Magnetic Particle testing is most common non-destructive weld
inspection technique used for the examination of surface flaws/openings in ferromagnetic
materials. Basic magnetic principles are used to make the indication of surface
cracks and other subsurface discontinuities. The test part is made magnetized with the
help of some magnetizing system and then some magnetic particles are applied on
the test surface. The behavior of those magnetic particles in the presence of
magnetic field, provides the obvious manifestation of surface discontinuities. Below
is the demonstration for How the Magnetic Particle Examination Works in evaluating the Weld components.
Magnetization
Magnetization of material can be done by different
techniques, normally the Yoke Technique is most common in weld and weld repair.
Magnetic field is generated in the test part by applying strong magnetic flux
to it. Say if the test material is clean and free from any kind of flaw then the
magnetic field lines will pass straight through it without any interruption. If
there is any breaking in surface, the magnetic field lines gets interrupted and
consequently cause the leakage of field which is often referred to as “Leakage
Field”. These leakage fields are responsible to attract any magnetic material
in surrounding.
Pre-Cleaning
As no strict conditions are required for cleaning the test
material before magnetic particle inspection. Less level of cleaning is needed
to remove dust, organic pollutants, oil and paint etc. Normally less level of
surface pollutants don’t create troublesome effects during inspection.
Introducing the Magnetic Particles
Different types of magnetic particles are used, depending on
the certain application and magnetizing technique. Fluorescent and colored
magnetic particles are available in market. Pigmented magnetic particles make
color contrast for indication while fluorescent particles are viewed by mean of
some black light (ultraviolet).
After applying on surface, excess particles are removed with
the help of low pressure air jet because in excess they can’t help proper
examination. The leakage fields if present due to discontinuities, tends to
attract these particles. The accumulation of particles make a clear
investigation of cracks, openings or breaking if present any.
Manipulation
The technique is easy to perform however there are some
critical points that need to be focused. Leakage fields are only formed if the
magnetic field lines are at ninety degrees to discontinuities. Say if the
surface breaking is parallel to these magnetic lines, the particles will not
get entrap thus giving no indication. For this, equipment is applied on test
weld part in two directions provided one direction is perpendicular to other. The
Inspector must make sure that test weld is examined in both directions. See the pre-requisites of visual inspection.
Indication and Evaluation
The accumulation of particle over the surface flaws, direct
the indication to the inspector. The weld discontinuities are evaluated (giving
the size and shape).
Post Cleaning
After examination, it is sometimes necessary to demagnetize
the test weld components and the magnetic particles are removed from the
surface.
Last comments
The evaluation of weld discontinuities using magnetic
particle technique, requires suitable light intensities. Codes/standards
codifies the minimum amount of light which must be fulfilled before the
examination to start. The test must be carried out in perpendicular directions
in order to provide adequate magnetic flux to discontinuities. Make sure the
equipment is kept in good condition and must be calibrated according to
standards.