8.3 Magnetic Particle (MT), Liquid Penetrant (PT) & Inspection Records

Magnetic Particle (MT), Liquid Penetrant (PT) & Inspection Records

While radiographic and ultrasonic examination provide volumetric inspection of structural welds, Magnetic Particle Testing (MT) and Liquid Penetrant Testing (PT) serve as the frontline non-destructive surface examination methods under AWS D1.1/D1.1M:2025. Governed by Clause 8 (Inspection), specifically Clause 8.10 (MT and PT), Clause 8.14 (NDT Procedures), Clause 8.15 (Extent of Testing), and industry consensus standards ASTM E709 and ASTM E165, these techniques detect fine surface-breaking cracks, lack of fusion, toe tears, and lamellar tears that visual inspection might miss.

Certified Welding Inspectors (CWIs) must understand the metallurgical and physical boundaries of MT and PT, the critical 48-hour delay rule for high-strength quenched and tempered steels, non-conformance documentation workflows, and code-mandated record retention rules.

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Magnetic Particle Testing (MT) Principles & Yoke Requirements (Clause 8.10 & ASTM E709)

Magnetic Particle Testing operates on the principle of magnetic flux leakage. When a ferromagnetic steel component is magnetized, magnetic lines of force (flux lines) pass through the material. A surface-breaking or near-surface discontinuity creates a localized disruption in magnetic permeability, forcing flux lines to leak out into the air above the flaw. Fine ferromagnetic iron oxide particles applied to the surface are attracted to these magnetic poles, creating a visible indication.

+-------------------------------------------------------------------------+
|                    MAGNETIC FLUX LEAKAGE AT SURFACE FLAW                |
+-------------------------------------------------------------------------+
|                                                                         |
|                        [ ELECTROMAGNETIC YOKE ]                         |
|                           Leg 1         Leg 2                           |
|                          +----+         +----+                          |
|                          |    |         |    |                          |
|          ================+====+=========+====+===============           |
|          BASE METAL      |  Magnetic Flux Flow  |   BASE METAL          |
|          ----------------+->->->->-+-<-<-<-<-<--+-------------          |
|                                    | Flux Leakage                       |
|                             \ | /  v                                    |
|                            -- X --  <-- Iron Particles Attracted        |
|                             / | \       to Surface Crack                |
+-------------------------------------------------------------------------+

1. Material Limitations

MT is applicable strictly to ferromagnetic materials (carbon steels and low-alloy structural steels). MT cannot be used on austenitic stainless steels (such as AISI 304 or 316), aluminum alloys, copper alloys, or nickel-base alloys because they lack ferromagnetic permeability.

2. Electromagnetic Yoke Requirements & The 10 lb / 40 lb Lift Test

The articulated-leg electromagnetic yoke is the primary magnetization tool in structural welding fabrication:

• Alternating Current (AC) Yokes: AC current generates an alternating magnetic field that concentrates on the outer skin of the steel (the skin effect). AC yokes produce superior particle mobility and are optimal for detecting fine, sharp, surface-breaking fatigue and shrinkage cracks. Under ASTM E709 and AWS D1.1, an AC yoke must have a dead-weight lifting power of at least 10 lb [4.5 kg] at the maximum pole spacing to be used.
• Direct Current (DC) / Rectified Yokes: DC magnetic fields penetrate deeper beneath the surface, offering limited detection of near-surface inclusions (up to approximately 1/8 in. [3 mm] depth). A DC yoke must have a dead-weight lifting power of at least 40 lb [18 kg] at the maximum pole spacing (or 30 lb [14 kg] for pole spacings between 2 to 4 in.). An annual or pre-shift lift test on a calibrated steel test weight is mandatory.

3. Inspection Media & The Continuous Method

• Dry Powder Method: Finely milled ferromagnetic particles coated with color pigments (red, yellow, black, or grey) applied via a powder blower or squeeze bulb. Dry powder is ideal for field inspections, rough as-welded surfaces, and elevated temperature applications up to 600°F [315°C].
• Wet Fluorescent Method: Particles suspended in a light petroleum distillate or conditioned water vehicle, viewed under ultraviolet radiation (UV-A / Black Light, λ = 365 nm) with an intensity of at least 1,000 µW/cm² at a distance of 15 in. [380 mm] in a darkened inspection area (ambient white light ≤ 20 lux / 2 foot-candles). Provides maximum sensitivity for machined surfaces.
• The Continuous Method Mandate: Under AWS D1.1, the continuous method is standard. The magnetizing current must remain energized WHILE the magnetic particles are applied to the joint and while excess powder is gently blown away with a low-pressure air stream. The residual method (applying particles after current is shut off) is prohibited unless specifically qualified.

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Liquid Penetrant Testing (PT) Principles & Controls (Clause 8.10 & ASTM E165)

Liquid Penetrant Testing utilizes capillary action to draw a low-viscosity liquid dye into clean, surface-breaking discontinuities. After removing excess surface penetrant, a porous white developer is applied, which acts as a blotter to draw the trapped penetrant back out of the defect cavity to form an enlarged, high-contrast bleed-out indication.

+-------------------------------------------------------------------------+
|                    LIQUID PENETRANT PROCESS SEQUENCE                    |
+-------------------------------------------------------------------------+
|                                                                         |
| 1. CLEAN SURFACE    2. APPLY PENETRANT   3. REMOVE EXCESS   4. DEVELOP  |
|    (Solvent/Dry)       (Dwell 10-20 min)    (Wipe clean)       (Bleed-out)|
|    +-----------+       +---vvvvvvv---+      +-----------+     +-w-w-w-w-w-+ |
|    |   |   |   |       |   |~~~~~|   |      |   |     | |     | | |RED| | | |
|    |   |   |   |       |   |~~~~~|   |      |   |~~~~~| |     | | |DYE| | | |
|    +---+---+---+       +---+-----+---+      +---+-----+---+   +-+-+---+-+-+ |
|        Crack               Crack                Crack             Crack     |
+-------------------------------------------------------------------------+

1. Penetrant Classifications (ASTM E165)

• Penetrant Types: Type I (Fluorescent Penetrant, viewed under UV-A black light) and Type II (Visible Dye Penetrant, typically bright red dye viewed in white light ≥ 100 foot-candles [1076 lux]).
• Penetrant Removal Methods:
  • Method A: Water-washable (built-in emulsifier).
  • Method B: Post-emulsifiable lipophilic (oil-based).
  • Method C: Solvent-removable (standard for structural field welding using aerosol cleaners and lint-free wiping rags).
  • Method D: Post-emulsifiable hydrophilic (water-based).
• Developer Forms: Form d (Nonaqueous developer in aerosol cans) is universally utilized on structural steel. It forms a uniform, ultra-thin white powder coating that provides high optical contrast against red dye.

2. Dwell Times & Temperature Windows

• Standard Temperature Range: The base metal and penetrant materials must be maintained between 40°F and 125°F [4°C to 52°C] during the entire examination.
• Penetrant Dwell Time: For structural steel welds and heat-affected zones, the minimum penetrant dwell time is 10 to 20 minutes (20 minutes recommended for fine cracks).
• Developer Developing Time: Developing begins as soon as the wet developer spray dries. Inspection of indications must be performed within 10 minutes minimum to 60 minutes maximum after developer application. Readings taken after 60 minutes are invalid due to excessive lateral dye bleed-out.

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The 48-Hour Delay Rule for High-Strength Q&T Steels (Clause 8.15.4)

In structural steel fabrication, high-strength quenched and tempered (Q&T) alloy steels—specifically ASTM A514, ASTM A517, and ASTM A709 Grade 100 / 100W (Fy = 90 to 100 ksi [620 to 690 MPa])—are exceptionally susceptible to Hydrogen-Induced Delayed Cracking (HIC).

+-------------------------------------------------------------------------+
|                 THE MANDATORY 48-HOUR NDT DELAY CLOCK                   |
+-------------------------------------------------------------------------+
|                                                                         |
| [ WELD COMPLETED ] --------> [ 48-HOUR HOLDING PERIOD ] ------> [ NDT ] |
|  - Cooling to ambient          - Ambient room temperature        - Final|
|  - High residual stress        - Diffusible hydrogen migrates      VT   |
|  - Susceptible microstructure  - Micro-cracks initiate & grow    - MT/PT|
|                                                                  - RT/UT|
+-------------------------------------------------------------------------+

Code Mandate (Clause 8.15.4 & Table 8.1 Notes)

Under Clause 8.15.4, for ASTM A514, A517, and A709 Gr 100 steels, final visual inspection and all non-destructive testing (MT, PT, RT, UT) shall NOT be performed until at least 48 hours have elapsed after the completed weld has cooled to ambient temperature.

Engineering Rationale: Diffusible hydrogen atoms trapped in the martensitic heat-affected zone require time to diffuse toward high triaxial tensile stress concentrations at weld toes or root notches. Micro-fissures frequently do not coalesce into detectable macroscopic cracks until 24 to 48 hours post-welding. Performing NDT immediately after welding creates a false sense of security and misses delayed hydrogen cracks.

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MT/PT Acceptance Criteria (Clause 8.10)

Under AWS D1.1 Clause 8.10, weldments examined by Magnetic Particle or Liquid Penetrant testing must satisfy the exact Visual Inspection Acceptance Criteria of Table 8.1. Clause 8.10.1 of the 2025 edition also defines how indications are classified: a linear discontinuity is one whose length exceeds three times its width; a rounded discontinuity is one whose length is three times its width or less:

1. Cracks: Zero tolerance. Any crack indication of any size or length is rejectable and must be excavated and repaired.
2. Relevant Indications: Only indications with a major dimension greater than 1/16 in. [1.6 mm] are considered relevant. Indications smaller than 1/16 in. are classed as non-relevant unless they form aligned clusters.
3. Fusion Defects & Undercut: Lack of fusion indications or sharp undercut indications revealed by MT/PT are rejectable per Table 8.1 limits.
4. Linear vs. Rounded Indications: Linear indications (length > 3 × width) are evaluated as cracks or lack of fusion. Rounded indications (length ≤ 3 × width) are evaluated under porosity criteria.

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Quality Documentation, Inspection Records & Retention Rules (Clause 8.5, 8.18 & 8.26)

Quality assurance under AWS D1.1 is rooted in thorough, contemporaneous documentation.

+-------------------------------------------------------------------------+
|                   QUALITY DOCUMENTATION LIFECYCLE                       |
+-------------------------------------------------------------------------+
| 1. CWI DAILY LOGBOOK:       Continuous record of fit-up, WPS, preheat   |
| 2. NDT EXAMINATION REPORT:  Detailed mapping of RT/UT/MT/PT results     |
| 3. NON-CONFORMANCE (NCR):   Formal hold on non-compliant welds          |
| 4. REPAIR VERIFICATION:     Re-examination of completed weld repairs    |
| 5. RECORD RETENTION:        Contractor retains RT/NDT for >= 1 YEAR     |
+-------------------------------------------------------------------------+

Why this section carries weight out of proportion to its length. Reports and Records is one of the five content areas AWS lists in the published test specification, with a minimum of 5% of the questions. Those items are not about metallurgy; they are about which document proves what, who signs it, and how long it is kept.

0. The Required Document Set

A recurring exam pattern is to describe a jobsite document and ask which of the four record types it is. The discriminator is simple: a PQR records what actually happened once, a WPS instructs what may happen repeatedly, and a WQR qualifies a person rather than a procedure.

1. Contractor Inspection vs. Verification Inspection (Clause 8.1.2)

• Contractor Inspection (Quality Control - QC): The primary quality control performed by the contractor's internal CWIs and NDT technicians to ensure all materials, fit-up, welding procedures, and workmanship conform strictly to contract documents.
• Verification Inspection (Quality Assurance - QA): Independent oversight performed by the Owner's Inspector or Engineer to verify that the contractor's QC program is effective. Verification inspection does not relieve the contractor of primary QC responsibility.

2. NDT Personnel Certification (Clause 8.14.6) and Examination Reports

The 2025 edition expands Clause 8.14.6 on how NDT personnel may be certified: 8.14.6.2 recognizes employer-based certification written to ASNT SNT-TC-1A or ANSI/ASNT CP-189, and 8.14.6.3 recognizes internationally recognized third-party certification such as ANSI/ASNT CP-9712, CAN/CGSB-48.9712, or ISO 9712.

Every NDT report generated on an AWS D1.1 project must record: (1) Project name and location, (2) Joint identification and weld number, (3) Base metal specification and thickness, (4) Welding process and WPS ID, (5) Specific NDT procedure and revision, (6) Equipment serial numbers, transducers/isotopes, and calibration blocks, (7) Detailed sketch or map of all acceptable and rejectable indications, and (8) Signature and NDT Level II/III certification details of the examiner.

3. Record & Radiograph Retention Mandate (Clause 8.18 & 8.26)

Unless explicitly superseded by more stringent project specifications in the contract documents, AWS D1.1 mandates that the contractor shall retain a complete copy of all radiographic films, digital radiographs, ultrasonic data, MT/PT examination reports, and inspection logs for a MINIMUM of ONE (1) YEAR following completion of the work, and shall make them available to the Engineer upon request.

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Worked Example: High-Strength Steel Inspection Audit

Problem Statement

A fabricator completes a heavy CJP moment-connection weld on an ASTM A514 Grade B crane runway girder at 4:00 PM on Tuesday afternoon. The welding was performed with low-hydrogen SMAW electrodes (E11018M). At 9:00 AM on Wednesday morning (17 hours after welding), the contractor's NDT technician performs an AC yoke magnetic particle inspection. The yoke was calibrated with a 5 lb dead weight. The technician reports zero indications and signs a final QC acceptance report. At 2:00 PM on Friday afternoon, the Owner's Verification Inspector audits the joint. What code violations have occurred under AWS D1.1?

Step-by-Step Code Evaluation

1. Evaluate the 48-Hour Delay Rule:
   • Base metal is ASTM A514, a high-strength quenched and tempered alloy steel (Fy = 100 ksi).
   • Under Clause 8.15.4, final visual inspection and all NDT cannot be performed until at least 48 hours have elapsed after weld completion.
   • Elapsed time on Wednesday at 9:00 AM was only 17 hours (17 < 48). The inspection was premature, invalid, and violates Clause 8.15.4.
2. Evaluate AC Yoke Lift Test Calibration:
   • Under ASTM E709 and AWS D1.1 Clause 8.10, an AC electromagnetic yoke must demonstrate a minimum lifting force of 10 lb [4.5 kg] at maximum pole spacing.
   • Calibrating the yoke with a 5 lb weight is non-compliant, as the yoke possessed insufficient magnetic field strength to guarantee defect detection.
3. Mandate Corrective Action:
   • The Wednesday MT acceptance report must be voided and an NCR issued.
   • The AC yoke must be re-calibrated and proven to lift a verified 10 lb test weight.
   • Because more than 48 hours have now elapsed (Tuesday 4:00 PM to Friday 2:00 PM = 70 hours), a full 100% MT re-examination of the weld and HAZ must be performed.