Magnetic particle testing finds cracks in iron and steel by magnetising the part and sprinkling fine iron powder on it: a crack lying across the magnetic field leaks flux into the air and the powder clusters along it as a visible line. It works only on ferromagnetic metals, so it cannot test aluminium, bronze or austenitic stainless steel, and a crack parallel to the field is invisible, so each part must be magnetised in two directions at right angles. Demagnetise the part afterwards.
Part of Non-Destructive Testing
Magnetic particle inspection detects surface and near-surface cracks in ferromagnetic metals by the way fine magnetic powder collects where a crack disturbs a magnetic field.
A Crack Is A Gap In A Magnetic Path
Inside a magnetised piece of steel the magnetic field runs through the metal. A crack is a thin gap of air, which carries magnetism poorly, so the field bulges out of the surface around the crack and re-enters beyond it. Fine iron powder dusted on the surface is attracted to that leakage and piles up along the crack. A crack that runs along the field disturbs it much less, which is why the direction matters.
This is a property of ferromagnetic metals. The Advanced Materials article says the method is for iron and steel only. Austenitic Steel is non-magnetic, as are aluminium and bronze, so use Dye Penetrant on those.
What You Need
- A way to magnetise: an electromagnet, or wire wound around the part carrying DC current, as the Advanced Materials article says.
- Fine magnetic powder or sifted iron filings.
- A bright light, a magnifier, and a reference piece with a known crack.
Method
From the Advanced Materials article:
- Clean the part. Oil, rust and scale hold the powder back.
- Magnetise it with the electromagnet or by wrapping it with wire carrying DC current.
- Sprinkle the powder gently on the surface while the current flows, or tap it on from a fine sieve.
- Look. Cracks perpendicular to the field make the particles cluster visibly.
- Turn the field 90 degrees and repeat, so cracks in every direction are seen.
- Demagnetise after the test.
For direction: a coil wrapped around the part along its length puts the field along the axis, which shows cracks that run across the part. Sending current through the part, along its length, puts a circular field around it, which shows cracks that run lengthwise. The wire-wrapping method can do the first.
Calibrate On A Known Crack
There is no safe general figure for how strong the field must be, because it depends on the steel and the shape. Find it empirically: make a reference piece of similar steel with a small known crack, and increase the current in steps until the powder defines the crack sharply. Stronger is not always better, since a very strong field holds powder everywhere. Use the same setup on the real parts.
Reading The Result
| Pattern | Likely meaning |
|---|---|
| Sharp, narrow line, powder held tightly | Crack |
| Fuzzy band at edges, sharp changes of section or threads | Field leakage from the geometry, not a crack |
| Scattered powder over the surface | Field too strong or surface dirty |
Lines along the part at a thread root, a keyway corner or a weld toe need special attention, since fatigue cracks start there (see Failure Modes).
Demagnetise Afterwards
A magnetised steel part attracts chips and filings and can damage bearings and sliding surfaces. Reduce the field gradually, by withdrawing the part slowly from an alternating-current coil, or heat it above its Curie point, about 770 °C according to Heat Treatment, which is not practical for finished parts.
Worked Example: Turns And Current
The strength of a coil field depends on ampere-turns, current multiplied by number of turns. A coil of 20 turns carrying 10 A gives 20 × 10 = 200 ampere-turns. Doubling the turns to 40 at the same current doubles it to 400, the same as doubling the current to 20 A with 20 turns. Winding more turns is safer than raising the current, because less current heats the wire and battery less.
Current, arcs, batteries and flammable carriers
Passing high current through wire or a part from a battery bank can arc, melt the wire and burn skin. Use properly sized insulated wire, secure connections and a fuse or breaker, and never switch the circuit on or off while holding a loose contact. A battery bank can deliver enough current to weld metal, so keep tools away from terminals. Do not use powder suspended in kerosene or any flammable liquid with arcing contacts. Wear eye protection, and avoid breathing iron dust.
Common Mistakes
- Testing in one direction only — misses half the cracks.
- Using it on stainless or aluminium — it cannot work.
- Forgetting to demagnetise — filings stick and wear parts.
What’s Next
- Learn next: Metallography — looking inside the metal itself
- Combine with: Non-Destructive Testing — choosing between methods