p2w QTI XIn small percentages (0.5–3%) silicon increases the fluidity of the molten iron, decreases
blow holes and increases the density of castings. It also reduces the solubility of carbon in iron
and shrinkage. When silicon is increased up to 6 per cent the iron becomes hard and has a
mirror-like fracture.
X Sulphur is an undesirable element in cast iron and is limited to less than 0.1 per cent.
It combines with manganese to form the sulphide (MnS) or, if the manganese is very low and
not sufficient to satisfy the sulphur, iron sulphide (FeS) may be formed. Since these sulphides
solidify at considerably lower temperatures, than cast iron, they tend to make castings brittle
and weak at higher temperatures. High sulphur content also increases shrinkage and causes
hard, brittle iron. These may be neutralized by proper additions of silicon.
XWhen phosphorus is less than 0.5 per cent, it has no marked effect on cast iron.
Usually it is present to the extent of 0.1 to 1.5 per cent. If more than 2 per cent, the iron is
embrittled and strength diminished. High phosphorus irons are much more fluid and shrink
less, which make them suitable for ornamental castings.
w XWhen present in range of 0.4–1.2%, manganese combines with sulphur, and—
having satisfied sulphur—with carbon to form manganese carbide. It increases the tensile
strength and hardness of iron. Manganese increases the solubility of carbon in iron and opposes
the liberation of graphite, which is a cementite stabilizer. High percentage of manganese
increases shrinkage and hardness. Thus in grey iron which is to be machined manganese
should be kept low.
On account of cheapness, strength, ease with which it may be melted and cast into more or less
intricate shapes, ease of machining, high damping capacity, and ease with which its hardness
may be varied, cast iron is the most used of the cast metals employed in engineering constructions
and machines. Some of the more common uses of cost iron are making ornamental castings
such as wall brackets, lamp posts; bathroom fittings such as cisterns, water pipes, sewers,
manhole covers, sanitary fittings and; rail chairs, carriage wheels and machine parts subjected
to shocks. It is used as basic material for manufacturing wrought iron and mild steel.
h
Checks, segregation, blow holes and coarse grain originate during the cooling of the castings.
Irons with high sulphur content are liable to have small cracks running transverse to longitudinal
axis, called checks, due to greater shrinkage and lack of strength. Segregation is pronounced in
high phosphorus iron. Carbon and silicon sometimes segregate in such manner that interior
portions of the metal are white and exterior parts are grey rendering it difficult to machine the
casting. Blow holes are caused due to improper venting of the mould or due to a high proportion
of sulphur. A coarse or open grain in the iron is caused by too slow cooling, or due it may be due
to a very high Phosphorus content. Spongy spots (the exaggerated forms of open grains) and
cold shuts — faults planes in the metal produced by the solidification of part of the casting
before the remaining molten metal was run into place — result from lack of fluidity in the iron
or from improper grating.