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Classification and Applications of Steel

 According to IS : 7598 - 1974 steels shall be classified as :

1. Unalloyed steels , commonly called plain carbon steels, and 

2. Alloy Steels.

UNALLOYED STEELS OR PLAIN CARBON STEELS :

The principal factors affecting the properties of plain carbon steels are the carbon content and the microstructure. Carbon is the principal determinant of many performance properties. It has a strengthening and hardening effect. At the same time, it lowers ductility , as evidence by decrease in elongation and reduction of area,. In addition, a rise in carbon content lowers machinability and decreases weldability. The amount of carbon present also affects physical properties and corrosion resistance. With an increase in carbon content, thermal and electrical conductivity decline, magnetic permeability decreases drastically , and corrosion resistance is lowered. 

The microstructure is determined by the composition of steel , i.e., carbon manganese, silicon, phosphorus and Sulphur which are always present and residual elements including oxygen, hydrogen, and nitrogen , and by final rolling , forging , or heat treating operation. However, most of the plain carbon steels are used without a final heat treatment and, consequently rolling and forging operations influence the microstructure.




Carbon steel are predominantly paralytic in the cast, rolled or forged conditions. The constituents of hypo eutectoid steels (steels containing from and above 0.87 percent carbon) are therefore ferrite and pearlite and of the hyper eutectoid steels ( containing from and above 0.87 per cent carbon ) cementite and pearlite. 

Types According to carbon content :

 On the basis of the carbon content plain carbon steels are commonly divided as :

 1.) Low carbon or mild steel - 0.05 - 0.30 per cent carbon

 2.) Medium Carbon Steel      - 0.03 - 0.06 per cent carbon 

3.) High Carbon Steel             - 0.60 - 1.50 per cent carbon 

4.) Tool Steel (high carbon)    - 0.90 - 1.50 per cent carbon 

It should be noted that carbon content of these four types of carbon steel are not rigid, and there may be a certain amount of overlapping from one type to next.

Types according to deoxidation practice : 

Steel are often identified as to the degree of deoxidation resulting during steel production :

1. Killed Steels , They are strongly deoxidized , and are characterized by high composition and proper uniformity. All forging steels and, in general all steels containing more than 0.25 per cent carbon are killed. The essential quality of killed steels in soundness (freedom from blow holes and segregation). When the steel is deoxidized sufficiently, there is no evolution of gas and the top surface of the ingot solidifies almost immediately. The symbol K stands for killed steel.

2. Semi - Killed Steels Or Balanced Steels .  They are intermediate between those of killed and rimmed steels and have variable degree of uniformity. Structural steels containing 0.15 to 0.25 per cent carbon are generally semi - killed. In semi - killed steel the aim is to produce metal free from surface blowhole and pipe. The surface should have a sound skin of considerable thickness. Plates and structural products are normally made from semi- killed steel. No symbol is used to denote semi-killed quality of steel which comprises 90 per cent of the total production of steel. 

3. Rimmed Steels . In Rimming, The steel is partially deoxidized . In rimmed steel, the aim is to produce a clean surface in carbon content. A wide variety of steels for deep drawing is made by the rimming process, specially where ease of forming and surface finish are major considerations. These steels are therefore, ideas for rolling. Sheets and strips made from rimmed steels have excellent surface - quality and cold forming characteristic. The symbol R is used for rimmed steel. 

APPLICATIONS OF CARBON STEELS :

The mild and medium carbon steels are widely used and high carbon steels are used mainly for tools and for specialized work requiring high quality are very good performance.

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