When does AWS D1.1:2025 require magnetic particle testing on structural welds?

AWS D1.1:2025 does not mandate MT on every weld. Contract documents or the engineer of record specify which welds require MT. The standard provides mandatory acceptance criteria and procedure requirements whenever MT is invoked on the project.

Can prod electrodes be used for MT on structural welds?

Prods are permitted under AWS D1.1 but restricted near the finished weld on quenched-and-tempered steels because prod contact points create arc burns that can cause local hardening and hydrogen-assisted cracking. Wet fluorescent yoke technique avoids this concern entirely.

What documentation is required after MT on a structural weld?

A written MT report recording the procedure, equipment, technique, coverage area, operator identity, and disposition of each indication — acceptable or rejected — is required and becomes a permanent part of the project quality record.

How does MT compare to UT for detecting weld discontinuities?

MT detects surface and near-surface discontinuities in ferromagnetic steel. UT reaches volumetric discontinuities at depth throughout the weld cross-section. On CJP groove welds where subsurface fusion flaws are the primary concern, UT is the workhorse; MT supplements it by catching surface-breaking cracks that UT can miss.

Aug 10, 20267 min read

MT Acceptance Criteria for Structural Welds: AWS D1.1:2025

AWS D1.1:2025 magnetic particle testing acceptance criteria for structural welds — linear indications, crack limits, technique requirements, and CWI documentation.

NDE MT AWS D1.1 Inspection CWI

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Magnetic particle testing (MT) is the go-to nondestructive examination (NDE) method for finding surface and near-surface discontinuities in ferromagnetic structural steel welds. When a fabricator calls for MT on a connection — whether driven by contract requirements, the engineer of record, or AWS D1.1:2025's NDE provisions — the CWI must know exactly what constitutes an acceptable indication, what gets rejected, and how to document the findings so the record holds up in a third-party audit.

Why MT, and When Does AWS D1.1:2025 Require It

MT works by magnetizing the part and applying iron particles (wet or dry). Where a discontinuity intersects or is near the surface, the magnetic field leaks out and the particles form a visible indication. The method is fast, inexpensive, and highly sensitive to surface-breaking cracks — which makes it ideal for inspecting:

Root pass and final pass surfaces of CJP groove welds before the joint is closed or painted
Fillet weld toes where fatigue cracks initiate on cyclically loaded connections
Repair welds after grinding, to confirm full crack removal before the repair fill pass
Heat-affected zones (HAZ) of high-restraint joints where hydrogen-assisted cracking risk is elevated

AWS D1.1:2025's inspection provisions do not automatically require MT on every structural weld. The contract documents govern — typically the structural steel specification (AISC, project spec, or owner's QC plan) calls out MT on specific joint types, weld categories, or tonnage tiers. However, when MT is invoked, AWS D1.1:2025 provides the acceptance criteria and the procedure requirements that govern how MT is performed and judged.

Projects governed by AWS D1.8:2016 (the seismic supplement) impose more demanding MT requirements on demand-critical welds — including mandatory MT on the root and cap passes of CJP groove welds at moment frame connections.

MT Techniques and Equipment Under AWS D1.1

AWS D1.1:2025 references ASTM E709 (Standard Guide for Magnetic Particle Testing) as the procedure framework. The inspector and the written MT procedure must address:

Magnetizing method:

Yoke technique — an AC or DC electromagnet placed across the weld. AC provides better surface sensitivity (skin effect); DC penetrates slightly deeper (up to roughly 1/4 in [6 mm]). Yokes are the preferred method for structural fabrication because they leave no arc burn.
Prod technique — two hand electrodes contact the base metal and pass current through the part, creating a circular magnetic field. Prods are effective but prohibited on quenched-and-tempered steels (ASTM A514, A517, A709 HPS 100W) and should not be used near finished weld surfaces where arc strikes are a quality concern.

Particle application:

Dry visible — iron powder applied by bulb or blower. Good for field work, hot surfaces, and rough geometry.
Wet fluorescent (WFMT) — particles suspended in liquid, inspected under UV (black) light. Superior sensitivity; the preferred method for shop fabrication when finding fine cracks or porosity chains matters.
Wet visible — particles in liquid, inspected in white light. Less sensitive than fluorescent but adequate for many structural applications.

Magnetization directions: A single magnetization direction only reveals discontinuities roughly perpendicular to the field. AWS D1.1 requires the weld to be examined in two perpendicular directions (or the full circumference of a tubular joint) to find both longitudinal and transverse discontinuities.

MT Acceptance Criteria

AWS D1.1:2025's acceptance criteria for MT distinguish between types of indications:

Relevant vs. non-relevant indications: An indication is relevant when it results from an actual discontinuity. Geometry changes (weld ripples, edge radii, thread roots) can also cause field leakage and produce non-relevant indications. The inspector must differentiate them — typically by examining the part after removing particles and re-examining the surface.

Cracks — none permitted: Any confirmed crack indication is cause for rejection, regardless of length or orientation. A crack must be ground out, confirmed removed by re-MT, and repaired before the weld is accepted.

Linear indications: A linear indication is one where the length exceeds three times its width. AWS D1.1:2025 imposes maximum length limits on linear indications for structural welds:

Linear indications associated with incomplete fusion, lack of penetration, or slag inclusions that would be rejectable by visual or volumetric criteria are rejectable by MT as well.
The standard's acceptance criteria for statically loaded and cyclically loaded connections differ — cyclically loaded connections, particularly in tension zones, carry tighter limits to account for fatigue crack initiation.

Rounded indications: A rounded indication has a length no greater than three times its width — typically porosity or small slag pockets at the surface. Small rounded indications are generally more permissive than linear ones, but clusters (aggregate area limits) are also evaluated.

A key principle: MT findings are disposition'd against the same underlying weld quality rules as visual acceptance criteria. An MT indication is rejectable when it reveals a discontinuity that would fail visual or mechanical acceptance — the MT just reveals it at the surface where vision alone might miss it (especially under coating or in tight geometry).

Magnetizing the Joint: Coverage and Technique Sequence

A written MT procedure approved by the responsible engineer or QC manager documents the sequence. For a structural CJP groove weld, a typical sequence is:

Clean the weld surface — remove loose scale, spatter, and any coating that would interfere with particle adhesion or magnetic flux.
Apply particles in the first magnetization direction (e.g., longitudinal, parallel to the weld axis — this detects transverse cracks perpendicular to the weld run).
Interpret and record any indications.
Apply particles in the second magnetization direction (transverse to the weld axis — this detects longitudinal cracks parallel to the weld run).
Interpret and record any indications.
Demagnetize if required by contract or if subsequent machining is planned.

Yoke adequacy must be confirmed before inspection begins. ASTM E709 specifies a minimum yoke lift force (10 lb [45 N] for AC, 40 lb [180 N] for DC; check the current procedure's values against the standard as these are procedure-governed requirements).

MT vs. PT: Choosing the Right Surface Examination Method

MT and penetrant testing (PT/LPT) both inspect surface discontinuities, but they work on different material properties. MT requires ferromagnetic material — it cannot be used on austenitic stainless steel, aluminum, or nickel alloys. PT works on any clean, non-porous surface regardless of magnetic properties.

For structural carbon and low-alloy steel fabrication under AWS D1.1, MT is almost always preferred over PT because it detects near-subsurface discontinuities (not just those open to the surface) and is faster to execute on large weldments. PT is reserved for stainless steel work (AWS D1.6) or cases where magnetization is impractical.

See also the article on UT acceptance criteria for structural welds and RT acceptance criteria for structural welds for the companion volumetric NDE methods.

Documentation Requirements for MT

The MT report — a permanent project quality record — must include at minimum:

Project name, weld ID or joint designation, and drawing reference
Date of examination and inspector name (with NDE certification level and employer)
Written MT procedure reference (document number and revision)
Equipment used: yoke model/serial, particle type and lot, UV lamp model (if WFMT)
Technique used (AC/DC, wet/dry, fluorescent/visible)
Magnetization directions and coverage
Results: sketch or photo of indication location, dimensions, and disposition (accept/reject)
Signature of the Level II or Level III MT examiner

A CWI reviewing the MT report should verify that coverage was complete (both magnetization directions documented), indications were dispositioned against the correct acceptance criteria tier (static vs. cyclically loaded), and any rejected indications were tracked to a repair and re-examination record.

For day-to-day NDE hold-point tracking, see the weld inspection hold points guide for CWIs. For the complete weld quality plan framework, see weld quality control plan for AWS D1.1.

Implications for Your WPS and PQR

MT acceptance criteria appear in the project inspection and test plan (ITP), not on the WPS itself. However, the WPS does affect MT-ability indirectly:

Surface profile and accessibility — a WPS that produces excessive weld reinforcement, spatter, or a very coarse bead surface makes MT interpretation harder. Specifying a smooth final pass profile in the WPS helps.
Steel grade — the WPS's base metal selection determines whether MT is even applicable. A514 and A517 WPSs must note that prod technique is prohibited.
Repair WPS — always include a note in the repair WPS requiring MT (or PT for stainless) on the excavated cavity before the repair fill pass begins, to confirm complete crack removal.

If your WPS pairs are set up in a digital system that generates the full audit packet — WPS, PQR, inspection reports, and NDE records in one export — see pricing for how to generate those records automatically.

Rule library based on AWS D1.1:2025; verify against your governing edition (the AHJ or contract may specify 2020 or earlier).

FAQ

Frequently asked.

When does AWS D1.1:2025 require magnetic particle testing on structural welds?: AWS D1.1:2025 does not mandate MT on every weld. Contract documents or the engineer of record specify which welds require MT. The standard provides mandatory acceptance criteria and procedure requirements whenever MT is invoked on the project.
Can prod electrodes be used for MT on structural welds?: Prods are permitted under AWS D1.1 but restricted near the finished weld on quenched-and-tempered steels because prod contact points create arc burns that can cause local hardening and hydrogen-assisted cracking. Wet fluorescent yoke technique avoids this concern entirely.
What documentation is required after MT on a structural weld?: A written MT report recording the procedure, equipment, technique, coverage area, operator identity, and disposition of each indication — acceptable or rejected — is required and becomes a permanent part of the project quality record.
How does MT compare to UT for detecting weld discontinuities?: MT detects surface and near-surface discontinuities in ferromagnetic steel. UT reaches volumetric discontinuities at depth throughout the weld cross-section. On CJP groove welds where subsurface fusion flaws are the primary concern, UT is the workhorse; MT supplements it by catching surface-breaking cracks that UT can miss.