Materials for thermionically emitting electrodes in arc welding

Abstract

Non-consumable electrodes are made of pure or alloyed tungsten for arc welding processes, and graphite for gouging. Yet the technical literature on arc welding has little consideration on the non-use of other materials such as thermionic emitters. Aiming to contribute to filling this knowledge gap, this study attempts to justify the unviability of the use of pure iron in the manufacture of non-consumable electrodes, and to demonstrate the thermionic emission potential of pure and thoriated tungsten and graphite electrodes. Initially, the emission areas and respective diameters for welding at 100 and 300 A at several temperatures were theoretically determined. Then, the maximum temperatures attained by the Joule effect on these electrodes at different diameters were calculated. The results were validated by means of comparison with the phenomena observed in practice. Non-consumable, pure iron electrodes were shown to be unviable, because they require extremely large diameters to emit at normal current levels below their melting temperatures. Furthermore, at these diameters, they would not be heated by the Joule effect up to the emission temperature. On the other hand, it has been demonstrated that pure and alloyed tungsten and graphite electrodes emit with smaller diameters at temperatures close to their melting/sublimation temperatures, and they quickly reach emission temperatures by the Joule effect. Despite the simplifications adopted, the need was shown for improvement of the electrodes for arc welding, and it was demonstrated that W electrode doping with lower uranium oxides limits the heating of the electrode tip, preventing the melting of its surface.