Magnetic Particle Testing (MT) – Complete NDT Guide to Flaw Detection

Magnetic Particle Testing (MT) is a widely used NDT or non-destructive testing method for detecting surface and near-surface defects in ferromagnetic materials. It plays a critical role in industries such as oil & gas, aerospace, automotive, and manufacturing, ensuring structural integrity and preventing failures.

In this guide, we will explore:
✔ Common defects detected by MT (in casting, forming, and welding processes)
✔ The working principle of MT and how magnetic fields reveal defects
✔ Different MT methods and their applications
✔ Types of magnetic particles and currents used in the inspection process
✔ Final steps after performing MT to ensure reliable results

Let’s dive into the details!

What is Magnetic Particle Testing (MT)?

Magnetic Particle Testing (MT) is a method that detects cracks, discontinuities, and surface defects by applying a magnetic field to a ferromagnetic material. When a defect disrupts the magnetic field, iron-based magnetic particles accumulate at the discontinuity, making it visible for inspection.

Key Benefits of Magnetic Particle Testing

✔ High Sensitivity – Detects even small surface and near-surface defects.
✔ Fast & Cost-Effective – Quick inspection process with minimal preparation.
✔ Versatile Applications – Used in casting, forging, welding, and machining industries.
✔ Portable & Flexible – Can be performed in a laboratory or on-site.

Types of Defects Detectable by Magnetic Particle Testing (MT)

Magnetic Particle Testing (MT) is particularly effective for identifying surface and near-surface defects that occur during manufacturing and fabrication processes such as casting, forming, and welding. These defects can compromise the structural integrity and performance of components, making early detection critical for quality control and safety.

Common Defects After Casting

Casting is a process where molten metal is poured into a mold to create a desired shape. However, various defects can arise due to material impurities, solidification issues, and cooling inconsistencies. MT helps in identifying such defects, which can be classified into three main categories:

1️⃣ Inherent Defects

These defects originate during the initial solidification of the metal and are often linked to material contamination or gas entrapment.

✅ Inclusions – Non-metallic impurities (such as oxides, sulfides, and silicates) trapped within the metal, leading to weak spots.
✅ Gas Porosity – Small gas bubbles that fail to escape before the metal solidifies, forming voids that weaken the material.

2️⃣ Shrinkage Defects

Shrinkage occurs when molten metal contracts unevenly during cooling, leaving voids or cavities that can be detected through MT.

✅ Shrinkage Cavities – Irregular voids inside the casting due to insufficient material filling.
✅ Hot Tears – Surface cracks that form due to thermal stresses when different parts of the casting cool at different rates.

3️⃣ Crack Formation

Cracks can develop in castings due to rapid cooling, stress concentration, or improper mold design.

✅ Surface Cracks – Visible cracks on the casting surface caused by thermal expansion and contraction.
✅ Cold Shuts – Occur when two streams of molten metal fail to fuse properly, leaving a weak joint that is prone to failure.

Common Defects After Forming

Forming processes like rolling, forging, and extrusion involve reshaping metal under mechanical force. However, these high-pressure processes can introduce internal and surface defects that affect product integrity.

✅ Laminations – Thin, layered separations within the metal caused by impurities or defects in the original billet. These can reduce strength and cause failure under stress.
✅ Rolling Defects – Surface cracks, laps, and wrinkles resulting from uneven rolling pressure or contaminated materials.
✅ Forging Laps – Overlapping layers of metal formed during improper die closure or excessive material flow, leading to structural weakness.
✅ Bursting Cracks – Internal cracks that occur when metal is overstretched during the forming process.

Common Defects After Welding

Welding is a critical process in fabrication and repair but can introduce defects if parameters like heat input, filler material, and joint preparation are not properly controlled. MT is widely used to detect surface-breaking defects in welds)welding inspection(, ensuring strong and durable joints.

1️⃣ Fusion Defects

✅ Lack of Fusion – Incomplete bonding between weld metal and base metal due to inadequate heat input or contamination.
✅ Cold Laps – Occur when molten weld metal fails to bond with the previously solidified weld pass, creating a weak joint.

2️⃣ Weld Contamination Defects

✅ Slag Inclusions – Non-metallic impurities trapped within the weld, reducing strength and increasing brittleness.
✅ Oxide Formation – Surface oxidation that weakens the weld and promotes corrosion.

3️⃣ Weld Cracking

✅ Hot Cracks – Form at high temperatures due to excessive heat input or improper cooling rates. Often occur along the weld bead or in the heat-affected zone (HAZ).
✅ Cold Cracks – Appear hours or days after welding, usually caused by hydrogen embrittlement or residual stresses.
✅ Underbead Cracks – Subsurface cracks located just beneath the weld bead, often difficult to detect without NDT methods like MT or UT (Ultrasonic Testing).

Why Identifying Defects with MT is Critical?

Detecting these defects before components go into service is essential to prevent costly failures, safety hazards, and production delays. MT provides a fast, reliable, and cost-effective way to ensure quality in metal components used in critical applications such as:

✔ Aerospace & Automotive – Ensuring cast and welded components are defect-free.
✔ Oil & Gas Industry – Ensuring the integrity and safety of critical assets through pipeline inspection services, pressure vessel inspection services, and comprehensive assessments of refinery equipment.

✔ Manufacturing & Heavy Equipment – Preventing fatigue failure in machinery and structures.

By understanding the types of defects MT can detect, inspectors and engineers can enhance product reliability, improve manufacturing processes, and extend the lifespan of critical assets.

Basic Principle of Magnetic Particle Testing

MT works on the principle of magnetic flux leakage. When a ferromagnetic component is magnetized, any surface or near-surface defect interrupts the magnetic field, causing the magnetic flux to leak out. This leakage attracts fine iron-based magnetic particles, forming a visible indication of the defect.

Magnetic Field Source

• Direct magnetization – Electric current passes directly through the component.
  • Indirect magnetization – Magnetic fields are introduced using probes, coils, or yokes.

Magnetic Field Orientation

Proper field alignment is crucial. Defects are best detected when the magnetic field is perpendicular to the defect. Two types of field orientations include:
✔ Longitudinal Magnetization – Detects transverse defects.
✔ Circular Magnetization – Detects longitudinal defects.

Methods of Performing Magnetic Particle Testing (MT)

Magnetic Particle Testing (MT) can be performed using direct or indirect magnetization methods, depending on the size, shape, material, and inspection requirements of the component. Choosing the right method ensures maximum defect detection accuracy while maintaining efficiency and ease of use.

Direct Magnetization Methods

Direct magnetization occurs when an electric current is passed directly through the component, generating a circular magnetic field around the area being tested. This method is highly effective for detecting surface and near-surface cracks.

1️⃣ Passing Current Directly Through the Component

✔ Best for components with simple shapes like rods and bars.
✔ Provides high sensitivity to detect fine cracks and discontinuities.
✔ Requires electrical contact with the part, which may not be suitable for all materials.

2️⃣ Magnetizing the Component Using Probes

✔ Suitable for localized inspections, such as pipeline inspection
✔ Uses specialized probes to generate a concentrated magnetic field in a specific area.
✔ Effective for detecting surface-breaking defects in hard-to-reach locations.

Indirect Magnetization Methods

Indirect magnetization does not require electrical contact with the component. Instead, an external magnetic field is applied, making it suitable for delicate, coated, or complex-shaped parts.

1️⃣ Creating a Magnetic Field Using Coils

✔ The component is placed inside a coil, which generates a strong longitudinal magnetic field.
✔ Ideal for inspecting cylindrical, tubular, and elongated components like pipes and shafts.
✔ Works well for batch inspections of multiple parts simultaneously.

2️⃣ Using a Yoke for Magnetic Field Application

✔ A portable yoke applies an external alternating or direct magnetic field to the component.
✔ Perfect for on-site inspections and irregularly shaped objects.
✔ Commonly used in welding inspection, structural assessments, and field applications.

Type of Current Used for Magnetization

The effectiveness of MT depends on the type of electrical current used for magnetization. The two main types are:

✔ Direct Current (DC) – Provides deep penetration, suitable for detecting subsurface defects.
✔ Alternating Current (AC) – Produces a strong surface field, ideal for surface crack detection.

💡 Choosing the right current type depends on the defect type and inspection requirements.

Magnetic Particles Used in MT

The magnetic particles used in MT play a crucial role in defect detection. They are available in two main types:

✔ Dry Particles – Best suited for outdoor inspections and rough surfaces.
✔ Wet Particles (Suspended in Liquid) – Provide higher sensitivity and contrast, making them ideal for fine crack detection.

💡 Fluorescent Magnetic Particles (visible under UV light) enhance defect visibility, improving inspection accuracy.

The Final Step After Magnetic Particle Testing

After the MT process, it is essential to:

✔ Interpret and document results – Record findings, take images, and classify defect severity.
✔ Demagnetize the component – Remove residual magnetism to prevent interference with further operations.
✔ Apply protective coatings – Prevent corrosion on exposed surfaces.
✔ Recommend corrective actions – Determine if repairs or further testing are needed.

Magnetic Particle Testing (MT) Inspection Checklist

A structured MT checklist ensures a systematic approach, reducing errors and improving reliability. The following table provides a clear framework for MT inspections while maintaining compliance with industry standards.

1️⃣ General Inspection Information

Field	Details
Company/Site Name	[Insert Name]
Inspector Name	[Insert Name]
Inspection Date	[Insert Date]
Component/Part ID	[Insert Details]
Material Type	[Steel, Cast Iron, Alloy, etc.]
Inspection Location	[Workshop, On-site, Field]

2️⃣ Pre-Inspection Requirements

Requirement	Check (✔/✖)
Review inspection procedures and standards (ASME, ASTM, ISO)
Verify calibration of testing equipment (Magnetizing equipment, Gauss meter)
Confirm appropriate lighting conditions (Minimum 1000 lux for visible particles)
Ensure surface preparation (Clean, dry, and free from oil, grease, or scale)
Check magnetization technique suitability (Direct, Indirect, AC, DC, Yoke, Coil)
Select appropriate magnetic particles (Wet/Dry, Visible/Fluorescent)
Confirm contrast levels for visible particles (Use white background paint if needed)

3️⃣ Inspection Process

A) Applying the Magnetic Field

Checkpoints	Check (✔/✖)
Verify correct magnetizing technique (Yoke, Coil, Prods, etc.)
Ensure proper current and field strength (Use Gauss meter)
Confirm complete coverage of inspection area

B) Applying Magnetic Particles

Checkpoints	Check (✔/✖)
Ensure uniform particle application (Dry powder, Wet suspension)
Maintain correct flow rate and coverage (for wet method)
Avoid excessive particle buildup, which may obscure indications

C) Interpretation & Evaluation of Indications

Checkpoints	Check (✔/✖)
Observe particle accumulation patterns (Linear, Rounded, Diffused)
Identify relevant defects (Cracks, Lack of Fusion, Porosity, Laminations)
Compare with acceptance criteria (ASME, AWS, ISO, customer specs)

D) Post-Inspection Procedures

Checkpoints	Check (✔/✖)
Verify proper demagnetization (If required)
Confirm residual magnetism is within acceptable limits
Remove magnetic particles from the surface
Ensure no damage or contamination remains

4️⃣ Documentation & Reporting

Requirement	Details
Magnetization Type	AC, DC, Half-wave rectified
Equipment Settings	Current, Field Strength, Magnetizing Time
Particles Used	Dry/Wet, Fluorescent/Visible
Surface Condition & Cleaning Method	[Describe]
Findings	Indication location, size, and orientation
Defect Severity & Acceptance Criteria	Based on ASME, AWS, ISO
Recommended Actions	[Insert Corrective Actions]
Supporting Evidence	Photos, Reference Drawings, Calibration Records

✔ Inspector Name: [Insert Name]
✔ Signature: [Sign Here]
✔ Supervisor Review & Approval: [Approval & Date]

Why Use This MT Checklist?

✅ Ensures compliance with ASME, ASTM, ISO, and AWS standards.
✅ Improves inspection reliability by covering all critical steps.
✅ Reduces errors and ensures thorough defect detection.
✅ Enhances documentation for traceability and quality assurance.

Need Expert Supervision for Your MT Inspections?

At NWE, we provide expert MT inspection supervision, ensuring compliance, accuracy, and defect-free evaluations.

✅ Contact us today for professional guidance and oversight!

🔗 Related Inspection Services

✔ Pipeline Inspection – Detecting surface and subsurface flaws in pipelines.
✔ Welding Inspection – Ensuring weld integrity and detecting discontinuities.
✔ Pressure Vessel Inspection – Assessing structural soundness in high-pressure equipment.

Frequently Asked Questions About Magnetic Particle Testing (MT)

What are the benefits of Magnetic Particle Testing?

✔ Detects surface and near-surface defects with high accuracy.
✔ Fast, cost-effective, and suitable for large-scale inspections.
✔ Portable and adaptable for on-site testing.

Which materials can be tested using MT?

Ferromagnetic materials such as steel, iron, and cobalt alloys.
Non-ferromagnetic materials (aluminum, copper) cannot be tested using MT.

What is the difference between MT and other NDT methods?

Compared to Ultrasonic Testing (UT): MT is better for surface cracks, while UT detects deep internal flaws.
Compared to Liquid Penetrant Testing (PT): MT is faster but limited to ferromagnetic materials.

Can Magnetic Particle Testing (MT) detect subsurface defects?

Yes, but only near-surface defects. MT can detect surface-breaking cracks and slightly subsurface flaws in ferromagnetic materials, but for deeper flaws, Ultrasonic Testing (UT) or Radiographic Testing (RT) is required.

What are the differences between wet and dry Magnetic Particle Testing?

Wet MT is more sensitive and uses magnetic particles suspended in a liquid for better defect visibility.
Dry MT is used on rough surfaces or where liquid may not spread properly.

What industries commonly use Magnetic Particle Testing (MT)?

Oil & Gas, Aerospace, Automotive, Power Plants, and Heavy Equipment Manufacturing. It is widely used for weld inspections, structural integrity checks, and detecting fatigue cracks.

Conclusion: Why Magnetic Particle Testing is Essential

Magnetic Particle Testing (MT) is a vital NDT method for uncovering surface and near-surface flaws in ferromagnetic materials. To ensure your MT inspections deliver accurate, reliable, and compliant results, you need end-to-end oversight—and that’s where NWE steps in.

At NWE, we offer specialized third-party NDT audit for Magnetic Particle Testing:

• Ensuring Compliance with international standards (ASNT, ISO, ASTM, API)

• Reviewing MT Procedures to guarantee accuracy, efficiency, and safety

• Verifying Defect Indications to distinguish between relevant and non-relevant findings

• Delivering Expert Recommendations for corrective actions and further evaluations

Ready to secure flawless MT inspections?
Partner with NWE’s certified supervisors for comprehensive NDT auditing and monitoring—contact us today for a customized consultation and boost your equipment integrity!