Relationship between furnace voltage signatures and the operational parameters arc power, arc current, CO pressure, and
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I.
INTRODUCTION
OPTIMIZATION of vacuum arc remelt ingot quality occurs when steady melting and solidification conditions are maintained. Melting and solidification are coupled to the metal vapor arc which supplies the energy for melting and generates the Lorentz and buoyancy forces which drive the liquid metal in the pool ahead of the dendritic solidification front. The maintenance of optimal steady melting conditions is accomplished by stabilizing the arc in a diffuse state so it behaves as a macro-uniform source of heat (on a macro time scale appropriate to the thermal diffusion speed) across the electrode face and generates a diffuse steady distribution of current on the anode surfaces including the pool. During melting the furnace operator can control, with good accuracy and resolution, the melting current and the electrode velocity. He strives to maintain steady conditions, usually by holding the melting current constant and matching the electrode velocity to the melt rate and thereby maintaining a constant electrode gap.* Because it is not possible *Electrode gap is arbitrarily defined as the average spacing between the electrodes and is determined by measuring the distance required to drive the electrode down to sustain a short of 0.1 second.t
to obtain a direct measurement of electrode gap during production melting, algorithms based on drop short and anode spike frequency or period ~ are sometimes utilized to estimate gap. Present gap control practice, then, consists of varying the electrode velocity or feed rate V~ (mm/s) around a nominal value which is selected based on previous experience. Adjustments to the feed rate AVe (mm/s) around the nominal *INCONEL is a trademark of the INCO family of companies. E J. ZANNER and L. A. BERTRAM are with Sandia National Laboratories, Albuquerque, NM 87185; R. HARRISON is with Cameron Iron Works, Houston, TX; and H. D. FLANDERS is with Special Metals Corporation, New Hartford, NY 13413. Manuscript submitted May 6, 1985.
METALLURGICAL TRANSACTIONS B
value are usually controlled by one of (1) drop short frequency, (2) anode spike frequency (commonly called hash), (3) furnace voltage, or (4) operator intervention. The basic geometry of the vacuum arc remelt process dictates that control will be difficult in part because of strong sensitivity of electrode gap ge (mm) to electrode uniformity. For example, suppose a uniform electrode of cross sectional area Ae (mm 2) and density, p (g/mm 3) is being melted at a constant mass melt rate M (g/s) into an ingot of area A, (mm2). Then constant electrode gap will result when ve =
-1
)
Ill
holds. This means that changes in electrode uniformity AAe cause fractional velocity changes hVe/Ve which are proportional to the ratio A A e / ( A , / A e - 1). For large melts (A,/Ae - 1) is a small quantity, and as a result electrode nonuniformity due to solidification pipe, porosity, and surface conditioning can require large changes of Ve in order to maintain constant gap. In the velocity Eq. [ 1], melt rate ~/appears as a factor. T
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