Engineering Materials Metals

Metals used in Civil Engineering

All metals used for engineering works are classified into

1. Ferrous metals
2. Non-Ferrous metals: Wherein iron is not the main constituent (Copper, Aluminum, Zinc and lead etc)

A) Ferrous metals:

Wherein iron is the main constituent (Cast iron, wrought iron and different forms of steels)

Ferrous metals ► not directly obtained from iron ores

A-1) PIG IRON:

From iron ore ► impure form of metal ► Pig iron

It is the pig iron which further yields “Ferrous metals”

Pig iron is not suitable for any mechanical use unless it is converted into cast iron, wrought iron or steel

A-2) CAST IRON:

Pig iron ► re melted with limestone and coke and poured into moulds of desired shapes and sizes to get purer product known as cast iron

Carbon content in cast iron varies from 2 to 5%

During re melting of pig iron ► scrap iron may also be added for economy

Properties of Cast Iron

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1. It is brittle, non ductile, non malleable and cracks when subjected to shocks
2. It cannot be magnetized
3. It does not rust
4. It is strong in compression but weak in tension and shear
5. Its melting point is 12000C
6. Its specific gravity is 7.5

USES

Weak in tension therefore cannot be used in construction

Can be used for parts of pumps, motors, engines etc

Because of corrosion resistance ►can be used for pipes to some extent

A-3) WROUGHT IRON

When pig iron is melted in such a way as to remove all of the carbon and other impurities, the result is wrought iron

Good quality wrought iron contains 99.5 % iron, less than 0.1 % of Silicon, 0.01 % of Sulfur, 0.07 % of phosphorus and 0.03 % of manganese

Properties of Wrought Iron

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1. Wrought iron is very malleable and ductile
2. Its tensile strength is 20-26 tons /in2
3. It is strong in compression but not so strong as steel
4. It can be easily worked, welded and is tough
5. Its melting point is 28000F
6. Wrought iron became pasty and very plastic at red heat and could be easily forged at about 16500F

USES:

Since mild steel has replaced the wrought iron, therefore it is no longer produced in large extent. Still in use for roof sheets, wires and metal ornaments etc

A-4) STEEL

Steel is an alloy of iron and carbon. Pure iron’s strength remarkably increases when alloyed with carbon. The tensile strength increases with increasing carbon content but the ductility reduces. Steel having its properties because of the presence of carbon alone is called “Plain carbon steel”

PLAIN CARBON STEEL can further be classified as

1. Low carbon steel or mild steel:

The carbon content does not increases 0.25%

Soft and ductile ► mostly used for construction purpose

Uses ► Sheets, rods, wires, pipes, hammers, chains, shafts etc

2. Medium-carbon steel :

The carbon content is 0.25 to 0.5 %

Stronger than the mild steel slightly less ductile

Uses ► Shafts, connecting rods and rails etc

3. High- carbon steel :

Carbon content is above 0.5%

Harder and stronger than mild steel and medium carbon steel

Uses ► Keys, knifes, drills etc

Properties of Mild Steel

1. Ductile and malleable
2. It corrodes quickly
3. It can be permanently magnetized
4. It is tough and more elastic than cast iron and wrought iron and withstands shocks and impacts well
5. It is equally strong in tension, compression and shear
6. Its specific gravity is 7.8
7. It is not much affected by Saline water

Properties of High-carbon Steel

1. Its structure is granular
2. It is more tough and elastic than mild steel
3. It is easier to harden and then to weld
4. It is more difficult to forge and then to weld
5. It can be permanently magnetized
6. Comparatively it is stronger in compression than in tension or in shear
7. It withstands vibration and shocks better

MANUFACTURE OF STEEL

Three basic raw materials are needed in large quantities for the production of steel

The first step in the steel manufacture begins at the blast furnace. To separate iron from iron ore ► coke (substance when gas is taken out of coal), limestone and dolomite are charged into the blast furnace

Temperature raised to 1600oF. This high temp causes the coke to burn and melt the iron. This red hot iron drained at an opening at the base of the furnace. Natural gas is often injected to reduce the amount of coke consumed. The dolomite and limestone combine with the non-ferrous elements of the ore to form a slag, which floats on the top of the molten iron and is removed separately. The product of the blast furnace is known as “Pig Iron” the basic ingredient of steel

It takes 2 tons of iron ore, 2/3 ton of coke, ½ ton of limestone, 4 tons of air to make 1 ton of Pig iron. Some of the pig iron goes to the foundries to make iron castings, but the vast majority is re melted and used in the production of steel in steel furnace. Several types of furnaces are used for the production of steel including