Radiographic testing uses penetrating radiation (X-ray or gamma ray) to examine the full cross-section of a weld. RT is a volumetric method — the symbol is typically centered on the reference line because the examination penetrates through the entire joint thickness. D1.1:2025 requires RT or UT for CJP groove welds when specified by the contract documents per Clause 8.15.
RT Symbol — Centered on Reference Line
Centered on ref line — no side preference
Combined — weld then inspect
Method
How Radiographic Testing Works
RT directs penetrating radiation (X-ray tube or gamma-ray isotope such as Ir-192 or Co-60) through the weld. The radiation passes through the weld metal and exposes film or a digital detector on the opposite side. Internal discontinuities — porosity, slag inclusions, incomplete fusion, cracks — appear as density variations on the resulting radiograph.
Because the radiation penetrates the full joint thickness, RT is classified as a volumetric method. The side from which the source is placed typically does not affect the examination result. Per A2.4 §17.5.5, the RT letter designation is centered on the reference line when there is no side preference.
D1.1:2025
When D1.1 Requires RT
CJP groove welds in tension (Clause 8.15): The extent of RT or UT is specified in the contract documents. For statically loaded structures, CJP groove welds in tension splices and connections are commonly specified for volumetric examination.
Cyclically loaded structures: All CJP groove welds require RT or UT, plus VT per Clause 8.9.
Procedure qualification (Clause 6): RT may be required to verify procedure qualification test plates.
RT is not typically required for fillet welds, PJP groove welds, or plug/slot welds. The engineer of record specifies RT on the drawing using the NDE symbol when full volumetric examination is needed.
RT vs UT choice: RT produces a permanent film record (archivable) but requires radiation safety precautions and area clearing. UT is faster, portable, and has no radiation hazard, but requires a skilled technician. D1.1 allows either per Clause 8.15 — the engineer selects based on joint geometry, access, and project requirements.
A2.4 §17.9.3
Radiation Direction Symbol
When the direction of penetrating radiation must be specified, A2.4 provides the radiation direction supplementary symbol. It is drawn at the required angle on the drawing, with the angle indicated in degrees. This is used when the radiation source position affects the quality of the radiograph — for example, specifying a 45-degree source angle on a T-joint to avoid geometric unsharpness.
CWI Exam Tip: RT and UT are both volumetric methods with centered symbol placement. The key difference to remember: RT produces a permanent image (film or digital), UT produces a real-time display requiring interpretation by the technician. Both detect internal discontinuities that surface methods (MT, PT) cannot find.
Common Questions
RT Symbol FAQ
What does RT mean on a welding drawing?
RT on a welding drawing stands for radiographic testing, a volumetric nondestructive examination method defined in AWS A2.4:2020 Table A6. When you see RT on the reference line of an NDE symbol, it means the weld must be examined using X-ray or gamma-ray radiation. The resulting radiograph reveals internal discontinuities such as porosity, slag inclusions, incomplete fusion, and cracks within the weld cross-section. Because RT penetrates the full thickness of the joint, the symbol is typically centered on the reference line per A2.4 section 17.5.5, indicating no preference for which side the examination is performed from.
Why is the RT symbol centered on the reference line?
The RT symbol is typically centered on the reference line because radiographic testing is a volumetric method. The radiation source penetrates through the full joint thickness to expose the film or digital detector on the opposite side. Because the examination covers the entire weld cross-section regardless of which side the source is placed on, there is typically no arrow-side or other-side significance. Per AWS A2.4:2020 section 17.5.5, when the letter designation has no side significance or there is no preference from which side the examination is to be made, the letter shall be centered on the reference line. However, when the radiation direction matters, the radiation direction supplementary symbol may be used to specify the angle per section 17.9.3.
When does D1.1 require radiographic testing?
D1.1:2025 Clause 8 specifies RT requirements based on joint type and loading. The extent of RT or UT examination is specified in the contract documents per Clause 8.15. For cyclically loaded structures, CJP groove welds subject to transverse tension commonly require volumetric examination. RT is also required for qualification of welding procedures under Clause 6 when volumetric examination is specified. The engineer of record specifies RT on the drawing using the NDE symbol when full volumetric examination is needed. RT is not typically required for fillet welds, PJP groove welds, or plug and slot welds. The choice between RT and UT depends on joint geometry, material thickness, and access constraints.
What is the difference between RT and UT symbols?
Both RT (radiographic testing) and UT (ultrasonic testing) are volumetric examination methods, and both symbols are typically centered on the reference line per AWS A2.4 section 17.5.5. The difference is the examination technique: RT uses penetrating radiation (X-ray or gamma ray) to produce an image on film or a digital detector, while UT uses high-frequency sound waves to detect reflections from internal discontinuities. On the drawing, the symbols look identical in placement but use different letter designations. RT produces a permanent film record but requires radiation safety precautions and area clearing. UT is faster, portable, and has no radiation hazard, but requires a skilled technician and does not produce a conventional image record. D1.1 allows either method for CJP groove weld examination per Clause 8.15.