Defects in Casting

DEFECTS IN CASTING Click to edit Master subtitle subtitle style

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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.

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Defects in castings may be of three basic types

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(i)

(ii)

(iii)

Major defects, which cannot be rectified, resulting in rejection of the casting and total losses Defects that can be remedied but whose cost of repair may not justify the salvage attempt Minor defects, which clearly allow the castings to be economically salvaged and thereby leave a responsible margin for profit

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Reasons for Casting Defects •

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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

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 Types of Casting Defects

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SHIFT •

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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.

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WARPED CASTING Warpage is an undesirable deformation in a casting which occurs during or after solidification.

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Large and flat sections or intersecting sections are prone to warpage

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Warpage is due to

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1.

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incorrectly placed rib  Too small flasks, which may cause rapid cooling of the edges or ends of casting Weak flasks, which may allow movement of the sand mould walls Insufficient gating system, which may not allow rapid pouring of metal

5/29/12 Non provision of camber allowance on the pattern

SWELL •

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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

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FIN •

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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 moulds e) improper clamping of flask

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

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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.

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

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Blowholes are due to following reasons

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i.

Low permeability of land

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High moisture content of sand

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 Too fineness of sand Sand is rammed too hard

Insufficient venting 5/29/12

PINHOLES •

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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.

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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.

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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.

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Gas holes may also raised due to the following reason

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Faulty and poor quality of metal

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Lack of controlled solidification

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Excessively moist sand

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SHRINKAGE CAVITY •

Shrinkage cavity is a void or depression in the casting.

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It is caused mainly by following reasons

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uncontrolled and haphazard solidification of the metal

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Wrong location or improperly sized gating system

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Inadequate risers

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Poor design of casting

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High pouring temparature

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 The defect can be eliminated by applying the principals of  directional solidification in mould design and by judicious use of chills, denseners and padding.

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POROSITY •

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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

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DROPS When the upper surface of the mould cracks and pieces of  sand fall into the molten metal, “drop” occurs

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Drops are due to following reasons

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Sand having low green strength

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Soft ramming

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Insufficient reinforcement of the mould

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DIRT Dirt generally appears in the form of foreign particles and sand embedded on the surface of casting

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Causes for this defects may be

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Crushing of mould due to mishandling Sand wash when the metal is poured because of low strength

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Soft ramming

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Insufficient fluxing of molten metal

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Presence of slag in the mould due to its incomplete separation from molten metal.

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METAL PENETRATION AND ROUGH SURFACES •

Appears as an uneven and rough surface of the casting.

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It may be caused when

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Sand has too high permeability

b.

Large grain size of sand

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Low strength of sand

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Soft ramming

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SLAG HOLES •

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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.

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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.

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Scabs can be recognised as rough, irregular projections on the surface containing embeddec sand

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Scab are of two types

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Expansion scab

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Erosion scab

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EXPANSION SCAB •

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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.

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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.

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

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Erosion scabs may caused by

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Hard and uneven ramming

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Improper selection of gates

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High moisture content in sand

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 Too low permeability of sand Intermittent pouring of metal

Insufficiently baked cores or moulds. 5/29/12

vi.

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.

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Hot tears are produced when

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Casting is poorly designed

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Chills are improperly placed

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Mould and core have poor collapsibility

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Ramming is too hard

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Incorrect pouring temperatures

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Improper placements of gates and risers

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Hot tears can be prevented by 5/29/12

COLD CRACKS •

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

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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.

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 The defect may appear like a crack or seam with smooth rounded edges.

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A mis –run casting is one that remains incomplete due to the failure of metal to fill the entire mould cavity.

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Reasons for cold shut or mis-run may be

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 Too thin sections and wall thickness

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Improper gating systems

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Damaged patterns

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slow and intermittent pouring

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