NDT Magnetic Particle Testing Methods 5 — Questions and Answers
Question 1: After magnetic particle testing, why is demagnetization often required?
- To remove fluorescent particles from the part surface
- To prevent residual magnetism from interfering with machining, gauging, or service performance (Correct answer)
- To reset the equipment for the next inspection
- To improve surface finish before coating
Correct answer: To prevent residual magnetism from interfering with machining, gauging, or service performance
Residual magnetism can attract metal chips during machining, cause erroneous compass or instrument readings, and interfere with arc welding and electronic equipment in service.
Question 2: Which demagnetization method is most effective for large, complex-shaped parts?
- Passing the part through a decreasing AC coil field (Correct answer)
- Repeatedly striking the part with a rubber mallet while heated
- Using a fixed DC yoke on each surface area
- Exposing the part to strong permanent magnets
Correct answer: Passing the part through a decreasing AC coil field
Passing a part through an AC coil while gradually reducing the field (or withdrawing the part from the coil) effectively randomizes magnetic domains across complex geometries.
Question 3: According to ASME Section V and ASTM E1444, what is the minimum gauss (mT) level at which a part is considered acceptably demagnetized?
- Less than 5 gauss (0.5 mT) (Correct answer)
- Less than 3 gauss (0.3 mT)
- Less than 10 gauss (1.0 mT)
- Less than 1 gauss (0.1 mT)
Correct answer: Less than 5 gauss (0.5 mT)
ASME Section V and most industry standards specify that residual field strength must be less than 3 oersteds (approximately 3 gauss or 0.3 mT), with some codes accepting up to 5 gauss unless lower levels are specified.
Question 4: What is the purpose of the pie-shaped (Berthold) gauge used in magnetic particle testing?
- Measuring the UV-A light intensity at the inspection surface
- Verifying adequate field strength and direction at the part surface (Correct answer)
- Checking the particle bath concentration
- Measuring the distance between prods
Correct answer: Verifying adequate field strength and direction at the part surface
The pie-shaped (Berthold) gauge, a circular shim with radiating slots, is placed on the part to verify that the applied magnetic field is sufficient and properly oriented to detect discontinuities.
Question 5: When using alternating current (AC) for magnetic particle testing, what is the primary limitation compared to direct current (DC)?
- AC cannot magnetize ferromagnetic materials
- AC is limited to surface and near-surface detection due to the skin effect (Correct answer)
- AC produces weaker fields than DC at all depths
- AC requires special particles not used with DC
Correct answer: AC is limited to surface and near-surface detection due to the skin effect
The skin effect causes AC to concentrate at the surface of a conductor, limiting its penetrating depth and making it less effective for detecting subsurface discontinuities compared to DC or HWDC.
Question 6: Half-wave DC (HWDC) is preferred over full-wave DC for which specific reason in wet fluorescent magnetic particle testing?
- HWDC generates less heat in the part
- HWDC provides a pulsating field that improves particle mobility and sensitivity (Correct answer)
- HWDC is safer for the operator
- HWDC allows longer inspection times between current applications
Correct answer: HWDC provides a pulsating field that improves particle mobility and sensitivity
The pulsating nature of HWDC creates a slight vibration in the magnetic particles, improving their mobility and migration to discontinuity sites, which enhances sensitivity especially for subsurface flaws.
Question 7: Which condition would most likely cause a false indication in magnetic particle testing?
- A tight fatigue crack on the surface
- Magnetic writing from contact with another magnetized part (Correct answer)
- A cold shut in a casting
- A seam running longitudinally in a bar
Correct answer: Magnetic writing from contact with another magnetized part
Magnetic writing occurs when a magnetized tool or part contacts the test surface, creating localized field distortions that attract particles and produce false indications unrelated to actual discontinuities.
NDT Magnetic Particle Testing Methods 5
After magnetic particle testing, why is demagnetization often required?