MAGNETIC PARTICLE TESTING
1.0.
INTRODUCTION
The magnetic particle method of non-destructive testing is one of the most
common methods for finding discontinuities in materials. This method is
used for detecting surface and sub-surface discontinuities in Ferro-magnetic
materials. Magnetic particle testing is a relatively easy and simple test
method that can be applied to finished components, billets hot rolled bars,
forgings and castings. It can also be used to check processing operations
such as machining, grinding and heat treatment.
The objective of magnetic particle testing is to ensure product reliability by
providing means of
a. Obtaining a visual of an indication related to a discontinuity in or the
surface of the material.
b. Disclosing the nature of discontinuities without impairing the material.
c. Separating acceptable and unacceptable materials in accordance with
pre-determined standards.
2.0.
PRINICIPLE OF MAGNETIC PARTICLE TESTING:
The basic principle in magnetic particle testing is that when a component is
magnetized, magnetic lines of force or magnetic flux will be created. Any
discontinuity in the material will cause set up of magnetic poles as well as
leakage of magnetic flux.
If some media which are Ferro-magnetic in nature such as iron powder is
dusted over the surface of the components, the powder accumulates at the
region of the leakage flux and the appearance of the powder build up will
resemble the nature of the discontinuity.
Fig: COIL METHOD
YOKE METHOD
HEAD SHOT METHOD
CENTRAL CONDUCTOR METHOD
PROD METHOD
FIG: FORMATION OF LEAKAGE FIELD AT DISCONTINUITY
2.1.
STEPS IN TESTING:
The method involves three essential steps:
a. Magnetising the material or a part under test.
b. Applying the Ferro-magnetic particles over the surface.
c. Examining the surface for powder patterns or indications.
3.0.
METHODS OF MAGNETISATION
Magnetisation of the materials can be carried out principally in two ways
viz. circular magnetization and longitudinal magnetization.
3.1.
CIRCULAR MAGNETISATION:
A circular magnetic field is induced into the component in two ways either
by directly passing the current through the component or indirectly through
a central conductor surrounded by a hollow article.
4.0.
4.1.
MAGNETISING CURRENTS:
Straight direct current (D.C) alternative current (A.C) and half wave
rectified direct current (HWDC) are all used. The following are their
relative merits and demerits.
DIRECT CURRENT (D.C.)
Straight DC is suitable for yokes and solenoids. The penetration power of
DC is more than that of AC. However, DC cannot be stepped up or down
easily.
4.2.
ALTERNATIVE CURRENT (A.C)
The current alternates at specified frequency. This type of current created a
maximum flux at the surface of the magnetized article and has relatively
less penetrating ability. The advantage of using AC is that the voltage can
be stepped up to down. Also the reversal of the magnetizing current cause
magnetic particles more mobile facilitates their collection at leakage fields.
AC is the best suited for locating surface discontinuities.
4.3.
J.W.D.C.
Half-wave Direct Current is produced by rectifying an alternative current
during the positive cycles. The use of rectified current has got the following
advantages.
1. AC at any commercial frequency can be used
2. Penetration is directly comparable to that of DC
3. The pulsating effect of the rectified wave is helpful in adding
mobility to the magnetic particles.
4. There is a definite advantage in locating deep seated
discontinuities.
5.0.
CURRENT REQUIREMENTS:
The required amount of magnetizing current is effected by the permeability
of the metal, the shape and thickness of article and the type of discontinuities sought. However, the following can be used as a ‘Rule of
Thumb’ method:1. Circular magnetization with Head-shot method. 800-1000 C
amps.per inch dia. Or cross section with HWDC 500-600
amps.per inch of dia. or cross section with A.C
2. For circular magnetization with prods:
90-110 amps.per inch of prod-spacing for thickness upto ¾”
100-125 amps.per inch of prod-spacing for thickness more than
¾”.
Prod-spacing can be kept between 4” and 8” for effective testing.
3. Longitudinal magnetization using coils:
AMPS. =
45.000
1
-------- x
--L/D
T
Where L is the Length and D is the dia.of the article and T is the
number of turns in the coil. (Limitations). L not more than 18”
and L/D between 2 & 15.
6.0.
TESTING MEDIUM, (MAGNETIC PARTICLES)
The particles used are in all cases finely divided Ferro-magnetic materials.
The properties of these materials vary over a wide range for different
applications-including magnetic properties, size shape, density, mobility
and visibility and contract. The medium Amy be dry powder with various
colours for better contract or liquid paste either black, red or fluorescent for
use with black light.
1. DRY METHOD: In dry method, the powder is sprinkled over by dosing
by hand bulbs or by mechanical blowers. The medium is applied while
current is on in continuous test method.
2. WET METHOD: In the wet method, the particles are suspended in a
carrier liquid such as kerosene and applied by spraying, brushing, or
sub-merging the article in the bath. The concentration of the particles in
the bath should be such that the indication of discontinuity is clear
without too much overall surface coverage. The recommended
concentration for 100cc of bath for non-fluorescent particles is 1.2 to
2.4. cc. The medium is applied before passing the current in continuous
method.
In general, the following are the requirements of the particles used for
magnetic particle testing:
i.
Non-toxic
ii.
Fine divided
iii.
Ferro magnetic
iv.
Free from contaminants
v.
High permeability
vi.
Low-retentivity
vii. High colour contrast
viii. Correct size range
7.0.
SURFACE PREPARATION:
Usually as welded, as cast, as forged or as formed surfaces are suitable for
magnetic particle testing. However, excess scales, ripples, slag etc., may
interfere with the interpretation of the powder patterns. Also, flakes heavy
built up of paints, rust, grease etc., are to be cleaned, sand blasting wire
brushing, filling, grinding etc are used for removing interfering substances.
8.0.
LOCATION OF DISCONTINUITIES:
It is possible with suitable technique to locate discontinuities on the surface
as well as below surface upto 6mm deep. Discontinuities located on the
surface appear as sharp and distinct lines whereas sub-surface discontinuities appear as irregular, rough and fuzzy indication varies with the
depth of its location below the surface. Correct interpretation of the pattern
revealed by sub-surface discontinuities requires a certain skill on the part of
the operator.
9.0.
DEMAGNETISATION:
Since all Ferro-magnetic materials possess retentively there will be certain
amount of residual magnetism left on the material tested. The amount of
residual magnetism depends o the magnetizing current, its strength and
nature (AC or DC) and the permeability of the material. De-magnetisation
is essential for components to be used in space crafts, aeroplanes, bearings
etc., where pick up of iron particles cause damage to the part. Demagnetisation is done by passing the component through an AC field of
gradually reducing current or by periodically reversing and reducing the
DC current.
10.0. FINAL CLEANING:
When the magnetic particle testing is completed, the components are
cleaned of the magnetic particle. This is accomplished by the use of air,
solvent washes and wiping etc.
11.0. TEST EQUIPMENT:
The equipment used for magnetic particle testing ranges from heavy
complex automated handling system to small light weight portable units.
The following particulars are considered in the selection of equipment:
i.
Wet or Dry method
ii.
AC or DC
iii.
Degree of automation
iv.
De-magnetisation
v.
Amperage required
vi.
Tank Capacity
vii. Air supply
viii. Line voltage requirements
ix.
Accessories needed
12.0. CLASSIFICATION OF DISCONTINUITIES:
The magnetic particle indications observed can be classified into three
categories:a) Dis- continuities (b) Non-relevant indications and (c) False indications
Dis-continuities can be classified as surface dis-continuities and sub-surface
dis-continuities. In magnetic particle testing, surface dis-continuities
produce sharp, distinct, clear cut and tightly held patterns. Typical
examples of this type of defects are hot cracks in welds, heat treatment
cracks, grinding cracks, forging laps, hot tears in castings, seams in bars
etc. These defects are objectionable from the stand point of service of
subsequent processing requirements. Sub-surface dis-continuities produce
indications which are less distinct, forming diffused or fuzzy patterns and
loosely held. Typical examples are lack of fusion in welds, slag line in
welds, laminations in plates etc. Non-relevant indications are caused by
flux leakage due to reasons other than discontinuities such as difference in
the permeability of materials, presence of other magnetic materials in the
surrounding formation of contractions in the objects such as keys and
splines in the shafts.
False indications are those wherein the magnetic particles are held for
reasons other than the flux leakage such as the magnetic particles getting
stored in the rough surfaces.
13.0. APPLICATION:
Magnetic particle Inspection is widely used in process control and in the
maintenance. The root welding of a header butt joint or drum
circumferential seam or similar type joints are magnetic partically tested
after lack of gouging before going for full welding. Magnetic particle
inspection is a widely used method for examining fillet welds. For detecting
any surface cracks after stress relieving magnetic particle inspection is the
best method. All such inspections can be carried-out prod-techniques. Prod
method is one way of circularly magnetising the component by directly
passing the current. Large area can be tested part by part and the direction
of the field can be changed very easily by relocating the prods. Care must
be taken to is see that the prod tips are cleaned properly and current is
switched only after establishing proper contacts; otherwise excessive arcing
will take place and can damage the surfaces. Fillet welds in columns and
beams are another area where magnetic particle inspection can be widely
used. During maintenance operation after shut down of a thermal power
plant again these methods can be put into service quite extensively.