Rail Steels: Microstructures and Properties of Pearlitic Rail Steels

Grade 700 rails that used to be the main product for railroads some 60 years ago, may be considered as the starting point for the development which since took place. The Grade 700, with about 0.5% C, has a microstructure of about 30% ferrite and 70% pearlite within the rail head, which is the relevant location for comparison.

The first step to raise strength, and consequently wear resistance, was to increase the carbon content to achieve a 100% pearlitic microstructure. This way Grade 900 rails were developed.

Microstructures and Properties of Pearlitic Rail Steels

Pearlite is an important feature of the microstructure because it possesses good wear resistance, hence making carbon an essential alloying element in rail steels. However, it is not only the amount of pearlite that is important but also its morphology, which means the shape and the distance between the cementite lamellae. The finer the structure of pearlite, the higher is its strength whilst still retaining reasonable toughness. Therefore the development of pearlitic rail steels has been focused on the refinement of pearlite.

Pearlite comprises a mixture of relatively soft ferrite and a hard, brittle iron carbide called cementite, taking the form of roughly parallel plates. It achieves a good resistance to wear because of the hard carbide and some degree of toughness as a result of the ferrite’s ability to flow in an elastic/plastic manner. Figure 1 shows the microstructure of a pearlitic railway rail steel. The cementite is white and the ferrite is black. The interlamellar spacing is about 0.3 microns.

Figure 1: Microstructures of pearlitic rail steels

Grade 700 rails that used to be the main product for railroads some 60 years ago, may be considered as the starting point for the development which since took place. The Grade 700, with about 0.5% C, has a microstructure of about 30% ferrite and 70% pearlite within the rail head, which is the relevant location for comparison.

Due to the rather slow cooling of the rail head on the cooling bed after rolling, the pearlite structure is relatively coarse. The first step to raise strength, and consequently wear resistance, was to increase the carbon content to achieve a 100% pearlitic microstructure. This way Grade 900 rails were developed.

The wear resistant rails of Grade 900 have a coarse pearlitic microstructure with sufficient ductility and toughness for general applications. Welding techniques were developed to replace fishplate connections and Grade 900 became the standard rail instead of Grade 700 for main lines. Nowadays Grade 700 rails are only used for tracks where low axle loads are applied, e.g. for trams. In some places like narrow curves and mountainous regions, but mainly for heavy haul ore and coal transportation, strengths greater than that exhibited by Grade 900 rails are needed; an increase in tensile strength of about 200 MPa doubles the wear resistance of the rails and consequently their service life.