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The answer’s obvious really; hot metal oxidises in the presence of air so the weld and any metal surrounding it will discolour until it has cooled. Welders know to keep the inert gas flowing from the torch to prevent oxidation around the surface. Why do they overlook the underside?
Whilst nominal oxidation, i.e a light straw shade, is quite acceptable in some cases and in any event can perhaps be cleaned at a later stage, some applications require absence from contamination of this type. With a little care it can be avoided. Simply continue the flow of inert purge gas over the hot metal until it has cooled—typically to below 150C ( 300F).
On the subject of purging, let’s have a closer look at purging practice and dispel a few myths that seem to abound. Choosing gas inlet and exit points Effective gas seals need to be provided on either side of the joint and these need to be far enough away to be unaffected by the temperature rise during welding. Gas can be admitted through the seal (A) or between the seal and the pipe or tube wall (B). Gas exit should be at C so that the lighter air is fully expelled by the denser argon.
Note that if helium is being used it is lighter than air and the air exit (C) should be at the bottom .
Sealing the pipe
Having recognised the need to use gas purging, many welders think that just passing inert gas through a pipe or tube is adequate. This is poor practice. Even with small diameter tubes, the cost of continuously passing gas through is often
greater than the cost of effective seals. Increasing flow rate simply leads to turbulence, trapping air around the joint.
It’s a fallacy that using crumpled paper or discs of cardboard or wood gives a good seal. Even if these apparent solutions appear to be a good fit, the chances of leaks are high. There is also the possibility of burning if the weld line is too close.
Bear in mind also that all three materials probably contain contamination and some residual moisture. It’s all bad for the welding procedure.
Proprietary sheet plastics have been developed which are contaminant free and can even be used when making joints for use for example in nuclear and pharmaceutical engineering. These are water soluble and can be removed effectively and easily after welding but care is needed during application. See Huntingdon Fusion Techniques’ Technical Note TN 14 ‘Purge Film’.
Use a commercial purge product
Huntingdon Fusion Techniques manufactures a unique re-usable range of purging equipment to meet every tube and pipe diameter from 25 to 1800 mm (1 to 72 in). They have all been designed specifically to meet exacting sealing demands, use the minimum quantity of inert gas, are easy to install and remove and greatly speed up the overall welding procedure. See Technical Notes TN 8 to TN 13. An example of the extensive range is shown here. This is the PurgElite® purge system;
Always remember, it’s more cost-effective to use a good purging technique than to resort to post weld grinding and cleaning to compensate for bad practice.