ASNT Level I Magnetic Particle Testing

Magnetic Fields and Flux Lines

A magnetic field is a region of space where a magnetic force can be detected. Every magnet - whether a permanent bar magnet, an electromagnet, or the field produced by current flowing through a conductor - generates a field that extends outward from one pole and returns to the other, forming continuous closed loops.

Magnetic flux lines (also called lines of force) are the visual representation of the field's direction and intensity. Understanding their behavior is essential for MT because the entire method depends on disrupting these flux lines at a discontinuity.

Properties of Flux Lines

• Flux lines form continuous closed loops - they never start or stop. Outside the magnet they travel from the north pole to the south pole; inside the magnet they travel from south back to north.
• They never cross each other.
• They follow the path of least reluctance (magnetic resistance). In a ferromagnetic material, reluctance is very low, so flux lines preferentially travel through the material rather than through air.
• Where flux lines are closely spaced, the field is stronger. Where they are spread apart, the field is weaker.
• Flux lines always try to take the shortest path through the material.

Flux Density (B)

Flux density (B) measures the amount of magnetic flux passing through a unit area, expressed in Tesla (T) or Gauss (G). The relationship is straightforward:

1 Tesla = 10,000 Gauss

For MT purposes, the tangential field strength at the part surface typically needs to be between 30 and 60 Gauss for adequate sensitivity. Below 30 Gauss, the leakage field at a small crack may be too weak to attract particles. Above approximately 60 Gauss, background noise from domain boundaries and surface roughness can begin to obscure real indications.

Magnetizing Force (H)

The magnetizing force (H), measured in Amperes per meter (A/m) or Oersteds (Oe), is the external driving force that creates the magnetic field. The fundamental relationship between B and H is:

B = μ × H

Where μ (mu) is the material's permeability - its ability to concentrate magnetic flux. For ferromagnetic materials, μ is very large (hundreds to thousands times that of air), meaning a modest magnetizing force produces a very dense flux field inside the part. This is why MT works - the ferromagnetic material concentrates flux so effectively that even a small interruption (a crack) forces flux out of the material, creating a detectable leakage field.

Material Classification by Magnetic Response

All materials interact with magnetic fields, but the nature and strength of that interaction varies dramatically. Understanding these classifications is fundamental for MT because the method only works on one category: ferromagnetic materials.

Ferromagnetic Materials

Ferromagnetic materials are strongly attracted to magnetic fields and can retain magnetization after the external field is removed. They have very high permeability - typically hundreds to thousands of times greater than air.

Common ferromagnetic materials suitable for MT:

• Iron (Fe) and all carbon steels (1018, 1045, 4130, 4140, 4340, A36, A572, etc.)
• Low-alloy steels used in pressure vessels and structural applications
• Ferritic stainless steels (400 series - 410, 430, 446)
• Martensitic stainless steels (410, 420, 440C)
• Nickel (Ni) and certain nickel alloys (Invar, Monel 400 in some conditions)
• Cobalt (Co) and certain cobalt alloys
• Cast irons (gray, ductile, malleable)

Materials that CANNOT be tested by MT (not ferromagnetic):

• Austenitic stainless steels (300 series - 304, 304L, 316, 316L, 321, 347)
• Aluminum and all aluminum alloys
• Copper, brass, bronze, and all copper alloys
• Titanium and titanium alloys
• Magnesium alloys
• Lead, tin, zinc

Paramagnetic Materials

Paramagnetic materials are very weakly attracted to magnetic fields. Their permeability is only slightly greater than 1 (approximately 1.00001 to 1.003). They do not retain magnetism. Examples include aluminum, platinum, and most austenitic stainless steels. The magnetic response is far too weak for particle accumulation, making MT impossible on these materials.

Diamagnetic Materials

Diamagnetic materials are very weakly repelled by magnetic fields. Their permeability is slightly less than 1. Examples include copper, gold, silver, and bismuth. These materials actively resist magnetization and cannot be tested by MT.

The Curie Temperature

Every ferromagnetic material has a Curie temperature above which it loses its ferromagnetic properties and becomes paramagnetic. For iron, the Curie temperature is approximately 770°C (1,418°F). For nickel, it is approximately 358°C (676°F).

This is relevant when testing parts that have been recently heat-treated, welded, or are in high-temperature service. If any portion of the material is above its Curie temperature, MT will not produce reliable results in that zone.

Magnetic Domains

Inside every ferromagnetic material, atoms are organized into microscopic regions called magnetic domains. Within each domain, all atomic magnetic moments are aligned in the same direction, making each domain a tiny permanent magnet.

In an unmagnetized piece of steel:

• Domains are randomly oriented throughout the material
• Their individual magnetic effects cancel out
• The material shows no net external magnetism

When an external magnetizing force (H) is applied, the domains respond in a progressive sequence:

1. Domain wall motion (low H) - Domains aligned favorably with the applied field grow at the expense of unfavorably aligned domains. The domain walls (boundaries between domains) shift, enlarging some domains and shrinking others. This process is largely reversible at low field strengths.

2. Domain rotation (moderate H) - At higher field strengths, domains begin to physically rotate their magnetization direction to align with the applied field. This process requires more energy and is partially irreversible.

3. Saturation (high H) - When all domains are fully aligned with the applied field, increasing the magnetizing force produces no further increase in flux density. The material is magnetically saturated. For most carbon steels, saturation occurs at approximately 20,000-22,000 Gauss (2.0-2.2 Tesla).

Why Domains Matter for MT

Understanding domains explains several practical MT phenomena:

• Residual magnetism - After the magnetizing force is removed, some domains do not return to random orientation. The material retains a net magnetization. The strength of this retained field depends on the material's retentivity.
• Demagnetization necessity - Residual magnetism must often be removed after testing. Demagnetization works by progressively randomizing domain orientation using an alternating and diminishing field.
• Overmagnetization background - If the applied field is too strong, domain boundary effects can create a fine background pattern of particles that obscures real discontinuity indications.

Hysteresis and the B-H Curve

The hysteresis loop (B-H curve) describes how a ferromagnetic material's flux density (B) responds to changes in magnetizing force (H) during a complete magnetization cycle.

Starting from a completely demagnetized state:
1. Increasing H causes B to rise along the initial magnetization curve, slowly at first (domain wall motion), then steeply (domain rotation), then leveling off (saturation).
2. Reducing H to zero does not return B to zero. The material retains residual flux density Br - this is the retentivity.
3. A reverse magnetizing force equal to Hc (the coercive force or coercivity) must be applied to bring B back to zero.
4. Continuing the reverse field drives B negative (opposite polarity magnetization).
5. Reversing again traces a mirror-image path back.

The area enclosed by the hysteresis loop represents the energy lost per magnetization cycle as heat.

Key Hysteresis Parameters

• Retentivity (Br): The flux density remaining after the magnetizing force is removed. High retentivity means the part holds a strong residual field - suitable for the residual MT technique.
• Coercivity (Hc): The reverse force needed to demagnetize the material. High coercivity means the material is harder to demagnetize, requiring more passes or stronger demagnetization equipment.
• Permeability (μ): The slope of the B-H curve. High permeability means the material magnetizes easily with low applied force.

The Magnetic Circuit Analogy

Just as electrical circuits have voltage, current, and resistance, magnetic circuits have magnetomotive force (MMF), magnetic flux, and reluctance.

MMF = NI (ampere-turns) - the driving force
Φ = MMF / ℛ - flux = driving force / reluctance

Reluctance (ℛ) depends on the path: ℛ = l / (μ × A)

Where l is the path length, μ is permeability, and A is the cross-sectional area. Ferromagnetic materials have very low reluctance (high μ), so flux concentrates inside them. Air has high reluctance, so flux avoids air gaps.

This is why prod contact is critical - an air gap at the prod tip dramatically increases the circuit reluctance, reducing the flux in the part beneath the contact point. Even a thin layer of mill scale or paint between the prod and the metal can create enough reluctance to reduce the effective magnetization significantly.

Applying Magnetic Principles to MT Operations

Every practical decision in MT - from amperage selection to particle choice - connects directly to the fundamental principles of magnetism covered in this chapter.

Why higher amperage for larger parts: Larger cross-sections require more current to achieve the same flux density at the surface. The same current spread over a 6-inch diameter shaft produces half the surface field strength as on a 3-inch shaft.

Why demagnetization works: The alternating, decreasing field progressively randomizes domain orientation. Each cycle moves domains toward randomness, and the decreasing amplitude ensures no new preferred direction is imposed.

Why temperature matters: As temperature increases toward the Curie point, thermal energy disrupts domain alignment. Permeability drops, more current is needed, and sensitivity decreases. Always check part temperature when testing recently welded or heat-treated components.

How Discontinuities Create Flux Leakage

The entire principle of magnetic particle testing depends on one physical phenomenon: flux leakage at a discontinuity.

When a ferromagnetic material is properly magnetized, magnetic flux flows through the material along the path of least reluctance. The material's high permeability means that flux strongly prefers to stay inside the material rather than pass through the surrounding air.

When a discontinuity - a crack, void, inclusion, or other interruption - lies perpendicular to the flux flow direction, it presents a high-reluctance barrier. The flux cannot easily cross the air gap inside the crack. Instead, the flux lines are forced to do one of three things:

1. Crowd together and pass through the remaining material beneath or around the discontinuity
2. Leak out of the material surface at the edges of the discontinuity, pass through the air above the crack, and re-enter the material on the other side
3. Some combination of both

This flux that exits and re-enters the material creates a localized magnetic field above the crack - the flux leakage field. This leakage field is what attracts and holds magnetic particles, forming the visible indication.

Factors Affecting Leakage Field Strength

The strength of the leakage field (and therefore the clarity of the indication) depends on several factors:

• Discontinuity depth: Deeper cracks produce stronger leakage fields because they force more flux out of the material.
• Discontinuity width: Very tight cracks can produce strong leakage if deep. Wide, shallow grooves may produce weaker leakage despite their size.
• Orientation to flux: Maximum leakage occurs when the discontinuity is perpendicular (90°) to the flux direction. At 45° orientation, leakage is significantly reduced. Below approximately 30° from the flux direction, the discontinuity may be undetectable.
• Depth below surface: Surface-breaking discontinuities produce the strongest leakage. Subsurface discontinuities produce progressively weaker leakage as depth increases. AC magnetization detects only the shallowest subsurface flaws (~0.5mm), while DC can reach approximately 6mm.
• Applied field strength: Insufficient magnetization produces weak leakage that may not attract enough particles. Excessive magnetization can produce background noise that masks the indication.
• Material permeability: Higher permeability materials concentrate flux more effectively, producing stronger leakage at discontinuities.

Two Fundamental Magnetization Directions

All magnetization techniques produce one of two fundamental field orientations:

Circular Magnetization

Circular magnetization creates a magnetic field that wraps around the circumference of the part, perpendicular to the current flow direction. The field forms concentric circles around the current path.

Created by:

• Direct contact (head shot) - current flows through the part lengthwise
• Central conductor - current flows through a bar inside a hollow part
• Prods - current flows between two contact points on the surface

Detects:

• Longitudinal discontinuities - cracks, seams, and laps running along the length of the part (parallel to the current flow, perpendicular to the circular field)

Key characteristic: The field exists only while current is flowing (for continuous technique) or as residual magnetism (for residual technique). There are no magnetic poles created at the part ends.

Longitudinal Magnetization

Longitudinal magnetization creates a magnetic field that runs along the length of the part, from one end to the other. The part effectively becomes a bar magnet with north and south poles at its ends.

Created by:

• Coil (solenoid) - part placed inside a current-carrying coil
• Cable wrap - flexible cable wrapped around the part
• Yoke - U-shaped electromagnet placed on the surface
• Permanent magnets

Detects:

• Transverse discontinuities - cracks and defects running across the part or perpendicular to the field direction

Key characteristic: The field produces poles at the ends of the part (or at the yoke contact points). These end poles create strong, non-relevant indications at the part extremities that must be recognized and disregarded.

Complete Coverage Requires Both

Since discontinuities can occur in any orientation, complete MT examination requires both circular and longitudinal magnetization - or a multidirectional technique that combines both in rapid sequence. The procedure specifies which techniques are required for each application.

Multidirectional (Swinging Field) Technique

Multidirectional magnetization applies circular and longitudinal fields in rapid alternation, creating a resultant field vector that sweeps through multiple directions. This provides detection capability for discontinuities in any orientation with a single examination sequence.

How It Works

A multidirectional bench unit applies two or more magnetization pulses in rapid sequence (typically within milliseconds). The pulses are timed so that while one field is at peak, the other is at zero, creating a rotating or swinging resultant field.

Advantages

• Single setup detects discontinuities in all orientations
• Faster than performing separate circular and longitudinal examinations
• Reduced handling (fewer part repositions)
• Commonly used in production environments for high throughput

Limitations

• More complex setup and verification
• Requires verification with a multi-directional QQI (cross-pattern or circle-pattern) to confirm all orientations are covered
• Dead spots between pulses may exist - verification is essential
• Not commonly used in field applications (requires bench unit with specialized circuitry)

The Skin Effect in MT

When alternating current flows through a conductor, it tends to concentrate near the surface rather than distributing uniformly through the cross-section. This is the skin effect, and it has direct implications for MT.

Skin Depth

The skin depth (δ) is the depth at which the current density has decreased to 37% of its surface value:

δ = √(2ρ / ωμ)

For steel at 60 Hz, the skin depth is approximately 0.5-1.0mm. This means that AC magnetization produces a field concentrated at the surface - ideal for detecting surface-breaking cracks but limited for subsurface defects.

MT Implications

• AC magnetization: The surface-concentrated field maximizes sensitivity for surface-breaking discontinuities. This is why AC is the default for most MT applications.
• DC magnetization: No skin effect - current distributes uniformly through the cross-section, creating a uniform field. Better subsurface penetration.
• HWDC: Intermediate behavior - some surface concentration but better penetration than AC.

The skin effect also explains why AC provides better particle mobility - the rapidly alternating surface field creates a vibration effect that helps particles move toward leakage fields.

B-H Relationship in Practice

The relationship between magnetic flux density (B) and magnetic field intensity (H) is the foundation for understanding why MT works differently on different materials.

For air and non-magnetic materials: B = μ₀H (linear relationship)
For ferromagnetic materials: B = μ₀μᵣH (nonlinear - μᵣ varies with H)

The nonlinear behavior of ferromagnetic materials means that doubling the applied current does not double the flux density once the material approaches saturation. This is why there is an optimal amperage range - too little gives insufficient flux, too much wastes energy and can cause overheating without proportionally improving sensitivity.

The practical significance: when a procedure specifies 300-800 A/inch for prod spacing, the lower value ensures minimum detectable field strength while the upper value avoids diminishing returns from approaching saturation.

Materials Commonly Examined by MT

As a Level I MT technician, you will encounter a wide variety of ferromagnetic materials across different industries. Understanding the basic material categories helps you anticipate how they will respond to magnetization and what types of discontinuities are most likely.

Carbon and Low-Alloy Steels

The most commonly tested materials in MT. Includes structural steels (A36, A572, A992), pressure vessel steels (SA-516, SA-387), piping steels (A106, A333), and machinery steels (1045, 4140, 4340).

• Excellent magnetic response - high permeability, moderate retentivity
• Magnetize easily with standard equipment settings
• Demagnetize readily with AC coils
• Common discontinuities: weld cracks, fatigue cracks, hydrogen-induced cracking, seams, laps

Cast Irons

Gray iron, ductile (nodular) iron, and malleable iron castings are ferromagnetic and testable by MT.

• Generally good magnetic response, but permeability varies with carbon content and microstructure
• Rough cast surfaces reduce MT sensitivity - surface preparation is critical
• Common discontinuities: shrinkage, porosity, hot tears, cold shuts, inclusions
• Gray iron's graphite flake structure can create background noise in some conditions

Ferritic and Martensitic Stainless Steels

400-series stainless steels (410, 420, 430, 440) are ferromagnetic.

• Moderate magnetic response - lower permeability than carbon steel
• May require higher amperage settings for equivalent magnetization
• Martensitic grades (410, 420, 440) after hardening have high coercivity - harder to demagnetize
• Common discontinuities: quench cracks, grinding cracks, stress corrosion cracking

High-Strength Low-Alloy (HSLA) Steels

Used in bridges, offshore structures, and heavy equipment. Grades like A588, A709, A514.

• Good magnetic response similar to carbon steels
• Higher susceptibility to hydrogen-induced cracking during welding
• Fatigue crack sensitivity at weld details in cyclic loading applications

Discontinuity Classification by Origin

Discontinuities are classified by when they form in the material's lifecycle. This classification helps the MT technician understand what to expect based on the part's manufacturing and service history.

1. Inherent (Primary Processing) Discontinuities

Formed during the original material production - casting, ingot solidification, and primary processing.

Porosity - Gas pockets trapped during solidification as dissolved gases come out of solution. Appear as scattered rounded indications. Most common in castings but can also be found in welds.

Shrinkage - Cavities formed as liquid metal contracts during solidification. Occurs at the last areas to solidify, typically at thick-to-thin transitions, hot spots, and the centers of heavy sections. May appear as scattered or clustered indications.

Non-metallic Inclusions - Oxides, sulfides, and slag particles trapped in the metal during melting and pouring. Appear as small, sometimes aligned, indications. In wrought products, inclusions become elongated in the rolling direction.

Hot Tears - Cracks formed during solidification when thermal contraction stresses exceed the material's hot strength. Found in castings at changes in section thickness and at re-entrant corners.

2. Processing (Secondary) Discontinuities

Formed during manufacturing - rolling, forging, machining, welding, heat treatment.

Seams - Longitudinal surface cracks in rolled products, caused by defects in the ingot or billet that are elongated during rolling. They run parallel to the rolling direction and may be intermittent. MT detects them as long, straight, linear indications aligned with the rolling direction.

Laps - Folds of metal rolled or forged into the surface. Created when overhanging metal is folded over and pressed into the base material. The folded surface does not metallurgically bond. Appear as curved or hooked linear indications.

Forging Bursts - Internal ruptures caused by excessive forging pressure or forging at incorrect temperature. May be internal (not detectable by MT) or surface-breaking.

Welding Discontinuities (most common MT application):

• Cracks: Hot cracks (solidification cracking from high sulfur/phosphorus), cold cracks (hydrogen-induced, delayed cracking in high-strength steels), crater cracks (star-shaped at weld stop points), underbead cracks (in HAZ of high-carbon or high-hardenability steels)
• Incomplete Fusion (Cold Lap): Weld metal does not fuse to the base metal or to the previous weld pass. Creates a linear indication at the fusion line.
• Undercut: A groove melted into the base metal at the weld toe, not filled by weld metal. Creates a linear indication at the weld toe.
• Porosity: Gas pockets in the weld metal from contamination, moisture, or shielding gas problems.

Heat Treatment Discontinuities:

• Quench Cracks: Cracks formed from thermal stress during rapid cooling. Typically found at stress risers - keyways, holes, sharp corners, section changes. Often follow grain boundaries.
• Grinding Cracks: Thermal cracks from excessive grinding heat. Appear as a network of fine, shallow, randomly oriented cracks confined to the ground area. The pattern is distinctive and recognizable.

3. Service (In-Service) Discontinuities

Develop during the part's operational life.

Fatigue Cracks - The most critical service discontinuity detected by MT. Caused by repeated cyclic loading. Initiate at stress concentrations (notches, holes, weld toes, keyways, surface defects) and propagate perpendicular to the applied stress. They grow with each load cycle until the remaining cross-section can no longer carry the load, resulting in catastrophic failure.

Stress Corrosion Cracks (SCC) - Result from the combined action of tensile stress and a corrosive environment. Typically branching, intergranular cracks. Common in caustic environments, chloride-containing atmospheres, and hydrogen sulfide (sour) service.

Hydrogen Embrittlement Cracks - Caused by hydrogen absorption during plating, pickling, cathodic protection, or sour service. Often delayed - cracking may occur hours or days after hydrogen exposure. Found at high-stress locations.

Recognizing Weld Discontinuities by MT Indication Pattern

As a Level I technician, you will most frequently perform MT on welded joints. Recognizing the characteristic MT indication patterns associated with different weld discontinuities helps you record accurate observations.

Common Weld Discontinuity Indication Patterns

Longitudinal crack: A straight or slightly curved linear indication running parallel to the weld axis. Often found at the weld centerline (solidification crack) or at the weld toe (cold crack, fatigue crack). The indication is sharp and well-defined, held tightly by the leakage field.

Transverse crack: A linear indication running perpendicular to the weld axis. Often caused by restraint stresses or thermal contraction. Found across the weld face or extending from the weld into the HAZ.

Crater crack: A star-shaped or branching indication at a weld termination point (crater). Caused by shrinkage of the weld pool at the end of a bead. Often appears as 2-4 short lines radiating from a central point.

Undercut: A linear indication at the weld toe, coinciding with a visible groove or channel in the base metal adjacent to the weld. The undercut is visible to the naked eye, and the MT indication confirms its extent.

Porosity: Scattered, rounded indications (dots) within the weld bead. Each dot represents a gas pocket near the surface. The pattern may be random (scattered porosity) or linear (piping porosity).

Incomplete fusion (cold lap): A linear indication at the interface between weld passes or between the weld and base metal. Found at the weld toe or inter-pass boundary. Similar in appearance to a crack indication - the Level II evaluates the significance.

Service-Induced Discontinuities

Parts develop new discontinuities during their operational life. These service discontinuities are often the most critical because they grow over time and can lead to sudden failure.

Fatigue Cracks

The most important service discontinuity detected by MT. Fatigue cracks initiate at stress concentrations (weld toes, notches, holes, keyways) and propagate perpendicular to the cyclic stress direction. They are progressive - each load cycle extends the crack slightly until the remaining cross-section cannot carry the load.

Characteristics:

• Start at the surface at a stress concentration
• Tight, sharp openings that produce excellent MT indications
• Grow during service - early detection by MT prevents catastrophic failure
• Found in bridges, pressure vessels, rotating equipment, aircraft structures

Stress Corrosion Cracking (SCC)

Results from the combined action of tensile stress + corrosive environment + susceptible material. Produces branching, intergranular crack networks. Common in:

• Caustic environments (boilers, chemical vessels)
• Chloride environments (stainless steels)
• Hydrogen sulfide environments (sour service piping)

Hydrogen-Induced Cracking

Hydrogen absorbed during welding, plating, or service diffuses to high-stress locations and causes delayed cracking. Often appears hours or days after the hydrogen exposure event. Found in the HAZ of high-strength steel welds.

Surface Preparation for MT

Surface condition directly and significantly affects MT sensitivity. Rough surfaces, scale, coatings, and contamination can all prevent particles from migrating to flux leakage sites and forming readable indications. Surface preparation is not just a procedural step - it is a critical factor in examination quality.

Why Surface Condition Matters

For particles to form a readable indication, several conditions must be met simultaneously:

1. The surface must be smooth enough for particles to move freely toward the leakage field
2. The surface must be clean enough that particles are attracted by magnetic forces, not held in place by mechanical entrapment, moisture, or contamination
3. Any coating must be thin enough that the leakage field from a subsurface crack can still reach the particles at the outer surface
4. The surface must provide adequate contrast for visual detection of the particle indication

Surface Roughness Guidelines

ASME Section V, Article 7 requires that examination surface roughness should not exceed 125 μin Ra (3.2 μm Ra) unless specifically authorized by the referencing code section.

Practical effects of surface roughness:

• Smooth machined surfaces (<63 μin Ra): Maximum MT sensitivity. Fine surface cracks as small as 0.001 inch (0.025mm) can be detected. Ideal for fluorescent wet particle MT.
• Standard machined finish (63-125 μin Ra): Good sensitivity. Adequate for most MT applications.
• Ground or sanded surfaces (125-250 μin Ra): Reduced sensitivity. Very shallow cracks may be masked. Dry particles may perform better than wet particles because they are less likely to be trapped in grooves.
• As-welded surfaces (varies, typically 250-500 μin Ra): Significantly reduced sensitivity for very fine cracks. Weld ripple marks create natural grooves that can trap particles. Many codes allow as-welded examination for certain applications.
• Rough cast surfaces (>500 μin Ra): Lowest sensitivity. Only larger discontinuities will be detected. Surface preparation (grinding) may be required by the procedure.

What Must Be Removed

• Loose rust, scale, and mill scale (unless tightly adherent and procedure allows)
• Weld spatter, slag, and flux residue
• Oil, grease, and machining coolant
• Dirt, sand, and debris
• Paint or coatings exceeding the maximum thickness specified in the procedure (typically >2 mils / 50μm)
• Moisture and condensation

What May Remain (if procedure allows)

• Tightly adherent mill scale (with acknowledged reduced sensitivity)
• Thin primer or paint below the specified maximum thickness
• As-welded surface profile (for procedures that specify as-welded examination)

Coatings and MT Sensitivity

Coatings on the examination surface create a physical barrier between the magnetic particles and the flux leakage field. The effect on sensitivity depends on the coating thickness, coating material, and the size/depth of the discontinuity being detected.

Maximum Coating Thickness

Most MT standards and procedures limit the maximum coating thickness to 1-2 mils (25-50 μm) for general examinations. Some standards allow up to 4 mils (100 μm) for less critical applications, but with acknowledged reduced sensitivity.

The physics are straightforward: the leakage field intensity decreases rapidly with distance from the material surface. A crack that produces a leakage field capable of attracting particles at 1 mil distance may not attract enough particles through 5 mils of paint to form a readable indication.

Types of Coatings Encountered

Contrast paint (white): Applied specifically for MT to provide visual contrast against dark particles. Must be the thinnest coating present - typically 0.5-1.5 mils per coat.

Shop primer: Many fabricated steel components arrive with a thin shop primer (0.5-1.5 mils). If the procedure allows testing through primer, verify thickness with a DFT gauge.

Service coatings: In-service inspections may encounter paint systems of 10-20+ mils total thickness. These must be removed from the examination area before MT.

Galvanizing: Hot-dip galvanized coatings are typically 3-5 mils thick. The zinc coating is non-magnetic and creates a significant barrier. MT through galvanizing is generally unreliable for fine cracks.

Temperature Considerations

Minimum temperature: Generally 40°F (4°C) for most particle types and carriers. Below this temperature:

• Condensation on the part surface can trap particles and create false indications
• Water-based wet particle baths may become too viscous
• Petroleum-based carriers may become too viscous
• Contrast paint may not dry properly

Maximum temperature for standard particles: Approximately 600°F (315°C) for special high-temperature dry particles. Standard particles lose their color-coding and may lose magnetic properties above this temperature.

Standard wet particle limit: Approximately 135°F (57°C) for water-based baths; higher for special formulations.

Part temperature affects magnetization: At elevated temperatures, permeability decreases, which means higher amperage may be needed. Near the Curie temperature, MT is ineffective.

Cleaning Methods for MT Surface Preparation

Mechanical Cleaning

• Wire brushing (manual): Removes loose rust, scale, and debris. Does not remove tightly adherent coatings. Use stainless steel brushes on stainless steel parts to avoid carbon contamination.
• Power wire brushing: Faster than manual. Be cautious of work hardening the surface or creating a smeared layer that could hide fine cracks.
• Grinding: Removes coatings, weld spatter, and surface irregularities. Use grinding discs appropriate for the material. Avoid excessive heat that could create thermal damage or mask discontinuities.
• Needle scaling: Removes heavy mill scale and rust. Creates a rougher surface than grinding.

Chemical Cleaning

• Solvent cleaning (wipe method): Apply solvent to a clean cloth and wipe the surface. Replace cloths frequently. Do not reuse contaminated cloths.
• Solvent spray: Spray directly on the surface, allow to dissolve contaminants, then wipe with clean cloth. Ensure solvent is fully evaporated before applying particles.
• Alkaline cleaning: Hot alkaline solutions remove oil and grease effectively. Requires rinse and dry after cleaning.
• Vapor degreasing: For precision parts in shop environments. Very effective at removing oil and grease from complex geometries.

Post-Examination Cleaning

After MT, remove all particles and contrast paint from the part unless the procedure specifies otherwise. Residual particles can mask subsequent examinations, interfere with coatings, or cause concern during in-service inspections.

Temperature Considerations for MT

Temperature affects every aspect of the MT examination: the magnetic properties of the part, the behavior of the particles, and the properties of the carrier fluid.

Cold Temperature Effects (Below 40°F / 4°C)

• Wet particle carriers become more viscous, reducing particle mobility
• Water-based carriers risk freezing
• Petroleum-based carriers become sluggish
• Dry particles are preferred in cold conditions
• Aerosol cans may not spray properly

Hot Temperature Effects (Above 135°F / 57°C for wet methods)

• Standard wet particles are limited to approximately 135°F part surface temperature
• Above this temperature, carrier fluid evaporates rapidly before particles can migrate to indications
• Dry particles can be used to approximately 600°F with standard formulations
• Special high-temperature dry particles extend the range to 800°F or higher
• Contrast paint may discolor or bubble at elevated temperatures

Magnetic Property Changes

As temperature increases toward the Curie point (approximately 1,414°F / 768°C for iron), permeability decreases progressively. At elevated temperatures, higher amperage may be needed to achieve the same field strength. Always verify with a field indicator when examining parts above ambient temperature.

Dry Magnetic Particles

Dry particles are finely divided ferromagnetic powder applied directly to the magnetized part surface. They are the simplest form of MT media and are the most commonly used particles for field inspection work.

Physical Properties

Particle size: 50-180 microns (much coarser than wet particles). The larger size helps particles bridge across discontinuities on rough surfaces and resist being blown away by wind in outdoor applications.

Particle shape: Elongated (rod-like or needle-shaped) rather than spherical. This elongated shape is intentional - the particles align with flux leakage fields more effectively than round particles, improving indication formation.

Magnetic properties:

• High permeability - particles must be easily magnetized by the weak leakage field at a crack
• Low retentivity - particles should not clump together due to residual magnetism. Particles that stick together lose their ability to migrate individually to leakage fields
• Low coercivity - particles should lose magnetism quickly for re-examination or cleanup

Available Colors

• Gray/Silver: Natural iron color. Good contrast on dark, oxidized, or rusty surfaces. Most economical.
• Red: High contrast on light-colored or machined surfaces. Good visibility on aluminum-painted steel.
• Yellow: Excellent contrast on dark welds, cast iron, and unpainted steel. Very good visibility.
• Black: Maximum contrast when used with white contrast paint - the most common combination for field weld inspection.

Application Methods

1. Squeeze bulb (rubber bulb applicator): The most common field application method. Squeeze gently to create a fine, diffuse powder cloud. Apply from 8-12 inches away - never blast the surface with a concentrated stream.

2. Powder spray can (aerosol): Provides consistent, fine application. Good for tight spaces and overhead work. More expensive per ounce than bulk powder.

3. Powder shaker (salt-shaker type): Simple, economical, low-tech. Adequate for small examination areas.

Application Rules

• Apply from 8-12 inches distance to create a light, even dusting
• Apply while the magnetizing current is flowing (continuous technique) - particles must be free to move while the field is active
• After particles have formed indications, gently remove excess with low-pressure air (1-2 psi maximum). Do NOT use high-pressure air - it will blow away indications
• Excess powder is the primary enemy of dry particle MT - more powder does NOT improve detection. It obscures indications under a blanket of particles.

Wet Magnetic Particles

Wet particles are much finer than dry particles (typically 1-30 microns) and are suspended in a liquid carrier - either petroleum-based oil or water-conditioned solution. The particle/liquid mixture is called the bath.

Wet Fluorescent Particles

Fluorescent particles are coated with a dye that glows bright yellow-green under ultraviolet (UV-A) light at 365nm wavelength (commonly called black light).

Key characteristics:

• Highest sensitivity of all MT methods - can detect very fine, tight surface cracks that other methods miss
• Requires darkened conditions - ambient visible light must be reduced to ≤2 foot-candles (20 lux) at the examination surface for adequate fluorescent contrast
• UV-A intensity requirement - The UV light must produce ≥1,000 μW/cm² at the examination surface per ASTM E1444
• Dark adaptation required - the examiner's eyes need 1-5 minutes to adapt to darkened conditions before examination can begin
• Best for: precision machined parts, aerospace components, critical welds where maximum sensitivity is required

Wet Visible (Non-Fluorescent) Particles

Visible wet particles are colored (typically black, brown, or red) and examined under normal white light.

Key characteristics:

• Lower sensitivity than fluorescent particles, but higher than dry particles for smooth surfaces
• No special lighting required - can be used under normal shop or field lighting
• Useful when UV equipment is impractical or unavailable
• Often used with white contrast paint for improved visibility

Carrier Fluids

Petroleum-based (oil carrier):

• Traditional carrier - excellent particle mobility and suspension
• Flash point must be ≥200°F (93°C) per ASTM E1444 - tested per ASTM D93
• Low-odor, low-viscosity formulations available
• Provides mild corrosion protection
• Fire hazard near welding, grinding, or open flame - maintain safe distance

Water-based (water conditioner):

• Environmentally preferred - no fire hazard, no petroleum vapors
• Requires conditioners (wetting agents) to prevent corrosion of the part and maintain particle suspension
• Water quality (hardness, contaminants, pH) must be monitored
• May not suspend particles as uniformly as oil carriers
• Corrosion risk if parts are not dried and protected after examination

Selecting the Right Particles

The Level I technician uses the particles specified in the written procedure. Understanding why specific particles are chosen for different applications helps you verify that the correct materials are being used.

Decision Factors

Surface condition → Particle size:

• Smooth surfaces (machined, ground): Fine wet particles - maximum sensitivity for tight cracks
• Rough surfaces (as-welded, cast): Coarser dry particles - bridge surface irregularities better

Lighting environment → Particle visibility:

• Controlled darkroom: Fluorescent wet particles - highest sensitivity
• Normal shop lighting: Visible (colored) particles - no UV needed
• Outdoor/field: Dry visible with contrast paint - works in all lighting conditions

Temperature → Carrier type:

• Normal (40-100°F): Standard wet or dry
• Cold (<40°F): Dry particles (wet carriers become too viscous)
• Hot (>135°F for wet, >600°F for standard dry): Special high-temperature dry particles

Access and orientation → Application method:

• Horizontal surfaces: Any method works
• Vertical surfaces: Wet particles (spray) adhere better than dry
• Overhead: Wet particles applied by spray - dry particles fall off before forming indications
• Confined spaces: Aerosol cans provide contained application with less mess

UV-A Light Requirements for Fluorescent Particle Examination

Fluorescent particles glow bright yellow-green when excited by ultraviolet light at 365nm wavelength (UV-A). The effectiveness of fluorescent MT depends entirely on proper UV-A illumination.

UV-A Intensity Requirements

• Minimum intensity at the examination surface: 1,000 μW/cm² (per ASTM E1444)
• Measurement: Use a calibrated UV-A radiometer positioned at the examination distance (typically 15 inches from the light source)
• Warm-up time: Allow UV-A lamps at least 5 minutes warm-up before measuring or examining

Ambient Light Requirements

• Maximum ambient visible light: 2 foot-candles (20 lux) at the examination surface
• Why: Visible light washes out the fluorescent glow, reducing contrast and detection capability
• Measurement: Use a calibrated visible light meter

Dark Adaptation

• Allow 1-5 minutes for eyes to adapt to darkened conditions before beginning examination
• Avoid looking at bright lights (phones, welding arcs, sunlight) during the examination - this resets your dark adaptation
• If you must leave the darkened area, allow re-adaptation time before resuming examination

UV Lamp Types

• Mercury vapor lamps: Traditional, reliable, 5-10 minute warm-up required
• LED UV lamps: Instant on, no warm-up, more energy efficient, increasingly common
• Both must meet the 1,000 μW/cm² minimum and produce adequate spectral output at 365nm

The Electromagnetic Yoke

The electromagnetic yoke is a U-shaped electromagnet that creates a longitudinal magnetic field between its poles when placed on a ferromagnetic surface. It is the single most commonly used piece of MT equipment in field applications, particularly for weld inspection.

Types of Yokes

AC Yokes:

• Powered by alternating current (50/60 Hz line frequency)
• Produce an alternating magnetic field that reverses direction with each cycle
• Best for detecting surface-breaking discontinuities
• The alternating field creates a "stirring" effect on wet particles, improving their mobility and ability to accumulate at leakage fields
• Maximum sensitivity limited to surface and very near-surface (approximately 0.5mm depth)
• Lightest and most commonly used type

DC Yokes (battery-powered or rectified):

• Produce a steady, unidirectional magnetic field
• Better penetration than AC - can detect subsurface discontinuities to approximately 6mm (¼ inch) depth
• Field does not alternate, so particles must be applied while the field is active (no stirring effect)
• Heavier due to batteries or rectifier circuitry
• Used when subsurface detection is specifically required

AC/DC Switchable Yokes:

• Can operate in either AC or DC mode
• Most versatile - allows the technician to select the appropriate mode for the application
• AC mode for surface cracks, DC mode for subsurface detection
• Increasingly common in modern equipment

Permanent Magnet Yokes:

• No power source needed - always "on"
• Field strength is fixed and cannot be adjusted
• Used in hazardous environments where electrical equipment is prohibited
• Heavy relative to their field strength
• Must meet the same lifting force requirements as electromagnetic yokes

Lifting Force Verification

Per ASTM E1444, yoke adequacy is verified by measuring the weight the yoke can lift with its poles spaced at the maximum distance that will be used during the examination:

• AC yokes: Minimum lifting force of 10 pounds (4.5 kg)
• DC yokes and permanent magnets: Minimum lifting force of 40 pounds (18 kg)

The higher DC requirement reflects the fact that DC yokes are expected to detect deeper subsurface discontinuities, which requires a stronger field.

Verification frequency: At least annually, and whenever the yoke is damaged, repaired, or its performance is questioned. Many specifications require daily or shift-start verification.

Yoke Placement and Coverage

The effective examination area for each yoke placement is the region between and immediately adjacent to the two poles. Outside this zone, field strength drops rapidly and detection is unreliable.

Effective Coverage Zone

• The field runs from pole to pole through the material, creating a longitudinal magnetic field in the part between the poles
• Discontinuities perpendicular to this field (perpendicular to the pole-to-pole line) will be detected
• The strongest field is directly between the poles; field strength diminishes toward the edges
• Practical coverage width extends approximately 1-2 inches beyond the pole-to-pole line on each side, depending on pole spacing and yoke strength

Two-Directional Coverage for Welds

For complete weld examination with a yoke, two passes at approximately 90° orientation are required:

Pass 1 - Transverse to weld:
Place the yoke with one pole on each side of the weld (poles straddling the weld). The field runs perpendicular to the weld axis.

• Detects: Longitudinal cracks in the weld, longitudinal incomplete fusion, longitudinal undercut, longitudinal HAZ cracks
• This pass catches the majority of welding defects because most weld cracks run parallel to the weld axis

Pass 2 - Parallel to weld:
Rotate the yoke 90° so both poles sit along the weld (parallel to the weld axis). The field now runs parallel to the weld.

• Detects: Transverse cracks across the weld, transverse fatigue cracks, crater cracks, transverse HAZ cracks
• This pass is critical for cyclically loaded structures where transverse fatigue cracks develop at weld details

Overlap Requirements

• Successive yoke placements must overlap the previous position by at least 1 inch (25mm) to ensure no gaps in coverage
• For the 90° rotated pass, overlap the previous placements by the same amount
• Track coverage systematically - mark examined areas with a grease pencil or chalk if the procedure allows

Curved Surface Considerations

• On pipe and vessel circumferential welds, both yoke legs must make full contact with the surface
• Rigid yokes may only contact at the pole tips on tight-radius curves, reducing the effective field
• Articulating yokes with flexible legs conform to curved surfaces and maintain full contact
• For small-diameter pipe (under approximately 4 inches NPS), alternative techniques (prods, coil, wrap) may be more effective than yokes

Permanent Magnet Yokes and Special Applications

Permanent Magnet Yokes

Permanent magnets create a magnetic field without electrical power. They are used in:

• Hazardous environments: Where electrical equipment is prohibited (explosive atmospheres, fuel storage areas)
• Remote locations: Where electrical power is unavailable
• Underwater MT: Specialized underwater permanent magnet assemblies

Advantages: No power required, no cables, no electrical hazard, always ready.

Disadvantages: Field strength is fixed (cannot adjust), heavy for their output, field cannot be turned off (creates handling challenges), must meet 40-lb DC lifting force requirement.

AC vs. DC Yoke Selection for Specific Crack Types

Use AC when searching for:

• Surface-breaking fatigue cracks at weld toes
• Grinding cracks on machined surfaces
• Surface-breaking stress corrosion cracks
• Any crack expected to be surface-breaking

Use DC when searching for:

• Subsurface hydrogen cracks in HAZ
• Near-surface inclusions or slag
• Subsurface porosity in castings
• Any discontinuity expected to be below the surface (up to ~6mm depth)

The Prod Technique

Prods are handheld electrodes that pass current directly through a localized area of the part surface. The current flowing between the two prods creates a circular magnetic field in the region between and around the prods.

When to Use Prods

• Large structures and weldments that cannot fit in a bench unit (structural steel, pressure vessels, storage tanks, piping, bridge members)
• Field inspections where portable equipment is required
• Localized examination of specific areas, repairs, or questionable zones
• In-service inspection of installed equipment

Prod Spacing and Its Effect

• Recommended spacing: 3 to 8 inches (75-200mm)
• Maximum per ASTM E709: 8 inches (200mm) - beyond this distance, field strength drops below detection threshold for most applications
• Minimum: 3 inches (75mm) - closer spacing concentrates current density too much, risking overheating and arc burns
• Optimal for most applications: 4-6 inches - balances field strength, coverage area, and practical handling

Amperage Guidelines

• Parts ¾ inch thick or greater: 100-125 amperes per inch of prod spacing
• Parts less than ¾ inch thick: 90-110 amperes per inch of prod spacing
• Example: 6-inch prod spacing on ¾" plate → 6 × 100 to 6 × 125 = 600-750 amperes

The Effective Examination Zone

The effective coverage area for each prod placement is an oval-shaped region between and immediately around the prods. Only this zone has adequate field strength for reliable detection.

Critical dead zones:

• At the prod contact points: Very high current density but distorted, unpredictable field. Indications here are unreliable.
• Beyond the prods: Field strength drops rapidly outside the prod-to-prod zone

This is why successive placements must overlap by at least 10% - to ensure the dead zones from one placement are covered by the next.

Contact Requirements

• Both prods must make firm, simultaneous contact with the surface before current is applied
• Use copper braid or lead contact pads between the prod tips and the part surface on machined, finished, or critical surfaces
• Prod tips must be clean, undamaged, and of appropriate size for the application
• Spring-loaded prods help maintain consistent contact pressure

Longitudinal Magnetization - Coils and Conductors

Coil (Solenoid) Technique

A coil consists of one or more turns of current-carrying conductor surrounding the part. The current flowing through the coil creates a longitudinal magnetic field inside the coil that runs along the part's length.

Amperage calculation - per ASTM E709:

NI = 45,000 / (L/D)

Where:

• NI = ampere-turns (current × number of coil turns)
• L = length of part (or effective length within the coil's field)
• D = diameter of part (or equivalent diameter for non-circular cross-sections)
• L/D = length-to-diameter ratio

Example: A 10-inch long, 2.5-inch diameter shaft in a 5-turn coil:

• L/D = 10/2.5 = 4
• NI = 45,000 / 4 = 11,250 ampere-turns
• Current = 11,250 / 5 turns = 2,250 amperes on the machine ammeter

The 6-9 inch rule: The effective longitudinal field from a coil extends approximately 6-9 inches in each direction from the coil center. For parts longer than 18 inches, multiple coil positions are required, with the part repositioned for each shot.

Fill factor: The part's cross-sectional area should be at least 10% of the coil's cross-sectional area for effective magnetization. If the part is too small relative to the coil, most flux passes through air and the field in the part is too weak.

Cable Wrap Technique

When a part is too large for a fixed coil, a flexible cable is wrapped around the part to create a temporary coil. The same NI formula applies, with the number of cable wraps serving as the number of turns.

Cable wrap guidelines:

• Wind the cable evenly and snugly around the part
• Keep wraps close together - widely spaced wraps create an uneven field
• Ensure the cable is in contact with or very close to the part surface
• Use insulated cable to prevent arc contact with the part

Central Conductor Technique

A central conductor is a conductive bar (usually copper) threaded through the bore of a hollow part (ring, bearing race, tube, cylinder). Current flows through the conductor, inducing a circular magnetic field in the surrounding hollow part.

Advantages:

• No current flows through the part - no risk of arc burns on the part surface
• Excellent for examining the internal diameter (ID) and external (OD) surfaces of hollow components
• Ideal for rings, bearing races, flanges, and cylindrical components

Positioning: Place the part near one end of the conductor for the first shot. The field is strongest close to the conductor surface and decreases with distance. After the first examination, move the part to the other end of the conductor for complete coverage.

Amperage: Based on the part's cross-section, not the conductor's diameter. Use the cross-section rules for circular magnetization.

Head Shot (Direct Contact) Technique

The head shot technique passes current directly through the part between two contact points (headstocks) on a bench unit. This creates a circular magnetic field around the part's cross-section.

Setup and Operation

1. Place the part between the headstocks with firm contact at both ends
2. Clamp securely - the part must not move during magnetization
3. Ensure contact surfaces are clean and conductive
4. Calculate amperage: 300-800 A/inch of part diameter
5. Select the appropriate value within the range based on material and sensitivity requirements

When to Use Head Shot

• Cylindrical parts (shafts, bolts, rods, pins, studs)
• Parts with uniform cross-section that fit between headstocks
• When looking for longitudinal discontinuities (seams, laps, longitudinal cracks)

Circular Field Pattern

The circular field wraps around the part's circumference. It is strongest at the surface and decreases toward the center (due to the current distribution in the conductor).

For solid parts, the field at the center is essentially zero - this is why head shots detect surface discontinuities effectively but have limited subsurface detection depth.

Cable Wrap Technique

When parts in the field are too large for a coil, a flexible cable wrapped around the part creates the same longitudinal magnetization effect.

Setup

• Wrap insulated welding cable around the part in tight, even turns
• Keep wraps close together - widely spaced wraps create uneven fields
• Use a portable power pack to supply the current
• Apply the NI formula: NI = 45,000 / (L/D)
• Divide the required NI by the number of wraps to determine the current setting

Example

A 30-inch long, 8-inch diameter pipe section needs longitudinal MT:

• L/D = 30/8 = 3.75
• NI = 45,000 / 3.75 = 12,000 ampere-turns
• With 4 wraps of cable: I = 12,000 / 4 = 3,000 amperes
• With 6 wraps: I = 12,000 / 6 = 2,000 amperes

More wraps = lower current needed = smaller, lighter power pack. In the field, using more wraps is often the practical choice because it allows use of available equipment.

Continuous vs. Residual Technique

The timing of particle application relative to the magnetizing current is a critical variable in every MT examination. Two approaches exist:

Continuous Technique

Particles are applied while the magnetizing current is flowing (or while the yoke is energized). The examination is performed while the magnetic field is at full strength.

Why continuous is more sensitive:

• The applied field strength is at its maximum during particle application
• Flux leakage at discontinuities is at its strongest
• Particles respond to the maximum available leakage field
• Works on ALL ferromagnetic materials regardless of their retentivity

When to use continuous:

• Default choice for most MT applications
• Mandatory for low-retentivity materials (low-carbon steel, mild structural steel) that do not hold enough residual field for the residual technique
• When maximum sensitivity is required (critical components, fatigue-loaded structures)
• All field work with yokes and prods
• When the procedure specifies continuous

Practical implementation:

• For bench units: Apply bath or powder while current is flowing. Current duration should be 0.5-1 second per shot minimum, and particles should be applied during this time.
• For prods: Apply particles while current is flowing between the prods.
• For yokes: Apply particles while the yoke is energized. AC yokes create a continuous alternating field while powered.

Residual Technique

The part is magnetized first, then the current is stopped, and particles are applied to the residual (retained) magnetic field.

When residual works:

• Only on high-retentivity materials that hold a strong residual field after the magnetizing force is removed
• Hardened steels, high-carbon steels, many tool steels, and some alloy steels
• The material must have sufficient retentivity (Br) to produce detectable flux leakage at discontinuities without the applied field

Advantages of residual:

• Parts can be magnetized in batch at one station and examined at a separate, dedicated examination station
• No electrical equipment needed at the examination station - reduces clutter, improves lighting control
• Reduced arc burn risk - no current flowing during handling and particle application
• Useful in production environments for high-throughput examination of small parts

Limitations of residual:

• Only works if the material retains sufficient magnetism - unusable on low-retentivity materials
• Generally less sensitive than continuous - the residual field is always weaker than the applied field
• If the procedure specifies continuous, residual is not an acceptable substitute

How to Know Which to Use

The written procedure specifies which technique applies. If you are unsure whether a material has adequate retentivity for the residual technique, use continuous - it is always at least as sensitive as residual, and usually more sensitive.

Verifying Field Adequacy

Calculated amperage is a starting point, but the actual magnetic field at the part surface must be verified. Three primary tools are available:

1. Pie Gauge (Magnetic Field Indicator)

A pie gauge is an octagonal steel disk approximately 1.5 inches in diameter, divided into 8 sectors by internal copper-brazed divisions. When placed on the magnetized surface with particles applied, the copper divisions create flux leakage lines that attract particles, forming visible radial lines.

Interpreting pie gauge results:

• Clear, sharp division lines in one direction = adequate field strength in that direction
• Lines visible in all directions = multidirectional field adequate
• Faint or absent lines = insufficient field strength - increase amperage
• Very heavy, broad lines with excessive background = possible overmagnetization - reduce amperage

Important: The pie gauge is qualitative - it tells you the field is adequate or not, but does not give a numerical measurement. It also shows the field direction, which helps verify correct technique setup.

2. Quantitative Quality Indicator (QQI) / Artificial Flaw Shim

A QQI is a thin (approximately 0.002-inch thick) steel shim with precisely machined grooves on one side. The shim is placed groove-side down on the part surface. When the part is magnetized and particles are applied, the grooves act as artificial discontinuities. Particles form clear lines over the grooves if the field is adequate.

Types of QQIs:

• Single-direction: grooves in one orientation - verifies one field direction
• Cross-pattern: grooves in two perpendicular directions - verifies two-directional coverage
• Circle pattern: radial grooves - verifies multidirectional coverage

Per ASTM E1444: System performance verification using a QQI, pie gauge, or equivalent indicator is required to demonstrate that the MT system (equipment + particles + technique) is functioning correctly.

3. Hall-Effect Gaussmeter

A Gaussmeter uses a Hall-effect sensor probe to measure flux density at the surface quantitatively.

Key specifications:

• Measures tangential field strength at the part surface
• Typical adequate range for MT: 30-60 Gauss (3-6 mT)
• The probe must be placed flat on the surface with the sensing element parallel to the surface
• Provides numerical readings - the most precise verification method available
• Required by some specifications (aerospace, nuclear) where quantitative field measurement is mandatory

When to Verify

• At the start of each examination sequence or shift
• Whenever technique parameters change (amperage, spacing, equipment)
• On complex geometries where field distribution may be uneven
• At the most difficult-to-magnetize location (farthest from current source, geometry transitions)
• When any doubt exists about field adequacy

Current Types Used in MT

The type of current used for magnetization significantly affects the examination's depth capability and particle mobility.

Alternating Current (AC)

AC reverses direction at the supply frequency (50 or 60 Hz). Due to the skin effect, AC current concentrates near the surface of the conductor, creating a surface-concentrated magnetic field.

• Detection depth: Surface only (~0.5mm / 0.020 inch)
• Particle mobility: Excellent - the alternating field creates a vibrating effect that helps particles migrate to leakage fields
• Best for: Surface-breaking cracks (the most common application)
• Equipment: Yokes, bench units, prod units

Direct Current (DC)

DC flows steadily in one direction, creating a uniform field through the full cross-section of the part.

• Detection depth: Surface + subsurface (up to ~6mm / 0.25 inch)
• Particle mobility: None - the steady field does not help mobilize particles. Must rely on particle application technique and gravity.
• Best for: Subsurface discontinuities (hydrogen cracks in HAZ, near-surface inclusions)
• Equipment: Battery-powered yokes, DC bench units, permanent magnets

Half-Wave DC (HWDC / Pulsating DC)

HWDC is rectified AC with only the positive (or negative) half-cycles passed. This creates a pulsating unidirectional field.

• Detection depth: Better than AC, approaching DC (~3-4mm)
• Particle mobility: Better than DC - the pulsations provide some particle movement
• Best for: Compromise between surface sensitivity and subsurface detection
• Equipment: Portable prod and bench units with half-wave rectification

Selection rule: AC for surface cracks (most common). DC or HWDC when subsurface detection is specifically required.

Types of MT Indications

Every particle accumulation on the part surface during MT is an "indication." The fundamental skill in MT is determining what caused each indication and whether it requires action. All indications fall into three categories:

Relevant Indications

Relevant indications are caused by actual discontinuities in the material - cracks, seams, laps, incomplete fusion, porosity, inclusions, or other metallurgical features that may affect the part's integrity.

Characteristics of relevant indications:

• Sharp, well-defined particle patterns with clear edges
• Held tightly in place by strong flux leakage - they resist being blown away by gentle air
• Reproducible - they appear in the same location and with the same appearance on repeated magnetization
• Oriented perpendicular to the applied magnetic field direction
• Located in areas where discontinuities are expected (weld toes, stress risers, heat-affected zones, section changes)

Classification of relevant indications:

• Linear: Length is 3 or more times the width (L ≥ 3W). Typically indicates cracks, incomplete fusion, seams, or laps. Linear indications are generally more serious than rounded.
• Rounded: Length is less than 3 times the width (L < 3W). Typically indicates porosity, inclusions, or shallow subsurface defects.

Non-Relevant Indications

Non-relevant indications are real particle accumulations caused by magnetic flux leakage, but the leakage is NOT from a harmful discontinuity. They are caused by geometry, material property variations, or design features.

Common causes:

• Geometry changes: Threads, keyways, splines, holes, sharp corners, changes in cross-section, press-fit boundaries. These abrupt geometry transitions create flux leakage at the boundary.
• Material property boundaries: Where hardened and unhardened zones meet (case-hardened parts, flame-hardened areas), where the heat-affected zone transitions to base metal.
• Dissimilar metal joints: Where two metals with different permeabilities are joined.
• Magnetic writing: Residual magnetism patterns from prior contact with magnetized objects or tools.
• Weld root geometry: On single-sided welds, the root profile change can create a non-relevant linear indication on the opposite side.

Critical rule per ASTM E1444: Any indication with a major dimension greater than 1/16 inch (1.6mm) that is initially classified as non-relevant must be treated as relevant until proven otherwise by additional examination, surface conditioning, or other means.

False Indications

False indications are particle accumulations NOT caused by magnetic flux leakage at all:

• Excessive particle application: Too much powder piles up mechanically without magnetic attraction
• Surface roughness: Particles mechanically trapped in pits, grinding marks, or machine grooves
• Contamination: Oil, moisture, or adhesive causing particles to stick regardless of magnetic field
• Gravity: Particles pooling in low spots on horizontal surfaces
• Scale or spatter debris: Ferromagnetic debris on the surface attracting particles independently

Recording vs. Evaluating Indications

This distinction defines the boundary of a Level I technician's authority and is one of the most important concepts in NDT certification.

Recording (Level I Responsibility)

Recording means documenting exactly what you observe - objectively, completely, and without interpretation beyond what you have been specifically trained to apply.

A Level I records:

• Location: Referenced to a fixed datum, landmark, or grid system. Example: "14 inches from the north end, 2 inches from the east edge, on the west side HAZ of weld W-3."
• Size: Measured length and width of the indication in the specified units
• Orientation: Longitudinal, transverse, or at a specified angle relative to the weld axis or part axis
• Type: Linear (L ≥ 3W), rounded (L < 3W), or cluster (multiple closely spaced indications)
• Number: How many separate indications in the examined area
• Sketch or photograph: Mark indication locations on a part drawing, weld map, or photograph for traceability

Evaluating (Level II Responsibility)

Evaluating means comparing indications against acceptance criteria to determine disposition - accept, reject, repair, or investigate further. This requires:

• Understanding the specific acceptance criteria table in the applicable code
• Determining whether an indication is relevant or non-relevant when judgment is required
• Applying multiple criteria simultaneously (maximum individual size, maximum cumulative size in a given length, minimum spacing between indications)
• Making disposition decisions that have safety, schedule, and cost implications

When Level I May Apply Criteria

If the written procedure includes specific, unambiguous acceptance criteria AND the Level I has been trained and authorized to apply them, the Level I may do so. Examples of clear-cut criteria:

• "No linear indications of any size" - binary, no judgment needed
• "No rounded indications exceeding 3/16 inch in largest dimension" - measurable, objective

Examples that REQUIRE Level II judgment:

• "No relevant indications" - requires determining relevance (a Level II function)
• "Evaluate per ASME Section VIII, Table UW-51" - requires applying multi-criteria acceptance standards
• "Engineer to review all questionable indications" - requires defining "questionable"

The Practical Rule

When in doubt, record the indication thoroughly and contact your Level II or Level III. You will never be criticized for recording a non-relevant indication. You will be held accountable for failing to record a relevant one. Thorough documentation is always correct, even when the indication turns out to be non-relevant.

Documenting MT Indications

Accurate documentation is as important as detection. A poorly documented indication - one that cannot be relocated or verified - has limited value. As a Level I, your documentation must be thorough enough for a Level II to evaluate your findings without being present.

Location Referencing Systems

Fixed datums: Reference indications to permanent, identifiable features: "3 inches from the north end, 2 inches from the east edge." Establish which end is "north" or "reference end" and document it.

Weld identification: Use the weld numbering system from the project drawings: "Joint W-7, 14 inches from mark A."

Grid systems: For large surfaces, establish a grid with marked coordinates. Report indications by grid location.

Photographs: Supplement written descriptions with photographs showing the indication location relative to identifiable landmarks. Include a scale reference (ruler, tape measure) in the photo.

Indication Description Elements

1. Type: Linear (L ≥ 3W) or Rounded (L < 3W)
2. Length: Measured major dimension
3. Width: Measured minor dimension (if required)
4. Orientation: Longitudinal, transverse, or angular (specify angle)
5. Location: Using the referencing system established above
6. Characteristics: Sharp/diffuse, strong/faint, single/clustered
7. Sketch: A simple drawing showing the indication on the part geometry

Common Non-Relevant Indications

Non-relevant indications are real magnetic particle accumulations caused by flux leakage from geometry or material property changes - NOT from discontinuities. Recognizing them prevents confusion and unnecessary rejection of good parts.

Thread Roots

Every thread root creates a flux leakage site. On threaded bolts and studs examined by MT, you will see particle accumulations at every thread root. These are non-relevant - they coincide exactly with the visible thread geometry.

Press-Fit Boundaries

Where two parts are press-fitted together (bearing in a housing, pin in a bore), the interface creates a permeability discontinuity. Particles accumulate at the press-fit line.

Section Changes

Abrupt changes in cross-section (steps, shoulders, undercuts, keyway openings) concentrate flux and create leakage at the transition. The indication coincides with the visible geometry change.

Weld-to-Base Metal Transitions

The weld metal and base metal may have different permeability due to different chemical composition and microstructure. Flux leakage at the fusion line can create non-relevant linear indications at the weld toe - these must be carefully distinguished from weld toe cracks.

Magnetic Writing

Prior contact with magnetized objects (tools, crane hooks, magnets) can leave residual magnetization patterns on the surface. These appear as random, non-geometric particle lines. Demagnetization before examination eliminates them.

The Critical Rule

Any indication exceeding 1/16 inch (1.6mm) in major dimension that you initially classify as non-relevant MUST be treated as relevant until proven otherwise. The Level II determines final classification.

Safety Hazards in MT

Magnetic particle testing involves several categories of hazards that every technician must understand and manage. Safety is not separate from technical competence - it is integral to it.

Electrical Hazards

MT equipment operates at high current levels - hundreds to thousands of amperes. While the voltage is typically low (4-16V open circuit for bench units, 60-80V for portable units), the hazards are serious:

Arc burns: The most common electrical hazard in MT. Caused by breaking electrical contact while current is flowing - lifting prods, loose headstock contact, damaged cable connections. Arc burns create localized surface damage (melted metal, heat-affected zones) that act as stress risers and can initiate fatigue cracks. On critical components, arc burns may require the part to be rejected.

Electrical shock: While low-voltage DC equipment presents minimal shock risk, portable AC equipment (yokes, prod units) operating at higher voltages can deliver dangerous shocks, especially in wet conditions. Damaged cable insulation, wet floors, and contact with grounded structures increase shock risk.

Cable heating: High current through damaged, kinked, or undersized cables causes resistive heating. Severely heated cables can melt insulation, cause burns to the operator, or start fires.

Precautions:

• Inspect all cables, connections, and prod tips before each use
• Never energize prods unless both are in firm contact with the part surface
• Replace damaged cables immediately - do not tape repairs
• Keep cables out of water and away from sharp edges
• Use insulated gloves when handling prods and cables during active testing
• Follow lockout/tagout procedures for all equipment maintenance

Chemical Hazards

Petroleum-based carriers:

• Flash point must be ≥200°F (93°C), but vapors can accumulate in enclosed spaces
• Prolonged skin contact causes irritation and drying
• Inhalation hazard in confined spaces - ensure adequate ventilation
• Keep away from welding, grinding sparks, and open flames

Dry particle powders:

• Inhalation of fine iron powder is a respiratory hazard - wear a dust mask rated for metal fume/dust
• Eye irritation from powder - wear safety glasses at all times

Contrast paints and cleaners:

• May contain volatile organic compounds (VOCs) - use in ventilated areas
• Some are flammable - check the Safety Data Sheet (SDS)
• Aerosol cans present pressure hazards - do not puncture or expose to heat

Ultraviolet Radiation (UV) Hazards

UV-A lights (365nm) used for fluorescent MT:

• UV-A is relatively low-energy but can cause eye fatigue, headaches, and long-term retinal effects with prolonged direct exposure
• NEVER look directly at the UV-A source - it is harmful to the eyes
• Wear UV-protective glasses (UV-absorbing amber or clear safety lenses) during prolonged fluorescent MT work
• Verify the UV lamp's filter is intact - cracked or missing filters allow harmful UV-B and UV-C radiation to escape
• UV-A does NOT cause sunburn, but some MT chemicals can cause photosensitivity when combined with UV exposure

Physical Hazards

• Heavy equipment: bench units, yokes, power packs, and cable reels - use proper lifting techniques
• Pinch points between headstocks, yoke legs, and clamps
• Hot surfaces: parts, prods, cables, and yoke cores can become hot during extended operation
• Slip hazards from wet particle bath on floors - clean spills immediately

MT Examination Documentation

Every MT examination must produce a written report. This report serves as a legal record of the examination, evidence of code compliance, and a quality record that may be required for the life of the component.

Required Report Content (per ASTM E1444 / ASME V)

1. Examination identification: Date, time, examination report number
2. Personnel: Examiner name, certification level, certificate number, employer
3. Part identification: Part number, serial number, heat number, material specification, drawing number
4. Procedure reference: Written procedure number and revision used
5. Equipment identification: Yoke serial number, bench unit ID, UV light ID, Gaussmeter serial number (if used)
6. Calibration/verification status: Ammeter verification date, yoke lifting force test date and result, UV-A intensity measurement, particle settling test results
7. Surface condition: As-received, cleaned, ground, coating type and thickness
8. Technique details:
- Magnetization method (prod, yoke, coil, head shot, central conductor)
- Current type (AC, DC, HWDC)
- Amperage or field strength applied
- Particle type (dry visible, wet fluorescent, wet visible)
- Carrier type (oil, water, aerosol)
- Continuous or residual technique
9. Environmental conditions: Part temperature, ambient light level (for fluorescent: ≤2 fc), UV-A intensity at surface (≥1000 μW/cm²)
10. Results: Location, size, orientation, type, and disposition of all indications found. Include a sketch or photograph for each indication.
11. Acceptance criteria reference: Code section, table, and revision applied
12. Disposition: Accept/reject per the criteria (if within Level I scope)
13. Post-examination: Demagnetization performed (yes/no), residual field measurement if applicable, part cleaning
14. Signature and date: Examiner's signature confirming accuracy of the report

Demagnetization

After MT examination, parts retain residual magnetism from the magnetization process. This residual field must be reduced to acceptable levels when required by the procedure, the customer specification, or when subsequent operations demand it.

When Demagnetization Is Required

• Before welding: Residual magnetism causes arc blow - the welding arc is deflected by the magnetic field, resulting in poor weld quality, porosity, and incomplete fusion
• Before machining: Magnetized parts attract cutting chips, contaminating coolant and scoring tool surfaces
• Before assembly: Magnetic components can interfere with instrumentation, compasses, or sensitive electronic equipment
• Per procedure requirements: Many MT procedures include demagnetization as a mandatory post-examination step
• Before subsequent NDT: Residual fields can interfere with other testing methods

AC Demagnetization (Most Common Method)

The part is passed slowly through an energized AC coil. The alternating field re-orients domains randomly, and as the part moves away from the coil center, the decreasing field amplitude progressively reduces domain alignment.

Key technique points:

• Pass the part through at a slow, steady speed - approximately 1 foot per second or slower
• Continue until the part is at least 3 feet (1 meter) beyond the coil on the exit side
• Do NOT de-energize the coil while the part is still inside - this re-magnetizes the part
• Multiple passes may be needed for high-coercivity materials
• Part orientation: align the part's long axis with the coil axis for most effective demagnetization

DC Demagnetization (Reversing DC)

Apply a DC field, reverse direction, and reduce amplitude. Repeat with progressively smaller field strengths until the residual field is acceptably low. This step-down process mimics the hysteresis loop at progressively smaller amplitudes.

Used when AC demagnetization is ineffective (very high-coercivity materials) or when AC equipment is not available.

Acceptable Residual Field Levels

• General industrial: < 3 Gauss (0.3 mT)
• Welding applications: < 5 Gauss at the weld joint
• Precision instruments/aerospace: < 1 Gauss
• Always follow the specification's stated requirement

Verification

Use a calibrated Gaussmeter (Hall-effect meter) to measure residual field. Measure at multiple locations: part ends, center, geometry transitions, and any location where residual fields tend to concentrate. A compass or field indicator can provide qualitative verification but is less precise.

NDT Personnel Qualification - SNT-TC-1A Overview

ASNT Recommended Practice No. SNT-TC-1A defines the framework for qualifying and certifying NDT personnel. As a Level I, you should understand the certification system you are part of.

Three Certification Levels

Level I: Can perform specific NDT operations under the direct supervision of a Level II or Level III. Can follow written instructions and record results per the procedure. Cannot independently evaluate indications or make accept/reject decisions (unless specifically authorized by the written procedure for clear-cut criteria).

Level II: Can set up and calibrate equipment, perform examinations independently, evaluate indications against acceptance criteria, make accept/reject decisions, and supervise Level I personnel. Can prepare and review written instructions.

Level III: Can develop procedures, establish and approve techniques, interpret codes and standards, designate NDT methods and techniques, train and examine Level I and Level II candidates. Responsible for the overall NDT program.

Certification Requirements

1. Training: Documented classroom instruction in the specific NDT method
2. Experience: Documented hands-on experience performing the method
3. Examinations: General (method principles), Specific (application/code knowledge), and Practical (hands-on demonstration)
4. Vision: Near vision acuity (Jaeger J1 or equivalent at 12 inches), color contrast perception (if applicable)

Key Points for Level I

• Your certification is employer-based - it is valid only while employed by the certifying employer
• If you change employers, your new employer must evaluate your qualifications and may require additional training, experience, or examination
• Certification is typically valid for 5 years (or per the employer's Written Practice) with annual vision checks
• You must work within the scope of your certification - do not attempt evaluations or dispositions reserved for Level II

MT Equipment Care

Properly maintained equipment performs consistently. Poorly maintained equipment produces unreliable results and can be a safety hazard.

Yoke Maintenance

• Clean pole faces regularly - debris reduces contact and lifting force
• Inspect power cord for damage - frayed cords are electrical hazards
• Verify articulating joints move freely and lock securely
• Check the on/off switch - it must function reliably as a dead man switch (field active only while trigger is held)

Prod Maintenance

• Inspect contact tips - worn or mushroomed tips create poor contact and arc burns
• Replace damaged tips per manufacturer's specifications
• Check spring mechanism (spring-loaded prods) - weak springs don't maintain adequate contact
• Inspect cable connections - loose or corroded connections create high-resistance joints

Particle Storage

• Keep dry powder containers sealed when not in use
• Store in a dry location away from moisture
• Check expiration dates - particles degrade over time
• Do not mix particles from different manufacturers or different batches

UV Lamp Care

• Allow adequate warm-up before examination
• Protect the filter from impact - replace if cracked
• Clean the filter periodically - particle dust reduces UV output
• Replace bulbs per manufacturer's recommended interval or when intensity drops below specification