Steel

Building Material Civil Engineering

Percentage of carbon

• Steel     0.2 – 2.1
• Stainless steel    < 1.2
• Cast iron    1.8 – 4.5
• Pig Iron    3.5 - 4.5
• The impure form of iron, containing carbon in between 2.0 to 4.5% is known as cast iron.

Different forms of iron are as follows:

Wrought iron: 

• It is a very pure iron where the iron content is of the order of 99.5%. It is produced by re-melting pig iron and some small amount of silicon, sulfur, or phosphorus may be present. It is tough, malleable and ductile and can easily be forged or welded. It cannot, however, take sudden shock. Chains, crane hooks, railway couplings, and such other components may be made of this iron.

Pig iron: 

• It is an intermediate product of the iron industry. It has a very high carbon content, typically 3.8–4.7% (3 to 4%), along with silica and other constituents of dross, which makes it very brittle, and not useful directly as a material except for limited applications.

Cast iron: 

• It is an alloy of iron, carbon, and silicon and it is hard and brittle. Carbon content in CI may be within 1.7% to 3% and carbon may be present as free carbon or iron carbide Fe3C. In general, the types of cast iron are (a) grey cast iron and (b) white cast iron (c) malleable cast iron, etc.

Steel :

• It is basically an alloy of iron and carbon in which the carbon content can be less than 1.7% and carbon is present in the form of iron carbide to impart hardness and strength. 

Two main categories of steel are

• (a) Plain carbon steel and 
• (b) alloy steel.

Name of Steel and its Carbon Content and Uses

Mild steel 

Up to 0.10% 

• Motor body, sheet metal,tin plate, etc.

Up to 0.25%

•  Boiler plates, structural steel, etc.

Medium carbon steel

Up to 0.45%

•  Rails, tyres, etc.

Up to 0.60%

•  Hammers, large stamping and pressing dies, etc.

High carbon steel or hard steel

Up to 0.75% 

• Sledge hammers, springs, stamping dies, etc.

Up to 0.90% 

• Miner’s drills, smith’s tools, stone mason’s tools, etc.

Up to 1.00% 

• Chisels, hammers, saws, wood working tools, etc.

Up to 1.10% 

• Axes, cutlery, drills, knives, picks, punches, etc.

Defects in Steel

The following four types of defects are found in steel.

• 1. Cavities/blow-holes: Cavities or blow-wholes are formed when gas is confined in the molten mass of metal. This confined gas produces bubbles or blow- holes on solidification of metal.

• 2. Cold shortness: Steel with this type of defect cracks when worked in cold state. This defect imparts due to presence of excess amount of phosphorusin the steel.

• 3. Red shortness: Steel having this defect cracks when worked in hot state. This defect happens due to presence of excess amount of sulphur in the steel.

• 4. Segregation: Some constituents of steel solidify at an early stage. Those are separated out from the main mass. This process is known as segregation.

Steel Manufacturing Processes

The following processes are used in manufacturing of steel.

• 1. Bessemer process

• 2. Cementation process

• 3. Crucible steel process

• 4. Duplex process

• 5. Electric process

• 6.LD Process (Linz-Donawitz process)

• 7. Open-hearth process

Market Form of Steel

Various forms of steel available in the market are:

• 1. Angle sections

• 2. Channel sections

• 3. Corrugated sheets

• 4. Expanded metal

• 5. T-sections

• 6. I-sections

• 7. Plates

• 8. Ribbed (HYSD) bars

• 9. Round bars

• 10. Square bars

• 11. Flat bars

• 12. Ribbed mild steel bars

• 13. Thermo-mechanically treated (TMT) bars

• 14. Cold twisted deformed (CTD) bars

• 15. Welded wire fabrics (WWF)

Mechanical Treatment of Steel

• • The purpose of giving mechanical treatment to steel is to give desired shape to the ingots to make steel available in marketable forms.
• • Mechanical treatment of steel requires the following

Types of  operations.

Drawing

• This operation is carried out to reduce the cross-section and to increase the length proportionately.
• This process is used to prepare wires and rods.

Forging

• This operation is carried out by giving repeated blows under a power hammer or press.
• This increases the density and improves grain size of
• metal.
• Forging is used for manufacture of bolts, cramps and in riveting, etc.

Pressing

• It is a slow process, carried out using in equipment known as press.
• The main advantage of this process is it does not involve any shock. Pressing is useful in manufacturing large  number of uniform type engineering products.

Rolling

• It is carried out in specially prepared rolling mills.
• Using rolling process, various shapes, such as angles,channels, flats, joists, rails, etc., are manufactured

Mild Steel

• It has fibrous structure.

• It can be easily hardened and tempered.

• It can be readily forged and welded.

• It can be permanently magnetised.

• Melting point of mild steelis about 1400°C.

• Its specific gravity is 7.80.

• Its ultimate compressive strength is 20 kN/cm 2 .

• Its ultimate tensile and shear strengths are about 60–80 kN/cm 2 .

• It is tougher and more elastic than wrought-iron.

• Used for all types of structural work.

Hard steel

• It cannot be easily hardened and tempered.

• Used for manufacturing cutlery,edge tools, parts exposed to shocks and vibrations.

• It is tougher and more elastic than mild steel.

• Its ultimate tensile strength is about 80–110 kN/cm 2 and ultimate shear strength is about 110 kN/cm 2 .

• Its ultimate compressive strength is 140–200 kN/cm 2 .

•  Its specific gravity is 7.90.

• Melting point hard steel is about 1300°C.

• It can be permanently magnetised.

• It cannot be readily forged and welded.

•  It has granular structure.