p2w QSWdure involves pouring molten metal into a cavity in a mass of packed sand. Wooden patterns
are used for moulds which are removed when the sand has dried. Each mould has a hole for
casting through which the molten iron is poured. Air and hot gases escape through another
hole. Cast material is taken out by breaking the mould after iron cools down.
r
2 g is used for making columns and piles. For hollow casting of the objects a solid
core is placed where the hollow is to be maintained. After casting the core and mould are taken
out. Cast material becomes hollow due to the core.
2 2 g The mould box and the solid core is kept in vertical position. Alter
cooling the core is taken out by crane. Good quality pipes can be obtained by this method.
g 2 g Many blow holes are left in ordinary casting because of little control over
temperature and the sand mould. The problem is overcome by centrifugal casting. Molten
material is poured in a revolving metallic cylindrical mould in a controlled manner, rotating at
the rate of 10,000 revolutions per minute. Large diameter pipes, gun barrels, etc. are cast by this
method. The castings are dense and have a fine-grained structure with uniform and high
physical properties. They are least subjected to directional variations on properties than static
castings.
h 2 g Die casting is cheap for commercial production. Casting is done under pressure,
which may be as high as 140 MN/mm^2 , into a split die cavity. Since the die is water cooled, the
molten metal solidifies quickly, permitting early removal of the casting.
g
Cast iron is classified as grey, white, malleable, mottled, chilled and toughened, and is described
in Table 13.1.The nomenclature white and grey signify the appearance of the fracture of a
casting. The rest of the terms signify the special properties developed in these classes by heat
treatment and alloying.
Cast iron is hard and brittle. It can neither be riveted nor welded. It is strong in compression
(600 N/mm^2 ) but weak in tension (150 N/mm^2 ) and shear. Its specific gravity is 7.50. It has low
melting point (1200°C) and is affected by sea water. It cannot be magnetized and is not suitable
for forging. Iron containing large amounts of manganese and chromium are likely to be
permanently white, while those having a high silicon content are grey. With proper adjustment
in composition, cast iron may be rendered white by cooling rapidly or grey by cooling slowly
from the molten state.
i2 2s
g XThe proportion of carbon and its form more or less influence most of the physical and
mechanical properties of cast iron. The melting temperature of cast iron is reduced as the
carbon content or the percentage of combined carbon is increased. Consequently white cast
iron has a lower melting point than grey cast iron. Shrinkage varies inversely as the carbon
content.