16.07.08
A novel and semi-automatic weld repair technique for rail defects
The complexity of vehicle track interaction and its dependency on a wide range of track design and traffic characteristics can lead to a variety of rail running surface defects such as squats and wheelburns which can significantly reduce the life of the rail, say Dr. Jay Jaiswal and Gareth Evans
Typically, major European rail networks remove 1000’s of discrete mid-rail defects each year. Defects are either weld repaired or removed by renewing the rail, either as a short closure or in longer lengths. Corus Rail Technologies has developed a novel weld repair technique to improve the integrity of repairs to discrete defects. The key strength of the novel process, for which a patent has already been filed, lies in the replacement of those aspects of the conventional MMA (manual metal arc) weld repair process that promote variability with more controlled automatic operations.
The Corus process
The process developed by Corus employs a portable milling head to produce a cavity of a standard shape and size. Milling of a 200mm x 10mm deep cavity (can be varied to suit the defect being removed) is completed within 15 minutes, significantly faster than the currently employed techniques of grinding or gas torch cutting. The cavity is then filled using semi-automatic flux cored arc welding and a programmed weave pattern. The process uses Network Rail-approved welding consumables. Welding is completed within 30 minutes.
A novel aspect of the Corus process is the significant reduction in the required weld preheat, from 3500C to just 800C - 1000C, for both Grade 220 and Grade 260 rail steels. This reduces the time required for heating and promotes more even heating of the area to be welded. The reduction in preheat has been achieved through careful metallurgical design of the welding process. The combination of rail and weld metallurgy and a ‘chill removal’ process produces a baintic surface weld repair structure with a hardness of 340-380 HV, a tough tempered heat affected zone (HAZ) and a high integrity, defect free interface between weld metal and parent rail.
The final stage of the Corus process is profile grinding of the weld-repaired rail, undertaken using conventional grinding equipment.
Weld properties
The combination of the above procedures has been shown to produce a robust and high integrity weld deposit as confirmed by detailed examination of the developed microstructure and hardness.
The primary failure mechanism of manual metal arc weld repaired sections is internal fatigue initiating from a flaw in the vicinity of the fusion line with the parent rail. In order to demonstrate the robustness of the newly developed process, Corus has undertaken laboratory fatigue testing of welded samples, placing the rail head in tension during cyclic loading. Using an applied stress range of 3 times the value typically experienced by the rail head in track under 25 tonne axle loads, failure did not occur after 5,000,000 load cycles. Applying an even greater stress range (8 times typical track conditions), the same test sample only failed after an additional 4,300,000 loading cycles.
Application of the Corus process
Following successful development and laboratory testing, Corus are currently in discussion with a number of mainline European railways that are interested in using the novel discrete rail defect weld repair process as part of their routine track maintenance procedures. Further information regarding the process can be obtained from the authors.
Acknowledgements
Corus Rail Technologies would like to acknowledge the support of Yorkshire Forward who provided 50% of the funding for the development of the discrete defect weld repair process.
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