Site Welding to Produce CWR
On arrival at site, long rails are welded to form CWR using the thermit or alumino thermic welding process. This method, which was discovered in 1896 by Hans Goldmidt, is based on the reduction of heavy metal oxides by aluminium. Thermit welding was first used in Hungary in 1904 and most of Europe had adopted the process for site rail joints by the late 1920s. The process was not used very widely in the UK however, until the 1950' Some light railways have used thermit welding of short rails throughout, without the use of FBW into long rails beforehand. Although this is cheaper and removes the need for a shop process, the practice is not recommended for railways carrying heavy axle loads. Thermit welds are completely satisfactory but have less consistency than FBW, being carried out in the open on site rather than in controlled workshop conditions.
Annual statistics, published on reported broken rails at welds in the UK over recent years, strongly bear out the better performance of FBW in practice.
In this process the rails to be joined are set in position, fixed in their baseplates, with the ends properly aligned and with a gap between them of between 22 and 26mm. A refractory mould is then placed around the joint and a thermit portion is ignited in a refractory crucible above the mould. The portion is a combination of powders which after reaction will produce a weld metal which matches the chemistry and metallurgy of the parent rails. When the reaction is complete the crucible is tapped and steel pours into the moulds to form the weld. Slag, being less dense than the steel, remains at the top of the mould. The weld is allowed to cool after which the excess metal, mould material and slag is trimmed away and the joint is ground to profile.
Stressing -up'or 'LockingofCWR
With jointed short rails, the object is to allow rails to expand and contract during extremes of temperature to avoid the build up of compressive and tensile stresses. In long welded rails and CWR however, the rail is constrained so that it cannot expand or contract. In this case, in order that the rail shall remain at its original length, the rail undergoes compressive and tensile strain, which is equal and opposite to thermal strain.
By simple calculation(F=strain ◊usingA◊E)it Hooke's Law can be seen that a restrained standard BS113A FB rail increased in temperature, by say 45?C, will produce a force of 76.5 tonnes in the rail.
A compressive force of such magnitude in hot weather is sufficient to cause a buckle of the track and it is essential for safety that development of such a force is prevented. Similarly, high tensile forces in extremely cold weather can cause brittle fracture of rails and must be avoided.
This is done on CWR by artificially extending the rail at the time of installation and fixing it down in a state of tension. The ideal is to fix the rail at a length that it will be at a temperature that is exactly halfway between the hottest and coldest likely rail temperature. In the UK this is generally accepted as a temperature of 27?C.
The rail may be artificially extended by rail warming or, as is now more usual, by stretching with a tensor.