Steel | Types and Manufacturing

Steel is an alloy of iron and carbon, with carbon content upto a maximum of 1.5%. Most of the steel produced now-a-days is plain carbon steel or simply carbon steel. It is divided into the following types depending upon the carbon content:
 
1. Dead mild steel — upto 0.15% Carbon
2. Low carbon or mild steel — 0.15% to 0.45% Carbon
3. Medium carbon steel — 0.45% to 0.8% Carbon
4. High carbon steel — 0.8% to 1.5% Carbon


According to Indian standards, the carbon steels are designated in the following order:

(a) Figure indicating 100 times the average percentage of carbon content,
(b) Letter 'C', and
(c) Figure indicating 10 times the average percentage of manganese content. The figure after multiplying shall be rounded off to the nearest integer.

For example 20C8 means carbon steel containing 0.15 to 0.25% (0.2% on an average) carbon and 0.60 to 0.90% (0.75% rounded off to 0.8% on an average) manganese.

The principal methods of manufacturing steel are as follows:

1. Cementation process. The steel made by this process is cement steel because ferrite in the wrought iron is converted into cementite (i.e. iron carbide). Since carbon combines with wrought iron and has its surface covered with blisters, therefore, the steel produced by this process is known as blister steel.

2. Crucible process. The steel produced by this method is very homogeneous, free from slag and dirt and much superior to cement steel. The steel so produced is known as crucible steel.

3. Bessemer process. In a bessemer process, following are the three distinctive stages used to convert molten pig iron to steel:
(a) In the first stage (known as charging position), the molten pig iron is poured into the converter.
(b) In the second stage (known as blowing position), the converter is tilted to the vertical position and the air blast turned on. In this stage, the silicon and manganese burns out which is indicated by the brown smoke rising up through the mouth of the converter. After this, the carbon is next to oxidise which is indicated by a white flame.
(c) In the third stage (known as pouring position), the white flame of the burning carbon drops and the contents of the converter are poured in a ladle. Now a small quantity of some alloy rich in carbon and manganese (i.e. spiegeleisen or ferro-manganese) is added to produce steel of quite good strength and ductility.

Note : The bessemer process may be acidic or basic depending upon the lining of furnace. In the acidic bessemer process, the furnace is lined with silica ricks. The slag produced in this process contains large amount of silica. Since phosphorus in a pig iron cannot be removed by this process, therefore acidic bessemer process is unsuitable for producing steel from pig iron containing large quantities of phosphorus.

In basic bessemer process, also known as Thomas process, the furnace is lined with a mixture of tar and burned dolomite. This process is applicable for making steel from pig iron which contains more than 1.5% phosphorus.

4. Open hearth process. The open hearth process of steel making is sometimes called `Siemens-Martin Process'. This process is more suitable than Bessemer process when a large quantity of mild steel, with definite quality and composition, is required.

5. Duplex process. The duplex process of steel making is a combination of acidic bessemer process and basic open hearth process. This process is in operation at Tata Iron and Steel works, Jamshedpur (Bihar).

6. L-D process (Linz-Donawitz process). It is the latest development in steel making processes and is now adopted at Rourkela steel plant where three converters of 40 tonnes capacity are working.

7. Electric process. This process is mainly used for the preparation of high quality and special alloy steels of high melting point, The electric process may be acidic or basic, but basic process is mostly used because it permits extensive elimination of impurities. The basic lined furnace of the Heroult type is especially adopted to the production of best quality carbon and alloy steels.

Note: The steel contains small amounts of impurities like silicon, sulphur, manganese and phosphorus. The effect of these impurities are as follows:
Silicon in the finished steel usually ranges from 0.05 to 0.30%. It is added in low carbon steels to prevent them from becoming porous. It removes the gases and oxides, prevents blow holes and thereby makes the steel tougher and harder.

Sulphur occurs in steel either as iron sulphide or manganese sulphide. Iron sulphide because of its low melting point produces red shortness whereas manganese sulphide does not effect so much.

Manganese serves as a valuable deoxidising and purifying agent, in steel. When used in ordinary low carbon steels, manganese makes the metal ductile and of good bending qualities. In high speed steels, it is used to tougher the metal and to increase its critical temperature.

Phosphorus makes the steel brittle, It also produces cold shortness in steel. In low carbon steels, it raises the yield point and improves the resistance to atmospheric corrosion. The sum of carbon and phosphorus usually does not exceed 0.25%.

2 comments:

  1. Excellent confined knowlege and understanding of materials and other mech. engg. topics...keep it up...

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