Steel

STEEL

Steel is an alloy of iron and carbon containing less than 2% carbon and 1% manganese and small amounts of silicon, phosphorus, sulphur and oxygen. Steel is the world's most important engineering and construction material. It is used in every aspect of our lives; in cars and construction products, refrigerators and washing machines, cargo ships and surgical scalpels. It can be recycled over and over again without loss of property.

PROPERTIES OF STEEL

Steel has a number of properties, including: hardness, toughness, tensile strength, yield strength, elongation, fatigue strength, corrosion, plasticity, malleability and creep.

❖ Hardness is the material's ability to withstand friction and abrasion, and it is different from strength and toughness in the context of metal properties.

❖ Toughness is the ability to absorb energy without fracturing or rupturing. It is also defined as a material's resistance to fracture when stressed. It is usually measured in Joules per sqcm.

❖ Yield strength is a measurement of the force required to start the deformation of the material (bending or warping).

❖ Tensile strength is a measurement of the force required to break the material.

❖ Elongation is the "degree" to which the material can be stretched or compressed before it breaks. It is expressed as a percent of the length being tested and the value lies between the tensile strength and yield strength.

❖ Fatigue strength is the highest stress that a material can withstand for a given number of cycles without breaking.

❖ Corrosion is the destruction of the steel material and its vital properties due to the electrochemical or chemical reaction of its surface to environmental factors such as acids, moisture and oxygen.

❖ Plasticity is the deformation of a material undergoing non-reversible changes of shape in response to applied forces.

❖ Malleability describes the property of a metal's ability to be distorted below compression. It is a physical property of metals by which they can be hammered, shaped and rolled into a very thin sheet without rupturing.

❖ Creep is a type of metal deformation that occurs at stresses below the yield strength of a metal, generally at elevated temperatures.

TYPES OF STEEL

❖ Mild steel: It is 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 magnetized. Its specific gravity is 7.3. Ultimate compressive and tensile strengths ranges from 800 to 1200 N/mm2 and 600 to 800 N/mm2 respectively. Mild steel is used in the form of rolled sections, reinforcing bars, roof coverings and sheet piles and in railway tracks.

❖ 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 Specific gravity is 7.9. 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.

❖ 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 %. High Tensile steel is used in prestressed concrete construction.

DEFECTS IN STEEL

The common defects in steel are as follows:

❖ Cavities or blow-holes

❖ Cold shortness

❖ Red shortness

❖ Segregation.

* Cavities or Blow-Holes: These are formed when gas is confined or imprisoned in the molten mass of metal. Such confined gas produces bubbles or blow-holes on solidification of metal.

* Cold Shortness: The steel, having this defect, cracks when being worked in cold state. This defect is due to the presence of excess amount of phosphorus.

* Red Shortness: The steel, having this defect, cracks when being worked in hot state. This defect is due to the presence of excess amount of sulphur.

* Segregation: Some constituents of steel solidify at an early stage and they separate out from the main mass. This is known as the segregation and it is prominent on the top surface of the ingots or castings

 
MECHANICAL TREATMENT OF STEEL

The purpose of giving mechanical treatment to the steel is to give desired shape, so as to make steel available in market forms. The mechanical treatment of steel may be hot working or cold working. The hot working is very common. Following are the operations involved in the mechanical treatment of steel:

* Drawing

* Forging

* Pressing

* Rolling

❖ Drawing: This operation is carried out to reduce the cross-section and to increase the length proportionately. In this operation, the metal is drawn through dies or specially shaped tools. The drawing is continued till wire of required diameter or cross-section is obtained. This process is used to prepare wires and rods.

❖ Forging: This operation is carried out by repeated blows under a power hammer or a press. The metal is heated above the critical temperature range. It is then placed on anvil and subjected to blows of a hammer. This process increases the density and improves grain size of metal. The riveting belongs to forging operations. The process is used for the manufacture of bolts, cramps, etc. The steel may be either forged free or die-forged. In the former case, the steel is free to spread in all directions as it is hammered. In the latter case, the steel flows under the blows of a hammer to fill the inside of a die and the excess material are forced out through a special groove and then it is cut off. The die- forged parts have very accurate dimensions.

❖ Pressing: This is a slow process and it is carried out in equipment known as the press. The main advantage of this process is that it does not involve any shock. A press consists mainly of a die and a punch. The die and punch are suitably shaped to get article of desired shape. The metal is placed on the die and punch is then lowered under a very heavy pressure. The metal is thus pressed between die and punch and article of desired shape is obtained. For preparing articles with wide changes of shape, the pressing is to be carried out in different stages. This process is useful when a large number of similar engineering articles are to be produced.

❖ Rolling: This operation is carried out in specially prepared rolling mills. The ingots, while still red hot, are passed in succession through different rollers until articles of desired shape are obtained. The various shapes such as angles, channels, flats, joists, rails, etc. are obtained by the process of rolling. It is possible to prepare joint-less pipe with the help of this process. The solid rod is bored by rollers in stages until the pipe of required diameter and thickness is obtained.

USES OF STEEL

The following are the uses of steel:

❖ Steel is environment-friendly, sustainable and possesses great durability.

❖ It requires a low amount of energy to produce lightweight steel construction.

❖ Steel is the world's most recycled material which can be recycled very easily, due to its unique magnetic properties.

❖ It gives better shape and edge than iron which is used to make weapons.

❖ Steel is highly used in the automobile industry. Different types of steels are used in a car body, doors, engine, suspension, and interior.

❖ Mild steel is used for building construction. It is also a highly favoured building frame material.

❖ Because of its easily welding capability and attractive finishing, steel has become a prominent feature in modern architecture.

❖Stainless steel gives a hygienic environment. That's why it is used for surgical implants.

❖ Renewable energy resources like solar, hydro and wind power use the stainless steel components.

❖ Steel has a wider range of temperature which is used to make large sheets.

❖ Stainless steels are used to produce offshore platforms and pipelines.