TPi September 2019

Attempts have been made to provide dams using paper, card, timber or even polystyrene plugs on the basis of economy but success is erratic – sealing is difficult, contamination is inevitable and effective removal of the debris following welding is difficult. The best solution, and the one now adopted globally by leading fabricators, is to employ commercially available integrated inflatable purging systems. For adequate protection of nickel alloys during welding, the internal purge atmosphere should have oxygen level reduced to 50 ppm (0.005 per cent) or less in order to obtain a root surface with little or no oxide. Control of purge gas oxygen content during welding is clearly an important aspect. Recent developments in monitoring instruments (Figure 3) have led to the introduction of equipment that not only measures oxygen level accurately but also provides facilities for continuous recording and even overrides the joining process if oxygen levels exceed those pre-set by the operator. Techniques for measuring oxygen content have been available for decades but only recently have instruments been developed specifically for welding applications. Users increasingly demand complete absence from discoloration and no loss of corrosion resistance and this implies purge gas oxygen content to be as low as 20 ppm (0.002 per cent). Very few oxygen purge monitors are capable of meeting this sensitivity, but the PurgEye ® (Figure 3) instruments cover all requirements.

Author profile Dr MJ Fletcher is a qualified metallurgist with extensive experience in welding and non-destructive testing. He works as an independent consultant, providing support to a wide range of manufacturing industry on a global basis. (barrow@post.com)

References

[1] Characterization of Inconel alloy 740H for Tube, Pipe and Fittings for Advanced Supercritical CO 2 Systems. de Barbadillo et al. 6 th International Supercritical CO 2 Power Cycles Symposium, March 2018, Pittsburgh. [2] Oxidation of nickel and cobalt based superalloys. Defense Metals Information Center. Battelle Memorial Institute Columbus, Ohio. Report 214 March 1, 1965. [3] Welding of nickel alloys – Part 1 – Job Knowledge 107 – The Welding Institute, Cambridge UK [4] Inert gas weld purging of pipes. M J Fletcher. Engineer Live, May 2017.

Acknowledgements

Images reproduced courtesy of Huntingdon Fusion Techniques Ltd, UK (HFT ® ).

Huntingdon Fusion Techniques – UK support@huntingdonfusion.com www.huntingdonfusion.com

Figure 3: PurgEye monitors include full-colour touchscreen control. The instruments support data logging and weld certification. Readings are accurate down to 10 ppm

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TUBE PRODUCTS INTERNATIONAL September 2019

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