API · ASME · ISO · NIST · UKAS
Magnetic particle inspection to detect surface and near-surface defects in ferromagnetic materials, ensuring the integrity of welds and industrial components.
Magnetic Particle Inspection (MT/MPI) detects surface and slightly subsurface discontinuities in ferromagnetic materials such as carbon steel and cast iron. When the part is magnetised, a discontinuity distorts the magnetic field and causes flux to leak at the surface; fine iron particles applied over the area are drawn to the leakage and gather to form a visible indication.
We magnetise using yokes, prods and coils, and apply dry powder or wet fluorescent particles depending on sensitivity and access, correctly orienting the field so defects in every direction are found.
A magnetic field is induced in the part using an electromagnetic yoke, prods that pass current through the material, or an encircling coil. A discontinuity lying across the field lines disturbs them and forces magnetic flux to bridge the gap through the air just above the surface — a leakage field.
Ferromagnetic particles dusted or flowed over the surface migrate to and pile up at the leakage field, marking the defect. Because leakage is strongest when the discontinuity is perpendicular to the field, the part is magnetised in at least two directions to catch flaws of any orientation.
MT reveals surface-breaking cracks and discontinuities open to the surface, and — unlike penetrant testing — indications lying a small distance below the surface such as sub-surface cracks, lack of fusion, laps, seams and inclusions in ferromagnetic parts.
It is especially effective on weld toe cracks, grinding and fatigue cracks, and on castings and forgings, making it a workhorse for both new fabrication and in-service inspection of steel.
Portable AC/DC yokes are the mainstay for field weld inspection; prod units and coils suit larger castings and bar stock, and bench units serve high-volume shop work. Particles are applied dry (coloured powder for good contrast) or wet — including fluorescent particles viewed under UV light for the highest sensitivity.
Field strength and direction are verified with pie gauges, shims or a Hall-effect meter, and contrast paint or magnetic ink is chosen for the surface condition.
Indications are classified, measured and evaluated against the code (for example ASME BPVC Section V Article 7 with Section VIII, or ASME B31.3). The report records the magnetising technique, current, particle type, field verification and each relevant indication with an accept/reject decision.
Where residual magnetism could interfere with later machining, welding or service, the part is demagnetised and the residual field checked. Inspectors are certified to ASNT SNT-TC-1A or ISO 9712.
Only ferromagnetic materials — carbon and low-alloy steels, cast iron and some nickel/cobalt alloys — because the method depends on magnetising the part. Austenitic stainless steel, aluminium, copper and titanium are non-magnetic and must be inspected by penetrant testing (PT) instead.
Yes, but only shallow ones. MT can detect discontinuities lying up to a few millimetres below the surface, with sensitivity falling rapidly with depth. For genuinely internal or deep volumetric defects an ultrasonic (UT/PAUT) or radiographic (RT) method is required.
A discontinuity produces the strongest leakage field, and therefore the clearest indication, when it lies perpendicular to the magnetic field. A flaw parallel to the field may be missed entirely. Magnetising in two mutually perpendicular directions ensures cracks of every orientation are detected.
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