10.2 Preparation and Qualification of MT Written Procedures

10.2 Preparation and Qualification of MT Written Procedures

Engineering Architecture: Written Procedure vs. Technique Sheet

In non-destructive examination, standard quality management frameworks enforce a strict hierarchical division between high-level general procedures and part-specific work instructions. An NDT Level III must clearly distinguish between a Written Procedure and a Part-Specific Technique Sheet:

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|                  NDT PROCEDURAL DOCUMENTATION HIERARCHY                     |
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| LEVEL 1: CODES & STANDARDS      | ASME Sec V, ASTM E1444, AWS D1.1, ISO 9934|
| LEVEL 2: WRITTEN PROCEDURE      | General methodology for product form/alloy|
| LEVEL 3: TECHNIQUE SHEET        | Exact shot sequence & amps for Part #1234 |
| LEVEL 4: INSPECTION REPORT      | Actual physical findings & disposition    |
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• NDT Written Procedure: An overarching technical engineering specification that governs the application of a test method across a broad product family, material group, or manufacturing discipline (e.g., "Magnetic Particle Examination of High-Pressure Carbon and Low-Alloy Steel Welds"). It establishes mandatory equipment tolerances, chemical controls, surface preparation rules, calibration intervals, evaluation criteria, and essential variable boundaries. It is qualified once for that range of variables.
• Part-Specific Technique Sheet (Technique Card): A concise, shop-floor instruction document dedicated to a single part number, drawing, or specific joint geometry. It details the exact operational settings: contact head clamping position, central conductor diameter, coil diameter and position, exact magnetizing current (in amperes), shot duration, particle application timing, and sequence of orthogonal shots. Technique sheets are authored by or under the direct direction of a Level III and must conform strictly to the boundaries of the governing Written Procedure.

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Ten Mandatory Technical Elements of an MT Written Procedure

To satisfy the stringent compliance requirements of ASME Section V Article 7 (T-150 / T-721), ASTM E1444, and ISO 9934-1, an industrial magnetic particle examination procedure must encompass ten distinct technical sections:

1. Scope and Applicable Documents

• Scope: Explicitly defines the components, manufacturing processes (e.g., forgings, castings, rolled plate, complete joint penetration welds), material types (ferromagnetic steels, ductile irons), and dimensional boundaries covered by the procedure.
• Applicable Documents: Cites all governing construction codes (ASME Section VIII, Section I, B31.3), testing standards (ASME Section V Art 7, ASTM E1444/E1444M, ASTM E709), specification references (AMS 2641, AMS 3044), and internal quality manual specifications. Establishes the contractual order of precedence.

2. Personnel Qualification Requirements

• Identifies the mandatory qualification standard (e.g., employer's Written Practice written to satisfy SNT-TC-1A, CP-189, or NAS 410).
• Establishes the minimum certification tier required to calibrate equipment, perform examinations, evaluate indications, and sign official inspection reports (typically certified NDT Level II or Level III; Level I allowed only to perform specific calibrations and tests under direct supervision).
• Re-states mandatory annual vision requirements (Jaeger 2 or Jaeger 1 near acuity, color differentiation).

3. Equipment and Materials Specifications

• Magnetizing Units: Lists approved commercial power sources (e.g., 6,000-Ampere stationary wet horizontal bench units, 2,000-Ampere mobile power packs, portable articulated AC/DC electromagnetic yokes). Cites allowable open-circuit voltage ranges (6 to 24 V).
• Magnetic Particles: Defines brand names, product numbers, and chemical specifications:
  • Wet Fluorescent: AMS 3044 (dry concentrate) or AMS 3045 (liquid concentrate) suspended in oil vehicle, or AMS 3046 (aerosol cans).
  • Dry Color-Contrast: AMS 3042 (gray, red, yellow, or black powders).
• Carrier Vehicles: Specifies carrier chemistry conforming to AMS 2641 Type I (dearomatized aliphatic petroleum distillates with flash point ≥ 200°F / 93°C) or conditioned water carriers containing corrosion inhibitors, anti-foaming agents, and wetting agents.
• Concentration Thresholds: Mandates daily centrifuge settling limits using a 100-mL pear-shaped centrifuge tube:
  • Fluorescent bath: 0.1 to 0.4 mL per 100 mL (after 30 min settle for oil, 60 min for water).
  • Visible wet bath: 1.2 to 2.4 mL per 100 mL.
• Sensors and Meters: Specifies calibrated photometers (for white light) and calibrated UV-A radiometers centered at 365 nm.

4. Surface Preparation and Cleanliness

• Initial Cleaning: Mandatory removal of all oil, grease, scale, rust, flux, spatter, and machining lubricants using solvent degreasing, vapor degreasing, or wire brushing.
• Abrasive Blasting Restrictions: Blast cleaning with coarse media or shot peening is strictly prohibited on precision components because it can peen over crack edges and mechanically seal surface-breaking discontinuities, preventing particle entrapment.
• Protective Coatings and Dry Film Thickness (DFT):
  • Bare metal inspection is standard.
  • Non-magnetic coatings (paint, zinc primer) may remain on the component only if the dry film thickness does not exceed 0.002 in (0.05 mm / 50 µm) per ASME Section V T-741.1(d).
  • If coating thickness exceeds 0.002 in, the procedure must be formally qualified in accordance with Mandatory Appendix I.

5. Magnetization Techniques and Electrical Parameters

Defines the electrical waveforms and quantitative magnetizing parameters for each geometry:

• Current Waveform Selection: Alternating Current (AC) for surface discontinuities and maximum particle mobility; Half-Wave Rectified DC (HWDC) for subsurface weld/casting flaws; Full-Wave Rectified DC (FWDC) for high-amperage central conductor inspection of forged rings.
• Direct Contact Head Shots (Circular): Enforces current density rules of 300 to 800 Amperes per inch (12 to 31 A/mm) of outer diameter per ASTM E1444.
• Central Conductor Shots (Circular): Requires non-ferromagnetic (copper) central conductors with current calculated based on outer component diameter. Off-center central conductors require multiple indexed shots to ensure 360° coverage within an effective radius of 4× the conductor diameter.
• Longitudinal Coil Shots: Establishes coil formulas for low fill-factor configurations (< 10% cross-sectional fill):
  • Parts centered in coil: NI = 45,000 / (L/D)
  • Parts placed against inside coil wall: NI = 43,000 / (L/D)
  • Maximum allowable ampere-turns: NI ≤ 50,000 A-t.
• Electromagnetic Yokes: Specifies articulated leg spacing (2 to 8 inches / 50 to 200 mm) and mandatory AC operation for surface fatigue crack detection.

6. System Performance Verification and Calibration

• Mechanical Lifting Force: AC yokes must demonstrate a minimum 10 lb (4.5 kg) dead-weight lift; DC yokes must lift 40 lb (18 kg) at maximum working pole spacing, verified prior to use each day under ASME Section V T-762.2(a) (ASTM E1444 Table 1 permits up to 6 months) and after any damage or repair.
• Field Verification Devices: Mandates Quantitative Quality Indicators (QQIs per SAE AS 5371) or calibrated Hall-effect Gaussmeters measuring tangential magnetic field strength (Ht = 30 to 60 Gauss / 2.4 to 4.8 kA/m). Prohibits ASME magnetic pie gauges from quantitative aerospace sensitivity determination.
• Ketos Ring Verification: ASME Section V T-766 requires the ring test at least once per week on horizontal units, at 1 400 A, 2 500 A, and 3 400 A FWDC through a central conductor, revealing a minimum of 3, 5, and 6 holes respectively; if the required holes do not appear, the equipment is taken out of service until the cause is found and corrected.
• Ammeter Calibration: Verification every 6 months (ASTM E1444 Table 1) or at least once a year (ASME Section V T-761), taking comparative readings at three current output levels across the usable range. The tolerances differ: ASTM E1444 allows ±10% of the reading or ±50 A, whichever is greater, while ASME Section V T-762.1 states ±10% of full scale.
• Lighting Verification: Visible light ≥ 100 fc (1 076 lux) at inspection plane; fluorescent UV-A ≥ 1000 µW/cm² at 15 inches with ambient booth white light ≤ 2 fc (21.5 lux).

7. Examination Protocol

• Application Timing (The Continuous Method): The procedure must state the sequence separately for each medium, because ASME Section V T-773 specifies opposite orders. For dry particles (T-773(a)), the magnetizing current remains on while the powder is applied and while excess is removed. For wet particles (T-773(b)), the current is turned on after the particles have been applied, and particle flow stops with the application of current; aerosol or pump-spray application may occur before and/or during magnetization. For wet baths, two consecutive current shots of 0.5 to 1.0 second duration are commonly specified.
• Directional Coverage: Every area must receive a minimum of two separate magnetic shots oriented orthogonally (at approximately 90° to each other) to detect discontinuities oriented in any physical direction.
• Scanning Overlap: Adjacent prod or yoke examination areas must overlap by a minimum of 1 inch (25 mm).

8. Evaluation and Acceptance Criteria

• Defines the minimum threshold for indication relevance: any particle accumulation with a major dimension exceeding 1/16 in (1.5 mm).
• Establishes mathematical aspect ratio classifications:
  • Linear Indication: L > 3W
  • Rounded Indication: L ≤ 3W
• Enforces governing code rejection standards (e.g., ASME Section VIII Mandatory Appendix 6-4: zero tolerance for relevant linear indications, and rejection of relevant rounded indications greater than 3/16 in. (5 mm) as a flat limit independent of plate thickness).
• Details diagnostic protocols for arbitrating false and non-relevant indications (solvent wiping, demagnetization and re-test at lower amperage, blend grinding).

9. Demagnetization and Post-Cleaning Protocol

• Demagnetization Mandate: When residual fields can interfere with subsequent electron beam or GTAW welding (causing arc blow), machining, or navigation instruments, components must be demagnetized.
• Demag Technique: Step-down continuous AC coil pullout or reversing DC step-down sequence.
• Residual Field Limit: Maximum residual field shall not exceed 3 Gauss (240 A/m or 0.3 mT) measured with a calibrated Hall-effect Gaussmeter or field indicator.
• Post-Cleaning: Complete solvent washing to remove oil carrier, particle residues, and fluorescent dyes; immediate application of water-displacing rust preventative oil.

10. Documentation and Reporting

Defines required log entries: part name, serial number, procedure number and revision, technique sheet number, equipment serial numbers, particle lot numbers, carrier type, amperage and shot durations, sketch and dimensions of all reportable indications, acceptance disposition, technician certification level, inspector signature, and date.

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Essential vs. Non-Essential Variables (ASME Section V Table T-721)

A critical technical duty of an NDT Level III authoring procedures under ASME Section V is categorizing variables in accordance with Table T-721:

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|         ASME SECTION V, TABLE T-721 PROCEDURAL VARIABLE CLASSIFICATION      |
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| VARIABLE DESCRIPTION                       | STATUS     | ACTION ON CHANGE  |
|--------------------------------------------+------------+-------------------|
| Magnetizing technique (prod, yoke, coil)   | Essential  | Re-qualification  |
| Current type (AC, HWDC, FWDC)              | Essential  | Re-qualification  |
| Particle type (dry, wet, visible, fluor.)  | Essential  | Re-qualification  |
| Amperage decrease / prod spacing increase  | Essential  | Re-qualification  |
| Surface preparation / coating > 0.002 in   | Essential  | Re-qualification  |
| Temperature outside allowable range        | Essential  | Re-qualification  |
| Demagnetization technique                  | Non-Essent.| Revision only     |
| Post-cleaning protocol                     | Non-Essent.| Revision only     |
| Lighting equipment brand                   | Non-Essent.| Revision only     |
| Carrier vehicle brand (within specs)       | Non-Essent.| Revision only     |
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Rule of Law: Any modification of an Essential Variable invalidates the existing procedure qualification, legally mandating that the Level III revise the procedure and perform a formal physical demonstration and re-qualification. Changing a Non-Essential Variable requires only an administrative revision to the written procedure without re-qualification.

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Procedure Qualification Demonstration and the PQR

Before a written procedure can be approved and issued to production, its technical adequacy must be validated through a physical qualification demonstration.

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|                PROCEDURE QUALIFICATION WORKFLOW (PQR TO RELEASE)            |
+-----------------------------------------------------------------------------+
| 1. DRAFT WRITTEN PROCEDURE    | Establish parameters, tolerances, techniques|
| 2. SELECT TEST SPECIMENS      | Fabricate samples with known target flaws   |
| 3. EXECUTE DEMONSTRATION      | Witnessed MT inspection per draft procedure |
| 4. VERIFY FLAW DETECTION      | Document 100% detection of critical flaws   |
| 5. COMPILE FORMAL PQR         | Record actual parameters, photos, data      |
| 6. LEVEL III & AIA SIGN-OFF   | Issue approved Procedure to production      |
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Step-by-Step Qualification Demonstration Protocol

1. Specimen Fabrication & Flaw Characterization:
   • The Level III must prepare representative qualification test specimens possessing identical metallurgical composition, product form (forging, casting, plate), heat treatment, and surface finish as production hardware.
   • The specimens must contain confirmed, documented surface-breaking flaws of the minimum rejectable dimensions established by the referencing code (e.g., thermal quench cracks, low-cycle fatigue cracks, or precision electro-discharge machined [EDM] notches).
   • Artificial notches (EDM) must have width ≤ 0.005 in (0.13 mm) and depth representative of threshold rejection limits (0.010 to 0.020 in).
2. Witnessed Demonstration:
   • The Level III executes the examination strictly in accordance with the draft written procedure in the presence of the Authorized Nuclear Inspector (ANI), Authorized Inspector (AI), or customer quality auditor.
   • System performance is verified using QQI shims and calibrated Gaussmeters.
   • The test pieces are magnetized, particles are applied using the continuous method, and indications are visually observed under calibrated lighting.
3. Flaw Detection Verification:
   • The procedure is deemed qualified only when 100% of the target qualification flaws are clearly and unambiguously detected with strong fluorescent or color-contrast indications.
4. Compilation of the Procedure Qualification Record (PQR):
   • The actual physical parameters used during the successful demonstration are recorded on a formal Procedure Qualification Record (PQR).
   • The PQR documents: base metal alloy, specimen dimensions, flaw types and locations, surface condition (or coating thickness), equipment model and serial numbers, particle batch number, actual measured magnetizing amperage, flux density (Gauss), lighting levels, and photographic documentation of indications.
   • The PQR is signed and certified by the NDT Level III and countersigned by the customer's quality representative or Authorized Inspector.
   • The Written Procedure references the supporting PQR number, establishing its legal pedigree.

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Level III Practical Engineering Scenario and Exam Traps

Worked Engineering Scenario

Scenario: A fabricator holds an ASME Section VIII MT procedure qualified for inspecting unpainted 1.0 in thick SA-516 Grade 70 shell welds using an AC electromagnetic yoke and dry gray particles. A new client contracts for offshore pressure vessels requiring an external marine epoxy coating with a nominal dry film thickness of 0.006 in (150 µm). The production manager requests an addendum to the existing procedure stating that testing will proceed through the paint using the same AC yoke.

Level III Technical Analysis & Mandatory Actions:

1. Essential Variable Violation: Under ASME Section V Table T-721 and paragraph T-741.1(d), an increase in coating thickness beyond 0.002 in (0.05 mm) is an Essential Variable.
2. Invalidity of Existing Procedure: The existing procedure cannot be applied to painted surfaces exceeding 0.002 in.
3. Mandatory Appendix I Qualification Protocol:
   • The Level III must prepare a steel test block matching SA-516 Grade 70 containing confirmed surface cracks of the minimum rejectable length (> 1/16 in).
   • The marine epoxy coating must be applied to the test block to a verified dry film thickness of at least 0.006 in (or the maximum expected production thickness, e.g., 0.008 in).
   • The Level III executes the yoke examination through the coating, demonstrating crisp, unambiguous detection of the target cracks.
   • The Level III documents the demonstration on a new Procedure Qualification Record (PQR) per Mandatory Appendix I.
   • The written procedure is formally revised to cite the PQR, specify the maximum qualified coating thickness (0.008 in), mandate daily verification of production coating thickness with a calibrated magnetic dry-film thickness (DFT) gauge, and define cleaning protocols for painted surfaces.