In this article we will discuss about:- 1. Composition of Steel 2. Uses of Steel 3. Factors Affecting the Physical Properties 4. Magnetic Properties 5. Defects 6. Properties of Mild Steel 7. Properties of Hard Steel 8. Mechanical Treatment.
Composition of Steel:
As far as the carbon content is concerned, the steel forms an intermediate stage between cast-iron and wrought-iron. The cast-iron contains carbon from 2 to 4 per cent. In wrought-iron, the carbon content does not exceed 0.15 per cent. In steel, the carbon content varies from anything below 0.25 per cent to 1.50 per cent maximum.
This is due to the fact that carbon, if in excess of 1.5 per cent, does not combine with iron and it is present as free graphite. Thus the dividing line of cast-iron and steel is the presence of free graphite. If there is no free graphite in the composition of a material, it is said to be steel. On the other hand, the presence of free graphite indicates that the material is cast-iron.
The steel becomes harder and tougher as its carbon content goes on increasing and at the maximum level of 1.5 per cent, all the carbon gets into chemical combination with iron and none of it exists in its free state.
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The cast-iron can take up only compressive stresses and its use is limited to the compression members only. The wrought-iron is of a fibrous nature and it is suitable to resist tensile stresses.
The steel is suitable for all constructional purposes in general and hence it has practically replaced cast-iron and wrought -iron in the present day practice of building construction. It is equally strong in compression as well as in tension.
If a drop of nitric acid is placed on steel, it will produce a dark grey stain due to the presence of higher percentage of carbon content. If the same procedure is carried out on wrought-iron, the stain will not be appreciable. Thus the mild steel and wrought-iron can be easily distinguished by this simple process.
The final battle between cast-iron, wrought-iron and steel was fought on the field of construction of skyscrapers. The columns of early skeleton of skyscrapers were of cast-iron and the beams were of wrought-iron.
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Sir Henry Bessemer (1813-98) of England invented his converter in 1857 and it came into use from 1880 or so. The introduction of the open-hearth process brought the final victory to the steel because it produced a better quality of steel that could take up higher working stresses.
The present, age is of steel and as a matter of fact, the industrial progress and financial stability of a nation are estimated directly by the volume of its yearly steel output. Japan, U.S.A. and former U.S.S.R. are producing nearly 1100 to 1250 million kN of steel each per year. Britain, West Germany and France produce about 350 to 400 million kN of steel each annually.
The steel produced by India is about 100 to 120 million kN per year. The overall control of the whole steel industry both in the public and private sectors is exercised by the Steel Authority of India Limited (SAIL) with its headquarters in New Delhi.
Uses of Steel:
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Depending upon the carbon content, the steel is designated as the mild steel or medium carbon steel or high carbon steel. The various uses of steel are governed by the amount of carbon contained in it.
The carbon content of mild steel is about 0.10 to 0.25 per cent. When carbon content is less than 0.10 per cent, it is known as the dead steel or very low carbon steel.
The carbon content of medium carbon steel is about 0.25 to 0.60 per cent.
The high carbon steel is also known as the hard steel and its carbon content varies from 0.60 to 1.10 per cent or so.
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Table 11-1 shows the various uses of steel of each category.
It is observed that the steel is required for the existence of the heavy and light engineering industries, for ship building, railways and rolling stock, automobiles, sheet metal industries, power generation and electrical industries, etc. It should also be noted that the entire range of electrical engineering industry depends upon the property of magnetism of steel.
Factors Affecting Physical Properties of Steel:
The physical properties of steel such as ductility, elasticity, strength, etc. are greatly influenced by the following three factors:
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(1) Carbon Content:
The variation in carbon percentage produces steel of different grades. The carbon always assists in increasing the hardness and strength of steel. But at the same time, it decreases the ductility of steel. The mild steel having carbon content of about 0.10 to 0.25 per cent is widely used for structural work.
(2) Presence of Impurities:
The usual impurities in steel are silicon, sulphur, phosphorus and manganese.
If silicon content is less than 0.20 per cent, it has no appreciable effect on the physical properties of steel. If silicon content is raised to about 0.30 to 0.40 per cent, the elasticity and strength of steel are considerably increased without serious reduction in its ductility.
If sulphur content is between 0.02 to 0.10 per cent, it has no appreciable effect on the ductility or strength of steel. It however decreases malleability and weldability of hot metal. The excess of sulphur decreases strength and ductility of steel.
The phosphorus produces detrimental effects on steel. It is desirable to keep its content below 0.12%. It reduces shock resistance, ductility and strength of steel.
The manganese helps to improve the strength of mild steel. Its desirable content is between 0.30 to 1.00 per cent. When its content exceeds about 1.50 per cent or so, the steel becomes very brittle and hence it loses its structural value.
(3) Heat Treatment Processes
Magnetic Properties of Steel:
The steel is also widely used in electrical machinery, generators, transformers, etc. For making steel suitable for such use, its magnetic properties are given supreme importance and these properties are obtained by carefully adjusting its chemical composition.
Following are the proportions of various elements in steel for making it to achieve better magnetic properties:
(1) Carbon – It is desirable to keep carbon content as low as possible and it should not exceed 0.10 per cent.
(2) Silicon – The presence of silicon results in considerable increase of electrical losses and hence it is highly undesirable.
(3) Sulphur and phosphorus – If combined content of sulphur and phosphorus exceeds about 0.30 per cent, the magnetic properties of steel are greatly affected.
(4) Manganese – If content of manganese exceeds about 0.30 per cent, it proves to be injurious to the magnetic properties of steel.
Defects in Steel:
Following are the four common defects in steel:
(1) Cavities or blow-holes
(2) Cold shortness
(3) Red shortness
(4) Segregation.
(1) 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.
(2) 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.
(3) 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.
(4) 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.
Properties of Mild Steel:
Following are the properties of mild steel:
(i) It can be magnetised permanently.
(ii) It can be readily forged and welded.
(iii) It cannot be easily hardened and tempered.
(iv) It has fibrous structure.
(v) It is malleable and ductile,
(vi) It is not easily attacked by salt water.
(vii) It is tougher and more elastic than wrought-iron.
(viii) It is used for all types of structural work.
(ix) It rusts easily and rapidly.
(x) Its melting point is about 1400°C.
(xi) Its specific gravity is 7.80.
(xii) Its ultimate compressive strength is about 80 to 120 kN per cm2.
(xiii) Its ultimate tensile and shear strengths are about 60 to 80 kN per cm2.
Properties of Hard Steel:
Following are the properties of hard steel:
(i) It can be easily hardened and tempered.
(ii) It can be magnetised permanently.
(iii) It cannot be readily forged and welded.
(iv) It has granular structure.
(v) It is not easily attacked by salt water.
(vi) It is tougher and more elastic than mild steel.
(vii) It is used for finest cutlery, edge tools and for parts which are to be subjected to shocks and vibrations.
(viii) It rusts easily and rapidly.
(ix) Its melting point is about 1300°C.
(x) Its specific gravity is 7.90.
(xi) Its ultimate compressive strength is about 140 to 200 kN per cm2.
(xii) Its ultimate shear strength is about 110 kN per cm2.
(xiii) Its ultimate tensile strength is about 80 to 110 kN per cm2.
Mechanical Treatment of Steel:
The purpose of giving mechanical treatment to the steel is to give desired shape to the ingots 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:
(1) Drawing
(2) Forging
(3) Pressing
(4) Rolling,
Each of these operations will now be briefly described.
(1) 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.
(2) 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.
(3) 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.
(4) 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.