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CHAPTER 4C - MPI (PART21-25) - Coggle Diagram
CHAPTER 4C - MPI (PART21-25)
Demagnetization
The amount of magnetism that remain in the part after magnetic inspection depends on the permeability of the materials & the strength & also the direction of the magnetizing force
After MPI, it is often necessary to demagnetize the part
Dry Detection Media
Dry particles are available in a wide variety of colors
Using the right color it is usually possible to work without contrast aid paint
Advantages
Virtually no lower limit on temperature
Upper temperature is limit to 65 degree Celsius (fluorescent particles) & 315 degree Celsius (for other colors)
No fire / explosion hazard
Residues are easily removed
Dry particles provide the best sensitivity for subsurface defects which depends on the brand
Disadvantages
Poor particle mobility
Reduce sensitivity for surface breaking defects
Not suitable to use with permanent magnet / DC magnetic field
Not suitable for residual testing
Greater operator skills are needed
Difficult to use on overheat surface
Difficult to use in windy condition
Testing Method Selection. Why?
MPI may be performed using
Either a continuous / residual method
Either fluorescent / visible detection media
Detection media which is either wet / dry
Not all combinations of the above are effective
Continuous method
Detecting media applied immediately prior to & during magnetization
Residual method
Detecting media used after the applied field has been removed
Required high retentivity
Less sensitive than continuous method
Useful for components such as ball bearing
Fluorescent Methods
Advantages
More contrast (more sensitive)
Less operator fatigue (batch inspection)
No contrast aid point needed
Disadvantages
Special lighting is needed
Not suit for rough component (high background could reduce the contrast)
Fluorescent coating is easily damage by higher temperature, acids & sunlight
Reason of Demagnetization
The part may be used in an area where a residual magnetic force will interfere with the operation of instruments that are sensitive to magnetic field, e.g.: compass
Abrasive particle may attract to the magnetized parts & could cause wear
Particles could also stick to the magnetized surfaces & interfere subsequent operations
During arc welding, strong magnetic field could deflect the arc / filler metal to be repelled from weld called the arc blow
Demagnetization is also required if the part is to be magnetized at the lower magnetizing force in a different direction than the previous test
How to Demagnetize
Use a reversing / reducing magnetizing force
The initial force applied must exceed the existing coercive force
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)
Visible Methods
Advantages
No special lighting is needed
Easier to use on rough surface
Colored particles are stable at surface temperature up to 315 degree Celsius
Disadvantages
Less contrast (less sensitive)
Contrast aid point may be / need to
Tiring to use (not suitable for batch inspection)
Wet Detection Media
Water based inks: contain detergent to improve wetting ability, corrosion inhibitors & anti-foaming agent
Paraffin based inks: Less affected by adverse surface conditions & generally more effective
Advantages
Particle mobility improved
Provide best sensitivity for surface defects
Magnetic ink is suitable for used with a permanent magnet / DC magnetic field
Suitable for residual methods
Less operator skill needed
Disadvantages
Limited temperature range
Possible fire / explosion due to paraffin based ink
Residues can be a problem for subsequent processing
Fluorescent Method
Advantages
The most sensitive methods
Cause less operator fatigue
Most suitable for batch inspection
Disadvantages
Require special lighting
Fluorescent ability can be severally reduce by exposure to UVA, sunlight, high temperature, acidity & alkalinity
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