ASME Section V Article 7 — Magnetic Particle Testing (MT) Requirements Explained

ASME Section V Article 7 governs magnetic-particle testing of ferromagnetic materials. This 2026 guide explains magnetisation technique, particle media, lighting, demagnetisation, and how Article 7 maps to ASME VIII, B31.3, AWS D1.1, and API 510/570/653 acceptance.

By Anoop Rayavarapu, ASNT NDT Level III · · Standards & Codes

ASME Section V Article 7 — Practical MT Guide

ASME Section V Article 7 is the ASME nondestructive examination standard for magnetic-particle testing (MT, MPI). It is the workhorse surface + near-surface inspection method for ferromagnetic materials — carbon steel, low-alloy steel, ferritic stainless, ductile iron, nickel-iron alloys. Article 7 is referenced by ASME Section VIII Div 1 § UW-51, B31.1 § 136.4, B31.3 § 344.3, AWS D1.1 Clause 8.14, API 1104 § 9.5, and API 510 / 570 / 653 alterations sections.

Scope and Applicability — T-710

Article 7 covers MT on ferromagnetic materials only (austenitic stainless and aluminum require liquid penetrant testing per Article 6). MT detects surface and slightly subsurface discontinuities — cracks, laps, seams, cold shuts, lack of fusion on the open root, hot cracking in welds. Penetration depth is typically 1–6 mm depending on current type, frequency, and material magnetic permeability.

Applicability examples: post-weld surface inspection of carbon-steel pressure-vessel welds (the dominant surface NDT method since carbon steel is ferro), heavy-fabrication structural welds per AWS D1.1, pipeline girth welds per API 1104, drilling-equipment inspection, API 510 Pressure Vessel Inspector alterations on carbon-steel vessels, and API 653 Tank Inspector tank-shell weld inspection.

Magnetisation Techniques — T-720 + T-740

Article 7 recognises 7 magnetisation techniques; the most common in pressure-equipment work:

  • Yoke (T-721.1): portable electromagnet; AC or DC; pole spacing 75–300 mm; lift test 4.5 kg (AC) / 18 kg (DC). The single most common field technique. Examples: Magnaflux Y-7 / Y-8, Magnatech 5C.
  • Prods (T-721.2): two contact electrodes; AC, HWDC, or DC; produces longitudinal magnetisation. Risk of arc burns — requires controlled contact, copper/brass tips, and post-test grinding of contact areas.
  • Central conductor (T-721.3): non-magnetic conductor through hollow part; circumferential magnetisation; preferred for bolts, hollow castings, and short pipe pieces.
  • Direct current induction / cable wrap: for long pipe lengths or large vessels; cable wrapped around the part energised with HWDC.
  • Multidirectional (T-721.5): simultaneous longitudinal + circumferential magnetisation; production-shop bench unit standard.

Current type matters: AC produces strongest surface field (skin effect concentrates current at surface), best for fine surface cracks. DC / HWDC penetrates 3–6 mm subsurface for incomplete-penetration root flaws and just-subsurface inclusions.

Field-Strength Verification — T-754 + T-764

The single most-rejected Article 7 audit point. Article 7 requires field-strength verification on every shift, on every part orientation, using one of:

  • ASTM E709 pie-gauge: 8-piece copper-clad slotted plate; place on the part being tested; magnetise; observe particle build-up across the eight slots. All eight must reveal indications for adequate field.
  • Ketos ring / Betz ring (T-764): reference ring with through-drilled holes at known depths; particles must reveal the required number of holes per the technique procedure (typically 3 holes for medium sensitivity, 5+ for high).
  • Hall-effect gaugemeter: reads tangential field strength in gauss / millitesla; requires 30 G / 3 mT minimum at the surface during indication build-up.

Particle Media — T-761

Article 7 recognises wet and dry particle systems:

  • Wet fluorescent (Magnaflux 14HF, 20B, 25B): highest sensitivity; bench-unit production standard; requires UV-A booth (1000 µW/cm² + dark booth ≤ 2 fc ambient). Suspended in oil or water-based carrier.
  • Wet visible (Magnaflux 7HF, Met-L-Chek WCP): easier field application; doesn't need UV; lower sensitivity than fluorescent.
  • Dry visible (Magnaflux Grey 8A, Black 1, Red 3): field workhorse for heavy-fabrication and structural-weld MT; particles applied via puffer-bottle or shaker; surface contrast critical (white-contrast paint or clean steel).

Lighting Requirements — T-776

Visible-particle MT: 100 fc (1000 lux) white light at the surface, photometer-verified. Fluorescent MT: ≤ 2 fc ambient + ≥ 1000 µW/cm² UV-A, radiometer-verified, with inspector dark-adapted 5 min before reading.

Demagnetisation — T-771

Post-MT demagnetisation is required when residual magnetism would interfere with subsequent operations (machining, welding, assembly). Methods: AC step-down (gradually reducing field), DC reverse with step-down, or thermal (above Curie point — 770 °C for carbon steel; rarely practical). Residual field measurement with field-indicator: typical maximum 3 gauss (0.3 mT) for assemblies that will be re-welded or machined.

Acceptance Criteria — From the Construction Code

Article 7 defines HOW to perform MT but not what's acceptable. Acceptance comes from:

  • ASME VIII Div 1 § UW-51: no cracks; linear indications ≤ 1/16 in. acceptable; rounded ≤ 1/8 in.
  • ASME B31.3 § 344.3 + Table 341.3.2A: stricter for Category M / Severe Cyclic
  • AWS D1.1 Clause 8.14: no cracks; specific length limits on linear indications per weld thickness
  • API 1104 § 9.5: pipeline-specific acceptance — defined per joint thickness

Personnel Qualification

Same path as Article 6 PT: SNT-TC-1A guide Level II MT, ISO 9712 Level 2 MT, or NAS 410 Level 2 MT for aerospace. Annual eye test (Jaeger J1 + Ishihara) mandatory. ASNT Level III consulting provides written-practice and procedure-approval support for combined MT + PT + UT + RT programs.

Frequently Asked Questions

Q1: Why does AC give a stronger surface indication than DC?

A: Skin effect — alternating current concentrates near the conductor surface (depth ∝ 1/√frequency). 60 Hz AC penetrates ≈ 0.3 mm in carbon steel, producing very high tangential field at the surface, ideal for fine fatigue cracks. DC distributes through the cross-section, penetrating ≈ 3–6 mm — better for just-subsurface flaws but lower tangential field at the surface itself.

Q2: When must I use a Ketos ring vs a pie-gauge?

A: Pie-gauge for technique verification on the actual part (every shift, every orientation). Ketos ring for procedure qualification — proves the technique can resolve known reference flaws at known depths. Most fab specs require both.

Q3: Are prods allowed on pressure equipment in service?

A: Yes, but with caution. Article 7 requires controlled contact pressure, copper/brass tips, and post-test grinding of contact marks to prevent micro-cracks from arc burn. Many refineries forbid prods on high-strength alloys (P91, 2.25Cr-1Mo) because arc burn can initiate stress-corrosion cracks. Yoke is the safer choice in service.

Q4: Can I MT a stainless-steel weld?

A: Only if it's ferritic stainless (e.g., 410, 430). Austenitic SS (304, 316) is non-magnetic — use liquid penetrant testing per Article 6 instead. Duplex (2205, 2507) is partly ferritic — MT may work but PT is the safer choice.

Q5: How does Article 7 MT integrate with API 579 FFS?

A: Surface cracks detected by MT are characterised and fed into API 579 FFS Level 1/2/3 crack-like flaw assessment. The flaw length from MT, the depth from follow-up ultrasonic testing Article 4, and the local material properties feed into the API 579 disposition — fitness-for-service or repair.

Q6: What's the difference between wet fluorescent and wet visible MT?

A: Fluorescent particles are coated with a fluorescent dye that emits visible light under UV — far higher contrast and sensitivity, especially for tight fatigue cracks. Visible particles rely on coloured particles + contrast paint — lower sensitivity but no UV booth needed. Aerospace + nuclear default fluorescent; pressure-equipment field work often defaults visible.

Q7: Are dry-particle aerosol cans accepted for Article 7?

A: Yes — most field MT uses Magnaflux 8A grey or 1B black aerosol applied via puffer bulb or shaker. Verify shelf life and batch traceability. The aerosol is essentially the same particle as bulk; only the carrier differs.

Q8: How long must I retain Article 7 MT records?

A: Life-of-equipment for pressure equipment; minimum 5 years per most referencing codes. Atlantis NDT Reporting Software stores SHA-256-hashed MT records with technique sheet, pie-gauge verification, particle batch + lot, lighting verification, and accept/reject record in a PDF/A-3 audit bundle.

Related Atlantis NDT Resources

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