DEFECTS IN CASTING

CASTING DEFECTS• Casting defects may be defined as those characteristics that create a

deficiency or imperfection contrary to the quality specifications imposed by the design and the service requirements.

• Defects in castings may be of three basic types(i) Major defects, which cannot be rectified, resulting in rejection of the

casting and total losses(ii) Defects that can be remedied but whose cost of repair may not justify the

salvage attempt(iii) Minor defects, which clearly allow the castings to be economically

salvaged and thereby leave a responsible margin for profit

Reasons for Casting Defects

• Unsuitable or unsatisfactory raw materials used in moulding, core making or casting

• The application of unsatisfactory moulding or casting practice by the individual worker or incorrect advice by the supervisor

• The use of improper tools , equipment, appliances or patterns• Unprofessional management policies relating to the fixing of incentive

plans and setting up of production procedures, faulty organisation and poor work discipline or lack of training

Types of Casting Defects

Shift Warped Casting Swell Fin

Blowhole Pinholes Gasholes Shrinkage cavity

Porosity Drops DirtMetal

Penetration and Rough surfaces

Slag holes Scrabs Hot tears(Pulls)

Cold Cracks

Cold shut and Mis-run

SHIFT• Results in a mis-match of the sections of a casting at a parting line.• Usually easy to identify• Defect cannot be rectified if mis-matching is not within the allowable

variation and the casting has to be scrapped• This defect can be prevented by ensuring proper alignment of the pattern

or die parts, moulding boxes, correct mounting of patterns on pattern plates and checking of pattern flasks, locating pins, etc. before use.

WARPED CASTING• Warpage is an undesirable deformation in a casting which occurs during or

after solidification.• Large and flat sections or intersecting sections are prone to warpage• Warpage is due to1. incorrectly placed rib2. Too small flasks, which may cause rapid cooling of the edges or ends of

casting3. Weak flasks, which may allow movement of the sand mould walls4. Insufficient gating system, which may not allow rapid pouring of metal5. Non provision of camber allowance on the pattern• If warping cannot• A proper casting design can reduce warpage of the casting

SWELL• Swell is an enlargement of the mould cavity by metal pressure• It results in localised or overall enlargement of the casting• Caused by insufficient ramming of sand and rapid pouring of molten metal• Insufficient weighting of the moulds during pouring may also cause the

cope to lift, giving a swell

FIN• Fin is a thin projection of metal, not intended as part of the casting.• Usually occur at the parting of the mould or core section• Fins are raised due to following reasons

a) incorrect assembly of moulds and cores.b) shorter length of “kiss core”.c) high metal pressure due to too long sprue.d) insufficient weighting of the mouldse) improper clamping of flask

BLOWHOLE• Blowholes are smooth and round holes clearly perceptible on the surface

of the casting• Blowholes may be either in the form of a cluster of a large number of

small holes having a diameter of about 3mm or less or in the form of one large and smooth depression.

• Caused in a casting by the generation and/or accumulation of gas or entrapped air in the mould cavity

• Blowholes are due to following reasonsi. Low permeability of landii. High moisture content of sandiii. Too fineness of sandiv. Sand is rammed too hardv. Insufficient venting

PINHOLES• Pinholes are numerous holes of small diameter, usually less than 2mm,

visible on the surface of the casting• Caused by the absorption of hydrogen or carbon monoxide when the

moisture content of sand is high or when steel is poured from wet ladles or is not sufficiently degasified.

• This defect can be minimised by using good melting and fluxing practices, by reducing the moisture content of the moulding sand and increased its permeability, and by promoting a faster rate of solidification.

GAS HOLES• Gas holes are those holes that appear when the surface of the casting is

machined or when the casting is cut into sections. • If the core prints are of inadequate size, gas cannot escape from the

mould as fast as it generates in the core. The accumulation of gas from the core may give rise to gas holes in the casting.

• Gas holes may also raised due to the following reasoni. Faulty and poor quality of metalii. Lack of controlled solidificationiii. Excessively moist sand

SHRINKAGE CAVITY• Shrinkage cavity is a void or depression in the casting.• It is caused mainly by following reasonsI. uncontrolled and haphazard solidification of the metalII. Wrong location or improperly sized gating systemIII. Inadequate risersIV. Poor design of castingV. High pouring temparature• The defect can be eliminated by applying the principals of directional

solidification in mould design and by judicious use of chills, denseners and padding.

POROSITY• Porosity is due to gas formation and gas absorption by the metal while it is

poured. Metal may dissolve some gas or air from the mould or core faces. These gases are liberated later when the metal cools, leaving behind porosity in the casting.

• Porosity defect may lead to leaking castings and reduce pressure tightness.

• Adequate fluxing of metal and controlling the amount of gas producing materials in the moulding and core making sand mixes can help in minimising this defect

DROPS• When the upper surface of the mould cracks and pieces of sand fall into

the molten metal, “drop” occurs• Drops are due to following reasonsI. Sand having low green strengthII. Soft rammingIII. Insufficient reinforcement of the mould

DIRT• Dirt generally appears in the form of foreign particles and sand embedded

on the surface of casting• Causes for this defects may bea. Crushing of mould due to mishandlingb. Sand wash when the metal is poured because of low strengthc. Soft rammingd. Insufficient fluxing of molten metale. Presence of slag in the mould due to its incomplete separation from

molten metal.

METAL PENETRATION AND ROUGH SURFACES

• Appears as an uneven and rough surface of the casting.• It may be caused whena. Sand has too high permeabilityb. Large grain size of sandc. Low strength of sandd. Soft ramming

SLAG HOLES• Slag holes are smooth depressions or cavities on the upper surface of the

casting or near it, usually near in-gates.• Produced when slag the slag tends to find the way in to the mould cavity

along with the molten metal.• Incorrect gating system and poor fluxing of metal are mainly responsible

for this defect.

SCABS• Scabs are sort of projection on the casting which occur when a portion of

the mould face or core lifts and the metal flows beneath in a thin layer.• Scabs can be recognised as rough, irregular projections on the surface

containing embeddec sand• Scab are of two typesi. Expansion scabii. Erosion scab

EXPANSION SCAB• Caused by the expansion of surface layers of the sand mould.• It may occur on any part of the mould, but more often it is found where

the sand gets strongly heated, such as top face of the mould which gets heated first by the radiation of heat from the molten metal rising upwards and then by actual contact with the molten metal.

• Heating by radiation causes a thin outer layer of sand to dry up and expand, leaving the interior green. This local expansion subjects the layer to severe and it eventually cracks.

EROSION SCAB• An erosion scab may occur where metal has been agitated or has partly

eroded the sand, leaving behind a solid mass of sand and metal at the spot where erosion took place.

• The sand that is eroded may find its way to the top part of the mould as dirt inclusion

• Erosion scabs may caused byi. Hard and uneven rammingii. Improper selection of gatesiii. High moisture content in sandiv. Too low permeability of sandv. Intermittent pouring of metalvi. Insufficiently baked cores or moulds.

HOT TEARS• Hot tears are internal or external ragged discontinuities or cracks on the casting

surface, caused by hindered contraction occurring immediately after the metal has solidified.

• Hot tears are produced wheni. Casting is poorly designedii. Chills are improperly placediii. Mould and core have poor collapsibilityiv. Ramming is too hardv. Incorrect pouring temperaturesvi. Improper placements of gates and risers• Hot tears can be prevented byi. Improving the casting designii. Achieving directional solidification and even rate of coolingiii. Selecting proper mould and core materials to suit the cast metaliv. Controlling the mould harness in relation to other ingredients of sand

COLD CRACKS• Cold cracks are similar in appearance and formation to hot tears• Breaks are less ragged and the cracks occur at temperature b.elow430oC.

COLD SHUT & MIS-RUN• A cold shut is a defect in which is formed a discontinuity due to the imperfect

fusion of two streams of metal in mould cavity.• The defect may appear like a crack or seam with smooth rounded edges.• A mis –run casting is one that remains incomplete due to the failure of metal

to fill the entire mould cavity.• Reasons for cold shut or mis-run may bei. Too thin sections and wall thicknessii. Improper gating systemsiii. Damaged patternsiv. slow and intermittent pouringv. Poor fluidity of metal caused by low pouring temperaturevi. Improper alloy composition• When the metal cavity is not completely filled because of insufficient

metal, the defect is called “Pour short”.