Weld Purging with Preheat
Some engineering alloys are prone to cracking during welding. Industry sectors having to overcome this problem are principally in power engineering and nuclear 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.
An example of a commonly used alloy benefitting from this treatment is SA 213 T91 or SA 335 P91.
This is a ferritic alloy steel that meets the condition of creep resistance required in high temperature steam generating plant.
The material, often simply referred to as P91, has been in successful use for the last two decades in power plant service.
|P91||0.08 - 0.12||0.30 - 0.60||0.020/0.010||0.20 - 0.50||8.00 - 9.50||0.85 - 1.05|
|V 0.18 - 0.25||N 0.03 - 0.07||Ni 0.40 max||Al 0.02 max||Nb 0.06 - 0.10||Ti 0.01 max|
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 P91 and similar alloys. Failure to follow the procedures can result in catastrophic failures in service.
Other high temperature creep resistant ferrous alloys requiring this type of heat treatment are;
ASTM A389 grade C24, A356 grade 9, DIN 21CrMoV 5-11, 15CrMoV 5-10, GS-17CrMoV 511, EN G17CrMoV5-10 and GE B50A224.
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 separate inert gas coverage and this is referred to as weld purging. It involves removal of oxygen from the vicinity of the joint to prevent contamination during the thermal cycle.
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.
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.
The only manufacturer that has studied materials and designed products suitable for use in weld purging at the high temperatures prevailing during pre- and post-heating is Huntingdon Fusion Techniques, HFT®. These systems, referred to as Argweld® HotPurge™ meet the requirement for thermal stability and operational reliability.
The inflatable seals at each end of HotPurge™ Systems are manufactured from flexible, thermally resistant engineering materials. The connecting collar is fabricated from high temperature resistant material. Gas delivery hoses are manufactured from engineering grade nylon and metal fittings comply with international standards.
As with all HFT® products, HotPurge™ is subject to continuous development and the latest innovative change has been to incorporate PurgeGate®. This exploits advanced valve technology to ensure that there is no possibility of over-inflation and hence failure of the seals. These heat resistant pipe purge systems are capable of withstanding temperatures up to 300°C (570°F) for 24 hours.
A wide range of pipe diameters can be accommodated with HotPurge™ offering sizes from 150 – 2440 mm (6 to 96 in). All products have an expansion range of +/- 12 mm on diameter.
The collapsed system is inserted into the pipe to be welded and positioned at the joint: pull loops are integrated with the system to help with this operation. When the selected purge gas is admitted the dams will first inflate to a preset pressure at which point the gas will be diverted into the purge volume. Gas flow will continue until the weld has been completed and cooled.
- The Argweld® HotPurge™ is positioned using the heat-resistant pull tags. A high visibility strip,called RootGlo®, is incorporated at the centre to facilitate accurate positioning below the joint under low light level conditions. RootGlo® gives up to 20 hours illumination for only 10 minutes of exposure to daylight.
- The seals are inflated using the inert gas supply.
- When inflation is complete the purge valve opens automatically. The inert gas then displaces the air between the seals.
- When the weld is completed and allowed to cool sufficiently to satisfy the metallurgical requirements set by the welding procedure the purge gas can be closed. Disconnection of the hose allows the system to deflate ready for removal.
Measurement of Oxygen Levels in the Purge Volume
It is of course essential to confirm that the oxygen content at the weld area has been reduced to the level set by the welding procedure. Acceptable oxygen levels may well be below those measurable with older instruments and HFT® has developed advanced monitoring instruments specifically for welding operations.
The PurgEye® 100 IP65 Sealed Weld Purge Monitor® has been designed to measure oxygen content as low as 0.01% accurately. For ferritic creep resistant materials an oxygen content below 0.1% is normally considered suitable.
BS EN ISO 13916:1997:
'Welding: Guidance on the measurement of preheating temperature, British Standards Institution, 1997.
BS EN 1011-2: 2001:
'Welding: Recommendations for welding of metallic materials. Arc welding of ferritic steels', British Standards Institution, 2001.
Bailey, N. Weldability of Ferritic Steels. The Welding Institute, 1995.
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