Studies of the pressure induced transition from diffuse to constricted arcing in large vacuum arc furnaces
Vacuum arc remelting (VAR) is a secondary melting process used to produce superalloys. A dc arc is utilized to melt the alloy at normal pressures of {approximately}1 Pa. However, in some instances the pressure in the region of the arc can reach abnormally high values due to CO evolution from the molten ingot, giving rise to a furnace condition known as glow.'' This phenomenon was investigated by studying metal vapor arcs burning on 152 mm diameter Alloy 718 electrodes at 2.3 kA in a modified VAR furnace in CO ambients of 1.3, 13.3, and 66.7 Pa. Electrical signatures and high speed photographic data were recorded at all pressures. At the lowest pressure, the arc was diffuse with cathode spots distributed randomly across the molten electrode surface. Under these conditions, the inter-electrode space appeared to be uniformly filled with plasma. Monochromatic images taken at this pressure show mobile ion channels connecting regions of cathode spot activity with the molten ingot pool. Because spot motion is fast compared to the thermal diffusion speed in the ingot, this arc constitutes a quasi-uniform heat source necessary for successful application of the process. At a CO pressure of 66.7 Pa, the arc was localized and normal cathode spot action had ceased, although normal appearing constricted plasma attachments were observed to spontaneously appear at the bottom of molten metal protuberances as they dripped from the electrode and drew within a few millimeters of the ingot surface. At this pressure, the arc was observed to operate in either of the two modes, one characterized by high'' voltage ({minus}24 V), and the other by low voltage ({minus}19 V). At 13.3 Pa CO the arc was not stable, although heating of the electrode remained quasi-uniform. 14 refs., 12 figs.
- Research Organization:
- Sandia National Labs., Albuquerque, NM (USA)
- Sponsoring Organization:
- DOE/DP
- DOE Contract Number:
- AC04-76DP00789
- OSTI ID:
- 6952361
- Report Number(s):
- SAND-90-1006C; CONF-9009192-9; ON: DE90015693
- Resource Relation:
- Conference: International symposium on discharges and electrical insulation in vacuum, Santa Fe, NM (USA), 17-20 Sep 1990
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ARC FURNACES
ELECTRIC ARCS
PRESSURE EFFECTS
VACUUM FURNACES
CARBON MONOXIDE
ELECTRODES
INCONEL 718
VACUUM MELTING
ALLOY-NI53CR19FE19NB5MO3
ALLOYS
ALUMINIUM ADDITIONS
ALUMINIUM ALLOYS
CARBON COMPOUNDS
CARBON OXIDES
CHALCOGENIDES
CHROMIUM ALLOYS
CORROSION RESISTANT ALLOYS
CURRENTS
ELECTRIC CURRENTS
ELECTRIC DISCHARGES
ELECTRIC FURNACES
FURNACES
HEAT RESISTANT MATERIALS
HEAT RESISTING ALLOYS
INCONEL ALLOYS
IRON ALLOYS
MATERIALS
MELTING
MOLYBDENUM ALLOYS
NICKEL ALLOYS
NICKEL BASE ALLOYS
NIOBIUM A
OXIDES
OXYGEN COMPOUNDS
PHASE TRANSFORMATIONS
360101* - Metals & Alloys- Preparation & Fabrication