CWI NDE Methods Study Guide: VT, PT, MT, RT and UT

Why NDE matters on Part A

A welding inspector rarely runs radiography or ultrasonic equipment. You are still expected to know what each method of nondestructive examination (NDE) can and cannot find. That lets you choose the right method, read a report and spot a method that cannot find the discontinuity in question. Part A is closed book, so you need the principles memorized.

A good way to study NDE is to ask the same four questions for every method:

• What physical principle does it use?
• Which materials can it test?
• Which discontinuities does it find well, and which does it miss?
• What mistakes in technique ruin the result?

Method	Materials	Finds well	Limits
Visual (VT)	Any	Surface discontinuities, size and profile	Surface only; needs proper light and access
Liquid penetrant (PT)	Most nonporous materials, magnetic or not	Discontinuities open to the surface	Nothing below the surface; porous parts mask indications
Magnetic particle (MT)	Ferromagnetic only (carbon steel, low-alloy steel, cast iron)	Surface and slightly subsurface discontinuities lying across the field	Not for austenitic stainless, aluminum or copper; field direction matters
Radiographic (RT)	Most materials	Volumetric discontinuities such as porosity and slag	Planar discontinuities found only when close to the beam direction
Ultrasonic (UT)	Most materials	Planar discontinuities facing the beam; depth and thickness	Coarse-grained austenitic welds, thin sections, rough surfaces
Eddy current (ET)	Electrically conductive materials	Surface and near-surface discontinuities; material sorting	Shallow depth only

Visual examination (VT)

Visual examination is the most widely used and lowest-cost method. Done before, during and after welding, it catches problems when they are still cheap to fix. The weld examination guide covers what to check at each stage. Codes and standards set minimum viewing conditions. ASME Section V, Article 9, for example, calls for the eye within about 24 in. (600 mm) of the surface. The viewing angle must be at least 30 degrees, with at least 100 fc of light. Mirrors, magnifiers and borescopes extend what the eye can reach.

Liquid penetrant testing (PT)

PT finds only discontinuities that are open to the surface. It works on most nonporous materials, magnetic or not. That makes it the usual choice for surface cracks in aluminum and austenitic stainless steel. Porous materials hold penetrant everywhere and mask indications.

The sequence is fixed, and exam questions often scramble it:

1. Preclean and dry the surface.
2. Apply penetrant and allow the dwell time.
3. Remove the excess penetrant.
4. Apply a thin, even coat of developer.
5. Inspect, then postclean.

Removing excess penetrant takes the most care. With solvent-removable penetrant, wipe with dry cloths, then a cloth lightly dampened with solvent. Spraying solvent straight onto the part can flush penetrant out of shallow discontinuities, and the indications disappear. Visible (red dye) penetrant is read in good white light. Fluorescent penetrant is read under UV-A ("black") light in a darkened area and is generally the more sensitive of the two.

Magnetic particle testing (MT)

MT works only on ferromagnetic materials such as carbon steel, low-alloy steel and cast iron. It cannot be used on austenitic stainless steel, aluminum or copper. It finds surface discontinuities and some that are slightly below the surface.

Field direction matters. A discontinuity shows best when it lies perpendicular to the magnetic field. It then forces flux to leak out of the surface and attract particles. A crack running parallel to the field may not show at all. That is why each area is examined twice, with the field turned about 90 degrees.

• Yoke: the field runs from leg to leg. With the legs on either side of the weld, the field crosses the weld and longitudinal cracks are found.
• Prods: current passes through the part, so poor contact can cause arc burns, which may harden or crack the surface.
• Current type: AC keeps the field near the surface and suits surface cracks. DC or half-wave rectified current reaches a little deeper for slightly subsurface discontinuities.

Radiographic testing (RT)

RT records how much radiation passes through the part. Less material in the path means more radiation reaches the film, which makes that area darker. Read every radiograph question with that rule:

• Porosity and slag show as dark spots or shapes.
• Incomplete joint penetration shows as a straight dark line along the center of the weld.
• Undercut shows as dark lines along the weld edges.
• Tungsten inclusions show as light spots, because tungsten is denser than steel.

RT is strongest on volumetric discontinuities such as porosity and slag. Planar discontinuities such as cracks and lack of fusion are found only when they lie close to the beam direction. Laminations parallel to the plate surface are a poor match for RT shot through the thickness.

An image quality indicator (IQI), hole-type or wire-type, proves that the technique is sensitive enough. It is not a ruler for sizing discontinuities.

Radiation safety rests on time, distance and shielding. Intensity follows the inverse square law: double the distance and the dose rate drops to one quarter. A dose rate of 40 mR/h at 10 ft becomes 10 mR/h at 20 ft.

Ultrasonic testing (UT)

UT sends high-frequency sound into the part and watches for reflections. A couplant (liquid or gel) is needed because air between the transducer and the part reflects almost all the sound.

• Straight beam (longitudinal waves) is used for laminations and thickness measurement.
• Angle beam (shear waves at angles such as 45, 60 or 70 degrees) is used for welds. The beam can strike groove faces nearly head-on, where lack of fusion lies.

On an A-scan display, the horizontal axis is time of flight, calibrated as distance, and the vertical axis is signal amplitude. UT is good at locating depth and at finding planar discontinuities facing the beam. It struggles with coarse-grained austenitic weld metal, thin sections, rough surfaces and complex shapes, and it needs a skilled technician.

Other methods

• Eddy current (ET): needs an electrically conductive material, magnetic or not. It finds surface and near-surface discontinuities and can sort materials or measure coating thickness.
• Leak testing (LT): ranges from simple bubble tests and pressure decay to helium mass spectrometer testing, which is generally the most sensitive.
• Acoustic emission (AET): sensors listen for stress waves released by active, growing discontinuities. The part must be under load, such as during a pressure test. It locates active flaws but does not size them precisely.

Matching methods to problems

Many exam items give a situation and ask for the best method. These pairings come up often:

• Surface cracks on aluminum or austenitic stainless steel: PT.
• Surface or near-surface cracks on carbon steel: MT (PT also works).
• Porosity and slag inside a butt weld: RT.
• Laminations in plate before welding: straight-beam UT.
• Sidewall lack of fusion in a thick groove weld: angle-beam UT.
• Leak tightness of a vessel: leak testing.

Mark J. Curry's tip: when a question names a material, check first whether the method can even be used on it. That alone often removes two choices.

Common traps

• Thinking PT can find anything below the surface.
• Forgetting that MT needs a ferromagnetic part.
• Mixing up light and dark on a radiograph.
• Treating the IQI as a sizing tool.
• Assuming RT finds every crack regardless of orientation.

How NDE methods are called out on drawings is covered in the welding symbols guide.

Work through the free NDE practice questions. To see which Part A topic to study first, take the free CWI diagnostic test.

Sources

The Test Methods: NDE questions cite these documents, by document and, where we can confirm it, chapter or section. We never reproduce them.

1. AWS B1.10 (22 questions)
2. ASME BPVC Section V (6 questions)
3. AWS D1.1 (1 question)

Found a mistake? Use "Report a problem" under any question, and Mark J. Curry checks it.

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