IMPROVEMENT OF ALUMINOTHERMIC WELDING CYCLE LIFE

Transcription

1 IMPROVEMENT OF ALUMINOTHERMIC WELDING CYCLE LIFE Luke Cherviakov Managing Director Railtech Australia, Brisbane, Australia Oliver.W Dolder Managing Director Railtech Boutet, Napoleon, USA Frédéric Delcroix Global ATW Technical Support Director, Raismes, France Fabrice Seewald Director of Railtech Welding & Equipment Railtech, Colombes, France SUMMARY New developments of rail s mechanical proprieties, reduction of track maintenance costs, and safety improvements are all major factors that constitute new challenges to address for developing dedicated products in relation with railroad expectations. Aluminothermic welding is a critical maintenance and construction tool for railroad owners, with its low capital cost and flexibility with which it can be installed, particularly for emergency repairs. Due to the relative inflexibility of other welding processes, Aluminothermic welds are often installed in the harshest environments such as entry and exits to crossings, and with often severely mismatched rails. The latest research and development conducted by Railtech Welding & Equipment have been focused on significantly improving Aluminothermic welding fatigue characteristics in order to match the life-cycle of the rail itself, ultimately utilising the rail asset to its full potential and generating profits for rail asset owners. These developments will allow asset owners to maintain the flexibility of using Aluminothermic welding whilst matching the fatigue performance of other capital intensive practices. The data regarding weld failures we have obtained from the different Heavy Haul Line customers indicate that failures which have occurred after relatively short service period are associated with either the process design or the procedures used when installing welds in track. Training and technical support are important for the Aluminothermic welding process and is the reason for continuing to improve welder training and audit programs that include frequent assessments of welding procedures. New products and processes have been realised to simplify the Aluminothermic welding process and minimize the risk of human error during installation. IHHA 2015 Conference

2 INTRODUCTION Aluminothermic welding continues to be used in heavy haul applications, due to its low capital cost and flexibility when dealing with joining of part-worn rails, and the speed with which welding gangs can attend to defective or broken rails. Railroad owners are demanding rail with increasing hardness and wearing characteristics. At the forefront of this demand is the heavy haul industry. Increasing rail-weld wear characteristics without sacrificing its mechanical performance has been a central factor to our research, and enabled a smooth integration into heavy haul networks. These changes to rail properties can be more easily adapted in the Aluminothermic welding process than other systems. MGT HB MPa Mega Gross Ton Brinell Hardness Mega Pascal 1. ALUMINOTHERMIC WELDING Today, Aluminothermic welding products are used for welding on most of the major Heavy Haul lines in the world (80%) such as the United States, Australia, Canada, Brazil, Mauritania.networks. Those lines run trains with some of the heaviest axle loads in the world (up to 42 tons.) Weld life Axle Load USA 370 MGT 39 tons BRAZIL 600 MGT 33 tons welding can be installed with minimal disruption to the construction or rehabilitation production. R&D programs are established especially with the input from welder experiences and adapted to on site track conditions. Customer feedback is essential in order to improve the practicality of the welding process and to ensure that the product remains user friendly. Improvement of the rail grades is a relevant parameter for our product development and our research teams are able to provide a dedicated welding solution for those new rails. This is more critical for high strength rails with hardness s reaching 440HB. Our research has provided products that can guarantee a weld steel ideally matched to the parent rail. We prefer to provide a weld hardness slightly below the parent rail hardness knowing that during train operations, the cast weld hardness will increase at a higher rate than the rolled steel parent rail, thereby achieving a value close to the parent rail steel hardness for the operational life of the rail. The data regarding weld failures we have obtained from the different Heavy Haul Line customers indicate that failures which have occurred after relatively short service period are associated with either the process design or the procedures used when installing welds in track. Failure statistics indicate that the majority of weld failures under heavy haul conditions occur within 6 months (~80MGT) of installation date. Early failures are often associated with poor weld quality resulting from incorrect welding procedures. Aluminothermic welding technology is an ideal welding solution when dealing with the joining of worn rails with its low capital cost and flexibility. The speed with which welding gangs can attend to defective or broken rails and complete a weld in less than one hour without stopping traffic on any type of rails including worn rails are additional advantages. Track time is an important factor to take into consideration in regards to track construction and/or rehabilitation. Products have been developed to ensure that Aluminothermic Through continuous research in the advancements of maintenance-of-way operations, new Aluminothermic welding processes have been developed such as Hybrid (HY) Mould Technology, Head Wash Repair 2 IHHA 2015 Conference

3 Weld (HWR) Technology and STARTWEL ignition Technology. Reduction of track maintenance costs, and safety improvements are all major factors which constitute new challenges to address for developing dedicated products in relation with railroad expectations. The latest developments in the field of Aluminothermic welding are focused on significantly improving its fatigue characteristics in order to match the life-cycle of the rail itself, ultimately utilising the rail asset to its full potential and generating profits for rail asset owners. Four major innovations have been developed and tested over the past 3 years: Hybrid (HY) Mould Technology, Head Wash Repair (HWR) Weld Technology, STARTWEL ignition Technology and WEL-D STRESS Technology. Reduction of track maintenance costs, and safety improvements are all major factors which constitute new challenges to address for developing dedicated products in relation with railroad expectations. The latest developments in the field of Aluminothermic welding are focused on significantly improving its fatigue characteristics in order to match the life-cycle of the rail itself, ultimately utilising the rail asset to its full potential and generating profits for rail asset owners. Four major innovations have been developed and tested over the past 3 years: Hybrid (HY) Mould Technology, Head Wash Repair (HWR) Weld Technology, STARTWEL ignition Technology and WEL-D STRESS Technology. Those new technologies are user-friendly for the welders and have significant advantages when it comes to in-track installation. 2. HYBRID Mould Technology: The Hybrid welding process is a mixture of a conventional aluminothermic welding process along with the use of our newly developed feltlined moulds for welding new and worn rail (Figure 1) Figure 1 : HYBRID Mould A major technical advantage to the Hybrid mould system is the overall design. A combination of felt lining and sand mould technologies are used to drastically reduce flashing along the web and the base of the rail, improving overall weld performance. The felt lining is attached to the web and base areas along the mould while the rail head area remains the same sand compound. The function of the felt lining greatly reduces any flashing caused from an improper fit or poor installation by improving the quality of the geometry around the web and base, particularly in the collar under the rail head of the rail. In addition, the need for packaging, distribution and storage of multiple kits in various sizes is greatly reduced by replacing it with one type of kit for similar size rail. Customers will notice these advantages with quicker turnaround times when ordering the kits, and therefore reducing the time of installation. Likewise, welders working with conventional moulds will have a higher degree of error when working on mismatched rail. With slight modifications from the welder, the Hybrid moulds are ready to be used and properly installed by following the same process and procedure as if welding uniform rail. Studies from independent facilities, as well as tests conducted by various railroads, have shown that flashing around the mould leads to 3 IHHA 2015 Conference

4 weld fatigue, fracturing due to stress and eventually weld failure. Welds performed on worn rails could have fatigue breakage due to flashing located on top of foot (Figure 2) [3] or under head radius location (Figure 3). maintained and no stressing of the rail is needed. The HWR process is very quick and not as dependent on the operator s skill. Once the defect is located in the rail, it will be marked and prepared for removal by one of two ways. Usually a maximum depth of 1 inch is allowed. A specific HWR mould was developed and designed for this application process (Figure 4). Figure 2: Underhead radius flashing Figure 4: HWR technology Figure 3: Top of the foot flashing 3. Head Wash Repair (HWR) Weld Technology HWR weld is an aluminothermic process providing railway networks with an efficient solution for repairing rail head defects. This process removes defects due to Rail Contact Fatigue such as squats, spalling, rail check, shelling and internal defects found with visual or with ultrasonic inspections. This innovative technology offers substantial savings by avoiding re-railing or manual metal Arc welding process. Also, neutral temperature is By incorporating these important features into its design, gap formation under the rail head is minimized. In turn, the risk of having flashing or cold lapping, which can act as stress risers and shorten the life of the weld, is virtually eliminated. The HWR moulds use Hybrid technology and fit new rail as well as worn rail up to 15 mm of head wear The moulds sit on the foot of the rail there is no gap underneath the rail head due to wear differential HWR welds have the same high metallurgical and mechanical characteristics of any conventional weld Fine pearlitic microstructures in accordance with the parent rail Hardness and chemical composition in accordance with network specifications 4 IHHA 2015 Conference

5 HWR weld has a large Fusion Zone guaranteeing the removal of the defect and a Heat Affected Zone in accordance with the standards and equivalent to a Wide Gap Weld (WGW). Many tests were performed on TTCI Pueblo and are in accordance with AREMA recommendations [4] Many tests were conducted in partnership with SNCF on French railways and now this process is approved for the entire French network and high-speed tracks. This welding process has a particular geometry for repairing the many running surface flash-butt welds defects. This process avoids the change of a piece of rail that has a significant cost, requires the use of a special train to bring the new rail in track, more welders, and other associated equipment At the end we can estimate that the cost of the HWR weld can be 5 times cheaper than a traditional procedure. The ignition is done by a dedicated electrical current with the STARTWEL handle. A really relevant aspect is that STARTWEL product is not classified as class 1 and can by carry and ship in every country without any transport dangerous good restriction. 5. WEL-D STRESS Shot peening Technology In sensitive areas, highly stressed, weld failures can occur over a quick period after installation. WEL-D STRESS Technology was developed to improve the fatigue behaviour of Aluminothermic welds. It is extremely common for defects to propagate from the weld collar area: - Under the head 4. STARTWEL ignition Technology STARTWEL is the electrical ignition system for Aluminothermic welding portions, developed and patented by RAILTECH. This technology was designed to guarantee a steady and reliable ignition. It is a very easy-touse system and consists of a drop of molten metal fallen inside the crucible and start the Aluminothermic powder. This way of ignition guaranty to have repeatability with the process behaviour by the fact that the molten metal fall always in the centre and on the portion s surface (Figure 5). - At the top of the foot Figure 5: STARTWEL ignition Technology 5 IHHA 2015 Conference

6 Figure 8: Rail break during fatigue test operation Shot peening locally lowers the amount of residual stress in the weld, and is intended to reduce nucleation and propagation of fatigue cracks. (Figure 6) At this level, we can consider that the fatigue life of the Aluminothermic welding came to the life of the rail and enables different networks to reduce the maintenance operations costs. 6. CONCLUSION Developments in our Aluminothermic welding technologies continue to be realised in order to improve the mechanical proprieties of the welds for use in heavy haul applications and also for all the tracks in the world. Figure 6: Shot peening operation This technology allows to significantly reduce the risk of premature fatigue. Welds treated by WEL-D'STRESS technology permit welds to withstand higher loads than the adjacent rail.(figures 7, 8) Training and technical support are important and are the reason for continuing our welder training and audit programs that include frequent assessments of welding procedures. Our new products and processes were also developed to simplify the installation process and minimize errors during installation. With these new product developments railroad owners are able to continue to utilise the benefits of Aluminothermic welding, without sacrificing operational performance. REFERENCES Figure 7: 68Kg PLK weld During the laboratory tests it is common to obtain a rail break with shot peened ATW weld as shown in the Figure 8. Rail break Fatigue initiation [1] Y. Chen, F.V Lawrence, C.P.L. Barkan, J.A Dantzing, Weld defect formation in rail thermite welds, Proc IMechE, Part F: J Rail Rapid Transit, 220(4) (2006) [2] Y. Chen, F.V Lawrence, C.P.L. Barkan, J.A Dantzing, Heat transfer modelling of rail thermite welding, Proc IMechE, Part F: J Rail Rapid Transit, 220 (3) (2006) [3] I.Salehi, P.Mutton, A.Kapoor, Analysis of straight break formation in aluminothermic rail welds under Heavy Axle Load conditions [4] Daniel Gutscher Railhead Repair Weld Residual Stress Investigation IHHA 2015 Conference