What was the issue?
The flash-butt welding (FBW) process is used extensively in the heavy haul rail industry to produce long and continuous rail sections for both construction and re-railing or repair activities.
Need to further understanding of the metallurgical response of new low-alloy and hyper-eutectoid head-hardened rail (HHR) steels, introduced to increase the load carrying capacity and service life of rails in heavy haul networks, to the FBW process.
What was done?
Literature review to establish current knowledge and identify any future research needs.
What was found?
Defects identified that may occur include differential wear due to heat-affected zone softening, martensite formation, weld dipping, hook crack formation, reduced fatigue resistance and generation of residual stresses.
Developments identified relating to the FBW process include generation and validation of procedures for producing narrow heat affected zone welds as well as the use of pulsed pre-heating and post-weld currents for control of heat input and post-weld cooling rates.
Technological advances include highly automated mobile welding units that use direct current power supplies, as well as having the ability to pre-tension rails during field welding and re-railing operations to counter buckling that result from thermal expansion.
Proposed future research to:
- Investigate four specific steel grades for the metallurgical response to thermal cycling, in particular the cooling rate of the FBW process.
- Determine if controlled cooling of welds is required and if so, to what extent as well as guidelines relating to the maximum allowable martensite contents in FBW joints for the grades of interest.
- Assess FBW joints in both high chromium and high silicon rail steels to establish if contaminant oxides are present in the weld bond line, a subject of concern for which no definitive research has been undertaken for these particular steels. Investigation of the source of such retained oxides and means to avoid their formation may also be beneficial.
Further research has followed on from this initial literature review. The Rail Flash Butt Welding research project is investigating the optimal combination of factors in the flash butt welding machine and the composition of the rail used in heavy haul rail operations.