EXPERIMENTAL TECHNIQUES
MAGNETIC PARTICLE TESTING
(Non Destructive Testing - NDT)
S. P. Ghiya
29-July- 2010
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Objective
To locate discontinuities on surface or on sub -surface
which can not be visibly seen / detectable. (chapter –1)
Discontinuities
visibly not seen
Discontinuity visible with
Magnetic Particle Test
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Principle
Flux Leakage
In this method, area to be examined is magnetized and ferromagnetic
particles are applied on its surface.These particles will form a pattern
on surface wherever there is a flux leakage due to discontinuity.
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Magnetization
The field strength outside the conductor is directly
proportional to the current strength. Inside the
conductor the field strength is dependent on the
current strength, magnetic permeability of the
material.
The field strength varies from zero at the center of
the thickness of component to a maximum on the
surface of component.
The field strength outside the conductor decreases
with distance from the conductor.
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Basics
Once
the
part
is
magnetized,finely
milled
iron particles, coated with a
dye pigment, are then
applied to the speciman.
These particles are attracted
to magnetic flux leakage
fields and will cluster to
form an indication directly
over the discontinuity.This
indication can be visually
detected
under
proper
lighting conditions.
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Basics
There are basically three types of
magnetic field: (chapter-3):
Longitudinal Magnetic field
Circular Magnetic Field
Multi-directional
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Basics
Longitudinal Magnetization(chapter-3): When the length of a
component is several time larger than its diameter, a
longitudinal magnetic field can be established in the
component. The component is often placed longitudinally in the
concentrated magnetic field that fills the center of a coil or
solenoid. This magnetization technique is often referred to as a
"coil shot."
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Longitudinal Magnetization
. Longitudinally magnetized the area to be
examined by using the magnetic yoke. Leg
spacing of the
yoke should not exceed 8”.
Spacing of less than 3”
shall not be used.
DIRECTION OF MAGNETISATION
All locations to be tested shall be tested at
least
in
two
directions
approximately
perpendicular to each other in order to permit
the detection of all non acceptable indications.
Examination shall be conducted with sufficient
overlap to assure 100% coverage
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Basics
Yoke for Longitudinal Magnetization
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Basics
2. Circular Magnetization (chapter-3): : when current
is passed through a solid conductor, a magnetic field
forms in and around the conductor.
The field strength varies from zero at the center of
the component to a maximum at the surface. The
field strength at the surface of the conductor
decreases as the radius of the conductor increases
when the current strength is held constant.
(However, a larger conductor is capable of carrying
more current.).
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Circular Magnetization
Circular magnetization is created in test piece by means of
contact prod using the HWDC machine. Prod spacing shall
be between 6” to 8” except when the geometry of the part
being examined does not permit.
Under such
circumstances the prod distance may be reduced to 3”.
Prod distance less than 3” are not permitted. Care should
be taken to avoid local overheating due to improper prod
contact especially on high carbon & low alloy steels. Care
should also be taken to ensure that local fields due to
proximity of prods does not cause the hiding of any relevant
indication. Magnetizing current should only be switched on
when the prods are in proper contact with the surface
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Magnetizing Current
• Magnetizing current may be from direct or
rectified source.
• The magnetizing current shall be 100 to 125
amp. / inch of prod spacing for the section
thickness of 19 mm and above
• The magnetizing current shall be 90 to 110
amp. / inch of prod spacing for section
thickness of less than 19 mm.
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Basics
Prods for Circular Magnetization (chapter-3)
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Basics
Circular Magnetic Field for
Longitudinal Defects.
Longitudinal
Magnetic field for
Circular Defects.
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Methods
Method Of Examination :
Visible
Fluorescent
Chapter-4
Medium Of Testing :
Dry
Wet
Chapter-4
Types Of Current :
AC
DC
HWDC
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Chapter-2
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Steps To Be Followed (Chapter-6)
Part
preparations (chapter-6)
Magnetizing
Application
using magnetizing currents (chapter-6)
of dry/wet particles (chapter-6)
Interpretation
Recording
Of Indications (chapter-7)
Of Indications (chapter-6)
Demagnetization,if
Post
any (chapter-9)
Cleaning (chapter-6)
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Application
Magnetic particle examination is a nondestructive method for detecting cracks and
other discontinuities at or near the surface in
ferromagnetic materials. Finely divided
magnetic particles are applied to the surface
of a part, which has been suitably
magnetised. The particles are attracted to
regions
of
magnetic
non-uniformity
associated with defects and discontinuities,
thus producing indications, which are
observed visually.
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Surface Preparation
Prior to magnetic particle examination, the surface
to be examined and all adjacent areas within at
least 1” shall be clean and free from oil, grease,
sand, loose rust, scale, lint, paint, weld spatter and
contaminations which could interfere with mobility
of magnetic particle/mask indications and/or
create false indications. The test surface shall not
be excessively rough. In case of any doubt, light
grinding of the surface may be carried out to
determine the actual indication.
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Insp. Medium (FLUORESCENT)
•Wet magnetic particles shall be used as the
examination medium.
•The material shall have a high permeability and low
retentivity and be of suitable size and shape to
produce magnetic indication of the defects.
•It shall be of a colour that will provide adequate
contrast with the background surface being tested.
Concentration and be suitable for examination under
black light.
•Concentration of wet medium as measured in pear
shaped centrifuge shall be 0.1 to 0.4 in 100 ml bath
sample. Concentration should be checked on daily
basis or prior to examinations applicable.
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Insp. Medium (NON-FLUORESCENT)
Examination shall not be performed (a) by dry particles if the
surface
temperature of the part exceeds 600 F (315 C) &
(b) by wet particles if the temperature of the wet particle
suspension & the surface of the part exceed 135 F (57 C).
Dry or wet magnetic particles shall be used as the
examination medium.
The material shall have a high
permeability and low retentivity and be of suitable size and
shape to produce magnetic indication of the defects. It shall be
of a colour that will provide adequate contrast with the
background surface being tested. Concentration of wet medium
as measured in pear shaped centrifuge shall be 1.2 to 2.4 ml. in
100ml bath sample. Concentration should be checked on daily
basis or prior to examinations applicable
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Magnetic medium to be used
Colour contrast consumables (Dry) :
Ferrochem make 246 Red
Magnaflux make 3A Black, 8A Red
P-MET make MP-71 Black, MP-71 Red.
Colour contrast consumables (Wet) :
Ferrochem make Magnalux Red 146-A
(Kerosene as medium)
Magnaflux make 29-I Red (Water as medium)
P-MET make ML-72 Red (Water as medium)
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Method Of Examination
Examination
shall
be
performed
by
continuous method, that is; current /
magnetizing field shall remain on while the
magnetic media is being applied and while
excess medium is being removed
• Wet medium shall be applied to the surface
by spraying, flowing or pouring to provide
abundant source of magnetic particles.
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Evaluation Of Indication
ILLUMINATION REQUIREMENT:
•On the surface of the part being examined, a
minimum light intensity of 100 fc (1000 lux) is
required to ensure adequate sensitivity during
examination and evaluation of indication
• A hand lamp with minimum 150 W bulb at
maximum 12 inch distance, is considered
adequate to provide above level of illumination.
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Black Light
•·
Fluorescent penetrant examination shall be
performed un in a darkened area.
• · The examiner shall be in the darkened area for
at least 1 minutes prior to performing examination to
enable his eyes to adopt to dark viewing. If the
examiner wears glasses or lenses, they shall not be
photosensitive.
• · Black light shall be allowed to warm for minimum
5 minutes prior to use.
• Black light intensity as measured with a black light
meter at least 1000 micro watt/sq. cm on the surface
of part being examined shall be required.
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Definition Of indication
An indication is the evidence of mechanical
imperfection.
Only
indications
with
major
dimensions greater than 1/16 inch (1.6 mm) shall be
consider relevant.
(a)
A linear indication is one having a length
greater
than three times the width.
(b)
A rounded indication is one of circular or
elliptical shape with the length equal to or
less than three time the width.
questionable or doubtful indications shall be
reexamined to determined whether or they are
relevant.
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Interpretation And Acceptance
•Relevant linear indications
•Relevant rounded indications greater than 3/16 in.
(4.8 mm)
•Four or more relevant rounded indications in a line
separated by 1/16 in. (1.6 mm) or less (edge to edge)
•An indication of an imperfection may be larger than
the imperfection that causes it; however, the size of
the indication is the basis for acceptance evaluation
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Demagnetization
Demagnetization shall be carried out
required by referencing code/ specification. In
demagnetization shall not be applicable.
only if
general
POST CLEANING:
On completion of testing and interpretation, the part
shall be cleaned in accordance with the applicable
specification.
SAFETY:
Care shall be taken to ensure that cables and
connections etc. are not loose and that the prods
are not getting overheated. Due care should be taken in
handling kerosene suspension to avoid risk of fire.
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Procedures / Acceptance
ASME Section V :
Procedure and Methods of Testing.
ASME Section VIII Div. 1 & Div. 2 :
Acceptance Criteria for Pressure
Vessels.
ASME B 31.3 :
Process Piping.
ASNT – TC – 1A :
Recommended Practice, Personnel
Qualification & Training
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Practical Examples
Indications of Cracks
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Practical Examples
Indication of a crack in a saw blade
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Practical Examples
Indication of cracks running between attachment
holes in a hinge
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Practical Examples
Indication of cracks originating at a fastener hole
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Practical Examples
Magnetic particle wet fluorescent indication of a
cracks in a drive shaft
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Practical Examples
Magnetic particle wet fluorescent indication of a
crack in a bearing
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Merits of MT

Large surface areas of complex parts can be
inspected rapidly

Can detect surface and subsurface flaws

Surface preparation is less critical than it is in
penetrant inspection

Magnetic particle indications are produced

directly on the surface of the part and form an
image of the discontinuity

Equipment costs are relatively low
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Demerits of MT
Only ferromagnetic materials can be inspected
Proper alignment of magnetic field and defect is
critical
Large currents may be needed for very large
parts
Requires relatively smooth surface
Paint or other nonmagnetic coverings adversely
affect sensitivity
Demagnetization and post cleaning is usually
necessary
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References
As attached handouts.
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