Defects in Castings

a)      Defects caused by pattern and molding box equipment :

1)      Mismatch or mold shift: it produces a casting which does not match at the parting lines. There is mismatching of top and bottom parts of the casting at the mold joint.

Causes:

a)      Worn or loose dowels in pattern made in halves.

b)      Faulty registering of top and bottom halves of pattern mounted on plates.

c)      Worn out, loose, bent or ill-fitting molding box clamping pins.

Remedies: remedies involve removing the causes listed above.

2)      Variation in wall thickness of the castings:

Causes:

a)      Worn core boxes giving oversize core dimension.

b)      Worn core print allowing a core to float or move.

c)      Inadequate core print area, permitting lifting of core due to buoyancy of molten metal.

3)      Fins, flash and strain:

-          Fins, flash and strain usually occurs at the parting lines and result in excess metal which has to be ground off. Flashes or fins commonly appear along the mold joint at the place where the mold halves do not fit together properly because of much wear or wrapping of flask halves or improper fastening of the cope to the drag.

-          Straining or movement of the mold makes a casting appreciably thicker than the pattern.

Causes:

a)      Fins or flashes at the mold joint may occur when,

-          Bottom boards are too flexible.

-          Pattern plates are not sufficiently rigid to keep straight during ramming.

b)      Pattern having insufficient taper and thus requiring excessive rapping for their withdrawal from the sand results in fins at the joint.

c)      Top part boxes inadequately weighted, permit the top box (i.e. cope) to lift slightly, when poured, thereby causing flash along the mold joint.

4)      Crush: it is the displacement of the sand while closing a mold, thereby deforming mold surfaces. A crush shows itself as an irregular sandy depression in the casting.

Causes:

a)      Excessive weighting of the green sand mold. (core portion)

b)      Core print too small for the cope.

c)      Core too large for the core print.

d)     Careless assembly of the core boxes and cores.

b)     Defects due to improper molding and core making materials. (improper sand condition)

1)      Blowholes:

-          Blowholes are smooth, rough holes.

-          Blowholes visible on the surface of the casting are called open blows whereas those occurring below the surface of the castings and not visible from the outside are termed as blowholes.

-          Blowholes may occur in clusters or there may be one large smooth depression.

-          Blowholes are entrapped bubbles of gas with smooth walls.

Causes:

a)      Excess moisture in the molding sand.

b)      Low permeability and excessive fine grain sand.

c)      Rusted and dump chills, chaplets and inserts.

d)     Cores neither properly baked nor adequately vented.

e)      Presence of gas producing ingredients in the mold or core sand.

f)       Extra hard rammed sands

g)      Mold being not adequately vented.

Remedies: remedies involve removing the causes promoting defects.   

2)      Drop: a drop occurs when cope surface cracks and breaks, thus the piece of sand fall into the molten metal.

Causes:

a)      Low green strength. (owing to less mulling time, moisture or clay content)

b)      Low mold hardness i.e. soft ramming

c)      Insufficient reinforcement of the sand projection in the cope.

3)      Scab: it occurs when a portion of the face of a mold lifts and metal flows underneath in a thin layer. In other words, liquid metal penetrate behind the surface layer of sand.

Causes:

a)      Too fine a sand

b)      Sand having low permeability.

c)      High moisture content of the sand.

d)     Uneven mold ramming.

e)      Intermittent or slow running of molten metal over the sand surface thereby producing intense local heating.

4)      Pinholes: pinholes are numerous very small holes revealed on the surface of the casting after the surface has been cleaned by shot blasting.

Causes:

a)      Sand with high moisture content.

b)       Sand containing gas generating ingredients.

c)      Faulty metal

d)     Gas dissolved in the alloy and alloy not being properly degassed.

e)      Metal mold reaction. (results pin holing in steel casting)

5)      Metal penetration and rough surface: molten metal enters into the space between the sand grains and results in metal penetration and rough casting surface.

Causes:

a)      High permeability

b)      Large grain size sands

c)      Low dry strength of sand

d)     Soft ramming

6)      Hot tears: they are internal and external cracks having ragged edge. Immediately after solidification, metal have low strength , if at this stage, solid shrinkage of the casting develops sufficiently high stresses, the metal fails with a resulting hot tears.

Causes:

a)      Very hard ramming and therefore excessive mold hardness.

b)      High dry and hot strength of sand mold.

c)      Insufficient collapsibility of core or of a portion of mold.

d)     To much shrinkage of the metal while solidification.

e)      Faulty design causing some portion of the casting to be restrained while cooling.

f)       Slow running of the molten metal due to small gates or metal lacking in fluidity.

g)      High sulphur content

h)      Too low pouring temperature.

c) Defects due to improper sand mixing and distribution:

- Improper sand mixing and distribution give rise to faulty sand condition various defects which result, have been discussed under section (b)

d)  Defects caused by molding, core making, gating etc.

1)  Hot tears

2)  Shifts

3) Fins and flash

4) Crush

5) Cold laps (shut) and misrun

- if molten metal is too cold or casting section is too thin, entire mold cavity may not filled during pouring before the metal starts solidifying and the result is misrun.

            If the molten metal enters mold cavity through two or more ingates or otherwise if two streams of metal which are too cold, physically meet in the mold cavity but do not fuse together, they develop cold shut defect.

Causes:

a)      Too cold molten metal

b)      Too thin casting section

c)      Too small gates

d)     Too many restriction in the gating system

e)      Metal lacking in the fluidity

Besides, misrun is often the result of interrupted flow of metal from ladle into the mold.

7)      Slag holes: they are smooth depression on the surface of the casting. They usually occur near the ingates.

Causes:

Slag holes results when slag enters the mold cavity hence they can be obviated by inserting slag traps in the gating system.

8)      Shrinkage defects: metal shrinks as they solidifies, if this shrinkage is not compensated by providing risers, etc. voids will occur on the surface (i.e. surface shrinkage) or inside (i.e. internal shrinkage) the casting.

f)       Defects due to improper mold drying and core baking:

1)      A layer of moisture collected under the impervious layer of mold point (on the surface of the mold) will cause sand and paint scab and peel, with the result that casting shows: - sand washes, scabs, blowholes.

2)      Oil sand cores, if over baked, are not strong enough to resist the flow of molten metal and cause:- sand washes, rough surface, metal penetration, undersized holes.

3)      Oil sand cores, if under baked, absorb moisture from the atmosphere or green sand mold and cause defects like:-core sand wash, core blow, blow holes.

g)      Defects occurring while closing and pouring the molds.

a)      Shift or mismatch

b)     Misrun

c)      Cold laps or cold shut

d)     Crush

e)      Run out: a run-out occurs when molten metal leaks out of the mold during pouring and results in an incomplete casting.

Causes:

a)      Faulty molding box equipment.

b)      Faulty molding

f)       Inclusion: any separate undesired foreign materials present in the metal of the casting are known as inclusion.

An inclusion may be-

a)      Oxides, slag, dirt etc. which enters the mold cavity along with the molten metal during pouring. Such inclusion should be skimmed off before pouring the molten metal into the mold cavity.

b)      Sand cracked and broken from gating system, mold cavity, cores etc. sand sinks in molten liquid metal and causes sand cavities in the drag whereas in heavier metal (e.g. steel) becomes entrapped within the casting itself.

Remedies:

1)      Proper molding

2)      Molding sand should possess adequate hot strength.

3)      Skimming off or screening of molten metal before pouring.

g)      Defects caused by molten metal:

1)      Misrun

2)      Cold shut

3)      Excessive penetration:

In heavy steel casting, penetration of metal into mold or core (sand) is the result of high casting temperature. In copper base alloy and cast iron, penetration occurs because of excessive fluidity of molten metal, due to high phosphorus content or high casting temperature.

4)      Tin and lead sweat:

It occurs in high leaded copper base alloys as spot and lumps.

The presence of an excessive amount of hydrogen dissolved in the molten metal may force lead to surface to cause inverse segregation.

Silicon in copper alloys and a low tin content cause tin and lead sweat.

When the surface of the copper base alloy is covered with discontinuous thin layer of metal containing a higher content of tin than the parent alloy, this is known as tin sweat.

5)      Hot tears

6)      Sand cut and washes:

Molten metal as it flows over the mold and core surface, erodes the same and results in defects known as cut and washes.

The place from where sand has been cut or washes is occupied by molten metal and thus excess metal appears on the casting surface in the form of rough jumps or ragged spot. These spots are also known as scabs.

Causes:

a)      Soft ramming

b)      Weak sand

c)      Insufficient draft on patterns

d)     Insufficient bonded or over baked cores

e)      Improper gating system.

7)      Fusion:

Sand may fuse or stick to the casting surface with a resultant rough glossy appearance.

Causes:

a)      Lack of refractoriness of sand

b)      To high molten metal temperature

c)      Faulty gating system.

8)      Gas porosity, gas-holes, sponginess:

Gas porosity, differs from blowholes which occurs due to the molding sand having low permeability, excessive moisture or having been rammed too hard.

Gas porosity is caused by the gases absorbed by the molten metal. The main gases dissolved by practically all metals are oxygen, nitrogen and hydrogen.

Hydrogen is responsible for gas porosity.

Molten metals (especially aluminium and copper base alloys) may absorb hydrogen from, unburnt fuel gases, moisture in the air, dampness n the furnaces and green sand mold.

As molten metal solidifies, many small voids distributed quite uniformly throughout the metal are found and it is known as pinhole porosity.

 In non-ferrous melting, hydrogen and sulphour dioxide in the molten metal cause gas holes just below the surface of the castings.

Causes:

a)      Excessive high pouring temperature.

b)      Damp ladles

c)      Low permeability of sand

d)     Hydrogen or sulphur dioxide dissolved in molten metal.

e)      High moisture content of the mold.

f)       Slow modification rate

In ferrous alloys, hydrogen, if present, forms gas holes in heavier section rather than in thinner more quickly cooled sections.

Oxygen is the more often cause of gas holes in ferrous castings.

Remedies:

a)      Remove the dissolved gases from the melt.

b)      Avoids the conditions as discussed above promoting pick up of gases by molten metal.

9)      Shot metal:

If the molten metal is at relatively lower temperature and during pouring into the mold, it splashes, a few small particles separate from the main stream, they solidify and form shots. These shots if do not fuse with rest of molten metal in the mold, get embedded in the casting and revealed on the fractured surface, thus causing a defects known as shot metal.

Causes:

a)      As explain above

b)      Excess sulphour content in the molten metal.

c)      Higher moisture content of the molding sand.

d)     Faulty gating system

e)      Improper pouring of molten metal.

10)  Rattails and buckles:

If molten metal having very high temperature is poured into the mold cavity, a thin outer sand layer of mold cavity expands, bulges, gets separated from the sand behind it and remains on the surface of the casting. The casting surface shows either a step or shallow indentation along the path of incipient mold failure, often with a short metal in presenting the original crack. This surface fissure or line defect is known as rattails and it appears as an irregular line across the surface of a castings.

A rattails is a result of slight compression failure of the thin layer molding sand.

Another defect known as buckles is more severe compression failure of the sand surface.

Buckles and scabs usually appear in the cope surface of the castings.

They are alike in appearance.

Buckles show extensive overlapping of metal whereas scabs are relatively small (than buckles).

Causes:

a)      Improper expansion characteristics of molding sand

b)      Excessive temperature of the molten metal (like than of irons and steels)

c)      Continuous large flat surface of mold cavity

d)     Excessive mold hardness

e)      Lack of combustible additives in molding sand.

11)  Swells:

A swell is an enlargement of the mold cavity (and hence that of the casting) due to molten metal pressure on the mold wall and the slides.

Causes:

a)      Soft ramming

b)      Insufficient weighting of the molding boxes during casting

c)      Low strength of mold

d)     Mold being not adequately supported.

Defects occurring during fettling etc:

            Defective casting may result from carelessness during the fettling operation, e.g.

1)      Sand and scale not properly removed from the casting surface to be machined later on.

2)      Sand not properly removed from the cavities where oil is to be circulated.

3)      Distorted castings not properly straightened.

4)      Cracks caused in brittle castings by too heavy grinding.

5)      Chisel work left on the castings.

6)      Heads burned off too low or too high, thereby requiring, building up by metal deposition or removal by machining operations.

Defects due to faulty heat treatment:

1)      Uncontrolled initial heating operation may cause cracking in brittle castings.

2)      Careless stacking of castings while they are in semi plastic state at the heat treatment temperature, will tend them (i.e. castings) to distort.

3)      Fast cooling rates may develop crack in the castings.

4)      Improper heat treatment furnace atmosphere may prove detrimental to the surface appearance of the castings.

Defects due to cast metal:

Hard spot:

Hard spot occurs in gray iron castings having insufficient silicon content.

Such castings get hardened by chilling action of molding sand.

Hard spot make machining of the casting difficult.

Causes:

a)      Faulty metal composition.

b)      Faulty casting design, leading to rapid cooling of some parts of the casting as compared to the other parts.

Warpage:

Casting warp (i.e. misalign) or deform because of the stresses set up in them internally, due to differential solidification rates experience by different sections of large , long and wide flat castings.

Causes:

a)      Faulty casting design

b)      Absence of directional solidification.