Magnetic Particle Testing of Welds: Procedure, Standards & Acceptance Criteria

Magnetic particle testing (MT) is the standard surface NDT method for ferromagnetic weld inspection. Fast, portable, and highly sensitive to surface-breaking cracks, it is referenced by every major fabrication code — EN 1090, AWS D1.1, ASME VIII, API 650, API 1104, EN 15085. Yet MT is routinely mis-specified: applied to austenitic stainless welds where it is physically impossible, used to “verify” internal soundness it cannot reach, and documented in ways that fail ISO 3834 and EN 1090 audit reviews. This guide covers the physics, procedure, standards, acceptance criteria, and documentation requirements a welding quality engineer needs to apply MT correctly.

How Magnetic Particle Testing Works

MT exploits the permeability contrast between intact ferromagnetic material and discontinuities. When a weld is magnetised, a surface or near-surface crack interrupts the flux path and creates a leakage field that projects above the surface. Ferromagnetic particles applied to the surface migrate toward this leakage field and accumulate, forming a visible indication.

Two primary magnetisation configurations are used for weld inspection:

Directional sensitivity: a crack parallel to the magnetic flux lines creates minimal leakage and will not produce a reliable indication. The induced field must be oriented perpendicular to the expected crack plane — this is the most common cause of missed defects in field MT.

Detection depth: reliable for surface-breaking and near-surface discontinuities to approximately 3 mm below the surface. MT is not a substitute for volumetric NDT (PAUT, RT) for internal flaws.

Material constraint: MT applies only to ferromagnetic materials — carbon steel, low-alloy steel, martensitic and ferritic stainless. It cannot inspect austenitic stainless steels, aluminium, titanium, or nickel alloys.

Applicable Standards Overview

EN ISO 17638 — Examination Method

EN ISO 17638:2016 (Non-destructive testing of welds — Magnetic particle testing) defines the examination procedure requirements:

• Magnetisation techniques and minimum tangential field strength (2 kA/m at the examination surface)
• Particle types: dry powder, wet colour contrast, wet fluorescent
• Surface preparation and maximum permitted roughness
• Lighting: 500 lux minimum for colour contrast; UV-A irradiance of at least 10 W/m² at 300–400 mm for fluorescent MT
• Examination sequence and overlap requirements
• Report content

EN ISO 23278 — Acceptance Levels

EN ISO 23278:2015 (Magnetic particle testing — Acceptance levels) defines three graded acceptance levels:

The level must be specified in the contract, the Inspection and Test Plan, or the welding quality plan — not left to the inspector’s judgment on-site.

North American Codes

When working under both European and North American codes (common in LNG, offshore, and energy projects), specify which code governs the MT inspection and do not mix acceptance tables.

Step-by-Step MT Procedure for Weld Inspection

1. Pre-Inspection Planning

Define in the Inspection and Test Plan (ITP):

• Joint references and weld map identifiers
• Applicable standard (EN ISO 17638) and acceptance level (EN ISO 23278 level 1, 2, or 3)
• Inspection stage: post-weld, post-PWHT, post-repair, or in-service
• Personnel qualification: ISO 9712 Level 2 MT (or ASNT Level II, PCN Level 2, as applicable)

2. Surface Preparation

• Remove weld spatter, heavy scale, oil, and grease from the weld bead, toes, and a minimum 25 mm band into the base material on each side.
• Surface roughness per EN ISO 17638: Ra ≤ 12.5 μm for wet fluorescent MT; smoother surfaces improve particle mobility and indication contrast.
• A thin coat of white contrast aid paint is permitted — and often necessary — for colour-contrast MT on dark-coloured weld surfaces.

3. Magnetisation

Position the yoke or prods so the induced field is perpendicular to the expected crack orientation:

• Transverse cracks (crossing the weld axis, typical in high-restraint joints and hydrogen cracking): magnetise longitudinally along the weld.
• Longitudinal cracks (along the weld bead, typical of solidification cracking and undercut cracking): magnetise transverse to the weld.
• For complete coverage, perform two separate magnetisations at approximately 90° to each other.

Yoke adequacy check (per EN ISO 17638):

• AC yoke: minimum lift force of 4.5 kg at the pole spacing used
• DC/permanent yoke: minimum lift force of 18 kg

Verify tangential field strength with a Hall-effect gaussmeter or a field indicator (pi-gauge): the reading at the examination surface must reach at least 2 kA/m.

4. Particle Application and Examination

Apply particles during active magnetisation (continuous method):

• Wet fluorescent: spray or flow bath, then inspect under UV-A (black light) with ambient white light below 20 lux. The UV-A irradiance at the surface must be confirmed at 10 W/m² or above at the working distance.
• Wet colour contrast: spray, then inspect under white light at 500 lux minimum; a lux meter at the examination surface is required for the inspection record.

Scan the weld bead, both toes, and the HAZ at least 150 mm either side of the weld centreline.

5. Indication Classification and Recording

Classify each indication before evaluating it:

• Linear indication: length is three times the width or more — crack-type, highest severity
• Non-linear / rounded indication: length is less than three times the width — pore-type, lower severity
• Pseudo-indication: caused by geometric features (weld toe radius, backing bar edge, fit-up gap) — confirmed non-relevant by re-magnetising at 90°

Record each relevant indication: position on the weld map, type (linear/non-linear), measured length and width, photograph. A photo with a scale reference next to the indication is required for any submission under EN 3834-2 or EN 1090.

6. Evaluation Against Acceptance Criteria

Compare measured indication dimensions against the applicable acceptance table. The simplified decision framework below is based on EN ISO 23278 Table 1:

Any linear indication at Level 1 or Level 2 exceeding these limits requires removal and re-inspection. The standard also specifies cumulative indication rules and minimum distances between indications — consult EN ISO 23278 directly for complete evaluation.

7. Demagnetisation

Components that will be subsequently arc-welded, or that are sensitive to residual magnetism (bearings, instruments, hydraulic components), must be demagnetised using a decreasing AC field after MT. Verify residual flux density below 0.3 mT with a calibrated gaussmeter. Record the result in the inspection report.

MT vs Other Surface and Near-Surface NDT Methods

MT is preferred over penetrant testing (PT) for ferritic and martensitic welds because it detects near-surface discontinuities (not just open-to-surface), covers more area per cycle, and does not require the drying time that PT demands on porous or heavily scaled surfaces. When austenitic stainless steel, aluminium, or titanium welds require surface NDT, PT is the mandatory choice. For volumetric defects or full-thickness inspection, PAUT or radiographic testing are required — MT does not complement them, it precedes them in the inspection sequence.

Digital Documentation and ISO 3834 Compliance

Paper-based MT reports — hand-drawn indication maps, unlabelled photographs, unsigned PDFs — are the single most frequent gap found in EN 3834-2 and EN 1090 third-party audits. The standard requires full traceability: joint reference, material heat number, operator name and qualification number, equipment calibration status, applicable standard and acceptance level, and the actual result (accept / reject with evidence).

A digital welding quality system links MT records directly to the weld traveller, the WPS, and the heat number of the joint material. This eliminates the transcription errors that generate non-conformances at certification bodies and customer audits.

Key Takeaways

• MT detects surface and near-surface cracks (down to ~3 mm) in ferromagnetic welds fast and at low cost — it is not a substitute for volumetric NDT for internal flaws.
• EN ISO 17638 governs the method; EN ISO 23278 provides acceptance levels 1 (most stringent) through 3.
• Always match the acceptance level to the structural classification in the governing code — EN 1090 EXC3 typically requires Level 2; EN 15085 CL1 requires Level 1.
• Magnetise perpendicular to the expected crack orientation — a yoke parallel to a transverse crack will miss it.
• Two orthogonal magnetisations (90° apart) are required for full-volume surface coverage.
• Demagnetise after MT whenever the component will be arc-welded or is sensitive to residual magnetism.
• Complete, traceable digital NDT records are a mandatory element of ISO 3834-2 and EN 1090 certification — not a nice-to-have.