This ppt is useful for Non destructive testing faculty

1. Magnetic particle inspection
Unit-1
K.KRISHNA KISHORE
kakarlakishore11@gmail.com

2. • Principle
• magnetization methods continuous and
residual methods
• sensitivities
• demagnetization
• magnetic particles
• applications advantages
• and limitations

3. What Can Be Inspected
• Magnetic particle inspection can detect
– production discontinuities
• seams, laps, grinding cracks and quenching cracks
&
– in-service damage
• fatigue and overload cracks

4. Introduction to Magnetism
• Magnetism is the ability of matter to attract other matter to itself.
• Objects that possess the property of magnetism are said to be magnetic
or magnetized and magnetic lines of force can be found in and around
the objects.
• A magnetic pole is a point where the a magnetic line of force exits or
enters a material.
Magnetic lines of force
around a bar magnet
Opposite poles attracting Similar poles repelling

5. Ferromagnetic Materials
• A material is considered ferromagnetic if it can be magnetized.
Materials with a significant Iron, nickel or cobalt content are
generally ferromagnetic.
• Ferromagnetic materials are made up of many regions in which
the magnetic fields of atoms are aligned. These regions are
call magnetic domains.
• Magnetic domains point randomly in demagnetized material,
but can be aligned using electrical current or an external
magnetic field to magnetize the material.
Demagnetized
Magnetized

6. How Does Magnetic Particle
Inspection Work?
• A ferromagnetic test specimen is magnetized with a strong magnetic
field created by a magnet or special equipment.
• If the specimen has a discontinuity, the discontinuity will interrupt the
magnetic field flowing through the specimen and a leakage field will
occur.

7. • Finely milled iron particles coated with a dye pigment are applied to the
test specimen.
• These particles are attracted to leakage fields and will cluster to form an
indication directly over the discontinuity.
• This indication can be visually detected under proper lighting conditions.

9. Magnetization Methods
• 1. Magnetic fields are induced by passing a heavy current through the
component
– Direct electrical contact at each end of a component, so that current passes through the
whole part
– Is a rapid and reliable method which is very suitable for the inspection of relatively small
components.
(a) current passed through complete part, inducing circular magnetisation;
b) part placed within coil, inducing longitudinal magnetisation ;

11. • Circular magnetisation is produced over the whole length of the work piece
and a good sensitivity can be achieved.
• Small to medium size components in which one dimension, namely length,
predominates can be readily magnetised in a longitudinal direction by
placing within a coil
• This technique is particularly useful for the location of transverse flaws in
such items as axles, crankshafts and camshafts.
• Large castings or forgings can be magnetised in the longitudinal direction by
winding a flexible cable around them.
• It may be necessary to repeat the process more than once, placing the
winding in different positions, to obtain complete inspection of a large and
complex part.

12. • 2. Flexible cables with prod contact pieces are widely used for the
inspection of large castings and forgings.
– If the contacts are placed at opposite ends of a large component, the entire piece will
be magnetised and inspection will be completed in a short time.
(c) prod contacts placed on surface of large
castings;

13. • The time taken for this form of inspection will,
– of course, be longer than if the entire component is magnetised at
once
– but the electrical power requirement is very much less.
• The two contacts can be placed close to the weld and by this method it is
possible to detect cracks, lack of weld penetration and, in some cases,
inclusions.
• When using prod contacts, care must be taken to ensure good electrical
contact otherwise
– arcing may occur between the prod and the work piece surface,
– resulting in over-heating and burn damage.

14. • 3. A component with a continuous hole through it can be magnetised by
energising a straight conducting cable passing through the hole
• This inspection technique is often used in the examination of parts such
as pipe connectors, hollow cylinders, gear wheels and large nuts.
(d) hollow section magnetised by threading a conducting cable through it;

15. (e) part magnetised within magnetic yoke.
• 4. Electro-magnetic yoke
• This technique is suitable for the examination of a variety of
shapes.

16. • forging laps and other surface defects in crane hooks.
• Good sensitivity can be achieved but it is important that the yoke be
positioned correctly in relation to the orientation of the anticipated flaws.

17. Continuous and residual methods
• Pure iron and low carbon steels in the annealed or normalised conditions
are magnetically 'soft' and are of low *coercivity and possess low
**remanence.
• * the resistance of a magnetic material to changes in magnetization
• ** the magnetization left behind in a ferromagnetic material (such as iron)
after an external magnetic field is removed.
• many alloy steels and hardened steels are magnetically 'hard‘
• They are more difficult to magnetise but possess high remanence

18. • CONTINUOUS METHOD
• Soft materials of low remanence must be tested using the continuous
method
– dry or wet magnetic particles used to indicate the presence of discontinuities
– applied to the component while the magnetising current is flowing through the
component or magnetising coil
• The current may be continuous
• The continuous method is very sensitive and will give indications of very
fine defects.

19. • RESIDUAL METHOD
• When a material possesses a high remanence the component may be
magnetised
• the field removed and then the magnetic particles applied and inspection
carried out
• Advatage
– inspection may be made away from the magnetising equipment
– The sensitivities possible in the residual method are generally less
than those of the continuous method.

20. Sensitivities
• orientation of the discontinuity with respect to the induced magnetic field
• sensitivity will be greater when the flaw lies at right angles to the field
– Size
– Shape
– general characteristics of the magnetic particles
– nature of the carrying fluid for these particles
– strength of the magnetic field
– detect cracks with a width of as little as 10-3 mm
– below the surface-3 to 7 mm when magnetisation by DC current
– below the surface-1 mm when magnetisation by AC current

21. Demagnetization
• Parts inspected by the magnetic particle method may sometimes have
an objectionable residual magnetic field that may interfere with
subsequent manufacturing operations or service of the component.
• Possible reasons for demagnetization include:
– May interfere with welding and/or machining operations
– Can effect gauges that are sensitive to magnetic fields if placed
in close proximity.
– Abrasive particles may adhere to components surface and cause
and increase in wear to engines components, gears, bearings
etc.

22. • If a magnetised part is machined
– chips could adhere to the surface being machined
– adversely affect
• the surface finish
• dimensions and tool life
• during any subsequent electric arc welding Operations
– strong residual magnetic fields could deflect the arc
from its point of application.

23. • Demagnetization requires that the residual
magnetic field is reversed and reduced by the
inspector.
• This process will scramble the magnetic domains
and reduce the strength of the residual field to an
acceptable level.
Magnetized
Demagnetized

24. Basic Procedure
Basic steps involved:
1.Component pre-cleaning
2.Introduction of magnetic field
3.Application of magnetic media
4.Interpretation of magnetic particle indications

25. Step-1-Component pre-
cleaning
• When inspecting a test part with the magnetic particle
method it is essential for the particles to have an unimpeded
path for migration to both strong and weak leakage fields
alike.
• The part’s surface should be clean and dry before inspection.
• Contaminants such as oil, grease, or scale may not only
prevent particles from being attracted to leakage fields, they
may also interfere with interpretation of indications.

26. Step-2 Introduction of the
Magnetic Field
• The required magnetic field can be introduced into a component in a
number of different ways.
1.Using a permanent magnet or an
electromagnet that contacts the
test piece

27. • Flowing an electrical current through
the specimen

28. • Flowing an electrical current through a coil of wire around
the part or through a central conductor running near the
part.

29. Direction of the Magnetic Field
• Two general types of magnetic fields (longitudinal and
circular) may be established within the specimen.

30. Importance of Magnetic Field
Direction
• Since defects may occur in various and unknown directions,
each part is normally magnetized in two directions at right
angles to each other.
Flux Leakage
No Flux Leakage

31. Producing a Longitudinal
Magnetic Field Using a Coil
• A longitudinal magnetic field is usually established by placing the part
near the inside or a coil’s annulus. This produces magnetic lines of force
that are parallel to the long axis of the test part.

32. Producing a Longitudinal Field Using Permanent or
Electromagnetic Magnets
• Permanent magnets and electromagnetic yokes are also often used to
produce a longitudinal magnetic field.
• The magnetic lines of force run from one pole to the other, and the
poles are positioned such that any flaws present run normal to these
lines of force.

33. Circular Magnetic Fields
• Circular magnetic fields are produced by passing current through the
part or by placing the part in a strong circular magnet field.
Magnetic Field
Electric
Current

34. • A headshot on a wet horizontal test unit and the use of prods are
several common methods of injecting current in a part to produce a
circular magnetic field.

35. • Placing parts on a central conductors carrying high current is another
way to produce the field.

36. Step-3- Application of Magnetic Media
• MPI can be performed using either dry particles, or particles
suspended in a liquid.

37. Magnetic particles
• The magnetic particles which are used for inspection may be made
from any
– ferromagnetic material of low remanence
– they are usually finely divided powders of either metal oxides
– or metals.
• Dry &wet
• Dry
– The normal carrier for dry particles is air
– the partic1e c10ud is produced using a mechanical powder blower, or a rubber spray
bulb
• powder should not be blown under pressure directly at the component
surface
• component surface be free from grease and other adhering
• deposits

38. • Dry Method (Magnetite, Fe3O4 )
• the particles are lightly dusted on to the surface.
• The dry method is more portable.
• Magnetic particles come in a variety of colors.
• A color that produces a high level of contrast against the
background should be used.

40. • Wet Method
• the part is flooded with a solution carrying the particles.
• The wet method is generally more sensitive since the liquid
carrier gives the magnetic particles additional mobility.
• Wet particles are normally employed in stationary equipment
• The liquid carrier is usually a light petroleum distillate such as kerosene,
but it may be water

41. • Wet particles are typically supplied as visible or fluorescent.
• Visible particles are viewed under normal white light and
fluorescent particles are viewed under black light.
• The particles are normally available in black and red pigments,
or as blue-green or yellow-green fluorescent powder.

42. Step-4-Interpretation of magnetic particle indications
• After applying the magnetic field, indications that form must
interpreted.

43. Crane Hook with
Service Induced Crack
Fluorescent, Wet Particle Method

44. Gear with
Service Induced Crack
Fluorescent, Wet Particle Method

45. Drive Shaft with
Heat Treatment Induced Cracks
Fluorescent, Wet Particle Method

46. Splined Shaft with
Service Induced Cracks
Fluorescent, Wet Particle Method

47. Threaded Shaft with
Service Induced Crack
Fluorescent, Wet Particle Method

48. Large Bolt with
Service Induced Crack
Fluorescent, Wet Particle Method

49. Crank Shaft with
Service Induced Crack Near Lube Hole
Fluorescent, Wet Particle Method

50. Lack of Fusion in Shielded metal arc welding
(SMAW) Weld
Indication
Visible, Dry Powder Method
Fusion-the process or result of joining two or more things together to form a single entity.

51. Toe Crack in SMAW Weld
Visible, Dry Powder Method

52. Throat and Toe Cracks in
Partially Ground Weld
Visible, Dry Powder Method

53. Applications of
Magnetic Particle Inspection
• industrial uses
– in-process inspection
– final inspection
– receiving inspection
– maintenance and overhaul
• inspected for cracks. Crankshafts, frames, flywheels,
crane hooks, shafts, steam turbine blades and fasteners

54. An internal combustion engine crankshaft in position in a magnetic test unit
The shaft is clamped between electrical contacts, which when energised will cause
circular magnetisation.

55. installation for the magnetic particle inspection of pipe couplings
The horizontal rod visible in the illustration is a conductor
The section of pipe will be positioned symmetrically about the conductor before
a magnetising current is passed through the rod.

56. A small adjustable magnetic yoke
This is suitable for use with a wide
variety of components.
The adjustable articulated legs of the
yoke will open up to a maximum
separation of 250 mm.

57. Advantages of
Magnetic Particle Inspection
• Can detect both surface and near sub-surface defects.
• Can inspect parts with irregular shapes easily.
• Pre cleaning of components is not as critical as it is for some other
inspection methods.
• Most contaminants within a flaw will not hinder flaw detectability.
• 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.

58. • Loading, conveying, magnetisation, manipulation and demagnetisation
can all be fully automated
• Automated inspection is used for
– ball and roller bearings, bearing races
– and rings,
– small castings and forgings,
– couplings,
– crankshafts
– and steel-mill billets.

59. Limitations of
Magnetic Particle Inspection
• 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 0.6” (under ideal conditions).
• Post cleaning, and post demagnetization is often necessary.
• Alignment between magnetic flux and defect is important.