Classification of Cast Iron:
CI having C in form of cementite is named as white cast iron. Microstructure of that CI consists of pearlite, cementite and ledeburite. Whether C content is less than 4.3 percent it is hypoeutectic CI and if C is superior to 4.3% it is hypereutectic CI. White cast iron has high hardness and wears resistance and is very complex to machine. This can be ground, even if. Hardness of white CI varies in between 300-500 BHN and UTS between 140-180 MPa. White CI is generally sand cast to produce such parts like pump liners, grinding balls, mill liners, etc.
Cast iron having carbon in form of graphite flakes dispersed in matrix of pearlite or ferrite is categorized as gray cast iron. The identity is derived from the fact such a fracture surface appears gray. Gray cast iron is different in percentage of Si from white cast iron while C percentage is almost similar. The liquid alloy of proper composition is cooled gradually in sand mould be decompose Fe3C into Fe and C out of that C is precipitated like graphite flakes. Addition of Si, Ni or Al accelerates graphitization. The graphite flakes differ in length from 0.01 to 1.0 millimeter. The flakes offer an easy passage to cracks hence not allowing softer microstructure to deform plastically. Superior flakes reduce ductility and strength. The best properties of gray cast iron are acquired with flakes oriented and distributed randomly. inoculant agent as like: metallic Al, Ti, Ca SiC, and Zr and CaSi while added in minute amount, cause formation of smaller graphite flakes and random orientation and distribution.
Gray cast iron is basically brittle along with hardness varying in between 149 to 320 BHN and UTS of 150 to 400 MPa. Various properties are acquired by varying cooling rate and quantity of inoculant agents. This has excellent fluidity, high damping machinability and capacity. If gray CI is repeatedly heated operating to about 400oC suffers from permanent expansion named as growth. As per with dimensional changes are less of ductility and strength as a consequence of growth. While locally heated to about 550oC various times this material enhances what are named as fire cracks resulting into failure.
High strength gray cast iron is acquired by addition of strong inoculating agent as like: CaSi to liquid metal before casting procedure. UTS in the range of 250 to 400 MPa are acquired. This cast iron is named as Meehanite iron and can be toughened via oil quenching treatment to UTS of 520 MPa.
If graphite in cast iron is illustrate in form of nodules or spheroids in the matrix of ferrite or pearlite the material is named as nodular cast iron. Such cast iron has marked ductility offering product the advantage of steel, and procedure the advantage of cast iron. This is mostly a gray cast iron whether C varies in between 3.2 to 4.1%, Si in between 1.0 to 2.8% whilst S and P are restricted to 0.03 to 0.1% respectively. Nickel and Mg are added as alloying elements. Crank shafts, sheet and punch metal, metal working rolls, dies and gears are made out of nodular CI. The defects as like: growth and fire cracks are not determined in such class of iron. It makes it proper for furnace doors, steam plants and sand casting. This also possesses good corrosion resistance making it helpful in marine applications and chemical plants, petroleum industry.
White CI having 2.0 to 3.0 percent C, 0.9 to 1.65 percent Si, < 0.18% S and P, some Mn and < 0.01% Bi and B can be heat treated for 50 hours to some days to generate temper carbon in the matrix of pearlite or ferrite imparting malleability to CI. This class is termed as malleable cast iron and can have as elevated as 100 MPa of UTS and 14 percent elongation. Because of such properties as strength, machinability, ductility, and wear resistance and convenience of casting in several shapes, malleable CI is largely employed for automotive parts as like: crank and cam shafts, shaft brackets, steering brackets, brake carriers and in electrical industry also like switch gear parts, fittings for low and high voltage distribution and transmissions system for railway electrification.
Addition of alloying elements as like: Ni and Cr give shock and impact resistance through corrosion and heat resistance of cast iron. Such are named as alloyed CI 3 to 5 percent Ni and 1 to 3% Chromium produce Ni-hard CI along with hardness upto 650 BHN and modified Ni-hard CI along with impact and fatigue resistance is generated by adding 4.8 percent Ni and 4.15% Cr. Ni-resist CI along with 14 to 36% Nickel and 1 to 5% Cr is alloy CI comprising good corrosion and heat resistance.
A large amount castings in CI should be stress relieved at 400-500oC since CI has a property to relieve locked in stresses after a time. CI can be annealed via heating to 800-900oC to enhance machin-ability. Cast iron can be quenched in oil to enhance hardness.