Magnetic Particle Testing (MT)
Magnetic Particle Testing (MT) is a surface and near-surface NDT method for ferromagnetic materials that detects discontinuities by applying a magnetic field and observing the leakage flux attract magnetic particles to the flaw indication.
Definition
Magnetic Particle Testing (MT) detects surface-breaking and slightly subsurface discontinuities in ferromagnetic materials. A magnetic field is induced in the part using a yoke, prods, coil, or central conductor. When a flaw interrupts the field, leakage flux at the surface attracts iron oxide particles, forming a visible indication.
Technical Context
Particles may be wet or dry, and either visible or fluorescent for inspection under UV-A light. Two orthogonal magnetizations are usually required to detect flaws of any orientation; multi-directional magnetization can achieve this in a single shot. Demagnetization after testing prevents residual magnetism issues.
When It Is Used
- Weld inspection on carbon and low-alloy steels
- Forging and casting inspection
- In-service crack detection on shafts, lifting lugs, and structural members
Related Standards
ASME Section V Article 7, ASTM E709, ASTM E1444, ISO 9934.
Synonyms
Also known as MPI (Magnetic Particle Inspection) or Magnaflux (trademark). Related: Penetrant Testing for non-ferromagnetic materials.
How it works
Magnetising a ferromagnetic component creates flux within it. Where a discontinuity interrupts that flux near the surface, it leaks out and forms local poles; fine magnetic particles applied to the surface are drawn to the leakage and pile up, making an indication far wider than the flaw itself and therefore visible.
What it finds
Surface-breaking and slightly subsurface discontinuities in ferromagnetic materials — cracks, laps, seams — including flaws too tight to see with the unaided eye. Sensitivity to surface cracking is excellent and the method is quick.
What it will not find
Everything in non-ferromagnetic material: austenitic stainless steel, aluminium, copper, titanium are all outside its scope. Flaws lying parallel to the flux direction produce little leakage, which is why examination is performed in two roughly perpendicular directions. Depth capability is marginal beyond the immediate subsurface.
How it is actually done
Field direction and adequacy matter more than particle choice. Field strength is verified — by a Hall probe, by an artificial flaw shim, or by a pie gauge depending on what the procedure calls for — and examination is repeated in a second direction. Demagnetisation follows where residual magnetism would interfere with later machining, welding or service.
Governing codes and standards
ASME Section V Article 7 covers magnetic particle examination; ASTM E709 is the standard guide; ASTM E1444 covers the process; ISO 9934 is the ISO series, with acceptance again coming from the referencing construction code.
Where it goes wrong
Examining in one direction only. A single magnetising shot finds flaws transverse to the flux and can miss a longitudinal crack entirely, which is why every serious procedure requires two directions and why auditors check for evidence that both were performed.
Frequently asked questions
Why does magnetic particle testing need two directions?
Flux leakage is strongest where a flaw lies across the field and weakest where it lies along it. One magnetising direction therefore has a blind orientation. Examining again at roughly ninety degrees covers what the first pass could not.
Does the surface need to be bare metal?
Not necessarily bare, but coatings reduce sensitivity because the particles sit further from the leakage field. Codes place limits on coating thickness and generally require it to be demonstrated that the coating does not impair detection, often by using a shim with a known artificial flaw.
Where Magnetic Particle Testing fits in an inspection programme
A term is only useful when it connects to a decision. Magnetic Particle Testing appears in written procedures, in technique sheets, and in the records an owner or accreditation body reviews afterwards — which means the way it is defined in your documentation has to match the way it is applied on site. Where the two drift apart, audits find it. Atlantis writes and reviews procedures against the governing codes, trains inspection personnel to apply them, and builds the record-keeping that makes the evidence retrievable years later. Procedure development and code consulting · NDT training and certification · Ask us about your programme.
Related terms
- Yoke — A yoke is a handheld electromagnet with two articulated legs used in magnetic particle testing to induce a longitudinal magnetic field between the leg tips for detecting transverse surface defects.
- Prods — Prods are a pair of handheld copper or copper-aluminum electrodes that pass high current through a localized area of a ferromagnetic part to induce a circular magnetic field for magnetic particle testing.
- Wet Particle Method — The wet particle method uses fine ferromagnetic particles suspended in a low-viscosity oil or water carrier, sprayed or flooded over the magnetized surface to achieve higher sensitivity than the dry method.
- Fluorescent Particles — Fluorescent particles are magnetic particles coated with a fluorescent pigment that emits visible light under UV-A (black light) illumination, providing higher sensitivity than visible particles by yielding bright indications against a dark background.
- Demagnetization — Demagnetization is the process of reducing the residual magnetic field in a part after magnetic particle testing, performed by passing the part through a decaying AC field or by AC current with reducing amplitude.
- ISO 9934 Magnetic Particle Testing — ISO 9934 is the international standard for magnetic particle testing, comprising Part 1 (general principles), Part 2 (detection media), and Part 3 (equipment), defining the European/international approach to MT inspection.
Further reading
magnetic particle inspection procedure interpretation · magnetic particle testing
More method terms
Ultrasonic Testing · Radiographic Testing · Penetrant Testing · Eddy Current Testing · Visual Testing · Acoustic Emission Testing · Leak Testing · Thermography / Infrared Testing · Microwave Testing · Phased Array Ultrasonic Testing
Where this comes up in practice
Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.
Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.