TPT November 2021

AR T I C L E

Huntingdon Fusion Techniques

Welding knowledge par t 6: monitoring weld purge gas while working on tubes and pipes By Michael Fletcher, PhD Metallurgy, Delta Consultants

The significance of using inert gas to protect the weld metal and heat affected zone against oxidation is well documented. How this is best effected is still a subject of debate and it is appropriate now to review the situation. When metals are exposed to high temperatures in air there is probability that they will oxidise, but also a danger that they will absorb nitrogen and other residual gases. Furthermore, they are also prone to ingestion of hydrogen from any water vapour present. An immediate indication is unsightly surface oxidation but other, often more significant, effects are loss of corrosion resistance in chromium-containing steels and degradation in mechanical properties in many high strength alloys [1] . When the GTAW process was first introduced in the 1940s it became immediately apparent that protection of the tungsten electrode by an inert gas was necessary to prevent rapid deterioration by oxidation. The use of inert gas in this way also protected the top of the weld deposit and any adjacent parent metal. It was some time before the need to also protect the weld underbead became apparent, principally because research began to indicate that lack of protection could lead to significant local reduction in mechanical properties in some materials. With flat plate joints, any locally affected material can be removed mechanically but with tube and pipe welding, where access to the weld root area is practically impossible, alternative solutions had to be found. Inert gas protection became known as weld purging and the last 20 years have witnessed significant development in purging techniques for tube and pipe joints.

Early attempts to provide protection employed the simple expedient of flooding the joint areawith inert gas, normally argon or helium. This was ineffective because ensuring adequate coverage proved too difficult; draughts and turbulence caused intermixing of any inert gas with atmospheric gases and moisture, thus defeating the objective. Isolating the pipe section at risk by locating cardboard, wooden or plastic sealing discs either side of the joint was an improvement, but all too often these provided inadequate seals and even occasionally burst into flames. Amazingly, these techniques are still used, even by some leading international operators. Notwithstanding the recognition that inert gas protection was necessary, users overlooked the need to monitor the purge gas to ensure that an acceptable maximum oxygen content, typically 50 parts per million (ppm), but with sensitive alloys as low as 20ppm, was maintained. High-performance quick connect-disconnect fittings provide security coupled with leak tightness and are used globally for high pressure connections in the pneumatics industry

PurgEye ® 600 computerised colour touchscreen weld purge monitor measuring from atmospheric oxygen levels down to 1 ppm. The instrument offers control of automatic welding equipment with a fast-response long life sensor and upper and lower ppm levels to trigger alarms

The concept of using inflatable pipe dams such as the PurgEye and PurgElite systems to ensure complete isolation of the joint and local material. These systems are available covering pipe diameters from 25 to 2,400mm diameter

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NOVEMBER 2021