13.3 Magnetic Particle Testing (MT)

13.3 Magnetic Particle Testing (MT)

Quick Answer: MT detects surface and near-surface discontinuities in ferromagnetic materials by flux leakage: magnetize the part, apply magnetic particles, and read accumulations at leaks. Yoke, prod, and coil methods are common. Austenitic stainless and aluminium cannot be MT-tested. Field direction should cut the expected crack path; demagnetize when required.

WI2.5 covers magnetic particle testing as a core surface NDT method for steel fabrication. For IWI-S, MT is often the default high-sensitivity surface method on carbon steel structures, pressure parts, and plant equipment—faster than PT for many shop welds, but useless on non-magnetic alloys.

Ferromagnetic Materials Only

MT requires the material to be capable of supporting a strong magnetic field and flux leakage at discontinuities. In welding inspection practice:

Exam hammer: “Can we MT this 316L pipe weld?” → No—not ferromagnetic. Choose PT (or other methods) for surface cracks.

Some hardfacings, clad overlays, and dissimilar joints create mixed magnetic responses. The written procedure and NDT Level 3 guidance govern—not improvisation.

Flux Leakage Principle

When a ferromagnetic piece is magnetized, magnetic field lines travel preferentially through the metal. A discontinuity that interrupts the field—especially a crack roughly perpendicular to the field—forces flux to leak into the air above the surface. Fine magnetic particles applied to the surface are attracted to that leakage field and form a visible indication outlining the discontinuity.

Depth capability:

• Surface-breaking discontinuities: highest sensitivity
• Near-surface subsurface flaws: possible if shallow and properly oriented
• Deep volumetric flaws: not an MT job—use UT/RT

Unlike PT, MT can find some near-surface flaws that are not open to the surface (tightly closed or slightly subsurface), provided flux leakage is strong enough. Paint or thin coatings may be allowed within procedure thickness limits; thick coatings kill sensitivity.

Orientation of Field vs Discontinuity

Maximum detectability occurs when the magnetic field is approximately perpendicular to the long axis of the discontinuity. A longitudinal crack may be missed if the field runs parallel to it.

Practical rule taught everywhere:

1. Magnetize in one direction and inspect
2. Magnetize in a roughly perpendicular second direction (or use multi-directional techniques where qualified)
3. Ensure overlap of magnetic field coverage along the weld length

For welds, expect cracks in various orientations (toe transverse, longitudinal centreline, crater). Two-directional coverage is standard unless a procedure justifies single-direction for a known single orientation risk.

Magnetization Methods — Overview

Electromagnetic yoke (AC/DC)

• Portable legs placed across the weld; field between poles
• AC yokes give good surface sensitivity and particle mobility; common for weld inspection
• DC or permanent-magnet styles emphasize somewhat deeper near-surface capability with different particle behaviour—follow procedure
• Lift-test / dead-weight checks verify yoke performance (procedure values)

Prods (current flow / direct contact)

• Current injected through contact prods; field circles the current path
• Useful on large structures; risk of arc strikes and local burning if poorly applied—inspectors watch for prod marks as imperfections
• Spacing and amperage per procedure

Coils / cable wraps

• Longitudinal magnetization of bars, shafts, pipes by coil
• Flexible cables wrapped around vessels or pipes for circumferential field components depending on setup
• More common in shop units and shaft inspection than every field weld, but concept appears in exams

Bench units / headshot–coil combinations

• Production NDT of components with combined circular and longitudinal magnetization
• Less central to site welding inspection but part of the MT method family

IWI-S focus: recognise yoke as the everyday weld tool; know prods and coils exist; demand two-direction coverage and calibrated equipment.

Dry vs Wet Particles

Fluorescent wet MT needs darkened viewing and UV checks similar in spirit to fluorescent PT. Colour contrast dry powders (grey, red, yellow) need good white light and colour contrast against the surface (sometimes a thin contrast paint is allowed by procedure).

Apply particles while the magnetizing force is applied (continuous method) or per residual method rules—procedure governs. Blowing dry powder gently avoids wiping away true indications.

Surface Preparation and Examination Zone

• Remove loose scale, thick paint, weld spatter that prevents particle contact or creates false patterns
• Thin adherent coatings: only if procedure permits a maximum thickness
• Examine weld metal plus adjacent parent band as specified
• Temperature limits apply for particles and for personnel safety on hot work

Demagnetization Notes

After MT, residual magnetism may:

• Interfere with welding (arc blow) on subsequent passes or nearby work
• Affect precision machining or instrumentation
• Attract debris in service (rotating equipment, hydraulic parts)

Demagnetization (AC decay, reversing DC, yoke techniques, tunnel demag units) reduces residual field to procedure limits, sometimes verified with a field indicator or gauss meter. Not every structural weld requires demag after MT—service specification and procedure decide. Critical rotating parts and many shop procedures do require it. Know that residual field is a real close-out item, not optional folklore.

Limitations and Common Traps

False sense of security: MT “clean” on only one field direction along a long seam is incomplete technique, not a quality miracle.

Inspector Review of MT Work and Reports

Parallel to PT review, check:

1. Correct method selection (ferro material confirmed)
2. Procedure and acceptance standard (e.g. ISO 17638 principles + ISO 23278 acceptance levels, or ASME/client equivalents—use the contract edition)
3. Equipment verification (yoke lift test, UV meters if fluorescent, ammeter for prods)
4. Two-directional magnetization / multi-directional coverage and overlap
5. Particle type (dry/wet, visible/fluorescent) suited to surface and temperature
6. Examination extent and weld identification
7. Indications listed with sketch/photo as required; relevant vs non-relevant rationale
8. Demagnetization and residual field check when specified
9. Post-clean if contrast paint or wet particles must be removed before coating/service
10. Personnel qualification (ISO 9712 MT Level as required)

MT vs PT — Decision Table for Inspectors

Integration with VT and the ITP

Typical flow on a steel butt weld requiring surface NDT:

1. VT final geometry and cleanliness
2. MT (or PT) for surface/near-surface cracks per ITP
3. UT or RT if volumetric examination is specified
4. Repair loop: excavate → weld repair → re-VT → re-MT/PT → re-volumetric as required

IWI-S ensures hold points are real, sequences are logical (do not coat before MT when MT is required bare), and reports match the physical work.

High-Yield Summary for Oral/Written Exams

• MT = ferromagnetic + flux leakage + particles
• Not for austenitic SS / Al
• Field ⟂ crack for best detection; two directions
• Yoke / prod / coil families
• Dry vs wet, visible vs fluorescent
• Demagnetize when required
• Review reports like a supervisor, not a spectator