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CHAPTER 4: Magnetic Particle Inspection (MPI) - Coggle Diagram
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