Types of Cast Iron | Cast Iron Properties | Uses of Cast Iron
Types of Cast Iron
Malleable Cast Iron
Malleable iron is obtained by heat treatment of white cast iron, which does not contain carbon in the free graphite form. There are two processes for the manufacture of malleable iron, which give rise to blackheart and whiteheart irons. These names are given because of the appearance of the surface of treated iron.
Blackheart Process
In the Blackheart process, white iron castings are heated at a temperature of about 9000C for 2-3 days and then cooled very slowly at the rate of 30C per hour. A neutral atmosphere is maintained in the furnace during the treatment cementite in the white iron structure breaks down into ferrite and spherical graphite aggregates. If the iron is cooled more rapidly, structures will consist of graphite in a ferrite and pearlite matrix.
Whiteheart Process
In the whiteheart process, white iron castings are packed into boxes with hematite ore and heated to a temperature of 9000C for about 2-5 days. Because of hematite, carbon is oxidized away from the surface. The structure obtained is composed of ferrite at the edge of the casting and ferrite, pearlite, and some graphite nodules at the center.
Properties of Malleable Cast Iron
Malleable cast iron is used where ductility, machinability, and high resistance to atmospheric corrosion is required.
It contains 2.0 to 3.0% carbon, 0.9 to 1.65% silicon, while sulphur and phosphorus are kept less than 0.18%. Manganese is added to take care of sulphur. Malleable iron has good machinability, castability, and cost economy. gives the typical microstructure of ferritic malleable iron. Distinctive properties of malleable cast iron are:
- Tensile strength : 270 to 690 N/mm2,
- Elongation is 2 to 12%,
- Young’s modulus (E) = 170 x 103 N/mm2,
- Hardness = 110 – 285 HB and
- Co-efficient of thermal expansion = 10 to 12.5 x 10-6 /0K
Uses of Malleable Cast Iron
Malleable cast iron is used for Steel brackets, brake carriers, crankshafts, camshafts, switchgear parts, and fittings for railway electrification systems.
Chilled Cast Iron
Chilled Cast Iron is produced by quick cooling of white cast iron. Chilling is carried out by putting the molten metal comes in contact with the chill, it gets cooled quickly, and a hard surface is formed. Typical applications are jaw crusher plates, running surface or rail carriage wheels, etc.
Alloy Cast Irons
Cast iron produced by the addition of alloying elements is known as alloy cast iron. It has increased strength, high wear, corrosion, and heat resistance depending upon the alloying element. Alloy cast iron has applications in the manufacture of crushing and grinding equipment parts, automobile components like cylinders, pistons, piston rings, brake drum, etc.,
Pearlitic Cast Iron
Pearlitic cast iron contains 2% chromium and is used for heavy-duty castings.
Martensitic cast iron
It contains 4.5% Ni and 1.5% Cr and is used to manufacture crusher rolls.
Ni-Cr-Si Cast Iron
It contains 18% Ni, 2$ Cr, 5%, and 2% carbon and is used for acid-resistant parts.
Ni-Mn Cast Iron
It contains 11% Ni, 1% Si, 6% Mn, and 2.8% and is used for the manufacture of pump parts to withstand the effect of corrosion, erosion, and abrasion.
Austenitic Cast Iron
It is non-magnetic, heat, and corrosion-resistant.
