9.1 Magnetic Particle Testing

9.1 Magnetic Particle Testing

Quick Answer: Magnetic particle testing (MT) finds surface and near-surface discontinuities in ferromagnetic materials by inducing a magnetic field so that flux leaks at cracks and attracts fine magnetic particles. It cannot inspect nonmagnetic alloys. Sensitivity depends on field direction, particle system (wet/dry, fluorescent/visible), surface condition, coating thickness, and control of nonrelevant leakage.

MT is a high-throughput surface method on the ASNT NDT Level III Basic outline. You are not writing a full method procedure here, but you must know when MT is valid, how magnetization direction relates to crack orientation, what equipment families do, and how false or nonrelevant indications arise. That Level III framing—method selection, process application, and relevant vs nonrelevant judgment—is what Basic items test.

Material Scope: Ferromagnetic Only

MT requires the test object (or a ferromagnetic coating thick enough to carry flux in specialized cases) to be ferromagnetic—able to support a strong magnetic flux density (high relative permeability). Typical MT candidates:

• Carbon and low-alloy steels
• Many tool steels and martensitic stainless steels
• Some cast irons and ferritic stainless grades
• Nickel alloys that are ferromagnetic in the condition used

Materials that are not MT candidates in normal practice:

• Austenitic stainless steels (e.g., 304/316) in the fully austenitic condition
• Aluminum, copper, brass, titanium, and most nonferrous alloys
• Many high-alloy nonmagnetic overlays and cladding

Exam trap: “Stainless steel” is not automatically nonmagnetic. Martensitic and some ferritic grades are ferromagnetic; austenitic grades generally are not. Choose PT, ET, or another method when permeability is too low for practical flux leakage.

Physics in One Paragraph

A magnetized part carries magnetic flux inside the material. A surface or near-surface discontinuity that interrupts or constricts that path forces flux to leave the part briefly—flux leakage. Fine ferromagnetic particles (iron oxide or iron powder, often coated and pigmented or fluorescent) collect in the leakage field and form an indication. Deep subsurface defects may not leak enough flux to the surface; MT is therefore a surface / near-surface method, not a volumetric substitute for UT or RT.

Longitudinal vs Circular Magnetization

Discontinuities produce the strongest leakage when they cut across the flux lines (ideally near 90°). Weak response occurs when a crack is nearly parallel to the field. Industry practice therefore magnetizes in two approximately perpendicular directions unless geometry or code allows a single direction for a known crack population.

Rule of thumb for exams: circular magnetization finds longitudinal cracks; longitudinal magnetization finds transverse cracks. Weld toe cracks can be parallel or transverse to the weld axis—technique must cover the expected orientations.

Equipment Families: Yokes, Prods, Coils, Conductors

Electromagnetic yokes (AC or DC/HWDC) create a field between portable poles. They are widely used for field welds and local inspection. AC yokes favor surface sensitivity and particle mobility; DC or rectified current improves near-surface depth slightly but can make particle mobility worse on rough surfaces.

Prods pass current into the part through contact points, producing circular fields between prods. They are effective for large plates and weld zones but risk arc strikes and burn marks if contact is poor—procedures control spacing, amperage, and contact condition.

Coils (rigid or flexible) produce longitudinal magnetization of bars, shafts, and elongated parts when the part is a core in a current-carrying coil (or a cable wrap). Central conductors and head shot (current through the part between contacts) produce circular magnetization on hollow or solid parts.

Stationary wet horizontal units combine head-shot and coil magnetization with recirculating bath for production volumes. Level III procedure work specifies current type, amperage or field strength verification (e.g., pie gauge, QQI/shim, Hall-effect meter where required), and coverage of both directions.

Wet vs Dry; Fluorescent vs Visible

Fluorescent inspection uses UV-A (black light) in a darkened area with limited ambient white light so indications fluoresce brightly. Visible particles need strong white light and surface contrast (sometimes a thin contrasting paint). Temperature limits apply: vehicles and powders have qualified ranges; hot surfaces may force dry powder or special products.

Process Sequence (Level III View)

1. Surface preparation — remove loose scale, thick paint, grease, and contaminants that block particle mobility or hide leakage. Coatings may be allowed only within thickness/procedure limits.
2. Magnetization — apply field(s) in required directions at specified strength.
3. Particle application — while magnetized (continuous method) or with residual field where residual technique is qualified (usually high-retentivity materials).
4. Interpretation — evaluate linear vs rounded indications, location (toe, root region, base metal), and relevance.
5. Demagnetization (when required) — reduce residual magnetism that could affect machining, instruments, welding, or service.
6. Post-clean and document results per procedure/code.

Continuous method applies particles while the magnetizing current or field is on—preferred for most AC and lower-retentivity work. Residual method relies on retained magnetism after current is removed—limited to materials that hold enough residual field and only when the written procedure allows it.

Relevant vs Nonrelevant Indications

Not every particle pattern is a crack.

Magnetic writing is a classic exam nonrelevant: accidental residual magnetization from stacking, lifting magnets, or prod misuse creates particle collection without a structural discontinuity. Demagnetization (AC decaying field, reversing DC, etc.) reduces residual field below a procedure limit (often verified with a field indicator).

Limitations

• Nonmagnetic materials — no MT.
• Orientation — cracks parallel to flux produce weak or no indications; two directions required for general search.
• Coating thickness — paint, plating, or insulation can attenuate leakage and particle contact; procedures set maximum coating thickness or require stripping.
• Deep subsurface — limited; near-surface only.
• Geometry — complex shapes may need multiple setups; internal surfaces of complex cavities may be inaccessible.
• Electrical safety and burns — prod/current techniques need controls.
• Residual magnetism — can affect subsequent processes if not demagnetized when required.

Process Applications

MT is widely applied to welds (toes, HAZ, crater cracks), forgings and bars (seams, laps, quench cracks), shafts and axles (fatigue cracks at shoulders and keyways), castings (surface cracks, hot tears), and in-service steel structures and pressure equipment where codes allow surface MT in lieu of or with PT. Level III selection asks: is the material ferromagnetic? Is the expected flaw surface or near-surface? Can both field directions and surface access be achieved? If coatings are thick or material is austenitic, pivot to PT, ET, UT, or RT as appropriate.

Exam trap: using MT on aluminum “because it is metal,” or claiming one longitudinal shot finds all weld cracks. Another trap: calling every particle line a reject without checking magnetic writing, geometry leakage, or acceptance criteria.

MT rewards clear physics: ferromagnetic path → field across the crack → controlled particles → interpret leakage vs noise → demagnetize when needed. That chain is the Basic-level story you will map to procedure and method-selection stems.