Rail steels are produced by hot rolling of steel blooms with a cross sectional area at least 9x that of the finished rail. They are then air cooled on special beds. It is during this cooling that the pearlitic microstructures shown previously are produced. The properties such as the hardness of the rail depend on the spacing of the iron/cementite lamellae in the microstructure, which in turn depend on the precise steel composition and the cooling rate.
The changes in cooling rates experienced by different parts of the rail result in residual stresses which cause the rail to distort during cooling. In order to counteract this, when the rails come on to the cooling beds they may be bent to a precise shape so that stresses produced during subsequent cooling cause the rails to straighten when cooling is complete as illustrated in the figure.
Rails on cooling beds after hot rolling – length over 120m
For applications where high hardness' are required the head of the rail may undergo a heat treatment process involving compressed air, water or polymer quenching to accelerate the cooling rate of the rail head to improve its wear performance.
Heat treatment of rails