The planned surge in new electricity power generation plant and refits across the world brings with it a demand for improvements in welding technology. This demand will be met by innovative developments in welding equipment to ensure consistently better quality joints, many of which are in the safety critical class.
All the programmes involve extensive fabrication of high-pressure steel pipes and tubes, the welding of which presents particular challenges.
The high pressures and temperatures used in steam generation circuits necessitate the use of creep resistant steels such as those based on chromium/ molybdenum/ vanadium alloys. These materials provide improved oxidation and corrosion resistance together with high strength and are widely used in both fossil fuel and nuclear power plants.
The demand for quality in all these safety critical joints is reflected in the stringent regulations laid down in welding procedures. Nevertheless, some welding practices can result in significant reduction both in corrosion resistance and mechanical strength.
Welding of high-pressure steam pipe
Some engineering alloys are prone to cracking during welding. Industry sectors having to overcome this problem are principally in the power generation sector and include low and medium alloy steels that have been specially developed for their high strength. Metallurgists have learned that heating the joint prior to and after welding (pre-heating and post-heating) can reduce the sensitivity to cracking quite significantly. It involves temperatures in the region of 200°C although this may be much higher for certain materials 3, 4, 5, 6.
Welding is one process that is widely used during manufacture. This affects the microstructure. Preheating, maintaining inter-pass temperatures, and post-weld heat treatment procedures are very critical for these creep resistant alloys. Failure to follow the procedures can result in catastrophic failures in service.
The preferred welding procedures in this type of fabrication are GTAW and GMAW and these offer protection of the exposed upper fusion zone. The joint around the underbead however needs to be protected by purging – the protection of exposed metal by applying a local inert gas atmosphere.
Meeting the requirements of inert gas purging when temperatures exceeding 200ºC are involved necessitates the use of purge systems capable of withstanding these temperatures throughout the heating and welding cycles. Typical thermal cycles can exceed 2 hours and it may be necessary to maintain the purge system in place throughout.
Purging system requirements
Specially engineered purge products have been designed over the past five years that are capable of withstanding the temperatures involved whilst at the same time maintaining adequate gas sealing characteristics. They are also rugged enough to survive multiple-use applications. These products are manufactured from thermally stable engineering polymers and can be provided with advanced gas valve control systems.
Few manufacturers are able to supply weld purging systems that can be used at the high temperatures prevailing during pre- and post-heating but some commercially available systems have been designed specifically to meet the requirements (Fig 1).
However, it is clear that many companies still employ paper, cardboard and polystyrene foam as dam materials. These are prone to outgassing during use, they are difficult to insert and remove and may even ignite at the prevailing temperatures.
FIG 1 Hotpurge® 7 systems cover the diameter range from 150 to 2440 mm. These systems are capable of withstanding temperatures up to 300°C for 24 hours. The inflatable seals are manufactured from flexible, thermally resistant engineering polymers.
Weld Purging Techniques
The most effective devices are those based on connected inflatable dams
The inert gas input can be programmed to control gas flow and pressure during inflation and purging and once placed in position require little more input from an operator. The dams are fabricated using advanced engineering polymers and are thus suitable for use where elimination of contamination is essential.
FIG 2 Example of the PurgElite® 7 range of fully integrated systems covering the 25 to 250 mm pipe range. Complementary systems are available covering the diameter range between 150 and 2440 mm.
Purge gas oxygen content can be controlled by using an oxygen monitor. These instruments not only measure oxygen levels but can inhibit welding if the level is above that set by the operator. Recording and analysing software provides information for quality control purposes.
Even very low oxygen concentrations in weld gases can give rise to discolouration, loss of corrosion resistance and reduction in mechanical strength. Controlling oxygen level in purge gas can be effected simply and efficiently using contemporary integrated purge systems.
FIG 3 Oxygen monitor designed specifically for use in a welding environment. The PurgEye® 7 range is capable of measuring residual oxygen content down to 10 ppm.
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