unit.2 radiography (x)

7/31/2019 Unit.2 Radiography (x)

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Radiograph Interpretation of

Castings


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Defects found by radiographic testing incasting

The major objective of radiographic testing of castings isthe disclosure of defects that adversely affect thestrength of the product.

Castings are a product form that often receiveradiographic inspection since many of the defectsproduced by the casting process are volumetric innature, and are thus relatively easy to detect with thismethod.

These discontinuities of course, are related to castingprocess deficiencies, which, if properly understood, can

lead to accurate accept-reject decisions as well as tosuitable corrective measures.


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Different types and sizes of defects have differenteffects of the performance of the casting,

It is important that the radiographer is able to identify thetype and size of the defects.

ASTM E155, Standard for Radiographs of castings hasbeen produced to help the radiographer make a betterassessment of the defects found in components.

The castings used to produce the standard radiographshave been destructively analyzed to confirm the size andtype of discontinuities present.


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Gas porosity or blow holes Gas porosity or blow holes are caused by

accumulated gas or air which is trappedby the metal.

These discontinuities are usually smooth-walled rounded cavities of a spherical,elongated or flattened shape.

If the sprue is not high enough to providethe necessary heat transfer needed toforce the gas or air out of the mold, thegas or air will be trapped as the moltenmetal begins to solidify.

Blows can also be caused by sand that is

too fine, too wet, or by sand that has a lowpermeability so that gas cannot escape. Too high a moisture content in the sand

makes it difficult to carry the excessivevolumes of water vapor away from thecasting.


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Sand inclusions

Sand inclusions and dross arenonmetallic oxides, which appear onthe radiograph as irregular, darkblotches.

These come from disintegrated portionsof mold or core walls and/or from oxides(formed in the melt) which have notbeen skimmed off prior to theintroduction of the metal into the moldgates.

Careful control of the melt, properholding time in the ladle and skimmingof the melt during pouring will minimizeor obviate this source of trouble


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Cavity shrinkage

Cavity shrinkage appears asareas with distinct jaggedboundaries.

It may be produced when metal

solidifies between two originalstreams of melt coming fromopposite directions to join acommon front.

Cavity shrinkage usually occursat a time when the melt has

almost reached solidificationtemperature and there is nosource of supplementary liquid tofeed possible cavities.


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Inclusions

Inclusions are nonmetallicmaterials in an otherwisesolid metallic matrix.

They may be less ormore dense than thematrix alloy and willappear on the radiograph,respectively, as darker orlighter indications.

The latter type is morecommon in light metalcastings.


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Sponge shrinkage

Sponge shrinkage shows itselfas areas of lacy texture withdiffuse outlines, generallytoward the mid-thickness ofheavier casting sections.

Sponge shrinkage may bedendritic or filamentaryshrinkage.

Filamentary sponge shrinkageappears more blurred becauseit is projected through therelatively thick coatingbetween the discontinuitiesand the film surface.


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Cracks

Cracks are thin(straight or jagged)linearly disposed

discontinuities thatoccur after the melthas solidified. Theygenerally appear

singly and originate atcasting surfaces.


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Filamentary shrinkage

Filamentary shrinkageusually occurs as acontinuous structure of

connected lines orbranches of variablelength, width and density,or occasionally as anetwork


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Defects in welding detect by

radiography


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Cold lap

Cold lap is a conditionwhere the weld fillermetal does not properlyfuse with the base metal

or the previous weld passmaterial (interpass coldlap).

The arc does not meltthe base metal sufficiently

and causes the slightlymolten puddle to flow intothe base material withoutbonding.


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Porosity

Porosity is the result of gasentrapment in the solidifying metal.

Porosity can take many shapes on aradiograph but often appears as darkround or irregular spots or specksappearing singularly, in clusters, or inrows.

Sometimes, porosity is elongated andmay appear to have a tail. This is theresult of gas attempting to escapewhile the metal is still in a liquid state

and is called wormhole porosity. All porosity is a void in the material

and it will have a higher radiographicdensity than the surrounding area.


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Cluster porosity

Cluster porosity is causedwhen flux coated electrodesare contaminated withmoisture.

The moisture turns into agas when heated andbecomes trapped in theweld during the weldingprocess.

Cluster porosity appear just

like regular porosity in theradiograph but theindications will be groupedclose together.


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Slag inclusions

Slag inclusions arenonmetallic solid materialentrapped in weld metalor between weld andbase metal.

In a radiograph, dark,jagged asymmetricalshapes within the weld or

along the weld joint areasare indicative of slaginclusions.


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Incomplete penetration (IP) or lack

of penetration (LOP) Incomplete penetration (IP) or

lack of penetration (LOP)occurs when the weld metalfails to penetrate the joint.

It is one of the most

objectionable welddiscontinuities. Lack of penetration allows a

natural stress riser from whicha crack may propagate.

The appearance on aradiograph is a dark area with

well-defined, straight edgesthat follows the land or rootface down the center of theweldment.


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Incomplete fusion

Incomplete fusion is acondition where the weldfiller metal does notproperly fuse with the

base metal. Appearance on

radiograph: usuallyappears as a dark line orlines oriented in the

direction of the weldseam along the weldpreparation or joiningarea.


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Internal concavity

Internal concavity is acondition where the weldmetal has contracted as itcools and has been

drawn up into the root ofthe weld.

On a radiograph it lookssimilar to a lack ofpenetration but the line

has irregular edges and itis often quite wide in thecenter of the weld image.


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Internal or root undercut

Internal or root undercutis an erosion of the basemetal next to the root ofthe weld.

In the radiographic imageit appears as a darkirregular line offset fromthe centerline of theweldment.

Undercutting is not asstraight edged as LOPbecause it does not followa ground edge.


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External or crown undercut

External or crownundercut is an erosion ofthe base metal next to thecrown of the weld.

In the radiograph, itappears as a darkirregular line along theoutside edge of the weld

area.


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Offset or mismatch

Offset or mismatch are termsassociated with a conditionwhere two pieces beingwelded together are notproperly aligned.

The radiographic image showsa noticeable difference indensity between the twopieces.

The difference in density iscaused by the difference inmaterial thickness.

The dark, straight line iscaused by the failure of theweld metal to fuse with theland area.


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Inadequate weld reinforcement

Inadequate weld reinforcementis an area of a weld where thethickness of weld metaldeposited is less than thethickness of the base material.

It is very easy to determine byradiograph if the weld hasinadequate reinforcement,because the image density inthe area of suspectedinadequacy will be higher

(darker) than the image densityof the surrounding basematerial.


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Excess weld reinforcement

Excess weld reinforcement isan area of a weld that has weldmetal added in excess of thatspecified by engineeringdrawings and codes.

The appearance on aradiograph is a localized,lighter area in the weld.

A visual inspection will easilydetermine if the weldreinforcement is in excess of

that specified by theengineering requirements.


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Cracks can be detectedin a radiograph only whenthey are propagating in adirection that produces a

change in thickness thatis parallel to the x-raybeam.

Cracks will appear asjagged and often very

faint irregular lines.Cracks can sometimesappear as "tails" oninclusions or porosity.

unit.2 radiography (x)

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