The welding process is in principle a regulated electric short circuit with the two rail ends functioning as D.C. poles. The high current produces an arc between the rail ends, the two rail cross-sections are heated to melting temperature due to the high input of current.
During the following upset stroke the rails are pressed together under high pressure causing the ends to coalesce. The upset metal is trimmed off immediately after the upset stroke.
The result is an accurate weld without filler metal with a very small heat-affected zone and a more or less consistent progression of the hardening process with a favourable crystal-line structure.
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Preparing the weld – the slight inclination
Due to the shape of the rail, there is an irregular distribution of material over the cross-section. Due to the effect of the electric current during flash-butt welding the rail is heated up to melting temperature. Consequently the rail does not cool down uniformly over the entire cross-section. The rail foot and the rail base cool down first.
Because the largest proportion of material is in the rail head, this cools down far more slowly. The natural consequence is a depression of the rail in the area of the welded joint.
However, this result would have very negative effects on the track geometry and the operational safety. For this reason, a slight upward inclination of the two rail ends is performed, adapted to the respective rail profile, so that an ideal position of the rail is achieved when the weld has cooled down.
Flash Butt Welding & Alumino Thermic Welding
a) Alumino Thermic Welding
– Strength of AT joint is app. only 56% of parent rail.
– More prone to corrosion
– High failure rate.
– Poor Quality of Weld.
b) Flash Butt Welding
– Strength of FB joint is almost equal to parent rail.
– Less prone to corrosion
– Failure rate < 10 %.
– Excellent Quality of Weld. The defects like porosity, inclusions and lack of fusion are eliminated.
Mobile Flash butt welding Machine
– It is rail cum road machine that can be driven to site.
– The mobile Flash butt welding machines are either built in Standard railway vehicle or as containerized units.
– In case of m/c built on Railway Vehicle the welding head is Positioned between the bogies and can be lowered for welding in running track..
– The containerized units can be used for welding in track or on cess on the other hand the standard vehicle machine can be used only on running line.
– New machine is fully computerized.
– Once the welding head is in position, the entire process is carried out automatically at the push of button.
– Welding current,
– Upset force Pressure,
– Welding time,
– Programmed identification and setting details.
Procedure Approval Tests
1) Weld Trimming
2) Weld alignment
3) Weld appearance.
4) Non-Destructive testing
5) Bend testing
6) Fatigue Testing
7) Macro Examination
8) Hardness testing
9) Statements of the result and procedures.
10) Maintenance of the records
Sources; Today published by Plasser & Theurer, indianrailways.gov.in
Related Standard; BS EN 14587-2 – Railway applications. Track. Flash butt welding of rails.
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