Rail Steels: Properties of Quality

So what makes rail steel superior to other steels? The simplest answer is its unique composition.

As we mentioned above, the rails are subject to heavy contact cyclic loading that accompanies increased car size and loading, to 100 and 125 ton capacity, increased train size, and increased train speeds used to transport bulk products over the last several decades. These increasing demands require manufacturing and metallurgical approaches that offset wear and other types of failure that limit rail life.

An early type of rail failure was associated with entrapped hydrogen that produced shatter crack or flakes in heavy rail sections, but that difficulty has been effectively controlled cooling and by vacuum degassing of liquid steel.

Typically, rail steels are produced in large BOS vessels and are vacuum degassed prior to being continuously cast into large blooms. Vacuum degassing, coupled with ladle trimming facilities, permits very tight control over chemical composition. After casting, the blooms are placed in insulated boxes, whilst still at a temperature of about 600°C, and are cooled at a rate of 1°C per hour for a period of three to five days. This treatment, coupled with prior vacuum degassing, reduces the hydrogen level in the finished rail to about 0.5 ppm, thereby reducing substantially the susceptibility to hydrogen cracking.

The blooms are then reheated and rolled directly to the finished rail profile. The rail produced by steel rail manufacturer from each bloom is hot sawn to specific lengths prior to passage through a rotary stamping machine en route to the cooling areas.

Depending upon the properties required, the rails are either cooled normally in air or subjected to enhanced cooling for the development of high strength. On cooling to room temperature, the rails are passed through a roller-straightener machine which subjects the section to a number of severe bending reversals and emerge with a very high degree of straightness. Finally, the rails pass through a series of ultrasonic, eddy current and laser inspection stations which monitor non-metallic inclusions, external defects and the flatness of the running surface.