Magnetic Particle Testing and Magnetic Particle Inspection (MT / MPI)

Reviewed by Atlantis NDT founder Anoop Rayavarapu, ASNT NDT Level III in five methods (UT, RT, MT, PT, VT), with 11+ years of international NDT field experience. MT is the workhorse surface-flaw method for ferromagnetic materials — under-loved by buyers, under-trained by suppliers, and over-relied on without rigour for calibration. This page is the operator-grade walk-through.

Method Physics — Magnetic Flux Leakage at Surface Discontinuities

MT works because ferromagnetic materials (carbon steel, low-alloy steel, ferritic and martensitic stainless) concentrate magnetic flux lines parallel to surface. A surface-breaking or near-surface discontinuity perpendicular to the flux field disrupts flux, generates a leakage field at the surface, and ferromagnetic particles (iron oxide dry powder or wet-suspension fluorescent / non-fluorescent) bridge across the discontinuity producing the visible indication. Maximum sensitivity requires the flux field perpendicular to the discontinuity orientation — so any procedure must magnetise in at least two orthogonal directions per ASME B&PV Section V Article 7 T-721 + ISO 17638 §6.3 + ASTM E1444 §6.2. Austenitic stainless, non-ferrous, and titanium are not ferromagnetic — they are out of MT scope and must be inspected with PT per ASME Section V Article 6 / ISO 3452-1 / ASTM E1417 instead. The frequent buyer error: calling MT on austenitic 304/316 stainless welds — invalid; calling MT on duplex 2205 — partially valid (ferrite content dependent, special acceptance criteria).

Magnetisation Techniques — Yoke, Prods, Coil, Central Conductor, Direct Contact

Yoke (electromagnetic AC or DC, hand-held) per ASME V Article 7 T-731 + ISO 17638 §6.4.2 — most common for field weld inspection, pole spacing 75-200 mm, lifting power ≥4.5 kg AC or ≥18 kg DC per ASTM E709 §6.4. Prods (electrical contact, current flow generates circular field) per T-732 — high sensitivity but arc-strike risk; not permitted on pressure-boundary welds without owner approval. Coil (longitudinal field through axial part) per T-733 — for shafts + tubes. Central conductor (through hollow part) per T-734 — for tube/pipe ID inspection without ID access. Direct contact (clamp at part ends) per T-735 — high current. Aramco SAES-W-012 §5 + Shell DEP 31.10.13.31 §6.4 + ExxonMobil GP 03-10-01 §4 mandate yoke method for field welds unless prod is specifically approved with arc-strike mitigation. Atlantis training pack includes hands-on with Magnaflux Y-7, Parker DA-400, Magwerks MX-4 yokes — see Atlantis Academy.

Wet Fluorescent vs Dry Powder vs Wet Visible — When to Use Each

Wet fluorescent (WFMT) per ASTM E1444 §8.3 + ISO 17638 §6.6 is highest-sensitivity — fluorescent magnetic particles in oil or water suspension, UV-A lamp ≥1000 µW/cm² at part surface per ASTM E709, white light ≤20 lux during inspection. Used on aerospace per Boeing D6-51991 §5 + Airbus AITM 6-4001, nuclear primary loop per ASME Section XI Appendix VII, and critical-service refinery welds. Dry powder per E1444 §8.2 — coloured magnetic particles applied dusted, good for rough surfaces, lower sensitivity, used on heavy fabrication. Wet visible (WMV) per E1444 §8.4 — coloured particles in suspension, mid-sensitivity, used when UV-A unavailable. The trap buyers fall into: specifying dry powder for critical-service aerospace welds where WFMT is the only adequate sensitivity. ASNT Level III procedure authorship per ASNT Level III consulting prevents this.

Calibration, Reference Standards, and the AS-5371 / Burmah-Castrol Block

Calibration per ASME Section V Article 7 T-743 + ASTM E1444 §7 + ISO 17638 §7 requires: lifting-power check daily for yokes (4.5/18 kg per ASTM E709), system performance check using Ketos Tool (AS-5371 ring), Pie-Field Indicator (T-754), Burmah-Castrol Strips, or Quantitative Quality Indicator (QQI / shim) to verify field direction + strength + particle suspension activity. Light intensity meter verification: UV-A ≥1000 µW/cm² at inspection surface measured at start of each shift + after lamp change. Suspension concentration per ASTM E1444 §7.5 — 0.1-0.4 mL of fluorescent particles per 100 mL of carrier in calibrated settling tube, replaced when contaminated.

Acceptance Criteria — ASME B31.3, AWS D1.1, API 1104, Aerospace

ASME B31.3 §344.3 surface-NDE acceptance for process piping welds — relevant indications shall be evaluated per Table 341.3.2; cracks any size = reject, linear ≥5 mm = reject, rounded depends on weld classification. AWS D1.1 §6.10 structural-steel welds — visual + MT acceptance per Table 6.1, cracks any = reject. API 1104 §9.6 pipeline-girth-weld acceptance. ASME Section VIII Div 1 UW-51 + Appendix 6 surface NDE — required after PWHT for critical service per UCS-56. Boeing D6-51991 + Airbus AITM 6-4001 aerospace acceptance — tighter than ASME, NDE before + after final heat treatment. Atlantis reporting layer carries acceptance criteria as structured fields with code-clause reference inside every report — see NDT reporting software.

Atlantis MT Training, Procedures and Records

Atlantis MT: ASNT SNT-TC-1A-based MT training, self-paced online with ASNT NDT Level III support and enrolment open all year (training and experience hours per the employer's written practice) — Training, Academy, LMS; ASNT Level III MT procedure authorship — Level III consulting; equipment-calibration ledger inside Atlantis ERP; ISO 9001 employer-infrastructure documentation — ISO 9001; cross-stack links — ASNT, API 510, API 570, API 653, AI defect detection, Digital Twins, 3D scanning, ISO 17024, Partner Program, Refining, LNG, Aerospace, Marine.

UV-A Lamp Selection, Black-Light Intensity, and the WFMT Sensitivity Cliff

Wet-fluorescent MT sensitivity drops off a cliff if UV-A intensity falls below 1000 µW/cm² at the inspection surface per ASTM E709 §6.7 + ASTM E1444 §7.4. Field-grade UV-A lamps — Magnaflux EV6000, Spectroline Pro, Labino MidLight UV-LED — must be intensity-verified at start of shift + after lamp change + at lamp filter degradation. White light at the inspection surface must be ≤20 lux per ASTM E1444 §7.4.2 to prevent fluorescence washout — a value most fluorescent overhead lighting violates by 50×, requiring inspection bay shading or off-hours scope. The buyer trap: specifying WFMT in a brightly-lit fabrication shop without darkening — sensitivity falls to PT-grade. ASNT Level III procedure authorship per Atlantis Level III consulting embeds the lighting specification in every WFMT procedure.

Aerospace + Nuclear Acceptance Criteria — Where MT Sensitivity Meets Regulatory Risk

Boeing D6-51991 §5 ferritic landing-gear forging acceptance for AISI 4340 (heat-treated to 270-300 ksi) demands WFMT after every machining stage + final heat treatment per AMS 2300 cleanliness + ASTM A484 surface finish. Crack-any-size = scrap. Airbus AITM 6-4001 mirrors. ASME Section XI Appendix VII nuclear primary-loop ferritic-weld acceptance under 10 CFR 50 Appendix B + ASME XI demands continuous PdM (preventive-defect monitoring) with WFMT or PT on every refuel outage at Sellafield, Hinkley, EDF Heysham, Bruce Power, Three Mile Island legacy decom, and US NRC fleet. NTPC ultra-supercritical main-steam ferritic-weld Indian Boiler Regulations 1950 acceptance + IBR Schedule III demand WFMT + PT on every Class A weld. Atlantis Academy trains to ASNT SNT-TC-1A; Level III consulting explains how the aerospace, nuclear and Indian IBR overlays sit beside the SNT-TC-1A, CP-189, ASNT 9712 (formerly ACCP) and ISO 9712 schemes.

Multi-Directional Field, Yoke Pole Spacing, and the Cross-Field Inspection Trap

Maximum MT sensitivity requires flux field perpendicular to discontinuity orientation — single-direction magnetisation misses up to 50% of indications oriented near-parallel to flux. ASME Section V Article 7 T-721 + ISO 17638 §6.3 + ASTM E1444 §6.2 all require at least two orthogonal directions per surface. Yoke pole spacing per ASTM E709 §6.4 — 75-200 mm spacing, lifting power 4.5 kg AC / 18 kg DC verified per test piece. Field-strength verification at the inspection surface via Pie-Field Indicator (ASME T-754), Quantitative Quality Indicator (QQI / shim) per ASME T-755, or Burmah-Castrol strips. The cross-field inspection trap: rotating yoke 90° at the same surface position without overlapping coverage — gaps near the pole feet under-inspect. Atlantis Academy practical-week drills the 90° rotation + 50% overlap protocol explicitly.

Frequently Asked Questions

Q1: Why use MT instead of PT for ferritic welds?

A: MT detects surface + near-surface (1-2 mm subsurface) discontinuities; PT detects only surface-breaking. On ferromagnetic materials MT is faster, cheaper, and more sensitive than PT for near-surface flaws.

Q2: Can MT be used on austenitic stainless?

A: No — austenitic 304/316 is non-magnetic. Use PT per ISO 3452-1 / ASTM E1417 / ASME Section V Article 6 instead.

Q3: AC or DC yoke for field weld inspection?

A: AC yoke for surface-breaking flaws (lifting power 4.5 kg minimum per ASTM E709); DC or pulsed-DC for subsurface (18 kg minimum). Aramco / Shell / ExxonMobil typically permit both; aerospace prefers DC.

Q4: How often do I calibrate the yoke?

A: Lifting-power check daily before start of inspection, after dropping or impact, and at shift change.

Q5: How are MT training fees quoted?

A: Fees depend on the region and the scope of the course. Contact us for fees in your region; we reply within one business day.

Q6: AI defect detection on MT?

A: Possible on imaged WFMT panels but ASNT Level II adjudication remains mandatory.

Q7: Does Atlantis offer outsourced ASNT Level III support for MT?

A: Yes. The founder is ASNT NDT Level III in MT, and the terms are agreed per engagement — see Level III consulting.

Free Consultation and Quote

Atlantis NDT founder Anoop Rayavarapu — ASNT NDT Level III in UT, RT, MT, PT and VT. We reply within one business day. Affordable. Accessible. Fully customizable. Contact, Training, Academy, ASNT, ERP, Digital Twins, About.

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Magnetic particle testing services: what you receive

Magnetic particle testing services find surface and near-surface cracks in ferromagnetic welds, forgings and castings, quickly and with simple equipment.

  • The deliverable. A report naming the procedure, magnetisation technique, particle type, indications and their acceptance.
  • What it will not find. Flaws in non-magnetic materials, or deep subsurface flaws. Use penetrant or ultrasonic testing there.
  • Who performs it. Certified technicians under a written procedure; the quotation names who performs each examination.

Wet fluorescent or dry visible particles?

Wet fluorescent particles are more sensitive for fine cracks; dry visible particles suit rough surfaces and field welds. The governing code and the part decide.

Practise magnetic particle testing

Magnetise in two directions, read the indications and demagnetise in the browser, and see how MT compares with PT.

Choose the magnetising technique from the crack orientation you expect, not from what is in the van. A yoke finds cracks lying across the pole-to-pole line and is blind to those parallel to it. A coil finds circumferential cracks; a central conductor finds axial ones. Every technique has a blind orientation, which is why each surface is magnetised in two orthogonal directions.

Field direction is the whole method. Leakage flux forms only where a discontinuity cuts across the flux path, so detection collapses as the crack rotates toward parallel with the field — which makes two orthogonal shots per surface a requirement rather than good practice. Current type sets depth: alternating current confines flux to a thin surface layer through skin effect, making an AC yoke a surface-breaking instrument, while direct and half-wave rectified current drive flux deeper and reach flaws sitting just beneath the surface. Lifting power reflects that split — 4.5 kg for AC and 18 kg for DC at maximum pole spacing. Neither number proves the field is adequate inside the part; only a field indicator does that. A pie gauge shows direction, a shim placed on the actual inspection surface shows a real flux level reached the actual geometry, and on complex shapes those two disagree often enough that the shim is the one to trust.

Source: ASME BPVC Section V (2023 Edition), Article 7 — Magnetic Particle Examination; ASTM E709 Standard Guide for Magnetic Particle Testing; ASTM E1444/E1444M Standard Practice for Magnetic Particle Testing; ISO 9934-1 and ISO 17638 for the ISO stack.

Technically reviewed by Anoop Rayavarapu — ASNT NDT Level III (UT, RT, MT, PT, VT)
Magnetising technique versus the crack it will and will not find
TechniqueField producedFinds cracks orientedMissesWhere specified
Yoke, ACLongitudinal field between the two poles, held near the surface by skin effectAcross the pole-to-pole lineCracks parallel to the pole line; anything below the surface skinASME V Article 7; ASTM E709; ISO 17638 — lifting power 4.5 kg
Yoke, DC or permanent magnetLongitudinal field between the poles, penetrating deeper into the sectionAcross the pole-to-pole line, including flaws slightly below the surfaceCracks parallel to the pole line; deep flawsASME V Article 7; ASTM E709 — lifting power 18 kg
ProdsCircular field encircling the current path between the prod tipsParallel to the prod-to-prod lineCracks across the prod line; anything outside the effective prod spacingASME V Article 7 — arc-strike risk; owner approval required on pressure-boundary welds
Encircling coilLongitudinal field along the part axisCircumferential, transverse to the part axisAxial cracks; part ends where fill factor and length-to-diameter ratio fall shortASME V Article 7; ASTM E1444/E1444M
Central conductorCircular field around the bore, strongest at the inner surfaceAxial, on bore and outer surfaceCircumferential cracks; the outer surface of heavy-wall parts where field strength falls awayASME V Article 7
Direct contact, head shotCircular field around the current path through the partAxial, along the current pathCircumferential cracks; burn and overheating risk at the contact padsASME V Article 7; ASTM E1444/E1444M
Every row has a blind orientation. That is the reason for the two-direction rule — no single technique covers a surface on its own.

AC or DC yoke for an in-service weld?

AC when the target is surface-breaking cracking — toe fatigue, hydrogen cracking, service-induced cracks — because skin effect concentrates flux exactly where those cracks open. DC or half-wave rectified when the concern is near-surface porosity, inclusions or lack of fusion under a thin skin. Fabrication acceptance regimes call surface-breaking, so AC covers the bulk of weld work.

The lifting-power check passed — is the field strong enough?

No. Lifting power qualifies the yoke, not the examination. It is measured on a flat test weight at maximum pole spacing, which no real component resembles. Field adequacy in the actual part is proved on the part: a pie gauge for direction and a shim on the inspection surface for level. Curvature, section change and pole contact all cut the delivered field.

Why can prods be refused on a pressure-boundary weld?

Prods pass examination current through the component at the contact tip, and a poor contact arcs. An arc strike leaves a localised hard, untempered martensitic spot carrying microcrack risk in the heat-affected zone — a new defect created by the inspection itself. Owner specifications require written approval, arc-strike mitigation, and removal plus re-examination of any strike found.

Duplex stainless — is MT valid?

Partially. Duplex grades contain ferrite and respond to a magnetic field, but the response tracks ferrite content and the weld metal behaves differently from the base metal. Do not assume yoke settings transfer from carbon steel. Qualify the technique on a representative sample with a shim on the actual weld and record the ferrite basis. Where response is weak, penetrant is the defensible method.

Does the part need demagnetising after examination?

Whenever residual field would interfere with the next operation — machining where chips cling to the cutting edge, subsequent arc welding where residual field deflects the arc, nearby instrumentation or bearings, or a customer specification setting a residual limit. Structural steel left in place after examination is the common exception. Measure with a field indicator and record the result either way.

Where in the fabrication sequence should MT run?

On the final surface condition and after any post-weld heat treatment, because PWHT can open cracks that were tight before and hydrogen cracking is delayed. Aerospace forging practice examines after each machining stage and again after final heat treatment, since machining exposes subsurface flaws and heat treatment initiates new ones. Examining before the last operation examines the wrong surface.

Related reading

Qualifying your own MT technicians

Many clients pair the service with capability: technicians certified under their own programme who handle routine examination between campaigns. magnetic particle testing training covers the level structure, the practical examination, and how on-site delivery under an ASNT Level III works.

MT by sector

What MT is used for differs sharply by industry — the assets, the damage mechanisms and the acceptance regime all change. Sector detail: Refining · Petrochemical · LNG · Offshore · Pipeline · Marine & Shipbuilding · Aviation & Aerospace · Power Generation · Mining & Minerals · Fabrication & Welding · Automotive Manufacturing · Steel & Primary Metals · Rail.