MT and PT surface NDE for structural welds under AWS D1.1
MT catches surface and near-surface weld flaws; PT catches surface-only. Here's when AWS D1.1:2025 requires each and how to document results for QC records.
Visual examination catches what the eye can see at the surface. Ultrasonic testing finds subsurface volumetric flaws. Between those two sits a class of methods — magnetic particle testing (MT) and liquid penetrant testing (PT) — that reveal surface and near-surface discontinuities that visual inspection misses but that UT is not optimized to find. Both are cost-effective, fast, and require no expensive capital equipment. For a CWI managing structural steel production, knowing when to deploy MT versus PT, and how to document results, is basic quality competency.
MT and PT: What Each Method Detects
Magnetic particle testing (MT) applies a magnetic field to the part and then introduces fine iron particles — either dry powder or suspended in a liquid carrier (wet method). Discontinuities at or near the surface create magnetic flux leakage, and the particles collect at those leakage points, making cracks and other flaws visible. The key capability: MT detects discontinuities that are slightly subsurface — up to roughly 1/4 in depth for favorable orientations — in addition to surface-breaking flaws.
MT only works on ferromagnetic materials: carbon steel and low-alloy steel are ideal. Austenitic stainless steel, aluminum, and copper alloys are non-ferromagnetic and cannot be examined with MT.
Liquid penetrant testing (PT) applies a dye (visible color-contrast or fluorescent) to the cleaned surface. Capillary action draws the penetrant into any surface-open crack or pore. After a dwell time, the excess penetrant is removed and a developer applied, which draws the entrapped dye back out and creates an indication. PT reveals what is literally open to the surface and nothing more.
PT works on virtually any non-porous material — carbon steel, stainless, aluminum, nickel alloys. It is the surface examination method when MT is not applicable.
Both methods are sensitive to flaw orientation. A tight crack running parallel to the applied magnetic field (for MT) or sealed by scale or paint (for PT) may not produce a detectable indication. Surface preparation is non-negotiable for either method.
When AWS D1.1:2025 Applies MT and PT
AWS D1.1:2025 does not prescribe MT or PT on every structural weld by default. The code's default inspection regime is visual examination (Clause 8.9) plus mandatory UT or RT for specific weld categories and joint types. MT and PT enter the picture through several channels:
Contract documents and engineer specification. The most common trigger. An engineer of record may specify MT on all CJP groove welds, PT on all stainless welds, or supplemental MT on weld repairs. This is contractual and the shop must comply regardless of whether the code mandates it.
Repair welds. AWS D1.1:2025 imposes additional inspection requirements on repaired welds. A weld repair that removes and replaces metal is often subject to the same NDE as the original weld, plus MT or PT to confirm the excavation removed the full extent of the discontinuity before rewelding.
AISC certification requirements. AISC Standard for Steel Building Structures (AISC 360 and the AISC quality certification program) references supplemental NDE including MT for specific connection categories in higher seismic design categories. Shops pursuing AISC certification need to document NDE protocols that satisfy these requirements.
AWS D1.8 seismic supplement. AWS D1.8 imposes additional requirements for demand-critical welds in seismic moment frames. MT (or PT for non-ferromagnetic materials) on backing removal areas and weld access hole surfaces is explicitly required. If your shop handles seismic-designed steel moment frames, MT requirements come from D1.8, not D1.1 alone.
Owner or third-party QC specification. Bridge owners, industrial plant operators, and heavy-industry owners often impose supplemental NDE beyond the governing code. Treat the most stringent requirement as controlling.
Rule library based on AWS D1.1:2025; verify against your governing edition.
Acceptance Criteria Under AWS D1.1
When MT or PT is performed, the acceptance or rejection of an indication must be referenced to a specific criterion. AWS D1.1:2025 Clause 8 provides visual acceptance criteria and references the applicable NDE methods and standards. For MT performed under AWS D1.1, the typical acceptance criterion used is:
Linear indications longer than 1/16 in (2 mm) are rejectable, particularly in CJP groove welds in tension
Rounded indications (porosity, inclusions) have size and spacing limits
Cracks of any length are rejectable
The specific acceptance table used depends on the weld category (statically loaded vs. cyclically loaded vs. tubular) and the joint type. The acceptance criteria for CJP groove welds in cyclically loaded structures are stricter than for fillet welds in statically loaded members. Consult Table 8.1 (visual acceptance) and the referenced NDE procedure for the weld category you are inspecting.
Two main techniques are used in structural weld inspection:
Dry powder MT (DA) — dry iron particles applied to the magnetized part. Practical for field use where liquid carrier is inconvenient. Sensitivity is slightly lower than wet methods. Used for outdoor structural erection and field repair inspection.
Wet fluorescent MT (WFMT) — iron particles suspended in a liquid carrier, viewed under UV (black) light. Higher sensitivity than dry powder. Preferred for shop work where controlled lighting conditions are achievable. Required for some specifications (notably ASME code work, though that standard governs pressure vessels, not structural steel).
Magnetization technique also matters. Yoke magnetization (an electromagnetic yoke) is the most common for weld inspection — the yoke is walked along the weld in two perpendicular directions to catch differently oriented flaws. Prod technique (prods pressed against the surface to pass current) is also used but leaves burn marks at prod contact points and is restricted or prohibited in some specifications. Coil technique is used for bar or shaft shapes, rarely for weld seams.
PT Technique Selection
For structural weld inspection, visible color-contrast PT (Type I, Method B — removable with solvent) is the workhorse. The steps:
Pre-clean the weld and adjacent base metal to bare metal; remove mill scale, spatter, and coatings.
Apply penetrant; hold dwell time per the product's data sheet (minimum 10 minutes is typical for Type I visible penetrant at ambient temperature).
Remove excess penetrant with a clean, lint-free cloth; avoid flooding with solvent, which pulls penetrant from the flaw.
Apply developer; allow development time.
Examine under adequate white light (minimum 100 fc / 1000 lx at the surface per most procedure references).
Fluorescent PT (Type II) requires UV illumination and is used when higher sensitivity is required or specified.
Temperature matters. PT has performance limits below approximately 40°F (5°C) and above 125°F (50°C) for standard penetrant systems. Cold-weather field inspections using standard PT chemicals need special low-temperature products or the results are not valid.
Documenting MT and PT Results
A complete MT or PT inspection report for a structural weld should include:
Inspector information: name, ASNT certification level (Level II or III), and certification expiration date
Date and location of inspection
Technique: MT-yoke/dry, MT-yoke/wet, PT-visible/solvent removable, etc.
Equipment used: yoke model, current settings (for prod technique), penetrant product name and batch number
Surface condition prior to inspection: as-welded, ground, etc.
Acceptance criteria reference: AWS D1.1:2025 Table 8.1 or contract specification
Findings: accept or reject, with location and description of any rejectable indications
Disposition of rejectable indications: repaired, re-inspected, accepted by engineering disposition
Avoid NDE reports that record only "pass" or "accept" without identifying the criteria used. An owner auditor or CWI reviewing the project file must be able to reconstruct what was inspected, what criteria applied, and what was found without asking the inspector to explain.
Keep MT and PT reports in the project quality file alongside the WPS, PQR, WPQ, and fit-up inspection records. See welding procedure library: audit-ready organization for how to structure a complete weld quality package.
Inspector Qualifications
MT and PT performed for acceptance purposes under AWS D1.1 require an inspector qualified to a recognized standard. The typical reference is ASNT SNT-TC-1A or ASNT CP-189, with Level II certification in the applicable method. A CWI can interpret and accept or reject based on visual examination, but MT and PT interpretation requires method-specific qualification.
Third-party NDE contractors must provide copies of their current certifications as part of project documentation. Do not accept an NDE report from an inspector whose certification has lapsed — the inspection result has no standing without a valid credential behind it.
When to Use MT Instead of UT
The decision between MT and UT is not always obvious. MT is faster and cheaper than UT for surface examination. UT detects subsurface volumetric flaws that MT will miss entirely. For the majority of structural groove welds required by AWS D1.1, UT is the volumetric NDE of choice precisely because it reaches through the full weld cross-section.
MT is the right call when:
Surface cracking is the specific concern (weld toe, HAZ cracking, repair excavation edges)
The weld geometry makes UT difficult (fillet welds, complex intersections)
The specification requires MT regardless of UT results
If you're building or auditing your NDE documentation package and want a platform that tracks weld-specific inspection records, NDE report status, and inspector qualifications against your WPS roster, see our welding QC platform.
Summary
MT and PT are the primary surface NDE methods used in structural weld inspection under AWS D1.1. MT reaches slightly subsurface and requires ferromagnetic material; PT is surface-only and works on any alloy. Neither is mandated by default for every weld — the trigger is the contract documents, the engineer of record, or supplemental standards such as AWS D1.8. When performed, both methods require qualified ASNT Level II inspectors, documented technique selection, and written inspection reports that identify the criteria, the finding, and the disposition of any rejectable indications.
FAQ
Frequently asked.
What's the difference between MT and PT for weld inspection?
Magnetic particle testing (MT) detects surface and near-surface discontinuities in ferromagnetic materials by applying a magnetic field and iron particles that cluster at flux leakage points. Liquid penetrant testing (PT) detects only surface-open discontinuities in any material by drawing a dye into cracks through capillary action. MT goes slightly subsurface; PT is limited to flaws open to the surface.
Does AWS D1.1:2025 specify when MT or PT must be used?
AWS D1.1:2025 does not mandate MT or PT on every weld as a default — the required NDE methods are specified by the contract documents, the engineer of record, or a referenced specification such as AISC 360. The code does provide acceptance criteria (Clause 8) and procedural requirements that govern MT and PT when they are specified. Some joint categories in AWS D1.1:2025 Clause 5 reference supplemental NDE including MT.
Can you use PT on carbon steel structural welds?
Yes. PT works on any non-porous material and can be used on carbon steel, stainless, aluminum, or other alloys. MT is generally preferred for carbon and low-alloy steel because it detects near-surface flaws that PT misses. PT is the correct choice when MT is not applicable — for instance, when welding austenitic stainless steel, which is non-ferromagnetic.
What records are required after an MT or PT inspection?
At minimum: the inspector name and certification level (ASNT), the date and weld identification, the technique used (wet fluorescent MT, color-contrast PT, etc.), acceptance or rejection finding, and any repair disposition. The NDE report should reference the WPS number and the acceptance criteria standard used (AWS D1.1:2025 Table 8.1 or equivalent). Keep records in the project quality file.