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, 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 not 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.
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 stress.
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. Sir Henry Bessemer 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 stee
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