4.3 Magnetic Particle Testing (MT)

Magnetic Particle Testing: Surface and Near-Surface Flaws

Magnetic Particle Testing (MT or MPI) detects surface and slightly subsurface (near-surface) discontinuities in ferromagnetic materials only — carbon steel, low-alloy steel, and ferritic/martensitic stainless (e.g., 400-series). It does not work on austenitic (300-series) stainless steel, aluminum, copper, titanium, or any non-magnetic material, because those cannot carry the magnetic field MT depends on. This material restriction is the single most-tested fact about MT.

MT's advantage over PT is depth reach: because it works through magnetic flux leakage rather than capillary bleed-out, it can reveal discontinuities lying just beneath the surface that PT — limited to surface-breaking flaws — would miss entirely. For ferromagnetic weldments, MT is therefore often the preferred surface/near-surface method.

How MT Works

1. A magnetic field is induced in the part.
2. A discontinuity perpendicular to the field interrupts the flux, forcing some field lines out of the part — a flux leakage field at the surface.
3. Finely divided ferromagnetic particles (iron powder) applied to the surface are attracted to the leakage field.
4. Particles cluster at the leakage site, forming a visible indication that outlines the flaw.

Field Direction and the 90° Rule

MT only reveals discontinuities oriented roughly perpendicular (within ~45°) to the magnetic field. A flaw lying parallel to the field produces little or no leakage and can be missed. Therefore two inspections, with the fields about 90° apart, are required for full coverage:

The portable AC yoke is the field workhorse for weld inspection: rotating it 90° between passes satisfies the two-direction rule on a single weld.

Media, Demagnetization, and Trade-offs

Particle Media

Wet fluorescent particles under UV-A give the highest sensitivity and are favored for critical shop work; dry powder suits field conditions and rough surfaces.

Current Type

• AC concentrates field near the surface (skin effect) — best for surface flaws and mobility.
• DC / half-wave DC penetrates deeper — better for near-surface flaws.
• Continuous (field applied while particles are wet) vs. residual (relying on retained magnetism) techniques affect sensitivity; continuous is more sensitive.

Demagnetization

After MT, parts are usually demagnetized, because residual magnetism can:

• Cause arc blow during later welding
• Attract chips during machining and hold abrasive debris
• Disturb instruments and compasses (a real issue in shipbuilding)

Advantages and Limitations

Exam trap: a prod-method arc burn is itself a rejectable surface condition the CWI must watch for. And remember the mirror-image of the PT/MT trade: MT beats PT on near-surface detection in steel, but PT beats MT on material range (aluminum, austenitic stainless).

Field Coverage, Indications, and Procedure Checks

The practical heart of MT is coverage. Because a flaw is revealed only when it lies roughly perpendicular to the field, an inspector running an AC yoke along a weld first orients the poles across the weld to find longitudinal (along-weld) cracks, then rotates the yoke 90° to place the poles along the weld and catch transverse cracks. Overlapping yoke placements ensure no length goes uninspected. The lifting (pull) test verifies a yoke is strong enough: an AC yoke is typically expected to lift about 10 lb at its maximum pole spacing, and a DC yoke about 40 lb, confirming adequate field strength before inspection begins.

The inspector distinguishes true (relevant) indications — particle build-up tracing a real flaw — from non-relevant indications caused by geometry, such as a sharp section change or a dimensional feature that concentrates flux without any crack present. False indications arise from loose particles, magnetic writing, or surface roughness. As with every NDE method, the CWI confirms the technique matches the written procedure (field method, current type, particle medium, and lighting), reads wet-fluorescent work under adequate UV-A intensity, and records each relevant indication.

The recorded indication is then judged a defect only if it exceeds the governing code's acceptance criteria.