Electrode erosion and arc stability in transferred arcs with graphite electrodes

Abstract

Arc stability and erosion behavior were studied on a hollow graphite DC cathode in an argon atmosphere at atmospheric pressure. It was found that the arc stability is associated with the electron emission mode transition of the cathode operation. Estimation of current densities, SEM pictures, Fast Fourier Transform (FFT) of total voltage, and measurement of cathode surface temperature supported this. Stable arcs are in the thermionic emission regime while unstable arcs in the thermofield emission regime. Higher argon gas flow rate is believed to cause the shift of the mode from the thermofield emission to the thermionic emission by increasing the arc root temperature through steepening the thermal gradient at the arc root and increasing ionization phenomena inside the arc. Sharp cathode tip geometry usually leads to the thermionic emission while a rounded tip geometry encourages the thermofield emission. For the unstable arcs, the high voltage fluctuation resulted from the jumping of the arc root between different cathode spots and changes in the arc length. In the stable arcs, however, the voltage was almost constant because of the absence of arc jumping. The standard deviation of the voltage was used as the arc stability indicator and was less than 3 V for the stable arc in this transferred arc system.The erosion rate of the cathode in this work ranged from 0.41 to 2.61 mug/C. At 150 A runs the arc stability strongly influenced the erosion rate; as the arc stability increased, the erosion rate decreased. Higher currents runs (300 and 400 A), however, showed the opposite trend because of the carbon vapor redeposition. The total erosion rates of 150 A runs were separated into the stable (Es) and the unstable (Eu) erosion rate. The Eu was more than 3 times higher in this work. It is believed that the thermofield emission of the unstable arcs produced more erosion because of the higher local heat flux to the cathode spots