Observations of melt rate as a function of arc power, CO pressure, and electrode gap during vacuum consumable arc remelt
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I.
INTRODUCTION
VACUUM consumable arc remelting is a process used to control the solidification of segregation sensitive alloys. This control is accomplished by manipulation o f the system thermodynamics in a manner such that the advancing solidification region has an abundant supply o f Iiquid metal while simultaneously minimizing local solidification time. Energy is added to the system by means o f a direct current vacuum arc, and the internal distribution o f this energy within the system determines melt rate, fluid flow within the molten pool, and the volume o f the molten pool. Fluid flow is particularly affected by transient behavior o f this energy distribution. Hence, the remelting operation must be controlled to give a steady melt rate. The purpose of this work is to evaluate the effect of variations of arc electrical power, carbon monoxide pressure, and electrode gap on melting rate. Arc electrical power is defined as the product o f arc voltage and melting current with different levels of power obtained by changing the level o f melting current. Because arc voltage increases with increasing current and decreases with increasing pressure, arc power input is nonlinear. Unlike melting current, furnace pressure and electrode gap are difficult to control while melting is underway. In situ gas analysis conducted with a differentially pumped and calibrated quadrupole gas analyzer show that furnace atmosphere consists mainly of contaminant gases such as O2, N2, and water vapor plus CO when vacuum consumable arc melting a family of rare earth deoxidized maraging steels.* Carbon monoxide typically comprised from 50 to
80 pet of the furnace atmosphere for this family of alloys. The CO evolved is directly proportional to the carbon loss incurred during melting (see Figure 1), and this suggests that a carbon boil occurs even though the electrode carbon concentrations are very low (0.003 to 0.007 wt pet C) and two o f the melts contain Ti (M22, 0.35 pct Ti, M24, 0.50 pct Ti). Thus, it is also likely that CO is generated when melting most Ni and Fe base alloys. Sources o f oxygen needed to produce CO resulting from vacuum chamber and seal leaks can be controlled with proper preventative maintenance. However, sources from within the electrode, both steady state and position variant, are a function o f the electrode chemistry, previous melting practice, and solidification conditions and cannot be controlled during arc remelting. In order to learn more about the effect o f this .006
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E J. ZANNER and L. A. BERTRAM are Members of Technical Staff, Sandia National Laboratories, Albuquerque, NM. C. ADASCZIK is with Special Metals Corporation, East Hartford, NY. T. O ' B R I E N is with Cameron Iron Works, Houston, TX. Manuscript submitted April 18, 1983.
METALLURGICAL
TRANSACTIONS B
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*Base alloy chemistry: 8 to 15 pct Ni, 15 pct CO, 5 to 10 pct Mo, and balance Fe (wt pct).
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E V O L V E D CO
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