Magnetic particle testing (MT) is one of the simplest and fastest NDE methods for detecting surface and near-surface discontinuities in ferromagnetic welds. For AWS D1.1 structural work, that makes it the go-to method for finding toe cracks, transverse cracks, and incomplete fusion that might not show up on visual but haven't grown deep enough for UT to catch efficiently.
The examination is fast. The documentation, if done wrong, becomes a source of inspection deficiencies that are harder to resolve than the defects themselves. This article covers what goes into a compliant MT examination record for structural steel welds and what QC managers should verify before accepting an MT report from an NDE subcontractor.
Rule library based on AWS D1.1:2025; verify against your governing edition (the AHJ or contract may specify 2020 or earlier).
When AWS D1.1 requires MT
MT is not blanket-required on all structural welds. AWS D1.1 specifies NDT requirements based on joint category and structural application. Key situations where MT is triggered:
- Repair welds — D1.1 requires NDE of repaired areas. MT is commonly specified for surface examination of the repair.
- Fracture-critical members (FCMs) — When the engineer designates fracture-critical members, enhanced NDE is typically required and MT is included.
- Owner or engineer-specified — Many structural drawing packages specify MT on particular joint types, connection categories, or base metal grades where cracking concern is elevated (high-strength steels, thick sections, highly restrained joints).
- Cause for concern after visual — A CWI who identifies visual anomalies during VT may call for MT to rule out surface cracking at cracks, laps, or areas of unusual profile.
If MT appears on the drawing NDT note or in the welding specification, it is required regardless of what the D1.1 baseline requires. See NDE method selection: when to use RT, UT, MT, or PT for a method comparison framework.
How MT works — the physics the documentation has to reflect
MT works by magnetizing the component and applying iron particles (dry powder or wet suspension). Magnetic flux leaks out at discontinuities — cracks, seams, incomplete fusion at the surface — and the iron particles accumulate at those flux-leakage points, revealing the indication.
Two things the examination technique must address, both of which appear in the documentation:
Magnetization direction. A discontinuity is most visible when it is perpendicular to the magnetic flux lines. A crack running parallel to the flux won't be revealed. This means each area must be examined in at least two approximately perpendicular directions. The examination report must document the directions used, not just note "MT performed."
Coverage. The magnetization is local — a yoke or prod pair covers the area between its poles. Moving the magnetization incrementally across the weld and HAZ ensures full coverage. The report must document the area covered, referenced to joint numbers or drawing callouts.
Magnetization methods: yoke vs. prods
Electromagnetic yoke — The yoke is placed on the surface, with the magnetic flux flowing through the component between the two yoke poles. AC yokes are sensitive to surface-only discontinuities; DC (permanent magnet or electromagnetic DC) yokes can detect slightly subsurface.
For structural welds, AC electromagnetic yoke is the standard choice. It is effective for the surface and near-surface zone that matters most for toe cracks and surface-breaking incomplete fusion.
Prod magnetization — Two prods are pressed against the surface and current passes through the component. Prod magnetization creates circular flux patterns useful for large flat areas.
The problem with prods on structural steel is arc strikes. The contact between the prod and the base metal can cause localized melting — the same condition AWS D1.1 prohibits under the arc strike provisions. Do not use prod magnetization on tension members or any member where arc strikes are prohibited. If you're dealing with a tension chord in a truss, a fracture-critical member, or any component where hardness from an arc strike is a structural concern, use a yoke. Document the method chosen and the reason.
Equipment calibration requirements
Every piece of MT equipment used must be within its calibration interval. The examination report must document:
- Equipment identification — make, model, and serial number of the yoke or power unit
- Calibration date and due date — calibration to be performed per the applicable standard (typically ASTM E1444 or equivalent)
- Yoke lifting power verification — an AC electromagnetic yoke must be able to lift at minimum a 10 lb (4.5 kg) weight at the pole spacing used; a DC yoke must lift at minimum 40 lb (18 kg). This is a field check that should be performed at the start of each shift and documented. If the yoke fails the lift test, it is removed from service.
- UV lamp calibration (for fluorescent particles) — If fluorescent MT is used, the UV-A lamp intensity must be verified at ≥1000 μW/cm² at the examination surface. The lamp meter calibration certificate must also be on file.
Calibration records don't need to be in the examination report itself, but they must be retrievable. Your QMS system should tie each examination to the equipment cal records for that date. When an auditor asks, you produce both.
Particle type and lighting
Dry particles — Colored powder (red, gray, black, yellow) applied in a light cloud over the magnetized area. Good for rough surfaces and large castings; less sensitive than wet method on smooth weld surfaces.
Wet particles — visible — Iron particles in a water or oil carrier. Better sensitivity than dry. Requires ≥100 foot-candles (1076 lux) of white light at the examination surface. Document ambient lighting verification.
Wet particles — fluorescent — Iron particles with fluorescent pigment, examined under UV-A (black light) illumination. Highest sensitivity for detecting fine cracks. Requires ≥1000 μW/cm² UV-A at surface and ambient white light ≤2 foot-candles (to make the fluorescent indication visible). Document both UV-A intensity and ambient light level.
For structural weld MT, wet fluorescent is the most sensitive technique and is commonly specified on critical applications. The choice of technique (dry vs. wet, visible vs. fluorescent) must appear in the examination report and must match the technique specified in the examination procedure document.
Personnel qualification
Personnel performing MT on AWS D1.1 structural work must be qualified and certified to a written employer-established practice. The industry standard is ASNT SNT-TC-1A (Recommended Practice for Personnel Qualification and Certification in Nondestructive Testing).
- Level I — Can set up and perform the examination following a written technique, but cannot independently interpret and accept/reject indications. A Level I can do the examination; a Level II must review.
- Level II — Qualified to set up the examination, calibrate equipment, perform the examination, interpret results, and accept or reject the component. Level II is the minimum for the technician who signs the final report.
- Level III — Qualified to establish examination procedures, interpret codes and standards, and provide technical direction. A Level III may sign the procedure documents that the Level II follows in the field.
The MT report must show the technician's name, certification level, the employer, and the date the certification was verified current. A technician whose certification has lapsed is not qualified to perform Level II-credited examinations. Your NDE subcontract should require current copies of technician certs before work begins, not after.
Acceptance criteria
AWS D1.1 establishes acceptance criteria for MT examinations of structural welds. The criteria distinguish between relevant and nonrelevant indications — not every mark from the MT process is a flaw.
Relevant indications are those whose greatest dimension exceeds 1/16 in (2 mm) and which are associated with a mechanical discontinuity (crack, incomplete fusion, seam). Scratches, machining marks, and magnetic writing can create nonrelevant indications.
AWS D1.1:2025 establishes the following as rejectable relevant indications:
- Any crack, regardless of size
- Linear indications exceeding a code-specified maximum length
- Rounded indications that are grouped or clustered within a defined pattern
The CWI reviewing the MT report must verify that the technician properly distinguished relevant from nonrelevant indications and applied the code acceptance criteria, not a more permissive or more restrictive standard. Do not let an NDE subcontractor apply ASME acceptance criteria to D1.1 structural work — they are not the same.
For a full comparison of NDE acceptance criteria across methods, see NDE documentation in the audit packet.
What a compliant MT report must contain
Each MT examination report for an AWS D1.1 submittal should include:
- Project and joint identification — job number, drawing reference, joint number or weld ID, heat number of base material if traceability is required
- Examination date — field examination date (not the report date if they differ)
- Applicable standard and acceptance criteria — state "AWS D1.1:2025 Clause 6" or the specific edition and clause your acceptance is based on
- Technique — magnetization method (AC yoke, DC yoke, prod), particle type (dry/wet, visible/fluorescent), magnetization directions
- Equipment identification and calibration — yoke make/model/serial, calibration due date, lift test result and date performed
- Lighting verification — for wet visible: ambient white light level; for fluorescent: UV-A intensity and ambient level
- Area examined — precisely identified, referenced to drawing or joint sketch
- Findings — each relevant indication described (location, orientation, length) or "no relevant indications"
- Disposition — accept or reject, per code criteria
- Technician identification — name, certification level, employer, and cert expiration date
- Reviewer (if Level I performed exam) — Level II name and signature
A report missing any of these items is incomplete and may not be accepted as a code-compliant examination record by a third-party inspector or authority having jurisdiction.
Audit trail integration
MT reports belong in the same documentation package as the PQR, WPS, WPQ, and production NDE records. When an AISC auditor or owner inspector requests the complete welding documentation package for a structure, MT reports are part of it.
See common WPS deficiencies found in third-party audits for the most frequent documentation gaps. MT report deficiencies — missing calibration records, technician certs not attached, acceptance standard not identified — appear regularly in audit findings.
Keeping MT reports, welder qualifications, WPSs, and PQRs cross-referenced and retrievable under a single audit-packet system is where manual filing fails under the volume of a busy shop. See how the platform manages NDE record traceability.