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What MT Detects on a Weld

Magnetic particle testing reveals surface and slightly subsurface discontinuities in ferromagnetic materials — carbon steel, low-alloy steel, ferritic stainless — that are too small, too tight, or too oriented to find by visual inspection alone. The method magnetizes the test area and applies fine ferromagnetic particles to the surface; at any discontinuity that interrupts the magnetic flux path, the leaked flux holds the particles in a visible indication.

Practical detection depth depends on the magnetizing technique, current type, and material permeability, but typical surface-and-near-surface coverage is on the order of a few millimeters from the surface. Tight, surface-breaking cracks — including hot cracks, hydrogen cracks at the toe of fillet welds, lamellar tears in T-joints — are the classic MT targets in welding inspection.

MT does not work on austenitic stainless steel, aluminum, copper, brass, or other non-ferromagnetic materials. For those, use liquid penetrant testing (PT) per Clause 8.14.5.

Where MT Sits in D1.1 Clause 8

AWS D1.1:2025 places magnetic particle testing inside Part C of Clause 8 (Acceptance Criteria) under Clause 8.10 jointly with penetrant testing (PT). The procedure for MT specifically lives in Part D (Procedures) under Clause 8.14.4. Personnel qualification for any NDT method, including MT, lives in Clause 8.14.6.

Per Clause 8.10 verbatim: "Welds that are subject to MT or PT shall be evaluated on the basis of the acceptance criteria in 8.10.1. The testing shall be performed in conformance with 8.14.4 or 8.14.5, whichever is applicable." So Clause 8.10 establishes the acceptance side; Clause 8.14.4 establishes the procedural side; together they define the complete MT-on-welds requirement under D1.1.

When MT Is Required

Per the framing of Clause 8.6.4 (Specified NDT Other than Visual), NDT — including magnetic particle testing — is specified by the contract documents furnished to bidders. There is no general blanket D1.1 requirement that all welds be MT-tested. The Engineer determines on each project whether MT is required, on which welds, at what stage, and to what acceptance level.

Common project specifications that call for MT on welds include: tack welds before they are covered by final passes, root passes in critical CJP groove welds, repair excavations that are about to be re-welded, fillet welds in dynamically loaded connections where surface-breaking toe cracks would propagate under cyclic load, and any weld in a lifting lug or attachment to a fracture-critical member.

Per Clause 8.6.5, if NDT other than visual is not in the original contract but is later requested by the Owner, the Owner is responsible for the associated costs unless the testing reveals an attempt to defraud or gross nonconformance to the code — in which case the repair work is at the contractor's expense.

MT Procedure: ASTM E3024 (Mandatory) and ASTM E709 (Informational)

Per Clause 8.14.4 verbatim: "When MT is used, the procedure and technique shall be in conformance with ASTM E3024/E3024M, and the standard of acceptance shall be in conformance with Clause 8, Part C, of this code." So D1.1:2025 mandates ASTM E3024/E3024M for the procedure.

The older ASTM E709 ("Standard Guide for Magnetic Particle Testing") is referenced only in Commentary C-8.14.4 as informational guidance: "ASTM E709 is a standard guide and may be used for information in conjunction with E3024/E3024M." This is a common practitioner confusion — E709 is widely cited in older project specifications and shop documents, but the current D1.1:2025 mandatory reference is E3024/E3024M, not E709.

“If your project spec still cites ASTM E709 as the mandatory MT procedure for D1.1 work, that spec was written against an older edition of the code. D1.1:2025 Clause 8.14.4 mandates E3024/E3024M; E709 is informational only per Commentary C-8.14.4.”

D1.1:2025 Clause 8.14.4 + Commentary C-8.14.4

Common MT Techniques

Within the E3024 procedure, several techniques are typical for weld inspection:

• Yoke method (AC or DC) — most common for field weld inspection. A handheld electromagnetic yoke applies a magnetic field across two contact points. Portable, fast, no risk of arc strikes on the part. The dominant technique for in-place MT on structural welds.
• Prod method — current is passed through the part via two electrode prods. High sensitivity but historically associated with arc-strike risk at the prod contact points; many specifications restrict or prohibit prods on critical members.
• Wet continuous — particles suspended in a fluid carrier are applied while the magnetizing current is on. Highest sensitivity, used for shop-controlled inspection of critical components; less practical for field use.
• Dry continuous — dry particles dusted on the surface during magnetization. Common with yoke testing for field work where wet bath handling is impractical.

Inspector Qualification

Per Clause 8.14.6.1, NDT personnel performing testing other than visual shall be certified as NDT Level II in the test method and technique, or as NDT Level I working under the supervision of an NDT Level II. The Level II inspector interprets indications, evaluates them against acceptance criteria, and signs off on the inspection record.

Per Clause 8.14.6.2, employer-based certification follows either ASNT SNT-TC-1A Recommended Practice or ANSI/ASNT CP-189. Certification of Level I and II individuals shall be performed by a Level III. Per Clause 8.14.6.3, third-party certification options include the ASNT Central Certification Program (ACCP), ASNT CP-9712, CAN/CGSB-48.9712, and ISO 9712. Per Clause 8.14.6.5, NDT personnel performing inspections under 8.14.6 are exempted from the AWS QC1 (CWI) requirements — an MT inspector does not need a CWI to perform MT on a D1.1 job, though many do hold both credentials.

For a side-by-side of CWI vs ASNT credentials, see the CWI vs ASNT Level 2 reference.

MT vs PT vs UT — Decision Matrix

MT Acceptance Criteria (Clause 8.10.1)

Per Clause 8.10.1: "All MT and PT indications shall meet the acceptance requirements of Table 8.1." So the MT inspector applies the same Table 8.1 criteria used for visual inspection — with two key classification rules added by Clause 8.10.1:

• Linear discontinuity: defined as one whose length exceeds three times its width. Per 8.10.1(1), linear discontinuities shall be considered equal to the size and shape of the indication itself.
• Rounded discontinuity: defined as one whose length is three times its width or less. May be round or irregular and may have tails. Per 8.10.1(2), prior to interpretation, the medium (particles for MT, developer for PT) shall be removed for accurate measurement.

Per Clause 8.10.2, the finishing of surfaces to improve detectability of discontinuities using MT or PT is permitted, provided the operation conforms to Clause 7. This is how shops legitimately grind weld toes or surfaces to remove visual obstructions before MT — subject to Clause 7 limits on grinding into base metal or weld throat.

Per Clause 8.11.1 (Time of Testing), MT may begin immediately after the completed welds have cooled to ambient temperature — with one critical exception: ASTM A514, A517, and A709 Grade HPS 100W [690W] steels require NDT to be performed not less than 48 hours after weld completion, to allow delayed (hydrogen-induced) cracking time to develop before inspection.

Related Standards Guides

• D1.1 NDE Requirements — Side-by-side reference for MT, PT, RT, UT, and PAUT methods
• Ultrasonic Testing of Welds — D1.1:2025 — UT for volumetric inspection per Clause 8.15
• Phased Array Ultrasonic Testing (PAUT) — Annex H mandatory PAUT requirements
• Visual Weld Inspection — D1.1 Clause 8.9 and Table 8.1 (the same acceptance criteria MT defers to)
• Weld Defects — What MT finds: cracks, lack of fusion, surface porosity, undercut
• CWI vs ASNT Level 2 — Inspector qualification crosswalk between visual (CWI) and method-specific NDT (ASNT Level II)
• Heat Input Calculator — Heat input drives HAZ properties; high heat input increases the cracking tendency MT is meant to catch