NDT TRAINING INSTITUTE IN CHENNAI
MAGNECTIC PARTICLE TESTING
INTRODUCTION
MPT is a Non Destructive Testing (NDT) process for detecting surface and subsurface discontinuities in ferroelectric materials such as iron, nickel, cobalt, and some of their alloys.
Surface & Sub Surface discontinuities can be find through this MPI
AC current = Only Surface
DC current = Up to 6mm from the surface
PROCEDURE
ASME Sec V Article 7
STANDARDS
ASME Sec V Article 25 SE 709
REFERENCE
ASME Code Section V / Article 25
ASTM-E709andard Test Method for Magnetic Particle Examination.
ASME Code Section I
ASME Code Section VIII
ANSI/ASME B31.1, B31.3
AWS D1.1
Clients specification requirements for acceptance criteria.
BASIC PRINCIPLE:
Magnetic Flux Leakage
MAGNETISM PROPERTIES :
Generally a magnet is having two poles namely North Pole and South Pole.
If a magnet is broken into number of pieces, Each piece forms as a an individual magnet (North pole & South Pole).
In a magnet opposite poles will attract and the same poles will repel.
The strength of the magnetic field is very high in the end poles of the magnet.
Inside the magnet, flux lines will travel from South Pole to North Pole and the outside of the magnet lines will travel from North Pole to South Pole.
The group of particle motion in a magnet is called as domains. In a material the domains are situated in its own direction
TYPES OF MAGNETIC MATERIALS: (According to the Magnetism)
Diamagnetic Materials ( -ve Susceptibility -Ability of material to be Magnetized )
This type of materials would have negative magnetism/ Susceptibility, So MPI can not be conducted on this materials. Examples: Gold, Copper, Aluminum
Paramagnetic Materials (Small Positive Susceptibility)
These materials would have small positive magnetism/ Susceptibility, MPT can’t be conducted on this materials also. Examples: Magnesium, lithium, Molybdenum.
Ferromagnetic Materials (Positive Susceptibility)
Ferro magnetic materials would have positive magnetism/ Susceptibility; MPI can be conduct on this type of materials. Examples: Iron, Cobalt, Nickel and Iron based materials.
ADVANDAGES
Can detect both surface and near sub-surface defects.
Can inspect parts with irregular shapes easily.
Fast method of inspection and indications are visible directly on the specimen surface.
Considered low cost compared to many other NDT methods.
Is a very portable inspection method especially when used with battery powered equipment.
DISADVANDAGES
Cannot inspect non-ferrous materials such as aluminum, magnesium or most stainless steels.
Inspection of large parts may require use of equipment with special power requirements.
Some parts may require removal of coating or plating to achieve desired inspection sensitivity.
Limited subsurface discontinuity detection capabilities. Maximum depth sensitivity is approximately 6mm (under ideal conditions).
Post cleaning, and post demagnetization is often necessary.
Alignment between magnetic flux and defect is important.