Rail Steels: Part Two

The further strengthening of pearlitic steel rails to 1100–1200 MPa tensile strength is based on increased pearlite refinement. As shown in Figure 2, the properties of rail steels are influenced by the spacing between cementite lamellae. The yield strength and tensile strength increase as the interlamellar spacing decreases while the elongation decreases with a reduction in the interlamellar spacing.

Figure 2: Effect of pearlite refinement on the properties of rail steel.

The continuous cooling transformation (CTT) diagram of Grade 900 steel shows two possibilities for achieving pearlite refinement. Firstly, as shown in Figure 3, the field of austenite to pearlite transformation may be moved to the right, e.g. through additions of chromium and other alloying elements, so that air cooling of the rail head transforms the austenite into fine pearlite with narrow interlamellar spacing. This type is the high strength and highly wear resistant alloy Grade 1100-1200, which cools at still air after rolling.

Figure 3: CCT diagram showing the effect of alloying to achieve pearlite refinement.

The second possibility is that the cooling speed of the rail head may be accelerated to move the austenite to pearlite transformation of the Grade 900 steel to the left in order to achieve a microstructure of fine pearlite; generating a 1100-1200MPa tensile strength with the same steel composition as shown in Figure 4.

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This type is a head-hardened rail. The heat treatment and accelerated cooling (slack quenching) of the rail head may be performed after reheating (offline) or using the still austenitic microstructure directly after rolling (in-line).

Figure 4: CCT diagram showing the effect of cooling rate on pearlite refinement.

The changes in pearlite transformation temperature in these two types of rails caused by changes in the cooling rate from the austenite region are schematically shown on the CCT diagram, Figure 5. To set the pearlite transformation temperature at about 600°C, slack quenching (cooling rate: 4-6°C/s) is required for the carbon steel rail, while natural air cooling (cooling rate: ~0.7°C/s) after hot rolling is sufficient for the alloy steel rail due to the high hardenability from alloying. These treatments could give the rails more than 1100MPa tensile strength.