Magnetic particle inspection for steel parts and welds

The part is magnetized. Where a crack or other discontinuity interrupts the magnetic field, some of the field leaks out of the surface. Fine iron particles applied to the surface, either dry powder or suspended in a liquid, gather at that leakage and form a visible line right over the defect. A trained technician then interprets the indication and evaluates it against acceptance criteria. The method is quick, works on rough and coated parts within limits, and gives an immediate answer at the part.

• Wet fluorescent particles viewed under UV-A light: highest sensitivity, standard for aerospace and critical machined parts
• Wet visible particles, usually black on a thin white contrast coat: common on welds in the field
• Dry powder: rough surfaces, hot parts and larger subsurface indications
• MPI inspection is also widely called magnaflux inspection, after a brand name that became shop shorthand

Field work usually uses an AC electromagnetic yoke: portable, safe on finished surfaces and good for surface cracks in welds. Prods pass current directly through the part and can cause arc strikes, so many specifications restrict them on finished or critical surfaces. In the lab, wet horizontal bench units magnetize parts with a head shot (current through the part) for longitudinal defects and a coil shot for transverse defects, and central conductors handle rings and tubes. Because MT finds defects best when they lie across the field, every area is inspected in two directions roughly 90 degrees apart. Field strength and direction are verified with indicators such as quantitative quality indicator shims or a pie gauge, and parts are demagnetized afterward where the next process or service requires it.

MT applies to carbon steels, low alloy steels, tool steels, cast iron and martensitic or ferritic stainless steels. Typical work includes structural and pressure vessel welds, crane hooks and lifting equipment, rail and truck components, crankshafts, axles and gears, oil country tubular goods and drilling equipment, and aerospace steel parts such as landing gear components. Overhaul shops use it at every maintenance interval on rotating and load bearing steel parts.

• ASTM E1444, standard practice for magnetic particle testing, widely called up in aerospace and defense work
• ASTM E709, the guide that explains MT technique choices in more depth
• ASME Section V Article 7 for pressure equipment, AWS D1.1 for structural steel welds
• Technician qualification to SNT-TC-1A or ANSI/ASNT CP-189, NAS 410 for aerospace, with Level II for interpretation and Level III oversight of procedures
• NADCAP accreditation for NDT when your aerospace customer requires it

Ask which written procedure the lab will use and which acceptance criteria apply. The method finds indications; the code or drawing decides which are rejectable.

1. Surface preparation: remove scale, heavy paint, grease and spatter as the procedure requires.
2. Magnetize in the first direction and apply particles.
3. Inspect under the right light: UV-A with low ambient light for fluorescent, good white light for visible.
4. Magnetize in the second direction and inspect again.
5. Evaluate, mark and record indications.
6. Demagnetize and clean if required.
7. Report with procedure, technique, equipment, results and technician level.

Turnaround is fast: a lab batch often returns in 1 to 3 business days, and a field technician reports welds on the day.

Price depends on part count and size, wet fluorescent or visible, lab or field, surface preparation, travel and the reporting standard.

Detailed cost drivers across methods are in our guide to NDT inspection cost.