Ultrasonic testing uses high-frequency sound waves to examine the full cross-section of a weld. UT is a volumetric method — the symbol is typically centered on the reference line because the examination penetrates the entire joint thickness. D1.1:2025 Clause 8.15 allows UT as an alternative to RT for CJP groove welds. Annex K governs UT techniques for structural steel.
UT Symbol — Centered on Reference Line
Centered on ref line — no side preference
Combined — weld then inspect
Method
How Ultrasonic Testing Works
UT directs high-frequency sound waves (typically 2–5 MHz) into the weld using a piezoelectric transducer coupled to the surface. The sound pulse travels through the weld metal, and when it encounters a discontinuity — lack of fusion, cracks, laminations, porosity — part of the energy reflects back to the transducer. The reflected signal is displayed as an amplitude spike on a real-time A-scan screen.
Because the sound waves propagate through the full joint thickness, UT is classified as a volumetric method. The transducer can be placed on either side of the weld to examine the entire cross-section. Per A2.4 §17.5.5, the UT letter designation is centered on the reference line when there is no side preference.
D1.1:2025
When D1.1 Requires UT
CJP groove welds in tension (Clause 8.15): Statically loaded structures require RT or UT for CJP groove welds in tension splices and connections. UT is allowed as an alternative to RT.
Cyclically loaded structures: All CJP groove welds require RT or UT, plus VT per Clause 8.9.
Annex K: D1.1 Clause 8 Annex K governs UT techniques for structural steel, specifying calibration procedures, scanning patterns, and acceptance/rejection criteria for ultrasonic indications.
UT is often preferred over RT for thicker sections, T-joints, and field conditions where radiation safety precautions are impractical. The engineer of record specifies UT on the drawing using the NDE symbol when volumetric examination is needed.
UT advantages over RT: UT is faster, portable, and requires no radiation safety clearance. It detects planar defects (cracks, lack of fusion) that RT may miss when oriented parallel to the radiation beam. However, UT does not produce a permanent film image — the A-scan display requires real-time interpretation by a skilled ASNT Level II or III technician. D1.1 allows either method per Clause 8.15.
Technician Qualification
ASNT Qualification for UT
Unlike RT, which produces a permanent film that can be reviewed by multiple interpreters, UT results depend heavily on the skill of the technician operating the equipment. D1.1 requires UT to be performed by personnel qualified to ASNT Level II or Level III in ultrasonic testing. The technician must calibrate the instrument, select the correct transducer angle and frequency, interpret the A-scan indications in real time, and classify each discontinuity against the acceptance criteria in Annex K.
CWI Exam Tip: UT and RT are both volumetric methods with centered symbol placement. The key difference to remember: UT produces a real-time A-scan display requiring skilled interpretation, RT produces a permanent film image. UT is better at finding planar defects (cracks, lack of fusion) while RT excels at detecting volumetric defects (porosity, slag). Both detect internal discontinuities that surface methods (MT, PT) cannot find.
Common Questions
UT Symbol FAQ
What does UT mean on a welding drawing?
UT on a welding drawing stands for ultrasonic testing, a volumetric nondestructive examination method defined in AWS A2.4:2020 Table A6. When you see UT on the reference line of an NDE symbol, it means the weld must be examined using high-frequency sound waves. The ultrasonic transducer sends sound pulses into the weld metal, and reflections from internal discontinuities such as lack of fusion, cracks, laminations, and porosity are displayed on an A-scan screen for interpretation by an ASNT Level II or III technician. Because UT examines the full joint thickness, 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 UT symbol centered on the reference line?
The UT symbol is typically centered on the reference line because ultrasonic testing is a volumetric method. The sound waves propagate through the full joint thickness, and the transducer can be placed on either side of the weld to examine the entire cross-section. Because the examination covers the full weld volume regardless of transducer placement, 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.
When does D1.1 require ultrasonic testing?
D1.1:2025 Clause 8.15 allows ultrasonic testing as an alternative to radiographic testing for CJP groove welds. For statically loaded structures, CJP groove welds in tension splices and connections require RT or UT. For cyclically loaded structures, all CJP groove welds require RT or UT. D1.1 Clause 8 Annex K governs UT techniques for structural steel, specifying calibration, scanning procedures, and acceptance criteria. UT is often preferred over RT for thicker sections, T-joints, and field conditions where radiation safety is impractical. The engineer of record specifies UT on the drawing using the NDE symbol when volumetric examination is needed.
What is the difference between UT and RT?
Both UT (ultrasonic testing) and RT (radiographic 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: UT uses high-frequency sound waves to detect reflections from internal discontinuities, while RT uses penetrating radiation (X-ray or gamma ray) to produce an image on film or a digital detector. UT is faster, portable, and has no radiation hazard, but requires a skilled ASNT Level II or III technician and does not produce a permanent film image. RT produces a permanent archivable record but requires radiation safety precautions and area clearing. D1.1 allows either method for CJP groove weld examination per Clause 8.15.