Abstract
Small diameter, thin-walled, metallic pipes are intended to be used for the introduction of a new, two-phase carbon dioxide (CO2) cooling system for the Compact Muon Solenoid (CMS) experiment at the European Organisation for Nuclear Research (CERN). This system will be operated at high pressures (up to 163 bar) and low temperatures (-45 °C) and be expected to operate for 10 years or more with no planned service. Previous design iterations of such pipes and their permanent, welded connections have been unable to meet these requirements.This work uses a number of experimental techniques to analyse substrate pipe materials and manufacturing routes as well as the performance of their welded connections. The experimental techniques utilised include: in-situ testing of strain evolution and performance of components using synchrotron radiation techniques, nanoindentation and focused ion beam milling to analyse residual stresses in components from the manufacturing processes, and optical techniques to analyse wear on components in service. Finally, the projected long-term performance of such components was studied by carrying out lifetime testing.
Under tensile loading, it was found that stainless steel piping outperforms copper nickel pipes of the same geometry, and they are able to withstand greater force and displacement prior to failure. It was found that orbital gas tungsten arc welding was the most effective joining method for stainless steel pipes, with such welds outperforming the substrate pipes under tensile loading. However, this joining process did induce significant residual strains within the joint. Laser welding produced similar magnitude strains during welding, however the formation of residual strains was found to be strongly driven by joint geometry. In contrast, soft soldering generated the lowest magnitude residual strains during the joining process.
Despite this, soft soldered joints could withstand much less force and displacement prior to complete failure, when subject to tensile loading. The
performance of a soft soldered joint has also been shown to be strongly driven by the residual stresses after the soldering process, with samples with lower magnitude residual stresses outperforming those with higher stresses. Voids and flaws within samples have also been shown to affect performance of soldered connections under tensile loading, however residual stresses have a greater effect on overall performance.
Reliability studies of permanent piping connections have shown that voids created during the manufacturing process of soldered connections are the critical factor in determining performance under cyclic, internal pressure loading. The initiation of cracks was seen to occur from existing voids, with crack propagation driven by the presence of smaller voids close to one another. These cracks then allowed gases to escape the connections, creating unacceptable leak rates that would negatively affect the performance of a two-phase CO2 cooling system.
Overall, this project makes recommendations for improving the design of small diameter, thin-walled, metallic pipe connections, and the critical factors that influence performance. New insights have been obtained by means of experimental methods, particularly in the area of fatigue performance by way of cyclic pressure loading. Recommendations have been made for further work arising from this project.
| Date of Award | 15 Nov 2023 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Alexander Lunt (Supervisor) & Andrew Rhead (Supervisor) |