mpt seminar

7/28/2019 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




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*Drive Shaft with

Heat Treatment Induced

Cracks



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*Splined Shaft with

Service Induced Cracks



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*Threaded Shaft with




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*Large Bolt with




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*Crank Shaft with Service

Induced Crack Near Lube Hole



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*Lack of Fusion in SMAW

Weld

Visible, Dry Powder Method

Indication


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*Toe Crack in SMAW Weld



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*Throat and Toe Cracks in

Partially Ground Weld



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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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