Erosion Phenomena and Evaluation of Life-Time of Tungsten Electrode for Plasma Torch in Nitrogen Gas Flow

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  • プラズマトーチ用タングステン電極の窒素ガス流中での損耗現象と寿命の評価
  • プラズマ トーチヨウ タングステン デンキョク ノ チッソ ガスリュウチュウ

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Abstract

Arc-cathode erosion has been studied for tungsten cathode with 2wt% of lanthanum oxide at the current level of 400A in DC. Main results are obtained in case of changing plasma gas from mainly used argon (Ar) to cheaper nitrogen (N2):<br>(1) Erosion weight increased by 25 to 50 times and the quantity of oxide additives decreased in the tip region. These results suggest that the amount of heat transferred to the cathode increased.<br>(2) Through 100 hours operation using N2, the electrode eroded with repeating formation and dissipation of a columnar projection at the tip. Even the shape changed in such complicated manner, the erosion weight was almost proportional to the operating time. Referring to the results, we made short electrodes simulating after long operation and estimated the lifetime would be 150 to 300 hours.<br>(3) Finally, the operation of the plasma torch using N2 was demonstrated in a pilot plant. The erosion weight could be decreased to about 1/3 compared with the results of basic experiments by increasing electrode-cooling capacity.<br>We conclude that it is possible to choose cheaper N2 instead of more expensive Ar as plasma gas to reduce total operating cost in some application fields such as wide area heating.

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