Mpt Seminar

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    -VINEETH V PILLAI

    SR QC INSPECTOR

    *

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    *Magnetic Particle Inspection

    (MPI)*Fast and relatively easy to apply and surfacepreparation is not as critical as for some otherNDT methods.*

    One of the most widely utilized NDT methods,commonly referred to as Magnaflux testing.*MPI uses magnetic fields and small magneticparticles, such as iron filings to detect flaws incomponents.

    *The only requirement is that the componentbeing inspected must be made of aferromagnetic material such as iron, nickel,cobalt, or some of their alloys.

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    Basic PrinciplesMagnetic Particle Inspection (MPI) is a relatively simple

    concept. It can be considered as a combination of two NDT

    methods:

    Magnetic flux leakage testing

    Visual testing

    Consider a bar magnet. It has a magnetic field in and

    around the magnet. Any place that a magnetic line of forceexits or enters the magnet is called a pole. A pole where a

    magnetic line of force exits the magnet is called a north pole

    and a pole where a line of force enters the magnet is called a

    south pole.

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    Types of Magnetic Materials

    Diamagnetic metals: Very weak and negative susceptibility tomagnetic fields. Diamagnetic materials are slightly repelled by a

    magnetic field and the material does not retain the magnetic

    properties when the external field is removed.

    Paramagnetic metals: Small and positive susceptibility tomagnetic fields. These materials are slightly attracted by a

    magnetic field and the material does not retain the magnetic

    properties when the external field is removed.

    Ferromagnetic materials: Large and positive susceptibility toan external magnetic field. They exhibit a strong attraction to

    magnetic fields and are able to retain their magnetic properties

    after the external field has been removed.

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    FERROMAGNETIC MATERIALS

    Ferromagnetic material becomes magnetized when the magnetic

    domains within the material are aligned.

    This can be done by placing the material in a strong external

    magnetic field or by passing electric current through the material.

    The more domains that are aligned, the stronger the magnetic field

    in the material. When all of the domains are aligned, the material issaid to be magnetically saturated.

    When a material is magnetically saturated, no additional amount of

    external magnetization force will cause an increase in its internal level

    of magnetization.

    Unmagnetized material Magnetized material

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    Properties of Magnetic Lines of

    ForceFollow the path of least resistancebetween opposite magnetic poles.

    Never cross one another.

    All have the same strength.

    Their density decreases (they spread

    out) when they move from an area of

    higher permeability to an area of lower

    permeability.

    Their density decreases withincreasing distance from the poles.

    Flow from the south pole to the

    north pole within the material and

    north pole to south pole in air.

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    When a bar magnet is broken in the center of its length, it willresult in two complete bar magnets with magnetic poles on each end of

    each piece.

    If the magnet is just cracked but not broken completely in two, a north

    and south pole will form at each edge of the crack.

    The magnetic field spreads out when

    it encounter the small air gap created by

    the crack because the air can not

    support as much magnetic field per unitvolume as the magnet can.

    When the field spreads out, it appears

    to leak out of the material and, it is

    called aflux leakage field.

    *How It Works!!

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    If iron particles are sprinkled on a cracked magnet, the

    particles will be attracted to and cluster not only at the

    poles at the ends of the magnet but also at the poles at the

    edges of the crack.

    This cluster of particles is much easier to see than the

    actual crack and this is the basis for magnetic particle

    inspection.

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    Cracks just below the

    surface can also berevealed.

    The magnetic particles form

    a ridge many times widerthan the crack itself, thus

    making the otherwise

    invisible crack visible.

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    The effectiveness of MPIdepends strongly on the

    orientation of the crack

    related to the flux lines.

    MPI is not sensitive to shallow

    and smooth surface defects.

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    *Testing Procedure of MPI

    Cleaning

    Magnetizing the object

    Addition of Magnetic particles

    Interpretation

    Demagnetization

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    *Steps of examination

    Preclean

    inspection area.

    Spray on Cleaner.Wipe off with

    cloth.

    Place Yoke on test

    piece

    perpendicular

    to direction of

    suspected cracks.

    Energize Yoke.

    Magnetic field

    will form in testpiece.

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    *Steps of examination

    Apply magnetic

    powder orprepared

    bath while Yoke is

    energized.

    Indications will

    form

    immediately.

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    *Surface condition

    Prior to examination, surface to be examined & alladjacent area within at list 1 inch(25mm) shell be dry &

    free of all dirt, grease, scale, welding flux & spatter, oil

    etc

    Cleaning prior to examination

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    *Pre-cleaning

    When inspecting a test part with the magneticparticle method it is essential for the particles tohave an unimpeded path for migration to both strongand weak leakage fields alike. The parts surface

    should be clean and dry before inspection.Contaminants such as oil,grease, or scale may notonly prevent particles frombeing attracted to leakage

    fields, they may alsointerfere with interpretationof indications.

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    *Importance of Magnetic

    Field DirectionBeing able to magnetize the part in two directionsis important because the best detection of defectsoccurs when the lines of magnetic force areestablished at right angles to the longest dimensionof the defect. This orientation creates the largest

    disruption of the magnetic field within the partand the greatest flux leakage at the surface of thepart. An orientation of 45 to 90 degrees betweenthe magnetic field and the defect is necessary toform an indication.

    Since defects may occur invarious and unknowndirections, each part isnormally magnetized in twodirections at right angles toeach other.

    Flux Leakage

    No Flux Leakage

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    *Techniques of magnetization

    Magnetization

    using coil

    CircularMagnetization

    Magnetizatingusing P rod

    Magnetizationusing Yoke

    MPI

    The required magnetic field can be introduced into

    component in a number of different ways, as given

    below:

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    *Magnetic Field Using a Coil

    When electric current is passedthrough a coil, the current flowsin circular direction in the coil

    which creates a longitudinalmagnetic field along the axis ofthe part which can detecttransverse (circumferential)discontinuities in the part.

    Coil on Wet Horizontal Inspection Unit

    Portable Coil

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    *Circular Magnetic Fields

    When current is passed directly through the axis of the compound,

    the current flows in longitudinal direction which creates a circular

    magnetization in the test part, which can detect longitudinal

    discontinuities.

    The field strength varies from zero at the center of the component

    to a maximum at the surface.

    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 thematerial.

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    Magnetic Field

    Electric

    Current

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    *Magnetization using prod

    *Prods creates circularmagnetization in testpart.

    *Discontinuities whichare in line with prod willbe detected.

    *Prods have arcing or

    burnout effect in thetest part which mayresult in cracking.

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    *Magnetization using Prod

    LOCALIZED AREA MAGNETIZATION USING PROD

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    *Magnetization using prod

    LOCALIZED AREA MAGNETIZATION USING PROD

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    *Using Yoke

    Permanent magnets andelectromagnetic yokes arealso often used to produce alongitudinal magnetic field.

    The magnetic lines of forcerun from one pole to theother, and the poles arepositioned such that anyflaws present run normal to

    these lines of force.

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    Application of Magnetic Media

    (Wet Versus Dry)MPI can be performed using eitherdry particles, or particles suspendedin a liquid. With the dry method,the particles are lightly dusted on to

    the surface. With the wet method,the part is flooded with a solutioncarrying the particles.

    The dry method is more portable.The wet method is generally more

    sensitive since the liquid carriergives the magnetic particlesadditional mobility.

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    *Dry Magnetic Particles

    Magnetic particles come in a variety of colors. A

    color that produces a high level of contrast against

    the background should be used.

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    MAGNETIC POWDERS FOR

    DRY & WET APPLICATION

    Available in Dry as well as wet application

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    *Wet Magnetic Particles

    Wet particles are typically supplied as

    visible or fluorescent. Visible particles

    are viewed under normal white lightand fluorescent particles are viewed

    under black light.

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    *Interpretation of Indications

    After applying the magnetic field, indications that

    form must be interpreted. This process requires that

    the inspector distinguish between relevant and non-

    relevant indications.

    The following series of images depict

    relevant indications produced from a

    variety of components inspected withthe magnetic particle method.

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    *Crane Hook with

    Service Induced Crack

    Fluorescent, Wet Particle Method

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    *Gear with

    Service Induced Crack

    Fluorescent, Wet Particle Method

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    *Drive Shaft with

    Heat Treatment Induced

    Cracks

    Fluorescent, Wet Particle Method

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    *Splined Shaft with

    Service Induced Cracks

    Fluorescent, Wet Particle Method

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    *Threaded Shaft with

    Service Induced Crack

    Fluorescent, Wet Particle Method

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    *Large Bolt with

    Service Induced Crack

    Fluorescent, Wet Particle Method

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    *Crank Shaft with Service

    Induced Crack Near Lube Hole

    Fluorescent, Wet Particle Method

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    *Lack of Fusion in SMAW

    Weld

    Visible, Dry Powder Method

    Indication

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    *Toe Crack in SMAW Weld

    Visible, Dry Powder Method

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    *Throat and Toe Cracks in

    Partially Ground Weld

    Visible, Dry Powder Method

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    Indication of a crack in a saw blade Indication of cracks in a weldment

    Before and after inspection pictures ofcracks emanating from a hole Indication of cracks running betweenattachment holes in a hinge

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    Demagnetization

    After conducting a magnetic particle inspection, it is usually

    necessary to demagnetize the component. Permanent magnetic

    fields can:

    Affect machining by causing cuttings to cling to a component.

    Interfere with electronic equipment such as a compass.

    Can create a condition known as "ark blow" in the welding process.

    Arc blow may causes the weld arc to wonder or filler metal to be

    repelled from the weld.

    Cause abrasive particle to cling to bearing or faying surfaces and

    increase wear.

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    *Demagnetization methods

    *Withdrawal from Alternating Current Coil

    *Decreasing Alternating Current

    *Demagnetizing With Yokes

    *Reversing Direct Current

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    *Withdrawal from Alternating

    Current Coil*The fastest and most simple technique is to pass the part

    through a high intensity alternating current coil and then

    slowly withdraw the part from the field of the coil.*A coil of 5,000 to 10,000 ampere turns is recommended.

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    *Decreasing Alternating Current

    *An alternative technique for part

    demagnetization is subjecting the part to thefield while gradually reducing its strength to a

    desired level.

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    *Demagnetizing With Yokes

    *Alternating current yokes may be used for local

    demagnetization by placing the poles on the surface, moving

    them around the area, and slowly withdrawing the yoke while

    it is still energized.

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    *Reversing Direct Current

    *The part to be demagnetized is subjected to consecutive

    steps of reversed and reduced direct current magnetization

    to a desired level.

    *Effective process to demagnetize large parts.

    *This technique requires.

    *Special equipment for reversing the current while

    simultaneously reducing it in small increments.

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    *Pie Field Indicator

    *The magnetic field indicator relies on the slots between

    the pie shaped segments to show the presence and the

    approximate direction of the magnetic field.

    *A suitable field strength is indicated when a clearly

    defined line of magnetic particles forms across the copper

    face of the indicator.

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    *Pie Field Indicator

    Failure to obtain an indication can result from:

    * Insufficient magnetic field, or

    * The magnetic properties of the material being examined

    or both.

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    *Pie

    Field

    Indicator

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    *Calibration

    *Magnetizing equipment shall be calibrated at least

    once a year, or in case of major electric repair or

    damage.

    *If equipment has not been in use for a year or more,

    calibration shell be done prior to first use.

    *Ad t f MPI

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    *Advantages of MPI

    *One of the most dependable and sensitive methods for

    surface defects.

    *Fast, simple and inexpensive.

    *Direct, visible indication on surface.

    *Can be used on painted objects.

    *Surface preparation not required.

    *Results readily documented with photo or tapeimpression.

    *

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    *Limitations of MPI

    *Only good for ferromagnetic materials.

    *Sub-surface defects will not always be indicated.

    *Relative direction between the magnetic field and

    the defect line is important.*Objects must be demagnetized after the

    examination.

    *The current magnetization may cause burn scars on

    the item examined.

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    *