Magnetic particle inspection (MPI), also called magnetic particle testing (MT), is a nondestructive testing method for detecting surface and near-surface discontinuities in ferromagnetic materials. The part is magnetized, and fine magnetic particles gather where a discontinuity creates flux leakage.
Important Material Limitation
MPI is not a general inspection method for every stainless steel grade. Common austenitic grades such as 304, 304L, 316, and 316L are normally non-ferromagnetic or only weakly magnetic and are generally unsuitable for reliable MPI. Material condition and magnetic response must be confirmed before selecting this method. For austenitic stainless steel, liquid penetrant testing, eddy-current testing, ultrasonic testing, or radiographic testing may be more appropriate depending on the defect type and specification.
Typical MPI Process
- Review the procedure: confirm material suitability, applicable standard, technique, acceptance criteria, and required surface condition.
- Clean the surface: remove contamination that could restrict particle movement or obscure indications.
- Magnetize the part: use a yoke, coil, prods, or current-flow technique appropriate to the geometry and procedure.
- Apply particles: use visible or fluorescent particles in a dry or wet carrier as specified.
- Inspect: evaluate indications under the required white-light or ultraviolet conditions.
- Reorient the field: use more than one field direction when necessary because sensitivity depends on the discontinuity orientation.
- Clean, demagnetize, and report: complete post-test requirements and document relevant indications and disposition.
Standards and Acceptance Criteria
Commonly referenced documents include ASTM E709 for guidance, ASTM E1444/E1444M for specified applications, ISO 9934, and ASME BPVC Section V Article 7. These documents describe examination practice; acceptance criteria normally come from the governing construction code, product standard, drawing, or purchase specification.
Suitable Applications
- Surface-breaking and shallow discontinuities in confirmed ferromagnetic components.
- Welds, castings, forgings, and machined parts when the approved procedure permits MPI.
- Follow-up examination of areas identified by visual inspection.
MPI FAQ
Can MPI inspect 304 or 316 stainless steel pipe?
It is generally not the correct primary method because these grades are austenitic and normally not sufficiently ferromagnetic. Confirm the alloy and magnetic response, then select an alternative NDT method if necessary.
Can MPI find internal defects?
MPI is most sensitive to surface and near-surface discontinuities. Deeper internal defects usually require ultrasonic or radiographic examination.
Does an MPI indication automatically mean rejection?
No. An indication must be evaluated against the acceptance criteria in the applicable code or purchase specification.
Compare ultrasonic testing and radiographic testing, or review our quality testing and inspection overview.