Protecting the underbead from oxidation ensures high-quality welds.
August 22, 2017
by Michael Fletcher
Weld purging of pipes involves removing gases, water and other vapours from the joint area and is only effective if oxygen is displaced from the purge zone prior to and during welding. Any residual oxygen significantly reduces corrosion resistance and joint strength, so it’s essential the pipe is sealed on either side of the joint and the seal is maintained throughout the process. The residual level of oxygen in the purge zone needs to be consistent with the welding procedure so continuously monitor it to ensure compliance.
Pass these seven welding purge tips along to welders to ensure joint welds are high quality:
1. Choose a dependable sealing material. Cheapest is seldom best so examine the options available. Don’t be tempted to use sealing discs made from polystyrene foam, wood and cardboard. At best, they leak or emit contaminants and at worst they catch fire during the hot weld cycle.
2. Use a complete purging system. Don’t try to economize by using whatever happens to be around at the time. Separate seals for the pipe and rubber tubing for the inert gas, held together with bits of tape, seldom succeeds. It’s time consuming and can only be used once.
Find systems using inflatable dams. Commercial equipment is now available in which gas flow and pressure and purge gas quality are all pre-set. Incorporating complete monitoring instrumentation ensures a high level of quality control. These systems have been designed for multiple use and are rugged enough to cope with site conditions while significantly reducing overall purging and welding time.
3. Establish an acceptable level of oxygen in the purge gas. There’s plenty of published information available that establishes what the maximum oxygen content needs to be to prevent loss of mechanical and physical properties in the weld. This depends on the material being welded but generally, some stainless steel welding requires a low level whereas most carbon steels are much less sensitive.
4. Take care with gas flow. Whatever system is selected ensure the inert gas enters slowly. Argon is heavier than air so introduce it slowly at the bottom of the weld purge space and discharge from the highest point. Helium is lighter than air and needs to be inserted at the top of a cavity and removed at the base.
5. Don’t rush into the welding sequence. Wait until all the air has been displaced before welding. For many metals this means ensuring a residual oxygen level below 100 ppm. Traditionally, and a practice still followed even by major fabricators, this is based on guesswork. If the oxygen content is too high or varies during welding, oxidation occurs and this often means rejection with the expense of re-machining, and production delays. It can also lead to loss of corrosion resistance in stainless steels.
6. Use a purge gas oxygen monitor. Don’t assume that allowing ‘plenty of time’ for purging removes all the oxygen. If there are leaks in the system, turbulence or poor quality purge gas, oxygen levels could be way above those necessary to prevent contamination. The best solution is to use a device capable of accurate and reliable measurement of the oxygen level.
7. Read published information about purging. Learn from the experience of others rather than using trial and error methods that could cost time and necessitate re-welding. (See Reading List.)
Michael Fletcher of Delta Consultants is a senior consultant with Huntington Fusion Technologies HFT, a manufacturer of weld purging products. He’s based near Phoenix, Ariz.