IIW White Paper

9 Needs and challenges of major industry sectors for future applications

to be taken into consideration include the service conditions (e.g. axle loads and train speeds), and the required performance or reliability. Of the above processes, flashbutt welding is generally considered to provide the most reliable and consistent weld performance; this process is used for fabrication of long- welded rail, and increasingly for in-track welding, the latter using mobile welding machines. In combinationwithother improvements in trackdesignand constructionprocedures, CWRhas contributed to a reduction in track maintenance requirements and increased rail lives. From a metallurgical and engineering perspective, however, flashbutt, gas pressure, aluminothermic and electric arc welding procedures will always result in a discontinuity in the rail section. Material characteristics such as microstructure, hardness ( or strength) and ductility will vary throughout the welded region. In addition, residual stress levels will be increased over those present in the parent rail, and the presence of the weld collar or reinforcement in aluminothermic welds alters the section dimensions, and hence the stress distribution under the action of wheel loads. Aluminothermic welds and electric arc welds are also more prone to welding defects than flashbutt and gas pressure welds, increasing the risk of service failures. The differences in material characteristics and quality between parent rail and weld may have a detrimental effect on rail performance, such that the service life of the weld will be less, and the risk of component failure higher, than that of the parent rail. This is of particular concern at the higher axle loads typical of heavy haulage operations, where the rate of weld deterioration may be much higher than under general freight and passenger operations. High speed passenger rail operations also impose tighter tolerances on weld quality, although in this case the major concern is longitudinal alignment, and minor irregularities, particularly in the running surface, can result in unacceptable impact loading factors at speeds of 200 kph and above. Typical deterioration and failure modes may include: Excessive weld batter (dipping) and rolling contact fatigue damage, associated with the variation in hardness and microstructure through the weld, and Fatigue failure, initiating in the head, web or foot of the rail. Weld batter contributes to increased impact loading, which in turn can result in localised breakdown in the rail support conditions (e.g. due to ballast crushing), hence increasing impact loading and the risk of fatigue failure. The consequences of unacceptable weld performance can therefore range from increased track maintenance costs to failed welds and increased risk of derailments. Significant improvements in the quality and service performance of rail welds have been achieved though a number of developments, including: More widespread use of mobile flashbutt welding equipment for field welding, and improved ( automated) process control during the flashbutt welding cycle. Improved process designs for aluminothermic welding, in particular the introduction of single-use crucible processes, and The availability of improved mathematical and experimental techniques that can be used for research into rail welding procedures, and which offer the potential for further optimisation of existing welding processes.

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Through Optimum Use and Innovation of Welding and Joining Technologies

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