Magnetic particle testing (MT): step by step

Magnetic particle testing (MT) is a surface nondestructive examination method for ferromagnetic materials. It reveals surface and near-surface discontinuities through the indication that fine magnetic particles form on a magnetized part. Its uses run from crack detection in steel welds to the examination of castings and forgings. Its main strengths are speed, low cost, and very high sensitivity to surface cracks. Its main limitation is that it works only on ferromagnetic materials.

1. How it works: flux leakage

When you magnetize a ferromagnetic part, magnetic flux travels through it. As long as the flux flows uninterrupted inside the material, nothing shows at the surface. A discontinuity in the flux path changes that. A crack or a pore splits the flux at that location, and part of it spills out of the part — especially when the discontinuity lies perpendicular to the flux. This local escaping field is called flux leakage, and the two edges of the discontinuity behave like small magnetic poles.

Fine ferromagnetic particles applied to the surface gather in this leakage field and bridge across it. They build a visible ridge of powder directly over the discontinuity — an indication. So what you see in MT is never the discontinuity itself, only the particles that its flux leakage has collected.

2. Ferromagnetic materials only

For MT to work, the material must have high magnetic permeability — it must magnetize easily up to the flux density the method needs. That covers carbon and low-alloy steels, cast iron, nickel, cobalt, and the ferritic, martensitic and precipitation-hardening (PH) grades of stainless steel (410, 420, 440C, 17-4 PH and similar). Duplex stainless steels are magnetic as well, thanks to their ferrite phase, so MT applies to them — although the austenite phase lowers sensitivity somewhat.

Austenitic stainless steels (most 300-series grades), aluminum, copper, titanium and other nonmagnetic materials are a different story. They cannot be magnetized to any useful degree, so MT cannot examine them. One caveat: cold work can form deformation-induced martensite in austenitic steels, which makes the area partially magnetic. That creates a risk of both weak and misleading indications. For surface discontinuities in these materials, use liquid penetrant testing (PT), or eddy current testing (ET) on conductive materials.

3. Orientation and magnetization directions

Orientation is the single most critical rule in MT. Flux leakage is strongest — and the indication clearest — when the discontinuity lies perpendicular to the magnetic flux. A discontinuity running parallel to the flux barely disturbs it and may go undetected.

No single magnetization direction is therefore enough on its own. Magnetize the part in at least two mutually perpendicular directions:

Circular magnetization: The magnetic field circles around the part. It reveals discontinuities running parallel to the axis (longitudinal).
Longitudinal magnetization: The magnetic field runs along the axis of the part. It reveals discontinuities running perpendicular to the axis (transverse/circumferential).

Never call an area clean until you have examined it in two perpendicular directions.

4. Magnetizing equipment

The equipment you choose sets the direction and strength of the field:

Yoke: A two-legged electromagnet (AC or permanent/DC) that produces a longitudinal field between its legs. It is portable, it passes no current through the part (no arcing or burn risk), and it is the most common field tool, especially for weld examination. On a weld, first set the legs perpendicular to the weld — the field then cuts cracks running parallel to the weld. Then rotate the yoke 90° and set it parallel to the weld line to catch transverse cracks. Overlap the scans by roughly 10%, and keep the leg spacing in the 75–200 mm range.
Prods (contact electrodes): Two electrodes touching the part pass current through it, and a circular field forms around that current. The high current works well on local areas, but the contact points carry a risk of arc strike. In weld examination an arc strike is not just an equipment hazard — most codes reject it in its own right. It can leave a locally hardened (martensitic) zone and microcracks at the contact point, so you must grind it out and re-examine the area. That is why yokes are preferred over prods on welds, and why prods are generally avoided on high-strength and quenched-and-tempered steels.
Bench unit (head/tailstock + coil): In the shop, a "head shot" passes current directly through the part for a circular field, while a coil around the part provides the longitudinal field.
Central conductor: For hollow parts such as pipe and rings, a conductor bar passed through the part produces a circular field on both the inside and outside surfaces.

5. Current types, dry/wet and visible/fluorescent

Current type. Alternating current (AC) concentrates the field at the surface because of the skin effect, which makes it most sensitive to surface discontinuities. Rectified current (half-wave or full-wave DC) drives the field somewhat deeper into the part and gives an advantage on near-surface discontinuities.

Particle medium. Particles reach the surface in one of two forms:

Dry method: The particles are applied as a dry powder. It suits rough surfaces, hot parts and field work, usually runs with rectified current, and favors near-surface discontinuities.
Wet method: The particles are applied suspended in a liquid (water- or oil-based). The finer particles give higher sensitivity to fine, shallow surface cracks. The suspension evaporates, though, so do not use it on hot surfaces — on hot welds, use the dry method within the applicable temperature limits.

Visible and fluorescent. Particles can be visible-color or fluorescent. Fluorescent particles glow bright yellow-green under UV-A light (about 365 nm) in a darkened area, giving very high contrast and sensitivity. They are the choice for critical work and fine cracks. The visible method uses a contrast paint under adequate white light and is practical in field conditions.

Magnetization timing splits the work in two as well: in the continuous technique the current stays on while you apply the particles (by far the most common), while the residual technique relies on the field the material retains after magnetization stops — an option only on materials with high remanence.

6. Step by step

Precleaning: Remove oil, grease, rust, slag and loose dirt from the surface. On welds, also remove spatter and loose slag. If surface roughness prevents you from interpreting indications, dress it lightly — blanket grinding is not required. Thick paint or coating can mask an indication by blocking both the flux leakage and particle movement. Depending on the application, codes typically accept nonmagnetic coatings up to about 50 µm without verification; above that, verify the coverage with reference shims.
Magnetize in the first direction: Apply the first direction with suitable equipment — a yoke, for example.
Apply the particles: With the continuous technique, apply the particles gently while the magnetizing current is on. Avoid over-application, which smothers indications.
Examine and evaluate: Examine the indications under appropriate lighting — adequate white light for the visible method, a darkened area plus adequate UV-A intensity for the fluorescent method. Separate relevant indications from non-relevant and false ones.
Magnetize in the second (perpendicular) direction and repeat: Rotate the equipment 90° or change technique to apply the perpendicular direction, then repeat the examination.
Record and assess acceptance: Evaluate the indications against the acceptance criteria of the governing construction or in-service code (linear versus rounded indication, size).
Demagnetize and postclean: Residual magnetism left in the part can affect later operations, welding, machining, or in-service behavior. Demagnetize the part where required and clean off the particle residue.

7. Field verification and quality control

The reliability of an MT examination rests on proving two things: that the field runs in the correct direction, and that it is strong enough.

Artificial flaw indicators (shims/QQI): Thin nickel shims carrying known artificial cracks are bonded to the part. They confirm that the field is strong enough to form indications and that it runs in the right direction.
Field direction indicator (pie gauge): A segmented steel gauge placed on the surface gives quick visual feedback on field direction. It does not represent the subsurface field, so use shims/QQI to verify real sensitivity.
Yoke lifting power: Per ASME, a yoke at its maximum leg spacing must lift at least 4.5 kg (10 lb) with AC and at least 18 kg (40 lb) with rectified or permanent DC. It is a quick way to confirm the yoke's output on site.
Lighting conditions: For the visible method, provide adequate white light on the examination surface (typically ≥ 1,000 lux). For the fluorescent method, provide adequate UV-A intensity at the surface (typically ≥ 1,000 µW/cm²) with low ambient white light (≤ 20 lux).
Gaussmeter/Hall probe: Where required, measure the tangential field strength directly.

8. Related standards

ASME BPVC Section V, Article 7: Magnetic particle examination method.
ASTM E709: Standard Guide for Magnetic Particle Testing.
ASTM E1444/E1444M: Standard Practice for Magnetic Particle Testing.
EN ISO 9934-1/-2/-3: MT — general principles / detection media (particles) / equipment.
EN ISO 17638: Non-destructive testing of welds — magnetic particle testing.
EN ISO 23278: Magnetic particle testing of welds — acceptance levels.

Acceptance criteria come from the construction or in-service code the examination is performed to (for example ASME Section VIII, or the applicable piping/welding code). The general framework for personnel qualification is ISO 9712 (Level I/II/III).

9. When to use MT, when to use PT

MT and PT are the two common methods for surface discontinuities, and the choice comes down to the material and to where the discontinuity sits. MT works only on ferromagnetic materials. In return, it catches near-surface discontinuities as well as surface-breaking ones — but only very close to the surface, within a limited depth — and it is usually faster. MT is not a volumetric method; sensitivity drops off quickly with depth. PT, by contrast, is material-independent and works on nonferromagnetic materials too, yet it detects only surface-breaking discontinuities. So when you are hunting surface cracks in a ferromagnetic steel weld, MT is usually the more practical and more sensitive option. On austenitic stainless, aluminum and similar materials, switch to PT.

From the field

The most common mistake in MT is neglecting orientation. A single magnetization direction will hide any crack that runs parallel to it, which is exactly why two perpendicular directions are mandatory. The second common mistake is over-applying particles: too much powder smothers the very indication you are looking for. Timing matters too. On steels prone to delayed (cold/hydrogen) cracking, run MT only after the waiting period the governing code requires — 48 hours for some high-strength steels. Examine too early and you miss a crack that has not appeared yet. Do not neglect the end of the job either: skip demagnetization and cleaning, and the residual magnetism left in the part will cause trouble in later fabrication or in service.

The best MT examination is the one run in the right directions, with verified field strength, under proper lighting — and cleaned up properly at the end. The equipment does not decide the result. Technique and discipline do.

Take it to the field: To keep these NDT methods, standard references and field steps in your pocket — completely offline and free — take a look at the Doawise NDT Guide app.

At DoaWise we perform nondestructive testing to international standards, magnetic particle testing included — from weld, casting and forging examination to field inspections — and we deliver recordable, auditable results.