MAINTAINING a railway network is an expensive business. The costs are not limited to the financial impost of maintaining overhead lines, signalling, rolling stock and the permanent way but also those associated with stopping trains, a major consideration in heavy-haul applications.
Maintaining the permanent way often means shutting down the network to complete work even when the problem may only be a tiny crack in the rail, no more than a few millimetres in size. This crack may require the removal and replacement of not just a section of rail but a major rail component such as a complete crossing.
While repair methods do exist, these tend to be geared towards light rail or tram networks and are generally regarded as temporary measures to ensure a component lasts until it is time for a complete replacement. For heavy-haul applications, there is no effective way of repairing defects in track other than a complete replacement of the component or the section of rail containing the defect.
However, this situation has changed. Hardface Technologys began experimenting with various repair methods about 15 years ago, and following development in laboratories and live field tests conducted with assistance from Australian Rail Track Corporation (ARTC) over several years, it was able to develop Hedkote.
Hedkote is not a single method or process but a collection of processes which are deployed depending on the circumstances of the defect.
Indeed one of the major difficulties faced during the development of Hedkote was comprehending the number of different types of rail that exist and are in use and the different maintenance techniques required depending on the type and makeup of the steel used. While the differences in the variety of steels such as carbon steel, austenitic manganese steel or chrome vanadium steel are clear, there are many different versions of each all of which contain all sorts of alloying elements in varying quantities.
Determining the correct metallurgy of a section of rail is crucial to identifying what process or consumable should be used. This brings up another issue; despite claims to the contrary, hardfacing consumables are not made specifically for rail but with mining, dredging, quarrying, rock crushing or other industries in mind where they are used by the tonne. In contrast, in rail many repairs may only use a few hundred grams of consumable, thus making it uneconomical to spend large amounts of time and money developing them especially for the rail industry.
Different types of wear
Further complexities are added when considering the difficulties of coping with different types of wear. This can include abrasion in switchblades or gauge wear situations, rolling contact wear on straight or plain rail and impact wear on crossings. Add to this the variations caused by ambient conditions, particularly in Australia's extreme climate where chilling winds can change to searing heat in just 12 hours.
With customer's paperwork on what steel is in use not always accurate, welders rely on visual identification to inform them of what technique they should use. There are many obvious differences between manganese monoblocks, RBMs, fabricated steel crossings, vanadium nose crossings, among others. Finding which manufacturer supplied the rail can inform this process because particular manufacturers are known to use certain grades, while rails often have markings which identify the grade and whether it is head-hardened. Some steels are magnetic while the hardness of the rail when grinding is another good indicator. Other less-conventional techniques include observing the colour of the sparks and the distance they travel and the way the grinding disks glaze up or burn. For some welders the smell of the steel when grinding is an indicator; manganese for example smells very different from other steels.
Once welders have identified what they are dealing with, they can select from the variety of techniques available to "hardface" components ranging from the thin fin of a switchblade to the very large chunk of steel in a heavy-haul crossing. This process essentially consists of using flux-cored welding wires to rebuild the worn sections of rail or to re-fill an area where a defect has been removed through grinding.
After testing for any remaining cracks, the area is built up by applying layers of hardfacing wire. This method is effective at removing rail defects, wheel burns, squats and cracks as well as wear, and can in some cases increase wear resistance on conventional track, points and crossings. The techniques used to achieve this can vary significantly and will depend on different conditions and the makeup of the track.
For example welds can use stringer beads, weaving, or a combination of the two techniques. Different stick-out lengths can also greatly affect the weld while heat is a critical factor; welders must be conscious of a specific steel's susceptibility to damage from very high temperatures. In some cases a tacking technique is used and it may be necessary to spray water on a weld to prevent it from overheating. In these circumstances repairs will often take much longer than on steels which can accommodate very high temperatures as heat can be applied during the weld. Often the heat may not be enough so an external source is required to maintain a certain temperature. In addition some steels must not cool too quickly so a controlled cooling process is required, which can again slow down the process.
Determining the correct process for the particular steel that is being welded and identifying the adjustments necessary for the size, weight, shape and location of the repair as well as those influenced by the conditions, are all skills that the welder needs to master. As well as variations in technique, the time taken also differs greatly but it is possible to conduct a complete repair or build-up in less than two hours.
The ability to conduct large repairs on the rail in-situ is what makes the process so advantageous. Traffic can run over a completed repair or rebuild immediately after it has been finished. In fact, if necessary, traffic can run over a semi-completed repair - at reduced speeds - without any detrimental effects. The repaired or rebuilt section has at least the same service life as the original rail.
Hardface has carried out various field tests with Hedkote to demonstrate its effectiveness. This includes tests at the BlueScope steelworks plant in Port Kembla, New South Wales. The facility's internal rail network sometimes transports steel between plants and can operate at up to 60-tonne axleloads travelling at around 5-10km/h.
Other trials include with ARTC, where Hedkote was applied to the transition area of a manganese insert which was installed near Newcastle, New South Wales, on April 30 2008. The standard-gauge track carried approximately 100 million gross-tonnes per annum with 32-tonne axleloads with trains travelling up to 100km/h. Normally the service life of the component is nine to 12 months. However, by applying Hedkote to the insert this was extended to 31 months, or more than 280 million tonnes of traffic, with the crossing removed for repair on December 20 2010, rebuilt using Hedkote, and reinstalled. Four-and-half years later, this component was returned to Hardface in April for another rebuild and was reinstalled at the end of May.
Similar repairs on switchblades or crossings are guaranteed against welding defects for one year while repairs on straight rail have a five-year guarantee. These guarantees also apply in heavy-haul applications and it is possible to repeat the Hedkote renewal process up to four times for a single component.
There are many add-on benefits of introducing the Hedkote process into a maintenance programme. Rail head defects can be removed as soon as they occur, preserving the overall safety and integrity of the rail. By keeping the rail head in good condition, other maintenance procedures such as packing, tamping and tightening of rail bolts are also reduced.
In addition aluminothermic welds used for the insertion of plug rails or closure rails are no longer needed which also adds to the integrity of the system. Noise from squats or wheelburns, a major concern in residential areas, can be prevented.
Other cost savings are related to the process itself. Hedkote requires a minimal amount of resources; a pickup truck can carry all the necessary equipment with a welding crew comprising only two people.
Hardface is currently working with Aurizon, BHP, and, outside the Pilbara, with Rio Tinto. Other customers include Metro Trains Melbourne, Yarra Trams, V-Line, DPTI Adelaide, Genesee and Wyoming and rail maintainers such as John Holland, Transfield, Downer, Fluor and Laing O'Rourke.
Hardface's customers now factor Hedkote into their regular maintenance schedule, with Hardface given the customer's annual shutdown plans and subsequently booking welding crews into certain times and locations. In the case of emergencies, welders are flown to the site and use equipment stored at depots in most state capitals.
Success in Australia has led to enquiries from other railways from around the world. Due to its limited resources at this stage, Hardface is exploring licensing the service, particularly for use in Asia. Here growing traffic levels in urban as well as heavy-haul applications, particularly in China, are demanding an innovative approach to track maintenance, and with Hedkote offering a cost-effective and proven solution, potential demand is significant.