Study ASNT NDT Level II Magnetic Particle Testing by linking every exam topic to a practical decision: which magnetization current and technique to select for a given flaw and geometry, when to use continuous versus residual application, and how to classify, evaluate, and document the indications you find. Work through the two scenarios and the yoke-direction exercise below, then use the readiness checks and the eight-week sequence to pace your review. Administrative details such as eligibility and scheduling are handled by ASNT; see the short note in the FAQ section.
AC versus HWDC: matching current type to flaw depth
Alternating current concentrates the magnetic field at the surface and is the choice for surface-opening discontinuities; half-wave direct current penetrates deeper and suits near-subsurface discontinuities. Selecting current type is therefore a depth decision, not a preference decision.
AC's skin effect keeps flux density high at the surface, which is exactly where a fatigue crack or a tight seam sits. Dry powder or wet fluorescent particles on an AC-magnetized part respond strongly to sharp, shallow surface breaks. When a procedure specifies AC for a weld inspection, it is usually because the acceptance criteria concern surface cracking, and mobile AC yokes are also the practical field option.
HWDC pulses charge the part with unidirectional current, driving flux deeper below the surface. Inclusions, subsurface slag, and voids a few millimeters under the surface of a casting or weld become detectable where AC would show nothing. The trade-off is reduced sensitivity to very tight surface cracks, because surface flux density is lower than with AC. A procedure that permits both currents is asking you to choose based on the discontinuity type in the acceptance criteria, and reasoning that way is exactly what to practice for scenario-style questions.
- AC: best for surface-opening cracks, tight seams, and field yoke work
- HWDC: best for near-subsurface inclusions and voids, at reduced surface sensitivity
- DC (full-wave): deeper penetration still, common in bench units on production parts
- The procedure, not habit, is the authority: read which current it requires and why
Field direction errors: why one magnetization pass can miss a crack
A discontinuity is detected best when its long axis lies perpendicular to the magnetic field. Parallel to the field, the same crack produces almost no indication. Inspections therefore need two field directions, so practice predicting which orientations a single setup can miss.
Scenario 1: You are examining a long welded plate and set up a coil so the part sits lengthwise in the coil, producing a longitudinal (lengthwise) field in the plate. A transverse crack across the weld, running crosswise, sits perpendicular to the flux and shows sharply. A longitudinal toe crack running along the weld, parallel to the field, shows weakly or not at all. The plausible mistake is concluding 'no relevant indications' from that single shot.
The better decision is a second, perpendicular field: circular magnetization around the part (for example by passing current through the part or a central conductor) so flux runs crosswise and longitudinal cracks cut across it. Why it matters: one direction of magnetization inspects for one family of orientations. When you study coils, prods, and central conductors, make the core exercise predicting which crack orientations each setup can and cannot reveal: sketch the field direction, then mentally rotate the assumed crack until the geometry is automatic.
Continuous versus residual method: retentivity decides, not habit
The continuous method applies particles while the magnetizing force is on, and it works on any steel. The residual method applies particles after the field is removed and only works on steels with high magnetic retentivity. Choose residual only when the material holds residual magnetism strongly.
Continuous inspection gives the strongest indications because full applied-field flux is present while particles migrate. It is the default for low-carbon and medium-carbon steels, for large parts where residual flux would be weak and non-uniform, and for complex shapes where flux distribution after de-energizing is unpredictable. Wet fluorescent horizontal bench work is nearly always continuous for these reasons.
Residual inspection is legitimate when the alloy has high retentivity and the geometry allows uniform residual flux: a finished, heat-treated, highly magnetizable part is the textbook case. Its advantage is that multiple particle applications, or inspection at a station away from the magnetizing unit, become possible. The plausible exam mistake is applying residual logic to a low-retentivity part and expecting indications that never form. The decision rule is material first, then geometry, then method. If the question gives you a low-carbon structural steel, residual is the wrong answer regardless of convenience.
Yoke, prods, central conductor, and coil: choosing the technique
Each technique creates circular or longitudinal fields in specific geometries. Yokes suit local weld-area work without electrical contact; prods give direct-contact circular field; central conductors inspect bore surfaces; coils induce longitudinal fields in long parts.
Direct contact techniques (prods, head-shot clamps) put current through the part and risk arc burns at the contacts, which is why procedures often restrict prod use or require contact pads and clean surfaces. Indirect techniques (yoke, central conductor, coil) magnetize without part contact and avoid burn damage entirely. A question giving you a finished machined part with a bore is steering you toward the central conductor; a question about a butt weld on plate is steering you toward yoke or prods.
Field direction follows from the technique: current through the part or a central conductor produces a circular field around that current, which reveals longitudinal discontinuities. Coils and yoke legs produce a field along the part or between the poles, revealing transverse discontinuities. Use the table to compare the options, then practice naming both the field type and the detectable crack orientation for each technique before moving on.
| Technique | Field produced | Typical use | Key caution |
|---|---|---|---|
| AC yoke | Longitudinal between poles | Surface cracks in welds and local areas, field work | Limited pole spacing; verify lifting power per the applicable standard |
| Prods | Circular around current path | Long discontinuities in large weldments | Direct contact; arc-burn risk on finished surfaces |
| Central conductor | Circular around conductor | Inside surfaces of tubes, rings, bores | Indirect; conductor must be centered for uniform flux |
| Coil | Longitudinal along part axis | Transverse discontinuities in shafts, bars | Long parts need multiple shots; fill factor and L/D ratio matter |
| Head shot (direct) | Circular through the part | Long discontinuities in bench parts | End-face contact damage possible; requires adequate contact |
Reading the indication: relevant, nonrelevant, and false
An indication is a particle accumulation; a discontinuity is the actual flaw. Classify each indication as relevant (from a real discontinuity), nonrelevant (from a harmless geometric or material feature), or false (from surface contamination or external fields) before any accept/reject call.
Nonrelevant indications come from features that leak field but are not flaws: sharp fillets and threads, keyways, changes in section, brazed or mechanical joints, and local hardness variations. False indications come from the surface condition or handling: particles clinging to a wet or oily surface, rough scale, residual magnetism in the part or holding fixtures attracting particles before inspection, and heavy cold working of the surface.
The verification step distinguishes them. Wipe the area and re-inspect: false indications usually do not reappear in the same pattern. Slightly demagnetize and re-inspect: nonrelevant indications from residual fields or geometry often fade or shift while a real discontinuity remains sharply defined. Only after an indication survives verification do you evaluate it against the acceptance criteria. In scenario answers, state the verification action explicitly; jumping straight to accept/reject skips the classification reasoning the question is testing.
Verifying the field: yoke direction exercise with expected observations
Field adequacy and direction are verified with artificial flaw devices such as a pie-shaped field indicator (pie gauge) or QQI shims placed on the part. This exercise trains you to read them and to spot the direction error before it costs you a missed discontinuity.
Exercise: Place a pie gauge on a scrap plate and magnetize it with an AC yoke, noting the pole orientation. A pie gauge shows indications as lines across its sectors where the field crosses them; the sectors that display tell you the field direction between the poles. Rotate the yoke 90 degrees and repeat. Expected observation: a different set of sectors lights up each time, demonstrating that the field direction changed with the yoke orientation. A sector that shows with neither orientation marks a direction gap you would need a third shot to cover.
Self-check rubric: score one point each for (1) correctly predicting which sectors display before energizing, (2) explaining why a crack parallel to the flux is weakly indicated, (3) naming a second setup that covers the missed orientation, and (4) stating that the gauge verifies direction and adequacy, not flaw size. Four of four means the direction concept is exam-ready. Note that specific devices and acceptance of their readings come from the standard referenced in your procedure; treat the numbers here as a learning example.
After the indications: demagnetization, post-cleaning, and records
Level II MT ends with three decisions: whether demagnetization is required, how the part is cleaned, and how results are recorded so a qualified reader can reconstruct the inspection. Each decision is procedure-driven, and the documentation must identify technique, extent, and disposition.
Residual magnetism can interfere with downstream machining (holding chips), nearby instruments, and later welding, so procedures specify demagnetization where needed, followed by verification that the field is acceptably low. Post-cleaning removes dried particles and any residue before the part returns to service or to a subsequent process. When a question pairs demagnetization with a service condition, reason from the consequence: what does the retained field do next?
Documentation at Level II means the report identifies the part, the technique and current used, the field direction(s) applied, particle type and application, indications found with their locations, relevant versus nonrelevant classification, and final disposition against the criteria. A reader with no access to you should be able to picture the inspection from the record. Practice rewriting a vague report entry ('indications found, part rejected') into that structure; the difference between the two versions is the level of completeness to aim for in written answers.
References and further reading
Use these references to explore the concepts and check the latest information from the relevant organizations.