Introduction to Basic Manufacturing Processes and ... - always yours

58 Introduction to Basic Manufacturing Processes and Workshop Technology
carbon does not change into graphite state but change in other form of free carbon called
tempered carbon.
Fe 3
C ——→ 3Fe + C
This makes the casting less brittle and malleable. The fracture portion of such a casting
is dark grey or black in appearance. These castings are specially used in automobile industries.
Black heart malleable iron casting
The castings packed in a drum of oxidizing media which is generally powdered iron ore
or powered scale (film of Fe 3
O 4
on surface). This close drum is kept in the furnace and heated
to 900°C. It is then maintained at this temperature to nearly 40 to 70 hours and allowed to
cool slowly in a furnace itself. The castings become malleable like white heart cast iron. The
percentage of carbon and silicon should be so selected that it can promote the development
of free carbon when these castings are annealed.
Properties
1. Malleable cast iron is like steel than cast iron.
2. It is costly than grey cast iron and cheaper than softer steel.
Applications
Malleable cast iron are generally used to form automobile parts, agriculture
implementation, hinges, door keys, spanners mountings of all sorts, seat wheels, cranks,
levers thin, waned components of sewing machines and textiles machine parts.
4.3.3.6 Meehanite cast iron
Meehanite cast iron is an inoculated iron of a specially made white cast iron. The
composition of this cast iron is graphitized in the ladle with calcium silicide. There are various
types of meehanite cast iron namely heat resisting, wear resisting and corrosion resisting
kind. These materials have high strength, toughness, ductility and good machinability. It is
highly useful for making castings requiring high temperature applications.
4.3.3.7 Alloy cast iron
The cast irons as discussed above contain small percentages of other constituents like
silicon, manganese, sulphur and phosphorus. These cast irons may be called as plain cast
irons. The alloy cast iron is produced by adding alloying elements like nickel, chromium,
molybdenum, copper and manganese in sufficient quantities in the molten metal collected in
ladles from cupola furnace. These alloying elements give more strength and result in
improvement of properties. The alloy cast iron has special properties like increased strength,
high wear resistance, corrosion resistance or heat resistance. The alloy cast irons are
extensively used for automobile parts like cylinders, pistons, piston rings, crank cases, brake
drums, parts of .crushing and grinding machinery etc.
4.3.3.8 Effect of impurities on cast iron
The cast iron contains small percentages of carbon, silicon, sulphur, manganese and
phosphorus. The affect of these impurities on the cast iron are as follows:
(1) Carbon. Carbon is one of the important elements in cast iron. It reduces melting
point of iron. Pure iron has a melting point of about 1500°C but iron with 3.50% C
has melting point of about 1350°C. When carbon is in free form i.e. as graphite form,