3. Special steels
page 15
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Photo of rail surface
Picture 3.9 The surface of a rail.
Rail steel
Steel is used to make rails for railway lines. This steel must be hard wearing and resistant to cracking. This is achieved by careful choice of the composition of the steel and by carefully controlling the way the hot rail is cooled.
Composition of rail steel
Before the development of heat treatment processes, the tensile strength and hardness of naturally cooled rails were controlled by the chemical composition of the steel. Both of these properties of steel depend upon the proportion of carbon and manganese that it contains. Carbon, up to a maximum of 0.82%, and manganese, to a maximum of 1.7%, are needed to produce hard, wear resisting rails.
Photo of structure
Picture 3.10a Micrograph of rail steel at 200 and 500 times magnification.
Graph
Picture 3.11 Graph showing effect of inter-lamellar spacing of pearlite on Brinell hardness of rails.
Microstructure of rail steel
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.

Photo of structure
Picture 3.10b Micrograph of mild steel at 200 and 500 times magnification.
Tensile strength of steel
The tensile strength of steel is measured in units of newtons per square millimetre. Mild steel has a tensile strength of about 430 N mm-2. (This means that a wire with a diameter of 1.4 mm could support the weight of a 70 kg man.) A high carbon rail steel, by comparison, may have a tensile strength from 900 N mm-2 to over 1200 N mm-2. (To support the same man, the wires diameter could be reduced to 0.8 mm.)
Hardness of steel
The hardness of steel is measured as a Brindle Hardness Number (HB). The typical hardness of a high carbon rail steel is about 280 HB.
Photo of structure
Picture 3.12 Microphoptographs of steel after Mill Hill Treatment has reduced the lamellar spacing.
Heat treatment of rail steel
The fineness of the pearlite structure (the inter-lamellar spacing) of steel is controlled by the rate at which the hot rail cools. Heat treatments have been developed to control the cooling rate and therefore control the microstructure which in turn controls the hardness and strength of the steel.

Some grades of steel are hot rolled and allowed to cool naturally in air. Other grades are cooled quickly from above 800 °C using water sprays. This accelerated cooling increases the base hardness from 260/300 HB to 355/390 HB.

The modern heat treatment system is called the Mill Heat Treatment. This greatly reduces the inter-lamellar distance in the pearlite and can produce steel with a hardness approaching 400 HB.

Crack resistance
Steel used to make rails must be free from internal cracks. These are caused by hydrogen trapped in the molten metal as it cools. The hydrogen can be removed from the steel either by a process called vacuum degassing of molten steel before it is cast or by letting hot rails cool down very slowly.
Interactive graphic of rail ware
Picture 3.13. A piece of rail showing the profile. The normal profile gets worn - usually on one side.
Rail fatigue
Steel used to make rails must be resistant to fatigue or surface cracking. The aim is for the steel to have a long fatigue life.

Rail steel is chosen to achieve a balance between fatigue life and wear resistance. If the steel is too hard, it will allow cracks to develop on the surface. Steel that is slightly softer, will wear the rail slightly and reveal a new surface before cracks can develop in the original surface.

The key to a long fatigue life is to make steel that has few non-metallic inclusions. Steel of this kind is said to be very ‘clean’.

Rail steel is designed to wear away very slowly to avoid surface cracking. The rails don’t wear evenly and this often produces an incorrect rail to wheel geometry. This uneven profile accelerates the wearing process. The problem is overcome by regular light grinding to restore the optimum rail profile.

You can find out more in the Making Tracks resource which is part of the Learning Zone on the coruseducation web site.
Question 3-4.
a) How can you tell the difference between a region of ferrite and a region of pearlite in a microphotograph of a piece of steel?

b) Why are steel rails sometimes cooled quickly using water sprays?


Summary                   Close
  • rail steel has to be hard wearing and resistant to cracking
  • the microstructure is all pearlite
  • hydrogen is removed from the microstructure to prevent cracking