Magnetic Particle Testing and Applications

1. Fundamentals of Magnetic Particle Testing

Magnetic Particle Testing (MT) is a method used to detect surface and near-surface discontinuities in ferromagnetic materials. Leakage flux occurs in the material when placed within a magnetic field, attracting iron particles and making discontinuities visible.

Classification of Materials by Magnetic Properties

• Diamagnetic: Materials like copper and gold, which are repelled by a magnetic field.
• Paramagnetic: Materials like platinum and lithium, which are weakly attracted to a magnetic field.
• Ferromagnetic: Materials like iron and nickel, which can retain magnetic properties.

2. Physical Principles of Magnetic Testing

Magnetic Field: The region where magnetism is effective.
Magnetic Flux: The number of magnetic field lines.
Magnetic Permeability (µ): The resistance of a material to magnetic flux.

Ferromagnetic materials retain their magnetic properties up to a certain temperature, known as the Curie Temperature. Beyond this, they lose their magnetism.

3. Magnetization Techniques and Equipment

Magnetization Methods

• Circular Magnetization: Achieved by passing current through the part or using a central conductor.
• Longitudinal Magnetization: Created using a yoke, coil, or flexible cable wrapping.
• Prod Technique: A magnetic field is created by applying current to the surface.

Types of Current Used

• DC (Direct Current): Effective for detecting subsurface discontinuities.
• AC (Alternating Current): Provides clearer surface indications.
• Half-Wave Rectified AC: Suitable for detecting near-surface discontinuities.

4. Magnetic Particle Testing Process

1. Surface Preparation

The surface must be cleaned of oil, rust, and debris before testing. Paint thickness should not exceed 50 µm.

2. Selection of Magnetic Particles

• Dry Method: Particles are sprayed or blown onto the surface.
• Wet Method: Particles are suspended in a liquid carrier.

3. Inspection Environment & Lighting

• Visible Light (Contrast Method): ≥ 500 Lx
• UV Light (Fluorescent Method): ≤ 20 Lx, ≥ 10 W/m² UV-A

4. Evaluation of Discontinuities

Indications are classified as:

• Linear Indications: Length is more than three times the width.
• Circular Indications: Length is equal to or less than three times the width.

5. Standards

TS EN ISO 9934-1: General principles of magnetic testing
TS EN ISO 9934-2: Magnetic particle properties & testing mediums
TS EN ISO 9934-3: Equipment for magnetic testing
TS EN ISO 3059: Inspection lighting conditions
TS EN ISO 23278: Welding inspections & acceptance criteria
TS EN ISO 10228-1: Forged steel inspection standards
TS EN ISO 1369: Casting materials magnetic testing

6. Demagnetization Process

After inspection, residual magnetism in welded parts should be ≤ 0.4 kA/m to prevent interference with further processing. Demagnetization can be achieved by:

• Applying an alternating current (AC) with decreasing intensity.
• Heating the material above its Curie temperature.

7. Inspection Reporting

• Inspection date, location, and personnel
• Standards applied
• Inspection technique & equipment
• Detected discontinuities
• Acceptance/rejection decisions