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

Atlantis NDT is led by Anoop Rayavarapu (ASNT NDT Level III, API 653 Authorized Inspector, ISO 9001 Lead Auditor). Free consultation for NDT inspection companies, training providers, and asset owners worldwide. request a demo — pricing varies by region and scope, quote on request.

Running this as a programme, not a one-off

If you are responsible for an inspection programme rather than a single job, the recurring problem is rarely the code — it is keeping measured thickness, damage-mechanism assignment and next-inspection dates in one defensible place. Asset integrity management software covers how RBI under API 580/581 and fitness-for-service under API 579 behave when they run on measured corrosion rates per CML instead of default rates, and what changes for the integrity team.

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ASME Section V Article 7 sets how magnetic particle testing is performed on ferromagnetic materials and defines no acceptance criteria. Acceptance comes from the referencing construction code: ASME Section VIII Division 1 UW-51, ASME B31.3 344.3, AWS D1.1 Clause 8.14, API 1104 Section 9.5. Article 7 recognises seven magnetisation techniques; the yoke lift test is 4.5 kg on AC and 18 kg on DC.

Article 7 applies to ferromagnetic materials only — carbon steel, low-alloy steel, ferritic stainless, ductile iron and nickel-iron alloys. Austenitic stainless and aluminium require liquid penetrant testing under Article 6. Detection reaches surface and slightly subsurface discontinuities, 1 to 6 mm deep depending on current type, frequency and material permeability. Current selection drives that depth: alternating current concentrates at the surface through skin effect, penetrating about 0.3 mm in carbon steel at 60 Hz and producing the highest tangential field for fine fatigue cracks, while direct current and half-wave DC distribute through the section to 3 to 6 mm for just-subsurface flaws. Field strength is verified every shift and in every part orientation using an ASTM E709 pie gauge on the part, a Ketos ring for procedure qualification, or a Hall-effect gaussmeter reading at least 30 gauss tangential at the surface during particle build-up.

Source: ASME Boiler and Pressure Vessel Code, Section V, Article 7 (Magnetic Particle Examination), paragraphs T-710 through T-776; ASTM E709 Standard Guide for Magnetic Particle Testing; ASTM E1444/E1444M Standard Practice for Magnetic Particle Testing.

Technically reviewed by Anoop Rayavarapu — ASNT NDT Level III (UT, RT, MT, PT, VT, ET) · API 653 · ISO 9001:2015 Lead Auditor
Article 7 magnetisation techniques — selection and constraint
TechniqueArticle 7 referenceCurrent typeField producedWhere it is usedMain constraint
YokeT-721.1AC or DCLongitudinal, between poles spaced 75 to 300 mmThe dominant field technique on pressure equipment and in-service weldsLift test must pass: 4.5 kg on AC, 18 kg on DC, verified each shift
ProdsT-721.2AC, HWDC or DCCircular field around each contact pointHeavy fabrication and structural weld workArc burn risk; copper or brass tips, controlled contact, post-test grinding of contact areas
Central conductorT-721.3AC or DC through a non-magnetic barCircumferential, around the boreBolts, hollow castings, short pipe sectionsPart must be hollow and fit over the conductor
Cable wrap / coilT-720 seriesHWDCLongitudinal along the part axisLong pipe lengths and large vesselsField falls off with distance from the coil; multiple shots required
MultidirectionalT-721.5Sequenced AC and DCLongitudinal and circumferential simultaneouslyProduction bench units in a shopRequires demonstration in every orientation being claimed
Field-strength verification under T-754 and T-764 is the most-rejected Article 7 audit point. Verify on every shift and in every part orientation, and record the pie gauge or gaussmeter result with the technique sheet.

If Article 7 has no acceptance criteria, where do they come from?

From the construction code that references it. ASME Section VIII Division 1 UW-51 rejects all cracks and permits linear indications to 1/16 inch and rounded to 1/8 inch. ASME B31.3 344.3 with Table 341.3.2A tightens for Category M and Severe Cyclic service. AWS D1.1 Clause 8.14 sets length limits by weld thickness. API 1104 Section 9.5 defines pipeline acceptance per joint thickness.

Should I use AC or DC to find a surface fatigue crack?

AC. Alternating current concentrates near the conductor surface through skin effect, penetrating roughly 0.3 mm in carbon steel at 60 Hz, which drives the tangential field at the surface as high as it goes and makes tight fatigue cracks reveal strongly. DC and half-wave DC spread current through the cross-section to 3 to 6 mm — the right choice for just-subsurface inclusions and incomplete-penetration root flaws, and the wrong one for fine surface cracking.

Pie gauge or Ketos ring — which does Article 7 require?

Both, for different purposes. The ASTM E709 pie gauge verifies the technique on the actual part being examined, every shift and every orientation: all eight slots must reveal particle build-up. The Ketos or Betz ring qualifies the procedure, proving the technique resolves known reference flaws at known depths — commonly three holes for medium sensitivity and five or more for high. Most fabrication specifications call for both.

Can I run magnetic particle testing on a stainless steel weld?

Only on ferritic grades such as 410 and 430. Austenitic 304 and 316 are non-magnetic and will not support a field — use liquid penetrant testing under Article 6 instead. Duplex grades such as 2205 and 2507 are partly ferritic, so MT can produce indications, but penetrant is the defensible choice because field strength across a two-phase microstructure is not reliably demonstrable.

What lighting does Article 7 require for fluorescent versus visible particles?

Visible-particle examination requires 100 foot-candles, equal to 1000 lux, of white light at the examination surface, verified with a photometer. Fluorescent examination requires ambient light at or below 2 foot-candles plus at least 1000 microwatts per square centimetre of UV-A at the surface, verified with a radiometer, and the inspector dark-adapted for five minutes before reading. Record both verifications on the technique sheet.

When is demagnetisation mandatory after magnetic particle testing?

When residual magnetism would interfere with a downstream operation — machining, welding, or assembly where a magnetised part deflects an arc or holds swarf. Methods are AC step-down with a gradually reducing field, DC reversal with step-down, or thermal treatment above the Curie point at 770 C for carbon steel, which is rarely practical. Verify with a field indicator; 3 gauss residual is the common ceiling for parts to be re-welded or machined.