CHAPTER 4: Magnetic Particle Inspection (MPI)

MPI

Is a method that can be used to detect surface & near surface defects/flaws/discontinuities in ferromagnetic materials (steel & iron)

Is fast & relatively easy to apply & part on the surface preparation is not as critical as for some of the NDT methods. These characteristics make MPI one of the most widely utilized NDT methods

Is based on the principle: magnetic lines of force (flux) will be distorted by the presence of the defect in a manner that will reveal its presence

Methods used

Inspect variety of product forms including:

  • Castings
  • Forgings
  • Welds
  • Many different industries used MPI to determine a component's fitness for used
  • Some examples of industries that use magnetic particle inspection are the steel (structural), automotive, petrochemical, power generation & aerospace industries
  • underwater inspection is another area which MPI is used to test the item such as (offshore, structures & underwater pipelines)

Ferromagnetic

Materials which have a large positive susceptibility to an external magnetic field

They exhibit a strong attraction to magnetic field & are able to retain magnetic properties after external force have been removed

Ferromagnetic materials have some unpaired electron, so their atoms have a net magnetic moment

Magnetic domain:

  • number of atoms moment (10^12 to 10^15)
  • aligned parallel so that magnetic force within the domain is strong

When a ferromagnetic material is in the unmagnetized state, domains are nearly randomly organized, the net magnetic field = 0

When a magnetizing force is applied, the domains become aligned which produce a strong magnetic field within the component

Iron, nickel, cobalt are examples of ferromagnetic materials components with these materials are commonly inspected using MPI

Paramagnetic

Materials which have a small positive susceptibility to the magnetic field

These materials are slightly attracted by a magnetic field and the material does not retained the magnetic properties when the external field is removed

Paramagnetic properties are due to the present of unpaired electron & from the realignment of the electron path caused by the external magnetic field

Paramagnetic materials, include magnesium, molybdenum, lithium & tantalum

Diamagnetic

Materials which weak & have negative susceptibility to magnetic field

Diamagnetic materials are slightly repelled by a magnetic field & the material does not retained the magnetic properties when the external field is removed

In diamagnetic materials all the electron are paired so there is no permanent net magnetic moment per atom

Diamagnetic properties arise from the realignment of electron path under the influence of external magnetic field

Most elements in the periodic table including, copper, silver & gold are diamagnetic

Steps:

Magnetized the component which will be inspected

If there is any defect on/near the surface present, the defect will create a leakage field

After the component is magnetized, the iron particles (dry/wet) are applied to the surface of the magnetized components/parts

Particles will be attracted & cluster at the flux leakage field which forms a visible indications that NDT officer can detect

Magnetic field orientation
& flow detectability

A longitudinal magnetic field has magnetic lines of force which is run parallel to the long axis of the part

A circular magnetic field has magnetic lines of force which is run around the perimeter of the parts

The type of magnetic field establish is determined by the method used to magnetize the specimen

Being able to magnetized the part in 2 directions is important because the best detection of defects occur when the lines of the magnetic force are established at the right angles to the longest dimension of the defect

This orientation creates the largest disruption of magnetic field within the parts and greatest flux leakage at the surface of the part

If the magnetic field is parallel to the defect, the field will see little disruption & no flux leakage field will be produced

Permeability (m)

is a material property that describes the ease which magnetic flux is establish in a component.
It is the ratio of the flux density (B) created within the material to magnetizing field (H) (m = B/H)

The permeability value is usually the maximum permeability of the maximum relative permeability

The maximum permeability is the point where the slope of B-H curve for the unmagnetized material is the greatest

Main steps
involved in MPI:

Cleaning the part from oil, grease / scale

Magnetization of the part to be inspected (e.g. using permanent magnet)

Application of magnetic particles such as iron (dry/wet) while the part is being magnetized

Inspect the part surface for any flux leakage field

Clean the part of any particle residue

Demagnetize the part (lose its magnetism)

Methods of Demagnetization

Aperture type coil reversing stepped DC

Aperture type coil reducing AC

AC / reverse DC aperture type coil, withdraw component along the coil axis

AC electromagnet (stroking with)

Heat above curie point (770 degree Celsius)

Advantages of MPI

Will detect subsurface defect

Rapid & simple to understand

Pre-cleaning is not as critical as LPI

Will work through thin coatings

Cheap rugged equipment

Direct test methods

Limitations of MPI

MPI cannot be used to non-magnetic material

Presence of surface coating may reduce the sensitivity of the test

Defect that do not break the surface give diffused indications

Dimension, rough surface & certain type of segregation / metallurgical change associated with the test item may give rise to misleading indications