casting defect (2)
TRANSCRIPT
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A
SEMINAR REPORT ONCASTING DEFECTS
Guided byDR.ANIL KUMAR SINGHPROFESSOR &(H.O.D)
FOUNDRY TECHNOLOGYNIFFT,HATIA,RANCHI
Submitted by:RAJU KUMAR
A.D.C.(foundry tech.)
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INTRODUCTION
CASTING DEFECTS:- The characteristics bywhich castings quality get affected & don’t fulfillthe predetermined design parameter are termed
as casting defects.
It may be classified broadly as:-
1>Major defects:-Not rectified i.e.directrejection.
2>Defects :-very costly if rectified
3>Minor defects:-economically repaired
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• WHY DEFECTIVE CASTINGS?
DUE TO 1.Casting & Pattern design2.Pattern equipment3.Flask equipment & rigging
4.Gating & risering5.Moulding sand & core sand6.Core making practice
7.Moulding practice8.Metal composition9.Melting practice
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CASTING DEFECTS
BLOW HOLESPIN HOLES
SCAB
RAT TAIL
HOT TEAR
SAND FUSION
BURN ON
METAL PENETRATION
SHRINKAGE DEFECT
COLD SHUT
MISRUNSHIFT & CROSS JOINT
FINS
CRUSH
MOULD WALL MOVEMENT
SWELL
INCLUSIONS
SAND CUTS &WASHERS DROP
SHOT METAL
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BLOW HOLE:-The holes found on surface or sub-surface of casting having dia=or>2mm are called
blow holes.
Causes:-
High moisture contentin the sand moulds
Less permeability of
mould or core sand Excess gas generating
binders & additivesused in core making.
Poor venting of mould &core
Rusty & damp chills andchaplets used
Hard ramming of sand
Defective gating system
Remedies:-
Moisture content shouldbe minimized.
Permeability can be
improved by properventing using coarsersand.
Gas generating binders &
additives should beavoided
Chills & chaplets shouldbe dry & clean
Ramming of sand should
not be too hard
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PIN HOLES:-The holes found on surface or sub-surface of casting having dia=or<2mm are called
blow holes.
Causes:-
High moisture content in themould.
Wet ladles
Wet refractory lining offurnace
Insufficient degassing ofmelt.
Metal mould reaction
Presence of Al(aluminium
present as impurity forms Al2O3 layer over casting anddoes not the trapped air tocome out).
Remedies:-
Moisture content of mouldshould be minimized
Ladle and lining should bepreheated
Proper degassing melt
To avoid metal mouldreaction, zircon wash isapplied, in cse steel castingand graphite wash in case of
cast irons.
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SCAB It is rough & irregular projections on thesurface of casting.SCAB are of two
type<1>expansion scab<2>erosion scab Causes :-
High thermal expansion ofsand
Uneven or hard ramming
High moisture content in themould
Low permeability
Flat surface on cope side ofthe mould
Slow pouring rate
Remedies:-
Sand should be of widedistribution
Ramming of sand should beproper &uniform.
Pouring rate should not betoo slow
Moisture content should beas less as possible
Enforcements can be used onthe cope side where chancesof scabbing in maximum.
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RAT TAIL:-It is also an expansion defect,similar to scab but of lower degree.
It causes lurinkles to appear on the top face
of the casting. Causes:-
Prolonged heating of
mould surface Excess moisture
content in the sand
Uneven ramming ofsand
Remedies:-
Moisture content
should beminimized.
Ramming of sand
should be proper Pouring should be
adequately fast.
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HOT TEAR :-Irregular cracks occurs on the
casting, during last stages of solidification.
The cracks have very rough surface.
Causes:-
Excess mould hardness
Improper design ofcasting
Insufficient collapsibilityof mould & cores
Long freezing range of
alloy, promotes hot tear Excess shrinkage of
metal
High sulphur contentpromotes hot tear
Remedies:-
Collapsibility of mould &core should be high
Ramming should not betoo hard
If excess sulphur ispresent, then
desulphurization can becarried out
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SAND FUSION
It occurs generally in ferrous castings where
pouring temperature is high. Sand particles stickson the surface of the casting, on all sides.
Reasons:-
High pouringtemperature
Low refractoriness
of sand
Remedies:-
Mould can bewashed by highrefractory material
Zircon sand can beused as facing sand,in the mould
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BURN ON It occurs in ferrous castings only where
temp. is very high. Metal mould reaction takes
place 2Fe + O2 ® 2FeO2FeO + SiO2 ® FeO. SiO2 (Fayalite)
The whole casting is covered with a layer offayalite.
Reasons:-
High pouringtemperature
Metal mould reaction
Remedies:-
In case of cast irons, coaldust is added in sand mix, itproduces a reducing
atmosphere which does notallow oxygen to react withmetal and form FeO.
Graphite mould wash canalso be done to stop metal
mould reaction
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METAL PENETRATION
Molten metal enters the space between sand
grains and thus results in rough surface of the casting.
Reasons:-
Soft ramming Coarse sand grains
High metallostaticpressure
Remedies:-
Proper and uniformramming
Wide distribution ofsand
Washes can beapplied on themould wall.
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SHRINKAGE DEFECT :-Irregular cavity formed in
casting due to insufficient metal
Reasons:-
Poor risering
Poor casting design
Lack of directionalsolidification
Remedies:-
Proper gating andrisering
achieve directionalsolidification for thatuse chills,exothermicmaterials, arcing theriser melt, etc.
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COLD SHUT When two streams of molten metal meet in
the mould cavity, but do not fuses with each
other perfectly &makes hair like crack is calledcold shut
Reasons:-
Low pouringtemperature
Slow pouring rate Poor gating design
Less fluidity of melt
Remedies:-
sufficient pouringtemp.
higher pouring rate Proper gating design
Increase the fluidityof melt
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MISRUN Incompletely filled mould cavities resulting in
incomplete casting is called misrun.
Reasons:-
Very thin sections in
casting Low pouring
temperature
Slow & intermittent
pouring Low permeability
Poor gating design
Insufficient metal
Remedies:-
Proper gating and
risering Proper venting
Sufficient metal shouldbe poured
Thin sections can bepreheated
Pouring should be fastenough
Pouring temperatureshould be correct
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SHIFT & CROSS JOINT It is a mismatch of the casting at the parting
line or a variation from specified dimensions of a
cored section due to change in position of core ormisalignment of core in assembling.
Reasons:-
Wrong assembly of mould &core
Incorrect locating pins in themould
Inadequate weighing of thecope part
Loose patterns on the plate
Pattern not properly alignedin the cope & drag part.
Inadequate provisions forcore print or core support
Remedies:-
Core should be properlyassembled. For this purposecore paste and other thingsshould be properly used.
Moulding boxes should beproperly clamped and
weighed. Loose piece of pattern should
be properly aligned withdowel pins
There should be adequatecore prints on pattern for thesupport of core.
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FINS They usually occur at the parting line all
along the castings in the form of extra metal
which has to be ground off. These are called fins.
Reasons:-
Incorrect mouldassembly
Insufficient weighing
of the mould Due to high
metallostatic
pressure
Remedies:-
Proper assembly ofcoke & drag
Extra weight should
be provided over themould box
Length of sprue pin
should not be toolon .
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CRUSH It is the displacement of sand while closing
or mould thereby deforming mould surface. Due
to this, irregular depressions occur on the surfaceof the casting.
Reasons:-
Excessive weighingof the cope
Careless mould &
core assembly Core print too small
to support the core
Improper ramming
Poor green strength
Remedies:-
Excess weighingshould be avoided
Mould & core should
be assembled,carefully
Proper ramming ofsand should be done
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MOULD WALL MOVEMENT The sides of the casting bulge out, due to
mould wall movement. This occurs generally in
gray iron and S.G. Iron castings.
Reasons:-
High moisturecontent in the mould
Less rigid mouldwalls
Remedies:-
Moisture contentshould be as low aspossible
Mould walls can be
made rigid by usingbinders andadditives such asdextrin, molasses
SWELL
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SWELL Some portion of the casting may small, due
to poor ramming at that part.
This is called small defect.
Reasons:-
Poor or non uniformramming
Lack of rigidity ofmould walls
High metallostaticpressure
Remedies:-
Ramming should beuniform
Sprue pin should notbe too long
Gating should bedone properly
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INCLUSIONS When slag enters the mould cavity, it
solidifies as inclusions. If large amount of slag
enters then it forms smooth depression on theupper surface of the casting.
Reason:-
Slag poured alongwith molten metal.
Remedy:-
Metallic or ceramicnets can be used tofilter the melt.
Skim bob gatingsystem can be used.
Bottom pouringladles can be usedfor slag free, clean
metal.
When molten metal flows over the mould &
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When molten metal flows over the mould &core surface it erodes some part of the sand andresults in cuts & washes. The place from where
sand has been eroded there extra metal solidifieson the surface of the casting.
Reasons:- Soft ramming
Insufficient draft on
patterns Improper gating
system
Improper baking of
cores
Remedies:- Ramming properly
Provide Draft
allowance Proper Gating
system
Core baked properly
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DROP When portion of sand drops from the cope
part or from other overhanging sections in the
molten metal, then this causes drop defect
Reasons:- Less green strength of
mould
Soft ramming
Improper casting design
Remedies:-
Green strength must beoptimum by havingoptimum moisturecontent & binder level
Ramming should bedone properly
Overhanging sectionscan be strengthened by
using enforcements
SHOT METAL OR COLD SHOT
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SHOT METAL OR COLD SHOT Due to sputtering of metal, premature
solidification of metal globules take place. This
premature solidified metal globules do not fusewithin the metal. This is called cold shot.
Reasons:-
Due to gating system which
causes turbulence. Multiple gating
High moisture content in themould creates boiling actionwhich causes metal sputter
Those metal which arereadily oxidized are moreprone to cold shots
Slow, interrupted pouring
Remedies:-
Amount of moisture in the mould
and core sand should be controlledso that it will not create boilingaction.
Gating system should be such thatit will prevent metal shooting intothe mould
Pouring should not be slow
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