mt level 2 course notes

7/29/2019 MT Level 2 Course Notes

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WELCOME TO MPT

Lesson



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Magnetic Particle Testing

Level II-SNT TC 1A (ASNT)



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GeneralMagnetic Particle Testing is relatively simple testmethod that can be applied to finished articles ,billets, rolled bars, castings forging. It can beused to check that the processing operationssuch as heat treat, Machining and grinding didnot uncover or cause discontinuities . Magneticparticle testing consists of Magnetization of thearticle , application of the particles, and

interpretation of the patterns formed by theparticles as they are attracted by magneticleakage fields. 1



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Theory of Magnetism An object is

Magnetized when

part or all of itsmolecules have theirnorth and southpoles oriented.



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Magnetic Poles

A Material that has its north and south polesoriented is called a Magnet

Magnets may be permanent - retaining their

magnetism more. Or Temporary - retainingtheir magnetism only as long as amagnetizing force is applied.

Each Magnet has at least two opposite poles

that are attracted by the earth’s magneticpoles; hence the poles are respectively calledthe north and south poles.



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Magnetic Materials Some materials are attracted by Magnet and

some are repelled.

Materials attracted by Magnetism are calledParamagnetic:

Materials repelled by Magnetism are calledDia Magnetic

Ferro Magnetic is a sub classification of Paramagnetic which are strongly attracted byMagnetism.


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Ferromagnetism Ferromagnetic materials are strongly attracted

by magnetic fields. These materials can bemagnetized and thus tested by MPI.Ferromagnetic can be explained using theidea of domain. Domains can be considered tobe minute internal magnets, each perhapscomprising 10^15 to 10^20 atoms. In

ferromagnetic atoms, the configurationdictates that more electron spin one way thanthe other. Simply a very small internal magnethaving a north and south pole.



Permanent Magnetism When the external

Magnetizing force isremoved from a

ferromagnetic material thedomains will remain in apartial alignment dependenton a number of factors, suchas Alloying elements, carboncontents Heat treatment,temperature.Strong barmagnets are made if alloyedwith aluminum ,nickel andcobalt.



Electromagnetism When a current flows

through a conductor, amagnetic field is set uparound the conductor ina direction at 90 to theelectric current. This isexplained by the right

hand rule.

7



Magnetic Hysterisis When a ferromagnetic

material is influenced byan AC magnetic forcethe variation of Magnetic flux density isrelated to aphenomenon called

Magnetic Hysteresis.



Magnetic Hysterisis On the curve aside

0-a is called virgin curve

Point a is the magneticsaturation

0-b is the point onResidual magnetism

b-c is the coercivity 0-c is the coercive force



Definitions Flux Density:

The number of

magnetic flux lines perunit area

Symbol= B

SI Unit = tesla = T

Previous unit is gauss 1 tesla = 10^4 gauss

Magnetizing force:

The total force

tending to set up amagnetic flux in amagnetic circuit.

Symbol = H

SI unit = ampereper meter = Am^-1



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Definitions Permeability:

The ease with which

a magnetic field orflux can be set up ina magnetic circuit.

Symbol= (mu)

= B/H

Where,

is the absolute

magneticpermeability inHenry/meter

B is the flux density

H is the magnetizingforce.



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Definitions Saturation:

The stage at which

any increase in themagnetising force Happlied to aspecimen, produces

no significant gain influx density B

Coercive force:

The reverse

magnetizing forcerequired to removeresidual magnetismfrom a material.



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Definitions Remanence: The

magnetic flux

density remaining ina material after themagnetising forcehas been removed.

Residual Magneticfield: The magnetic

field remaining in amaterial after themagnetising forcehas been reduced to

zero.



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Definitions Reluctance: A measure

of the degree of difficulty with which a

component can bemagnetised that isanalogous to resistancein a electric circuit. It is

reciprocal of permeability.

Retentivity: Themagnetic flux

density remaining ina material after themagnetising forcehas been removed,

synonymous toremanence.



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Flux Leakage A flux leakage is a

break or adiscontinuity in amagnetic circuit.

Any abrupt changeof permeabilitywithin a magnetic

specimen willchange the numberof flux lines.



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Flux Leakage



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Flux Leakage



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Flux Leakage



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Types of magnetization fields Basically there are two types of

magnetization fields

1. Longitudinal field

2. Circular field



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Longitudinal field Longitudinal field on

a linear motion

e.g are Yokes, Coil, Threaded bar



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Longitudinal Field



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Circular field Circular field runs on

the circumferences

of the component,No poles exist.Examples are Prod,Head shot, central

conductor.



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Circular Magnetization



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Equipment The equipment used for magnetic particle

inspection can be divided up according to sizeand purpose. The magnetizing force may besupplied by anything from a small permanentmagnet to a highly sophisticated fixedinstallation.

When electricity is introduced it is usuallytransformed into a low voltage, highamperage supply. There is no electricaldanger but the specimens get hot due toelectrical resistance.



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Equipment flow chart

Flexible cable&Closed loop

Coils

Prods

Electo Magnet

Permanent magnet

Portable Mobile

Current flow

Induced current

Rigid coil

Magnetic flow

Threading bar

Fixed Bench Units

Stationary

Types of MPI Equipments



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Permanent magnet Permanent magnets

produce a longitudinalmagnetic field between

the poles. Modernhorseshoe type withadjustable arms areavailable

Optimum defect detectabilityis at 90degrees to the poles



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Electro Magnets Electromagnets are

made from soft iron

laminates to reduceeddy current losses,if powered by ACfields.

The legs of theequipment arearticulated.



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Electro MagnetsThis is basically a piece of soft ferromagnetic

steel that has an electrical coil, with a large

number of wraps, around the handle area. This

type of magnet generates a very strongmagnetic field in a local area where the poles of

magnet touch the part to be inspected. Someyokes are so strong that they can lift weights inexcess of 40 pounds.



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Portable Yoke- AC/DC



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Electro Magnet



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Complete Set of Portable Unit



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Prods Prods induce a

circular magneticfield by sending ahigh amperagecurrent through testpiece

The high Amps cancause arcing on thetest piece surface

Typical Amps:1000A



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Prods Let us see how the Prod works to

Identify flaws

Click

http://www.cnde.iastate.edu/ncce/MT_CC/Principles.html

http://www.cnde.iastate.edu/ncce/MT_CC/Principles.html



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Prod Set Up



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Flexible coil In this technique the

current carrying

cable is woundtightly around thecomponent.

It is a longitudinal

magnetization andfind defects lyingparallel to the cable



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Mobile Equipment As the name implies, mobile equipment is too

bulky and heavy to carry and yet needs to be

moved to the work. Some mobile units arecapable of supplying output currents up to20,000A, although 5000A is a more normalfigure.

In addition to the normal portable unit, themobile is likely to have better current controland a step control to allowing demagnetizing.



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Bench Units Bench Units are

fixed installations

used to test largenumbers of manufacturedspecimens. They

range in size andoutput.



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Stationary Bench Unit



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Coil Method



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Coil - Photo



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Current flow The component is fixed

firmly between contactheads which have a soft

conductive surface,such as copperbraiding. A low voltagehigh current is passed

through the componentand create circularmagnetic field.



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Threading Bar Magnetization by

the threading bar

technique is inducedby passing currentthrough an insulatednon-magnetic

conductor (Al, Cu orBrass)



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Threading Bar or Central

Conductor



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Magnetic Flow Energized solenoids in

the bench heads createa longitudinal magnetic

field in a component,which is clampedbetween the heads,completing themagnetic circle.Defect

where the major axis lietransverse to a line joining the heads, arefound best.



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Head Shot for Crank Shaft



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Rigid Coil The component is

placed in a currentcarrying rigid coil with

its longitudinal axis at90 to the direction of the windings on thecoil. 4 to 8 turns are

usual the specimen isplaced in the bottom of the coil whereverpossible.



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Various magnetization

currents Types;

Alternating current

Half wave directcurrent

single-Phase, fullwave direct current

Three phase FWDC

Direct Current (DC)



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AC Current As a Magnetization source, AC has three main advantages

1. The current reversal causes a phenomenon calledthe skin effect , where an inductive effect

concentrates the magnetic field at the surface of thetest item

2. The residual field can be removed easily bydemagnetization.

3. The pulsating effect of the current reversalagitates the magnetic particles applied to the surface



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Half wave direct current The HWDC does not produce skin effect

Its main advantage is it can locate deeper

lying discontinuities than AC generated fields.HWDC also experiences as a pulsating effectthat agitates the magnetic particles applied tothe surface.

Refer sketch b for the wave pattern



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Direct Current(DC) The primary advantage of using DC

magnetization for MT is because of its ability

to locate subsurface discontinuities. Theuniform distribution of current across theentire cross section of test part allows DC todetect subsurface anomalies better.



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Difference Between AC & DCThe main difference between these two types of current is that

alternating current flows back and forth in a circuit and direct

current flow in only one direction. A very important difference

between the two types of magnetic fields created by each type of

current has to do with how far each field will penetrate in the

component being inspected. When alternating current is used, the

magnetic field does not penetrate very far below the surface of the

material. Therefore, alternating current is primarily used to detectsurface defects. Magnetic fields created with direct currentpenetrate more deeply into the part and can detect defects underthe surface of the part (generally up to 1/4").



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Relative Depth of Discontinuity

A m p e

r e s C ur r e n t

1000

200

400

600

800

2 4 6 12



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Flux Indicators Field strength meters based on the hall effect

are the best way of ascertaining adequatefield strength at the surface of the test

component. However they are expensive andthe probes used tend to be fragile. Portableflux indicators are an acceptable alternative,They are simple to use

a. segment type b. foil type e.g segment- USA/Berthold flux indicator

Foil type - Burmah castrol flux indicator



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Typical design of fluxIndicator(Segment)

4 or 8 identicalsegments of ferrousmetal are joined with

non-magneticcompound of eventhickness into the shapeof a flat disc. Onesurface of the disc is

covered with non-magnetic particlesgetting to the surfaceand prevent themgiving misleading

indications.



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Foil Type

These Indicatorsconsist of a

magnetic foilcontaining slots tosimulatediscontinuities,

sandwiched betweennonmagnetic foils.



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Powders

Powders are finelyground ferromagneticparticles, often iron,

coated or heated to atemperature which willgive a distinctive color.Most standards say the

particles size shall beless than 200

The particles shallhave a globularcenter and sharp

edges. Typical colors are

Black

Red Grey

Yellow



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Particles

The particles that are used for magnetic particle inspection are

a key ingredient as they form the indications that alert the

inspector to defects. Particles start out as tiny milled (a

machining process) pieces of metal that can vary in size from

0.000005 inch to 0.0025 inch. After these particles are milled,

a pigment (somewhat like paint) is bonded to their surfaces to

give the particles color. Particles may come in a dry form or ina wet suspension such as water or oil. Colors that dry particles

can typically be purchased in are red, black gray, yellow etc.



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Dry powders

Dry powders are dispersed on to the testcomponent either through a puffer or a dry

spray can. The chosen color is the one whichgives the best contrast against the specimenbackground.

Powders are usually applicable to site work

such as welds and castings, often as an initialcheck on a weld root pass, where wetmaterials would cause contamination



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Dry Powders



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Magnetic Inks

Magnetic Inks aresubdivided into

1. Contrast andfluorescent

2. Kerosene andwater based

Magnetic Ink concentration %

Non fluoro- 1.2 -2.4%

Fluorescent-0.1 -0.7%



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Evaluation

The Evaluation of an MPI testing ispurely based on Operators skill such as

vision, experience etc. Basically the Indications are classified

into

False Indications

True Indications

Relevant Indications



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Evaluation

What is False Indication?

Powders stick due to gravity

Excessive Magnetization

Variations of Permeability



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Evaluation

True Indication

The Indications which are seen due to

the leakage flux from a artificial flaw ordiscontinuities is True Indication

Relevant Indication are discontinuities

which you as an operator looking for!



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Evaluation of Weld Ments



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Indications from forging



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Fluorescent MPI in casting

Fl t MPI i C



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Fluorescent MPI in CraneHook



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Measuring the Indication



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Test Procedure

Generally the Test procedure is basedon a Published standard such as

BSEN1290:1998 (BS6072:1981) Let us discuss, Items Present in a

Procedure,

How to Read Or Use a procedureManual.



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Calibration

Yokes – using test block ( Before use)

Light Meters- Monthly/Ref-Yearly

Inspection Tools, rulers, Magnifiers Ink – Bath Concentration (

representative sample from a batch)

Sensitivity of Ink – Using Standardblock

Lifting Blocks – 2years



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Interpretation with Standards

BS 5135:1984

BSEN25817 (BS EN1291:1998)

Linear Indications , Rounded Indication.



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Examination Details

General : 40 Questions

Specific : 20 Questions

Practical : Prepare Technique Sheet

Using a Procedure & Perform test & Prepare test Report – 2 Samples



END of MPI Level II

Good Luck for your Exams

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