Classification of Cast Iron
CI containing C in form of cementite is called white cast iron. Microstructure of such CI consists of pearlite, cementite and ledeburite. If C content is less than 4.3% it is hypoeutectic CI and if C is greater than 4.3% it is hypereutectic CI. White cast iron has high hardness and wear resistance and is very difficult to machine. It can be ground, though. Hardness of white CI varies between 300-500 BHN and UTS between 140-180 MPa. White CI is normally sand cast to produce such parts as pump liners, mill liners, grinding balls, etc.
Cast iron containing carbon in form of graphite flakes dispersed in matrix of ferrite or pearlite is classified as gray cast iron. The name is derived from the fact that a fracture surface appears gray. Gray cast iron differs in percentage of Si from white cast iron while C percentage is almost same. The liquid alloy of suitable composition is cooled slowly in sand mould be decompose Fe3C into Fe and C out of which C is precipitated as graphite flakes. Addition of Si, Al or Ni accelerates graphitesation. The graphite flakes vary in length from 0.01 to 1.0 mm. The flakes provide an easy passage to cracks thus not allowing softer microstructure to deform plastically. Larger flakes reduce strength and ductility. The best properties of gray cast iron are obtained with flakes distributed and oriented randomly. Inoculant agents such as metallic Al, Ca, Ti, Zr and SiC and CaSi when added in small amount, cause formation of smaller graphite flakes and random distribution and orientation.
Gray cast iron is basically brittle with hardness varying between 149 to 320 BHN and UTS of 150 to 400 MPa. Different properties are obtained by varying cooling rate and quantity of inoculant agents. It has excellent fluidity, high damping capacity and machinability. If gray CI is repeatedly heated in service to about 400oC suffers from permanent expansion called growth. Associated with dimensional changes are less of ductility and strength as a result of growth. When locally heated to about 550oC several times this material develops what are called fire cracks resulting into failure.
High strength gray cast iron is obtained by addition of strong inoculating agent like CaSi to liquid metal before casting process. UTS in the range of 250 to 400 MPa is obtained. This cast iron is called Meehanite iron and can be toughened by oil quenching treatment to a UTS of 520 MPa.
If graphite in cast iron is present in form of nodules or spheroids in the matrix of pearlite or ferrite the material is called nodular cast iron. This cast iron has marked ductility giving product the advantage of steel, and process the advantage of cast iron. It is basically a gray cast iron in which C varies between 3.2 to 4.1%, Si between 1.0 to 2.8% while S and P are restricted to 0.03 to 0.1% respectively. Ni and Mg are added as alloying elements. Crank shafts, metal working rolls, punch and sheet metal dies and gears are made out of nodular CI. The defects like growth and fire cracks are not found in this class of iron. This makes it suitable for furnace doors, sand casting and steam plants. It also possesses good corrosion resistance making it useful in chemical plants, petroleum industry and marine applications.
White CI containing 2.0 to 3.0% C, 0.9 to 1.65% Si, < 0.18% S and P, some Mn and < 0.01% Bi and B can be heat treated for 50 hours to several days to produce temper carbon in the matrix of ferrite or pearlite imparting malleability to CI. This class is known as malleable cast iron and can have as high as 100 MPa of UTS and 14% elongation. Due to such properties as strength, ductility, machinability and wear resistance and convenience of casting in various shapes, malleable CI is largely used for automotive parts such as crank and cam shafts, steering brackets, shaft brackets, brake carriers and also in electrical industry as switch gear parts, fittings for high and low voltage transmissions and distribution system for railway electrification.
Addition of alloying elements such as Ni and Cr provide shock and impact resistance along with corrosion and heat resistance of cast iron. These are called alloyed CI 3 to 5% Ni and 1 to 3% Cr produce Ni-hard CI with hardness upto 650 BHN and modified Ni-hard CI with impact and fatigue resistance is produced by adding 4.8% Ni and 4.15% Cr. Ni-resist CI with 14 to 36% Ni and 1 to 5% Cr is alloy CI having good corrosion and heat resistance.
Most castings in CI must be stress relieved at 400-500oC because CI has a property to relieve locked in stresses after sometime. CI can be annealed by heating to 800-900oC to improve machinability. Cast iron can be quenched in oil to improve hardness. Such quenching treatment is often followed by heating to 300oC and cooling slowly.