MATERIALS AND METALLURGY - TYPES OF CAST IRON

 

TYPES OF CAST IRON

Cast Iron

Contents show

The cast iron is obtained by re-melting pig iron with coke and limestone in a furnace known as a cupola. It is an alloy of iron and carbon. The carbon contents in cast iron differ from 1.7% to 4.5%. Also, it contains a very small amount of silicon, sulfur, manganese, and phosphorus.

Carbon is present in the following two forms:

1. Free carbon or graphite
2. Combined carbon or cementite

Following are the important properties of cast iron that make it a valuable material for engineering purposes.

1. Low cost.
2. Good casting characteristics.
3. High compressive strength.
4. Wear resistance.
5. Excellent machinability.

The cast iron compressive strength is greater than tensile strength.

Following are the values of the ultimate strengths of cast iron.

1. Tensile – 100 to 200 N/mm2
2. Compressive – 400 to 100 N/mm2
3. Shear – 120 N/mm2

Types of Cast Iron

The following are types of cast iron used for engineering purposes:

1. Grey cast iron
2. White cast iron
3. Chilled cast iron
4. Mottled cast iron
5. Malleable cast iron
6. Nodular cast iron
7. Alloy cast iron

#1 Grey Cast Iron

Grey cast iron is commercial iron has the following compositions:

It consists of Carbon – 3 to 3.5%, Silicon – 1 to 2.75%, Manganese – 0.40 to 1%, Phosphorus – 0.15 to 1%, Sulphur – 0.02 to 0.15% and the remaining is iron.

By Hb tuw – Own work, CC BY-SA 4.0,
https://commons.wikimedia.org/w/index.php?curid=45350398

The grey color is due to the fact that carbon is present as free graphite. Grey cast iron can be easily machined because it has high compressive strength, low tensile strength, and no ductility.

A good property of grey cast iron is that free graphite in its structure acts as a lubricant. For this reason, it is highly suitable for parts where sliding action is desired. The grey iron castings are broadly used for machine tool bodies, pipes and pipe fittings, automobile cylinder blocks, and agricultural implements.

Grey cast iron composition: casting quality – PDF

#2 White Cast Iron

White cast iron shows a white fracture and have the following relative compositions:

It consists of Carbon – 1.75 to 2.3%, Silicon – 0.85 to 1.2%, Manganese – 0.10 to 0.40%, Phosphorus – 0.05 to 0.20%, Sulphur – 0.12 to 0.35%

Image credit: exportersindia.com

Because the carbon is in the form of cementite in metals the white color occurs. Cementite is the hardest element of iron and it causes quick cooling of the molten iron. The white cast iron has high tensile and low compressive strength therefore it cannot be machined.

It is utilized for inferior casting and places where the hard coating is required, such as the outer surface of car wheels. In the production of malleable cast iron and wrought iron white cast iron is also used as a raw material.

#3 Chilled Cast Iron

It is white cast iron produced by quick cooling of molten iron. The quick cooling is generally called chilling and the iron so produced is known as chilled iron. All casting moulds have contact with molten iron with cool sand on their outer skin. But on most casting, this hardness penetrates to a very small depth (less than mm).

Image Credit: indiamart.com

Sometimes casting is chilled intentionally and sometimes chilled becomes accidentally to a considerable depth. Intentional chilling is done by inserting iron or steel inserts into the mold. When the molten metal comes into contact with the chill, its heat is rapidly conducted away and the hard surface is formed.

Chills are used on any face of the casting that has to be hard to withstand wear and friction. The process of chilling is used in the casting of roles for crushing grains and jaw crusher plates. The running surface of rail-carriage wheels is also chilled.

4. Mottled Cast Iron

It is a product between grey cast iron and white cast iron in composition, color, and properties. It is obtained in casting where certain wearing surfaces have been chilled.

5. Malleable Cast Iron

The malleable cast iron is obtained from white cast iron by a suitable heat treatment process (i.e., annealing). The annealing process separates the combined carbon of the white cast iron into nodules of free graphite.

Image credit: homedepot.com

The malleable cast iron is ductile and may be bent without breaking or fracturing the section. Its tensile strength is higher than grey cast iron and has excellent machining qualities. It is used to make machine parts for which steel forgings will be expensive and must have a fair degree of mental accuracy.

Example: hubs of wagon wheels, small fitting for railway rolling stock, brake supports, parts of agricultural machinery, pipe fitting, door hinges, locks, etc.

#6 Nodular Cast Iron

It is also known as ductile cast iron, spheroidal graphite (i.e., S.G.) cast iron or high-strength cast iron. The nodular cast iron is produced by adding magnesium to the molten cast iron. Magnesium converts graphite of cast iron from a flake form to a spherical or nodular form.

By Michelshock – McGill University, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6464476

This way, the mechanical properties are considerably improved. The nodular cast iron behaves like steel. It is usually used for pressure-resisting casting, hydraulic cylinders, cylinder heads, rolls for rolling mill, and centrifugally cast products.

7. Alloy Cast Iron

The cast iron discussed above is called plain cast irons. The alloy cast iron is produced by adding elements like nickel, chromium, molybdenum, copper, silicon, and manganese.

Image credit: indiamart.com

These alloys give greater strength and result in improved element properties.

The alloy cast iron has,

1. Carbon – 0.02%
2. Silicon – 0.15%
3. Manganese – 0.03%
4. Phosphorus – 0.12%
5. Sulphur – 0.02%
6. Slag – 3% (by weight) and the remaining is iron.

Effect of Impurities on Cast Iron

We have discussed that cast iron contains small percentages of silicon, sulfur manganese, and phosphorus. The effect of these impurities on cast iron is as follows:

1. Silicon
2. Sulfur
3. Manganese
4. Phosphorus

1. Silicon

It is present in cast iron up to 4%. Silicon provides the formation of free graphite that makes the iron soft and easily machinable. Silicon produces sound casting free from blow holes in the material due to its high affinity for oxygen.

2. Sulfur

It makes the cast iron hard and brittle. Since too much sulfur gives unsound castings, therefore, it should be kept well below 0.1% for most foundry purposes.

3. Manganese

Manganese makes the cast iron white and hard. It is often kept below 0.75%. This helps to control influence over the harmful effect of sulfur.

5. Phosphorus

It helps fusibility and fluidity in cast iron but causes brittleness. It is rarely allowed to exceed 1%. Phosphoric irons are the useful casting of intricate design and for many light engineering casting when cheapness is essential.

Properties

Following are the properties of cast iron:

1. Cast iron has good corrosion resistance, so it can be used for water-carrying pipes, etc.
2. It does not get attracted to a magnet.
3. The melting point is about 1250 ºC.
4. It has a specific gravity of 7.5.
5. Cast iron contains tensile strength of about 150N/mm2 and the compressive strength is about 600N/mm2. Therefore, it is good at compression.
6. It becomes soft when kept in saltwater and shrinks when cooled.
7. Cast iron may not be useful for forging work due to its lack of plasticity.

Advantages of Cast iron

1. It has good high machinability and casting properties.
2. Cast iron has good wear resistance as well as good vibration damping.
3. It can withstand greater loads and has a good degree of resistance against corrosion.
4. Cast iron has low tensile strength and elongation properties.
5. It can be given any complicated shapes and sizes without using expensive machining operations.
6. Cast iron is available at low cost and has low durability.

Disadvantages of Cast iron

1. Cast iron has poor tensile strength.
2. Cast iron parts are section sensitive, due to the slow cooling of thick sections.
3. The failure of its parts is sudden and total, it does not reflect the yield point.
4. When compared to steel it has poor machinability
5. Cast iron has high brittleness.

Applications of Cast iron

1. It is used in making pipes and for carrying suitable fluids.
2. The castiron is used for making various machines.
3. Used for making automotive parts.
4. To build the anchor for ships cast iron is used.
5. Cast iron is also used to make cooking appliances such as pots and pans and utensils.