13.2 Magnetic Particle Testing: Flux Leakage, Field Geometry & Prod Amperage

3. Magnetic Particle Testing (MT): Ferromagnetic Mechanics & Field Configurations

Magnetic Particle Testing (ASME Section V Article 7, ASTM E709, ASTM E1444, AWS D1.1 Clause 8) detects surface and near-surface discontinuities in ferromagnetic materials (carbon steels, low-alloy steels, ferritic and martensitic stainless steels). It is completely ineffective on austenitic stainless steels (e.g., AISI 304, 316), aluminum, copper, or titanium alloys.

Magnetic Flux Leakage (MFL) Physics

Ferromagnetic materials possess high magnetic permeability ($\mu_r \gg 1$). Discontinuities (cracks, lack of fusion, slag inclusions) represent voids of non-magnetic permeability ($\mu_r \approx 1$, high magnetic reluctance). When magnetic flux encounters a boundary perpendicular to its vector path, the lines of flux cannot crowd entirely through the remaining metal matrix without saturating. A portion of the flux bridges the gap by leaking out into the air above the surface, establishing local North and South magnetic poles.

                  MAGNETIC FLUX LEAKAGE (MFL) AT CRACK APEX

                   Leakage Flux Trapping Magnetic Particles
                              S            N
                           .-'--------------'-.
                          /                    \
     Magnetic Flux: B    V                      V     Magnetic Flux: B
     ==================> +----------------------+ ==================>
                         |   Low Permeability   |
                         |    Reluctance Gap    |
                         |   (Crack Fissure)    |
     --------------------+                      +--------------------
                               Ferromagnetic Matrix

Finely divided ferromagnetic particles (iron oxides, $\text{Fe}_3\text{O}_4$) dusted or sprayed over the region are drawn toward these localized magnetic poles by the gradient field force:

$\mathbf{F} = \nabla (\mathbf{m} \cdot \mathbf{B})$

creating a distinct visible indication that outlines the flaw profile.

Magnetization Methods: Continuous vs. Residual

1. Continuous Magnetization (Mandatory Standard): Ferromagnetic particles (dry powder or wet suspension) are applied to the test surface while the magnetizing force is actively applied, and particle application terminates prior to current cutoff. This technique utilizes maximum magnetic flux density ($B_{\text{max}}$) along the material's hysteresis loop, providing maximum sensitivity for tight planar cracks and subsurface flaws.
2. Residual Magnetization: Particles are applied after the magnetizing current has been terminated, relying strictly on the material's retentivity ($B_r$). This method is strictly prohibited on low-carbon structural steels (such as ASTM A36 or A572), which have low coercivity and near-zero magnetic retentivity. It is permissible only on high-carbon, hardened alloy steels with high remanence.

Field Geometries: Longitudinal vs. Circular Fields

To trigger flux leakage, the magnetic field must intersect the planar flaw at an angle between $45^\circ$ and $90^\circ$ (ideally orthogonal, $90^\circ$). Because parallel flaws do not distort flux lines, two separate, perpendicular inspections are mandatory for full coverage:

      CIRCULAR MAGNETIZATION                      LONGITUDINAL MAGNETIZATION
    (Current Through Component)                   (Coil / Solenoid or Yoke)

             Current (I)                                Magnetic Flux (B)
                ====>                                        ======>
          +---------------+                           +-------------------+
          |    ( B )      |                           |                   |
          |   /     \     |                           | ~~~~~~~~~~~~~~~~~ | Flux
          |  (   *   )    | Circular Flux             |                   |
          |   \     /     |                           +-------------------+
          +---------------+                                     ^
                 V                                              |
       Detects LONGITUDINAL Flaws                     Detects TRANSVERSE Flaws

• Longitudinal Fields: Induced by solenoid coils or electromagnetic yokes. Flux lines travel parallel to the longitudinal axis of the part or yoke legs. Detects transverse discontinuities oriented perpendicular to the axis.
• Circular Fields: Induced by passing direct electric current through the component (headstock/tailstock), through a central conductor bar, or via localized prods. Flux lines form concentric circles centered on the current path ($B = \frac{\mu I}{2 \pi r}$). Detects longitudinal discontinuities oriented parallel to current flow.

Yokes, Prods & Field Generation Equipment

        ARTICULATED ELECTROMAGNETIC YOKE              PROD MAGNETIZATION SYSTEM

               [ AC / DC Switch ]                        [ High Current Source ]
               +----------------+                            |             |
               |      Coil      |                            V             V
               +-------+--------+                         Prod 1        Prod 2
                      / \                                    |             |
                     /   \ Articulated                       V             V
                    /     \ Legs                         ===*===============*===
                   V       V                              Arc Strike Risk Zone!
              ===+===========+===                        Local Martensite Formation
                 Inspection Zone

Field Verification Media: Pie Gages, Strips & Hall-Effect Probes

• Pie Gage (ASME / AWS Standard): An octagonal copper-faced disk containing eight low-carbon steel pie segments brazed together with non-ferromagnetic copper lines. Placed copper-side up on the weldment during magnetization; crisp particle lines across the seams confirm adequate field strength and verify flux direction.
• Flexible Flux Indicator Strips (Castrol / ASTM E1444 Type I/II): Thin, flexible polymeric shims containing slotted foils protected by brass. Taped directly across the weld seam; particles orient over internal slots when tangential flux reaches $\ge 30\text{ Gauss}$ ($2.4\text{ kA/m}$).
• Tangential Hall-Effect Gaussmeter: Digital magnetometer with transverse Hall probe held flush against the plate. Code mandates a minimum tangential field of $30\text{ to }60\text{ Gauss}$ ($2.4\text{ to }4.8\text{ kA/m}$) throughout the inspection volume.
• Demagnetization: High residual magnetism interferes with subsequent GTAW or GMAW arcs (inducing severe arc blow), distorts nearby navigational equipment, and traps metal shavings during machining. Demagnetization is executed by pulling the weldment through an AC solenoid coil while decaying current to zero, confirming residual fields $\le 3\text{ Gauss}$ ($0.24\text{ kA/m}$).

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4. Worked Engineering Examples

Example 1: Capillary Driving Pressure in Crack Fissures (PT)

Problem: A liquid penetrant formulated with surface tension $\gamma = 0.035\text{ N/m}$ and a wetting contact angle $\theta = 8^\circ$ on clean structural steel is applied to two distinct surface-breaking discontinuities:

1. A tight fatigue microcrack with an effective fissure half-width $r_1 = 0.5\ \mu\text{m}$ ($0.5 \times 10^{-6}\text{ m}$).
2. A wide surface gas pore with radius $r_2 = 250\ \mu\text{m}$ ($250 \times 10^{-6}\text{ m}$).

Calculate the capillary driving pressure ($\Delta P$) forcing penetrant into each discontinuity, and contrast the resulting capillary suction forces.

Solution:

• Step 1: Capillary Pressure Formula $\Delta P = \frac{2 \gamma \cos \theta}{r}$ Calculate $\cos(8^\circ) = 0.99027$.

• Step 2: Microcrack Entry Pressure ($r_1 = 0.5 \times 10^{-6}\text{ m}$) $\Delta P_1 = \frac{2 \times (0.035\text{ N/m}) \times 0.99027}{0.5 \times 10^{-6}\text{ m}} = \frac{0.069319}{0.5 \times 10^{-6}} = 138,638\text{ Pa} \approx 138.6\text{ kPa} \approx 20.1\text{ psi}$

• Step 3: Broad Gas Pore Entry Pressure ($r_2 = 250 \times 10^{-6}\text{ m}$) $\Delta P_2 = \frac{2 \times (0.035\text{ N/m}) \times 0.99027}{250 \times 10^{-6}\text{ m}} = \frac{0.069319}{2.5 \times 10^{-4}} = 277.3\text{ Pa} \approx 0.277\text{ kPa} \approx 0.040\text{ psi}$

• Engineering Evaluation: The capillary suction driving penetrant into the tight fatigue crack is $500\text{ times greater}$ than that entering the broad pore. However, during the post-dwell wash phase, the broad pore will be easily evacuated by overwashing, whereas penetrant trapped within the high-capillarity fatigue crack remains secured against wash pressures up to $40\text{ psi}$.

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Example 2: Prod Magnetization Amperage Sizing per AWS D1.1

Problem: An AWS Certified Welding Engineer is establishing a dry powder magnetic particle inspection procedure for a $32\text{ mm}$ ($1.25\text{ in}$) thick butt joint in an ASTM A572 Grade 50 steel plate using dual prods. The prod spacing is fixed at $L_p = 8.0\text{ inches}$ ($203\text{ mm}$). Under AWS D1.1 / ASME Section V Article 7, the recommended current for prod spacing $\ge 6\text{ in}$ and thickness $\ge 3/4\text{ in}$ is $100\text{ to }125\text{ Amperes}$ per inch of prod spacing. Calculate the required inspection amperage range and evaluate the electrical parameters.

Solution:

• Step 1: Calculate Minimum Amperage $I_{\min} = 100\text{ A/in} \times 8.0\text{ in} = 800\text{ A}$

• Step 2: Calculate Maximum Amperage $I_{\max} = 125\text{ A/in} \times 8.0\text{ in} = 1000\text{ A}$

• Engineering Evaluation: The power source must deliver $800\text{ to }1000\text{ A}$ of Half-Wave Rectified Direct Current (HWDC) or AC. To avoid arc strikes at this high amperage, the operator must confirm firm contact before energizing the prods and must de-energize the circuit before breaking physical contact. If prods are used on quenched and tempered steels, this procedure should be rejected in favor of an electromagnetic AC yoke to prevent arc strike martensite damage.

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5. Industrial Case Studies & CWEng Examination Traps

Real-World Field Disaster Scenario

During the fabrication of an offshore production platform node connecting heavy $75\text{ mm}$ thick high-strength low-alloy structural tubulars, NDE technicians conducted MT using direct contact prods at $900\text{ A}$. Several prod tips slipped on the curved pipe surface, producing micro-arc strikes outside the weld zone. The technicians did not flag the arc strikes because the weld itself exhibited zero indications. Three weeks later, during hydrostatic proof testing at $1.5\times$ design pressure, a catastrophic brittle rupture initiated directly from one of the prod arc strikes. Metallurgical failure analysis revealed that the instantaneous high current melted a localized hemisphere of base metal ($d \approx 1.5\text{ mm}$) that was rapidly self-quenched by the surrounding $75\text{ mm}$ steel heat sink, forming an untempered martensite micro-zone with a microhardness of $580\text{ HV}$ and severe microscopic hot/cold cracking. The incident resulted in $2.4\text{ million}$ in structural repairs and prompted a code revision banning prod testing on high-strength offshore structural members without mandatory mechanical grinding and etching of prod touch-points.

Certified Welding Engineer Exam Pitfalls

Exam Trap 1: AC vs. DC Yoke Lift Test Capacities Examination candidates routinely transpose the lift test capacities for electromagnetic yokes. Remember: an AC yoke requires a $10\text{ lb}$ ($4.5\text{ kg}$) lift capacity due to the skin effect concentrating flux at the surface. A DC yoke requires a $40\text{ lb}$ ($18.1\text{ kg}$) lift capacity because DC flux penetrates deep into the core of the plate, requiring a much higher total magnetic energy density to generate equivalent surface leakage fields.

Exam Trap 2: Direct Solvent Spraying in Method C Penetrant Testing In Method C (solvent-removable) PT, technicians must never spray solvent directly onto the weldment to remove excess penetrant. Direct solvent application immediately dissolves and washes the penetrant entrapped within shallow or tight crack fissures, yielding fatal false-negative inspection results. The solvent must be sprayed onto a clean, lint-free cloth, which is then used to manually wipe the surface until pink background staining ceases.

Exam Trap 3: Subsurface Discontinuity Detection with PT A classic multiple-choice trap asks whether Liquid Penetrant Testing can detect subsurface inclusions, laminations, or incomplete joint penetration in a sound weld with a pristine crown. The answer is an absolute no. PT operates exclusively via capillary entry; if a discontinuity is not physically open to the exterior surface, PT has zero detection capability regardless of penetrant sensitivity level.