TPT September 2012

Technology News

Post weld purging WHILE most welders recognise the need for inert gas purging when making quality joints it is not obvious why purging should be continued after the weld has been completed. The answer is 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? While nominal oxidation, ie 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 150°C ( 300°F).

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’. Huntingdon Fusion Techniques manufactures a unique re-usable range of purging equipment to meet every tube and pipe diameter from 25 to 1,800mm (1 to 72"). 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. Huntingdon Fusion Techniques – UK Email: hft@huntingdonfusion.com Website: www.huntingdonfusion.com

between the seal and the pipe or tube wall. Gas exit should be available 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 should be at the bottom. 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 is 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 is all bad for the

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 or

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S eptember 2012