(2) iit mt may 7
TRANSCRIPT
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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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