It is possible to join rails together with modern welding machines in a fully automated operation. Most of the times, flash-butt welding is used for this purpose. In comparison to the alumino-thermic welding process, this welding technology has the advantage that no foreign material is introduced into the welded joint. In particular when a high grade rail steel is used, the discontinuity in the rail steel can be reduced to a minimum.
The first developments of mobile electrical welding technology were carried out at the Paton Electric Welding Institute in Ukraine in the1960s. Since then, these ideas have been further developed by different manufacturers and the quality and effectiveness of the welding have been optimised.
Although no filler material is added to the welded joint, there is a change in the steel structure. The soft annealing caused by the application of heat changes the hardness of the rail steel. With severely heat-affected zones and very hard rail steels localized irregularities in hardness are produced, which depending on their size, are prone to wear and tear.
The highest quality standards must be maintained in the welding of rails in order to prevent artificial trouble spots in the weld. Modern welding robots which work in accordance with the welding standard (EN14587-2 for flash-butt welding machines) make it possible to produce rail welds automatically and in a secure way.
The welding robot is lowered to the point of welding (e.g. Plasser APT 1500 RL). Auxiliary clamps grab the rails and align them in respect of their height which is done completely automatically with modern machinery. The usually difficult manual alignment of the rail heads to each other is no longer necessary. In order to counteract the material shrinkage due to the uneven cooling of the rail profile, the rail ends are aligned to each other in a slightly inclined way. After the alignment of the rail sections, clamping jaws fix the rails in the end position. The current carrying clamps are put on and the welding process can begin. A contact grinding of the rail web (to eliminate rust and dirt) is necessary in order to enable the electric current to flow satisfactorily. On some machines it may also be necessary to remove the rolling marks with a Rail grinding machine , otherwise not enough energy can be transferred.
The welding process is divided into three distinct phases:
- The Unstable phase (irregularities are burned off, rail ends are pre-heated).
- The Stable phase (even burn-off).
- The Progressive phase (increase in energy and burn-off of impurities).
Carrying out the flash-butt welding
Once these phases have been done, the upset stroke is carried out in which the rail ends are pressed together with hydraulic cylinders. Excess material is pressed outward and sheared off automatically immediately after welding. The clamping jaws remain closed during this process and hold the rail during the cooling process. This is particularly important in the case of closure welds (final welds).
Modern flash-butt welding machines, such as the APT 1500 of Plasser & Theurer, have a maximum drawing distance of 150 mm and a pulling force of 1 500 kN and enable not only common rail connection welds, but also the final welding of the closure weld.