Magnetic Particle Inspection

What is Magnetic Particle Inspection?

Magnetic Particle Inspection is a non-destructive inspection that involves small magnetic fields and fine magnetic particles to detect linear flaws on the surfaces or slightly below surfaces of ferromagnetic materials such as steel, cobalt, nickel and their alloys. 

Magnetic Particle Inspection is an excellent method for detecting defects such as cracks, porosity, cold shuts, laps etc. on castings, forgings, and weldments. Magnetic Particle Inspection is used in a variety of industries including aerospace, automotive, petrochemical, power generation and structural steel. It may also be used in offshore structures and underwater pipelines. 

Magnetic Particle Inspection can be performed mobile (on-site) and in a laboratory.

For mobile inspections, electromagnetic yokes are commonly used. These yokes are U-Shaped laminated soft iron, with an electrical coil wrapped around the base of the U. When the coil is turned on, an electrical current passes through the coil creating a magnetic field around the legs/poles of the yoke.  

For laboratory inspections, a wet horizontal bench unit is very commonly used. These bench units are excellent for production environments because it allows for batch inspections of a variety of components, big and small. The bench units have a head and tailstock with electrical contacts, where the part are clamped, to produce circular magnetization and also have a movable coil for producing longitudinal magnetic fields.  

Magnetic Particle Inspection can be broken down into four steps.
  1. Induce a magnetic field into the item being inspected.

  2. Apply the fine magnetic particles to the inspection surface.

  3. Inspect and examine the inspection surface.

  4. Demagnetize and clean the item. 

Advantages
  1. Excellent for detecting surface or slightly sub-surface discontinuities 

  2. A relatively fast method of inspection compared to other NDT methods

  3. Surface Preparation is not as critical compared to other NDT methods

  4. Can be performed on-site or in a laboratory environment

  5. An inexpensive compared to other NDT Methods 

limitations
  1. Only Ferrous materials can be inspected. Non-Ferrous materials including aluminum, magnesium and most stainless steels cannot be inspected.

  2. Thick coatings or platings on inspection surface are required to be remove to achieve desired sensitivity.

  3. Limited capability to detect sub-surface discontinuities.

  4. Post-demagnetization is usually required.

  5. Inspections must be performed in two different directions. 

  6. Magnetic flux and indications alignment is very critical to the inspection. 

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