Abstract:
The behavior of a metal vapor plasma arc in a vacuum arc remelting (VAR) furnace is
believed to contribute to the formation of defects in reactive metal and super-alloy
ingots. Industry standard instrumentation, which includes electric current and voltage
measurements, can asses the stability of an arc but cannot predict the location of an
arc. It is known that Maxwell’s equations predict a magnetic flux density at a distance
from an arc. It is shown that a single arc’s location can be uniquely determined in a
cross section by using an externally mounted 2-axis Hall Effect magnetic flux density
sensor provided that the system’s electric current is also measured and the geometry of
the VAR furnace is known. The solution is based on the Biot-Savart Law with finite
element modeling assisting the analysis. The methodology is validated using
controlled, static experiments. The measurement system is deployed on a small scale,
experimental VAR furnace to investigate arc behavior. Results from VAR operation
show a time averaged arc distribution that does not significantly change over the
course of a melt. By comparing the results from multiple sensors, observed arc
motions are categorized as being either retrograde or sympathetic. The former is
characterized by large periodic motions, and the latter either small random motions or
motions associated with an event such as a liquid metal drip short. Significant
alternating currents are found to exist in the DC VAR furnace. A magnetostatic single
arc model is not sufficient to describe the current distribution in the VAR furnace at an
instant but it may be an effective means to detect quasi static non-axisymmetry or
slow time varying current profile changes during VAR operation.
