Cast Iron

This iron plays very significant role in engineering practice, as this is one of the commonest materials for manufacturing of machines and its parts. It is being utilized because 14th century while steel came in employ during 19th century. Casting is a process whether molten metal is poured in a mould and on solidifying the casting of the shape of mould is acquired. Common properties of cast iron are as:

(a) Cheap material.
(b) Lower melting point at 1200°C as compared to steel 1,380 to 1,500°C.
(c) Good casting properties, for illustration: high fluidity, sound casting, low shrinkage, ease of production in large number.
(d) Good in compression but Cast iron along with ductility is available also.
(e) Cast iron is machine-able is most cases.
(f) Abrasion resistance is remarkably high.
(g) Extremely significant property of Cast iron is its damping characteristic that isolates vibration and makes such good material for foundation and housing.
(h) Alloy Cast iron may be better against corrosion.

Cast iron is prepared from melting pig iron in cupola furnace or in electric furnace. Electric furnace gives better quality.

Cast iron contains various elements in addition to Fe. The carbon content of Cast iron is more than 2%. Si Varies in between 0.5 to 3.0%. This is very significant as this controls the form of C in Cast iron. S content in Cast iron varies in between 0.06 to 0.12% and is largely present as FeS, which tends to melt at comparatively low temperature causing hot shortness. Manganese inhibits formation of FeS.

Although P increases fluidity of Cast iron

-a property useful for pouring
- It has to be restricted to 0.1 to 0.3% since it reduces toughness. P is present in the form of FeP.

Mn in cast iron varies from 0.1 to 1 .O% while such a small Mn, does not affect properties of cast iron. This certainly helps enhance upon hot shortness by taking care of S.

Some other alloying elements as like Nickel, Chromium, Mo, Mg, Copper and Vanadium may be added to cast iron to acquire several desirable properties.

Cast iron containing C in form of cementite is named as white cast iron. Microstructure of this Cast iron consists of cementite pearlite, and ledeburite. If Carbone content is less than 4.3% it is hypoeutectic Cast iron and if C is greater than 4.3% it is hypereutectic Cast iron. White cast iron has high stiffness or hardness and wear resistance and is extremely difficult to machine, this can be ground. Though, hardness of white Cast iron varies in between 350-500 BHN and UTS in between 140-1 80 Mpa. White Cast iron is normally sand cast to produce this part as pump liners, grinding balls, mill liner.

Cast iron containing carbon in form of graphite flakes dispersed in matrix of ferrite or pearlite is classified like grey cast iron. The title is derived from the fact that a fracture surface shows gray. Gray Cast iron differs in %age of Si from white cast iron while C %age is almost similar. The liquid alloy of proper composition is cooled slowly or gradually in sand mould to decompose Fe3C into Fe and C out of which C is precipitated as graphite flakes. Addition of Si, Alluminium or Nickel accelerates graphitisation. The graphite flakes vary in length from 0.01 to 1.0 millimeter. Larger flake decreases strength and ductility. The good properties of gray Cast iron are acquired with flakes distributed and oriented in random. Inoculants agents such as metallic Alluminium, Ca, Zr, Ti, and SiC and CaSi while added in small amount reason formation of smaller graphite flakes and randomly orientation and distribution.

Grey Cast iron is basically brittle with stiffness or 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 inoculants agents. This has excellent fluidity, high damping capacity and machine ability. If grey Cast iron is repeatedly heated in service to about 400°C, this suffers from permanent expansion named as growth. Associated along with dimensional change are loss of ductility and strength like a result of growth or development. When locally heated is about 550°C sometime this material enhances what are named as fire cracks resulting into failure.
Grey Cast iron is used for clutch plates, for brake drums, and for beds of machine tools and for equipments, for counter weights in elevator and for furnace, as well as for gear reducer casing, pump housing, motor housing, turbine housing, engine frames, pistons and cylinders.

Iron castings are several times cooled rapidly on the surface to get white Cast iron structure while inside is permitted to cool slowly to obtain gray Cast iron structure. This combination is named as chilled or mottled Cast iron. Metal or graphite chillers are utilized to chill the casting on outer surface. High stiffness or hardness and wear resistance on the surface and low stiffness or hardness and Engineering Design strength at the core are acquired in chilled Cast iron. Railroads freight car wheel, gram mill rolls, rolls for crushing ores, for hammers are made in chilled Cast iron.

High strength grey Cast iron is acquired by addition of strong inoculating agent as like CaSi to liquid alloy before casting process. In the range of 250 to 400 Mpa UTS is acquired. This Cast iron is called meehanite iron and can be toughened by oil quenching treatment to UTS of 520 Mpa.

If graphite in cast iron is present in form of nodules or spherods in the matrix of pearlite or ferrite the material is named as nodular cast iron. The Cast iron has a marked ductility providing product the advantage of steel and process benefit of Cast iron. It is mostly a grey Cast iron whether Carbon varies in between 3.2 to 4.1%, Si in between 1 .O to 2.8% while S and P are restricted to 0.03 and 0.1%, respectively. Nickel and Mg are added as alloying elements. Crankshafts, nietal working rolls, punch and sheet metal dies and gears are made out of nodular Cast iron. The defects as like growth and fire cracks are not found in such class of iron. It makes it appropriate for steam plants, sand casting and furnace doors. This possesses also good corrosion resistance making this useful in chemical plans, petroleum industry and marine applications.

White Cast iron containing 2.0 to 3.0% carbon, 0.9 to 1.65% Silicon, < 0.1 8% S and P, some Manganese and < 0.01 % Bi and B can be heat treated for 50 hours to some days to produce temper carbon in the matrix of ferrite or pearlite imparting malleability to Cast iron. This class is termed as malleable cast iron and can contain a high as 100 MPs of UTS and 14% elongation. Because of such properties as strength, ductility, machine ability and wear resistance and convenience of casting in different shapes, malleable Cast iron is largely utilized for automotive parts such a crank and can? Shafts, steering brackets, shaft brackets, brake curriers and in electrical industry also as switch gear parts, fittings for low and high voltage transmissions and distribution system for railway electrification.

Addition of alloying elements as like Nickel and Chromium provide shock and impact resistance with corrosion and heat resistance to Cast iron. These are named as alloyed Cast iron 3 to 5% Nickel and 1 to 3% Chromium produces Nickel-hard Cast iron with hardness upto 650 BHN and modified Nickel-hard Cast iron with impact and fatigue resistance is produced by adding 4-8% Nickel and 4-15% Chromium. Nickel-resist Cast iron with 14 to 36% Nickel and 1 to 5% Cr is alloy Cast iron having good corrosion and heat resistance.