| ||Steel used for rails has a very different microstructure from the low carbon mild steel used for railway sleepers. The microstructure of low carbon steel will be made up of ferrite and cementite. The microstructure of rail steel, however, will consist fully of pearlite. |
If you look through an optical microscope at mild steel, you will see light coloured grains of soft ferrite with only a small amount of darker coloured pearlite.
If you look at rail steel through the optical microscope you will see only pearlite. If the microphotograph of rail steel is magnified, the pearlite is seen to be made up of lines. It is said to be lamellar. That is, it looks as if it is laminated.
The hardness and tensile strength of steel are determined by the proportion of pearlite in its microstructure. They are also determined by the 'fineness' of the pearlite structure. The shorter the inter-lamellar spacing, the higher the hardness, wear resistance and tensile strength.
|Picture 3.10b Micrograph of mild steel at 200 and 500 times magnification.|