Steel and Other Materials

STEEL

Steel is the most suitable building material among metallic materials. This is due to a wide range and combination of physical and mechanical properties that steels can have. By suitably controlling the carbon content, alloying elements and heat treatment, a desired combination of hardness, ductility and strength can be obtained in steel.

As per as carbon content is concerned, steel forms anIntermediate stage between cast iron and wrought iron. Cast ironContains carbon from 2 to 4 percent and wrought iron contains 0.15Percent. In steel the carbon content varies from 0.25 to 1.5 percent.

Type of steel	Carbon content (%)
Dead mild steel	< 0.15
Mild steel	0.15–0.3
Medium carbon steel	0.3–0.8
High carbon steel	0.8–1.5
or hard steel	(> 1 is also called cast steel or tool steel

A. The prominent steel-making processes are:

Bessemer process
Cementation process
Crucible process
Open Hearth process
Electric Smelting process
Duplex process
Lintz and Donawitz (L.D.) process

B. Properties and uses

1. Mild steel: Also known as low carbon or soft steel. It is ductile, malleable; tougher and more elastic than wrought iron. Mild steel can be forged and welded, difficult to temper and harden. It rusts quickly and can be permanently magnetised. The properties are: Sp. gr. = 7.30, ultimate compressive and tensile strengths 800–1200N/mm2 and 600–800N/mm2 . Mild steel is used in the form of rolled sections, reinforcing bars, roof coverings and sheet piles and in railway track.

2. High carbon steel: The carbon content in high carbon steel varies from 0.55 to 1.50%. It is also known as hard steel. It is tougher and more elastic than mild steel. It can be forged and welded with difficulty. Its ultimate compressive and tensile strengths are 1350 N/mm2 and 1400–2000 N/mm2 , respectively. Its Sp. gr. is 7.90. High carbon steel is used for reinforcing cement concrete and prestressed concrete members. It can take shocks and vibrations and is used for making tools and machine parts.

3. High Tensile Steel: The carbon content in high tensile steel is 0.6–0.8%, manganese 0.6%, silicon 0.2%, sulphur 0.05% and phosphorus 0.05%. It is also known as high strength steel and is essentially a medium carbon steel. The ultimate tensile strength is of the order of 2000 N/mm2 and a minimum elongation of 10 per cent. High Tensile steel is used in prestressed concrete construction

C. Properties of Steel:

The factors influencing the properties of steel are chemical composition, heat treatment, and mechanical work.

1.Chemical Composition:

The presence of carbon in steel gives high degree of hardness and strength. The addition of carbon to iron decreases the malleability and ductility of the metal, and reduces its permeability to magnetic forces.

The tensile strength of hot rolled steel bars is maximum between 1.0 and 1.2 per cent carbon.

The ductility of steel decreases as the carbon content increases.

The resistance of steel to heavy shocks or blows decreases with increase of carbon content.

2.Effect of principal impurities on steel:

It is not feasible to entirely remove impurities in making either iron or steel. The final product always contains small percentages of the metallic impurities like silicon, manganese, sulphur, and phosphorus besides iron and carbon.

In well made steel these impurities generally range between 0.2 and 1.0 per cent.

Of the common impurities, Phosphorus cannot be eliminated in the process of manufacture, whereas most of the silicon and manganese are introduced to improve the metal.

Copper: increases resistance to corrosion when present in small percentage.
Arsenic: has a tendency to raise the strength and brittleness.
Non-Metallic Impurities: are mechanically suspended in the metal and are often called slag inclusions causing brittleness.

3.Heat Treatment: The object of heat treatment is to develop desired properties in steel.

A steel of given composition may be made soft, ductile and tough by one heat treatment, and the same steel may be made relatively hard and strong by another. Heat treatment affects the nature, amount, and character of the metallographic properties.

Some of the principle purposes of heat treatment are as follows:

To enhance properties such as strength, ductility, hardness and toughness.
To relieve internal stresses and strains.
To refine the grain.
To remove gases.
To normalize steel after heat treatment.

4. Hardening:

The objective of this treatment may be to secure a given hardness to a desired depth in steel.

Fully hardened steel are not suitable for most commercial uses because they are hard and brittle and have poor toughness.

5. Tempering:

Since hardened steels do not usually have the combination of properties desired for specific uses, modification is affected by tempering.

When a thick piece of steel is cooled rapidly it develops additional strains as the surface cools quicker than the interior. To relieve this strain, steel is subjected to the process tempering which consists in slowly heating the steel to a predetermined subcritical temperature and then cooling it slowly.

6. Annealing:

It is a general term used for heating and slow cooling of metal, glass or any other material, which has developed strain due to rapid cooling.

D. Rusting and Corrosion:

When steel is exposed to atmosphere, it is subjected to action of atmospheric agencies. The humid air causes the rusting of steel (the formation of oxides on the surface of steel), also the atmospheric conditions along with rain produces oxidation and corrosion. Consequently, the physical and mechanical properties are affected.

To safeguard iron and steel from rusting and corrosion some of the prevalent methods are enamelling; applying metal coatings – galvanizing, tin plating, electroplating; and applying organic coatings – painting and coal tarring. Of these methods painting is the most common. Enamelling consists in melting a flux on the surface of iron in muffle furnace and then coating it with a second layer of more fusible glaze. Galvanising is the process of coating iron with a thin film of zinc, whereas in tin plating a film of tin is coated.

E. Alloy Steel:

In general, the properties desired in a metal to be used as building material are not present to the best advantage in any single metal. To develop specific properties a combination of metals or metallic substances is done and are classed as alloys. Some of the most common alloys, their properties and uses are given in Table

S. NO.	Alloy steel	Composition	Properties	Uses
1.	Stainless steel	Chromium 16%	Very hard and tough High elastic and ultimate strength Acid and rust proof	Ball bearings, dies, crushing machines, razors.
2.	Nickle steel	Nickel 3.5%	More elastic Higher tensile strength Lesser brittle than mild steel Improved hardness and ductility	Automobile and airplane parts
3.	Invar steel	Nickel 30 – 40%	Low coefficient of thermal expansion	Delicate instruments
4.	Vanadium steel	Vanadium 0.1 – 2%	High tensile and yield strength Resistance to softening at high temperatures	High speed tools, Locomotive castings, auto parts, chassis
5.	Tungsten steel	Tungsten 14 – 20%	High cutting hardness Resistant to abrasion	Drilling machines, high speed tools
6.	Manganese steel	Manganese 12 – 15%	Hard, tough and strong Difficult to machine high electrical resistance	Points and crossing in railways, rollers, jaws of crushers, Heavy earth and mining equipments
7.	Molybdenum steel	Molybdenum 0.2 – 0.3%	Maintains tensile strength at high temperatures	Gears, axles, shafts.

The important reasons for alloy additions are:

To increase the hardenability of steel. The steel in this group are usually heat treated by quenching and tempering, for it is only this way that the added expanse of the alloys can be justified through the better combination of properties that is obtained.
To strengthen the steel when it is to be used without special heat treatment. The steels that fall in this category are designed specifically for constructional purposes.
To confer some special property such as machinability, corrosion resistance wear resistance, etc.

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