14.05.08
Production of long-welded and continuous-welded rail
Welding is a versatile engineering tool used to join lengths of rail to form long-welded rail (LWR) or “welded strings” and then to further join these LWRs when installed into track to form continuous-welded rail (CWR), says Bill Mosley
Production of LWR
LWR is produced from rail of varying lengths (36, 72 and 108m) by flashbutt welding. The preferred method is by static plant in fixed installations, but can also be produced by mobile flashbutt welding (MFBW) machines, which will be discussed later. Currently, there are two static welding plants producing LWR for Network Rail, namely Corus Scunthorpe (relocated from Castleton) and NR’s own welding plant located in Eastleigh, Hampshire. Corus’ rail service centre in Scunthorpe which opened in 2007 is now capable of manufacturing long lengths of rail (108m) and producing 216m long welded strings which meet NR’s current policy of producing LWR with as few welds as possible. Following manufacture, these lengths of LWR are transported to site on a purpose-built train. NR’s plant also produces 216m LWR from rail supplied by Scunthorpe, which is also supplemented by rail supplied from Corus Hayange (France), Voest-Alpine and Lucchini.
Both static plants operate a Schlatter GASS 80/580 AC welding machine, which also comprises a rail end cleaning station, an automated press and an automatic grinding facility.
Rails are prepared for welding at the rail end cleaning station and then travel on to the welding head where the rails are aligned along the centreline of the head and are also checked for lateral alignment. The rails are clamped on the head and under the foot prior to welding commencing. One of the rails is held statically, whilst the other is allowed to move which permits forging to take place. Depending on the rail profile being welded, the welding program is selected which has the welding parameters (volts, amps, number of preheats, forging pressure etc) programmed into it, which means that it cannot be altered by the operator.
Initial flashing takes place which removes any surface irregularities and squares the rail ends and is followed by a series of preheats (usually seven). On completion of preheating, final flashing takes place followed by forging. The voltage used is quite low (~14V), amperage is in the region of 65-75kA and forging pressure is 65t. The welding data recorder which automatically records the details for each weld, will reject any weld that falls outside of its preset parameters and turns the graph red in colour. The weld then has to be cut out and the joint re-welded.
Approximately 26mm of rail in total is consumed in making the weld. On completion of welding, one side of the rail is unclamped which permits the weld upset to be stripped automatically.
Once stripping is completed, the weld travels to the press station for any corrective pressing that may be required, prior to the weld being ground to final profile.
MFBW machines are also used to produce LWR, usually in brown field sites, where access means that it is not possible to deliver normally produced LWR. In this instance, it is only possible to supply 18m lengths of rail by road and a derogation against the Line Standard can be granted to permit LWR to be produced from these shorter rail lengths.
Production of CWR
Following installation into track, the LWR is then welded together to form CWR with the usual method of joining being aluminothermic welding. Two processes are currently approved by NR - Thermit (GB) and Railtech (UK). The Thermit process is the more established process and was first introduced into the UK in 1958. However, the Railtech process, which has only been in use since 1998, is gaining popularity, particularly amongst the renewals contractors.
Both processes are similar in their execution. The welding gap is produced by either disc-cutting or oxy-fuel gas cutting (nominally 28mm and 25mm for Thermit and Railtech respectively) and the rail ends peaked by 1.5mm measured over 1m. At this point, the two processes vary in so far as Thermit utilise a two-piece refractory mould that is adjusted by rubbing and filing, whilst Railtech utilise a three-piece refractory mould that has built in felted strips which seal against the rail and require no adjustment apart from local cutting of the felt to aid alignment.
The moulds are then fitted centrally about the gap and secured in position with mould shoes and clamps before being sealed or “luted” with refractory sand (Thermit) or paste applied with a sealing gun (Railtech).
The recent introduction of the Thermit SkV-E (enhanced) process means that both processes now employ low pressure preheating systems using oxygen and propane fuel gas for normal applications and acetylene fuel gas in tunnels. The gas pressures and preheat times for Thermit and Railtech respectively (CEN 56E1 rail profile) are; 0.7bar propane/3.0bar oxygen for 3.5 minutes and 0.6bar propane/1.2bar oxygen for 4 minutes. Both processes now employ a re-useable crucible, although for the Thermit SkV-L80 wide gap process, the long-life crucible is still used mainly because of health & safety considerations due to the large volume of molten steel that would be left unsupported on the moulds.
Following preheating, the crucible is placed in position, the aluminothermic portion ignited and the molten steel produced is then subsequently tapped into the moulds. The weld is left undisturbed for a set time before the equipment is broken down and the weld trimmed by machine to leave approximately 3mm of surplus material around the railhead, which is then ground to profile with a trolley grinder.
As mentioned previously, MFBW machines are used to produce LWR but are being increasingly used to produce CWR. MFBW machines have disadvantages, particularly with site access due to their relative size and also the unit cost per shift, which normally requires in the order of 30-40 welds to be produced to make them economically attractive compared to aluminothermic welding. However, this high cost can be offset against the superior weld quality and lower weld rejection rates.
Two types of machine are currently approved by NR - the containerised version which has the generator and hydraulics located in a cabinet mounted on a bogie and towed behind a conventional road/rail vehicle.
The column acts as the umbilical for the hydraulic pipework and power cables, with the welding head suspended from the jib. The second type is a lorry-mounted MFBW machine, with the generator, hydraulics and welding head contained within a conventional 3 or 4 axle lorry.
A third type is currently undergoing approval - an all-terrain machine which is a conventional road/rail vehicle but where the generator and hydraulics are mounted at the back of the cab which also acts as a counterbalance. The cables and pipework are contained inside the column itself and the welding head is suspended from the jib as before. This offers advantages over the other two types as it has a 360º capability and can also work in either direction.
With MFBW currently, at the end of a renewals length a closure rail still requires to be installed using conventional aluminothermic welds and stressing, but NR is currently working in partnership with one of its contractors to look at the possibility of making the final closure weld and stressing simultaneously with a MFBW machine.
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