In brief
- Volumetric method — radiography images the internal structure of the joint
- Detects best — gas pores, slag and tungsten inclusions, lack of penetration
- Frequently misses — tight cracks and lack of side-wall fusion lying perpendicular to the beam
- Access — source and detector on opposite sides of the wall
- Permanent record — the radiograph remains readable and auditable years later
Radiographic testing (RT) passes ionising radiation through a welded joint and records the transmitted intensity on film or a digital detector. Material that is thinner or less dense — a pore, a slag inclusion, a missing root — absorbs less radiation and appears darker on the radiograph.
It is one of two volumetric methods used routinely on welds, and the only common NDT method that produces a permanent, re-readable image. Its complement is ultrasonic testing, which is superior for the planar defects radiography struggles with.
Radiography does not decide whether a weld passes. It produces an image. Acceptance is judged by a certified inspector against the criteria of the governing standard — a distinction that matters when a client asks why an indication was reported but the joint was accepted.
Radiation sources: X-ray tube or gamma isotope
X-ray tubes — energy adjustable, better contrast, and the radiation stops entirely when the unit is switched off. There is no radioactive source to license, transport under ADR, store securely, or recover if it fails to retract. For a client admitting a contractor to a live plant, that materially simplifies the risk assessment. The trade-offs are a mains power requirement, bulk in confined spaces, and a ceiling on penetrable thickness set by tube voltage.
Gamma isotopes — fully portable, no power needed, and effective on thicker sections. Iridium-192 covers the general range; Selenium-75, being lower in energy, is often preferred where floor space is tight because it requires a materially smaller controlled area; Cobalt-60 handles heavy sections. The cost is fixed energy, a licensed source, and the full apparatus of transport, storage and emergency procedure.
Our capability. Gamma radiography is available for projects in Poland, where we hold the required authorisation for radioactive sources. Outside Poland we currently operate X-ray equipment up to 250 kV. Where a section exceeds the practical range of that equipment, we inspect by PAUT or TOFD instead — which is often the better answer regardless, since ultrasonic techniques handle planar defects that radiography misses.
Film, CR or DDA
- Film — highest spatial resolution, universally accepted, wide latitude for variable thickness. Requires chemical processing, a darkroom and physical archiving
- Computed radiography (CR) — reusable phosphor plates, flexible enough to wrap around pipe. Much wider exposure latitude than film, so fewer retakes
- Digital detector arrays (DDA / DR) — image in seconds, best signal-to-noise, lowest dose. Highest capital cost; rigid panels limit access geometry
CR and DDA both fall under ISO 17636-2 and shorten the loop from hours to minutes — which matters when a welder is standing beside the joint waiting to know whether to grind it out. Lower dose per exposure and fewer repeats also make digital techniques an ALARA measure, not just a convenience.
See digital radiographic testing for our CR and DDA capability.
What RT detects — and how it looks on the radiograph
Reliably detected:
| Discontinuity | Appearance on the radiograph |
|---|
| Gas pores | Dark, rounded spots with smooth edges |
| Clustered / wormhole porosity | Groups of dark spots, or elongated dark channels |
| Slag inclusions | Darker, irregular areas, often slightly elongated along the weld axis |
| Tungsten inclusions | Bright spots — tungsten is denser than steel and absorbs more radiation |
| Lack of penetration | Sharp, dark line at the weld root |
| Root concavity / burn-through | Dark, irregular area at the root |
| Excess penetration | Light area at the root |
| Undercut | Dark line along the weld toe |
Detected only when favourably oriented:
- Cracks — visible as fine dark lines when roughly aligned with the beam
- Lack of side-wall fusion — appears as a narrow, elongated band at the bevel face, but only if the fusion plane is inclined enough to present measurable thickness change to the beam
That last row is the single most important limitation of the method, and it deserves stating plainly.
What radiography misses
RT measures differences in absorbed thickness along the beam path. A planar defect with near-zero opening, lying perpendicular to the beam, presents almost no thickness difference — and produces almost no contrast. This is physics, not a matter of operator skill or equipment quality.
In practice this affects:
- Tight cracks perpendicular to the beam
- Lack of side-wall fusion in bevelled joints, where the fusion plane can be close to vertical
- Laminations in the parent plate, lying parallel to the surface
Where planar defects are the governing concern — thick-section butt welds, fatigue-loaded joints, high-integrity pressure work — ultrasonic testing is the more capable method, and PAUT or TOFD more capable still, with the added advantage of through-wall sizing that radiography cannot provide at all.
The two methods are complementary rather than competing: RT for volumetric defects and a permanent image, UT for planar defects and depth sizing. Many codes require both. An inspection scope specifying one without considering the other is usually incomplete.
Preparing the joint and setting up the exposure
- Visual testing first. VT rules out gross surface defects that would otherwise confuse the radiograph, and is required by most inspection plans as the preceding step.
- Clean the joint — paint (especially lead-bearing primers, which absorb strongly), grease, oil, loose scale and weld spatter. Spatter in particular is routinely misread as an indication.
- Position source and detector on opposite sides of the wall. For small-bore pipe this may mean a double-wall technique (DWDI, elliptical) performed entirely from outside; with internal access, a panoramic exposure covers the full circumference in one shot. Access to the inside of a pipe is not required.
- Place lead identification markers and mark the component physically, so the radiograph can be tied unambiguously to a location. On pipe, a lead measuring tape locates indications around the circumference.
- Place the image quality indicator (IQI, penetrameter) — normally on the source side; where that is impossible it is marked with a lead letter F.
- Expose, process, evaluate.
What governs image quality
Three parameters the operator controls:
Contrast — depends largely on radiation energy. Lower energy gives greater contrast between weld metal and defect, at the cost of longer exposure.
Geometric unsharpness — reduced by keeping the detector close to the component and the source far away: U₉ = d · t / (F − t), where d = focal spot size, t = object-to-detector distance, F = source-to-detector distance.
Graininess — a property of the film system (classified under ISO 11699-1, with processing control under ISO 11699-2) or of the digital detector.
None of this is assumed — it is demonstrated. The IQI on the radiograph must show the wire or hole diameter required by the standard. If the required IQI element is not legible, the radiograph is not valid, whatever it appears to show, and the exposure is repeated with different parameters.
ISO 17636 defines class A (basic) and class B (improved) techniques. Class B demands better geometry and longer exposures and is what most demanding specifications call for.
Interpretation
Evaluation proceeds in a fixed order. The inspector first confirms that markers are correctly placed and the film shows no mechanical damage or processing artefacts, then verifies IQI sensitivity. Only once the radiograph is established as valid does assessment of indications begin — using shape, edge definition and optical density to distinguish ordinary weld face irregularity from a genuine, disqualifying defect.
Evaluation against acceptance criteria requires ISO 9712 Level 2 as a minimum; procedure approval requires Level 3.
When to radiograph
Inspect at the weld-joint stage, not on the finished assembly — but only after any code-required hold time, and after PWHT where PWHT is specified.
Inspecting early is sound economics: a defect found in a fresh, accessible joint is ground out and re-welded in hours; the same defect found after assembly may require dismantling.
But “as soon as welding stops” is wrong for high-strength steels, which are susceptible to hydrogen-induced (delayed) cracking appearing hours or days later. Codes therefore impose an inspection hold time:
- AWS D1.1 — acceptance criteria for ASTM A514, A517 and A709 Gr. 100/100W must be based on NDT performed not less than 48 hours after completion of the weld
- DNV-OS-C401 — final NDT of structural steel welds not before 48 hours after completion, except where PWHT is required
- ABS hull welds — minimum 24 hours up to 400 N/mm² yield, minimum 72 hours above
Where a repair is made, the hold clock restarts on the repair weld. Build the hold time into the schedule as a fixed constraint, then inspect at the earliest point after it.
Radiation safety
Radiography is safe when the exposure is planned and the area controlled — and the risk is managed by time, distance and shielding, not by the absence of the beam alone.
ALARA in practice. Of the three levers, distance is the most powerful: dose rate falls with the square of distance, so doubling the separation reduces it fourfold. Time and shielding then refine what distance cannot achieve.
Scattered radiation is what reaches bystanders, not the primary beam. Standing off the beam line is not protection. This is the single most misunderstood point on a working shop floor. A practical check: a lead letter B placed behind the cassette should not appear on the radiograph — if it does, backscatter is excessive.
Controlled and supervised areas are established from calculated and then measured dose rate at the boundary, demarcated physically, and verified with a survey meter before every exposure. They are not estimated from the beam direction.
Collimation restricts the beam to the required angle and substantially shrinks the exclusion radius — often the difference between stopping a shift and not.
Dosimetry operates on two levels: passive dosimeters (TLD) form the legal record, while electronic personal dosimeters give real-time alarm. A dose rate meter confirms the boundary before, during and after each exposure, and — with isotope work — confirms that the source has returned to its container.
Legal framework. In the EU, dose limits derive from Directive 2013/59/Euratom: 20 mSv/year for classified workers and 1 mSv/year for members of the public, with IAEA GSR Part 3 as the international basis. Equivalent regimes apply in other jurisdictions.
In Poland, work with X-ray equipment and with radioactive sources is supervised by the National Atomic Energy Agency (PAA) and requires an authorisation, a qualified Radiation Protection Officer, individual dose records, and — for isotopes — compliance with ADR transport rules and secure storage.
These requirements distinguish companies genuinely authorised to perform radiography from those merely offering it. It is worth asking a prospective contractor for their authorisation reference.
Standards
| Scope | Standard |
|---|
| RT of welds — film technique | ISO 17636-1 |
| RT of welds — digital techniques (CR and DDA) | ISO 17636-2 |
| Acceptance levels — steel, nickel, titanium | ISO 10675-1 |
| Acceptance levels — aluminium | ISO 10675-2 |
| Image quality indicators | ISO 19232 series |
| Film system classification | ISO 11699-1 |
| Film processing control | ISO 11699-2 |
| Weld quality levels (B / C / D) | ISO 5817 |
| Classification of weld imperfections | ISO 6520-1 |
| Personnel certification | ISO 9712 |
| US practice | ASME BPVC Section V, Article 2 |
| Acceptance — pressure vessels | ASME BPVC Section VIII |
| Structural steel welding | AWS D1.1 |
Frequently asked questions
Can radiography detect cracks in welds?
Only when the crack is favourably oriented relative to the beam. Tight cracks lying perpendicular to the beam are frequently missed. For reliable crack detection use ultrasonic testing — PAUT or TOFD where through-wall sizing is also required.
Does RT require access to both sides of the component?
The source and detector must be on opposite sides of the wall, which is not the same as access to the inside of an object. Small-bore pipe is radiographed entirely from outside using a double-wall technique.
How long after welding can radiography be performed?
Immediately for most carbon steels once cooled. For high-strength quenched-and-tempered grades, codes impose a hold time — 48 hours under AWS D1.1 and DNV-OS-C401, 24 to 72 hours under ABS depending on yield strength. Where PWHT is specified, final RT follows the heat treatment.
Does production have to stop during an exposure?
Only within the controlled area, whose extent is calculated and measured, and which is typically smaller than people expect — particularly with collimation and a lower-energy source. Work outside the boundary continues normally.
What is an image quality indicator and why does it matter?
A reference device of known dimensions placed on the component. If the wire or hole diameter required by the standard is not legible on the radiograph, the required sensitivity was not achieved and the radiograph is invalid — regardless of what it appears to show.
Which standard applies to weld radiography?
ISO 17636-1 for film and ISO 17636-2 for digital techniques in Europe; ASME BPVC Section V Article 2 in North America. Acceptance criteria come separately, from ISO 10675-1 or the governing construction code.
Is X-ray weld testing safe for people on site?
Yes, when the area is properly controlled. The key point often missed is that scattered radiation, not the primary beam, is what reaches bystanders — so the boundary is set from measured dose rate, not from the beam direction.