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MAGNETIC PARTICLE TESTING

One of the most effective surface NDTmethods for examination of material

forms in ferro-magnetics To detect surface & sub-surface flaws

Sensitivity is greatest for surfacediscontinuities that are linear andoriented normal to induced flux-lines

Sensitivity decreases for sub-surfacediscontinuities located beneath withincreasing depth

Typical flaws detected: Cracks, Lack of fusion, Incomplete

penetration, Lamination on weldgroove faces

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PRINCIPLE IN BRIEF

Weld area to be examined ismagnetized, establishing magnetic fluxlines on surface & near surface

During magnetization ferromagneticparticles (wet or dry) are applied on theweld and adjacent base metal surface

Discontinuities oriented normal to

magnetic flux lines distort the fieldsinduced, cause flux-leakages on materialsurfaces to attract magnetic particlesand form patterns (indications)

These patterns are characteristic of typeof flaw revealed

Each weld area is examined twice so thatLines of flux during one examination isapproximately perpendicular to the fluxlines of other examination

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First choice When welds areFerro-magnetic

When groove faces need beexamined for laminations

To assess Root conditions inmulti-pass groove welds

When the intention is to find outspherical flaws such as porosity

CHOOSING MAGNETICPARTICLE TEST

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SELECTION OF EQUIPMENT

Magnetization method

Magnetisation current (AC,HWDC or both)

Amperage requirement

Wet or Dry Method

Flaws that are soughtSurface orsubsurface

Part requirementsAccessibility &

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PORTABLE KITS-YOKES

For site locations & parts locatedat heights

When very fine surface flaws aresought to be detected

When No arcing is required onweld/base metal

When Wet medium is required tobe used

When Smooth & fine surfaceconditions exist

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MOBILE EQUIPMENT

PRODS INSPECTION UNITS

For Shop-floor fabrication When Sub-surface flaws are

required to be detected

When Dry powder use is required

When surfaces are Rough (Weldsurfaces)

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EXAMINATION MEDIUM

Both dry & wet Particles used

Particles imparted color to provide

sufficient contrast for increased visibilityagainst background examination surface

DRY PARTICLES

Have high permeability & low retentivity Mixture of spherical & elongated shapes

Particle sizes range 0.060 mm to 0.150mm; 75% of which in the range of 0.080mm 0.100 mm

Used under extreme environmentalconditions;

Heat Resistant & can be used upto 315

C Some particles lose color at high

temperatures & lose contrast Not affected by cold

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WET PARTICLES

Fluorescent &Non-fluorescent

Oxides of iron & have relatively lower

permeability than Dry particles Particle sizes range 0.005 mm to 0.030

mm Suspended in Conditioned water/light

petroleum distillate

When suspended in petroleum basesused upto 50C

Affected by freezing cold conditions

CONCENTATION

Verified with ASTM centrifuge tube

Non-fluorescent suspension: 1.2 to 2.4

mL/100 mL Fluorescent suspension: 0.2 to 0.4

mL/100 mL

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APPLICATION AREAS

Welds in Boiler, Piping & Pressure vessels

Finished Butt & Corner welds & intermediate

stage examination for thick welds to avoidmajor repairs at later time

Branch welds in piping

SURFACE CONDITION

General Surface Condition: As welded issufficient

Excessive ripples & undercut mask indicationsfrom discontinuities lying beneath

Shall be clean, dry & free from organic andinorganic contaminants that interfere withtest

Examination area: 25 mm on each side of weld, HAZ & base

metall

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SURFACE CONDITIONING

Grinding, Machining, grit blasting or abrasivemethods for organic contaminants

Organic solvents for organic contaminants

CONDUCTIVE & NON-CONDUCTIVECOATINGS:

SURFACE CONTRAST ENHANCEMENTcoatings- may be applied tentatively onsurfaces to improve contrast

For non-conductive paintingcoatingthickness not to exceed 50 m

Coating removed at points where contact isto be made by prods

For coating thickness that exceeds 50 m ormaterial forms that have CONDUCTIVE

coatings Procedure verified with cracks introduced in

blocks with such coating & coating thicknessfor detection

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MAGNETISING CURRENT

Rectified current HWDC used for deeppenetration of parts

Current selection primarily depends on prodspacing

For Material thick < 20 mm 90 to 110amps/25 mm prod spacing

For Material thick > 20 mm 110 to 125amps/25 mm prod spacingProd spacing: Min.75 mm; Max: 200 mm

MAGNETISING STRENGTH:

The prods technique introduce circularmagnetic field on the surface around theprods

The field strength developed on part isinfluenced by

Material permeability, Weld size & shape,Magnetization technique.

Field adequacy is verified utilizingGauss/Tesla meters or ASTM pie-gauge fieldindicator

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TECHNIQUES FOR WELD

EXAMINATION

PROD TECHNIQUE:

Prod Technique-Dry

Yoke Technique-wet non fluorescent Prod tips to be kept clean & dressed free

from oxides

Al or Pb or Fe tipped prods used to avoidcu deposits on material

If Cu prods used open-circuit voltageneed be < 24V

Prods pressed against test surfacenormal to it

Angulation of prods results distortedfields

Prods provided with remote contactswitch to make electrical contact afterfirmly positioned

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FIELD STRENGTH & DIRECTION

VERIFICATION of field:

ASTM Pie gauge is positioned on the

examination surface with brazed pie-sections toward the examination surface.

A clearly defined line or lines whenparticles are applied simultaneously withmagnetizing force indicates adequatestrength.

MEASURING FIELD STRENGTH:

Hall-effect tangential probe positioned onthe surface

The peak-value of tangential field is

measured when magnetizing force isapplied

Value of 30 to 60 Gauss (2.4 k to 4.8 kAmps /meter) indicates adequacy of field

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MAGNETISING DIRECTION

The direction of magnetization isverified using ASTM pie-gauge

Pie gauge is positioned so that

brazed pie-sections toward theexamination surface

Cu plate away form surface. A clearly defined line or lines in the

desired direction when particlesreapplied simultaneously withmagnetizing force

fulfills the requirement of fielddirection

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DRY CONTINUOUS

APPLICATION OF DRY PARTICLES :

Flow of magnetizing currentinitiated Cloud of Dry particles floated with

low velocity upto the surface

Application of current terminatedafter the application of powderhas been completed and anyexcess has been blown-off.

Build-up of indication observed as

the particles are being appliedused for examination.

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YOKE TECHNIQUE

To detect discontinuities open to surface

Yokes have TWO adjustable legs

Adjustable legs permit changing thecontact space in between the legs andimproved contact area with surface

Legs of yoke are positioned similar toprods & pressed against surface

The discontinuities anticipated should becentrally located in the area between pole

pieces The effective inspection area

space in between the legs is the area ofexamination

Lateral area of 38 mm on each side ofthe center-line of yoke legs

Depends on part geometry andqualified using Hall-effect gauss meter

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ELECTRO-MAGNETIC YOKES

A single leg used in areas of tight access

Flux lines for single leg yoke flow

radially producing radial magnetization

ELECTRO-MAGNETIC YOKES: DC yoke has greater penetration while

AC yoke concentrates the field on thesurface of weld providing greater

sensitivity At maximum pole spacing used for

examination the minimum Lifting powerof AC yokes shall be:4.5 kg; DCyokes:18.1 kg.

The yoke techniques produceLONGITUDINAL magnetic field on thesurface in between the pole-pieces

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PERMANENT YOKES

Permanent yokes used where noelectric power is available orwhere arcing is not permitted

The strength of any yokes isdetermined by its lifting power

At maximum pole spacing used

for examination he minimumLifting power of Permanent yokeshall be:18.1 kg

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MAGNETISING STRENGTH

Field adequacy is verified utilizing

Gauss/Tesla meter or artificialflaw shims Yoke can be switched ON or OFF

at will which helps to remove theyoke-leg from part

The flux density introduced intopart can be varied by varying theamount of current in the coil

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YOKE-FIELD STRENGTH

VERIFYING USING SHIMS The shims are foils of low carbon steel

50 m thick that contains a notch ornotches chemically etched or machinedto 30% of foil thickness.

Shims are attached to part such that theartificial flaw is toward the examinationsurface.

A clearly defined line or lines when wetparticles are flowed over the shimsimultaneously with magnetizing forceindicates adequate field strength

MEASURING USING GAUSS METER

Hall-effect tangential probe positioned on the

surface The peak-value of tangential field is

measured when magnetizing force is applied

Value of 30 to 60 Gauss (2.4 k to 4.8 k Amps/meter) indicates adequacy of field

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MAGNETISING DIRECTION

FIELD DIRECTION

The direction of magnetization isverified using ASTM pie-gauge

Pie gauge is positioned

brazed pie-sections are towardthe examination surface

Cu plate away form surface.

A clearly defined line or lines in thedesired direction when particles areapplied simultaneously with

magnetizing force fulfills therequirement of field direction

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WET PARTICLE APPLICATION

Wet continuous

Flow of magnetizing currentinitiated

After thorough agitation wetparticles are applied by sprayingor flowing over the entire weld

area to be examined Multiple current shots (2 or 3)

applied. The last shot appliedwhile particle bath is still on part.

Wet particles accumulate at thesite of discontinuities & produceindication characteristic of itsnature

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DETECTABLE DISCONTINUITY

LIGHTING CONDITIONS

Day light or artificial lights used for dry

powder and non-fluorescent applications Min 500 Lux /1000 lux needed for critical

applications

Black Lightfor fluorescent wet particlesDETECTABLE DISCONTINUITIES:

Discontinuity at right angles to thesurface and its Depth five times itsopening width ;Length > width & >1 mm

flaw width creating small air gap

Incipient cracks less than 0.025 mmdeep and have surface openings of one-

tenth of its width are detectable by wetparticle inspection

Correct interpretation of indicationscaused by subsurface discontinuitiesrequires skill.

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CONDITIONS FOR DIFFICULTY

IN DETECTION

If the faces of a crack tightlyforced together by compressivestresses

the absence of air gap produceslittle leakage field that no particleindication is formed.

If the crack is not close-lipped butwide open at the surface ShallowCracks

Magnetic Reluctance of air gap

reduces strength of the leakagefield, combined with inability ofthe particles to bridge the gap,results in a weaker indication.

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INTERPRETATION

INTERPRETATION OF INDICATIONS: The accumulation pattern of

dry/wet particles that forms onmaterial surface duringexamination is INDICATION

Any indication has to beascertained whether RELEVANT,false or non-relevant

Discontinuities located on surfaceappear sharp & distinct

Discontinuities located below the

surface appear broad & fuzzy NDICATION is characterized

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FALSE & NON-RELEVANT

INDICATIONS

False Indications

Do not arise from flaws as there is no

magnetic attraction Particle patterns are held by gravity or

surface roughness.

Non-relevant indications

Caused by flux leakage, not due tobreaks in metal

Excessive magnetization force, partconfiguration, mill scales are possiblecauses

Abrupt changes in magnetic propertiesbetween base metal & weld metal

Toe of the welds, Edges of fillet welds, inHeat Affected Zone

Indications appear wide, loose & lightlyadhering particle pattern

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RELEVANT INDICATIONS

Relevant indications

Relevant indications arise fromdiscontinuity sizes as defined instandards

APPEARANCE OF FLAW INDICATIONS

Porosity

Appear more or less roundindications, as spherical in shape

Cracks in weld/Base Metal Longitudinal, Transverse & crater

Fairly straight & twisty ascontinuous linear indication

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EVALUATION

Evaluation

Determine severity of discontinuity,whether detrimental for intendeduse

Pass judgment based on welldefined acceptance standards

Provide Disposition of examinedweld-Accept/Repair/Reject

Post-cleaning

Sometimes demagnetized, Blownoff using compressed air for Drypowder

Sometimes demagnetized, handwipe with solvents and cleaned forwet particles

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