§3.2B · Under NDE methods
Radiography or ultrasonics: which finds what on the CWI
Radiography (RT) and ultrasonics (UT) both look inside a weld, but they don't see the same shapes. RT is a shadow picture, so it likes a flaw with some body to it. UT bounces sound off faces, so it likes thin, flat flaws and can tell you where they sit in the wall.
- Exam
- CWI
- Clause
- §3.2B
- Tags here
- 8
- Parent
- NDE methods
3.2B.1Flaw shape decides it
3.2B.2Why shape matters more than size
RT is a shadow picture. Radiation goes through the weld and hits film or a digital plate. Where there's less metal in the path, more gets through and the image goes darker. A gas pore or a slag pocket takes away thickness, so it shows. Porosity is the textbook RT find.
A tight crack or a lack-of-fusion plane is a different animal. It's thin in one direction. If the beam crosses it side-on, the thickness change is close to nothing, and the image stays quiet. RT finds it only when the beam runs down its plane.
UT works the other way round. A probe sends sound into the steel and listens for what bounces back. A flat face square to the beam sends most of the sound straight back, so planar flaws ring loud. A round pore scatters sound in all directions and returns a weak echo. The return time gives you depth, and moving the probe gives you length.
The job-site differences
- Access. RT usually needs the source on one side and the film on the other. UT works from one surface.
- Thickness. RT exposure time climbs with thickness. UT handles heavy sections well but gets awkward on very thin material.
- Record. A radiograph is a picture anyone can re-read later. Manual UT leaves a written report unless the scan is encoded.
- Safety. RT means radiation, an exclusion zone and licensed operators. UT needs couplant and a trained technician.
Neither method names the flaw for you. A radiograph shows shape and length, and the reader infers the type from them. A UT technician infers it from echo height, shape and how the signal moves as the probe moves. Both take trained judgment, which is why NDE reports carry the technician's level and procedure.
The root is where the two methods overlap most. Incomplete joint penetration sits right in the beam path for RT, and it gives UT a clean corner to bounce off. Visual and the two surface methods round out the NDE methods clause.
3.2B.3Film or sound?
Name the flaw's shape before you name the method. The surface methods are covered on MT vs PT if a tag drifts that way.
0 of 8 tagged · 0 accepted
Tag 01
What is the primary advantage of ultrasonic testing over radiographic testing?
Remarks on every option
- A Correct: UT uses sound, not ionizing radiation. No exclusion zone and no radiation safety program.
- B Equipment cost isn't the defining advantage, and costs vary widely either way.
- C Speed depends on the job. Either method can be faster.
- D UT isn't a surface method. Its strength is internal, especially planar flaws.
Pick an option. The remarks on all 4 open here.
Tag 02
Which NDE method is best suited for detecting internal defects in thick metal sections?
Remarks on every option
- A VT only sees the surface.
- B PT only finds discontinuities open to the surface.
- C MT reaches just below the surface, not into thick sections.
- D Correct: RT images the full thickness and shows internal volumetric discontinuities like porosity and slag. Of these choices, it's the one for internal defects.
Pick an option. The remarks on all 4 open here.
Tag 03
What is the primary limitation of radiographic testing for crack detection?
Remarks on every option
- A Correct: RT sees cracks only when they line up with the beam. A tight crack at the wrong angle can disappear on the image.
- B Exposure time is a productivity issue, not a detection limit for cracks.
- C Density is accounted for when setting the exposure. It isn't the key limitation for crack detection.
- D Film quality affects every indication, but orientation is what hides cracks from RT.
Pick an option. The remarks on all 4 open here.
Tag 04
Which NDE method is most suitable for detecting laminations in steel plate?
Remarks on every option
- A Laminations are inside the plate. PT only sees them if they break the edge.
- B VT only sees them where they show at a cut edge.
- C MT might catch them where they reach the surface, but not inside the plate.
- D Correct: straight-beam UT finds laminations as planar reflectors parallel to the plate surface.
Pick an option. The remarks on all 4 open here.
Tag 05
Which NDE method is most effective for detecting lack of fusion in thick-walled pressure vessels?
Remarks on every option
- A Correct: lack of fusion is planar, and UT detects planar flaws well. It also gives depth through thick walls.
- B VT only sees the surface. Lack of fusion is usually buried.
- C MT only reaches near the surface, not through a thick wall.
- D PT only finds flaws open to the surface.
Pick an option. The remarks on all 4 open here.
Tag 06
Which NDE method is most effective for detecting subsurface corrosion in non-magnetic materials?
Remarks on every option
- A PT only shows flaws open to the surface, not hidden corrosion.
- B VT can't see corrosion on the far side or inside.
- C Correct: UT thickness readings find hidden wall loss from the accessible side, on any material that carries sound.
- D MT doesn't work on nonmagnetic materials.
Pick an option. The remarks on all 4 open here.
Tag 07
Butt welds on a job have two NDE concerns: gas porosity from a drafty site, and lack of sidewall fusion in a narrow bevel. Which assignment plays to each method's strength?
Remarks on every option
- A Correct: RT images volumetric discontinuities like porosity well. UT is strongest on planar ones like lack of fusion, and it also gives their depth.
- B This swaps the strengths. Round pores return weak echoes, and a thin, planar lack of fusion can be missed on film unless the beam lines up with it.
- C RT depends on orientation. A planar discontinuity that isn't lined up with the beam barely changes film density and can go unseen.
- D UT response depends heavily on shape and orientation. A flat reflector facing the beam echoes strongly; a round pore echoes weakly.
Pick an option. The remarks on all 4 open here.
Tag 08
On a radiograph of a pipe weld with a GTAW root, you see a small, sharply defined spot that is lighter than the weld metal around it. What's the most likely cause?
Remarks on every option
- A A gas pore is less dense than steel, so it images dark on film, not light.
- B Slag is less dense than weld metal and images dark. A GTAW root doesn't make slag in the first place.
- C Correct: Tungsten is denser than steel, so a fragment from the electrode absorbs more radiation and shows as a light, sharp spot.
- D Incomplete root penetration is missing metal, so it images as a dark line along the root, not a light spot.
Pick an option. The remarks on all 4 open here.
3.2B.4RT and UT on real jobs
Can I swap UT in when the job calls for RT?
Not on your own say-so. The contract documents and the code set the method, and an alternate goes to the Engineer for approval. The inspector duties clause covers who decides what.
Why does a CWI need to know RT and UT at all?
NDE Fundamentals is its own Part A domain in B5.1:2025 (AWS B5.1:2025, checked October 2026). In practice you review NDE reports and decide which method fits the flaw you suspect.
Will a radiograph show undercut?
Often, as a dark line along the edge of the weld image. You still judge undercut by visual examination and a gauge, because depth is what the acceptance criteria care about and one exposure can't give you depth.
What does phased array add to UT?
Phased array (PAUT) steers and focuses the beam electronically and can store an encoded scan, so the data can be reviewed later. D1.1:2020 added an annex for it, and the 2025 edition keeps one.
Ten questions on a break
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