Railroad tracks, also called railroad rails or railway tracks, are essential for railway systems for its function of leading trains move forward and carrying loads then transfer it to the lower structure. With the development of railways in the direction of high-speed and heavy-load, modern requirements for rail quality are getting higher and higher. As the speed of the rail vehicle increases, rail life becomes shorter and shorter. Because of the inherent defects of the traditional process, problems such as the appearance of straightness and the blind zone of the internal quality inspection, the asymmetry in size, the scratches on the surface, and the decarburization, etc., all seriously hinder the improvement of the quality of the railroad tracks.
Now the railroad tracks are made of high manganese steel. If 13% of manganese is added, it will become both strong and tough. Besides, there are new requirements in the railroad track manufacturing process. The steel rails produced by the traditional die-casting process can no longer meet the requirements of high-speed railways for the internal quality and mechanical properties of the rail dimensional accuracy. To meet the requirements, the current steel rail production method has gradually used forging or rolling to replace the casting method.
The rail section is in an “I” shape. It’s divided into the railhead in contact with the wheels, the rail web and the rail base. Rolling and forging are carried out on the rolling mill. Different lines have different requirements for the strength, stability and wear resistance of the rail, so the rail also has different specifications.
Railway Track Manufacturing Process
The manufacturing process of railway tracks has evolved. The following steps are the modern level production process.
It starts with the pretreatment of molten iron. The main purpose is to further reduce the content of harmful impurities in the molten iron. The usual way is to desiliconize at the tap hole of the blast furnace, dephosphorize and desulfurize the molten iron in the hot metal tank car. Some also adopt the “tandem method” of steelmaking, that is, the dephosphorization and desulfurization of molten iron are first carried out, and then go to the converter for decarburization and smelting.
Adopt the refining technology outside the furnace. The main function is to further adjust the molten steel composition and molten steel temperature, reduce the inclusion content, and ensure the accuracy of the molten steel composition during tapping.
- Vacuum Degassing
Vacuum degas the molten steel after steelmaking to further reduce the content of hydrogen, oxygen and other gases dissolved in the molten steel. By doing so, the influence of these gases on the performance of rail steel can be reduced. Usually degassing is realized by the VD method or by the RH method. After vacuum degassing, the hydrogen content (volume fraction) in the steel can be less than 2.5PPM, and the oxygen content (volume fraction) can be less than 20PPM, which is more conducive to improving the toughness of the steel.
- Continuous Bloom Casting
Put the molten steel into the ladle, and lift it to the top of the continuous casting machine by the overhead crane. Pour the liquid steel into the continuous casting machine to cast a slab. Without cooling, the steel slab or “blooms” is directly heated in the soaking furnace for a certain period. This softens the steel for further shaping. It is then get squirted with water at over 3700 PSI. the water jets strip off any iron oxide that’s formed on the steel’s surface leaving the steel slab ready to be shaped into railroad tracks.
The continuous casting process has been gradually improved in the past 20 years. Due to the use of protective casting, it can effectively prevent the secondary oxidation of the molten steel during casting. Besides, it greatly reduces the inclusions in the steel, and significantly improve the surface quality of the steel slab. Major steel rail manufacturers in the world have adopted continuous casting process to produce steel rails.
After taking the above measures, the smelting level of rail steel has reached a high degree of purity. For example, the fluctuation of carbon composition is controlled at about 0.06% (mass fraction).
It is discovered that all manufacturers that use melton iron pretreatment, out-of-furnace refining, vacuum degassing and continuous casting technologies have gained the very good physical quality of their rails. The content of hydrogen, oxygen, phosphorus, and sulfur are all very low, and the content of inclusions: Type A inclusions are less than class 1.5, and B, C, and D inclusions are less than class 1.0.
The steel slab is then directly entering a machine called rolling mill which squeezes the rectangular steel into T-shaped tracks and stretches the rails out more than quadrupling its length.
- Heat treatment
The popular heat treatment is quenching and tempering or slack quenching. Some countries tried to improve the yield strength and tensile strength of railroad tracks by controlling the temperature of rolling or online heat treatment. Full heat treatment stabilizes the microstructure of the rail.
After cooling, a straightening machine works on the railroad tracks from both horizontal and vertical directions.
Railroad track inspection is also an important step. Check the surface and inner defect of railroad tracks through ultrasonic and eddy current inspection.
- Sawing and drilling
Use a carbide machine with hard alloy to process and drill the railhead to obtain high accuracy.