Industrial Magnetic Particle Testing for Surface and Near-Surface Defects

Magnetic particle testing (MT) is a nondestructive examination method used to find certain surface and near-surface discontinuities in ferromagnetic materials. It is commonly applied to welds, castings, forgings, shafts, and other components where cracking may affect service reliability. The method is not suitable for every material or every defect orientation, so the inspection plan must be matched to the component and acceptance criteria.

MT relies on magnetising a suitable part. A discontinuity that interrupts the magnetic field can create a leakage field, attracting visible or fluorescent particles and forming an indication. The quality of the result depends on surface condition, magnetisation direction, field strength, particle application, lighting, and the inspector's competence.

Confirm that MT is the right method

First identify the material and its magnetic properties. Austenitic stainless steels and many non-ferromagnetic alloys cannot be examined effectively using conventional MT. For those materials, another method may be needed.

Review the component drawing, material specification, weld procedure where relevant, service history, suspected defect type, and applicable examination code or acceptance standard. MT can reveal surface-breaking and some near-surface discontinuities, but it does not provide a complete volumetric assessment of thick sections. Ultrasonic or radiographic methods may be needed for internal defects.

Surface preparation and access

Remove loose scale, dirt, grease, and coatings to the extent required by the approved procedure. Excessive roughness or thick coating can obscure indications or reduce sensitivity. Do not grind away a suspected crack before it has been documented and evaluated by the responsible authority.

Confirm that the area can be magnetised in the necessary directions. A discontinuity is most readily detected when its orientation is suitably related to the magnetic field; a single magnetisation direction may miss a defect aligned unfavourably. Complex geometry may require more than one technique or setup.

Choose the magnetisation method

Common approaches include yokes, prods, coils, or other equipment specified for the component. Each creates a different field pattern and has limitations around geometry, contact, access, and heat or arc-damage risk. Use the method authorised by the written procedure and verify that the equipment can produce the required field for the part.

The inspector should check equipment condition and required performance verification before examination. Where the procedure calls for field indicators or another verification device, use it as specified. Do not assume that a powered yoke automatically provides adequate magnetisation at every location.

Examination workflow

1. Verify part identity, material suitability, examination extent, and acceptance criteria.
2. Review the written procedure and confirm personnel qualification requirements.
3. Prepare the surface without erasing relevant evidence.
4. Verify the equipment and required viewing conditions.
5. Apply the specified magnetisation and particle medium using the approved sequence.
6. Observe indications under the required lighting conditions; fluorescent methods require suitable ultraviolet-A equipment and controlled viewing conditions.
7. Reorient the field or repeat the examination where required to improve coverage.
8. Mark and photograph relevant indications where permitted, and record their location and orientation.
9. Evaluate indications against the governing acceptance criteria.
10. Complete post-examination cleaning and demagnetisation where required by the component or process.

The exact sequence and technical parameters must come from the applicable procedure and standard. They should not be improvised from a general guide.

Interpreting indications carefully

An indication is not automatically a crack. Geometry changes, edges, threads, permeability variations, surface contamination, and other conditions can create nonrelevant indications. Conversely, a weak or absent indication does not prove that a component is defect-free if coverage or magnetisation was inadequate.

Record the indication's location, length or other required dimensions, orientation, and appearance. The qualified examiner must distinguish relevant from nonrelevant indications and apply the acceptance criteria specified for the job. Do not invent a universal permissible crack size; acceptance depends on component class, code, service, and contract requirements.

Common sources of missed defects

Only one field direction is used. A defect aligned with the field may produce a weak indication. Follow the procedure's coverage requirements and change field direction as specified.

Surface condition is poor. Scale, paint, grease, or roughness can mask indications. Prepare the surface to the required condition and document any limitation.

Viewing conditions are unsuitable. Inadequate visible light, incorrect UV-A conditions, or uncontrolled ambient lighting can impair detection. Verify the required conditions before evaluation.

Equipment checks are skipped. Damaged cables, poor contacts, or inadequate field strength can compromise the examination. Record required equipment checks.

The report says only “MT passed.” Such wording provides little traceability. Report coverage, method, limitations, and relevant indications.

Safety and reporting

Follow electrical safety precautions, the chemical safety data for particle media, and the site's controls for working at height, hot surfaces, or restricted spaces. Some techniques can create local heating or arcing if used incorrectly; comply with the approved procedure and equipment instructions.

The report should identify the component, material where known, examination location and extent, procedure revision, equipment, particle method, viewing conditions, personnel qualification, indications, acceptance basis, limitations, and disposition. Any repair should be followed by the specified re-examination.

Practical takeaway

MT is effective when the material is suitable, the magnetic field is correctly oriented, the surface and viewing conditions are controlled, and indications are evaluated against the correct criteria. The report must show what was examined and what the method could reasonably detect.