Magnetic Particle Testing (MT) principle, techniques, types, advantages, limitations and standards


What is Magnetic Particle Testing (MT)

The Magnetic Particle Test (MT Test) is used for magnetizable materials (E.g. Iron, Carbon Steel & low-alloy steel). Magnetic Particle Test or MT and Magnetic Particle Inspection or MPI are the same NDT Method. MPI detection sensitivity (to find the flaws) is usually significantly higher compared with the dye penetrant test (DPT or PT).

The MT test is cheaper compared to PT but is only limited to ferromagnetic materials such as carbon steel, low alloy steel, and nickel alloys. Furthermore, it is a practical way of testing fillet welds in steel construction. The test can detect only surface and limited sub-surface discontinuities in the material due to the limited penetrating power of the magnetic field that is induced in the material during testing.

Magnetic particle testing (MT) is a type of electromagnetic testing. Another NDT method that correlates to the MT is the Magnetic Flux Leakage MFL. The main difference between MT and MFL is the flaw detecting media, while the former uses magnetic particles to detect the flaws in the material, the latter (MFL) uses a Hall Effect Sensor to locate the flaws. The magnetic Particle Testing (MPI) technique is however used in a wider range compared to MFL limited applications.

How does Magnetic Particle Examination work?

During the magnetic particle test, a strong magnetic field is introduced into the component. Manual yoke magnets are often used. The conductivity for magnetic fields (permeability) is very high in steel which is commonly used for machine, and steel plant construction.

Any discontinuities, such as lack of fusion or cracks, create significant resistance for a magnetic field. At such discontinuities, the magnetic field escapes at the surface of the test piece (leakage flux). Extremely fine magnetic particles, which are mixed to a paste in water or oil, show the location of the leakage flux and/or the cracks or lack of fusion.
Fig. below shows schematically the pattern of the field lines and the crack indication when testing a weld.

Magnetic Particle Testing Principle

Magnetic Particles Testing (MT) Equipment

Various types of equipment & techniques of Magnetic testing are used in different applications. The ASME Section V, Article 7, specified five MT methods or techniques as given below:

  1. Prod Method
  2. Longitudinal magnetization method
  3. Circular magnetization method
  4. Yoke method
  5. Multidirectional magnetization method

For testing, two different examination media are used which are the Wet Particle type and Dry particle type. They are available as:

  1. Fluorescent &
  2. Non-fluorescent.

The most commonly used MT Method

  1. Portable MT yoke Method (permanent magnet or externally supplied power type).
  2. The stationary horizontal type.

Ferromagnetic Particles for MPI

As stated in the introduction, MPI uses magnetic particles as a flaw detection media. So, they are the essential requirements to complete magnetic particle testing. Magnetic particles are available in two forms:

  1. Dry Particles
  2. Wet Particles

Dry particles are available in varied sizes and colors to suit different sensitivity levels in MPI. Dry particles are applied by the dusting method. The colored particles are used to enhance the sensitivity level. They are very portable and suit best for field MPI. Dry particles are made from iron fillings.

Wet magnetic particles are used for stationary MPI mostly but are also used in field applications. In a wet particle system, iron oxides particles are suspended with water in petroleum-based liquids. They are applied by spraying or pouring on the test surface.

MT Inspection of the Long Seam using Yoke Method

MPI using yoke is the most commonly used MT technique for inspection of welds. Inspection using a yoke is a continuous magnetic field application to identify and locate the flaws. The magnetic particles are applied prior to the field application by spraying or using a bulb as shown in the above picture. This is followed by the application of a magnetic field using a portable yoke.

In a magnetic yoke device, the two poles of an electromagnet are attached in a systematic arrangement where the circuit is completed once both the poles touch the workpiece. Any flaws that lie across the magnetic field are shown due to breakage of magnetic line flow as represented in the below picture.

To ensure an effective magnetic inspection using yoke method, it is essential to apply the magnetization in two different directions so that all flaws if present on the test surface are detected regardless of their orientation.

Magnetic Particle Testing (MT/MPI) Kit and accessories

Magnetic particle testing requires additional accessories to verify the test sensitivity, calibration, field strength measurements, and testing the effectiveness of the MT test. The main accessories used in Magnetic Particle Testing or MT (MPI) are:

  1. Pie gauge– For checking the magnetic field direction
  2. Berthold Penetrameter– For measuring the magnetic field strength, it’s direction & depth of penetration.
  3. Gaussmeter (Pocket magnetometer)– For measuring the magnetic field strength.
  4. Tool steel test ring- For checking the MT system performance.
  5. Quantitative quality indicators (QQI)–  For verification of field direction and strength.
  6. Centrifuge tube– for magnetic particles settling test
  7. Magnetic stripe card- For evaluating the sensitivity of the MT/ MPI method.

Advantages and Disadvantages of Using Magnetic Particle Examination

Advantages of Magnetic Particle Testing

Magnetic Particle Testing is one of the most used surfaces NDT methods. The test is used for many ferromagnetic materials irrespective of their shape and sizes ranging from welding, machining, forging, and other manufacturing process components. The main advantages or benefits of Magnetic Particle Testing (MT) are listed below:

  1. Fast and highly sensitive surface & sub-surface NDT method.
  2. Portable and suits best for field inspection using portable yokes.
  3. Less training & skills requirements for NDT Technician.
  4. Direct inspection & evaluation of parts.
  5. Moderate surface cleaning is only required.
  6. Post-cleaning after the test is also not high as Penetrant testing.
  7. MT can be applied for varied material surface temperatures.
  8. The test can be carried out in any position or shape.

Disadvantages (Limitations) of Magnetic Particle Testing

Magnetic Particle Testing, MT or MPI has also some drawbacks or disadvantages like other NDT methods. The topmost important limitations of Magnetic Particle Testing(MT) are:

  1. The test can be applied only for ferromagnetic materials i.e. suits only for materials that can be magnetized.
  2. MT is not possible for non-ferrous materials.
  3. Proper yoke placement is mandatory for effective inspection.
  4. MT is only limited to the surface and limited to sub-surface material inspection.
  5. After the test, demagnetization is required to eliminate the residual magnetism.
  6. Surface coating such as painting will affect the depth of magnetism and hence it will reduce the sensitivity of the test.

Magnetic Particle Testing Standards

The main important magnetic particle inspection (MPI/ MT) American, Canadian & ISO standards & codes are listed below:

  1. ASME Section V
  2. ASTM E3024– Standard Practice for MPI.
  3.  ISO 9934-1: MPI General principles
  4. ISO 17638: For inspection of welds. (Used in Canada also).
  5. ISO 3059: MPI viewing conditions
  6. ASTM E1444: MPI for aerospace applications
  7. CSA W59
  8. AWS D1.1

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