ASME V references; covers exposure technique, screen selection, processing chemistry and digital workflow.

ASTM E1742 / E1742M — Standard Practice for Radiographic Examination, the contracting practice for purchasers. ASTM E1648, E155, E186, E280, E446 are reference radiographs for aluminum castings, steel castings up to 2", 4.5"-12", and 12"+ — used to grade casting indications.

ASTM E2698, E2737, E2736 — Digital Detector Array (DR) standards. E2033, E2007, E2445, E2446 — Computed Radiography (CR) standards governing system characterization.

ISO 17636-1 (film) and ISO 17636-2 (digital detectors and CR) define class A (basic) and class B (improved) techniques for weld radiography in Europe and adopted globally for ISO 9712 work. ISO 17636-2 class B is the digital equivalent of ASME V with enhanced SNR requirements. ISO 11699-1 classifies industrial film (C1-C6); ISO 19232-1/-2/-3 covers IQIs.

Pipeline RT is governed by API 1104 Section 11.1 and API Std 1163 for in-line inspection cross-validation. Welded structures invoke AWS D1.1 Clause 10; aerospace work is governed by NAS 410, SAE AMS 2635 and primes’ specifications such as Boeing BAC 5424 and Rolls-Royce RPS 700. Nuclear RT is dual-coded under ASME Section III and 10 CFR 50 Appendix B. The European Pressure Equipment Directive (PED 2014/68/EU) requires RT to ISO 17636 with EN ISO 9712 certified personnel.

Radiographer Certification Pathways

RT certifies the highest barriers to entry of any common NDT method because of radiation safety. Two main schemes apply globally.

ASNT SNT-TC-1A (employer-based recommended practice) and ANSI/ASNT CP-189 (central certification) are the dominant North American frameworks, both administered through your company's qualified Level III. Level I requires 40 hours classroom + 210 hours on-the-job experience for RT (the highest hour requirement of all methods). Level II adds another 40 hours classroom + 630 hours total experience. Level III requires a passing score on the basic, method and specific exams plus 4 years documented work history. See our full ASNT certification roadmap and the comparison guide on SNT-TC-1A vs CP-189.

ISO 9712 / PCN / CSWIP — third-party central schemes used in Europe, the Middle East and Asia. ISO 9712 requires 80 hours of training for Level II RT plus the same 630-hour experience minimum, with an external proctored exam at an accredited body (e.g. BINDT for PCN, TWI for CSWIP). Certificates are valid for 5 years with annual vision testing and a 10-year renewal exam.

For pressure-vessel and piping inspectors who must specify and review radiographs but not necessarily shoot them, see our API 510 inspector, API 570 inspector and API 653 inspector certification guides. These API certifications require knowledge of ASME V Article 2 acceptance criteria and IQI interpretation.

Radiation safety adds an additional licensing layer in every jurisdiction — U.S. NRC or Agreement State Radiographer license (Part 34), Canadian CNSC Class IIB, UK HSE IRR17 Qualified Person, Indian AERB RP-II.

Applications by Industry

Oil & Gas Pipelines: Single-wall and dual-wall ellipse RT of girth welds on 4" - 60" line pipe per API 1104. Internal pipe crawlers shoot 100 % of cross-country welds; Ir-192 panoramic exposures complete a 36-inch weld in 4 minutes. Refineries radiograph all class 300+ piping butt welds per ASME B31.3.

Pressure Vessels & Boilers: ASME Section VIII Division 1 mandates full or spot RT for category A/B/C/D welds depending on joint efficiency. ASME B31.1 power piping invokes the same. Computed radiography has largely replaced film in fabrication shops since 2018.

Aerospace Castings & Welds: Investment castings of jet-engine turbine blades and structural airframe nodes are 100 % radiographed against ASTM E155/E186 reference standards. Real-time radioscopy in production lines images 30+ parts per minute.

Power Generation: HRSG (heat-recovery steam generator) tube butt welds, high-pressure feedwater piping, steam-header attachment welds — all conventionally radiographed during outages.

Nuclear: Reactor coolant piping, primary vessel circumferential welds, internals welds — RT augmented by UT under ASME Section XI ISI. The double-coded nature drives heavy use of high-energy linear accelerators for reactor pressure vessel girth welds approaching 300 mm thickness.

Manufacturing: Steel and ductile-iron sand castings, aluminum die castings (automotive engine blocks, hydraulic manifolds), powder-metal sintered parts — all graded by RT against ASTM reference radiograph sets.

Shipbuilding & Marine: Hull plate butt welds, propulsion shaft housings, LNG carrier containment welds — Lloyd's Register, DNV, ABS and BV all require RT on class-survey welds. See our ship hull inspection guide and our Houston, Sabine Pass and Galveston shipyard NDT pages.

Common Defects Detected by Radiography

Radiography is exceptional for volumetric defects (porosity, slag, lack of penetration) and good for planar defects favorably oriented to the beam. The table below maps the typical indications a Level II reader interprets.

Defect Type	Radiographic Appearance	Severity	Typical Acceptance (ASME B31.3 Normal)
Porosity (gas pores)	Round dark spots, scattered or clustered	Low-Moderate	≤ 1/4 T diameter individual; sum < 2T in any 6T length
Slag inclusions	Irregular dark elongated indications, parallel to weld axis	Moderate	≤ T/3 width; length ≤ T; total ≤ T in any 12T length
Lack of fusion (LOF)	Sharp straight dark line at fusion face, often with tail	High — planar & crack-prone	Not permitted
Lack of penetration (LOP)	Continuous dark line along root, very straight	High	≤ 1.5 mm total in any 150 mm (limited applications only)
Cracks (transverse, longitudinal, crater)	Sharp, jagged, often branched dark line	Critical	Not permitted in any direction
Tungsten inclusions (GTAW)	Small bright (light) spots — higher density than steel	Moderate	≤ 1/4 T diameter; ASME and AWS as porosity
Undercut / excess penetration	Density variation at toe / root	Low if within geometric tolerance	Per visual acceptance criteria, not RT
Burn-through	Local dark spot in root area, often with adjacent excess penetration	High	Not permitted
Casting shrinkage	Filamentary, dendritic dark patterns — "ribbon" appearance	Moderate-High	Per ASTM E186 / E446 reference radiograph severity level
Casting gas porosity	Round / oval clusters	Low-Moderate	Per reference radiograph severity Level 1-5

Advantages & Limitations of Radiographic Testing

Advantages: RT produces a permanent, archivable, visually intuitive image of the defect, which makes it uniquely defensible in court, regulator review and warranty disputes. It detects volumetric defects (porosity, slag, voids) extremely well — sensitivities of 1-2 % of wall thickness routine for properly executed shots. It is geometry-insensitive (does not need couplant, conductivity or surface preparation comparable to UT). It works on any material (steel, aluminum, copper, plastic, composite) and on any joint geometry — pipe, plate, tee, nozzle. The technique is well understood, codified, and has 80+ years of acceptance criteria backed by reference radiograph sets.

Limitations: Ionising radiation safety requires evacuation of the inspection zone (typical 60-100 m exclusion around an Ir-192 source) which is operationally disruptive in a live plant. Capital cost of digital systems is high. Planar defects (cracks, lack of fusion) unfavorably oriented to the beam may be missed — UT is far better for crack detection in thick sections. RT has limited capability above 75-100 mm steel unless high-energy sources are used. It does not size defect depth, only length and width in the plane of the film. License, dosimetry, and disposal obligations add ongoing cost. Film consumables are environmentally regulated (silver recovery, fixer disposal).

RT vs UT vs PAUT: Method Comparison for Weld Inspection

For weld inspection, the practitioner's perennial choice is among radiography, conventional ultrasonic and phased array ultrasonic. The table below is the operational comparison every Level III draws on the back of a procedure sheet.

Criterion	RT (Film / DR)	Conventional UT	PAUT
Best at detecting	Volumetric defects (porosity, slag)	Planar defects (LOF, cracks)	Planar & volumetric defects
Defect sizing — length	Excellent	Moderate	Excellent
Defect sizing — depth	Not provided	Excellent	Excellent
Permanent record	Yes — image	Trace + log (legacy)	Yes — encoded data file
Safety hazard	Ionising radiation	None	None
Production downtime	High (evacuation)	Low	Low
Thickness range	2 - 300 mm	5 - 300 mm	6 - 200 mm typical
Cost per weld (typical 6")	$45 - $90	$25 - $50	$60 - $110
Code acceptance	ASME V Art 2, B31.3, AWS D1.1	ASME V Art 4, B31.3	ASME V Art 4 + Mandatory App IV
Speed	4-10 min per 6" weld	3-8 min	1-3 min

For thick-wall pipeline (≥ 12 mm), modern operators run PAUT as the primary method per PAUT inspection practices and use RT only as a backup for arbitration. For thin-wall (< 8 mm) and process piping < 4" NPS, RT remains the most economical. For more on the head-to-head, see our film vs DR comparison and the ultrasonic testing complete guide.

Cost & ROI of Radiographic Testing

Pricing varies by source, geometry, output and access. Typical 2026 market rates (USD) for a competent ASNT Level II crew in the U.S. Gulf Coast:

Pipeline crawler shot, 6" - 36" pipe, Ir-192: $35 - $65 per weld. Field RT of process piping, dual-wall ellipse on 1.5" - 3" NPS: $25 - $45 per weld. Pressure-vessel longitudinal seam RT (12 mm wall, 1 m exposure): $80 - $150 per shot. Aerospace casting RT: $200 - $800 per part. Digital radiography (DR) typically commands a 25-40 % premium over film but eliminates film cost, chemistry, dark-room and storage. Computed radiography (CR) is roughly cost-neutral to film but turns the same cost into faster turnaround. ROI for converting a 4-crew shop from film to DR breaks even in 12-20 months at > 4,000 shots/year, driven by 60 % less consumable spend and 70 % faster report cycle. Owner-operator cost savings are larger: a 24-hour turnaround saves an offshore platform $100,000+/day in deferred startup.

Recent Innovations in Radiographic Testing (2024 - 2026)

The biggest shift is the maturation of digital detector arrays (DDA) with 75-100 μm pixel pitch and 16-bit dynamic range. Field-deployable battery-powered DR panels (Varex 4343HE-I, Carestream IndustreX HPX-DR) now shoot up to 1.5" steel in 30 seconds with no consumables and immediate viewing. AI-assisted defect recognition — CNN models such as Vidisco AI, GE Smart Radiography and academic open-source TorchRT — flag candidate indications with sensitivity matching or exceeding ASNT Level II readers; Level II adjudication remains required by code (ASME V Mandatory Appendix IX). Direct conversion CdTe and a-Se detectors deliver SNR > 200 versus 130 minimum, enabling 1.5 % IQI sensitivity from a single 30-second exposure. X-ray computed tomography (XCT) in industrial bays has moved from a $2M lab tool to a $400k turnkey cell — used heavily in additive-manufactured (AM) parts where internal lack-of-fusion porosity is hidden from surface NDT. The DR migration guide covers the workflow change in depth. DICONDE archival has become the de-facto standard, replacing proprietary formats.

Radiation Safety, ALARA & the Radiation Safety Officer (RSO)

Radiation safety is the single most regulated aspect of radiographic operations and the leading cause of fatal NDT-related incidents historically. Every radiography service must operate under a written radiation safety program approved by the regulator (U.S. NRC or Agreement State per 10 CFR 34, Canadian CNSC, UK HSE under IRR17, Indian AERB, Saudi Arabian NTRC). The program is owned by a designated Radiation Safety Officer (RSO) — a Level III radiographer or equivalent who controls source inventory, dosimetry records, area surveys, transport documentation and emergency response.

Three pillars of ALARA (As Low As Reasonably Achievable) drive operational practice. Time: minimize the duration any worker spends in elevated dose-rate zones — pre-rig the camera, pre-position the film cassettes, use the source drive cable only as long as needed. A typical Ir-192 panoramic exposure lasting 90 seconds delivers < 1 mSv whole-body dose to the radiographer if the operator remains > 30 m away during exposure. Distance: dose rate falls with the inverse-square law, so doubling distance reduces exposure 4×. Every shot is barricaded to the 0.02 mSv/h boundary (high-radiation area, restricted access) and the 0.0025 mSv/h boundary (controlled area). For a 100 Ci Ir-192 source those boundaries are ≈ 50 m and ≈ 140 m respectively. Shielding: lead, depleted-uranium and tungsten source-camera shielding reduce dose at the operator's position; collimators direct the beam only at the part and not at people on the back side; concrete bunker walls in fixed-cell shops are typically 200 mm dense concrete for ≤ 200 kV X-ray, scaling up with energy.

The triple-D dosimetry rule — every radiographer wears (1) a passive OSL (optically stimulated luminescence) or TLD badge processed monthly, (2) an electronic personal dosimeter (EPD) read at each shift, and (3) an audible alarming pocket dosimeter set to alarm at 2 mSv/h. Federal and most state regulations cap occupational exposure at 50 mSv/year whole body and 500 mSv/year extremities, with reportable thresholds at 5 mSv/quarter. The ALARA program target is typically < 5 mSv/year per worker — far below the legal limit. Source transport requires Type B containers (DOT 7A or IAEA Type B(U)), shipping papers, and emergency response plans per 49 CFR / IAEA SSR-6.

The most common radiography incident is a stuck source — the source pigtail jams in the guide tube or fails to retract fully into the shielded camera. Response: do not approach the camera; place a survey meter at 1 m to confirm source location; barricade the area at > 0.02 mSv/h boundary; notify the RSO; manual recovery is a rehearsed two-person procedure using lead-blanket shielding and remote-handling tools. Reportable to the regulator within 24 hours.

Image Quality Verification & Acceptance Workflow

A radiograph is only as valuable as its image-quality verification. Every shot must demonstrate, on the image itself, that it meets the procedure requirements before any defect interpretation is performed. The Level II reader works through a checklist:

Identification. Part number, weld number, side identifier (clock-face position for pipe), date and inspector ID must be legible in lead-foil characters in the image. ASME V T-275 requires identification on every shot.

Location markers. Lead arrows or numerical markers every 250 mm (or as procedure specifies) overlap between adjacent shots — verifies coverage with no skip.

IQI. Source-side wire or hole IQI present, in clear unobstructed area adjacent to the weld (not over the weld bead), and the smallest required wire or hole is just-visible. Per ASME V T-276 the IQI sensitivity required varies by thickness and material — Table T-276 lists the wire diameter or essential hole. The 1-2T sensitivity (2 % equivalent) is standard for code work; 1-1T (1 %) for nuclear and aerospace.

Density. Film density measured at three points across the area of interest with a calibrated densitometer; required range 2.0-4.0 H&D, with limits 1.8 minimum and 4.0 maximum per ASME V T-282 (some codes allow up to 4.5). For digital DR, signal-to-noise ratio (SNR_N) > 130 per ISO 17636-2 class B or ASME V Mandatory Appendix VIII.

Geometric unsharpness. Verified by measuring the IQI shadow penumbra and back-calculating U_g. The IQI shadow edge should be sharp; a fuzzy IQI means Ug is too high and the shot must be re-taken.

Backscatter check. Lead letter "B" placed on the back of the cassette; if the image of the "B" is darker than the surroundings, backscatter is excessive (acceptable per ASME V T-277 only if "B" is lighter or invisible).

Processing artifacts. No pressure marks, fog, water-spots, streaks (film); no detector calibration artifacts, lag, banding (DR/CR). Re-shoot if any artifact obscures the weld.

Only after all six checks pass does the reader proceed to defect interpretation. Many service errors trace back to acceptance of marginal-density or low-IQI-sensitivity radiographs — these are the radiographs that pass inspections but later admit missed defects to litigation.

Pipeline RT Workflows: Internal Crawler vs External Single-Wall

The two dominant pipeline radiography workflows are internal crawler RT and external single-wall (or dual-wall) RT, each used at different stages of the pipeline lifecycle.

Internal crawler (panoramic) RT is the default for new-construction long-distance pipelines ≥ 8" NPS. A tracked crawler carrying an Ir-192 source rolls inside the pipe to each weld, with external film or DR wraps in a circumferential cassette belt outside the pipe. The crawler is positioned by a battery-powered drive and located by external magnetic-flux sensors or radioactive locator beacons. A single panoramic exposure (typically 60-120 seconds for Ir-192 at 60-100 Ci) radiographs the entire 360° weld onto the external belt in one shot — fast, low operator dose, excellent geometry. Approval per API 1104 Section 11.1.

External single-wall single-image (SWSI) is used when the pipe is not crawlerable (small bore, complex geometry, U-bends, in-service pipelines without pigging access). The source is positioned outside the pipe and shoots through one wall onto a film cassette positioned on the inside (via inspection port) or onto a second external cassette positioned on the back side (double-wall single-image, DWSI). DWSI is the standard for 2"-3.5" NPS pipe; the elliptical double-wall double-image (DWDI) shoots through both walls onto a single cassette at an offset angle (typically 15° from perpendicular) producing the two weld images side-by-side — used on ≤ 3" pipe.

External SWSI/DWSI typically requires 6-10 shots per girth weld versus 1 for internal panoramic — driving cost differential. However, internal crawler requires the pipe to be pre-pigged, clean, dry, and is usually only feasible during construction. In-service pipeline RT (line out of service, mothballed weld checks, repair-weld verification) is almost always external.

Casting Radiography & Reference Radiograph Sets

RT of castings is a separate discipline from weld RT, governed by reference-radiograph grading. The ASTM Eseries reference radiograph standards provide graded image examples of common casting defects — gas porosity, inclusion, shrinkage, cold-shut, hot tear, internal chill — at increasing severity Level 1 (light) to Level 5 (severe). Inspectors visually compare the production radiograph to the reference at the contractually-specified maximum severity level. ASTM E155 covers aluminum castings; E186 covers heavy-walled (4.5"-12") steel castings; E280 covers very heavy (> 12") steel castings; E446 covers 0-2" steel castings; E2422 covers aluminum die castings. Aerospace casting prime specs (e.g. Boeing BAC 5301, RR RPS 700) typically specify Level 1 or Level 2 maximum severity for grade-A safety-critical components — driving 100 % RT inspection and routine reject rates of 5-15 % for first-article castings.

RT Field Case Study: Refinery Crude Tower Repair

A 28-foot diameter crude distillation tower on a Gulf Coast refinery exhibited a chloride-stress-corrosion-cracking through-wall leak on a 38 mm-thick longitudinal seam at elevation 65 ft during a routine turnaround. The repair was an in-situ weld build-up followed by 100 % radiographic inspection of the repair weld and 1 meter on each side. The radiography crew used a 90 Ci Ir-192 source with a single-wall external technique, shielded with 100 mm Pb collimators to limit area-evacuation to a 60 m boundary (active refinery, adjacent units online). DR was used in lieu of film to cut turnaround time by 40 %, with all 24 shots imaged, reviewed by Level II onsite and approved by Level III remotely via the digital archive within the same 8-hour shift. Total cost: $42,000 for 3 days of work; alternative film workflow would have added 36 hours of darkroom processing and an estimated $180,000 of incremental deferred-production cost. This is a typical 2026 ROI of moving from film to DR for in-service refinery work.

Frequently Asked Questions about Radiographic Testing

Q1: What is the difference between X-ray and gamma ray radiography?

A: X-rays are generated electrically — turning the machine off stops the radiation. Gamma rays come from radioactive isotopes (Ir-192, Co-60, Se-75) that emit continuously and must be shielded. X-rays give better contrast and tunable energy; gamma sources are smaller, battery-free and used in confined-access fieldwork.

Q2: When do I use Ir-192 vs Se-75 vs Co-60?

A: Se-75 for 5-30 mm steel (best contrast on thin walls). Ir-192 for 20-75 mm steel (general workhorse). Co-60 for 50-200 mm steel (heavy castings, thick pipe). High-energy X-ray (linac) for > 150 mm. Choose by half-value layer thickness: HVL Se ≈ 13 mm Pb, Ir ≈ 16 mm Pb, Co ≈ 22 mm Pb.

Q3: How do I calculate geometric unsharpness Ug?

A: U_g = F · t / d, where F is source focal spot (typical 2-4 mm for Ir-192), t is object-to-film distance, and d is source-to-object distance. Keep U_g ≤ 0.5 mm below 50 mm wall and ≤ 0.76 mm at 100 mm wall per ASME V T-274.

Q4: What density range is acceptable on industrial film?

A: 2.0 - 4.0 transmitted optical density per ASME V T-282 in the area of interest, when read on a calibrated viewer with luminance ≥ 20,000 cd/m². For composite-density shots (DWDI on small pipe) up to 4.5 is allowed by some codes.

Q5: Where do I place the IQI?

A: Source-side of the part at the area of interest. If source-side is impossible (e.g. inside a closed vessel), film-side with a lead letter "F" placed on the cassette per ASME V T-277.2.

Q6: What is the safe distance from an Ir-192 source?

A: Depends on activity. For a 100 Ci Ir-192 source unshielded, the dose rate is ≈ 50 mSv/h at 1 m. Inverse-square law gives the high-radiation boundary (> 0.02 mSv/h) at ≈ 50 m and the restricted-area boundary (> 0.0025 mSv/h) at ≈ 140 m. Use survey meter to verify.

Q7: Can radiography detect cracks?

A: Yes — if the crack is favorably oriented to the beam (within 5-10° of being parallel to the beam direction). Tight, transverse cracks perpendicular to the beam may be missed. UT or PAUT is more reliable for crack detection in thick sections.

Q8: What is the ALARA principle?

A: As Low As Reasonably Achievable — the regulatory mandate that radiation dose shall be minimized below the legal limit. ALARA drives the time-distance-shielding triangle: spend less time, increase distance, increase shielding.

Q9: How do I qualify an RT procedure to ASME V?

A: Write the procedure (source, kV/Ci, SFD, film/detector class, IQI, density, technique). Have a Level III approve. Demonstrate on a representative test piece that the required IQI sensitivity is achieved and image quality meets Article 2. Keep the procedure under change control.

Q10: Is computed radiography (CR) accepted by ASME?

A: Yes — ASME V Article 2 Mandatory Appendix III covers CR. Image must meet SNR-normalized requirements equivalent to film density 2.0 - 4.0.

Q11: How long must I retain radiographs?

A: Per ASME V T-292, retain for the life of the equipment or per jurisdictional requirements — typically 5 years minimum for piping/vessels, 10 years for nuclear ISI, permanently for some primary nuclear components. Digital DICONDE archives are now standard.

Q12: What is back-scatter and how do I detect it?

A: Radiation scattered from objects behind the film back to the cassette, lowering image contrast. Test by placing a lead letter "B" (12 mm tall) on the back of the cassette. If the "B" image appears lighter than the surroundings on the developed film, back-scatter is present and unacceptable.

Q13: Can I radiograph stainless steel and aluminum?

A: Yes — adjust energy. Aluminum (low Z, low μ) needs lower energy (Se-75 or 120-160 kV X-ray) for adequate contrast. Stainless steel is essentially identical to carbon steel for RT purposes.

Q14: What is the minimum IQI sensitivity required for ASME work?

A: 2 % equivalent penetrameter sensitivity (2-1T or 2-2T per ASME V Table T-276) for most weld work, and 1-2T for critical nuclear and aerospace applications.

Q15: When is radioscopy (real-time radiography) acceptable instead of film/DR?

A: Radioscopy is acceptable per ASME V Article 2 Mandatory Appendix VI when image quality is demonstrated equivalent to film/DR — used heavily in production-line casting inspection and additive manufacturing.

About Atlantis NDT

Atlantis NDT is an ASNT Level III-led inspection technology and services company headquartered in Houston, Texas with engineering operations in Hyderabad, India. We help oil & gas operators, EPCs, fabrication shops, and asset owners modernize their inspection programs across all surface and volumetric NDT methods. Our offerings include the Atlantis NDT Reporting Software, an Odoo 18 ERP pre-configured for NDT operations, and the Atlantis Digital Twins platform that overlays live inspection data on a 3D model of the asset. We also provide ASNT Level III consulting, training and procedure development for ASME Section V, ISO, API and AWS code work. Speak to a Level III directly: +1 (281) 840-8969 or email sales@atlantisndt.com.