When does AWS D1.1 require magnetic particle testing?

AWS D1.1 does not require MT on every weld. MT is required when called out on structural drawings, in the project specification, or when visual inspection reveals indications warranting further examination. AWS D1.8 (seismic) mandates MT for demand-critical CJP welds in special moment frames. The governing document determines when MT is required.

Can MT detect subsurface defects?

MT detects surface and slightly subsurface indications — typically within about 1/4 in [6 mm] of the surface for wet fluorescent techniques. It cannot detect deep embedded defects. For volumetric inspection of embedded flaws, UT or RT is required.

Is the prod method allowed on finished structural welds?

AWS D1.1 permits the prod method but restricts it: prods must not be used on machined surfaces, and arc strikes at prod contact points are a concern on fracture-critical or demand-critical members. The yoke method is preferred on finished welds and on materials where arc strikes would be a nonconformance.

What does a rejectable MT indication look like?

Relevant linear indications longer than 3/16 in [5 mm] are rejectable under AWS D1.1. Relevant rounded indications with a major dimension greater than 3/16 in [5 mm] are also rejectable. Indications with a major dimension of 1/16 in [2 mm] or less are not considered relevant. Check the applicable clause in your governing edition for the exact thresholds.

Magnetic particle testing (MT) for structural welds: AWS D1.1 requirements

MT finds surface and near-surface cracks in structural welds fast. Here's how AWS D1.1:2025 specifies MT procedures, acceptance criteria, and documentation for fab shops.

Magnetic particle testing (MT) is one of the fastest NDE methods available to a structural fab shop. It requires no film processing, no couplant, and no radiation safety perimeter — a trained technician can check a completed weld in minutes. AWS D1.1:2025 references MT as an accepted inspection method for structural welds, with specific requirements for technique, acceptance criteria, and documentation.

Understanding when MT is required, how to apply it correctly, and what the acceptance criteria mean is part of the CWI and QC manager's toolkit — particularly on seismic, bridge, or fracture-critical work where surface crack detection requirements are explicit.

Rule library based on AWS D1.1:2025; verify against your governing edition.

How magnetic particle testing works

MT uses a magnetic field to reveal surface and near-surface discontinuities in ferromagnetic materials. Steel structural members and weld deposits are ferromagnetic — MT applies to most carbon and low-alloy structural steels used in AWS D1.1 work. It does not apply to austenitic stainless steel (which is non-magnetic) or aluminum; for stainless structural welds under AWS D1.6, liquid penetrant testing (PT) is the surface-inspection method of choice.

When a magnetic field is established in the test material, any discontinuity that interrupts the field — a crack, linear slag inclusion near the surface, or lack of fusion — creates a flux leakage at that location. Iron particles (dry powder or wet suspension) applied to the surface migrate to the leakage field and form a visible indication at the discontinuity location.

The key operational variables are:

Magnetization method (how the field is established)
Particle type (dry or wet; white-light visible or fluorescent)
Field strength and orientation (must be within roughly 45–90° of the indication to detect it)
Viewing conditions (adequate light for dry; UV/black light for fluorescent wet)

Magnetization methods: prod vs. yoke

Prod technique

Two electrical prods are placed in contact with the test surface a defined distance apart, and a direct current flows between them. The current creates a circular magnetic field in the material between and around the prods.

Advantages: portable, effective, and generates strong fields across wide areas.

Restrictions: prod placement leaves burn marks or arc strikes at the contact points if care is not taken — or if the prods arc when applied. On demand-critical welds, fracture-critical bridge members, and any surface where arc strikes are a nonconformance, the yoke method is preferred. AWS D1.1 requires that prods not be used on finished surfaces (as defined in the project spec) or machined surfaces without explicit authorization.

Prod spacing is typically 3–8 in [75–200 mm]. The gap and field orientation must be reset to check each 90° perpendicular direction, since a crack parallel to the current path will not be detected.

Yoke technique

A horseshoe-shaped electromagnet (AC or DC) is placed across the area to be examined. The field runs through the material between the yoke legs.

Advantages: no contact burn risk, compact, quiet. The AC yoke is preferred for surface indications because AC creates a stronger surface field; DC yokes penetrate deeper for near-surface flaws.

Limitations: yokes produce a directional field, so each examination area must be checked in two orientations at approximately 90° to each other. Yoke legs must maintain good contact with the surface.

A yoke must be checked periodically for minimum lifting power: AC yoke ≥ 10 lb [4.5 kg]; DC or permanent magnet yoke ≥ 40 lb [18 kg]. This is usually confirmed with a steel test piece of known weight.

Dry powder vs. wet fluorescent

Dry powder MT (DPT)

Dry colored particles (usually red, gray, or black) are dusted onto the examination surface while the field is applied. Dry powder is used outdoors, in field erection, or where wet suspension is impractical.

Dry powder examination is done under visible white light. The indication contrast depends on particle color vs. surface color — gray or red particles can be hard to read on mill scale or rusty steel. Surface preparation (removal of heavy mill scale and rust) improves sensitivity.

Wet fluorescent MT (WFMT)

Iron particles suspended in a liquid carrier (water or oil) are applied to the surface, and the test is conducted under ultraviolet (UV) black light. The particles fluoresce brightly — indications are highly visible against the dark background even when small.

WFMT is the preferred method for critical weld inspection because its sensitivity for small indications is significantly higher than dry powder. AISC 341 (seismic) and many fracture-critical member specifications require WFMT, not dry MT. The tradeoff is equipment cost, the need for a darkened inspection environment, and couplant fluid handling.

When MT is required under AWS D1.1

AWS D1.1:2025 does not mandate MT on every structural weld. MT is specified by:

The structural drawings or project specification. Engineering drawings for bridges, high-seismic moment frames, and similar structures will typically call out NDE methods for each weld category. MT may be listed for surface inspection of CJP flange-to-web welds, splice welds, or other critical connections.

AWS D1.8 seismic supplement. For demand-critical welds in special moment frames (SMFs) and special concentrically braced frames (SCBFs), AWS D1.8:2016 (the seismic supplement to AWS D1.1) requires MT or PT of completed welds in addition to visual inspection and UT. MT is the preferred method on the carbon steel base metals used in seismic moment frames because it offers higher sensitivity for surface cracks.

Inspector-discretionary MT. The CWI may request MT when visual inspection reveals an indication that warrants closer examination — a color change, an unusual surface texture, or a linear mark that could be a crack or could be a machining mark. This is judgment-based, not code-mandated.

For demand-critical weld NDE requirements and how they relate to UT, see UT acceptance criteria for AWS D1.1 structural welds.

Acceptance criteria under AWS D1.1

AWS D1.1:2025 sets acceptance criteria for MT indications based on indication type and size:

Relevant vs. non-relevant indications. Indications with a major dimension of 1/16 in [2 mm] or less are considered non-relevant and do not require further evaluation (they may be caused by geometry, permeability changes at the weld toe, or surface roughness, not actual discontinuities). Any indication greater than 1/16 in [2 mm] is treated as relevant and evaluated against the acceptance criteria.

Linear indications. An indication is linear when its length is at least three times its width. A relevant linear indication longer than 3/16 in [5 mm] is rejectable. Linear indications shorter than 3/16 in are accepted.

Rounded indications. A relevant rounded indication with a major dimension greater than 3/16 in [5 mm] is rejectable.

Adjacent indications. Multiple indications in a line — even if each is individually acceptable — may need to be evaluated as a single linear indication if the separation between them is small relative to the length of the longer indication. The specific aggregation rule is in the applicable inspection clause; refer to the current code for the spacing threshold.

Repair and re-inspection follow the same acceptance criteria. The governing weld must pass after repair before the joint is accepted.

MT procedure documentation

MT examinations on structural welds must be performed by certified personnel. AWS D1.1 references the qualification requirements of applicable personnel certification standards (ASNT SNT-TC-1A or ANSI/ASNT CP-189 are the most common references in structural fab).

The written MT procedure should document:

Magnetization technique (prod or yoke, AC or DC)
Particle type and application method (dry or wet fluorescent)
Field strength and verification method (lift check for yoke)
Coverage overlap requirements (typically 10% minimum)
Viewing conditions (minimum light levels for visible; UV intensity for fluorescent)
Acceptance criteria reference
Reporting requirements (which indications must be recorded)

MT examination records should capture: the joint identifier, the technician and certification level, the date and technique, a sketch or photograph of any relevant indications, and the disposition (accept/repair/reject). These records become part of the weld package and are subject to audit.

MT vs. PT for structural steel

Liquid penetrant testing (PT) is the alternative surface-inspection method. PT detects the same types of surface-breaking indications as MT, but through a different mechanism — penetrant seeps into surface-breaking discontinuities and is drawn out by developer.

For carbon and low-alloy structural steel welds (the domain of AWS D1.1), MT is generally preferred over PT for several reasons:

MT detects slightly subsurface indications (within a few millimeters); PT is limited to true surface-breaking flaws only.
MT is faster on large weld areas — a yoke pass covers a wide area quickly; PT requires dwell time and multiple application steps.
MT is less sensitive to surface contamination — heavy mill scale, paint, or coating can interfere with PT but has less effect on MT sensitivity.

PT is used on stainless steel and non-ferromagnetic materials where MT is not applicable. For AWS D1.6 stainless structural welds, PT replaces MT.

Documentation and audit readiness

For AISC certification or owner audits, MT records should be organized by weld joint ID and linked to the corresponding WPS, welder certification, and visual inspection records. A complete weld package for a demand-critical joint typically includes:

Visual inspection record (pass)
MT examination record (technique, date, technician, result)
UT examination record (if required)
Welder qualification certificate
WPS reference

Keeping these records in a connected digital system — rather than scattered PDFs — dramatically reduces the time to assemble an audit package. See welding procedure library audit readiness for how to structure this documentation, and WeldingWPS pricing for the platform that manages it.