CAST IRONS

Cast irons are basically iron carbon alloys containing more than 2.04%C. Since high carbon content makes them brittle the carbon content is generally restricted to 2.5 -4%.

Si is always associated with it.

In as cast condition cannot be worked at any temperatures.

Properties in general:

(i) Poor ductility

(ii) Most of them are not malleable at any temperature.

(iii) Casting properties are very good.

Low melting point,

good flow properties;

can be cast into any complicated shape.

(iv) Cheap

(v) Properties can be enhanced by alloying, foundry control and heat treatment.

Classification:

According to metallographic structure ( considering Fe-C

content, purity, cooling behaviours, heat treatment after

casting);

(i) White cast iron: All carbon is in the combined form

(ii)Malleable cast iron: Most or all carbon is in uncombined

form, temper carbon.

(iii)Grey cast iron: Most or all carbon is in the form of

graphite flakes.

(iv)Spheroidal graphite (SG) Iron :Carbon is present as

nodules of graphite.

White Cast Iron:

They are hypoeutectic in nature with cementite particles instead of

graphite particles

Cementite is in large amount and is a continuous inter dendritic network

Fractured surface shows white appearance bcoz of presence of more

cementite-hence called white cast irons.

Solidification follows Fe-C diagram rather than Fe-graphite diagram.

The proeuctectoid cementite makes the material hard, wear resistant but

brittle. Hence it has limited applications.

Typical Composition:C: 2.5-3.5% Si: 0.4-1.5%, Mn: 0.4-0.6%,P: 0.1-0.4%, S: 0.15, Fe: balance

Uses: raw material for malleable cast iron (Si <1.4%), cement mixers liners, ball mills, extrusion nozzles.Blades, pulverizers, rail wheels

T.S: 140-490 MPa, C.S: 1400-1750 MPa, H: 375-600 BHN.

Malleable Cast iron Fe3C is a metastable phase. There is a tendency to decompose according to,

Fe3C = 3Fe + C

Production:

White Cast iron is heated slowly to 900-950 °C

Fe3C + pearlite = Fe3C + γ

During this heating, pearlite transforms to γ and graphite nucleates at the γ-

Fe3C Interface.

As T increases more Fe3C is dissolved.

Holding at 900-950oC (20-70 hrs), graphitization continues.

As graphitization occurs γ gets depleted of C and draws C from Fe3C which

results in more dissolution of Fe3C.

At the end of this stage we will have Fe3C in γ matrix. This is called first

stage of graphitization (FSG).

After FSG, the casting is cooled rapidly to 730 -740oC. From this stage the castings are cooled at a very very slow rate (5-10oC/hr) – called second stage graphitization (SSG). During this slow cooling γ transforms to ferrite and graphite. We end up in following microstructure.

Pearlitic malleable cast iron can be obtained either by adding Mn which promotes pearlite or after SSG the castings are quenched. Property Ferritic Pearlitic

T. S. (MPa) 390 450-700

Y. S. (MPa) 250 280-500

% elongation 15 % 16 %

Hardness (BHN)

115 160-210

Applications;Ferritic: Automotive industry, Agriculture, Pipe fittingsPearlitic: Rolls, pumps, Nozzles, Crank shafts, clamps, wrenches.

Typical Composition: C: 2.0-2.65% , Si: 0.9-1.4%, Mn: 0.25-0.55%, P: <0.18%, S: 0.05, Fe: balance.

Grey Cast Iron

Because of high Si, Fe3C becomes unstable and the system

follows Fe-graphite phase diagram.

Generally hyper eutectic alloys

Eutectic reaction: L= γ + graphite.

Solidification sequence, ( very slow cooling)

L = L + γ = γ + eutectic = graphite + pearlite.

Large amount of free carbon in free state as graphite flakes.

This gives grey colour appearance on the fractured surface.

Hence called so.

Some Fe3C of pearlite also may decompose giving graphite

flakes surrounded by ferrite in a pearlitic matrix. If Si and C

content is too high then matrix will be entirely ferritic.