Cast Iron and Types

CAST IRON

What is Cast Iron?

The iron carbon alloy with more than 2% of carbon and considerable amount of silicon is called Cast Iron.

It is produced by melting of pig iron in cupola furnace and its properties are modified by addition of their elements. The resulting product is cast and hence the name Cast Iron.

Composition of Cast Iron

• Carbon 2.5 to 3.7%;
• Silicon 1.0 to 3.0%;
• Manganese 0.5 to 1%;
• Phosphorous 0.1 to 0.9%;
• Sulphur 0.007 to 0.1%.

Carbon is present in iron as free graphite or as cementite. "Graphite in cast iron makes it soft and acts as lubricant, while cementite in cast iron makes it brittle and hard."

Silicon has strong graphitising effect; it helps to change cementite to graphite and also it helps from sound castings.

Manganese helps to remove sulphur to slag.

Phosphorous helps in increasing metal fluidity, but weakens the castings.

Sulphur is harmful impurity in cast iron. Sulphur decreases fluidity and increases shrinkage and cracking.

Advantages of Cast Iron.

• Low melting point ~1200 C, high fluidity and low shrinkage.
• Cheapness casting are produced with swiftly and inexpensively.
• Free graphite in cast iron acts as a lubricant.
• High compression strength with food damping capacity.
• Better corrosion and wear resistance.

Limitations of Cast Iron

• It is brittle.
• It cannot be forged or hardened.
• It is weak in tension.

Applications of Cast Iron

• Most of the castings.
• Brake drums
• Cylinder heads
• Machine tool beds
• Pulleys and Agriculture Implements.

Classifications of Cast Iron:

White Cast Iron

Composition of White Cast Iron

• Carbon 1.86 to 3.6%;
• Silicon 0.5 to 1.9%;
• Manganese 0.5 to 0.8%;
• Phosphorous 0.15%;
• Sulphur 0.06 to 0.18%.

White cast iron is usually made by limiting Si content below 1.3%. Here all the carbon content exist as cementite and the name white refers to the brittle fracture produced by the brittle constitute.

It is brittle and very hard. This makes it difficult to machine and also, they cannot be welded.

They must be grouped to shape. Because of this, use of white cast iron is limited to the parts requiring wear resistance.

Applications Include: It is used for rollers for crushers, plough shares, brake shoes and other similar parts subjected to heavy wear. The major use of white cast iron is to produce malleable cast iron.

Grey Cast Iron

Composition of Grey Cast Iron

• Carbon 2.4 to 4.0%;
• Silicon 1.0 to 3.0%;
• Manganese 0.25 to 1.0%;
• Phosphorous 0.05 to 1.0%;
• Sulphur 0.02 to 0.25%.

In the grey cast iron, the carbon is present in the form of graphite flakes in the matric of ferrite or pearlite. The pearlite one is stronger than ferrite one. Because of the grey appearance of its freshly fractured surface, it is called as Grey Cast Iron.

The shape, size and distribution of graphite flakes can have effect on the properties. The strength of the grey cast iron increases as the amount if graphite is reduced and fitness of the graphite flakes in increased.

Grey cast irons possess excellent compressive strength and machinability.

Very good wear resistance and very good damping capacity.

They also possess excellent casting properties such as low melting point, good fluidity and low shrinkage,

Further, the presence of graphite flakes provides lubrication effect to reduce friction.

Applications include: grates, pipes, agriculture implements, cylinder blocks, automobiles engines, brake drums and various machine parts.

Ductile Cast Iron

Composition of Ductile Cast Iron

• Carbon 3.0 to 4.0%;
• Silicon 1.8 to 2.8%;
• Manganese 0.1 to 1.0%;
• Phosphorous 0.1% max;
• Sulphur 0.03% max;
• Magnesium 0.015 to 0.1%.

In the ductile cast iron, the carbon is present in the form of Spheres or nodules. Hence it is also called as spheroidal or nodular cast iron.

The addition of magnesium or cerium in the melt prevents formation of graphite flakes during the cooling.Thus, the structure of ductile iron possess graphite nodules in the ferrite of pearlite matrix.

The resulting cast iron is more ductile and stronger than grey cast iron. “Sharp edges of grey cast iron create stress concentration areas.” Spheroidal shapes graphite nodules prevent stress concentrations.

If facilities ductility while the strength approaches as high as steel.

It also possesses significant toughness,

Superior wear resistance, good shock resistance and because of graphite presence, can also acts as solid lubricant.

Application includes: Hydraulic Cylinders, Valves Cylinder heads, Connecting rods and high-pressure pipes. Because of good compression and corrosion resistance it is used for steam plans and marine applications.

Malleable Cast Iron

Composition of Malleable Cast Iron

• Carbon 2.0 to 2.5%;
• Silicon 1.1 to 1.6%;
• Manganese 0.2 to 1.0%;
• Phosphorous 0.18% max;
• Sulphur 0.04 to 0.18%.

Malleable cast irons are produced by the heat treatment of white cast iron.

They possess good ductility combined with good strength.

They has high yield strength, high young’s modulus and very low thermal expansion coefficient.

Possess good wear and vibration damping resistance. 

Compact Graphite Cast Iron

It is also called as Vermicular Graphite Iron is named after the worm looking appearance of the graphite flakes.

The graphite looks different from ductile Iron because of its shorter and thicker appearance.

This results in higher adhesion between iron and graphite particles, hence its high tensile strength.

The material is light weight than cast iron and its application include necessity of higher strength

Applications includes: High speed train Disc brakes. Diesel V engines, Heavy automobiles engines where heavy combustion pressures are expected.

Alloy Cast Iron

Alloying elements are added to cast iron to give the desired qualities for special applications.

The common alloying elements are nickel, chromium, molybdenum and copper.

Cast iron consisting of these alloying elements are called as Alloy Cast Iron.

Alloy cast iron possess high corrosion and heat resistance. They also possess shock resistance combined with wear and abrasion. The specific effects of alloying elements are described here:

Nickel: (Austenite stabilizer and graphitizer) increases hardenability, impact strength and lowers the melting point of cast iron.

Chromium: (Carbide former) Increases hardenability, tensile strength, and hardness. High chromium content Cr> 12% imparts wear, corrosion and oxidation resistance.

Molybdenum: (Carbide former) Increases tensile strength, hardness and imparts resistance to corrosion at elevated temperature.

Copper: (Graphitiser) is generally added in combination with other alloying elements. It increases strength, hardness and resistance to atmospheric corrosion.

Element	Ductile Cast Iron%	Gray Cast Iron%	Malleable Cast Iron %	White Cast Iron%
Carbon	3.0 – 4.0%	2.5 – 4.0%	2.0 – 2.6%	1.8 – 3.6%
Manganese	0.1 – 1.0%	0.25 – 1.0%	0.2 – 1.0%	0.25 – 0.8%
Phosphorus	0.1  % Maximum	0.05 – 1.0%	0.18% Maximum	0.06 – 0.18%
Silicon	1.8 – 2.8%	1.0 – 3.0%	1.1 – 1.6%	0.5 – 1.9%
Sulphur	0.03% Maximum	0.02 – 0.25%	0.04 – 0.18%	0.06 – 0.20%

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