MT Acceptance Criteria for AWS D1.1 Structural Welds

Magnetic particle testing finds surface and near-surface discontinuities through a deceptively simple mechanism: magnetize the part, apply ferromagnetic particles, and observe where flux leaks at a discontinuity pull those particles into a visible indication. But executing MT on structural welds in compliance with AWS D1.1:2025 requires more precision than that description implies — from surface preparation through acceptance classification, the details matter.

This article covers the technique requirements, acceptance criteria, and common field errors that determine whether an MT examination is compliant with AWS D1.1:2025 Clause 9.

Where MT fits in the D1.1 inspection scheme

AWS D1.1 Clause 9 is the inspection clause. It specifies which NDE methods apply to which joint types and inspection categories:

• Visual testing (VT): Required for all welds, all categories.
• Ultrasonic testing (UT): Required for complete joint penetration (CJP) groove welds in tension zones of cyclically loaded structures (Inspection Category C), and commonly specified for statically loaded structures by contract or specification.
• Radiographic testing (RT): An alternative to UT for CJP groove welds; specific thickness limits apply.
• Magnetic particle testing (MT): Specified for surface discontinuity detection; particularly relevant for HAZ cracking in cyclically loaded applications, repair welds, and base metal evaluation.

MT's particular strength is detecting tight surface cracks — fatigue cracks, hydrogen-assisted cracking in the HAZ, crater cracks, and root cracks on accessible surfaces — that VT might miss on surfaces with weld spatter, mill scale, or other visual interference. It is not a replacement for volumetric NDE on CJP welds subject to tension.

ASTM E709 as the technique base

AWS D1.1:2025 references ASTM E709, Standard Guide for Magnetic Particle Testing, as the governing document for MT procedure and technique selection. The examiner must understand the relationship: E709 covers technique; D1.1 Clause 9 provides the acceptance criteria. Where D1.1 states specific requirements, they take precedence.

An MT procedure for D1.1 structural work should address:

• Magnetization technique (wet or dry method; yoke, prod, or coil)
• Particle type and application method (wet fluorescent, wet visible, dry visible)
• Current type (AC or DC, or half-wave DC for deeper penetration)
• Minimum field strength verification (tangential field intensity ≥ 30 Gauss, verified with a Hall-effect meter or pie gauge)
• Surface condition requirements
• Lighting requirements
• Personnel qualification

Personnel performing and interpreting MT on AWS D1.1 structural work should hold ACCP Level II or SNT-TC-1A Level II certification in MT. The certifying employer must have a documented qualification and certification program.

Surface preparation requirements

Surface condition is the most common source of MT technique deficiency. Indications can be masked or false indications generated by surface irregularities, and the standard inspection methods require a reasonably smooth surface.

Remove before MT:

• Weld spatter (chip or grind)
• Mill scale in the direct examination zone if it will interfere with particle mobility
• Heavy rust, paint, or coatings thicker than 0.003 inch (0.08 mm) for wet method; thicker coatings impede field entry
• Oil, grease, or other contamination that would prevent particle migration

Do not remove:

• Light tight mill scale that does not impede field entry or particle mobility — removal is often unnecessary on the weld HAZ itself if the surface is smooth

The inspection surface must be accessible to the magnetizing equipment and to the inspector's view without obstructions. Examining a fillet weld toe at the root of a girder flange in a tight box section requires tooling and lighting arranged before the examination begins, not after.

Magnetization technique selection

Yoke method

An electromagnetic yoke induces a longitudinal field between its poles. It is the most common technique for weld examination because it is safe (no arcing risk), portable, and does not require prod contact. The yoke is positioned so that discontinuities oriented perpendicular to the expected flaw direction are detectable — meaning a second examination at 90° is required to detect flaws in both orientations.

Verify adequate field strength with a lift-test: an AC yoke must lift at least a 10-lb (4.5-kg) weight; a DC yoke at least a 40-lb (18-kg) weight. Perform this test before and after each examination shift.

Prod technique

Prods pass current directly through the part, creating a circular magnetic field around the prod-to-prod axis. They can develop high field intensity and work on irregular surfaces where a yoke cannot bridge. Disadvantages: prod contact points can arc and damage the surface, and prod burning on finished structural steel may be a contract deficiency. AWS D1.1 does not prohibit prods, but project specifications often do.

Wet fluorescent MT (FWMT)

Fluorescent wet magnetic particle examination, conducted under UV-A (black) light with fluorescent particles in a carrier oil or water, provides the highest sensitivity of standard MT methods. The background fluorescence of the carrier provides contrast against which bright-yellow-green particle indications are immediately visible. Required minimum UV-A irradiance at the inspection surface is 1000 µW/cm²; visible light level must be less than 2 foot-candles.

FWMT is often specified for demand-critical welds and CVN-required applications where tight crack detection sensitivity is paramount. For routine weld inspection on statically loaded structures, dry visible or wet visible MT is typically adequate.

Acceptance criteria under AWS D1.1:2025

AWS D1.1 Clause 9 establishes acceptance criteria for MT indications by type and category:

Cracks — reject regardless of size

Any indication characterized as a crack is rejectable without dimensional threshold. This includes:

• Crater cracks at weld terminations
• Root cracks
• Longitudinal weld cracks
• Transverse cracks in the weld or HAZ
• Hydrogen-induced HAZ cracking (typically manifests as delayed cracking 24–72 hours after welding on high-strength or high-restraint applications)

An indication that cannot be classified as non-crack (porosity, slag inclusion, geometric) must be treated as a crack until proven otherwise by sectioning or further investigation.

Linear indications — length limit

A linear indication has a length-to-width ratio greater than 3:1. Under AWS D1.1:2025 Clause 9, linear indications in welds and the heat-affected zone that exceed 3/8 inch (10 mm) in length are rejectable.

Cyclically loaded structures (Inspection Category C and D) receive tighter criteria — verify the applicable acceptance level against the specific Clause 9 table for the inspection category. The contract or specification may impose criteria more stringent than the code minimum.

Rounded indications

A rounded indication has a length-to-width ratio of 3:1 or less. Rounded indications are subject to the same porosity acceptance criteria as visual inspection — the size limits in Clause 9 apply. Individual rounded indications and aggregate lengths within a given weld length are both bounded.

Laminations and base metal indications

MT is also used to screen plate and structural shapes for laminations, particularly on ends of members and burn-cut surfaces. Laminations running parallel to the surface and perpendicular to the weld thickness (i.e., through-thickness) are particularly dangerous under through-thickness tensile stress from flange-to-web fillet welds on column sections. These are characterized as planar discontinuities and are rejectable without a dimensional threshold if they are continuous.

Post-weld delay for hydrogen cracking

Hydrogen-assisted cracking (HAC) is a delayed phenomenon — it typically initiates below 250°F (120°C) and may not produce a detectable indication for 24–72 hours after welding. On high-strength steels (minimum yield strength ≥ 100 ksi), thick sections, high-restraint joints, or any application with elevated hydrogen potential, AWS D1.1 Clause 9 requires that MT not be performed until a minimum holding time after weld completion has elapsed. Verify the required hold time in Clause 9 for the specific application; examining immediately after cool-down and calling the weld clear is not compliant for these material and condition combinations.

Documentation requirements

Each MT examination requires a written record. At minimum, document:

• Joint or weld identification (WPS number, joint ID, weld map reference)
• Date and time of examination
• Examiner name, certification level, and employer
• Equipment used (make, model, calibration status)
• Technique applied (yoke or prod; AC/DC/HWDC; wet or dry, visible or fluorescent)
• Surface condition at time of examination
• Field strength verification result
• Indications found (location, dimensions, orientation, classification)
• Disposition (accept, reject, repair required)
• Examiner signature

A verbal pass at the end of a shift without written documentation is not a quality record. For AISC-certified fabricators and shops subject to audit, undocumented NDE is treated as NDE not performed.

For the broader context of how NDE fits into a project weld file and audit package, see weld inspection hold points and NDE documentation and common WPS deficiencies found in third-party audits. For shops managing NDE requirements across multiple WPSs and qualification records, the audit-ready welding procedure library article covers the documentation structure. And if your project involves cyclically loaded structures with CVN-required joints, see CVN supplementary essential variables: AWS D1.1 Table 6.8 for the examination timing and post-weld inspection requirements.

Managing NDE records alongside WPSs, PQRs, and welder qualifications in one system eliminates the audit-prep scramble; see what a complete audit packet looks like for how the tool exports the full weld file.

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