NDT Controls (MT)

Magnetism is the ability of certain materials (known as ferromagnetic materials) to attract other materials or themselves. The magnetic field is typically characterized by its flux lines, which report the following features

• At a magnetic pole level, the lines either exit from or enter into the material itself.
• They form closed paths.
• They do not intersect.
• They follow the path with the least magnetic reluctance.
• They have the same intensity.
• They have a direction due to the forces of attraction or repulsion between the poles.

Principle of the Magnetic Particle Testing Method

An intense magnetic field is applied to the part under examination using a special device.
If the part has a discontinuity, it will interrupt the magnetic flux lines, creating a flux leakage on the surface.
The developer is applied to the surface to be examined.
Pigment coated Ferromagnetic particles, are attracted to the localized flux leakage, forming an indication at the discontinuity.
The magnetic testing method allows for the detection of defects that are directly on the surface or just below the same.

Control Procedures

The key points of the control procedure are as follwos:

1. Preliminary Surface Cleaning
   The main purpose of this phase is to ensure the flow of the developer and, therefore, the formation of any flux leakage.
   Surfaces must be clean and dry before inspection.
   Substances such as oil, grease, or rust must be removed because   as they limit the interaction between the developer and the dispersed flux.
2. Application of the Magnetic Field
   The magnetic field can be induced in the part using various systems:

• By putting A permanent magnet or electromagnet in contact with the part.
• By letting an electric current flow through the part (e.g., a bench).
• By Using a coil near the part, through which electric current passes.
  It is necessary to magnetize the part in two main directions: longitudinal and circular fields, as the direction of the discontinuities is usually unknown in advance. The condition of highest sensitivity  is at An angle of 90° between the flux lines and the plane of the discontinuity, with a tolerance of about 30°. In case of joints, appropriate magnetization techniques and sequences must be adopted.

1. Application of the Developer
   The MT method uses either dry or wet (suspension) developers.

• Dry Developers: They are very practical but have  with some limitations,

They  are ferromagnetic powders available in a wide range of colors. Usually,there is a tendency to choose colors that provide the highest contrast with the surface to be examined. The powder should be applied simultaneously with the magnetization of the part to prevent it from falling off before it is attracted to the areas with dispersed flux

• Wet Developers: They are generally more sensitive and can be applied in any position

 They are typically color-contrast or fluorescent. The suspension medium is usually a light hydrocarbon, although water-based developers also exist. The liquid developer should be sprayed immediately before or during the early stages of magnetization to prevent the liquid from washing away the indications due to the force of the spray.

Interpretation of Indications
After applying the developer, the indications can be interpreted:

• False indications: Caused by accumulations of the developer, which deposits in grooves, corners, or surface roughness.
• Indications caused by imperfections: These are classified as either acceptable or unacceptable.

1. Demagnetization of the Part (if required)
   Parts subjected to MT testing may sometimes have residual magnetic induction levels that interfere with subsequent operations or the operating conditions of the component, for example:

• Welding operations.
• Machining processes.
• Placement of sensors sensitive to magnetic fields.
• Increased wear conditions in certain mechanical parts due to mutual attraction between the part and ferromagnetic particles.