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CHAPTER 4a & 4b - MPI (Part 13-20) - Coggle Diagram
CHAPTER 4a & 4b - MPI (Part 13-20)
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)
Magnetizing Methods
The required magnetic field can be introduced into a component in a number of different ways:
Permanent magnet that contact with the test specimen
Flow an electrical current through the specimen
Flow the electrical current through a coil of wire around the part / through a central conductor running near the part
The magnetizing methods are classified based on:
2. Based on the character of the magnetizing field used, into:
Circular
Longitudinal
*The probability that a defect will detected is depends on its orientation in relation to the direction of magnetic field
1. Whether or not the magnetizing force is maintained into:
Continuous methods
Used on materials with low remanence
Magnetic particle are applied on the part while being
magnetized
Residual methods
Used on materials with high remanence
Magnetic particles are applied after the part has been
magnetized & the magnetizing force is removed
Relies on the residual magnetism retained in the part
Measuring Magnetic
Fields / Flux Density
The devices commonly
used in MPI
The field indicator
The Hall-effect meter, which is also called a gauss meter
Pie gauges (ASME) is device that is often used to provide an indication of the field direction & strength but do not actually yield a quantitative measure
Flexible Cable
Advantages
Simple to operate
No danger of burning
AC, DC / rectified
Current adjustable
It suited for the underwater application
Limitations
Difficult to keep cable in place
High current required
Transformer required
Permanent magnet / DC Yoke
A permanent magnet / DC Yoke suitable for MPI should be capable of lifting a steel weight of 18kg
Flux indicators become permanently magnetized in a DC field & are unreliable when used with permanent magnet
Permanent magnet / DC yokes are not generally permitted by BS EN 9934-1 (they can be used if the contracting parties agree)
(Permanent magnet)
Advantages
No power supply
No electrical contacts problem
Inexpensive
No damage to test piece
Light weight
Can detect sub-surface defects
Limitations
Direct field only
Deteriorates over time
No control of field strength
Poles attract detecting media
Tiring to use
DC YOKE
Advantages
No electrical contact problems
Relatively inexpensive
No damage to the test piece
Light weight
Operates on low voltage (12V)
Can detect sub-surface defects
Limitations
Direct field only
Not suited for the used with dry powder detection media
Dry magnetic particle
Iron powder / magnetic iron oxide (magnetite)
Typically 5-200 microns, rounded elongated shapes
Colors vary for contrast against component
Can be used on hot surface
Poor particle mobility
Greater operator skill required
Difficult to apply to overheat surface especially in field conditions
Generally less sensitive than the wet particles
Suitable for used on very cold surface & effectively no lower temperature limit
Non-fluorescent magnetic particles can be used on hot surface up to 315 degree Celsius
Fluorescent powder is available but rarely used
Wet Magnetic Particles
Magnetic iron oxide (magnetite) / iron powder
Typically 0.1-100 microns rounded & elongated shapes
Color contrast / fluorescent
Water / kerosene based
Concentration important
Good particle mobility
Easier to used
More sensitive
Typical concentration for a visible ink is 1.25%-3.5% by volume
Typical concentration for a fluorescent ink is 0.1%-0.3% by volume
Steps in Performing an Inspection
using dry particles
Prepare the part surface
Apply the magnetizing force
Dust on the dry magnetic particles
Gently blow off the excess powder
Terminate the magnetizing force
Inspect for indications
Wet Suspension Inspection
Prepare the part surface
Apply the suspension
Apply the magnetizing force
Inspect for indications